fitting techniques, coupled with a broad wavelength coverage, are an
important approach to statistically understand the origins of UDGs
and their impact on the field of galaxy formation and evolution.

Software: ASTROPY (Astropy Collaboration 2013, 2018), FSPS (Conroy & Gunn 2010; Conroy et al. 2010), PYTHON-FSPS (Johnson et al. 2021a), PROSPECTOR (Leja et al. 2017; Johnson et al. 2021b).

ACKNOWLEDGEMENTS

We thank the anonymous referee for suggestions that greatly im-
proved the manuscript.

We are thankful to Patrick Durrell for his contributions to the MATLAS survey. We thank Pavan Uttarkar for all the help with KMeans and other clustering algorithms tested and used in this work. We also thank Amelia Fraser-McKelvie for her colloquium at Swinburne University which gave us the idea of applying clustering algorithms to our data set. We thank Yimeng Tang for important discussions about PROSPECTOR'S configurations and outputs.

This research was supported by the Australian Research Coun-
cil Centre of Excellence for All Sky Astrophysics in 3 Di-
mensions (ASTRO 3D), through project number CE170100013.
DAF, JPB, JP, and WJC thank the ARC for support via
DP220101863. AJR was supported by National Science Foundation
grant AST-2308390. AFM has received support from CEX2019-
00920-S, RYC2021-031099-I, and PID2021-123313NA-100
of MICIN/AEI/10.13039/501100011033/FEDER,UE, NextGenera-
tionEU/PRT. SL acknowledges the support from the Sejong Science
Fellowship Program through the National Research Foundation of
Korea (NRF-2021R1C1C2006790).

This paper is based in part on observations from the Legacy Survey, which consists of three individual and complementary projects: the Dark Energy Camera Legacy Survey (DECaLS; Proposal ID #2014B-0404; PIs: David Schlegel and Arjun Dey), the Beijing–Arizona Sky Survey (BASS; NOAO Prop. ID #2015A-0801; PIs: Zhou Xu and Xiaohui Fan), and the Mayall z-band Legacy Survey (MzLS; Prop. ID #2016A-0453; PI: Arjun Dey). This publication uses data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.

DATA AVAILABILITY

DECaLS data are available via the Legacy survey portal. WISE data
are available via the WISE archive. MATLAS data are available from
the CDS archives and through the MATLAS portal.

REFERENCES

Alabi A. et al., 2018, MNRAS, 479, 3308

Amorisco N. C., Loeb A., 2016, MNRAS, 459, L51

Astropy Collaboration, 2013, A&A, 558, A33

Astropy Collaboration, 2018, AJ, 156, 123

Barbosa C. E. et al., 2020, ApJS, 247, 46

Beasley M. A., Romanowsky A. J., Pota V., Navarro I. M., Martinez Delgado
D., Neyer F., Deich A. L., 2016, ApJ, 819, L20

Benavides J. A. et al., 2021, Nat. Astron., 5, 1255

Bertin E., Mellier Y., Radovich M., Missonnier G., Didelon P., Morin B., 2002, in Bohlender D. A., Durand D., Handley T. H., eds, ASP Conf. Ser. Vol. 281, Astronomical Data Analysis Software and Systems XI. Astron. Soc. Pac., San Francisco, p. 228

Bilek M. et al., 2020, MNRAS, 498, 2138

Burkert A., Forbes D. A., 2020, AJ, 159, 56

Buzzo M. L. et al., 2021, MNRAS, 504, 2146

Buzzo M. L. et al., 2022, MNRAS, 517, 2231(B22)

Buzzo M. L., Forbes D. A., Brodie J. P., Janssens S. R., Couch W. J., Romanowsky A. J., Gannon J. S., 2023, MNRAS, 522, 595

Calzetti D., Armus L., Bohlin R. C., Kinney A. L., Koornneef J., Storchi-Bergmann T., 2000, ApJ, 533, 682

Cappellari M., 2017, MNRAS, 466, 798

Cardona-Barrero S., Di Cintio A., Brook C. B. A., Ruiz-Lara T., Beasley M. A., Falcón-Barroso J., Macciò A. V., 2020, MNRAS, 497, 4282

Carleton T., Errani R., Cooper M., Kaplinghat M., Peñarrubia J., Guo Y., 2019, MNRAS, 485, 382

Carleton T., Guo Y., Munshi F., Tremmel M., Wright A., 2021, MNRAS, 502, 398

Carnall A. C., McLure R. J., Dunlop J. S., Davé R., 2018, MNRAS, 480, 4379

Chan T. K., Kereš D., Wetzel A., Hopkins P. F., Faucher-Giguère C. A., El-Badry K., Garrison-Kimmel S., Boylan-Kolchin M., 2018, MNRAS, 478, 906

Chartab N., Newman A. B., Rudie G. C., Blanc G. A., Kelson D. D., 2024, ApJ, 960, 73

Chilingarian I. V., Afanasiev A. V., Grishin K. A., Fabricant D., Moran S., 2019, ApJ, 884, 79

Coccato L., Fraser-McKelvie A., Jaffé Y. L., Johnston E. J., Cortesi A., Pallero D., 2022, MNRAS, 515, 201

Collins M. L. M., Read J. I., 2022, Nat. Astron., 6, 647

Conroy C., Gunn J. E., 2010, ApJ, 712, 833

Conroy C., Gunn J. E., White M., 2009, ApJ, 699, 486

Conroy C., White M., Gunn J. E., 2010, ApJ, 708, 58

Danieli S. et al., 2022, ApJ, 927, L28

Davis M., Geller M. J., 1976, ApJ, 208, 13

Dey A. et al., 2019, AJ, 157, 168

Di Cintio A., Brook C. B., Dutton A. A., Macciò A. V., Obreja A., Dekel A., 2017, MNRAS, 466, L1

Doppel J. E., Sales L. V., Benavides J. A., Toloba E., Peng E. W., Nelson D., Navarro J. F., 2024, MNRAS, 529, 1827

Dressler A., 1980, ApJ, 236, 351

Dressler A. et al., 1997, ApJ, 490, 577

Duc P.-A., 2020, preprint (arXiv:2007.13874)

Duc P.-A., Paudel S., McDermid R. M., Cuillandre J.-C., Serra P., Bournaud F., Cappellari M., Emsellem E., 2014, MNRAS, 440, 1458

Duc P.-A. et al., 2015, MNRAS, 446, 120

Faber S. M. et al., 2003, in Iye M., Moorwood A. F. M., eds, Proc. SPIE Conf. Ser. Vol. 4841, Instrument Design and Performance for Optical/Infrared Ground-based Telescopes. SPIE, Bellingham, p. 1657

Ferré-Mateu A. et al., 2018, MNRAS, 479, 4891

Ferré-Mateu A., Gannon J. S., Forbes D. A., Buzzo M. L., Romanowsky A. J., Brodie J. P., 2023, MNRAS, 526, 4735(FM23)

Forbes D. A., Gannon J., 2024, MNRAS, 528, 608

Forbes D. A., Alabi A., Romanowsky A. J., Brodie J. P., Arimoto N., 2020, MNRAS, 492, 4874

Forbes D. A., Gannon J. S., Romanowsky A. J., Alabi A., Brodie J. P., Couch W. J., Ferré-Mateu A., 2021, MNRAS, 500, 1279

Forbes D. A. et al., 2023, MNRAS, 525, L93

Gannon J. S., Forbes D. A., Romanowsky A. J., Ferré-Mateu A., Couch W. J., Brodie J. P., 2020, MNRAS, 495, 2582

Gannon J. S. et al., 2021, MNRAS, 502, 3144

Gannon J. S. et al., 2022, MNRAS, 510, 946

Gannon J. S., Forbes D. A., Brodie J. P., Romanowsky A. J., Couch W. J., Ferré-Mateu A., 2023a, MNRAS, 518, 3653

Gannon J. S., Buzzo M. L., Ferré-Mateu A., Forbes D. A., Brodie J. P., Romanowsky A. J., 2023b, MNRAS, 524, 2624

Garcia-Dias R., Allende Prieto C., Sánchez Almeida J., Ordovás-Pascual I., 2018, A&A, 612, A98

Gordon K. D., Clayton G. C., Misselt K. A., Landolt A. U., Wolff M. J., 2003, ApJ, 594, 279

Harris W. E., Harris G. L. H., Alessi M., 2013, ApJ, 772, 82

Haslbauer M., Dabringhausen J., Kroupa P., Javanmardi B., Banik I., 2019, A&A, 626, A47
Häußler B. et al., 2013, MNRAS, 430, 330

Heesters N. et al., 2023, A&A, 676, A33

Higson E., Handley W., Hobson M., Lasenby A., 2019, Stat. Comput., 29, 891

Hubble E., Humason M. L., 1931, ApJ, 74, 43

Iodice E. et al., 2023, A&A, 679, A69

Jarrett T. H. et al., 2012, AJ, 144, 68

Jarrett T. H. et al., 2013, AJ, 145, 6

Jarrett T. H., Cluver M. E., Brown M. J. I., Dale D. A., Tsai C. W., Masci F., 2019, ApJS, 245, 25

Johnson B. et al., 2021a, dmf/python-fsps: python-fsps v0.4.1rc1 (v0.4.1rc1). Zenodo. Available at: https://doi.org/10.5281/zenodo.4737461

Johnson B. D., Leja J., Conroy C., Speagle J. S., 2021b, ApJS, 254, 22

Jones M. G. et al., 2023, ApJ, 942, L5

Kado-Fong E., Greene J. E., Huang S., Beaton R., Goulding A. D., Komiyama Y., 2020, ApJ, 900, 163

Kado-Fong E. et al., 2021, ApJ, 920, 72

Kadowaki J., Zaritsky D., Donnerstein R. L., RS P., Karunakaran A., Spekkens K., 2021, ApJ, 923, 257

Kroupa P., 2001, MNRAS, 322, 231

La Marca A. et al., 2022, A&A, 665, A105

Leja J., Johnson B. D., Conroy C., van Dokkum P. G., Byler N., 2017, ApJ, 837, 170

Leja J. et al., 2019, ApJ, 877, 140

Li J. et al., 2022, MNRAS, 515, 5335

Li J., Greene J. E., Greco J., Beaton R., Danieli S., Goulding A., Huang S., Kado-Fong E., 2023, ApJ, 955, 2

Lim S., Peng E. W., Côté P., Sales L. V., den Brok M., Blakeslee J. P., Guhathakurta P., 2018, ApJ, 862, 82

Lim S. et al., 2020, ApJ, 899, 69

Ma X., Hopkins P. F., Faucher-Giguère C.-A., Zolman N., Muratov A. L., Kereš D., Quataert E., 2016, MNRAS, 456, 2140

MacQueen J., 1967, in Cam Le L. M., Neyman N., eds, Berkeley Symp. on Math. Statist. and Prob., Vol. 5.1, Some Methods for Classification and Analysis of Multivariate Observations. California, p. 281

Mancera Piña P. E., Aguerri J. A. L., Peletier R. F., Venhola A., Trager S., Choque Challapa N., 2019a, MNRAS, 485, 1036

Mancera Piña P. E. et al., 2019b, ApJ, 883, L33

Mancera Piña P. E., Fraternali F., Oosterloo T., Adams E. A. K., Oman K. A., Leisman L., 2022, MNRAS, 512, 3230

Marigo P., Girardi L., 2007, A&A, 469, 239

Marigo P., Girardi L., Bressan A., Groenewegen M. A. T., Silva L., Granato G. L., 2008, A&A, 482, 883

Marleau F. R. et al., 2021, A&A, 654, A105

Martin D. C. et al., 2005, ApJ, 619, L1

Moore B., Katz N., Lake G., Dressler A., Oemler A., 1996, Nature, 379, 613

Moore B., Lake G., Katz N., 1998, ApJ, 495, 139

Moore B., Lake G., Quinn T., Stadel J., 1999, MNRAS, 304, 465

Morrissey P. et al., 2007, ApJS, 173, 682

Müller O. et al., 2020, A&A, 640, A106

Müller O. et al., 2021, ApJ, 923, 9

Oemler A., Jr, 1974, ApJ, 194, 1

Pandya V. et al., 2018, ApJ, 858, 29

Pedregosa F. et al., 2011, J. Mach. Learn. Res., 12, 2825

Peng E. W., Lim S., 2016, ApJ, 822, L31

Penny S. J., Conzelice C. J., de Rijcke S., Held E. V., Gallagher J. S., O'Connell R. W., 2011, MNRAS, 410, 1076

Phillips S., Driver S. P., Couch W. J., Smith R. M., 1998, ApJ, 498, L19

Planck Collaboration VI, 2020, A&A, 641, A6

Poulain M. et al., 2021, MNRAS, 506, 5494

Prochaska J. et al., 2020, J. Open Source Softw., 5, 2308

Prole D. J., van der Burg R. F. J., Hilker M., Davies J. I., 2019, MNRAS, 488, 2143

Román J., Trujillo I., 2017, MNRAS, 468, 4039

Rong Y. et al., 2020, ApJ, 899, 78

Ruiz-Lara T. et al., 2018, MNRAS, 478, 2034

Saifollahi T., Zaritsky D., Trujillo I., Peletier R. F., Knapen J. H., Amorisco N., Beasley M. A., Donnerstein R., 2022, MNRAS, 511, 4633

Sales L. V., Wetzel A., Fattahi A., 2022, Nat. Astron., 6, 897

Sánchez Almeida J., Aguerri J. A. L., Muñoz-Tuñón C., de Vicente A., 2010, ApJ, 714, 487

Sánchez-Janssen R. et al., 2019, MNRAS, 486, L1

Sawicki M., 2012, PASP, 124, 1208

Schlafly E. F., Finkbeiner D. P., 2011, ApJ, 737, 103

Schlegel D. J., Finkbeiner D. P., Davis M., 1998, ApJ, 500, 525

Simon J. D., 2019, ARA&A, 57, 375

Skilling J., 2004, in Fischer R., Preuss R., Toussaint U. V., eds, AIP Conf. Proc. Vol. 735, Bayesian Inference and Maximum Entropy Methods in Science and Engineering: 24th International Workshop on Bayesian Inference and Maximum Entropy Methods in Science and Engineering. Am. Inst. Phys., New York, p. 395

Smith R., Sánchez-Janssen R., Fellhauer M., Puzia T. H., Aguerri J. A. L., Farias J. P., 2013, MNRAS, 429, 1066

Smith R. et al., 2015, MNRAS, 454, 2502

Speagle J. S., 2020, MNRAS, 493, 3132

Toloba E. et al., 2018, ApJ, 856, L31

Toloba E. et al., 2023, ApJ, 951, 77

Tremmel M., Wright A. C., Brooks A. M., Munshi F., Nagai D., Quinn T. R., 2020, MNRAS, 497, 2786

Trujillo-Gomez S., Kruijssen J. M. D., Keller B. W., Reina-Campos M., 2021, MNRAS, 506, 4841

Trujillo I. et al., 2019, MNRAS, 486, 1192

van Dokkum P. G., Abraham R., Merritt A., Zhang J., Geha M., Conroy C., 2015, ApJ, 798, L45

van Dokkum P. et al., 2017, ApJ, 844, L11

van Dokkum P. et al., 2018, Nature, 555, 629

van Dokkum P. et al., 2019, ApJ, 880, 91

van Dokkum P. et al., 2022, Nature, 605, 435

Van Nest J. D., Munshi F., Wright A. C., Tremmel M., Brooks A. M., Nagai D., Quinn T., 2022, ApJ, 926, 92

Vazdekis A. et al., 2015, MNRAS, 449, 1177

Vika M., Bamford S. P., Häußler B., Rojas A. L., Borch A., Nichol R. C., 2013, MNRAS, 435, 623

Villaume A. et al., 2022, ApJ, 924, 32

Webb K. A. et al., 2022, MNRAS, 516, 3318

Wright E. L. et al., 2010, AJ, 140, 1868

Zaritsky D., Donnerstein R., Dey A., Karunakaran A., Kadowaki J., Khim D. J., Spekkens K., Zhang H., 2023, ApJS, 267, 27

Zepf S. E., Whitmore B. C., 1993, ApJ, 418, 72

## APPENDIX A: DEIMOS SPECTROSCOPIC DATA

Our spectroscopic data were obtained from the Keck II telescopes with the Deep Imaging Multi-Object Spectrograph (DEIMOS, Faber et al. 2003) spectrograph. MATLAS-342, MATLAS-368, and MATLAS-1059 were the selected targets due to their higher surface brightness and/or presence of a bright nucleus. They were observed with DEIMOS on the nights of 2022 November 23 and 2022 November 24, with good seeing ranging from 0.4 to 0.8 arcsec most of the nights, with a short interval of seeing ~2 arcsec. MATLAS-342 and MATLAS-368 were observed with the blue-sensitive 1200 lines $mm^{-1}$ grating (1200B), GG400 filter, central wavelength 5850 Å and wavelength coverage of 4500–7200 Å for 1 h, with individual exposures of 1800 s with a custom-designed 3 arcsec-wide longslit. MATLAS-1059 was observed with the same set-up, but only for 1800 s.

The data were reduced with the open-source reduction package PypeIt (version 1.12.0, Prochaska et al. 2020). The arc image obtained in the same night as our observations turned out to have signs of ghosting, and thus, additional arc images were obtained in the same configurations on the night 2023 March 23 and were the ones used to reduce these data.
Häußler B. et al., 2013, MNRAS, 430, 330

Heesters N. et al., 2023, A&A, 676, A33

Higson E., Handley W., Hobson M., Lasenby A., 2019, Stat. Comput., 29, 891

Hubble E., Humason M. L., 1931, ApJ, 74, 43

Iodice E. et al., 2023, A&A, 679, A69

Jarrett T. H. et al., 2012, AJ, 144, 68

Jarrett T. H. et al., 2013, AJ, 145, 6

Jarrett T. H., Cluver M. E., Brown M. J. I., Dale D. A., Tsai C. W., Masci F., 2019, ApJS, 245, 25

Johnson B. et al., 2021a, dmf/python-fsps: python-fsps v0.4.1rc1 (v0.4.1rc1). Zenodo. Available at: https://doi.org/10.5281/zenodo.4737461

Johnson B. D., Leja J., Conroy C., Speagle J. S., 2021b, ApJS, 254, 22

Jones M. G. et al., 2023, ApJ, 942, L5

Kado-Fong E., Greene J. E., Huang S., Beaton R., Goulding A. D., Komiyama Y., 2020, ApJ, 900, 163

Kado-Fong E. et al., 2021, ApJ, 920, 72

Kadowaki J., Zaritsky D., Donnerstein R. L., RS P., Karunakaran A., Spekkens K., 2021, ApJ, 923, 257

Kroupa P., 2001, MNRAS, 322, 231

La Marca A. et al., 2022, A&A, 665, A105

Leja J., Johnson B. D., Conroy C., van Dokkum P. G., Byler N., 2017, ApJ, 837, 170

Leja J. et al., 2019, ApJ, 877, 140

Li J. et al., 2022, MNRAS, 515, 5335

Li J., Greene J. E., Greco J., Beaton R., Danieli S., Goulding A., Huang S., Kado-Fong E., 2023, ApJ, 955, 2

Lim S., Peng E. W., Côté P., Sales L. V., den Brok M., Blakeslee J. P., Guhathakurta P., 2018, ApJ, 862, 82

Lim S. et al., 2020, ApJ, 899, 69

Ma X., Hopkins P. F., Faucher-Giguère C.-A., Zolman N., Muratov A. L., Kereš D., Quataert E., 2016, MNRAS, 456, 2140

MacQueen J., 1967, in Cam Le L. M., Neyman N., eds, Berkeley Symp. on Math. Statist. and Prob., Vol. 5.1, Some Methods for Classification and Analysis of Multivariate Observations. California, p. 281

Mancera Piña P. E., Aguerri J. A. L., Peletier R. F., Venhola A., Trager S., Choque Challapa N., 2019a, MNRAS, 485, 1036

Mancera Piña P. E. et al., 2019b, ApJ, 883, L33

Mancera Piña P. E., Fraternali F., Oosterloo T., Adams E. A. K., Oman K. A., Leisman L., 2022, MNRAS, 512, 3230

Marigo P., Girardi L., 2007, A&A, 469, 239

Marigo P., Girardi L., Bressan A., Groenewegen M. A. T., Silva L., Granato G. L., 2008, A&A, 482, 883

Marleau F. R. et al., 2021, A&A, 654, A105

Martin D. C. et al., 2005, ApJ, 619, L1

Moore B., Katz N., Lake G., Dressler A., Oemler A., 1996, Nature, 379, 613

Moore B., Lake G., Katz N., 1998, ApJ, 495, 139

Moore B., Lake G., Quinn T., Stadel J., 1999, MNRAS, 304, 465

Morrissey P. et al., 2007, ApJS, 173, 682

Müller O. et al., 2020, A&A, 640, A106

Müller O. et al., 2021, ApJ, 923, 9

Oemler A., Jr, 1974, ApJ, 194, 1

Pandya V. et al., 2018, ApJ, 858, 29

Pedregosa F. et al., 2011, J. Mach. Learn. Res., 12, 2825

Peng E. W., Lim S., 2016, ApJ, 822, L31

Penny S. J., Conzelice C. J., de Rijcke S., Held E. V., Gallagher J. S., O'Connell R. W., 2011, MNRAS, 410, 1076

Phillipps S., Driver S. P., Couch W. J., Smith R. M., 1998, ApJ, 498, L19

Planck Collaboration VI, 2020, A&A, 641, A6

Poulain M. et al., 2021, MNRAS, 506, 5494

Prochaska J. et al., 2020, J. Open Source Softw., 5, 2308

Prole D. J., van der Burg R. F. J., Hilker M., Davies J. I., 2019, MNRAS, 488, 2143

Román J., Trujillo I., 2017, MNRAS, 468, 4039

Rong Y. et al., 2020, ApJ, 899, 78

Ruiz-Lara T. et al., 2018, MNRAS, 478, 2034

Saifollahi T., Zaritsky D., Trujillo I., Peletier R. F., Knapen J. H., Amorisco N., Beasley M. A., Donnerstein R., 2022, MNRAS, 511, 4633

