# The Impact of the Group Environment on the Molecular Gas and Star Formation Activity

Bumhyun Lee, Jing Wang, Aeree Chung, Luis C. Ho, Juan Molina, Yongjung Kim, Shun Wang, Bi-Qing For, Bärbel S. Koribalski, Kristine Spekkens, et al.

► To cite this version:

Bumhyun Lee, Jing Wang, Aeree Chung, Luis C. Ho, Juan Molina, et al. The Impact of the Group Environment on the Molecular Gas and Star Formation Activity. Resolving the Rise and Fall of Star Formation in Galaxies. Proceedings of the International Astronomical Union, 0000, à renseigner, Unknown Region. pp.136-139, 10.1017/S1743921322004392. insu-04479119

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# The Impact of the Group Environment on the Molecular Gas and Star Formation Activity

Bumhyun Lee¹,²,³ ID, Jing Wang², Aeree Chung³, Luis C. Ho²,⁴, Juan Molina⁵,², Yongjung Kim⁶, Shun Wang²,⁴, Bi-Qing For⁷,⁸, Bärbel S. Koribalski⁹,¹⁰ ID, Kristine Spekkens¹¹, Albert Bosma¹², Benne W. Holwerda¹³ and Lourdes Verdes-Montenegro¹⁴

¹Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Daejeon 34055, Republic of Korea
bhlee301@gmail.com

²Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People's Republic of China

³Department of Astronomy, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea

⁴Department of Astronomy, School of Physics, Peking University, Beijing 100871, People's Republic of China

⁵Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden

⁶Department of Astronomy and Atmospheric Sciences, College of Natural Sciences, Kyungpook National University, Daegu 41566, Republic of Korea

⁷International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia

⁸ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia

⁹CSIRO Space and Astronomy, Australia Telescope National Facility, P.O. Box 76, NSW 1710, Australia

¹⁰School of Science, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia

¹¹Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, Ontario, K7K7B4, Canada

¹²Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France

¹³University of Louisville, Department of Physics and Astronomy, 102 Natural Science Building, 40292 KY Louisville, USA

¹⁴Instituto de Astrofísica de Andalucía (CSIC), Spain

**Abstract.** At least half of the local galaxies reside in galaxy groups, which indicates that the group is the common environment where galaxies evolve. Therefore, it is important to probe how significantly galaxies are affected by group environmental processes, in order to obtain a better understanding of galaxy evolution. We carried out a new CO imaging survey for 31 galaxies in the IC 1459 and NGC 4636 groups, using the Atacama Compact Array, to study the effect of the group environment on the molecular gas properties and the star formation activity. With our resolved CO data, combined with high-resolution H I images, we find asymmetric CO and H I distributions in the group galaxies. Compared to isolated galaxies, group members have relatively low molecular gas fraction and low star formation rate. These results suggest that

© The Author(s), 2023. Published by Cambridge University Press on behalf of International Astronomical Union. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
the group environment can change the properties of cold gas components and star formation in
group galaxies.

**Keywords.** Molecular gas, Galaxies, Galaxy groups, Galaxy evolution

**1. Introduction**

Galaxy groups are common places where at least 50% of a whole population of galaxies in the local universe live and evolve. In particular, many galaxies experience external perturbations for the first time in the group environment before they join larger structures such as galaxy clusters. To begin with, tidal interactions among group galaxies are expected to commonly occur due to the low velocity dispersion of the group. In addition, the detection of X-ray in some groups indicates that a hot intragroup medium (IGrM) can also affect galaxies, mostly the interstellar medium (e.g., Cortese, Catinella, & Smith 2021). Those environmental processes can change the physical properties of group galaxies, making their color redder, and stopping their star formation. The combination of these processes can also account for many quenched galaxies at the outskirts of a cluster (known as “preprocessing”, Morokuma-Matsui et al. 2021). Therefore, probing the influence of the group environment on member galaxies is a key to better understanding galaxy evolution in general. Recently, we carried out a CO imaging study of group galaxies to investigate how the group environment affects the cold molecular gas content and star formation processes. In this work, we present CO images of group members showing signs of external perturbations, together with H I images. We also compare the global properties (e.g., star formation rate (SFR), H₂ gas fraction) of group galaxies with those of isolated galaxies. The details of this work are presented in Lee et al. (2022).

