Rey M. P., Pontzen A., Saintonge A., 2019a, MNRAS, 485, 1906

Rey M. P., Pontzen A., Agertz O., Orkney M. D. A., Read J. I., Saintonge A., Pedersen C., 2019b, ApJ, 886, L3

Robles S., Gómez J. S., Ramírez Rivera A., Padilla N. D., Dujovne D., 2022, MNRAS, 514, 3692

Rosdahl J., Blaizot J., 2012, MNRAS, 423, 344

Rosdahl J., Blaizot J., Aubert D., Stranex T., Teyssier R., 2013, MNRAS, 436, 2188

Roth N., Pontzen A., Peiris H. V., 2016, MNRAS, 455, 974

Ruchti G. R. et al., 2015, MNRAS, 450, 2874

Ruiz-Lara T., Gallart C., Bernard E. J., Cassisi S., 2020, Nat. Astron., 4, 965

Sanchez N. N., Tremmel M., Werk J. K., Pontzen A., Christensen C., Quinn T., Loebman S., Cruz A., 2021, ApJ, 911, 116

Sanderson R. E. et al., 2020, ApJS, 246, 6

Sawala T. et al., 2016, MNRAS, 457, 1931

Schmidt M., 1959, ApJ, 129, 243

Sellwood J. A., 2013, ApJ, 769, L24

Sellwood J. A., Binney J. J., 2002, MNRAS, 336, 785

Sestito F. et al., 2021, MNRAS, 500, 3750

Somerville R. S., Davé R., 2015, ARA&A, 53, 51

Springel V., Hernquist L., 2005, ApJ, 622, L9

Springel V. et al., 2005, Nature, 435, 629

Stern J. et al., 2021, MNRAS, 911, 88

Stopyra S., Pontzen A., Peiris H., Roth N., Rey M. P., 2021, ApJS, 252, 28

Teyssier R., 2002, A&A, 385, 337

Thomas G. F. et al., 2021, ApJ, 910, 17

Toro E. F., Spruce M., Speares W., 1994, Shock Waves, 4, 25

Virtanen P. et al., 2020, Nat. Methods, 17, 261

Wetzel A. et al., 2022, preprint(arXiv:2202.06969)

Woosley S. E., Heger A., 2007, Phys. Rep., 442, 269

Wu J., Struck C., Elmegreen B. G., D'Onghia E., 2022, MNRAS, 517, 4417

Xiang M., Rix H.-W., 2022, Nature, 603, 599

Zel'dovich Y. B., 1970, A&A, 5, 84

Zolotov A. et al., 2015, MNRAS, 450, 2327

van Donkelaar F., Agertz O., Renaud F., 2022, MNRAS, 512, 3806

van der Walt S., Colbert S. C., Varoquaux G., 2011, Comput. Sci. Eng., 13, 22

Fig. A1 shows the distributions of $E_{\text{tot}}$ and [α/Fe] of each GSE subpopulation identified in Section 4.2. As in Section 4, we recover strong similarities in the overall GSE-like population (purple) between the different merger scenarios, with similar peaks in abundances – ([α/Fe]) = 0.48, 0.48, 0.48, 0.47, 0.47, from left to right, respectively – and broad coinciding distributions in $E_{\text{tot}}$.

Breaking down the overall population into its subpopulations (colours matching Fig. 7), we first confirm that, for all merger scenarios, the contaminating population (grey) has high $E_{\text{tot}}$ and low [α/Fe], combining with its higher [Fe/H]. This is consistent with our interpretation that this contamination primarily arises from outer disc and halo stars that formed after the ancient structures of interest.

Focusing on the subpopulations originating at high-redshift (colours), we recover a strong overlap in $E_{\text{tot}}$ and [α/Fe], adding to the kinematic and chemical overlap already highlighted in Section 4. In particular, stars formed during early interactions (light blue and green) can populate high-$E_{\text{tot}}$ orbits and have similar [α/Fe] ratios than the accreted population of the genetically modified merger at $z \approx 2$ ('B'; red).

To further visualize this, we show in Fig. A2 the positions of subpopulations in the $E_{\text{tot}}-L_z$, [α/Fe]-[Fe/H], and $v_r - v_\phi$ planes (top, middle, and bottom, respectively) compared to the overall population of stars within the galaxy (grey background). We estimate respective 50 per cent isodensity contour using a 2D kernel density estimate with Gaussian smoothing according to Scott's rule. The small sample sizes, large spread and non-Gaussian nature of chemodynamical distributions can lead to noisy KDE estimates,<fnref n="4" /> and generates the large isodensity contours for very early populations that have large spreads in [α/Fe] and small sample sizes. For completeness, we choose to show all subpopulations, even when noisy, but stress that they might not contribute a large fraction of the total GSE-like feature (absolute contributions can be estimated from Fig. 7 and A1).

