arXiv subjects

Richard D'Souza

Publications and source records attributed to Richard D'Souza.

At least 19 recordsLinked to original sources

arXiv2026

The Star Formation History and Evolution of the Ultra-Diffuse M81 Satellite F8D1

We present deep HST imaging of one of the nearest ultra-diffuse galaxies (UDGs) outside of the Local Group: F8D1, a satellite of M81 known to be tidally disrupting. UDGs are an enigmatic and diverse population, with evolutionary pathways ranging from tidal processing to bursty feedback and high initial angular momentum. To determine F8D1's evolutionary drivers, we resolve stars in F8D1's central $\sim$1 kpc and in a parallel field $\sim$6 kpc along its major axis to deep photometric limits, reaching below the Red Clump. We also image eight shallower fields along F8D1's major and minor axes. We calculate the star formation history (SFH) in the two deep fields, finding that while currently quiescent, both regions experienced a substantial burst $\sim$2 Gyr ago and a smaller burst $\sim$500 Myr ago, which likely formed F8D1's nuclear star cluster. In the shallow fields, using the ratio of evolved Asymptotic Giant Branch and Red Giant Branch stars out to $\sim$13 kpc along F8D1's known stellar stream, we confirm that F8D1 was globally star-forming until at least $\sim$2 Gyr ago. We estimate a total progenitor stellar mass, including the stream, of $\sim$1.3$\times$10$^8 M_{\odot}$, with an average [M/H] $\sim$ $-$0.8. We compare F8D1's properties to those of Local Group galaxies with similar initial stellar mass. We find that F8D1 is consistent with a progenitor star-forming galaxy similar to NGC 6822 that is in the midst of a transition to a Sagittarius-like system. Notably, this evolutionary sequence can be accomplished through tidal processing alone in galaxies that have experienced sufficiently bursty feedback to have created cored profiles.

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arXiv2026

Galaxy mergers and disk angular momentum evolution: stellar halos as a critical test

We investigate the role of hierarchical assembly in the angular momentum (AM) evolution of galaxies using a sample of 471 Milky Way-mass galaxies from the TNG-50 simulation. While galaxy orientation is often attributed to tidal torques and the cooling of gas within halos, we demonstrate that galaxy reorientation (tilting) is a common consequence of satellite accretion. Specifically, 80+/-2% of galaxies show alignment between their present-day AM and the orbital AM of their most massive (dominant) merger progenitor. This reorientation typically results in changes of around 50% in the galaxies' specific AM, with the most significant shifts occurring in galaxies that were initially highly misaligned. We find only a weak influence from the second most massive merger, and negligible impacts from surviving satellites. We show that accreted stellar halos encode the history of this reorientation. Driven by the same accretion event, the main bodies of galaxies and their stellar halos tend to co-align, with 81+/-2% of TNG-50 stellar halos showing prograde rotation relative to the galaxy. This signature will be detectable through major-axis kinematics with 30-meter class telescopes for Milky Way mass galaxies, offering a valuable observational test of this picture. While halo rotation directly constrains the specific AM of mergers within the last ~7 Gyr, this kinematic `memory' is largely erased for older and more radial events. Consequently, the Milky Way itself appears to be a notable exception to the general merger-driven trend: TNG-50 analogs with early, radial, and low angular momentum dominant mergers affect present-day disk orientation minimally. The current MW disk orientation may instead reflect the accumulated influences of gas accretion or dark matter torques.