Sales L. V., Wetzel A., Fattahi A., 2022, Nat. Astron., 6, 897

Sánchez Almeida J., Aguerri J. A. L., Muñoz-Tuñón C., de Vicente A., 2010, ApJ, 714, 487

Sánchez-Janssen R. et al., 2019, MNRAS, 486, L1

Sawicki M., 2012, PASP, 124, 1208

Schlafly E. F., Finkbeiner D. P., 2011, ApJ, 737, 103

Schlegel D. J., Finkbeiner D. P., Davis M., 1998, ApJ, 500, 525

Simon J. D., 2019, ARA&A, 57, 375

Skilling J., 2004, in Fischer R., Preuss R., Toussaint U. V., eds, AIP Conf. Proc. Vol. 735, Bayesian Inference and Maximum Entropy Methods in Science and Engineering: 24th International Workshop on Bayesian Inference and Maximum Entropy Methods in Science and Engineering. Am. Inst. Phys., New York, p. 395

Smith R., Sánchez-Janssen R., Fellhauer M., Puzia T. H., Aguerri J. A. L., Farias J. P., 2013, MNRAS, 429, 1066

Smith R. et al., 2015, MNRAS, 454, 2502

Speagle J. S., 2020, MNRAS, 493, 3132

Toloba E. et al., 2018, ApJ, 856, L31

Toloba E. et al., 2023, ApJ, 951, 77

Tremmel M., Wright A. C., Brooks A. M., Munshi F., Nagai D., Quinn T. R., 2020, MNRAS, 497, 2786

Trujillo-Gomez S., Kruijssen J. M. D., Keller B. W., Reina-Campos M., 2021, MNRAS, 506, 4841

Trujillo I. et al., 2019, MNRAS, 486, 1192

van Dokkum P. G., Abraham R., Merritt A., Zhang J., Geha M., Conroy C., 2015, ApJ, 798, L45

van Dokkum P. et al., 2017, ApJ, 844, L11

van Dokkum P. et al., 2018, Nature, 555, 629

van Dokkum P. et al., 2019, ApJ, 880, 91

van Dokkum P. et al., 2022, Nature, 605, 435

Van Nest J. D., Munshi F., Wright A. C., Tremmel M., Brooks A. M., Nagai D., Quinn T., 2022, ApJ, 926, 92

Vazdekis A. et al., 2015, MNRAS, 449, 1177

Vika M., Bamford S. P., Häußler B., Rojas A. L., Borch A., Nichol R. C., 2013, MNRAS, 435, 623

Villaume A. et al., 2022, ApJ, 924, 32

Webb K. A. et al., 2022, MNRAS, 516, 3318

Wright E. L. et al., 2010, AJ, 140, 1868

Zaritsky D., Donnerstein R., Dey A., Karunakaran A., Kadowaki J., Khim D. J., Spekkens K., Zhang H., 2023, ApJS, 267, 27

Zepf S. E., Whitmore B. C., 1993, ApJ, 418, 72

## APPENDIX A: DEIMOS SPECTROSCOPIC DATA

Our spectroscopic data were obtained from the Keck II telescopes with the Deep Imaging Multi-Object Spectrograph (DEIMOS, Faber et al. 2003) spectrograph. MATLAS-342, MATLAS-368, and MATLAS-1059 were the selected targets due to their higher surface brightness and/or presence of a bright nucleus. They were observed with DEIMOS on the nights of 2022 November 23 and 2022 November 24, with good seeing ranging from 0.4 to 0.8 arcsec most of the nights, with a short interval of seeing ~2 arcsec. MATLAS-342 and MATLAS-368 were observed with the blue-sensitive 1200 lines $mm^{-1}$ grating (1200B), GG400 filter, central wavelength 5850 Å and wavelength coverage of 4500–7200 Å for 1 h, with individual exposures of 1800 s with a custom-designed 3 arcsec-wide longslit. MATLAS-1059 was observed with the same set-up, but only for 1800 s.

The data were reduced with the open-source reduction package PypeIt (version 1.12.0, Prochaska et al. 2020). The arc image obtained in the same night as our observations turned out to have signs of ghosting, and thus, additional arc images were obtained in the same configurations on the night 2023 March 23 and were the ones used to reduce these data.
<figure><img src="image_13.png" /><figcaption>**Figure A1.** PPXF fitting of MATLAS-342, one MATLAS UDG with available spectra from Keck/DEIMOS. The black curve shows the spectrum of each source, and the red curve shows the best fit. The green dots are the residuals from the fit. The grey regions show the masked areas in the fit. Legend box within the plot shows the recovered radial velocity of the UDG ($V = 2142 \pm 31$ km s⁻¹) and the radial velocity of its host galaxy ($V = 2153$ km s⁻¹). Results indicate that this UDG is at the same redshift as its host.</figcaption></figure>

<table><caption>**Table A1.** PPXF fitting results for the MATLAS UDGs studied in this work. Columns stand for (1) Galaxy ID; (2) signal-to-noise ratio of the resulting spectrum; (3) recovered recessional velocity; (4) name of host; (5) recessional velocity of host galaxy.</caption><thead><tr><th>Galaxy</th><th>S/N (Å<sup>-1</sup>)</th><th>V (km s<sup>-1</sup>)</th><th>Candidate host</th><th>V<sub>host</sub> (km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>MATLAS-342</td><td>8.2</td><td>2142 ± 31</td><td>NGC 2481</td><td>2153</td></tr><tr><td>MATLAS-368</td><td>3.8</td><td>2035 ± 97</td><td>NGC 2577</td><td>2073</td></tr><tr><td>MATLAS-1059</td><td>3.2</td><td>1248 ± 360</td><td>NGC 3683</td><td>1700</td></tr></tbody></table>

We fit each spectrum with PPXF (Cappellari 2017), using the MILES stellar library (Vazdekis et al. 2015), assuming a Kroupa initial mass function (Kroupa 2001) and the BaSTI isochrones. In Fig. A1, we show one example of PPXF fit for the galaxy MATLAS-342. Results from the fits of all three galaxies are summarized in Table A1. The results indicate that all three probed UDGs are at the same redshifts as their massive neighbours.

We note that MATLAS-368 was selected to be followed up because it was thought to be nucleated, which would facilitate recovering its

velocity because of its brightness. However, the central point source was found to be at $z = 0$, thus likely being a foreground star. As an alternative, we studied the galaxy light coming from the outer parts of the slit, but this rendered low S/N data. The identification of the Hα line was possible, resulting in the recessional velocity shown in Table A1. We caution, none the less, that the result is based on a single line, which is highly uncertain.

The spectrum of the galaxy that was found to be closer than its host, MATLAS-1059, has very low signal-to-noise ($3.8$ Å⁻¹) due to its low exposure time, resulting in a much more uncertain velocity than the other studied UDGs. Even in this extreme case of low S/N, the recovered velocity is consistent with the one of the host. Additionally, from CFHT and DECaLS imaging, we can see that this UDG seems tidally disrupted by the host, further confirming its proximity to it.

These findings show that all three UDGs studied are at the redshifts of their hosts. This is in agreement with the recent study of Heesters et al. (2023) who found, after studying 56 MATLAS dwarfs, that 75 per cent of them were indeed at the same redshifts as their hosts, including the three UDGs present in their sample. Putting together our results and the ones from Heesters et al. (2023), we have a cumulative result that six out of six MATLAS UDGs followed up with spectroscopy are confirmed as associated with their parent host galaxy. We thus conclude that assuming the redshift of the host for the UDGs that have not been spectroscopically followed up is a reasonable assumption, bearing in mind that these are based on ensemble properties and may not hold for every individual galaxy.

## APPENDIX B: TABLES

### B1 MATLAS UDGs photometry

### B2 GALFITM results and physical properties of MATLAS UDGs

### B3 Stellar population properties from PROSPECTOR
<figure><img src="image_12.png" /><figcaption>**Figure A1.** PPXF fitting of MATLAS-342, one MATLAS UDG with available spectra from Keck/DEIMOS. The black curve shows the spectrum of each source, and the red curve shows the best fit. The green dots are the residuals from the fit. The grey regions show the masked areas in the fit. Legend box within the plot shows the recovered radial velocity of the UDG ($V = 2142 \pm 31$ km s⁻¹) and the radial velocity of its host galaxy ($V = 2153$ km s⁻¹). Results indicate that this UDG is at the same redshift as its host.</figcaption></figure>

<table><caption>**Table A1.** PPXF fitting results for the MATLAS UDGs studied in this work. Columns stand for (1) Galaxy ID; (2) signal-to-noise ratio of the resulting spectrum; (3) recovered recessional velocity; (4) name of host; (5) recessional velocity of host galaxy.</caption><thead><tr><th>Galaxy</th><th>S/N (Å<sup>-1</sup>)</th><th>V (km s<sup>-1</sup>)</th><th>Candidate host</th><th>V<sub>host</sub> (km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>MATLAS-342</td><td>8.2</td><td>2142 ± 31</td><td>NGC 2481</td><td>2153</td></tr><tr><td>MATLAS-368</td><td>3.8</td><td>2035 ± 97</td><td>NGC 2577</td><td>2073</td></tr><tr><td>MATLAS-1059</td><td>3.2</td><td>1248 ± 360</td><td>NGC 3683</td><td>1700</td></tr></tbody></table>

We fit each spectrum with PPXF (Cappellari 2017), using the MILES stellar library (Vazdekis et al. 2015), assuming a Kroupa initial mass function (Kroupa 2001) and the BaSTI isochrones. In Fig. A1, we show one example of PPXF fit for the galaxy MATLAS-342. Results from the fits of all three galaxies are summarized in Table A1. The results indicate that all three probed UDGs are at the same redshifts as their massive neighbours.

We note that MATLAS-368 was selected to be followed up because it was thought to be nucleated, which would facilitate recovering its

velocity because of its brightness. However, the central point source was found to be at $z = 0$, thus likely being a foreground star. As an alternative, we studied the galaxy light coming from the outer parts of the slit, but this rendered low S/N data. The identification of the Hα line was possible, resulting in the recessional velocity shown in Table A1. We caution, none the less, that the result is based on a single line, which is highly uncertain.

The spectrum of the galaxy that was found to be closer than its host, MATLAS-1059, has very low signal-to-noise (3.8 Å⁻¹) due to its low exposure time, resulting in a much more uncertain velocity than the other studied UDGs. Even in this extreme case of low S/N, the recovered velocity is consistent with the one of the host. Additionally, from CFHT and DECaLS imaging, we can see that this UDG seems tidally disrupted by the host, further confirming its proximity to it.

These findings show that all three UDGs studied are at the redshifts of their hosts. This is in agreement with the recent study of Heesters et al. (2023) who found, after studying 56 MATLAS dwarfs, that 75 per cent of them were indeed at the same redshifts as their hosts, including the three UDGs present in their sample. Putting together our results and the ones from Heesters et al. (2023), we have a cumulative result that six out of six MATLAS UDGs followed up with spectroscopy are confirmed as associated with their parent host galaxy. We thus conclude that assuming the redshift of the host for the UDGs that have not been spectroscopically followed up is a reasonable assumption, bearing in mind that these are based on ensemble properties and may not hold for every individual galaxy.

## APPENDIX B: TABLES

### B1 MATLAS UDGs photometry

### B2 GALFITM results and physical properties of MATLAS UDGs

### B3 Stellar population properties from PROSPECTOR
<table><caption>Table B1. Optical, near- and mid-IR photometry of the MATLAS UDGs.</caption><thead><tr><th>ID</th><th>g<br>[mag]</th><th>r<br>[mag]</th><th>g-r<br>[mag]</th><th>i<br>[mag]</th><th>g-i<br>[mag]</th><th>z<br>[mag]</th><th>g-z<br>[mag]</th><th>W1<br>[mag]</th><th>W2<br>[mag]</th><th>W3<br>[mag]</th><th>W4<br>[mag]</th></tr></thead><tbody><tr><td>MATLAS-42</td><td>17.46 ± 0.02</td><td>17.03 ± 0.03</td><td>0.43</td><td>—</td><td>—</td><td>16.70 ± 0.03</td><td>0.76</td><td>17.10 ± 0.15</td><td>17.92 ± 0.24</td><td>&gt;18.01</td><td>—</td></tr><tr><td>MATLAS-48</td><td>18.94 ± 0.05</td><td>18.26 ± 0.07</td><td>0.67</td><td>—</td><td>—</td><td>18.00 ± 0.07</td><td>0.94</td><td>18.39 ± 0.17</td><td>18.92 ± 0.13</td><td>—</td><td>&gt;18.57</td></tr><tr><td>MATLAS-141</td><td>20.09 ± 0.07</td><td>19.60 ± 0.08</td><td>0.50</td><td>—</td><td>—</td><td>19.33 ± 0.12</td><td>0.76</td><td>19.24 ± 0.26</td><td>20.97 ± 0.77</td><td>&gt;20.86</td><td>&gt;17.29</td></tr><tr><td>MATLAS-149</td><td>20.10 ± 0.07</td><td>19.66 ± 0.08</td><td>0.44</td><td>—</td><td>—</td><td>19.41 ± 0.16</td><td>0.68</td><td>19.91 ± 0.13</td><td>20.98 ± 1.52</td><td>&gt;18.21</td><td>&gt;16.41</td></tr><tr><td>MATLAS-177</td><td>19.18 ± 0.08</td><td>18.52 ± 0.06</td><td>0.66</td><td>—</td><td>—</td><td>&gt;19.73</td><td>—</td><td>18.76 ± 0.06</td><td>18.97 ± 0.21</td><td>&gt;20.30</td><td>&gt;16.18</td></tr><tr><td>MATLAS-262</td><td>19.96 ± 0.09</td><td>19.27 ± 0.08</td><td>0.69</td><td>19.03 ± 0.32</td><td>0.93</td><td>18.95 ± 0.16</td><td>1.01</td><td>19.86 ± 0.68</td><td>20.18 ± 1.38</td><td>—</td><td>&gt;15.81</td></tr><tr><td>MATLAS-342</td><td>18.78 ± 0.03</td><td>18.18 ± 0.03</td><td>0.60</td><td>17.79 ± 0.04</td><td>0.99</td><td>17.37 ± 0.05</td><td>1.41</td><td>18.54 ± 0.18</td><td>18.82 ± 0.80</td><td>—</td><td>&gt;19.32</td></tr><tr><td>MATLAS-365</td><td>19.75 ± 0.09</td><td>19.15 ± 0.07</td><td>0.59</td><td>18.96 ± 0.08</td><td>0.79</td><td>19.12 ± 0.16</td><td>0.63</td><td>19.18 ± 0.32</td><td>19.86 ± 1.47</td><td>&gt;17.79</td><td>&gt;17.37</td></tr><tr><td>MATLAS-368</td><td>18.06 ± 0.08</td><td>17.50 ± 0.08</td><td>0.55</td><td>17.39 ± 0.08</td><td>0.67</td><td>17.29 ± 0.13</td><td>0.76</td><td>18.56 ± 0.08</td><td>19.58 ± 1.08</td><td>&gt;19.23</td><td>&gt;15.03</td></tr><tr><td>MATLAS-405</td><td>18.88 ± 0.05</td><td>18.31 ± 0.05</td><td>0.57</td><td>—</td><td>—</td><td>&gt;19.76</td><td>—</td><td>18.43 ± 0.20</td><td>19.11 ± 0.81</td><td>&gt;21.73</td><td>&gt;19.95</td></tr><tr><td>MATLAS-478</td><td>17.69 ± 0.02</td><td>16.91 ± 0.02</td><td>0.78</td><td>—</td><td>—</td><td>&gt;17.60</td><td>—</td><td>16.74 ± 0.09</td><td>16.86 ± 0.04</td><td>—</td><td>&gt;15.42</td></tr><tr><td>MATLAS-585</td><td>18.72 ± 0.03</td><td>18.19 ± 0.03</td><td>0.53</td><td>17.90 ± 0.04</td><td>0.82</td><td>17.87 ± 0.08</td><td>0.85</td><td>18.46 ± 0.18</td><td>18.94 ± 0.21</td><td>&gt;20.83</td><td>—</td></tr><tr><td>MATLAS-658</td><td>20.53 ± 0.25</td><td>19.77 ± 0.28</td><td>0.76</td><td>19.20 ± 0.50</td><td>1.32</td><td>19.10 ± 0.50</td><td>1.43</td><td>19.09 ± 0.21</td><td>19.56 ± 0.41</td><td>—</td><td>—</td></tr><tr><td>MATLAS-799</td><td>17.40 ± 0.03</td><td>16.73 ± 0.03</td><td>0.67</td><td>16.51 ± 0.02</td><td>0.88</td><td>16.48 ± 0.04</td><td>0.92</td><td>16.85 ± 0.02</td><td>17.44 ± 0.06</td><td>—</td><td>—</td></tr><tr><td>MATLAS-898</td><td>18.15 ± 0.05</td><td>17.58 ± 0.05</td><td>0.57</td><td>17.43 ± 0.05</td><td>0.72</td><td>17.21 ± 0.07</td><td>0.94</td><td>17.94 ± 0.06</td><td>18.22 ± 0.80</td><td>&gt;20.68</td><td>—</td></tr><tr><td>MATLAS-951</td><td>17.97 ± 0.06</td><td>17.18 ± 0.05</td><td>0.79</td><td>16.87 ± 0.05</td><td>1.10</td><td>16.54 ± 0.06</td><td>1.43</td><td>17.78 ± 0.26</td><td>18.09 ± 0.10</td><td>—</td><td>—</td></tr><tr><td>MATLAS-984</td><td>19.60 ± 0.04</td><td>18.93 ± 0.05</td><td>0.67</td><td>—</td><td>—</td><td>&gt;19.75</td><td>—</td><td>18.69 ± 0.03</td><td>20.53 ± 0.25</td><td>&gt;18.52</td><td>&gt;16.76</td></tr><tr><td>MATLAS-1059</td><td>17.88 ± 0.05</td><td>17.17 ± 0.05</td><td>0.72</td><td>—</td><td>—</td><td>&gt;18.34</td><td>—</td><td>17.69 ± 0.10</td><td>18.35 ± 0.31</td><td>&gt;23.98</td><td>&gt;15.99</td></tr><tr><td>MATLAS-1174</td><td>19.47 ± 0.16</td><td>18.96 ± 0.13</td><td>0.51</td><td>18.64 ± 0.10</td><td>0.83</td><td>18.40 ± 0.18</td><td>1.07</td><td>19.09 ± 0.19</td><td>19.30 ± 1.14</td><td>—</td><td>&gt;19.31</td></tr><tr><td>MATLAS-1177</td><td>18.29 ± 0.01</td><td>17.54 ± 0.01</td><td>0.74</td><td>—</td><td>—</td><td>&gt;19.51</td><td>—</td><td>17.76 ± 0.03</td><td>17.91 ± 0.09</td><td>&gt;16.63</td><td>&gt;15.62</td></tr><tr><td>MATLAS-1205</td><td>19.49 ± 0.09</td><td>18.94 ± 0.09</td><td>0.55</td><td>18.60 ± 0.08</td><td>0.89</td><td>18.71 ± 0.14</td><td>0.78</td><td>19.85 ± 0.25</td><td>20.81 ± 0.80</td><td>&gt;18.59</td><td>—</td></tr><tr><td>MATLAS-1216</td><td>19.05 ± 0.27</td><td>18.96 ± 0.16</td><td>0.08</td><td>18.71 ± 0.28</td><td>0.34</td><td>18.33 ± 0.19</td><td>0.72</td><td>19.19 ± 0.61</td><td>20.07 ± 1.27</td><td>&gt;20.23</td><td>&gt;18.39</td></tr><tr><td>MATLAS-1225</td><td>18.51 ± 0.11</td><td>17.74 ± 0.12</td><td>0.78</td><td>17.17 ± 0.11</td><td>1.35</td><td>16.99 ± 0.17</td><td>1.53</td><td>17.37 ± 0.05</td><td>17.94 ± 0.20</td><td>&gt;20.83</td><td>—</td></tr><tr><td>MATLAS-1245</td><td>19.58 ± 0.05</td><td>18.93 ± 0.05</td><td>0.64</td><td>18.80 ± 0.11</td><td>0.78</td><td>18.69 ± 0.15</td><td>0.88</td><td>18.97 ± 0.43</td><td>—</td><td>&gt;20.62</td><td>&gt;17.98</td></tr><tr><td>MATLAS-1246</td><td>17.88 ± 0.05</td><td>17.23 ± 0.03</td><td>0.64</td><td>17.04 ± 0.03</td><td>0.84</td><td>16.94 ± 0.07</td><td>0.94</td><td>17.47 ± 0.09</td><td>18.73 ± 0.65</td><td>&gt;17.42</td><td>&gt;16.21</td></tr><tr><td>MATLAS-1248</td><td>17.49 ± 0.01</td><td>16.86 ± 0.01</td><td>0.63</td><td>16.64 ± 0.01</td><td>0.85</td><td>16.53 ± 0.03</td><td>0.96</td><td>16.89 ± 0.09</td><td>17.50 ± 0.06</td><td>&gt;16.73</td><td>&gt;18.37</td></tr><tr><td>MATLAS-1249</td><td>19.18 ± 0.04</td><td>19.00 ± 0.04</td><td>0.17</td><td>18.82 ± 0.05</td><td>0.35</td><td>18.53 ± 0.07</td><td>0.65</td><td>18.02 ± 0.03</td><td>18.31 ± 0.23</td><td>—</td><td>&gt;15.97</td></tr><tr><td>MATLAS-1262</td><td>18.71 ± 0.02</td><td>18.11 ± 0.02</td><td>0.60</td><td>17.88 ± 0.03</td><td>0.83</td><td>17.82 ± 0.07</td><td>0.89</td><td>18.33 ± 0.17</td><td>19.49 ± 1.03</td><td>—</td><td>&gt;16.20</td></tr><tr><td>MATLAS-1274</td><td>17.52 ± 0.02</td><td>17.03 ± 0.02</td><td>0.50</td><td>16.86 ± 0.02</td><td>0.66</td><td>16.68 ± 0.06</td><td>0.84</td><td>17.55 ± 0.09</td><td>18.36 ± 0.48</td><td>—</td><td>&gt;19.61</td></tr><tr><td>MATLAS-1302</td><td>20.73 ± 0.07</td><td>20.13 ± 0.07</td><td>0.59</td><td>19.82 ± 0.06</td><td>0.91</td><td>19.63 ± 0.14</td><td>1.10</td><td>20.10 ± 0.24</td><td>20.33 ± 0.82</td><td>&gt;18.46</td><td>&gt;16.50</td></tr><tr><td>MATLAS-1337</td><td>18.47 ± 0.04</td><td>18.22 ± 0.04</td><td>0.25</td><td>18.08 ± 0.04</td><td>0.39</td><td>17.82 ± 0.09</td><td>0.65</td><td>17.74 ± 0.17</td><td>18.04 ± 0.37</td><td>&gt;16.84</td><td>&gt;14.61</td></tr><tr><td>MATLAS-1413</td><td>18.43 ± 0.13</td><td>18.11 ± 0.15</td><td>0.32</td><td>17.84 ± 0.21</td><td>0.59</td><td>17.65 ± 0.28</td><td>0.78</td><td>18.55 ± 0.35</td><td>18.71 ± 0.43</td><td>—</td><td>—</td></tr><tr><td>MATLAS-1493</td><td>19.71 ± 0.08</td><td>19.28 ± 0.08</td><td>0.43</td><td>19.04 ± 0.08</td><td>0.67</td><td>18.96 ± 0.14</td><td>0.75</td><td>20.43 ± 1.92</td><td>20.86 ± 1.64</td><td>—</td><td>—</td></tr><tr><td>MATLAS-1494</td><td>18.81 ± 0.05</td><td>18.29 ± 0.01</td><td>0.51</td><td>18.04 ± 0.03</td><td>0.77</td><td>17.94 ± 0.08</td><td>0.87</td><td>19.18 ± 1.44</td><td>20.03 ± 1.58</td><td>—</td><td>&gt;19.67</td></tr><tr><td>MATLAS-1534</td><td>20.26 ± 0.10</td><td>19.58 ± 0.08</td><td>0.67</td><td>—</td><td>—</td><td>&gt;20.45</td><td>—</td><td>19.11 ± 0.60</td><td>19.85 ± 1.00</td><td>&gt;19.02</td><td>&gt;16.39</td></tr><tr><td>MATLAS-1550</td><td>19.71 ± 0.05</td><td>19.07 ± 0.06</td><td>0.64</td><td>—</td><td>—</td><td>&gt;20.38</td><td>—</td><td>19.66 ± 0.32</td><td>—</td><td>—</td><td>&gt;17.28</td></tr><tr><td>MATLAS-1558</td><td>19.59 ± 0.06</td><td>19.12 ± 0.06</td><td>0.47</td><td>—</td><td>—</td><td>&gt;20.05</td><td>—</td><td>19.92 ± 0.14</td><td>20.15 ± 0.50</td><td>&gt;23.39</td><td>—</td></tr><tr><td>MATLAS-1589</td><td>19.98 ± 0.13</td><td>19.59 ± 0.14</td><td>0.40</td><td>—</td><td>—</td><td>19.05 ± 0.93</td><td>0.93</td><td>19.85 ± 0.39</td><td>20.05 ± 0.30</td><td>&gt;18.85</td><td>—</td></tr><tr><td>MATLAS-1606</td><td>18.36 ± 0.05</td><td>17.69 ± 0.05</td><td>0.67</td><td>—</td><td>—</td><td>&gt;19.74</td><td>—</td><td>18.15 ± 0.18</td><td>19.01 ± 0.66</td><td>&gt;18.83</td><td>&gt;23.44</td></tr><tr><td>MATLAS-1615</td><td>18.26 ± 0.04</td><td>17.82 ± 0.03</td><td>0.44</td><td>—</td><td>—</td><td>&gt;19.21</td><td>—</td><td>18.16 ± 0.30</td><td>18.78 ± 0.61</td><td>&gt;21.47</td><td>—</td></tr><tr><td>MATLAS-1616</td><td>18.65 ± 0.06</td><td>18.08 ± 0.05</td><td>0.57</td><td>—</td><td>—</td><td>&gt;19.50</td><td>—</td><td>18.92 ± 0.18</td><td>19.48 ± 0.71</td><td>—</td><td>&gt;15.50</td></tr><tr><td>MATLAS-1630</td><td>19.28 ± 0.08</td><td>18.23 ± 0.07</td><td>1.05</td><td>—</td><td>—</td><td>&gt;19.74</td><td>—</td><td>18.61 ± 0.17</td><td>19.36 ± 0.15</td><td>&gt;17.27</td><td>&gt;16.97</td></tr><tr><td>MATLAS-1647</td><td>20.80 ± 0.06</td><td>20.17 ± 0.07</td><td>0.63</td><td>—</td><td>—</td><td>&gt;20.31</td><td>—</td><td>20.37 ± 0.15</td><td>20.21 ± 0.93</td><td>—</td><td>—</td></tr><tr><td>MATLAS-1779</td><td>18.66 ± 0.09</td><td>18.03 ± 0.08</td><td>0.63</td><td>18.00 ± 0.12</td><td>0.66</td><td>17.70 ± 0.17</td><td>0.96</td><td>18.67 ± 0.68</td><td>19.60 ± 1.56</td><td>&gt;22.42</td><td>&gt;20.48</td></tr><tr><td>MATLAS-1794</td><td>19.02 ± 0.02</td><td>18.57 ± 0.01</td><td>0.44</td><td>18.38 ± 0.04</td><td>0.63</td><td>18.24 ± 0.02</td><td>0.78</td><td>19.01 ± 0.30</td><td>—</td><td>&gt;21.14</td><td>—</td></tr><tr><td>MATLAS-1824</td><td>18.36 ± 0.04</td><td>17.87 ± 0.05</td><td>0.50</td><td>—</td><td>—</td><td>&gt;19.42</td><td>—</td><td>18.22 ± 0.36</td><td>19.27 ± 0.32</td><td>&gt;18.60</td><td>—</td></tr><tr><td>MATLAS-1847</td><td>19.28 ± 0.06</td><td>18.66 ± 0.05</td><td>0.62</td><td>18.36 ± 0.07</td><td>0.91</td><td>18.82 ± 0.11</td><td>0.46</td><td>19.65 ± 0.30</td><td>—</td><td>&gt;16.72</td><td>—</td></tr><tr><td>MATLAS-1855</td><td>18.97 ± 0.05</td><td>18.44 ± 0.05</td><td>0.54</td><td>18.34 ± 0.06</td><td>0.63</td><td>18.30 ± 0.10</td><td>0.67</td><td>19.77 ± 1.66</td><td>—</td><td>—</td><td>&gt;15.90</td></tr><tr><td>MATLAS-1865</td><td>18.93 ± 0.02</td><td>18.39 ± 0.02</td><td>0.54</td><td>—</td><td>—</td><td>&gt;18.88</td><td>—</td><td>18.83 ± 0.12</td><td>19.60 ± 0.64</td><td>—</td><td>&gt;19.08</td></tr><tr><td>MATLAS-1907</td><td>18.11 ± 0.01</td><td>17.57 ± 0.01</td><td>0.53</td><td>17.30 ± 0.02</td><td>0.80</td><td>17.20 ± 0.02</td><td>0.91</td><td>17.74 ± 0.13</td><td>18.07 ± 0.34</td><td>&gt;22.37</td><td>&gt;20.08</td></tr><tr><td>MATLAS-1957</td><td>18.17 ± 0.04</td><td>17.43 ± 0.04</td><td>0.74</td><td>17.25 ± 0.07</td><td>0.91</td><td>17.07 ± 0.08</td><td>1.10</td><td>17.01 ± 0.04</td><td>17.75 ± 0.03</td><td>—</td><td>&gt;19.91</td></tr><tr><td>MATLAS-1975</td><td>19.08 ± 0.11</td><td>18.50 ± 0.10</td><td>0.59</td><td>18.06 ± 0.17</td><td>1.02</td><td>17.80 ± 0.16</td><td>1.29</td><td>19.14 ± 0.55</td><td>19.14 ± 0.94</td><td>&gt;21.64</td><td>&gt;20.86</td></tr><tr><td>MATLAS-1985</td><td>18.06 ± 0.10</td><td>17.26 ± 0.07</td><td>0.80</td><td>17.03 ± 0.19</td><td>1.03</td><td>16.85 ± 0.20</td><td>1.21</td><td>18.04 ± 0.28</td><td>—</td><td>—</td><td>&gt;16.60</td></tr><tr><td>MATLAS-1991</td><td>17.92 ± 0.02</td><td>17.36 ± 0.01</td><td>0.56</td><td>17.08 ± 0.03</td><td>0.84</td><td>16.98 ± 0.03</td><td>0.95</td><td>17.56 ± 0.09</td><td>18.35 ± 2.00</td><td>&gt;18.82</td><td>&gt;18.24</td></tr><tr><td>MATLAS-1996</td><td>19.91 ± 0.11</td><td>19.47 ± 0.10</td><td>0.44</td><td>19.20 ± 0.21</td><td>0.70</td><td>19.08 ± 0.22</td><td>0.82</td><td>22.35 ± 10.73</td><td>23.41 ± 2.51</td><td>&gt;18.28</td><td>—</td></tr><tr><td>MATLAS-2019</td><td>17.82 ± 0.02</td><td>17.20 ± 0.02</td><td>0.61</td><td>16.95 ± 0.04</td><td>0.87</td><td>16.81 ± 0.04</td><td>1.01</td><td>18.13 ± 0.14</td><td>18.45 ± 0.19</td><td>—</td><td>&gt;16.54</td></tr><tr><td>MATLAS-2021</td><td>19.37 ± 0.11</td><td>18.79 ± 0.09</td><td>0.58</td><td>18.74 ± 0.25</td><td>0.63</td><td>18.49 ± 0.21</td><td>0.88</td><td>24.23 ± 1.86</td><td>—</td><td>—</td><td>&gt;18.53</td></tr><tr><td>MATLAS-2103</td><td>17.94 ± 0.01</td><td>17.41 ± 0.01</td><td>0.53</td><td>17.16 ± 0.04</td><td>0.78</td><td>17.08 ± 0.03</td><td>0.86</td><td>18.06 ± 0.13</td><td>18.41 ± 1.95</td><td>&gt;19.70</td><td>&gt;20.36</td></tr><tr><td>MATLAS-2184</td><td>19.59 ± 0.08</td><td>19.07 ± 0.10</td><td>0.52</td><td>—</td><td>—</td><td>&gt;19.30</td><td>—</td><td>21.62 ± 1.17</td><td>—</td><td>&gt;22.56</td><td>&gt;15.93</td></tr></tbody></table>