As with 1D distributions, we find substantial similarities between merger scenarios (left to right). Individual subpopulations have distinct peaks and a clear progressing sequence in [α/Fe]-[Fe/H]. However, the main contributors to the metal-poor GSE-like population (blue formed during the earlier interaction 'A', and red brought with the merger body 'B') all significantly overlap in these planes. We thus do not find differences in chemodynamics of the GSE-like population that unequivocally allows us to differentiate merger mass ratio at $z \approx 2$.

## APPENDIX A: ADDITIONAL CHEMODYNAMICAL CHARACTERIZATION OF THE GSE-LIKE POPULATIONS

In Section 4, we establish that, despite a large diversity in final morphology and past merger mass ratio at $z \approx 2$, each galaxy has similar GSE-like kinematic features at fixed stellar mass (Section 4.1). We further show in Section 4.2 that GSE-like stars, in all merger scenarios at $z \approx 2$, have broad age and [Fe/H] distributions that peak around the same median, due to overlapping populations with varying cosmological origins. In this Appendix, we show that distinguishing subpopulations within each merger scenario, or merger scenarios from one another, remains difficult even when leveraging additional chemodynamical data for the GSE-like stars.

<footnote n="4">⁴We find that going for larger isodensity contours (e.g. 68 per cent for one sigma) exacerbates this issue.</footnote>
Rey M. P., Pontzen A., Saintonge A., 2019a, MNRAS, 485, 1906

Rey M. P., Pontzen A., Agertz O., Orkney M. D. A., Read J. I., Saintonge A., Pedersen C., 2019b, ApJ, 886, L3

Robles S., Gómez J. S., Ramírez Rivera A., Padilla N. D., Dujovne D., 2022, MNRAS, 514, 3692

Rosdahl J., Blaizot J., 2012, MNRAS, 423, 344

Rosdahl J., Blaizot J., Aubert D., Stranex T., Teyssier R., 2013, MNRAS, 436, 2188

Roth N., Pontzen A., Peiris H. V., 2016, MNRAS, 455, 974

Ruchti G. R. et al., 2015, MNRAS, 450, 2874

Ruiz-Lara T., Gallart C., Bernard E. J., Cassisi S., 2020, Nat. Astron., 4, 965

Sanchez N. N., Tremmel M., Werk J. K., Pontzen A., Christensen C., Quinn T., Loebman S., Cruz A., 2021, ApJ, 911, 116

Sanderson R. E. et al., 2020, ApJS, 246, 6

Sawala T. et al., 2016, MNRAS, 457, 1931

Schmidt M., 1959, ApJ, 129, 243

Sellwood J. A., 2013, ApJ, 769, L24

Sellwood J. A., Binney J. J., 2002, MNRAS, 336, 785

Sestito F. et al., 2021, MNRAS, 500, 3750

Somerville R. S., Davé R., 2015, ARA&A, 53, 51

Springel V., Hernquist L., 2005, ApJ, 622, L9

Springel V. et al., 2005, Nature, 435, 629

Stern J. et al., 2021, MNRAS, 911, 88

Stopyra S., Pontzen A., Peiris H., Roth N., Rey M. P., 2021, ApJS, 252, 28

Teyssier R., 2002, A&A, 385, 337

Thomas G. F. et al., 2021, ApJ, 910, 17

Toro E. F., Spruce M., Speares W., 1994, Shock Waves, 4, 25

Virtanen P. et al., 2020, Nat. Methods, 17, 261

Wetzel A. et al., 2022, preprint(arXiv:2202.06969)

Woosley S. E., Heger A., 2007, Phys. Rep., 442, 269

Wu J., Struck C., Elmegreen B. G., D'Onghia E., 2022, MNRAS, 517, 4417

Xiang M., Rix H.-W., 2022, Nature, 603, 599

Zel'dovich Y. B., 1970, A&A, 5, 84

Zolotov A. et al., 2015, MNRAS, 450, 2327

van Donkelaar F., Agertz O., Renaud F., 2022, MNRAS, 512, 3806

van der Walt S., Colbert S. C., Varoquaux G., 2011, Comput. Sci. Eng., 13, 22

## APPENDIX A: ADDITIONAL CHEMODYNAMICAL CHARACTERIZATION OF THE GSE-LIKE POPULATIONS

In Section 4, we establish that, despite a large diversity in final morphology and past merger mass ratio at $z \approx 2$, each galaxy has similar GSE-like kinematic features at fixed stellar mass (Section 4.1). We further show in Section 4.2 that GSE-like stars, in all merger scenarios at $z \approx 2$, have broad age and [Fe/H] distributions that peak around the same median, due to overlapping populations with varying cosmological origins. In this Appendix, we show that distinguishing subpopulations within each merger scenario, or merger scenarios from one another, remains difficult even when leveraging additional chemodynamical data for the GSE-like stars.