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arXiv2026

The role of mass loss in constraining quenching time in dwarf galaxies from AGB and RGB star counts

The capability of reconstructing the past star formation history of dwarf elliptical galaxies out of the Local Volume relies on modelling bright stellar populations currently evolving through the red giant branch (RGB) and the asymptotic giant branch (AGB) phases. Recent studies proposed the use of the relative fractions of RGB and AGB stars populating specific boxes in the observational colour-magnitude plane to infer the epoch within which 90\% of the stellar population of the galaxy formed (T90). We aim at understanding the physical process of stellar evolution that are constrained by the relationship between the relative fraction of AGB and RGB stars of dwarf galaxies and the T90 epoch. We use updated stellar models that include the description of dust formation in the wind, to undertake a population synthesis approach, to allow monitoring the variation of the NAGB/NRGB ratio with time. The effects of some specific ingredients, such as the mass loss experienced by low-mass stars during the RGB phase, and the details of the time variation of the star formation rate, are extensively explored and tested against data. The mass lost by low-mass stars during the RGB evolution proves the most relevant ingredients affecting the time variation of NAGB/NRGB: at metallicities ~ 1/10 solar, a mass loss ~ 0.25Msun is required to reproduce the observations. This analysis allows to derive a relationship between NAGB/NRGB and T90, with a ~ 1 Gyr uncertainty on T90.

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arXiv2025

The low-mass and structured stellar halo of M83 argues against a merger origin for its starburst and extended neutral hydrogen disk

A merger origin has been suggested for M83's massive, metal-rich extended HI disk and nuclear starburst. We observe M83's stellar halo to test this idea. We train nearest-neighbor star-galaxy separation on wide-area Subaru imaging with Hubble Space Telescope data to map M83's halo in resolved stars. We find that M83 has an extended, very low density smooth stellar halo of old and metal-poor [M/H]$\sim -1.15$ RGB stars with a mass between 15 and 40 kpc of $\log_{10}M_{*,15-40,maj}/M_{\odot}=8.02\pm0.10$. In addition to M83's well-known Northern Stream, our ground-based Subaru imaging reveals a new stream to M83's south, which modeling suggests could be its trailing arm. The combined stream masses are $\log_{10}M_{stream}/M_{\odot}=7.93\pm0.10$, with metallicity [M/H]$= -1.0\pm0.2$. The stream progenitor was only recently accreted, as its stellar populations suggest that it formed stars until $2.1\pm1.3$ Gyr ago. M83 lies on the stellar halo mass-metallicity correlation seen for other Milky Way mass galaxies, albeit with low stellar halo mass. We infer a total accreted mass of $\log_{10}M_{*,accreted}/M_{\odot}=8.78^{+0.22}_{-0.28}$, with the most massive past merger having $\log_{10}M_{*,dom}/M_{\odot}=8.5\pm0.3$. We identify plausible M83 analogs in TNG-50 with similar stellar halos, finding that while a recent accretion can create a prominent stellar stream, such accretions do not trigger starburst activity, nor do they deliver enough gas to form M83's extended Hi disk. We conclude that other non-merger mechanisms, such as secular evolution or accretion of gas from the IGM, are likely to be responsible for M83's remarkable properties.

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arXiv2025

An integral field spectroscopic study of stellar and ionized gas properties around edge-on disk galaxies in the stellar mass range 9<log M*<11

We analyze the stellar light, 4000 Angstrom break and emission line profiles of 82 edge-on disk galaxies from the MaNGA survey. We characterize the stellar light profiles perpendicular to the disk plane using two parameters: a) the power law slope of the thick disk component, 2) the transition radius where the profile flattens. The 4000 Angstrom break profiles perpendicular to the plane are characterized by the number of significant changes in slope (breaks) and by the change in D_n(4000) from inner to outer disk. The slope correlates tightly with the stellar mass of the galaxy over the stellar mass range 10^9<log M*<10^10 M_sun. More massive galaxies have more extended thick disks. The slope and transition radius exhibit large scatter for galaxies more massive than 10^{10} M_sun. Half the sample have older stellar populations in their thick disks, a third have flat D$_n$(4000) profiles and 15% have younger thick disks. The D$_n$(4000) profiles exhibit as many as 4 separate breaks. There are more breaks in massive galaxies with bulges and more extended thick disks. This may indicate that the breaks are produced by more frequent accretion events in such systems. The extra-planar Halpha EQW correlates most strongly with the specific star formation rate of the galaxy, and the [OII]/Halpha ratio increases with distance from the disk plane. This increase is most apparent for massive galaxies with extended thick disk components and low SFR/M* These findings support the hypothesis that the larger [OII]/Halpha ratios may be caused by ionization from evolved stars.