Note. Columns are (1) Galaxy ID; (2) GALFITM DECaLS g-band magnitude; (3) GALFITM DECaLS r-band magnitude; (4) g-r colour; (5) GALFITM DECaLS i-band magnitude; (6) g-i colour; (7) GALFITM DECaLS z-band magnitude; (8) g-z colour; (9) WISE 3.4μ-band magnitude; (10) WISE 4.6μ-band magnitude; (11) WISE 12μ-band magnitude; (12) WISE 22μ-band magnitude. ‘-’ stands for unavailable data. ‘>’ denote upper limit magnitudes.
<table><caption>Table B1. Optical, near- and mid-IR photometry of the MATLAS UDGs.</caption><thead><tr><th>ID</th><th>g<br>[mag]</th><th>r<br>[mag]</th><th>g-r<br>[mag]</th><th>i<br>[mag]</th><th>g-i<br>[mag]</th><th>z<br>[mag]</th><th>g-z<br>[mag]</th><th>W1<br>[mag]</th><th>W2<br>[mag]</th><th>W3<br>[mag]</th><th>W4<br>[mag]</th></tr></thead><tbody><tr><td>MATLAS-42</td><td>17.46 ± 0.02</td><td>17.03 ± 0.03</td><td>0.43</td><td>—</td><td>—</td><td>16.70 ± 0.03</td><td>0.76</td><td>17.10 ± 0.15</td><td>17.92 ± 0.24</td><td>&gt;18.01</td><td>—</td></tr><tr><td>MATLAS-48</td><td>18.94 ± 0.05</td><td>18.26 ± 0.07</td><td>0.67</td><td>—</td><td>—</td><td>18.00 ± 0.07</td><td>0.94</td><td>18.39 ± 0.17</td><td>18.92 ± 0.13</td><td>—</td><td>&gt;18.57</td></tr><tr><td>MATLAS-141</td><td>20.09 ± 0.07</td><td>19.60 ± 0.08</td><td>0.50</td><td>—</td><td>—</td><td>19.33 ± 0.12</td><td>0.76</td><td>19.24 ± 0.26</td><td>20.97 ± 0.77</td><td>&gt;20.86</td><td>&gt;17.29</td></tr><tr><td>MATLAS-149</td><td>20.10 ± 0.07</td><td>19.66 ± 0.08</td><td>0.44</td><td>—</td><td>—</td><td>19.41 ± 0.16</td><td>0.68</td><td>19.91 ± 0.13</td><td>20.98 ± 1.52</td><td>&gt;18.21</td><td>&gt;16.41</td></tr><tr><td>MATLAS-177</td><td>19.18 ± 0.08</td><td>18.52 ± 0.06</td><td>0.66</td><td>—</td><td>—</td><td>&gt;19.73</td><td>—</td><td>18.76 ± 0.06</td><td>18.97 ± 0.21</td><td>&gt;20.30</td><td>&gt;16.18</td></tr><tr><td>MATLAS-262</td><td>19.96 ± 0.09</td><td>19.27 ± 0.08</td><td>0.69</td><td>19.03 ± 0.32</td><td>0.93</td><td>18.95 ± 0.16</td><td>1.01</td><td>19.86 ± 0.68</td><td>20.18 ± 1.38</td><td>—</td><td>&gt;15.81</td></tr><tr><td>MATLAS-342</td><td>18.78 ± 0.03</td><td>18.18 ± 0.03</td><td>0.60</td><td>17.79 ± 0.04</td><td>0.99</td><td>17.37 ± 0.05</td><td>1.41</td><td>18.54 ± 0.18</td><td>18.82 ± 0.80</td><td>—</td><td>&gt;19.32</td></tr><tr><td>MATLAS-365</td><td>19.75 ± 0.09</td><td>19.15 ± 0.07</td><td>0.59</td><td>18.96 ± 0.08</td><td>0.79</td><td>19.12 ± 0.16</td><td>0.63</td><td>19.18 ± 0.32</td><td>19.86 ± 1.47</td><td>&gt;17.79</td><td>&gt;17.37</td></tr><tr><td>MATLAS-368</td><td>18.06 ± 0.08</td><td>17.50 ± 0.08</td><td>0.55</td><td>17.39 ± 0.08</td><td>0.67</td><td>17.29 ± 0.13</td><td>0.76</td><td>18.56 ± 0.08</td><td>19.58 ± 1.08</td><td>&gt;19.23</td><td>&gt;15.03</td></tr><tr><td>MATLAS-405</td><td>18.88 ± 0.05</td><td>18.31 ± 0.05</td><td>0.57</td><td>—</td><td>—</td><td>&gt;19.76</td><td>—</td><td>18.43 ± 0.20</td><td>19.11 ± 0.81</td><td>&gt;21.73</td><td>&gt;19.95</td></tr><tr><td>MATLAS-478</td><td>17.69 ± 0.02</td><td>16.91 ± 0.02</td><td>0.78</td><td>—</td><td>—</td><td>&gt;17.60</td><td>—</td><td>16.74 ± 0.09</td><td>16.86 ± 0.04</td><td>—</td><td>&gt;15.42</td></tr><tr><td>MATLAS-585</td><td>18.72 ± 0.03</td><td>18.19 ± 0.03</td><td>0.53</td><td>17.90 ± 0.04</td><td>0.82</td><td>17.87 ± 0.08</td><td>0.85</td><td>18.46 ± 0.18</td><td>18.94 ± 0.21</td><td>&gt;20.83</td><td>—</td></tr><tr><td>MATLAS-658</td><td>20.53 ± 0.25</td><td>19.77 ± 0.28</td><td>0.76</td><td>19.20 ± 0.50</td><td>1.32</td><td>19.10 ± 0.50</td><td>1.43</td><td>19.09 ± 0.21</td><td>19.56 ± 0.41</td><td>—</td><td>—</td></tr><tr><td>MATLAS-799</td><td>17.40 ± 0.03</td><td>16.73 ± 0.03</td><td>0.67</td><td>16.51 ± 0.02</td><td>0.88</td><td>16.48 ± 0.04</td><td>0.92</td><td>16.85 ± 0.02</td><td>17.44 ± 0.06</td><td>—</td><td>—</td></tr><tr><td>MATLAS-898</td><td>18.15 ± 0.05</td><td>17.58 ± 0.05</td><td>0.57</td><td>17.43 ± 0.05</td><td>0.72</td><td>17.21 ± 0.07</td><td>0.94</td><td>17.94 ± 0.06</td><td>18.22 ± 0.80</td><td>&gt;20.68</td><td>—</td></tr><tr><td>MATLAS-951</td><td>17.97 ± 0.06</td><td>17.18 ± 0.05</td><td>0.79</td><td>16.87 ± 0.05</td><td>1.10</td><td>16.54 ± 0.06</td><td>1.43</td><td>17.78 ± 0.26</td><td>18.09 ± 0.10</td><td>—</td><td>—</td></tr><tr><td>MATLAS-984</td><td>19.60 ± 0.04</td><td>18.93 ± 0.05</td><td>0.67</td><td>—</td><td>—</td><td>&gt;19.75</td><td>—</td><td>18.69 ± 0.03</td><td>20.53 ± 0.25</td><td>&gt;18.52</td><td>&gt;16.76</td></tr><tr><td>MATLAS-1059</td><td>17.88 ± 0.05</td><td>17.17 ± 0.05</td><td>0.72</td><td>—</td><td>—</td><td>&gt;18.34</td><td>—</td><td>17.69 ± 0.10</td><td>18.35 ± 0.31</td><td>&gt;23.98</td><td>&gt;15.99</td></tr><tr><td>MATLAS-1174</td><td>19.47 ± 0.16</td><td>18.96 ± 0.13</td><td>0.51</td><td>18.64 ± 0.10</td><td>0.83</td><td>18.40 ± 0.18</td><td>1.07</td><td>19.09 ± 0.19</td><td>19.30 ± 1.14</td><td>—</td><td>&gt;19.31</td></tr><tr><td>MATLAS-1177</td><td>18.29 ± 0.01</td><td>17.54 ± 0.01</td><td>0.74</td><td>—</td><td>—</td><td>&gt;19.51</td><td>—</td><td>17.76 ± 0.03</td><td>17.91 ± 0.09</td><td>&gt;16.63</td><td>&gt;15.62</td></tr><tr><td>MATLAS-1205</td><td>19.49 ± 0.09</td><td>18.94 ± 0.09</td><td>0.55</td><td>18.60 ± 0.08</td><td>0.89</td><td>18.71 ± 0.14</td><td>0.78</td><td>19.85 ± 0.25</td><td>20.81 ± 0.80</td><td>&gt;18.59</td><td>—</td></tr><tr><td>MATLAS-1216</td><td>19.05 ± 0.27</td><td>18.96 ± 0.16</td><td>0.08</td><td>18.71 ± 0.28</td><td>0.34</td><td>18.33 ± 0.19</td><td>0.72</td><td>19.19 ± 0.61</td><td>20.07 ± 1.27</td><td>&gt;20.23</td><td>&gt;18.39</td></tr><tr><td>MATLAS-1225</td><td>18.51 ± 0.11</td><td>17.74 ± 0.12</td><td>0.78</td><td>17.17 ± 0.11</td><td>1.35</td><td>16.99 ± 0.17</td><td>1.53</td><td>17.37 ± 0.05</td><td>17.94 ± 0.20</td><td>&gt;20.83</td><td>—</td></tr><tr><td>MATLAS-1245</td><td>19.58 ± 0.05</td><td>18.93 ± 0.05</td><td>0.64</td><td>18.80 ± 0.11</td><td>0.78</td><td>18.69 ± 0.15</td><td>0.88</td><td>18.97 ± 0.43</td><td>—</td><td>&gt;20.62</td><td>&gt;17.98</td></tr><tr><td>MATLAS-1246</td><td>17.88 ± 0.05</td><td>17.23 ± 0.03</td><td>0.64</td><td>17.04 ± 0.03</td><td>0.84</td><td>16.94 ± 0.07</td><td>0.94</td><td>17.47 ± 0.09</td><td>18.73 ± 0.65</td><td>&gt;17.42</td><td>&gt;16.21</td></tr><tr><td>MATLAS-1248</td><td>17.49 ± 0.01</td><td>16.86 ± 0.01</td><td>0.63</td><td>16.64 ± 0.01</td><td>0.85</td><td>16.53 ± 0.03</td><td>0.96</td><td>16.89 ± 0.09</td><td>17.50 ± 0.06</td><td>&gt;16.73</td><td>&gt;18.37</td></tr><tr><td>MATLAS-1249</td><td>19.18 ± 0.04</td><td>19.00 ± 0.04</td><td>0.17</td><td>18.82 ± 0.05</td><td>0.35</td><td>18.53 ± 0.07</td><td>0.65</td><td>18.02 ± 0.03</td><td>18.31 ± 0.23</td><td>—</td><td>&gt;15.97</td></tr><tr><td>MATLAS-1262</td><td>18.71 ± 0.02</td><td>18.11 ± 0.02</td><td>0.60</td><td>17.88 ± 0.03</td><td>0.83</td><td>17.82 ± 0.07</td><td>0.89</td><td>18.33 ± 0.17</td><td>19.49 ± 1.03</td><td>—</td><td>&gt;16.20</td></tr><tr><td>MATLAS-1274</td><td>17.52 ± 0.02</td><td>17.03 ± 0.02</td><td>0.50</td><td>16.86 ± 0.02</td><td>0.66</td><td>16.68 ± 0.06</td><td>0.84</td><td>17.55 ± 0.09</td><td>18.36 ± 0.48</td><td>—</td><td>&gt;19.61</td></tr><tr><td>MATLAS-1302</td><td>20.73 ± 0.07</td><td>20.13 ± 0.07</td><td>0.59</td><td>19.82 ± 0.06</td><td>0.91</td><td>19.63 ± 0.14</td><td>1.10</td><td>20.10 ± 0.24</td><td>20.33 ± 0.82</td><td>&gt;18.46</td><td>&gt;16.50</td></tr><tr><td>MATLAS-1337</td><td>18.47 ± 0.04</td><td>18.22 ± 0.04</td><td>0.25</td><td>18.08 ± 0.04</td><td>0.39</td><td>17.82 ± 0.09</td><td>0.65</td><td>17.74 ± 0.17</td><td>18.04 ± 0.37</td><td>&gt;16.84</td><td>&gt;14.61</td></tr><tr><td>MATLAS-1413</td><td>18.43 ± 0.13</td><td>18.11 ± 0.15</td><td>0.32</td><td>17.84 ± 0.21</td><td>0.59</td><td>17.65 ± 0.28</td><td>0.78</td><td>18.55 ± 0.35</td><td>18.71 ± 0.43</td><td>—</td><td>—</td></tr><tr><td>MATLAS-1493</td><td>19.71 ± 0.08</td><td>19.28 ± 0.08</td><td>0.43</td><td>19.04 ± 0.08</td><td>0.67</td><td>18.96 ± 0.14</td><td>0.75</td><td>20.43 ± 1.92</td><td>20.86 ± 1.64</td><td>—</td><td>—</td></tr><tr><td>MATLAS-1494</td><td>18.81 ± 0.05</td><td>18.29 ± 0.01</td><td>0.51</td><td>18.04 ± 0.03</td><td>0.77</td><td>17.94 ± 0.08</td><td>0.87</td><td>19.18 ± 1.44</td><td>20.03 ± 1.58</td><td>—</td><td>&gt;19.67</td></tr><tr><td>MATLAS-1534</td><td>20.26 ± 0.10</td><td>19.58 ± 0.08</td><td>0.67</td><td>—</td><td>—</td><td>&gt;20.45</td><td>—</td><td>19.11 ± 0.60</td><td>19.85 ± 1.00</td><td>&gt;19.02</td><td>&gt;16.39</td></tr><tr><td>MATLAS-1550</td><td>19.71 ± 0.05</td><td>19.07 ± 0.06</td><td>0.64</td><td>—</td><td>—</td><td>&gt;20.38</td><td>—</td><td>19.66 ± 0.32</td><td>—</td><td>—</td><td>&gt;17.28</td></tr><tr><td>MATLAS-1558</td><td>19.59 ± 0.06</td><td>19.12 ± 0.06</td><td>0.47</td><td>—</td><td>—</td><td>&gt;20.05</td><td>—</td><td>19.92 ± 0.14</td><td>20.15 ± 0.50</td><td>&gt;23.39</td><td>—</td></tr><tr><td>MATLAS-1589</td><td>19.98 ± 0.13</td><td>19.59 ± 0.14</td><td>0.40</td><td>—</td><td>—</td><td>19.05 ± 0.93</td><td>0.93</td><td>19.85 ± 0.39</td><td>20.05 ± 0.30</td><td>&gt;18.85</td><td>—</td></tr><tr><td>MATLAS-1606</td><td>18.36 ± 0.05</td><td>17.69 ± 0.05</td><td>0.67</td><td>—</td><td>—</td><td>&gt;19.74</td><td>—</td><td>18.15 ± 0.18</td><td>19.01 ± 0.66</td><td>&gt;18.83</td><td>&gt;23.44</td></tr><tr><td>MATLAS-1615</td><td>18.26 ± 0.04</td><td>17.82 ± 0.03</td><td>0.44</td><td>—</td><td>—</td><td>&gt;19.21</td><td>—</td><td>18.16 ± 0.30</td><td>18.78 ± 0.61</td><td>&gt;21.47</td><td>—</td></tr><tr><td>MATLAS-1616</td><td>18.65 ± 0.06</td><td>18.08 ± 0.05</td><td>0.57</td><td>—</td><td>—</td><td>&gt;19.50</td><td>—</td><td>18.92 ± 0.18</td><td>19.48 ± 0.71</td><td>—</td><td>&gt;15.50</td></tr><tr><td>MATLAS-1630</td><td>19.28 ± 0.08</td><td>18.23 ± 0.07</td><td>1.05</td><td>—</td><td>—</td><td>&gt;19.74</td><td>—</td><td>18.61 ± 0.17</td><td>19.36 ± 0.15</td><td>&gt;17.27</td><td>&gt;16.97</td></tr><tr><td>MATLAS-1647</td><td>20.80 ± 0.06</td><td>20.17 ± 0.07</td><td>0.63</td><td>—</td><td>—</td><td>&gt;20.31</td><td>—</td><td>20.37 ± 0.15</td><td>20.21 ± 0.93</td><td>—</td><td>—</td></tr><tr><td>MATLAS-1779</td><td>18.66 ± 0.09</td><td>18.03 ± 0.08</td><td>0.63</td><td>18.00 ± 0.12</td><td>0.66</td><td>17.70 ± 0.17</td><td>0.96</td><td>18.67 ± 0.68</td><td>19.60 ± 1.56</td><td>&gt;22.42</td><td>&gt;20.48</td></tr><tr><td>MATLAS-1794</td><td>19.02 ± 0.02</td><td>18.57 ± 0.01</td><td>0.44</td><td>18.38 ± 0.04</td><td>0.63</td><td>18.24 ± 0.02</td><td>0.78</td><td>19.01 ± 0.30</td><td>—</td><td>&gt;21.14</td><td>—</td></tr><tr><td>MATLAS-1824</td><td>18.36 ± 0.04</td><td>17.87 ± 0.05</td><td>0.50</td><td>—</td><td>—</td><td>&gt;19.42</td><td>—</td><td>18.22 ± 0.36</td><td>19.27 ± 0.32</td><td>&gt;18.60</td><td>—</td></tr><tr><td>MATLAS-1847</td><td>19.28 ± 0.06</td><td>18.66 ± 0.05</td><td>0.62</td><td>18.36 ± 0.07</td><td>0.91</td><td>18.82 ± 0.11</td><td>0.46</td><td>19.65 ± 0.30</td><td>—</td><td>&gt;16.72</td><td>—</td></tr><tr><td>MATLAS-1855</td><td>18.97 ± 0.05</td><td>18.44 ± 0.05</td><td>0.54</td><td>18.34 ± 0.06</td><td>0.63</td><td>18.30 ± 0.10</td><td>0.67</td><td>19.77 ± 1.66</td><td>—</td><td>—</td><td>&gt;15.90</td></tr><tr><td>MATLAS-1865</td><td>18.93 ± 0.02</td><td>18.39 ± 0.02</td><td>0.54</td><td>—</td><td>—</td><td>&gt;18.88</td><td>—</td><td>18.83 ± 0.12</td><td>19.60 ± 0.64</td><td>—</td><td>&gt;19.08</td></tr><tr><td>MATLAS-1907</td><td>18.11 ± 0.01</td><td>17.57 ± 0.01</td><td>0.53</td><td>17.30 ± 0.02</td><td>0.80</td><td>17.20 ± 0.02</td><td>0.91</td><td>17.74 ± 0.13</td><td>18.07 ± 0.34</td><td>&gt;22.37</td><td>&gt;20.08</td></tr><tr><td>MATLAS-1957</td><td>18.17 ± 0.04</td><td>17.43 ± 0.04</td><td>0.74</td><td>17.25 ± 0.07</td><td>0.91</td><td>17.07 ± 0.08</td><td>1.10</td><td>17.01 ± 0.04</td><td>17.75 ± 0.03</td><td>—</td><td>&gt;19.91</td></tr><tr><td>MATLAS-1975</td><td>19.08 ± 0.11</td><td>18.50 ± 0.10</td><td>0.59</td><td>18.06 ± 0.17</td><td>1.02</td><td>17.80 ± 0.16</td><td>1.29</td><td>19.14 ± 0.55</td><td>19.14 ± 0.94</td><td>&gt;21.64</td><td>&gt;20.86</td></tr><tr><td>MATLAS-1985</td><td>18.06 ± 0.10</td><td>17.26 ± 0.07</td><td>0.80</td><td>17.03 ± 0.19</td><td>1.03</td><td>16.85 ± 0.20</td><td>1.21</td><td>18.04 ± 0.28</td><td>—</td><td>—</td><td>&gt;16.60</td></tr><tr><td>MATLAS-1991</td><td>17.92 ± 0.02</td><td>17.36 ± 0.01</td><td>0.56</td><td>17.08 ± 0.03</td><td>0.84</td><td>16.98 ± 0.03</td><td>0.95</td><td>17.56 ± 0.09</td><td>18.35 ± 2.00</td><td>&gt;18.82</td><td>&gt;18.24</td></tr><tr><td>MATLAS-1996</td><td>19.91 ± 0.11</td><td>19.47 ± 0.10</td><td>0.44</td><td>19.20 ± 0.21</td><td>0.70</td><td>19.08 ± 0.22</td><td>0.82</td><td>22.35 ± 10.73</td><td>23.41 ± 2.51</td><td>&gt;18.28</td><td>—</td></tr><tr><td>MATLAS-2019</td><td>17.82 ± 0.02</td><td>17.20 ± 0.02</td><td>0.61</td><td>16.95 ± 0.04</td><td>0.87</td><td>16.81 ± 0.04</td><td>1.01</td><td>18.13 ± 0.14</td><td>18.45 ± 0.19</td><td>—</td><td>&gt;16.54</td></tr><tr><td>MATLAS-2021</td><td>19.37 ± 0.11</td><td>18.79 ± 0.09</td><td>0.58</td><td>18.74 ± 0.25</td><td>0.63</td><td>18.49 ± 0.21</td><td>0.88</td><td>24.23 ± 1.86</td><td>—</td><td>—</td><td>&gt;18.53</td></tr><tr><td>MATLAS-2103</td><td>17.94 ± 0.01</td><td>17.41 ± 0.01</td><td>0.53</td><td>17.16 ± 0.04</td><td>0.78</td><td>17.08 ± 0.03</td><td>0.86</td><td>18.06 ± 0.13</td><td>18.41 ± 1.95</td><td>&gt;19.70</td><td>&gt;20.36</td></tr><tr><td>MATLAS-2184</td><td>19.59 ± 0.08</td><td>19.07 ± 0.10</td><td>0.52</td><td>—</td><td>—</td><td>&gt;19.30</td><td>—</td><td>21.62 ± 1.17</td><td>—</td><td>&gt;22.56</td><td>&gt;15.93</td></tr></tbody></table>