Fig. A1 shows the distributions of $E_{\text{tot}}$ and [α/Fe] of each GSE subpopulation identified in Section 4.2. As in Section 4, we recover strong similarities in the overall GSE-like population (purple) between the different merger scenarios, with similar peaks in abundances – $([\alpha/Fe]) = 0.48, 0.48, 0.48, 0.47, 0.47$, from left to right, respectively – and broad coinciding distributions in $E_{\text{tot}}$.

Breaking down the overall population into its subpopulations (colours matching Fig. 7), we first confirm that, for all merger scenarios, the contaminating population (grey) has high $E_{\text{tot}}$ and low [α/Fe], combining with its higher [Fe/H]. This is consistent with our interpretation that this contamination primarily arises from outer disc and halo stars that formed after the ancient structures of interest.

Focusing on the subpopulations originating at high-redshift (colours), we recover a strong overlap in $E_{\text{tot}}$ and [α/Fe], adding to the kinematic and chemical overlap already highlighted in Section 4. In particular, stars formed during early interactions (light blue and green) can populate high-$E_{\text{tot}}$ orbits and have similar [α/Fe] ratios than the accreted population of the genetically modified merger at $z \approx 2$ ('B'; red).

To further visualize this, we show in Fig. A2 the positions of subpopulations in the $E_{\text{tot}}-L_z$, [α/Fe]-[Fe/H], and $v_r - v_\phi$ planes (top, middle, and bottom, respectively) compared to the overall population of stars within the galaxy (grey background). We estimate respective 50 per cent isodensity contour using a 2D kernel density estimate with Gaussian smoothing according to Scott's rule. The small sample sizes, large spread and non-Gaussian nature of chemodynamical distributions can lead to noisy KDE estimates,<fnref n="4" /> and generates the large isodensity contours for very early populations that have large spreads in [α/Fe] and small sample sizes. For completeness, we choose to show all subpopulations, even when noisy, but stress that they might not contribute a large fraction of the total GSE-like feature (absolute contributions can be estimated from Fig. 7 and A1).

As with 1D distributions, we find substantial similarities between merger scenarios (left to right). Individual subpopulations have distinct peaks and a clear progressing sequence in [α/Fe]-[Fe/H]. However, the main contributors to the metal-poor GSE-like population (blue formed during the earlier interaction 'A', and red brought with the merger body 'B') all significantly overlap in these planes. We thus do not find differences in chemodynamics of the GSE-like population that unequivocally allows us to differentiate merger mass ratio at $z \approx 2$.

<footnote n="4">⁴We find that going for larger isodensity contours (e.g. 68 per cent for one sigma) exacerbates this issue.</footnote>
<figure><img src="image_12.png" /><figcaption>**Figure A1.** 9$E_{\text{tot}}$ (top) and [α/Fe] (bottom) distributions of all radially biased, GSE-like stars identified in Fig. 4 (purple) and their high-redshift origin (colours matching Fig. 7). In all merger scenarios (increasing mass ratio from left to right), in-situ and ex-situ contributors substantially overlap chemically and kinematically, and cannot be easily distinguished (see also Fig. A2). The contaminating population (grey) lies preferentially at high $E_{\text{tot}}$ and low [α/Fe].</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure A2.** 2D distributions in orbital (top), chemical (middle), and kinematic (bottom) space of each subpopulation identified in Fig. 7. Contours show 50 per cent isodensities in these planes, normalized to each subpopulation’s total (their absolute contribution to the overall GSE-like feature can be estimated from Fig. 7 and A1). The most important subpopulations to the metal-poor GSE feature (blue and red) always overlap in all planes, making it difficult to distinguish increasing merger mass ratios at $z \approx 2$ in the GSE-like progenitor (left to right). The grey histogram shows the distribution of all inner stars ($r < 50$ kpc).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Figure A1.** 9$E_{\text{tot}}$ (top) and [α/Fe] (bottom) distributions of all radially biased, GSE-like stars identified in Fig. 4 (purple) and their high-redshift origin (colours matching Fig. 7). In all merger scenarios (increasing mass ratio from left to right), in-situ and ex-situ contributors substantially overlap chemically and kinematically, and cannot be easily distinguished (see also Fig. A2). The contaminating population (grey) lies preferentially at high $E_{\text{tot}}$ and low [α/Fe].</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure A2.** 2D distributions in orbital (top), chemical (middle), and kinematic (bottom) space of each subpopulation identified in Fig. 7. Contours show 50 per cent isodensities in these planes, normalized to each subpopulation’s total (their absolute contribution to the overall GSE-like feature can be estimated from Fig. 7 and A1). The most important subpopulations to the metal-poor GSE feature (blue and red) always overlap in all planes, making it difficult to distinguish increasing merger mass ratios at $z \approx 2$ in the GSE-like progenitor (left to right). The grey histogram shows the distribution of all inner stars ($r < 50$ kpc).</figcaption></figure>
<figure><img src="image_14.png" /></figure>