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arXiv2025

Andromeda XXXV: The Faintest Dwarf Satellite of the Andromeda Galaxy

We present the discovery of Andromeda XXXV, the faintest Andromeda satellite galaxy discovered to date, identified as an overdensity of stars in the Pan-Andromeda Archaeological Survey and confirmed via Hubble Space Telescope imaging. Located at a heliocentric distance of $927^{+76}_{-63}$ kpc and $158^{+57}_{-45}$ kpc from Andromeda, Andromeda XXXV is an extended ($r_h = 53\,^{+13}_{-11}$ pc), elliptical ($ε= 0.4\, \pm 0.2$), metal-poor ($[\text{Fe}/\text{H}]\sim-1.9$) system, and the least luminous ($M_V=-5.2 \pm 0.3$) of Andromeda's dwarf satellites discovered so far. Andromeda XXXV's properties are consistent with the known population of dwarf galaxies around the Local Group, bearing close structural resemblance to the Canes Venatici II and Hydra II Milky Way (MW) dwarf satellite galaxies. Its stellar population, characterized by a red horizontal branch or a red clump feature, mirrors that of other Andromeda satellite galaxies in showing evidence for a spread in age and metallicity, with no signs of younger stellar generations. This age-metallicity spread is not observed in MW satellites of comparable stellar mass, and highlights the persistent differences between the satellite systems of Andromeda and the MW, extending even into the ultrafaint regime.

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arXiv2024

Star formation beyond galaxies: widespread in-situ formation of intra-cluster stars

We study the fraction of the intra-cluster light (ICL) formed in-situ in the three most massive clusters of the TNG50 simulation, with virial masses $\sim 10^{14}$ M$_{\odot}$. We find that a significant fraction of ICL stars ($8\%$-$28\%$) are born in-situ. This amounts to a total stellar mass comparable to the central galaxy itself. Contrary to simple expectations, only a sub-dominant fraction of these in-situ ICL stars are born in the central regions and later re-distributed to more energetic orbits during mergers. Instead, many in-situ ICL stars form directly hundreds of kiloparsecs away from the central galaxy, in clouds condensing out of the circum-cluster medium. The simulations predict a present-date diffuse star formation rate of $\sim$1 $\mathrm{M}_{\odot}$/yr, with higher rates at higher redshifts. The diffuse star forming component of the ICL is filamentary in nature, extends for hundreds of kiloparsecs and traces the distribution of neutral gas in the cluster host halo. We discuss briefly how numerical details of the baryonic treatment in the simulation, in particular the density threshold for star formation and the equation of state, may play a role in this result. We conclude that a sensitivity of $1.6 \times 10^{-19} - 2.6 \times 10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ in H$_α$ flux (beyond current observational capabilities) would be necessary to detect this diffuse star-forming component in galaxy clusters.

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arXiv2024

Exploring the Diversity of Faint Satellites in the M81 Group

In the last decade, we have been able to probe further down the galaxy luminosity function than ever before and expand into the regime of ultra-faint dwarfs (UFDs), which are some of the best probes we have of small-scale cosmology and galaxy formation. Digital sky surveys have enabled the discovery and study of these incredibly low-mass, highly dark-matter dominated systems around the Local Group, but it is critical that we expand the satellite census further out to understand if Milky Way and M31 satellites are representative of dwarf populations in the local Universe. Using data from HST/ACS, we present updated characterization of four satellite systems in the M81 group. These systems - D1005+68, D1006+69, DWJ0954+6821, and D1009+68 - were previously discovered using ground-based Subaru HSC data as overdensities in M81's halo and are now confirmed with HST/ACS by this work. These are all faint (M_V >= -7.9) and consistent with old (~13 Gyr), metal-poor ([M/H] < -1.5) populations. Each system possesses relatively unusual features - including one of the most concentrated satellite galaxies with a Sersic index of n ~ 5, one of the most elliptical galaxies outside the Local Group with an e ~ 0.6, and one of the most compact galaxies for its magnitude. Two of the satellites have very low surface brightness, lower than most known galaxies in this absolute magnitude range. This work previews the scientific promise of the upcoming Rubin Observatory and Roman Telescope for illuminating the diversity of UFDs in the Local Volume and beyond.