Note. Columns are (1) Galaxy ID; (2) GALFITM DECaLS g-band magnitude; (3) GALFITM DECaLS r-band magnitude; (4) g-r colour; (5) GALFITM DECaLS i-band magnitude; (6) g-i colour; (7) GALFITM DECaLS z-band magnitude; (8) g-z colour; (9) WISE 3.4μ-band magnitude; (10) WISE 4.6μ-band magnitude; (11) WISE 12μ-band magnitude; (12) WISE 22μ-band magnitude. ‘-’ stands for unavailable data. ‘>’ denote upper limit magnitudes.
<table><caption>Table B2. GALFITM morphological parameters and physical properties of the MATLAS UDGs.</caption><thead><tr><th>ID</th><th>R<sub>e</sub><br>[arcsec]</th><th>n</th><th>b/a</th><th>PA<br>[degrees]</th><th>R<sub>e</sub><br>[kpc]</th><th>(μ<sub>g, c</sub>)<br>[mag arcsec<sup>-2</sup>]</th><th>μ<sub>g, 0</sub><br>[mag arcsec<sup>-2</sup>]</th><th>Candidate host</th><th>Host distance<br>[Mpc]</th><th>KMeans class<br>[with N<sub>GC</sub>, w/o N<sub>GC</sub>]</th></tr></thead><tbody><tr><td>MATLAS-42</td><td>19.44 ± 0.43</td><td>0.45 ± 0.00</td><td>0.60 ± 0.00</td><td>-43.05 ± 0.30</td><td>3.11</td><td>25.94</td><td>25.64</td><td>NGC0502</td><td>35.9</td><td>B,B</td></tr><tr><td>MATLAS-48</td><td>10.07 ± 0.72</td><td>0.95 ± 0.02</td><td>0.71 ± 0.01</td><td>-85.12 ± 1.20</td><td>1.72</td><td>25.98</td><td>24.94</td><td>NGC0502</td><td>35.9</td><td>–B</td></tr><tr><td>MATLAS-141</td><td>9.87 ± 1.00</td><td>0.52 ± 0.02</td><td>0.52 ± 0.01</td><td>-69.09 ± 1.30</td><td>1.73</td><td>27.14</td><td>26.76</td><td>NGC0770</td><td>36.7</td><td>A,A</td></tr><tr><td>MATLAS-149</td><td>11.41 ± 1.12</td><td>0.55 ± 0.03</td><td>0.45 ± 0.01</td><td>-84.63 ± 1.00</td><td>2.00</td><td>27.46</td><td>27.04</td><td>NGC0770</td><td>36.7</td><td>A,A</td></tr><tr><td>MATLAS-177</td><td>15.74 ± 1.33</td><td>0.73 ± 0.02</td><td>0.81 ± 0.01</td><td>-45.93 ± 2.50</td><td>1.69</td><td>27.20</td><td>26.51</td><td>NGC0936</td><td>22.4</td><td>A,A</td></tr><tr><td>MATLAS-262</td><td>9.37 ± 1.07</td><td>1.00 ± 0.02</td><td>0.31 ± 0.00</td><td>85.97 ± 0.51</td><td>1.36</td><td>26.92</td><td>25.79</td><td>NGC1248</td><td>30.4</td><td>A,A</td></tr><tr><td>MATLAS-342</td><td>9.65 ± 0.49</td><td>1.15 ± 0.01</td><td>0.76 ± 0.00</td><td>27.79 ± 0.08</td><td>1.48</td><td>25.54</td><td>24.16</td><td>NGC2481</td><td>32.0</td><td>B,B</td></tr><tr><td>MATLAS-365</td><td>5.13 ± 0.56</td><td>1.00 ± 0.02</td><td>0.74 ± 0.01</td><td>-62.48 ± 2.60</td><td>0.75</td><td>25.34</td><td>24.22</td><td>NGC2577</td><td>30.8</td><td>A,A</td></tr><tr><td>MATLAS-368</td><td>17.88 ± 1.59</td><td>1.15 ± 0.02</td><td>0.78 ± 0.01</td><td>-48.24 ± 1.20</td><td>2.63</td><td>26.35</td><td>24.98</td><td>NGC2577</td><td>30.8</td><td>A,A</td></tr><tr><td>MATLAS-405</td><td>17.21 ± 1.05</td><td>0.78 ± 0.01</td><td>0.53 ± 0.00</td><td>50.77 ± 0.62</td><td>2.31</td><td>27.08</td><td>26.32</td><td>UGC04551</td><td>28.0</td><td>A,A</td></tr><tr><td>MATLAS-478</td><td>19.26 ± 0.52</td><td>1.06 ± 0.01</td><td>0.49 ± 0.00</td><td>-65.97 ± 0.10</td><td>2.02</td><td>26.15</td><td>24.92</td><td>NGC2768</td><td>21.8</td><td>A,A</td></tr><tr><td>MATLAS-585</td><td>11.61 ± 0.45</td><td>0.59 ± 0.01</td><td>0.61 ± 0.00</td><td>3.21 ± 1.00</td><td>1.51</td><td>26.14</td><td>25.66</td><td>IC0560</td><td>27.2</td><td>B,B</td></tr><tr><td>MATLAS-658</td><td>12.56 ± 4.57</td><td>1.54 ± 0.08</td><td>0.39 ± 0.01</td><td>16.92 ± 0.56</td><td>1.99</td><td>28.04</td><td>25.96</td><td>NGC3193</td><td>33.1</td><td>A,A</td></tr><tr><td>MATLAS-799</td><td>16.98 ± 0.53</td><td>0.73 ± 0.01</td><td>0.87 ± 0.00</td><td>-4.23 ± 0.13</td><td>2.00</td><td>25.56</td><td>24.87</td><td>NGC3414</td><td>24.5</td><td>B,B</td></tr><tr><td>MATLAS-898</td><td>13.97 ± 0.85</td><td>1.00 ± 0.02</td><td>0.67 ± 0.01</td><td>34.34 ± 0.94</td><td>1.33</td><td>25.89</td><td>24.77</td><td>NGC3599</td><td>19.8</td><td>B,B</td></tr><tr><td>MATLAS-951</td><td>20.83 ± 1.15</td><td>1.00 ± 0.03</td><td>0.45 ± 0.00</td><td>-70.71 ± 0.20</td><td>2.62</td><td>26.11</td><td>24.98</td><td>NGC3640</td><td>26.3</td><td>–B</td></tr><tr><td>MATLAS-984</td><td>9.45 ± 0.53</td><td>0.64 ± 0.02</td><td>0.73 ± 0.01</td><td>40.64 ± 0.66</td><td>1.49</td><td>26.49</td><td>25.93</td><td>NGC3665</td><td>33.1</td><td>B,A</td></tr><tr><td>MATLAS-1059</td><td>18.94 ± 1.39</td><td>1.64 ± 0.02</td><td>0.60 ± 0.00</td><td>-69.50 ± 0.58</td><td>3.02</td><td>26.18</td><td>23.92</td><td>NGC3674</td><td>33.4</td><td>B,B</td></tr><tr><td>MATLAS-1174</td><td>14.09 ± 2.39</td><td>1.00 ± 0.02</td><td>0.29 ± 0.01</td><td>57.06 ± 0.57</td><td>2.56</td><td>27.24</td><td>26.11</td><td>NGC4078</td><td>38.1</td><td>A,A</td></tr><tr><td>MATLAS-1177</td><td>12.28 ± 0.07</td><td>1.00 ± 0.02</td><td>0.94 ± 0.01</td><td>39.37 ± 0.01</td><td>1.45</td><td>25.15</td><td>24.02</td><td>NGC4036</td><td>24.6</td><td>–B</td></tr><tr><td>MATLAS-1205</td><td>10.14 ± 1.17</td><td>0.83 ± 0.02</td><td>0.71 ± 0.00</td><td>-38.53 ± 1.90</td><td>1.89</td><td>26.53</td><td>25.69</td><td>NGC4191</td><td>39.2</td><td>–A</td></tr><tr><td>MATLAS-1216</td><td>8.72 ± 2.75</td><td>0.80 ± 0.04</td><td>1.00 ± 0.03</td><td>0.11 ± 0.98</td><td>1.63</td><td>26.76</td><td>25.97</td><td>NGC4191</td><td>39.2</td><td>B,B</td></tr><tr><td>MATLAS-1225</td><td>15.01 ± 1.63</td><td>1.00 ± 0.02</td><td>0.54 ± 0.01</td><td>-67.13 ± 0.80</td><td>1.38</td><td>26.41</td><td>25.29</td><td>NGC4251</td><td>19.1</td><td>A,A</td></tr><tr><td>MATLAS-1245</td><td>14.15 ± 0.91</td><td>0.75 ± 0.02</td><td>0.49 ± 0.00</td><td>-88.12 ± 0.60</td><td>2.13</td><td>27.35</td><td>26.62</td><td>NGC4215</td><td>31.5</td><td>–A</td></tr><tr><td>MATLAS-1246</td><td>15.23 ± 0.88</td><td>1.08 ± 0.01</td><td>0.96 ± 0.00</td><td>32.36 ± 0.82</td><td>2.70</td><td>25.81</td><td>24.55</td><td>NGC4259</td><td>37.2</td><td>–B</td></tr><tr><td>MATLAS-1248</td><td>15.24 ± 0.22</td><td>0.96 ± 0.00</td><td>0.68 ± 0.00</td><td>-64.26 ± 0.20</td><td>2.70</td><td>25.42</td><td>24.35</td><td>NGC4259</td><td>37.2</td><td>–B</td></tr><tr><td>MATLAS-1249</td><td>11.39 ± 0.71</td><td>0.86 ± 0.02</td><td>0.50 ± 0.00</td><td>59.25 ± 0.55</td><td>2.02</td><td>27.08</td><td>26.18</td><td>NGC4259</td><td>37.2</td><td>–B</td></tr><tr><td>MATLAS-1262</td><td>11.65 ± 0.36</td><td>0.88 ± 0.01</td><td>0.80 ± 0.00</td><td>-81.22 ± 0.90</td><td>1.75</td><td>26.06</td><td>25.13</td><td>NGC4215</td><td>31.5</td><td>B,B</td></tr><tr><td>MATLAS-1274</td><td>14.41 ± 0.31</td><td>0.80 ± 0.00</td><td>0.79 ± 0.00</td><td>41.54 ± 0.61</td><td>2.56</td><td>25.33</td><td>24.53</td><td>NGC4259</td><td>37.2</td><td>–B</td></tr><tr><td>MATLAS-1302</td><td>9.85 ± 0.60</td><td>0.23 ± 0.01</td><td>0.28 ± 0.00</td><td>19.20 ± 0.41</td><td>1.75</td><td>27.71</td><td>27.64</td><td>NGC4259</td><td>37.2</td><td>A,A</td></tr><tr><td>MATLAS-1337</td><td>12.56 ± 0.60</td><td>0.81 ± 0.01</td><td>0.61 ± 0.00</td><td>-69.72 ± 0.60</td><td>2.15</td><td>25.99</td><td>25.17</td><td>NGC4251</td><td>19.1</td><td>–A</td></tr><tr><td>MATLAS-1413</td><td>18.19 ± 4.11</td><td>1.00 ± 0.02</td><td>1.00 ± 0.02</td><td>89.23 ± 0.43</td><td>3.52</td><td>26.56</td><td>25.43</td><td>PGC042549</td><td>40.7</td><td>B,B</td></tr><tr><td>MATLAS-1493</td><td>9.69 ± 1.01</td><td>0.99 ± 0.03</td><td>0.62 ± 0.01</td><td>73.25 ± 0.38</td><td>1.86</td><td>26.67</td><td>25.55</td><td>NGC4690</td><td>40.2</td><td>–A</td></tr><tr><td>MATLAS-1494</td><td>12.49 ± 0.66</td><td>0.65 ± 0.01</td><td>0.64 ± 0.00</td><td>-80.86 ± 0.50</td><td>2.39</td><td>26.46</td><td>25.89</td><td>NGC4690</td><td>40.2</td><td>–A</td></tr><tr><td>MATLAS-1534</td><td>5.22 ± 0.63</td><td>1.00 ± 0.02</td><td>0.61 ± 0.01</td><td>-25.30 ± 1.90</td><td>0.98</td><td>26.06</td><td>24.94</td><td>NGC5198</td><td>39.6</td><td>A,A</td></tr><tr><td>MATLAS-1550</td><td>10.58 ± 0.75</td><td>0.86 ± 0.02</td><td>0.51 ± 0.00</td><td>-18.97 ± 0.60</td><td>1.59</td><td>26.85</td><td>25.95</td><td>NGC5308</td><td>31.5</td><td>A,A</td></tr><tr><td>MATLAS-1558</td><td>14.16 ± 0.99</td><td>0.68 ± 0.02</td><td>0.62 ± 0.01</td><td>-33.43 ± 1.10</td><td>2.13</td><td>27.36</td><td>26.74</td><td>NGC5308</td><td>31.5</td><td>A,A</td></tr><tr><td>MATLAS-1589</td><td>11.74 ± 1.52</td><td>1.00 ± 0.02</td><td>0.53 ± 0.01</td><td>-27.34 ± 1.30</td><td>1.70</td><td>26.74</td><td>25.61</td><td>NGC5322</td><td>30.3</td><td>A,A</td></tr><tr><td>MATLAS-1606</td><td>9.77 ± 0.56</td><td>1.00 ± 0.02</td><td>0.85 ± 0.01</td><td>58.20 ± 0.56</td><td>1.73</td><td>25.31</td><td>24.19</td><td>NGC5355</td><td>37.1</td><td>–B</td></tr><tr><td>MATLAS-1615</td><td>16.11 ± 0.64</td><td>0.79 ± 0.01</td><td>0.77 ± 0.00</td><td>-49.48 ± 1.00</td><td>2.85</td><td>26.30</td><td>25.52</td><td>NGC5355</td><td>37.1</td><td>–A</td></tr><tr><td>MATLAS-1616</td><td>17.09 ± 1.33</td><td>0.98 ± 0.02</td><td>0.58 ± 0.00</td><td>-47.39 ± 0.70</td><td>2.45</td><td>26.82</td><td>25.73</td><td>NGC5379</td><td>30.0</td><td>B,B</td></tr><tr><td>MATLAS-1630</td><td>13.16 ± 1.40</td><td>1.19 ± 0.03</td><td>0.56 ± 0.01</td><td>0.27 ± 0.43</td><td>1.91</td><td>26.89</td><td>25.44</td><td>NGC5322</td><td>30.3</td><td>B,B</td></tr><tr><td>MATLAS-1647</td><td>7.52 ± 0.64</td><td>0.54 ± 0.03</td><td>0.33 ± 0.01</td><td>86.86 ± 0.64</td><td>1.33</td><td>27.19</td><td>26.78</td><td>NGC5355</td><td>37.1</td><td>A,A</td></tr><tr><td>MATLAS-1779</td><td>14.50 ± 1.34</td><td>1.00 ± 0.02</td><td>0.47 ± 0.01</td><td>22.12 ± 0.12</td><td>2.68</td><td>26.50</td><td>25.37</td><td>NGC5493</td><td>38.8</td><td>A,A</td></tr><tr><td>MATLAS-1794</td><td>9.02 ± 0.21</td><td>1.05 ± 0.01</td><td>0.32 ± 0.00</td><td>44.44 ± 0.10</td><td>1.23</td><td>26.06</td><td>24.84</td><td>NGC5507</td><td>28.5</td><td>A,A</td></tr><tr><td>MATLAS-1824</td><td>13.31 ± 0.63</td><td>0.98 ± 0.01</td><td>0.67 ± 0.00</td><td>71.43 ± 0.74</td><td>2.93</td><td>25.99</td><td>24.89</td><td>NGC5557</td><td>38.8</td><td>–A</td></tr><tr><td>MATLAS-1847</td><td>15.58 ± 0.84</td><td>0.32 ± 0.01</td><td>0.52 ± 0.01</td><td>20.84 ± 0.76</td><td>1.73</td><td>27.27</td><td>27.13</td><td>NGC5574</td><td>23.2</td><td>–A</td></tr><tr><td>MATLAS-1855</td><td>11.72 ± 0.74</td><td>0.80 ± 0.01</td><td>0.73 ± 0.01</td><td>-44.86 ± 1.40</td><td>1.30</td><td>26.35</td><td>25.55</td><td>NGC5574</td><td>23.2</td><td>–A</td></tr><tr><td>MATLAS-1865</td><td>11.17 ± 0.28</td><td>0.71 ± 0.01</td><td>0.35 ± 0.00</td><td>-71.96 ± 0.10</td><td>1.44</td><td>25.98</td><td>25.32</td><td>NGC5631</td><td>27.0</td><td>A,A</td></tr><tr><td>MATLAS-1907</td><td>12.83 ± 0.28</td><td>0.90 ± 0.01</td><td>0.45 ± 0.00</td><td>-48.91 ± 0.10</td><td>1.49</td><td>25.77</td><td>24.82</td><td>IC1024</td><td>24.2</td><td>B,B</td></tr><tr><td>MATLAS-1957</td><td>7.64 ± 0.33</td><td>1.00 ± 0.02</td><td>0.80 ± 0.00</td><td>-74.43 ± 1.30</td><td>1.14</td><td>24.64</td><td>23.51</td><td>NGC5813</td><td>31.3</td><td>–B</td></tr><tr><td>MATLAS-1975</td><td>13.00 ± 1.80</td><td>1.58 ± 0.04</td><td>0.80 ± 0.01</td><td>-40.81 ± 2.10</td><td>1.64</td><td>26.71</td><td>24.54</td><td>NGC5831</td><td>26.4</td><td>A,A</td></tr><tr><td>MATLAS-1985</td><td>15.36 ± 1.30</td><td>1.00 ± 0.02</td><td>0.72 ± 0.01</td><td>38.87 ± 0.12</td><td>1.94</td><td>26.04</td><td>24.92</td><td>NGC5831</td><td>26.4</td><td>B,B</td></tr><tr><td>MATLAS-1991</td><td>12.63 ± 0.26</td><td>0.73 ± 0.01</td><td>0.68 ± 0.00</td><td>10.37 ± 0.41</td><td>1.53</td><td>25.48</td><td>24.79</td><td>NGC5845</td><td>25.2</td><td>–B</td></tr><tr><td>MATLAS-1996</td><td>14.87 ± 0.36</td><td>1.00 ± 0.01</td><td>0.29 ± 0.01</td><td>-62.51 ± 0.80</td><td>1.56</td><td>27.82</td><td>26.69</td><td>NGC5838</td><td>21.8</td><td>–A</td></tr><tr><td>MATLAS-2019</td><td>15.74 ± 0.36</td><td>0.61 ± 0.06</td><td>0.97 ± 0.03</td><td>-78.49 ± 5.30</td><td>1.51</td><td>25.85</td><td>25.34</td><td>NGC5845</td><td>25.2</td><td>B,B</td></tr><tr><td>MATLAS-2021</td><td>17.79 ± 0.45</td><td>0.76 ± 0.03</td><td>0.84 ± 0.02</td><td>-40.85 ± 6.60</td><td>1.86</td><td>27.67</td><td>26.94</td><td>NGC5838</td><td>21.8</td><td>A,A</td></tr><tr><td>MATLAS-2103</td><td>15.34 ± 0.21</td><td>0.81 ± 0.00</td><td>0.69 ± 0.00</td><td>-89.43 ± 0.20</td><td>2.62</td><td>25.90</td><td>25.10</td><td>NGC6014</td><td>35.8</td><td>–B</td></tr><tr><td>MATLAS-2184</td><td>9.46 ± 1.04</td><td>1.00 ± 0.03</td><td>0.74 ± 0.01</td><td>83.62 ± 0.52</td><td>1.33</td><td>26.55</td><td>25.42</td><td>NGC7465</td><td>29.3</td><td>A,A</td></tr></tbody></table>