**Figure A3.** Same as Fig. 6, but comparing the galactic and GSE-like star formation histories. Peaks of star formation in GSE-like stars broadly map onto galaxy-wide star formation enhancements during early interactions (marked at the top). However, the opposite is not verified—galaxy-wide starbursts do not necessarily form stars that end up on radial halo orbits at *z* = 0. This is particularly visible for the largest mass ratio (right-most panel) where interaction ‘B’ drives a large galaxy-wide starburst that is missing in the GSE-like population.

## APPENDIX B: COMPARING GALAXY-WIDE AND GSE STAR FORMATION HISTORIES

In Section 4.2, we show that star formation peaks in the GSE-like population coincide in timing with the coalescence of early mergers (Fig. 6), when a significant fraction of its stars are formed (Fig. 7). In this Appendix, we quantify whether this surplus in star formation reflect a general galaxy-wide enhancement, or is specific to stars on halo-like, radial orbits at $z=0$.

We show in Fig. A3 the star formation histories of all inner stars ($r \le 50$ kpc; black) for each merger scenario (mass ratio at $z \approx 2$ increasing from left to right), compared to those of GSE-like stars (purple; see also Fig. 6). We recover that early interactions ($z_{\text{infall}}$ to $z_{\text{coalescence}}$ marked at the top) drives large, galaxy-wide star formation enhancements consistent with merger induced starbursts. For lower mass ratios at $z \approx 2$ (first three panels), the elevated star

formation coincide with peaks in star formation in the GSE-like population, reflecting the dominant contribution of starburst stars in this population at $z=0$ (Fig. 7).

For larger mass ratios (right-hand panels), the mapping between galaxy-wide and GSE star formation is less clear, particularly for interaction ‘B’ (red). This is particularly visible for the largest mass ratio (right-most panel), where the galaxy undergoes a large starburst during interaction ‘B’ (black), but stars formed at this time do not populate radially biased orbits at $z=0$ (purple). In fact, the origin of GSE-like at $z=0$ population is dominated by accreted stars previously formed in the merging body, rather than stars formed during the interaction (Fig. 7).

This paper has been typeset from a TeX/LATeX file prepared by the author.
<figure><img src="image_15.png" /></figure>

**Figure A3.** Same as Fig. 6, but comparing the galactic and GSE-like star formation histories. Peaks of star formation in GSE-like stars broadly map onto galaxy-wide star formation enhancements during early interactions (marked at the top). However, the opposite is not verified—galaxy-wide starbursts do not necessarily form stars that end up on radial halo orbits at *z* = 0. This is particularly visible for the largest mass ratio (right-most panel) where interaction ‘B’ drives a large galaxy-wide starburst that is missing in the GSE-like population.

## APPENDIX B: COMPARING GALAXY-WIDE AND GSE STAR FORMATION HISTORIES

In Section 4.2, we show that star formation peaks in the GSE-like population coincide in timing with the coalescence of early mergers (Fig. 6), when a significant fraction of its stars are formed (Fig. 7). In this Appendix, we quantify whether this surplus in star formation reflect a general galaxy-wide enhancement, or is specific to stars on halo-like, radial orbits at $z=0$.

We show in Fig. A3 the star formation histories of all inner stars ($r \le 50$ kpc; black) for each merger scenario (mass ratio at $z \approx 2$ increasing from left to right), compared to those of GSE-like stars (purple; see also Fig. 6). We recover that early interactions ($z_{\text{infall}}$ to $z_{\text{coalescence}}$ marked at the top) drives large, galaxy-wide star formation enhancements consistent with merger induced starbursts. For lower mass ratios at $z \approx 2$ (first three panels), the elevated star

formation coincide with peaks in star formation in the GSE-like population, reflecting the dominant contribution of starburst stars in this population at $z=0$ (Fig. 7).

For larger mass ratios (right-hand panels), the mapping between galaxy-wide and GSE star formation is less clear, particularly for interaction ‘B’ (red). This is particularly visible for the largest mass ratio (right-most panel), where the galaxy undergoes a large starburst during interaction ‘B’ (black), but stars formed at this time do not populate radially biased orbits at $z=0$ (purple). In fact, the origin of GSE-like at $z=0$ population is dominated by accreted stars previously formed in the merging body, rather than stars formed during the interaction (Fig. 7).

This paper has been typeset from a TeX/LATeX file prepared by the author.