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arXiv2024

A Timeline of the M81 Group: Properties of the Extended Structures of M82 and NGC 3077

Mergers of and interactions between galaxies imprint a wide diversity of morphological, dynamical, and chemical characteristics in stellar halos and tidal streams. Measuring these characteristics elucidates aspects of the progenitors of the galaxies we observe today. The M81 group is the perfect galaxy group to understand the past, present, and future of a group of galaxies in the process of merging. Here we measure the end of star formation (t$_{90}$) and metallicity ([M/H]) of the stellar halo of M82 and the eastern tidal stream of NGC 3077 to: 1) test the idea that M82 possesses a genuine stellar halo, formed before any interaction with M81, 2) determine if NGC 3077's tidal disruption is related to the star formation history in its tails, and 3) create a timeline of the assembly history of the central trio in the M81 group. We argue that M82 possesses a genuine, metal poor ([M/H] ~ -1.62 dex) stellar halo, formed from the merger of a small satellite galaxy roughly 6.6 Gyr ago. We also find that the stars present in NGC 3077's tails formed before tidal disruption with M81, and possesses a roughly uniform metallicity as shown in Okamoto et. al. 2023 implying that NGC 3077's progenitor had significant population gradients. Finally, we present a timeline of the central trio's merger/interaction history.

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arXiv2024

First resolved stellar halo kinematics of a MW-mass galaxy outside the Local Group: A flat counter-rotating halo in NGC 4945

Stellar halos of galaxies, primarily formed through the accretion of smaller objects, are important to understand the hierarchical mass assembly of galaxies. However, the inner regions of stellar halos in disk galaxies are predicted to have an in-situ component that is expected to be prominent along the major axis. Kinematic information is crucial to disentangle the contribution of the in-situ component from the accreted stellar halos. The low surface brightness of stellar halos makes it inaccessible with traditional integrated light spectroscopy. In this work, using a novel technique, we study the kinematics of the stellar halo of the edge-on galaxy NGC 4945. We couple new deep Multi Unit Spectroscopic Explorer spectroscopic observations with existing Hubble Space Telescope imaging data to spectroscopically measure the line-of-sight (LOS) heliocentric velocity and velocity dispersion in two fields at a galactocentric distance of 12.2 kpc (outer disk field) and 34.6 kpc (stellar halo field) along NGC 4945 major axis, by stacking individual spectra of red giant branch and asymptotic giant branch stars. We obtain a LOS velocity and dispersion of 673+/-11 km/s and 73+/-14 km/s, respectively, for the outer disk field. This is consistent with the mean HI velocity of the disk at that distance. For the halo field we obtain a LOS velocity and dispersion of 519+/-12 km/s and 42+/-22 km/s. The halo fields' velocity measurement is within ~40 km/s from the systemic LOS velocity of NGC 4945, which is 563 km/s, suggesting that its stellar halo at 34.6 kpc along the major axis is counter-rotating and is of likely accretion origin. This provides the first ever kinematic measurement of the stellar halo of a Milky Way-mass galaxy outside the Local Group from its resolved stellar population, and establishes a powerful technique for measuring the velocity field of the stellar halos of nearby galaxies.