Note. Columns are (1) Galaxy ID; (2) GALFITM effective radius in arcsec; (3) GALFITM Sérsic index; (4) GALFITM axis ratio; (5) GALFITM position angle; (6) Effective radius in kpc; (7) Mean surface brightness; (8) Central surface brightness; (9) Candidate host of UDGs; (10) Distance to host (and assumed distance to UDG); (11) KMeans clustering algorithm class (first class is for the determination using N<sub>GC</sub> – only 38 galaxies; the second is for the classification without N<sub>GC</sub> – all 59 UDGs.
<table><caption>Table B2. GALFITM morphological parameters and physical properties of the MATLAS UDGs.</caption><thead><tr><th>ID</th><th>R<sub>e</sub><br>[arcsec]</th><th>n</th><th>b/a</th><th>PA<br>[degrees]</th><th>R<sub>e</sub><br>[kpc]</th><th>(μ<sub>g, c</sub>)<br>[mag arcsec<sup>-2</sup>]</th><th>μ<sub>g, 0</sub><br>[mag arcsec<sup>-2</sup>]</th><th>Candidate host</th><th>Host distance<br>[Mpc]</th><th>KMeans class<br>[with N<sub>GC</sub>, w/o N<sub>GC</sub>]</th></tr></thead><tbody><tr><td>MATLAS-42</td><td>19.44 ± 0.43</td><td>0.45 ± 0.00</td><td>0.60 ± 0.00</td><td>-43.05 ± 0.30</td><td>3.11</td><td>25.94</td><td>25.64</td><td>NGC0502</td><td>35.9</td><td>B,B</td></tr><tr><td>MATLAS-48</td><td>10.07 ± 0.72</td><td>0.95 ± 0.02</td><td>0.71 ± 0.01</td><td>-85.12 ± 1.20</td><td>1.72</td><td>25.98</td><td>24.94</td><td>NGC0502</td><td>35.9</td><td>–B</td></tr><tr><td>MATLAS-141</td><td>9.87 ± 1.00</td><td>0.52 ± 0.02</td><td>0.52 ± 0.01</td><td>-69.09 ± 1.30</td><td>1.73</td><td>27.14</td><td>26.76</td><td>NGC0770</td><td>36.7</td><td>A,A</td></tr><tr><td>MATLAS-149</td><td>11.41 ± 1.12</td><td>0.55 ± 0.03</td><td>0.45 ± 0.01</td><td>-84.63 ± 1.00</td><td>2.00</td><td>27.46</td><td>27.04</td><td>NGC0770</td><td>36.7</td><td>A,A</td></tr><tr><td>MATLAS-177</td><td>15.74 ± 1.33</td><td>0.73 ± 0.02</td><td>0.81 ± 0.01</td><td>-45.93 ± 2.50</td><td>1.69</td><td>27.20</td><td>26.51</td><td>NGC0936</td><td>22.4</td><td>A,A</td></tr><tr><td>MATLAS-262</td><td>9.37 ± 1.07</td><td>1.00 ± 0.02</td><td>0.31 ± 0.00</td><td>85.97 ± 0.51</td><td>1.36</td><td>26.92</td><td>25.79</td><td>NGC1248</td><td>30.4</td><td>A,A</td></tr><tr><td>MATLAS-342</td><td>9.65 ± 0.49</td><td>1.15 ± 0.01</td><td>0.76 ± 0.00</td><td>27.79 ± 0.08</td><td>1.48</td><td>25.54</td><td>24.16</td><td>NGC2481</td><td>32.0</td><td>B,B</td></tr><tr><td>MATLAS-365</td><td>5.13 ± 0.56</td><td>1.00 ± 0.02</td><td>0.74 ± 0.01</td><td>-62.48 ± 2.60</td><td>0.75</td><td>25.34</td><td>24.22</td><td>NGC2577</td><td>30.8</td><td>A,A</td></tr><tr><td>MATLAS-368</td><td>17.88 ± 1.59</td><td>1.15 ± 0.02</td><td>0.78 ± 0.01</td><td>-48.24 ± 1.20</td><td>2.63</td><td>26.35</td><td>24.98</td><td>NGC2577</td><td>30.8</td><td>A,A</td></tr><tr><td>MATLAS-405</td><td>17.21 ± 1.05</td><td>0.78 ± 0.01</td><td>0.53 ± 0.00</td><td>50.77 ± 0.62</td><td>2.31</td><td>27.08</td><td>26.32</td><td>UGC04551</td><td>28.0</td><td>A,A</td></tr><tr><td>MATLAS-478</td><td>19.26 ± 0.52</td><td>1.06 ± 0.01</td><td>0.49 ± 0.00</td><td>-65.97 ± 0.10</td><td>2.02</td><td>26.15</td><td>24.92</td><td>NGC2768</td><td>21.8</td><td>A,A</td></tr><tr><td>MATLAS-585</td><td>11.61 ± 0.45</td><td>0.59 ± 0.01</td><td>0.61 ± 0.00</td><td>3.21 ± 1.00</td><td>1.51</td><td>26.14</td><td>25.66</td><td>IC0560</td><td>27.2</td><td>B,B</td></tr><tr><td>MATLAS-658</td><td>12.56 ± 4.57</td><td>1.54 ± 0.08</td><td>0.39 ± 0.01</td><td>16.92 ± 0.56</td><td>1.99</td><td>28.04</td><td>25.96</td><td>NGC3193</td><td>33.1</td><td>A,A</td></tr><tr><td>MATLAS-799</td><td>16.98 ± 0.53</td><td>0.73 ± 0.01</td><td>0.87 ± 0.00</td><td>-4.23 ± 0.13</td><td>2.00</td><td>25.56</td><td>24.87</td><td>NGC3414</td><td>24.5</td><td>B,B</td></tr><tr><td>MATLAS-898</td><td>13.97 ± 0.85</td><td>1.00 ± 0.02</td><td>0.67 ± 0.01</td><td>34.34 ± 0.94</td><td>1.33</td><td>25.89</td><td>24.77</td><td>NGC3599</td><td>19.8</td><td>B,B</td></tr><tr><td>MATLAS-951</td><td>20.83 ± 1.15</td><td>1.00 ± 0.03</td><td>0.45 ± 0.00</td><td>-70.71 ± 0.20</td><td>2.62</td><td>26.11</td><td>24.98</td><td>NGC3640</td><td>26.3</td><td>–B</td></tr><tr><td>MATLAS-984</td><td>9.45 ± 0.53</td><td>0.64 ± 0.02</td><td>0.73 ± 0.01</td><td>40.64 ± 0.66</td><td>1.49</td><td>26.49</td><td>25.93</td><td>NGC3665</td><td>33.1</td><td>B,A</td></tr><tr><td>MATLAS-1059</td><td>18.94 ± 1.39</td><td>1.64 ± 0.02</td><td>0.60 ± 0.00</td><td>-69.50 ± 0.58</td><td>3.02</td><td>26.18</td><td>23.92</td><td>NGC3674</td><td>33.4</td><td>B,B</td></tr><tr><td>MATLAS-1174</td><td>14.09 ± 2.39</td><td>1.00 ± 0.02</td><td>0.29 ± 0.01</td><td>57.06 ± 0.57</td><td>2.56</td><td>27.24</td><td>26.11</td><td>NGC4078</td><td>38.1</td><td>A,A</td></tr><tr><td>MATLAS-1177</td><td>12.28 ± 0.07</td><td>1.00 ± 0.02</td><td>0.94 ± 0.01</td><td>39.37 ± 0.01</td><td>1.45</td><td>25.15</td><td>24.02</td><td>NGC4036</td><td>24.6</td><td>–B</td></tr><tr><td>MATLAS-1205</td><td>10.14 ± 1.17</td><td>0.83 ± 0.02</td><td>0.71 ± 0.00</td><td>-38.53 ± 1.90</td><td>1.89</td><td>26.53</td><td>25.69</td><td>NGC4191</td><td>39.2</td><td>–A</td></tr><tr><td>MATLAS-1216</td><td>8.72 ± 2.75</td><td>0.80 ± 0.04</td><td>1.00 ± 0.03</td><td>0.11 ± 0.98</td><td>1.63</td><td>26.76</td><td>25.97</td><td>NGC4191</td><td>39.2</td><td>B,B</td></tr><tr><td>MATLAS-1225</td><td>15.01 ± 1.63</td><td>1.00 ± 0.02</td><td>0.54 ± 0.01</td><td>-67.13 ± 0.80</td><td>1.38</td><td>26.41</td><td>25.29</td><td>NGC4251</td><td>19.1</td><td>A,A</td></tr><tr><td>MATLAS-1245</td><td>14.15 ± 0.91</td><td>0.75 ± 0.02</td><td>0.49 ± 0.00</td><td>-88.12 ± 0.60</td><td>2.13</td><td>27.35</td><td>26.62</td><td>NGC4215</td><td>31.5</td><td>–A</td></tr><tr><td>MATLAS-1246</td><td>15.23 ± 0.88</td><td>1.08 ± 0.01</td><td>0.96 ± 0.00</td><td>32.36 ± 0.82</td><td>2.70</td><td>25.81</td><td>24.55</td><td>NGC4259</td><td>37.2</td><td>–B</td></tr><tr><td>MATLAS-1248</td><td>15.24 ± 0.22</td><td>0.96 ± 0.00</td><td>0.68 ± 0.00</td><td>-64.26 ± 0.20</td><td>2.70</td><td>25.42</td><td>24.35</td><td>NGC4259</td><td>37.2</td><td>–B</td></tr><tr><td>MATLAS-1249</td><td>11.39 ± 0.71</td><td>0.86 ± 0.02</td><td>0.50 ± 0.00</td><td>59.25 ± 0.55</td><td>2.02</td><td>27.08</td><td>26.18</td><td>NGC4259</td><td>37.2</td><td>–B</td></tr><tr><td>MATLAS-1262</td><td>11.65 ± 0.36</td><td>0.88 ± 0.01</td><td>0.80 ± 0.00</td><td>-81.22 ± 0.90</td><td>1.75</td><td>26.06</td><td>25.13</td><td>NGC4215</td><td>31.5</td><td>B,B</td></tr><tr><td>MATLAS-1274</td><td>14.41 ± 0.31</td><td>0.80 ± 0.00</td><td>0.79 ± 0.00</td><td>41.54 ± 0.61</td><td>2.56</td><td>25.33</td><td>24.53</td><td>NGC4259</td><td>37.2</td><td>–B</td></tr><tr><td>MATLAS-1302</td><td>9.85 ± 0.60</td><td>0.23 ± 0.01</td><td>0.28 ± 0.00</td><td>19.20 ± 0.41</td><td>1.75</td><td>27.71</td><td>27.64</td><td>NGC4259</td><td>37.2</td><td>A,A</td></tr><tr><td>MATLAS-1337</td><td>12.56 ± 0.60</td><td>0.81 ± 0.01</td><td>0.61 ± 0.00</td><td>-69.72 ± 0.60</td><td>2.15</td><td>25.99</td><td>25.17</td><td>NGC4251</td><td>19.1</td><td>–A</td></tr><tr><td>MATLAS-1413</td><td>18.19 ± 4.11</td><td>1.00 ± 0.02</td><td>1.00 ± 0.02</td><td>89.23 ± 0.43</td><td>3.52</td><td>26.56</td><td>25.43</td><td>PGC042549</td><td>40.7</td><td>B,B</td></tr><tr><td>MATLAS-1493</td><td>9.69 ± 1.01</td><td>0.99 ± 0.03</td><td>0.62 ± 0.01</td><td>73.25 ± 0.38</td><td>1.86</td><td>26.67</td><td>25.55</td><td>NGC4690</td><td>40.2</td><td>–A</td></tr><tr><td>MATLAS-1494</td><td>12.49 ± 0.66</td><td>0.65 ± 0.01</td><td>0.64 ± 0.00</td><td>-80.86 ± 0.50</td><td>2.39</td><td>26.46</td><td>25.89</td><td>NGC4690</td><td>40.2</td><td>–A</td></tr><tr><td>MATLAS-1534</td><td>5.22 ± 0.63</td><td>1.00 ± 0.02</td><td>0.61 ± 0.01</td><td>-25.30 ± 1.90</td><td>0.98</td><td>26.06</td><td>24.94</td><td>NGC5198</td><td>39.6</td><td>A,A</td></tr><tr><td>MATLAS-1550</td><td>10.58 ± 0.75</td><td>0.86 ± 0.02</td><td>0.51 ± 0.00</td><td>-18.97 ± 0.60</td><td>1.59</td><td>26.85</td><td>25.95</td><td>NGC5308</td><td>31.5</td><td>A,A</td></tr><tr><td>MATLAS-1558</td><td>14.16 ± 0.99</td><td>0.68 ± 0.02</td><td>0.62 ± 0.01</td><td>-33.43 ± 1.10</td><td>2.13</td><td>27.36</td><td>26.74</td><td>NGC5308</td><td>31.5</td><td>A,A</td></tr><tr><td>MATLAS-1589</td><td>11.74 ± 1.52</td><td>1.00 ± 0.02</td><td>0.53 ± 0.01</td><td>-27.34 ± 1.30</td><td>1.70</td><td>26.74</td><td>25.61</td><td>NGC5322</td><td>30.3</td><td>A,A</td></tr><tr><td>MATLAS-1606</td><td>9.77 ± 0.56</td><td>1.00 ± 0.02</td><td>0.85 ± 0.01</td><td>58.20 ± 0.56</td><td>1.73</td><td>25.31</td><td>24.19</td><td>NGC5355</td><td>37.1</td><td>–B</td></tr><tr><td>MATLAS-1615</td><td>16.11 ± 0.64</td><td>0.79 ± 0.01</td><td>0.77 ± 0.00</td><td>-49.48 ± 1.00</td><td>2.85</td><td>26.30</td><td>25.52</td><td>NGC5355</td><td>37.1</td><td>–A</td></tr><tr><td>MATLAS-1616</td><td>17.09 ± 1.33</td><td>0.98 ± 0.02</td><td>0.58 ± 0.00</td><td>-47.39 ± 0.70</td><td>2.45</td><td>26.82</td><td>25.73</td><td>NGC5379</td><td>30.0</td><td>B,B</td></tr><tr><td>MATLAS-1630</td><td>13.16 ± 1.40</td><td>1.19 ± 0.03</td><td>0.56 ± 0.01</td><td>0.27 ± 0.43</td><td>1.91</td><td>26.89</td><td>25.44</td><td>NGC5322</td><td>30.3</td><td>B,B</td></tr><tr><td>MATLAS-1647</td><td>7.52 ± 0.64</td><td>0.54 ± 0.03</td><td>0.33 ± 0.01</td><td>86.86 ± 0.64</td><td>1.33</td><td>27.19</td><td>26.78</td><td>NGC5355</td><td>37.1</td><td>A,A</td></tr><tr><td>MATLAS-1779</td><td>14.50 ± 1.34</td><td>1.00 ± 0.02</td><td>0.47 ± 0.01</td><td>22.12 ± 0.12</td><td>2.68</td><td>26.50</td><td>25.37</td><td>NGC5493</td><td>38.8</td><td>A,A</td></tr><tr><td>MATLAS-1794</td><td>9.02 ± 0.21</td><td>1.05 ± 0.01</td><td>0.32 ± 0.00</td><td>44.44 ± 0.10</td><td>1.23</td><td>26.06</td><td>24.84</td><td>NGC5507</td><td>28.5</td><td>A,A</td></tr><tr><td>MATLAS-1824</td><td>13.31 ± 0.63</td><td>0.98 ± 0.01</td><td>0.67 ± 0.00</td><td>71.43 ± 0.74</td><td>2.93</td><td>25.99</td><td>24.89</td><td>NGC5557</td><td>38.8</td><td>–A</td></tr><tr><td>MATLAS-1847</td><td>15.58 ± 0.84</td><td>0.32 ± 0.01</td><td>0.52 ± 0.01</td><td>20.84 ± 0.76</td><td>1.73</td><td>27.27</td><td>27.13</td><td>NGC5574</td><td>23.2</td><td>–A</td></tr><tr><td>MATLAS-1855</td><td>11.72 ± 0.74</td><td>0.80 ± 0.01</td><td>0.73 ± 0.01</td><td>-44.86 ± 1.40</td><td>1.30</td><td>26.35</td><td>25.55</td><td>NGC5574</td><td>23.2</td><td>–A</td></tr><tr><td>MATLAS-1865</td><td>11.17 ± 0.28</td><td>0.71 ± 0.01</td><td>0.35 ± 0.00</td><td>-71.96 ± 0.10</td><td>1.44</td><td>25.98</td><td>25.32</td><td>NGC5631</td><td>27.0</td><td>A,A</td></tr><tr><td>MATLAS-1907</td><td>12.83 ± 0.28</td><td>0.90 ± 0.01</td><td>0.45 ± 0.00</td><td>-48.91 ± 0.10</td><td>1.49</td><td>25.77</td><td>24.82</td><td>IC1024</td><td>24.2</td><td>B,B</td></tr><tr><td>MATLAS-1957</td><td>7.64 ± 0.33</td><td>1.00 ± 0.02</td><td>0.80 ± 0.00</td><td>-74.43 ± 1.30</td><td>1.14</td><td>24.64</td><td>23.51</td><td>NGC5813</td><td>31.3</td><td>–B</td></tr><tr><td>MATLAS-1975</td><td>13.00 ± 1.80</td><td>1.58 ± 0.04</td><td>0.80 ± 0.01</td><td>-40.81 ± 2.10</td><td>1.64</td><td>26.71</td><td>24.54</td><td>NGC5831</td><td>26.4</td><td>A,A</td></tr><tr><td>MATLAS-1985</td><td>15.36 ± 1.30</td><td>1.00 ± 0.02</td><td>0.72 ± 0.01</td><td>38.87 ± 0.12</td><td>1.94</td><td>26.04</td><td>24.92</td><td>NGC5831</td><td>26.4</td><td>B,B</td></tr><tr><td>MATLAS-1991</td><td>12.63 ± 0.26</td><td>0.73 ± 0.01</td><td>0.68 ± 0.00</td><td>10.37 ± 0.41</td><td>1.53</td><td>25.48</td><td>24.79</td><td>NGC5845</td><td>25.2</td><td>–B</td></tr><tr><td>MATLAS-1996</td><td>14.87 ± 0.36</td><td>1.00 ± 0.01</td><td>0.29 ± 0.01</td><td>-62.51 ± 0.80</td><td>1.56</td><td>27.82</td><td>26.69</td><td>NGC5838</td><td>21.8</td><td>–A</td></tr><tr><td>MATLAS-2019</td><td>15.74 ± 0.36</td><td>0.61 ± 0.06</td><td>0.97 ± 0.03</td><td>-78.49 ± 5.30</td><td>1.51</td><td>25.85</td><td>25.34</td><td>NGC5845</td><td>25.2</td><td>B,B</td></tr><tr><td>MATLAS-2021</td><td>17.79 ± 0.45</td><td>0.76 ± 0.03</td><td>0.84 ± 0.02</td><td>-40.85 ± 6.60</td><td>1.86</td><td>27.67</td><td>26.94</td><td>NGC5838</td><td>21.8</td><td>A,A</td></tr><tr><td>MATLAS-2103</td><td>15.34 ± 0.21</td><td>0.81 ± 0.00</td><td>0.69 ± 0.00</td><td>-89.43 ± 0.20</td><td>2.62</td><td>25.90</td><td>25.10</td><td>NGC6014</td><td>35.8</td><td>–B</td></tr><tr><td>MATLAS-2184</td><td>9.46 ± 1.04</td><td>1.00 ± 0.03</td><td>0.74 ± 0.01</td><td>83.62 ± 0.52</td><td>1.33</td><td>26.55</td><td>25.42</td><td>NGC7465</td><td>29.3</td><td>A,A</td></tr></tbody></table>