**2. Sample and Observations**

From the Group Evolution Multiwavelength Study (GEMS)-H I survey (Kilborn et al. 2009) with the Parkes radio telescope, we selected 31 target galaxies with H I detections from the IC 1459 (I1459G) and NGC 4636 (N4636G) groups. Recent H I observations with the Australian Square Kilometre Array Pathfinder (ASKAP, Serra et al. 2015; Koribalski et al. 2020) covering these two groups provide high-resolution imaging data which we use to identify the environmental effects. Our sample includes dwarf and spiral galaxies with stellar masses ranging from 1.6 × 10⁶ – 4.9 × 10¹⁰ M☉. Using the Atacama Compact Array (ACA), we observed ¹²CO(J=1–0) of 31 group members from October 2019 to December 2019. To detect the edge of the molecular disk, we aimed to reach a 5σ gas surface density sensitivity of 4 M☉ pc⁻².

**3. Results**

**3.1. Peculiar CO and H I structures**

Sixteen out of the 31 galaxies in these two groups observed were detected in CO, such that imaging data could be obtained at ~1 kpc scale. Figure 1 displays both CO (color scale) and H I (contours) distributions of two interesting cases (IC 5264 and NGC 4632) that show signs of violent external perturbations (for other group members, see Lee et al. 2022). IC 5264 (1459G) has an asymmetric CO distribution with CO disk size of 6.5 kpc in the west and 7.5 kpc in the east. The CO disk is also more extended toward the south. The H I morphology is similar to the CO morphology, showing an extended H I tail-like structure in the east and a marginally truncated H I disk in the west. IC 5264 is thought to be experiencing both ram pressure stripping and tidal interaction. NGC 4632
<figure><img src="image_2.png" /></figure>

**Figure 1.** (a) & (b): CO (color scale) and H I (contours) distributions of two group members (left: IC 5264 (I1459G) and right: NGC 4632 (N4636G)) overlaid on the DSS2 blue images. (c) & (d): comparisons of SFR and H₂ gas fraction ($M_{H2}/M_*$) between our group sample and the field sample. In panel (c), the star-forming main sequence (Saintonge et al. 2016) and the 0.3 dex scatter are shown in a solid line and dashed lines, respectively. In panel (d), a solid line and dashed lines indicate the median trend and the 1σ standard deviation, respectively, which are derived from the field sample. This figure is reconstructed from results presented in Lee et al. (2022).

(N4636G) has a large H I polar ring-like structure. Its CO distribution is irregular with a more extended disk in the east and a local peak. For this galaxy, tidal interaction or a minor merger may cause these complex structures of H I and CO.

## 3.2. Comparisons of SFR and H₂ gas fraction between group and isolated galaxies

We compare the SFR and H₂ gas fraction ($M_{H2}/M_*$) of our group members with those of 240 isolated galaxies (i.e., the field sample) from the extended CO Legacy Database for GASS (xCOLD GASS; Saintonge et al. 2017). In Figure 1(c), the fraction (48%) of the star-forming galaxies in our group sample is similar to that (50%) in the field sample. However, many low-mass members ($\log(M_*/M_\odot) < 10$) in our group sample have suppressed SFRs (i.e., below the star-forming main sequence). For the H₂ gas fraction (Figure 1(d)), our group sample shows a high fraction (57%) of H₂ deficient galaxies compared to the field sample (26%). In particular, most low-mass group galaxies tend to be deficient in H₂. Interestingly, these low-mass galaxies are not deficient in H I. One possible scenario is that the conversion of H I to H₂ in the low-mass members is severely suppressed due to the environmental effects, resulting in low SFRs of the low-mass group galaxies. Further discussion of this is given in Section 5.2.1 of Lee et al. (2022).

The results in this work indicate that the group environmental effects can change not only the cold gas (CO and H I) distribution but also the global properties (e.g., SFR and H₂ gas fraction) of group members. This is particularly true for the N4636G, which is falling into the Virgo cluster. Our findings from the N4636G seem supporting evidence for the group preprocessing, but this study has been conducted with a small sample (only 31 galaxies). Follow-up studies observing various group members are required to obtain a more concrete and robust result of group environmental effects. In particular, more CO imaging observations with the ACA will be very helpful to reveal the environmental effects on the molecular gas of group galaxies.

**Acknowledgements.** This work was supported by the Korea Astronomy and Space Science Institute grant funded by the Korea government (MSIT) (Project No. 2022-1-840-05). Support for this work was also provided by the National Research Foundation of Korea to the grant No. 2018R1D1A1B07048314. BL acknowledges support from the National Science Foundation of China (12073002, 11721303, 11991052) and the National Key R&D Program of China (2016YFA0400702). LVM acknowledges financial support from grants SEV-2017-0709, RTI2018-096228-B-C31 (MCIU/AEI/FEDER,UE), and IAA4SKA (Ref. R18-RT-3082).
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