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arXiv2023

The progenitors of the intra-cluster light and intra-cluster globular clusters in galaxy groups and clusters

We use the IllustrisTNG50 cosmological hydrodynamical simulation, complemented by a catalog of tagged globular clusters, to investigate the properties and build up of two extended luminous components: the intra-cluster light (ICL) and the intra-cluster globular clusters (ICGC). We select the 39 most massive groups and clusters in the box, spanning the range of virial masses $5 \times 10^{12} < \rm M_{200}/\rm M_{\odot} < 2 \times 10^{14}$. We find good agreement between predictions from the simulations and current observational estimates of the fraction of mass in the ICL and its radial extension. The stellar mass of the ICL is only $\sim10\%-20\%$ of the stellar mass in the central galaxy but encodes useful information on the assembly history of the group or cluster. About half the ICL in all our systems is brought in by galaxies in a narrow stellar mass range, $M_*=10^{10}-10^{11}$ $\rm M_{\odot}$. However, the contribution of low-mass galaxies ($M_*<10^{10}$ $\rm M_{\odot}$) to the build-up of the ICL varies broadly from system to system, $\sim 5\%-45\%$, a feature that might be recovered from the observable properties of the ICL at $z=0$. At fixed virial mass, systems where the accretion of dwarf galaxies plays an important role have shallower metallicity profiles, less metal content and a lower stellar mass in the ICL than systems where the main contributors are more massive galaxies. We show that intra-cluster GCs are also good tracers of this history, representing a valuable alternative when diffuse light is not detectable.

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arXiv2023

Constraining the assembly time of the stellar haloes of nearby Milky Way-mass galaxies through AGB populations

The star formation histories (SFHs) of galactic stellar haloes offer crucial insights into the merger history of the galaxy and the effects of those mergers on their hosts. Such measurements have revealed that while the Milky Way's most important merger was 8-10 Gyr ago, M31's largest merger was more recent, within the last few Gyr. Unfortunately, the required halo SFH measurements are extremely observationally expensive outside of the Local Group. Here we use asymptotic giant branch (AGB) stars brighter than the tip of the red giant branch (RGB) to constrain stellar halo SFHs. Both stellar population models and archival datasets show that the AGB/RGB ratio constrains the time before which 90% of the stars formed, $t_{90}$. We find AGB stars in the haloes of three highly-inclined roughly Milky Way-mass galaxies with resolved star measurements from the Hubble Space Telescope; this population is most prominent in the stellar haloes of NGC 253 and NGC 891, suggesting that their stellar haloes contain stars born at relatively late times, with inferred $t_{90}\sim 6\pm1.5$Gyr. This ratio also varies from region to region, tending towards higher values along the major axis and in tidal streams or shells. By combining our measurements with previous constraints, we find a tentative anticorrelation between halo age and stellar halo mass, a trend that exists in models of galaxy formation but has never been elucidated before, i.e, the largest stellar haloes of Milky-Way mass galaxies were assembled more recently.

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arXiv2023

Origins of the Evil Eye: M64's Stellar Halo Reveals the Recent Accretion of an SMC-mass Satellite

M64, often called the "Evil Eye" galaxy, is unique among local galaxies. Beyond its dramatic, dusty nucleus, it also hosts an outer gas disk that counter-rotates relative to its stars. The mass of this outer disk is comparable to the gas content of the Small Magellanic Cloud (SMC), prompting the idea that it was likely accreted in a recent minor merger. Yet, detailed follow-up studies of M64's outer disk have shown no evidence of such an event, leading to other interpretations, such as a "flyby" interaction with the distant diffuse satellite Coma P. We present Subaru Hyper Suprime-Cam observations of M64's stellar halo, which resolve its stellar populations and reveal a spectacular radial shell feature, oriented $\sim$30$^{\circ}$ relative to the major axis and along the rotation axis of the outer gas disk. The shell is $\sim$45 kpc southeast of M64, while a similar but more diffuse plume to the northwest extends to $>$100 kpc. We estimate a stellar mass and metallicity for the southern shell of $M_{\star} {=} 1.80~{\pm}~0.54{\times}10^8~M_{\odot}$ and [M/H] $=$ $-$1.0, respectively, and a similar mass of $1.42~{\pm}~0.71{\times}10^8 M_{\odot}$ for the northern plume. Taking into account the accreted material in M64's inner disk, we estimate a total stellar mass for the progenitor satellite of $M_{\rm \star,prog}~{\simeq}~5{\times}10^8~M_{\odot}$. These results suggest that M64 is in the final stages of a minor merger with a gas-rich satellite strikingly similar to the SMC, in which M64's accreted counter-rotating gas originated, and which is responsible for the formation of its dusty inner star-forming disk.