Note. Columns are (1) Galaxy ID; (2) GALFITM effective radius in arcsec; (3) GALFITM Sérsic index; (4) GALFITM axis ratio; (5) GALFITM position angle; (6) Effective radius in kpc; (7) Mean surface brightness; (8) Central surface brightness; (9) Candidate host of UDGs; (10) Distance to host (and assumed distance to UDG); (11) KMeans clustering algorithm class (first class is for the determination using N<sub>GC</sub> – only 38 galaxies; the second is for the classification without N<sub>GC</sub> – all 59 UDGs.
<table><caption>**Table B3.** PROSPECTOR stellar population properties of the MATLAS UDGs.</caption><thead><tr><th>ID</th><th>log(M<sub>*</sub>/M<sub>☉</sub>)</th><th>[M/H]<br>[dex]</th><th>τ<br>[Gyr]</th><th>t<sub>M</sub><br>[Gyr]</th><th>A<sub>v</sub><br>[mag]</th><th>M<sub>*</sub>/L<sub>V</sub><br>[M<sub>☉</sub>/L<sub>☉</sub>, V]</th></tr></thead><tbody><tr><td>MATLAS-42</td><td>8.17<sup>+0.08</sup><sub>-0.10</sub></td><td>-1.34<sup>+0.30</sup><sub>-0.19</sub></td><td>1.57<sup>+1.70</sup><sub>-1.08</sub></td><td>5.97<sup>+5.46</sup><sub>-3.59</sub></td><td>0.07<sup>+0.09</sup><sub>-0.05</sub></td><td>0.97</td></tr><tr><td>MATLAS-48</td><td>7.89<sup>+0.07</sup><sub>-0.10</sub></td><td>-1.31<sup>+0.30</sup><sub>-0.20</sub></td><td>1.19<sup>+1.13</sup><sub>-0.76</sub></td><td>8.90<sup>+3.50</sup><sub>-4.30</sub></td><td>0.07<sup>+0.08</sup><sub>-0.05</sub></td><td>1.99</td></tr><tr><td>MATLAS-141</td><td>7.31<sup>+0.14</sup><sub>-0.16</sub></td><td>-1.06<sup>+0.58</sup><sub>-0.40</sub></td><td>2.85<sup>+3.79</sup><sub>-2.01</sub></td><td>5.13<sup>+5.78</sup><sub>-2.94</sub></td><td>0.15<sup>+0.20</sup><sub>-0.11</sub></td><td>1.44</td></tr><tr><td>MATLAS-149</td><td>7.25<sup>+0.12</sup><sub>-0.14</sub></td><td>-1.42<sup>+0.26</sup><sub>-0.13</sub></td><td>3.32<sup>+3.36</sup><sub>-2.33</sub></td><td>4.04<sup>+5.87</sup><sub>-2.16</sub></td><td>0.04<sup>+0.06</sup><sub>-0.03</sub></td><td>1.25</td></tr><tr><td>MATLAS-177</td><td>7.49<sup>+0.09</sup><sub>-0.13</sub></td><td>-1.19<sup>+0.41</sup><sub>-0.28</sub></td><td>1.43<sup>+1.63</sup><sub>-0.93</sub></td><td>8.34<sup>+3.84</sup><sub>-4.33</sub></td><td>0.16<sup>+0.14</sup><sub>-0.10</sub></td><td>2.52</td></tr><tr><td>MATLAS-262</td><td>7.40<sup>+0.19</sup><sub>-0.19</sub></td><td>-1.21<sup>+0.68</sup><sub>-0.54</sub></td><td>2.74<sup>+3.53</sup><sub>-1.98</sub></td><td>6.02<sup>+5.46</sup><sub>-3.31</sub></td><td>0.50<sup>+0.30</sup><sub>-0.30</sub></td><td>2.31</td></tr><tr><td>MATLAS-342</td><td>7.81<sup>+0.09</sup><sub>-0.10</sub></td><td>-1.22<sup>+0.33</sup><sub>-0.26</sub></td><td>1.48<sup>+1.65</sup><sub>-0.98</sub></td><td>7.49<sup>+4.51</sup><sub>-4.20</sub></td><td>0.12<sup>+0.11</sup><sub>-0.08</sub></td><td>1.77</td></tr><tr><td>MATLAS-365</td><td>7.12<sup>+0.13</sup><sub>-0.16</sub></td><td>-1.16<sup>+0.51</sup><sub>-0.32</sub></td><td>2.86<sup>+3.41</sup><sub>-1.98</sub></td><td>5.33<sup>+5.69</sup><sub>-2.93</sub></td><td>0.15<sup>+0.19</sup><sub>-0.11</sub></td><td>0.96</td></tr><tr><td>MATLAS-368</td><td>7.23<sup>+0.08</sup><sub>-0.10</sub></td><td>-1.42<sup>+0.22</sup><sub>-0.13</sub></td><td>2.59<sup>+4.35</sup><sub>-2.29</sub></td><td>4.44<sup>+0.89</sup><sub>-0.16</sub></td><td>0.03<sup>+0.01</sup><sub>-0.00</sub></td><td>0.26</td></tr><tr><td>MATLAS-405</td><td>7.64<sup>+0.09</sup><sub>-0.12</sub></td><td>-1.21<sup>+0.41</sup><sub>-0.27</sub></td><td>1.57<sup>+1.77</sup><sub>-1.06</sub></td><td>7.90<sup>+4.24</sup><sub>-4.19</sub></td><td>0.12<sup>+0.13</sup><sub>-0.08</sub></td><td>1.75</td></tr><tr><td>MATLAS-478</td><td>8.05<sup>+0.04</sup><sub>-0.06</sub></td><td>-0.69<sup>+0.39</sup><sub>-0.28</sub></td><td>0.77<sup>+0.56</sup><sub>-0.48</sub></td><td>10.92<sup>+1.99</sup><sub>-2.98</sub></td><td>0.22<sup>+0.10</sup><sub>-0.13</sub></td><td>2.43</td></tr><tr><td>MATLAS-585</td><td>7.53<sup>+0.07</sup><sub>-0.08</sub></td><td>-1.36<sup>+0.27</sup><sub>-0.16</sub></td><td>1.12<sup>+1.18</sup><sub>-0.69</sub></td><td>8.91<sup>+3.44</sup><sub>-4.31</sub></td><td>0.06<sup>+0.07</sup><sub>-0.04</sub></td><td>1.24</td></tr><tr><td>MATLAS-658</td><td>7.39<sup>+0.26</sup><sub>-0.24</sub></td><td>-1.08<sup>+0.71</sup><sub>-0.62</sub></td><td>3.18<sup>+3.76</sup><sub>-2.32</sub></td><td>5.28<sup>+5.85</sup><sub>-3.03</sub></td><td>1.17<sup>+0.31</sup><sub>-0.33</sub></td><td>3.16</td></tr><tr><td>MATLAS-799</td><td>7.87<sup>+0.05</sup><sub>-0.06</sub></td><td>-1.56<sup>+0.06</sup><sub>-0.03</sub></td><td>0.69<sup>+0.84</sup><sub>-0.43</sub></td><td>3.67<sup>+2.32</sup><sub>-1.76</sub></td><td>0.01<sup>+0.01</sup><sub>-0.00</sub></td><td>0.97</td></tr><tr><td>MATLAS-898</td><td>7.35<sup>+0.09</sup><sub>-0.10</sub></td><td>-1.51<sup>+0.13</sup><sub>-0.07</sub></td><td>1.83<sup>+2.92</sup><sub>-1.39</sub></td><td>3.85<sup>+6.63</sup><sub>-2.31</sub></td><td>0.01<sup>+0.02</sup><sub>-0.01</sub></td><td>0.90</td></tr><tr><td>MATLAS-951</td><td>8.25<sup>+0.10</sup><sub>-0.15</sub></td><td>-1.33<sup>+0.56</sup><sub>-0.45</sub></td><td>0.88<sup>+0.96</sup><sub>-0.57</sub></td><td>9.19<sup>+3.32</sup><sub>-4.54</sub></td><td>0.37<sup>+0.17</sup><sub>-0.22</sub></td><td>3.46</td></tr><tr><td>MATLAS-984</td><td>7.34<sup>+0.09</sup><sub>-0.09</sub></td><td>-1.50<sup>+0.15</sup><sub>-0.08</sub></td><td>1.50<sup>+2.01</sup><sub>-1.04</sub></td><td>5.70<sup>+5.58</sup><sub>-3.21</sub></td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>1.20</td></tr><tr><td>MATLAS-1059</td><td>7.84<sup>+0.10</sup><sub>-0.12</sub></td><td>-1.50<sup>+0.15</sup><sub>-0.07</sub></td><td>2.07<sup>+2.84</sup><sub>-1.47</sub></td><td>4.47<sup>+6.32</sup><sub>-2.60</sub></td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>0.77</td></tr><tr><td>MATLAS-1174</td><td>7.57<sup>+0.15</sup><sub>-0.18</sub></td><td>-1.23<sup>+0.44</sup><sub>-0.27</sub></td><td>2.39<sup>+3.25</sup><sub>-1.65</sub></td><td>6.12<sup>+5.31</sup><sub>-3.48</sub></td><td>0.09<sup>+0.13</sup><sub>-0.07</sub></td><td>1.38</td></tr><tr><td>MATLAS-1177</td><td>6.92<sup>+0.18</sup><sub>-0.22</sub></td><td>-0.92<sup>+0.61</sup><sub>-0.71</sub></td><td>6.33<sup>+2.50</sup><sub>-2.95</sub></td><td>2.70<sup>+3.20</sup><sub>-1.43</sub></td><td>0.37<sup>+0.12</sup><sub>-0.12</sub></td><td>0.25</td></tr><tr><td>MATLAS-1205</td><td>7.37<sup>+0.13</sup><sub>-0.14</sub></td><td>-1.39<sup>+0.30</sup><sub>-0.16</sub></td><td>3.11<sup>+3.52</sup><sub>-2.25</sub></td><td>5.27<sup>+5.81</sup><sub>-2.76</sub></td><td>0.04<sup>+0.07</sup><sub>-0.03</sub></td><td>0.83</td></tr><tr><td>MATLAS-1216</td><td>7.45<sup>+0.13</sup><sub>-0.21</sub></td><td>-0.94<sup>+0.60</sup><sub>-0.69</sub></td><td>0.96<sup>+1.41</sup><sub>-0.63</sub></td><td>9.00<sup>+3.50</sup><sub>-5.24</sub></td><td>0.04<sup>+0.07</sup><sub>-0.03</sub></td><td>0.66</td></tr><tr><td>MATLAS-1225</td><td>7.30<sup>+0.12</sup><sub>-0.16</sub></td><td>-1.35<sup>+0.32</sup><sub>-0.18</sub></td><td>1.48<sup>+2.40</sup><sub>-1.01</sub></td><td>7.28<sup>+4.75</sup><sub>-4.39</sub></td><td>0.07<sup>+0.09</sup><sub>-0.05</sub></td><td>1.21</td></tr><tr><td>MATLAS-1245</td><td>7.39<sup>+0.09</sup><sub>-0.12</sub></td><td>-1.13<sup>+0.46</sup><sub>-0.33</sub></td><td>1.71<sup>+2.43</sup><sub>-1.16</sub></td><td>7.66<sup>+4.35</sup><sub>-4.37</sub></td><td>0.18<sup>+0.22</sup><sub>-0.13</sub></td><td>1.46</td></tr><tr><td>MATLAS-1246</td><td>8.17<sup>+0.08</sup><sub>-0.08</sub></td><td>-1.49<sup>+0.16</sup><sub>-0.08</sub></td><td>1.04<sup>+0.95</sup><sub>-0.63</sub></td><td>7.92<sup>+4.01</sup><sub>-3.74</sub></td><td>0.03<sup>+0.04</sup><sub>-0.02</sub></td><td>1.31</td></tr><tr><td>MATLAS-1248</td><td>8.49<sup>+0.02</sup><sub>-0.05</sub></td><td>-1.28<sup>+0.22</sup><sub>-0.11</sub></td><td>0.50<sup>+0.53</sup><sub>-0.30</sub></td><td>11.81<sup>+1.31</sup><sub>-2.86</sub></td><td>0.07<sup>+0.05</sup><sub>-0.05</sub></td><td>1.93</td></tr><tr><td>MATLAS-1249</td><td>6.61<sup>+0.24</sup><sub>-0.27</sub></td><td>-1.05<sup>+0.86</sup><sub>-0.71</sub></td><td>6.30<sup>+2.55</sup><sub>-2.99</sub></td><td>1.15<sup>+1.80</sup><sub>-0.64</sub></td><td>1.06<sup>+0.21</sup><sub>-0.20</sub></td><td>0.12</td></tr><tr><td>MATLAS-1262</td><td>7.89<sup>+0.06</sup><sub>-0.08</sub></td><td>-1.27<sup>+0.34</sup><sub>-0.23</sub></td><td>0.92<sup>+0.99</sup><sub>-0.61</sub></td><td>9.40<sup>+3.13</sup><sub>-4.34</sub></td><td>0.14<sup>+0.10</sup><sub>-0.09</sub></td><td>2.08</td></tr><tr><td>MATLAS-1274</td><td>8.26<sup>+0.08</sup><sub>-0.07</sub></td><td>-1.51<sup>+0.13</sup><sub>-0.07</sub></td><td>0.96<sup>+1.07</sup><sub>-0.59</sub></td><td>5.97<sup>+4.51</sup><sub>-2.98</sub></td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>1.17</td></tr><tr><td>MATLAS-1302</td><td>7.08<sup>+0.11</sup><sub>-0.15</sub></td><td>-1.01<sup>+0.50</sup><sub>-0.41</sub></td><td>1.70<sup>+2.67</sup><sub>-1.18</sub></td><td>7.66<sup>+4.50</sup><sub>-4.28</sub></td><td>0.22<sup>+0.19</sup><sub>-0.15</sub></td><td>1.48</td></tr><tr><td>MATLAS-1337</td><td>7.04<sup>+0.15</sup><sub>-0.17</sub></td><td>-0.91<sup>+0.70</sup><sub>-0.47</sub></td><td>4.98<sup>+3.41</sup><sub>-2.81</sub></td><td>2.43<sup>+3.38</sup><sub>-1.32</sub></td><td>0.12<sup>+0.13</sup><sub>-0.09</sub></td><td>0.64</td></tr><tr><td>MATLAS-1413</td><td>7.85<sup>+0.21</sup><sub>-0.27</sub></td><td>-1.40<sup>+0.60</sup><sub>-0.20</sub></td><td>4.38<sup>+3.60</sup><sub>-3.00</sub></td><td>4.08<sup>+5.69</sup><sub>-2.37</sub></td><td>0.38<sup>+0.35</sup><sub>-0.26</sub></td><td>0.88</td></tr><tr><td>MATLAS-1493</td><td>7.21<sup>+0.15</sup><sub>-0.22</sub></td><td>-1.03<sup>+0.70</sup><sub>-0.40</sub></td><td>4.40<sup>+3.84</sup><sub>-2.89</sub></td><td>3.57<sup>+5.28</sup><sub>-2.25</sub></td><td>0.31<sup>+0.31</sup><sub>-0.21</sub></td><td>0.66</td></tr><tr><td>MATLAS-1494</td><td>8.00<sup>+0.11</sup><sub>-0.17</sub></td><td>-1.10<sup>+0.56</sup><sub>-0.56</sub></td><td>0.90<sup>+1.06</sup><sub>-0.56</sub></td><td>9.15<sup>+3.29</sup><sub>-4.71</sub></td><td>0.04<sup>+0.06</sup><sub>-0.03</sub></td><td>1.80</td></tr><tr><td>MATLAS-1534</td><td>7.65<sup>+0.20</sup><sub>-0.33</sub></td><td>-0.76<sup>+0.52</sup><sub>-0.77</sub></td><td>1.00<sup>+1.50</sup><sub>-0.65</sub></td><td>9.00<sup>+3.42</sup><sub>-5.11</sub></td><td>0.31<sup>+0.33</sup><sub>-0.22</sub></td><td>3.14</td></tr><tr><td>MATLAS-1550</td><td>7.47<sup>+0.10</sup><sub>-0.14</sub></td><td>-1.16<sup>+0.43</sup><sub>-0.30</sub></td><td>1.37<sup>+1.87</sup><sub>-0.93</sub></td><td>8.20<sup>+4.08</sup><sub>-4.53</sub></td><td>0.18<sup>+0.18</sup><sub>-0.10</sub></td><td>1.98</td></tr><tr><td>MATLAS-1558</td><td>7.33<sup>+0.14</sup><sub>-0.14</sub></td><td>-1.29<sup>+0.40</sup><sub>-0.23</sub></td><td>2.22<sup>+2.55</sup><sub>-1.56</sub></td><td>5.97<sup>+5.53</sup><sub>-3.41</sub></td><td>0.07<sup>+0.10</sup><sub>-0.05</sub></td><td>1.27</td></tr><tr><td>MATLAS-1589</td><td>7.11<sup>+0.17</sup><sub>-0.22</sub></td><td>-0.94<sup>+0.60</sup><sub>-0.71</sub></td><td>1.54<sup>+2.00</sup><sub>-1.01</sub></td><td>7.82<sup>+4.28</sup><sub>-4.50</sub></td><td>0.21<sup>+0.22</sup><sub>-0.14</sub></td><td>1.20</td></tr><tr><td>MATLAS-1606</td><td>8.09<sup>+0.09</sup><sub>-0.11</sub></td><td>-1.35<sup>+0.30</sup><sub>-0.18</sub></td><td>1.17<sup>+1.32</sup><sub>-0.75</sub></td><td>8.63<sup>+3.67</sup><sub>-4.60</sub></td><td>0.08<sup>+0.09</sup><sub>-0.06</sub></td><td>1.70</td></tr><tr><td>MATLAS-1615</td><td>7.87<sup>+0.11</sup><sub>-0.14</sub></td><td>-1.09<sup>+0.54</sup><sub>-0.36</sub></td><td>2.95<sup>+2.74</sup><sub>-2.01</sub></td><td>4.63<sup>+6.09</sup><sub>-2.66</sub></td><td>0.15<sup>+0.16</sup><sub>-0.11</sub></td><td>0.94</td></tr><tr><td>MATLAS-1616</td><td>7.73<sup>+0.11</sup><sub>-0.12</sub></td><td>-1.26<sup>+0.40</sup><sub>-0.24</sub></td><td>1.54<sup>+2.06</sup><sub>-1.03</sub></td><td>7.77<sup>+4.33</sup><sub>-4.43</sub></td><td>0.13<sup>+0.14</sup><sub>-0.09</sub></td><td>1.51</td></tr><tr><td>MATLAS-1630</td><td>7.28<sup>+0.08</sup><sub>-0.11</sub></td><td>-1.34<sup>+0.30</sup><sub>-0.18</sub></td><td>1.12<sup>+1.22</sup><sub>-0.72</sub></td><td>9.07<sup>+3.35</sup><sub>-4.34</sub></td><td>0.06<sup>+0.08</sup><sub>-0.05</sub></td><td>0.94</td></tr><tr><td>MATLAS-1647</td><td>6.96<sup>+0.11</sup><sub>-0.14</sub></td><td>-1.04<sup>+0.55</sup><sub>-0.40</sub></td><td>1.93<sup>+2.80</sup><sub>-1.33</sub></td><td>7.09<sup>+4.79</sup><sub>-3.97</sub></td><td>0.33<sup>+0.23</sup><sub>-0.23</sub></td><td>1.22</td></tr><tr><td>MATLAS-1779</td><td>7.91<sup>+0.14</sup><sub>-0.17</sub></td><td>-0.91<sup>+0.57</sup><sub>-0.47</sub></td><td>2.52<sup>+3.26</sup><sub>-1.79</sub></td><td>5.63<sup>+5.79</sup><sub>-3.35</sub></td><td>0.36<sup>+0.32</sup><sub>-0.23</sub></td><td>1.37</td></tr><tr><td>MATLAS-1794</td><td>7.40<sup>+0.12</sup><sub>-0.20</sub></td><td>-1.24<sup>+0.60</sup><sub>-0.49</sub></td><td>0.98<sup>+1.69</sup><sub>-0.63</sub></td><td>8.83<sup>+3.56</sup><sub>-5.20</sub></td><td>0.01<sup>+0.02</sup><sub>-0.01</sub></td><td>1.09</td></tr><tr><td>MATLAS-1824</td><td>7.95<sup>+0.12</sup><sub>-0.13</sub></td><td>-0.88<sup>+0.39</sup><sub>-0.22</sub></td><td>1.70<sup>+2.32</sup><sub>-1.17</sub></td><td>6.79<sup>+5.02</sup><sub>-4.07</sub></td><td>0.07<sup>+0.09</sup><sub>-0.05</sub></td><td>1.14</td></tr><tr><td>MATLAS-1847</td><td>7.36<sup>+0.10</sup><sub>-0.12</sub></td><td>-1.08<sup>+0.43</sup><sub>-0.36</sub></td><td>1.67<sup>+2.57</sup><sub>-1.16</sub></td><td>7.38<sup>+4.65</sup><sub>-4.26</sub></td><td>0.20<sup>+0.19</sup><sub>-0.14</sub></td><td>1.91</td></tr><tr><td>MATLAS-1855</td><td>7.28<sup>+0.12</sup><sub>-0.15</sub></td><td>-1.20<sup>+0.50</sup><sub>-0.30</sub></td><td>2.45<sup>+2.92</sup><sub>-1.71</sub></td><td>5.47<sup>+5.76</sup><sub>-3.33</sub></td><td>0.16<sup>+0.21</sup><sub>-0.11</sub></td><td>1.19</td></tr><tr><td>MATLAS-1865</td><td>7.43<sup>+0.09</sup><sub>-0.11</sub></td><td>-1.48<sup>+0.18</sup><sub>-0.09</sub></td><td>2.49<sup>+2.83</sup><sub>-1.92</sub></td><td>3.80<sup>+6.71</sup><sub>-2.37</sub></td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>1.21</td></tr><tr><td>MATLAS-1907</td><td>7.55<sup>+0.18</sup><sub>-0.22</sub></td><td>-1.68<sup>+0.37</sup><sub>-0.23</sub></td><td>1.31<sup>+4.41</sup><sub>-0.91</sub></td><td>7.34<sup>+4.68</sup><sub>-4.85</sub></td><td>0.00<sup>+0.00</sup><sub>-0.00</sub></td><td>0.92</td></tr><tr><td>MATLAS-1957</td><td>8.00<sup>+0.05</sup><sub>-0.07</sub></td><td>-1.47<sup>+0.12</sup><sub>-0.09</sub></td><td>0.84<sup>+0.74</sup><sub>-0.52</sub></td><td>10.04<sup>+2.64</sup><sub>-3.90</sub></td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>1.65</td></tr></tbody></table>
<table><caption>**Table B3.** PROSPECTOR stellar population properties of the MATLAS UDGs.</caption><thead><tr><th>ID</th><th>log(M<sub>*</sub>/M<sub>☉</sub>)</th><th>[M/H]<br>[dex]</th><th>τ<br>[Gyr]</th><th>t<sub>M</sub><br>[Gyr]</th><th>A<sub>v</sub><br>[mag]</th><th>M<sub>*</sub>/L<sub>V</sub><br>[M<sub>☉</sub>/L<sub>☉</sub>, V]</th></tr></thead><tbody><tr><td>MATLAS-42</td><td>8.17<sup>+0.08</sup><sub>-0.10</sub></td><td>-1.34<sup>+0.30</sup><sub>-0.19</sub></td><td>1.57<sup>+1.70</sup><sub>-1.08</sub></td><td>5.97<sup>+5.46</sup><sub>-3.59</sub></td><td>0.07<sup>+0.09</sup><sub>-0.05</sub></td><td>0.97</td></tr><tr><td>MATLAS-48</td><td>7.89<sup>+0.07</sup><sub>-0.10</sub></td><td>-1.31<sup>+0.30</sup><sub>-0.20</sub></td><td>1.19<sup>+1.13</sup><sub>-0.76</sub></td><td>8.90<sup>+3.50</sup><sub>-4.30</sub></td><td>0.07<sup>+0.08</sup><sub>-0.05</sub></td><td>1.99</td></tr><tr><td>MATLAS-141</td><td>7.31<sup>+0.14</sup><sub>-0.16</sub></td><td>-1.06<sup>+0.58</sup><sub>-0.40</sub></td><td>2.85<sup>+3.79</sup><sub>-2.01</sub></td><td>5.13<sup>+5.78</sup><sub>-2.94</sub></td><td>0.15<sup>+0.20</sup><sub>-0.11</sub></td><td>1.44</td></tr><tr><td>MATLAS-149</td><td>7.25<sup>+0.12</sup><sub>-0.14</sub></td><td>-1.42<sup>+0.26</sup><sub>-0.13</sub></td><td>3.32<sup>+3.36</sup><sub>-2.33</sub></td><td>4.04<sup>+5.87</sup><sub>-2.16</sub></td><td>0.04<sup>+0.06</sup><sub>-0.03</sub></td><td>1.25</td></tr><tr><td>MATLAS-177</td><td>7.49<sup>+0.09</sup><sub>-0.13</sub></td><td>-1.19<sup>+0.41</sup><sub>-0.28</sub></td><td>1.43<sup>+1.63</sup><sub>-0.93</sub></td><td>8.34<sup>+3.84</sup><sub>-4.33</sub></td><td>0.16<sup>+0.14</sup><sub>-0.10</sub></td><td>2.52</td></tr><tr><td>MATLAS-262</td><td>7.40<sup>+0.19</sup><sub>-0.19</sub></td><td>-1.21<sup>+0.68</sup><sub>-0.54</sub></td><td>2.74<sup>+3.53</sup><sub>-1.98</sub></td><td>6.02<sup>+5.46</sup><sub>-3.31</sub></td><td>0.50<sup>+0.30</sup><sub>-0.30</sub></td><td>2.31</td></tr><tr><td>MATLAS-342</td><td>7.81<sup>+0.09</sup><sub>-0.10</sub></td><td>-1.22<sup>+0.33</sup><sub>-0.26</sub></td><td>1.48<sup>+1.65</sup><sub>-0.98</sub></td><td>7.49<sup>+4.51</sup><sub>-4.20</sub></td><td>0.12<sup>+0.11</sup><sub>-0.08</sub></td><td>1.77</td></tr><tr><td>MATLAS-365</td><td>7.12<sup>+0.13</sup><sub>-0.16</sub></td><td>-1.16<sup>+0.51</sup><sub>-0.32</sub></td><td>2.86<sup>+3.41</sup><sub>-1.98</sub></td><td>5.33<sup>+5.69</sup><sub>-2.93</sub></td><td>0.15<sup>+0.19</sup><sub>-0.11</sub></td><td>0.96</td></tr><tr><td>MATLAS-368</td><td>7.23<sup>+0.08</sup><sub>-0.10</sub></td><td>-1.42<sup>+0.22</sup><sub>-0.13</sub></td><td>2.59<sup>+4.35</sup><sub>-2.29</sub></td><td>4.44<sup>+0.89</sup><sub>-0.16</sub></td><td>0.03<sup>+0.01</sup><sub>-0.00</sub></td><td>0.26</td></tr><tr><td>MATLAS-405</td><td>7.64<sup>+0.09</sup><sub>-0.12</sub></td><td>-1.21<sup>+0.41</sup><sub>-0.27</sub></td><td>1.57<sup>+1.77</sup><sub>-1.06</sub></td><td>7.90<sup>+4.24</sup><sub>-4.19</sub></td><td>0.12<sup>+0.13</sup><sub>-0.08</sub></td><td>1.75</td></tr><tr><td>MATLAS-478</td><td>8.05<sup>+0.04</sup><sub>-0.06</sub></td><td>-0.69<sup>+0.39</sup><sub>-0.28</sub></td><td>0.77<sup>+0.56</sup><sub>-0.48</sub></td><td>10.92<sup>+1.99</sup><sub>-2.98</sub></td><td>0.22<sup>+0.10</sup><sub>-0.13</sub></td><td>2.43</td></tr><tr><td>MATLAS-585</td><td>7.53<sup>+0.07</sup><sub>-0.08</sub></td><td>-1.36<sup>+0.27</sup><sub>-0.16</sub></td><td>1.12<sup>+1.18</sup><sub>-0.69</sub></td><td>8.91<sup>+3.44</sup><sub>-4.31</sub></td><td>0.06<sup>+0.07</sup><sub>-0.04</sub></td><td>1.24</td></tr><tr><td>MATLAS-658</td><td>7.39<sup>+0.26</sup><sub>-0.24</sub></td><td>-1.08<sup>+0.71</sup><sub>-0.62</sub></td><td>3.18<sup>+3.76</sup><sub>-2.32</sub></td><td>5.28<sup>+5.85</sup><sub>-3.03</sub></td><td>1.17<sup>+0.31</sup><sub>-0.33</sub></td><td>3.16</td></tr><tr><td>MATLAS-799</td><td>7.87<sup>+0.05</sup><sub>-0.06</sub></td><td>-1.56<sup>+0.06</sup><sub>-0.03</sub></td><td>0.69<sup>+0.84</sup><sub>-0.43</sub></td><td>3.67<sup>+2.32</sup><sub>-1.76</sub></td><td>0.01<sup>+0.01</sup><sub>-0.00</sub></td><td>0.97</td></tr><tr><td>MATLAS-898</td><td>7.35<sup>+0.09</sup><sub>-0.10</sub></td><td>-1.51<sup>+0.13</sup><sub>-0.07</sub></td><td>1.83<sup>+2.92</sup><sub>-1.39</sub></td><td>3.85<sup>+6.63</sup><sub>-2.31</sub></td><td>0.01<sup>+0.02</sup><sub>-0.01</sub></td><td>0.90</td></tr><tr><td>MATLAS-951</td><td>8.25<sup>+0.10</sup><sub>-0.15</sub></td><td>-1.33<sup>+0.56</sup><sub>-0.45</sub></td><td>0.88<sup>+0.96</sup><sub>-0.57</sub></td><td>9.19<sup>+3.32</sup><sub>-4.54</sub></td><td>0.37<sup>+0.17</sup><sub>-0.22</sub></td><td>3.46</td></tr><tr><td>MATLAS-984</td><td>7.34<sup>+0.09</sup><sub>-0.09</sub></td><td>-1.50<sup>+0.15</sup><sub>-0.08</sub></td><td>1.50<sup>+2.01</sup><sub>-1.04</sub></td><td>5.70<sup>+5.58</sup><sub>-3.21</sub></td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>1.20</td></tr><tr><td>MATLAS-1059</td><td>7.84<sup>+0.10</sup><sub>-0.12</sub></td><td>-1.50<sup>+0.15</sup><sub>-0.07</sub></td><td>2.07<sup>+2.84</sup><sub>-1.47</sub></td><td>4.47<sup>+6.32</sup><sub>-2.60</sub></td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>0.77</td></tr><tr><td>MATLAS-1174</td><td>7.57<sup>+0.15</sup><sub>-0.18</sub></td><td>-1.23<sup>+0.44</sup><sub>-0.27</sub></td><td>2.39<sup>+3.25</sup><sub>-1.65</sub></td><td>6.12<sup>+5.31</sup><sub>-3.48</sub></td><td>0.09<sup>+0.13</sup><sub>-0.07</sub></td><td>1.38</td></tr><tr><td>MATLAS-1177</td><td>6.92<sup>+0.18</sup><sub>-0.22</sub></td><td>-0.92<sup>+0.61</sup><sub>-0.71</sub></td><td>6.33<sup>+2.50</sup><sub>-2.95</sub></td><td>2.70<sup>+3.20</sup><sub>-1.43</sub></td><td>0.37<sup>+0.12</sup><sub>-0.12</sub></td><td>0.25</td></tr><tr><td>MATLAS-1205</td><td>7.37<sup>+0.13</sup><sub>-0.14</sub></td><td>-1.39<sup>+0.30</sup><sub>-0.16</sub></td><td>3.11<sup>+3.52</sup><sub>-2.25</sub></td><td>5.27<sup>+5.81</sup><sub>-2.76</sub></td><td>0.04<sup>+0.07</sup><sub>-0.03</sub></td><td>0.83</td></tr><tr><td>MATLAS-1216</td><td>7.45<sup>+0.13</sup><sub>-0.21</sub></td><td>-0.94<sup>+0.60</sup><sub>-0.69</sub></td><td>0.96<sup>+1.41</sup><sub>-0.63</sub></td><td>9.00<sup>+3.50</sup><sub>-5.24</sub></td><td>0.04<sup>+0.07</sup><sub>-0.03</sub></td><td>0.66</td></tr><tr><td>MATLAS-1225</td><td>7.30<sup>+0.12</sup><sub>-0.16</sub></td><td>-1.35<sup>+0.32</sup><sub>-0.18</sub></td><td>1.48<sup>+2.40</sup><sub>-1.01</sub></td><td>7.28<sup>+4.75</sup><sub>-4.39</sub></td><td>0.07<sup>+0.09</sup><sub>-0.05</sub></td><td>1.21</td></tr><tr><td>MATLAS-1245</td><td>7.39<sup>+0.09</sup><sub>-0.12</sub></td><td>-1.13<sup>+0.46</sup><sub>-0.33</sub></td><td>1.71<sup>+2.43</sup><sub>-1.16</sub></td><td>7.66<sup>+4.35</sup><sub>-4.37</sub></td><td>0.18<sup>+0.22</sup><sub>-0.13</sub></td><td>1.46</td></tr><tr><td>MATLAS-1246</td><td>8.17<sup>+0.08</sup><sub>-0.08</sub></td><td>-1.49<sup>+0.16</sup><sub>-0.08</sub></td><td>1.04<sup>+0.95</sup><sub>-0.63</sub></td><td>7.92<sup>+4.01</sup><sub>-3.74</sub></td><td>0.03<sup>+0.04</sup><sub>-0.02</sub></td><td>1.31</td></tr><tr><td>MATLAS-1248</td><td>8.49<sup>+0.02</sup><sub>-0.05</sub></td><td>-1.28<sup>+0.22</sup><sub>-0.11</sub></td><td>0.50<sup>+0.53</sup><sub>-0.30</sub></td><td>11.81<sup>+1.31</sup><sub>-2.86</sub></td><td>0.07<sup>+0.05</sup><sub>-0.05</sub></td><td>1.93</td></tr><tr><td>MATLAS-1249</td><td>6.61<sup>+0.24</sup><sub>-0.27</sub></td><td>-1.05<sup>+0.86</sup><sub>-0.71</sub></td><td>6.30<sup>+2.55</sup><sub>-2.99</sub></td><td>1.15<sup>+1.80</sup><sub>-0.64</sub></td><td>1.06<sup>+0.21</sup><sub>-0.20</sub></td><td>0.12</td></tr><tr><td>MATLAS-1262</td><td>7.89<sup>+0.06</sup><sub>-0.08</sub></td><td>-1.27<sup>+0.34</sup><sub>-0.23</sub></td><td>0.92<sup>+0.99</sup><sub>-0.61</sub></td><td>9.40<sup>+3.13</sup><sub>-4.34</sub></td><td>0.14<sup>+0.10</sup><sub>-0.09</sub></td><td>2.08</td></tr><tr><td>MATLAS-1274</td><td>8.26<sup>+0.08</sup><sub>-0.07</sub></td><td>-1.51<sup>+0.13</sup><sub>-0.07</sub></td><td>0.96<sup>+1.07</sup><sub>-0.59</sub></td><td>5.97<sup>+4.51</sup><sub>-2.98</sub></td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>1.17</td></tr><tr><td>MATLAS-1302</td><td>7.08<sup>+0.11</sup><sub>-0.15</sub></td><td>-1.01<sup>+0.50</sup><sub>-0.41</sub></td><td>1.70<sup>+2.67</sup><sub>-1.18</sub></td><td>7.66<sup>+4.50</sup><sub>-4.28</sub></td><td>0.22<sup>+0.19</sup><sub>-0.15</sub></td><td>1.48</td></tr><tr><td>MATLAS-1337</td><td>7.04<sup>+0.15</sup><sub>-0.17</sub></td><td>-0.91<sup>+0.70</sup><sub>-0.47</sub></td><td>4.98<sup>+3.41</sup><sub>-2.81</sub></td><td>2.43<sup>+3.38</sup><sub>-1.32</sub></td><td>0.12<sup>+0.13</sup><sub>-0.09</sub></td><td>0.64</td></tr><tr><td>MATLAS-1413</td><td>7.85<sup>+0.21</sup><sub>-0.27</sub></td><td>-1.40<sup>+0.60</sup><sub>-0.20</sub></td><td>4.38<sup>+3.60</sup><sub>-3.00</sub></td><td>4.08<sup>+5.69</sup><sub>-2.37</sub></td><td>0.38<sup>+0.35</sup><sub>-0.26</sub></td><td>0.88</td></tr><tr><td>MATLAS-1493</td><td>7.21<sup>+0.15</sup><sub>-0.22</sub></td><td>-1.03<sup>+0.70</sup><sub>-0.40</sub></td><td>4.40<sup>+3.84</sup><sub>-2.89</sub></td><td>3.57<sup>+5.28</sup><sub>-2.25</sub></td><td>0.31<sup>+0.31</sup><sub>-0.21</sub></td><td>0.66</td></tr><tr><td>MATLAS-1494</td><td>8.00<sup>+0.11</sup><sub>-0.17</sub></td><td>-1.10<sup>+0.56</sup><sub>-0.56</sub></td><td>0.90<sup>+1.06</sup><sub>-0.56</sub></td><td>9.15<sup>+3.29</sup><sub>-4.71</sub></td><td>0.04<sup>+0.06</sup><sub>-0.03</sub></td><td>1.80</td></tr><tr><td>MATLAS-1534</td><td>7.65<sup>+0.20</sup><sub>-0.33</sub></td><td>-0.76<sup>+0.52</sup><sub>-0.77</sub></td><td>1.00<sup>+1.50</sup><sub>-0.65</sub></td><td>9.00<sup>+3.42</sup><sub>-5.11</sub></td><td>0.31<sup>+0.33</sup><sub>-0.22</sub></td><td>3.14</td></tr><tr><td>MATLAS-1550</td><td>7.47<sup>+0.10</sup><sub>-0.14</sub></td><td>-1.16<sup>+0.43</sup><sub>-0.30</sub></td><td>1.37<sup>+1.87</sup><sub>-0.93</sub></td><td>8.20<sup>+4.08</sup><sub>-4.53</sub></td><td>0.18<sup>+0.18</sup><sub>-0.10</sub></td><td>1.98</td></tr><tr><td>MATLAS-1558</td><td>7.33<sup>+0.14</sup><sub>-0.14</sub></td><td>-1.29<sup>+0.40</sup><sub>-0.23</sub></td><td>2.22<sup>+2.55</sup><sub>-1.56</sub></td><td>5.97<sup>+5.53</sup><sub>-3.41</sub></td><td>0.07<sup>+0.10</sup><sub>-0.05</sub></td><td>1.27</td></tr><tr><td>MATLAS-1589</td><td>7.11<sup>+0.17</sup><sub>-0.22</sub></td><td>-0.94<sup>+0.60</sup><sub>-0.71</sub></td><td>1.54<sup>+2.00</sup><sub>-1.01</sub></td><td>7.82<sup>+4.28</sup><sub>-4.50</sub></td><td>0.21<sup>+0.22</sup><sub>-0.14</sub></td><td>1.20</td></tr><tr><td>MATLAS-1606</td><td>8.09<sup>+0.09</sup><sub>-0.11</sub></td><td>-1.35<sup>+0.30</sup><sub>-0.18</sub></td><td>1.17<sup>+1.32</sup><sub>-0.75</sub></td><td>8.63<sup>+3.67</sup><sub>-4.60</sub></td><td>0.08<sup>+0.09</sup><sub>-0.06</sub></td><td>1.70</td></tr><tr><td>MATLAS-1615</td><td>7.87<sup>+0.11</sup><sub>-0.14</sub></td><td>-1.09<sup>+0.54</sup><sub>-0.36</sub></td><td>2.95<sup>+2.74</sup><sub>-2.01</sub></td><td>4.63<sup>+6.09</sup><sub>-2.66</sub></td><td>0.15<sup>+0.16</sup><sub>-0.11</sub></td><td>0.94</td></tr><tr><td>MATLAS-1616</td><td>7.73<sup>+0.11</sup><sub>-0.12</sub></td><td>-1.26<sup>+0.40</sup><sub>-0.24</sub></td><td>1.54<sup>+2.06</sup><sub>-1.03</sub></td><td>7.77<sup>+4.33</sup><sub>-4.43</sub></td><td>0.13<sup>+0.14</sup><sub>-0.09</sub></td><td>1.51</td></tr><tr><td>MATLAS-1630</td><td>7.28<sup>+0.08</sup><sub>-0.11</sub></td><td>-1.34<sup>+0.30</sup><sub>-0.18</sub></td><td>1.12<sup>+1.22</sup><sub>-0.72</sub></td><td>9.07<sup>+3.35</sup><sub>-4.34</sub></td><td>0.06<sup>+0.08</sup><sub>-0.05</sub></td><td>0.94</td></tr><tr><td>MATLAS-1647</td><td>6.96<sup>+0.11</sup><sub>-0.14</sub></td><td>-1.04<sup>+0.55</sup><sub>-0.40</sub></td><td>1.93<sup>+2.80</sup><sub>-1.33</sub></td><td>7.09<sup>+4.79</sup><sub>-3.97</sub></td><td>0.33<sup>+0.23</sup><sub>-0.23</sub></td><td>1.22</td></tr><tr><td>MATLAS-1779</td><td>7.91<sup>+0.14</sup><sub>-0.17</sub></td><td>-0.91<sup>+0.57</sup><sub>-0.47</sub></td><td>2.52<sup>+3.26</sup><sub>-1.79</sub></td><td>5.63<sup>+5.79</sup><sub>-3.35</sub></td><td>0.36<sup>+0.32</sup><sub>-0.23</sub></td><td>1.37</td></tr><tr><td>MATLAS-1794</td><td>7.40<sup>+0.12</sup><sub>-0.20</sub></td><td>-1.24<sup>+0.60</sup><sub>-0.49</sub></td><td>0.98<sup>+1.69</sup><sub>-0.63</sub></td><td>8.83<sup>+3.56</sup><sub>-5.20</sub></td><td>0.01<sup>+0.02</sup><sub>-0.01</sub></td><td>1.09</td></tr><tr><td>MATLAS-1824</td><td>7.95<sup>+0.12</sup><sub>-0.13</sub></td><td>-0.88<sup>+0.39</sup><sub>-0.22</sub></td><td>1.70<sup>+2.32</sup><sub>-1.17</sub></td><td>6.79<sup>+5.02</sup><sub>-4.07</sub></td><td>0.07<sup>+0.09</sup><sub>-0.05</sub></td><td>1.14</td></tr><tr><td>MATLAS-1847</td><td>7.36<sup>+0.10</sup><sub>-0.12</sub></td><td>-1.08<sup>+0.43</sup><sub>-0.36</sub></td><td>1.67<sup>+2.57</sup><sub>-1.16</sub></td><td>7.38<sup>+4.65</sup><sub>-4.26</sub></td><td>0.20<sup>+0.19</sup><sub>-0.14</sub></td><td>1.91</td></tr><tr><td>MATLAS-1855</td><td>7.28<sup>+0.12</sup><sub>-0.15</sub></td><td>-1.20<sup>+0.50</sup><sub>-0.30</sub></td><td>2.45<sup>+2.92</sup><sub>-1.71</sub></td><td>5.47<sup>+5.76</sup><sub>-3.33</sub></td><td>0.16<sup>+0.21</sup><sub>-0.11</sub></td><td>1.19</td></tr><tr><td>MATLAS-1865</td><td>7.43<sup>+0.09</sup><sub>-0.11</sub></td><td>-1.48<sup>+0.18</sup><sub>-0.09</sub></td><td>2.49<sup>+2.83</sup><sub>-1.92</sub></td><td>3.80<sup>+6.71</sup><sub>-2.37</sub></td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>1.21</td></tr><tr><td>MATLAS-1907</td><td>7.55<sup>+0.18</sup><sub>-0.22</sub></td><td>-1.68<sup>+0.37</sup><sub>-0.23</sub></td><td>1.31<sup>+4.41</sup><sub>-0.91</sub></td><td>7.34<sup>+4.68</sup><sub>-4.85</sub></td><td>0.00<sup>+0.00</sup><sub>-0.00</sub></td><td>0.92</td></tr><tr><td>MATLAS-1957</td><td>8.00<sup>+0.05</sup><sub>-0.07</sub></td><td>-1.47<sup>+0.12</sup><sub>-0.09</sub></td><td>0.84<sup>+0.74</sup><sub>-0.52</sub></td><td>10.04<sup>+2.64</sup><sub>-3.90</sub></td><td>0.02<sup>+0.03</sup><sub>-0.01</sub></td><td>1.65</td></tr></tbody></table>
<table><caption>Table B3 – continued</caption><thead><tr><th>ID</th><th>log(M<sub>*</sub>/M<sub>☉</sub>)</th><th>[M/H]<br>[dex]</th><th>τ<br>[Gyr]</th><th>t<sub>M</sub><br>[Gyr]</th><th>A<sub>v</sub><br>[mag]</th><th>M<sub>*</sub>/L<sub>V</sub><br>[M<sub>☉</sub>/L<sub>☉</sub>, V]</th></tr></thead><tbody><tr><td>MATLAS-1975</td><td>7.58<sup>+0.13</sup><sub>-0.15</sub></td><td>-0.76<sup>+0.68</sup><sub>-0.57</sub></td><td>2.74<sup>+3.25</sup><sub>-1.94</sub></td><td>6.09<sup>+5.36</sup><sub>-3.27</sub></td><td>0.37<sup>+0.30</sup><sub>-0.24</sub></td><td>2.04</td></tr><tr><td>MATLAS-1985</td><td>7.96<sup>+0.10</sup><sub>-0.13</sub></td><td>-1.40<sup>+0.25</sup><sub>-0.14</sub></td><td>1.25<sup>+1.65</sup><sub>-0.82</sub></td><td>7.94<sup>+4.10</sup><sub>-4.42</sub></td><td>0.05<sup>+0.08</sup><sub>-0.04</sub></td><td>1.92</td></tr><tr><td>MATLAS-1991</td><td>7.88<sup>+0.04</sup><sub>-0.06</sub></td><td>-1.35<sup>+0.19</sup><sub>-0.16</sub></td><td>0.75<sup>+0.70</sup><sub>-0.48</sub></td><td>10.84<sup>+2.13</sup><sub>-3.75</sub></td><td>0.09<sup>+0.06</sup><sub>-0.06</sub></td><td>1.53</td></tr><tr><td>MATLAS-1996</td><td>6.69<sup>+0.18</sup><sub>-0.23</sub></td><td>-0.79<sup>+0.74</sup><sub>-0.58</sub></td><td>4.99<sup>+3.14</sup><sub>-2.99</sub></td><td>3.50<sup>+4.71</sup><sub>-2.13</sub></td><td>0.37<sup>+0.40</sup><sub>-0.26</sub></td><td>0.83</td></tr><tr><td>MATLAS-2019</td><td>8.01<sup>+0.03</sup><sub>-0.05</sub></td><td>-1.40<sup>+0.10</sup><sub>-0.12</sub></td><td>0.67<sup>+0.63</sup><sub>-0.42</sub></td><td>11.22<sup>+1.79</sup><sub>-3.23</sub></td><td>0.04<sup>+0.04</sup><sub>-0.03</sub></td><td>1.88</td></tr><tr><td>MATLAS-2021</td><td>7.08<sup>+0.16</sup><sub>-0.20</sub></td><td>-0.88<sup>+0.65</sup><sub>-0.52</sub></td><td>3.95<sup>+3.54</sup><sub>-2.74</sub></td><td>4.55<sup>+5.69</sup><sub>-2.81</sub></td><td>0.41<sup>+0.39</sup><sub>-0.28</sub></td><td>1.22</td></tr><tr><td>MATLAS-2103</td><td>8.21<sup>+0.05</sup><sub>-0.06</sub></td><td>-1.40<sup>+0.14</sup><sub>-0.13</sub></td><td>0.92<sup>+0.69</sup><sub>-0.57</sub></td><td>9.79<sup>+2.81</sup><sub>-3.84</sub></td><td>0.06<sup>+0.05</sup><sub>-0.04</sub></td><td>1.66</td></tr><tr><td>MATLAS-2184</td><td>7.48<sup>+0.18</sup><sub>-0.20</sub></td><td>-0.77<sup>+0.79</sup><sub>-0.60</sub></td><td>3.11<sup>+3.71</sup><sub>-2.11</sub></td><td>4.64<sup>+5.98</sup><sub>-2.83</sub></td><td>0.38<sup>+0.46</sup><sub>-0.27</sub></td><td>2.09</td></tr></tbody></table>

Note. Columns are (1) Galaxy ID; (2) PROSPECTOR stellar mass; (3) PROSPECTOR metallicity; (4) PROSPECTOR star formation time-scale; (5) PROSPECTOR mass-weighted age; (6) PROSPECTOR dust attenuation; (7) mass-to-light ratio.

## APPENDIX C: THE EFFECTS OF UNDERESTIMATING UNCERTAINTIES

Galaxy fitting codes are known for severely underestimating the uncertainties of their recovered parameters. In this appendix, we show the effects on the stellar populations properties obtained with PROSPECTOR if the photometric uncertainties from GALFITM were underestimated by 10 per cent, 20 per cent, 50 per cent, and 100 per cent.

In Fig. C1, we compare the stellar populations properties of MATLAS-2019 using the nominal uncertainties coming from GALFITM and if they were underestimated. We can see that if the uncertainties were underestimated by 10 or 20 per cent, the recovered stellar populations barely change. However, if they were severely underestimated, such as by 50 or 100 per cent, the changes start to get important. We emphasize, however, that we do not expect the uncertainties to be this highly underestimated. The biggest of these changes is in the mass-weighted ages reaching a change of 5 Gyr. The biggest

change in metallicity is 0.1 dex, 0.2 dex in mass and 0.02 in $A_v$. The minor changes in metallicity and mass give us confidence that even if the uncertainties are underestimated, the major conclusions in this paper (i.e. the bimodality in the mass–metallicity plane) would maintain.

## APPENDIX D: ALTERNATIVE VIEW OF FIG. 7

In Fig. 7 we show the MATLAS UDGs overlaid in the mass–metallicity plane in an attempt to compare their positioning to known MZRs and to start inferring some of their likely formation histories. We colour-code the MATLAS UDGs by several key parameters and show that these galaxies distill into two main types: some that are consistent with the classical dwarf MZR and some that are consistent with the high-redshift MZR. As the colour gradients may be hard to visualize in some cases, in this appendix we provide an alternative view of Fig. 7, shown in Fig. D1. In this case, the key properties are directly plotted against their distance to both the dwarf MZR and the high-redshift MZR.

<figure><img src="image_14.png" /></figure>

**Figure C1.** Differences in the recovered stellar populations of MATLAS-2019 if the photometric uncertainty in the optical obtained from GALFITM was underestimated by 10 per cent (red), 20 per cent (blue), 50 per cent (green), and 100 per cent (orange). Left: differences in metallicity and mass-weighted ages obtained. Right: differences in stellar mass and dust attenuation. We conclude that the photometry being underestimated by 50 per cent or 100 per cent can severely change the ages of the galaxies, reaching a change of 5 Gyr. The maximum change in metallicity is 0.11 dex, in stellar mass is 0.2 dex and in $A_v$ is 0.02. Due to the small changes in mass and metallicity, the main conclusions of the paper regarding the bimodality in the mass–metallicity plane remain even if the uncertainties are found to be extremely underestimated.
<table><caption>Table B3 – continued</caption><thead><tr><th>ID</th><th>log(M<sub>*</sub>/M<sub>☉</sub>)</th><th>[M/H]<br>[dex]</th><th>τ<br>[Gyr]</th><th>t<sub>M</sub><br>[Gyr]</th><th>A<sub>v</sub><br>[mag]</th><th>M<sub>*</sub>/L<sub>V</sub><br>[M<sub>☉</sub>/L<sub>☉</sub>, V]</th></tr></thead><tbody><tr><td>MATLAS-1975</td><td>7.58<sup>+0.13</sup><sub>-0.15</sub></td><td>-0.76<sup>+0.68</sup><sub>-0.57</sub></td><td>2.74<sup>+3.25</sup><sub>-1.94</sub></td><td>6.09<sup>+5.36</sup><sub>-3.27</sub></td><td>0.37<sup>+0.30</sup><sub>-0.24</sub></td><td>2.04</td></tr><tr><td>MATLAS-1985</td><td>7.96<sup>+0.10</sup><sub>-0.13</sub></td><td>-1.40<sup>+0.25</sup><sub>-0.14</sub></td><td>1.25<sup>+1.65</sup><sub>-0.82</sub></td><td>7.94<sup>+4.10</sup><sub>-4.42</sub></td><td>0.05<sup>+0.08</sup><sub>-0.04</sub></td><td>1.92</td></tr><tr><td>MATLAS-1991</td><td>7.88<sup>+0.04</sup><sub>-0.06</sub></td><td>-1.35<sup>+0.19</sup><sub>-0.16</sub></td><td>0.75<sup>+0.70</sup><sub>-0.48</sub></td><td>10.84<sup>+2.13</sup><sub>-3.75</sub></td><td>0.09<sup>+0.06</sup><sub>-0.06</sub></td><td>1.53</td></tr><tr><td>MATLAS-1996</td><td>6.69<sup>+0.18</sup><sub>-0.23</sub></td><td>-0.79<sup>+0.74</sup><sub>-0.58</sub></td><td>4.99<sup>+3.14</sup><sub>-2.99</sub></td><td>3.50<sup>+4.71</sup><sub>-2.13</sub></td><td>0.37<sup>+0.40</sup><sub>-0.26</sub></td><td>0.83</td></tr><tr><td>MATLAS-2019</td><td>8.01<sup>+0.03</sup><sub>-0.05</sub></td><td>-1.40<sup>+0.10</sup><sub>-0.12</sub></td><td>0.67<sup>+0.63</sup><sub>-0.42</sub></td><td>11.22<sup>+1.79</sup><sub>-3.23</sub></td><td>0.04<sup>+0.04</sup><sub>-0.03</sub></td><td>1.88</td></tr><tr><td>MATLAS-2021</td><td>7.08<sup>+0.16</sup><sub>-0.20</sub></td><td>-0.88<sup>+0.65</sup><sub>-0.52</sub></td><td>3.95<sup>+3.54</sup><sub>-2.74</sub></td><td>4.55<sup>+5.69</sup><sub>-2.81</sub></td><td>0.41<sup>+0.39</sup><sub>-0.28</sub></td><td>1.22</td></tr><tr><td>MATLAS-2103</td><td>8.21<sup>+0.05</sup><sub>-0.06</sub></td><td>-1.40<sup>+0.14</sup><sub>-0.13</sub></td><td>0.92<sup>+0.69</sup><sub>-0.57</sub></td><td>9.79<sup>+2.81</sup><sub>-3.84</sub></td><td>0.06<sup>+0.05</sup><sub>-0.04</sub></td><td>1.66</td></tr><tr><td>MATLAS-2184</td><td>7.48<sup>+0.18</sup><sub>-0.20</sub></td><td>-0.77<sup>+0.79</sup><sub>-0.60</sub></td><td>3.11<sup>+3.71</sup><sub>-2.11</sub></td><td>4.64<sup>+5.98</sup><sub>-2.83</sub></td><td>0.38<sup>+0.46</sup><sub>-0.27</sub></td><td>2.09</td></tr></tbody></table>

Note. Columns are (1) Galaxy ID; (2) PROSPECTOR stellar mass; (3) PROSPECTOR metallicity; (4) PROSPECTOR star formation time-scale; (5) PROSPECTOR mass-weighted age; (6) PROSPECTOR dust attenuation; (7) mass-to-light ratio.

## APPENDIX C: THE EFFECTS OF UNDERESTIMATING UNCERTAINTIES

Galaxy fitting codes are known for severely underestimating the uncertainties of their recovered parameters. In this appendix, we show the effects on the stellar populations properties obtained with PROSPECTOR if the photometric uncertainties from GALFITM were underestimated by 10 per cent, 20 per cent, 50 per cent, and 100 per cent.

In Fig. C1, we compare the stellar populations properties of MATLAS-2019 using the nominal uncertainties coming from GALFITM and if they were underestimated. We can see that if the uncertainties were underestimated by 10 or 20 per cent, the recovered stellar populations barely change. However, if they were severely underestimated, such as by 50 or 100 per cent, the changes start to get important. We emphasize, however, that we do not expect the uncertainties to be this highly underestimated. The biggest of these changes is in the mass-weighted ages reaching a change of 5 Gyr. The biggest

change in metallicity is 0.1 dex, 0.2 dex in mass and 0.02 in $A_v$. The minor changes in metallicity and mass give us confidence that even if the uncertainties are underestimated, the major conclusions in this paper (i.e. the bimodality in the mass–metallicity plane) would maintain.

## APPENDIX D: ALTERNATIVE VIEW OF FIG. 7

In Fig. 7 we show the MATLAS UDGs overlaid in the mass–metallicity plane in an attempt to compare their positioning to known MZRs and to start inferring some of their likely formation histories. We colour-code the MATLAS UDGs by several key parameters and show that these galaxies distill into two main types: some that are consistent with the classical dwarf MZR and some that are consistent with the high-redshift MZR. As the colour gradients may be hard to visualize in some cases, in this appendix we provide an alternative view of Fig. 7, shown in Fig. D1. In this case, the key properties are directly plotted against their distance to both the dwarf MZR and the high-redshift MZR.

<figure><img src="image_13.png" /></figure>

**Figure C1.** Differences in the recovered stellar populations of MATLAS-2019 if the photometric uncertainty in the optical obtained from GALFITM was underestimated by 10 per cent (red), 20 per cent (blue), 50 per cent (green), and 100 per cent (orange). Left: differences in metallicity and mass-weighted ages obtained. Right: differences in stellar mass and dust attenuation. We conclude that the photometry being underestimated by 50 per cent or 100 per cent can severely change the ages of the galaxies, reaching a change of 5 Gyr. The maximum change in metallicity is 0.11 dex, in stellar mass is 0.2 dex and in $A_v$ is 0.02. Due to the small changes in mass and metallicity, the main conclusions of the paper regarding the bimodality in the mass–metallicity plane remain even if the uncertainties are found to be extremely underestimated.
<figure><img src="image_15.png" /><figcaption>**Figure D1.** Alternative view of Fig. 7. Distribution of the physical properties of the MATLAS UDGs in the stellar mass–metallicity plane. The plus signs show the results obtained from SED fitting with PROSPECTOR for the 59 UDGs in our sample. The Simon (2019) MZR for classical Local Group dwarf galaxies is shown with the dashed black line. The dash-dotted line is the evolving MZR at z = 2.2 from Ma et al. (2016). In all panels, the properties are plotted against distance to the dwarf (left-hand side y-axis) and high-redshift MZRs (right-hand side y-axis). Top row: surface brightness (left) and effective radii (right). Second row: axis ratio and local environment (Marleau et al. 2021), respectively. Third row: mass-weighted ages and star formation time-scales. Fourth row: number of GCs (Marleau et al., submitted) and GC specific frequencies (per unit stellar mass), respectively. Results are as described in the caption of Fig. 7.</figcaption></figure>

This paper has been typeset from a TEX/LATEX file prepared by the author.
<figure><img src="image_14.png" /><figcaption>**Figure D1.** Alternative view of Fig. 7. Distribution of the physical properties of the MATLAS UDGs in the stellar mass–metallicity plane. The plus signs show the results obtained from SED fitting with PROSPECTOR for the 59 UDGs in our sample. The Simon (2019) MZR for classical Local Group dwarf galaxies is shown with the dashed black line. The dash-dotted line is the evolving MZR at z = 2.2 from Ma et al. (2016). In all panels, the properties are plotted against distance to the dwarf (left-hand side y-axis) and high-redshift MZRs (right-hand side y-axis). Top row: surface brightness (left) and effective radii (right). Second row: axis ratio and local environment (Marleau et al. 2021), respectively. Third row: mass-weighted ages and star formation time-scales. Fourth row: number of GCs (Marleau et al., submitted) and GC specific frequencies (per unit stellar mass), respectively. Results are as described in the caption of Fig. 7.</figcaption></figure>

This paper has been typeset from a TEX/LATEX file prepared by the author.