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arXiv2023

Saying Hallo to M94's Stellar Halo: Investigating the Accretion History of the Largest Pseudobulge Host in the Local Universe

It is not yet settled how the combination of secular processes and merging gives rise to the bulges and pseudobulges of galaxies. The nearby ($D\sim$ 4.2 Mpc) disk galaxy M94 (NGC 4736) has the largest pseudobulge in the local universe, and offers a unique opportunity for investigating the role of merging in the formation of its pseudobulge. We present a first ever look at M94's stellar halo, which we expect to contain a fossil record of M94's past mergers. Using Subaru's Hyper Suprime-Cam, we resolve and identify red giant branch (RGB) stars in M94's halo, finding two distinct populations. After correcting for completeness through artificial star tests, we can measure the radial profile of each RGB population. The metal-rich RGB stars show an unbroken exponential profile to a radius of 30 kpc that is a clear continuation of M94's outer disk. M94's metal poor stellar halo is detectable over a wider area and clearly separates from its metal-rich disk. By integrating the halo density profile, we infer a total accreted stellar mass of $\sim 2.8 \times 10^8 M_\odot$, with a median metallicity of [M/H] $=-$1.4. This indicates that M94's most-massive past merger was with a galaxy similar to, or less massive than, the Small Magellanic Cloud. Few nearby galaxies have had such a low-mass dominant merger; therefore we suggest that M94's pseudobulge was not significantly impacted by merging.

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arXiv2022

Ultrafaint Dwarf Galaxy Candidates in the M81 Group: Signatures of Group Accretion

The faint and ultrafaint dwarf galaxies in the Local Group form the observational bedrock upon which our understanding of small-scale cosmology rests. In order to understand whether this insight generalizes, it is imperative to use resolved-star techniques to discover similarly faint satellites in nearby galaxy groups. We describe our search for ultrafaint galaxies in the M81 group using deep ground-based resolved-star data sets from Subaru's Hyper Suprime-Cam. We present one new ultrafaint dwarf galaxy in the M81 group and identify five additional extremely low surface brightness candidate ultrafaint dwarfs that reach deep into the ultrafaint regime to $M_V \sim -6$ (similar to current limits for Andromeda satellites). These candidates' luminosities and sizes are similar to known Local Group dwarf galaxies Tucana B, Canes Venatici I, Hercules, and Boötes I. Most of these candidates are likely to be real, based on tests of our techniques on blank fields. Intriguingly, all of these candidates are spatially clustered around NGC 3077, which is itself an M81 group satellite in an advanced state of tidal disruption. This is somewhat surprising, as M81 itself and its largest satellite M82 are both substantially more massive than NGC 3077 and by virtue of their greater masses, would have been expected to host as many or more ultrafaint candidates. These results lend considerable support to the idea that satellites of satellites are an important contribution to the growth of satellite populations around Milky Way-mass galaxies.

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arXiv2022

New Globular Cluster Candidates in the M81 group

The study of outer halo globular cluster (GC) populations can give insight into galaxy merging, globular cluster accretion and the origin of GCs. We use archival Subaru Hyper Suprime-Cam (HSC) data in concert with space-based GALEX, IRAC and Gaia EDR3 data to select candidate Globular clusters (GCs) in the outer halo of the M81 group for confirmation and future study. We use a small sample of previously-discovered GCs to tune our selection criteria, finding that bright already-known GCs in the M81 group have sizes that are typically slightly larger than the Subaru PSF in our fields. In the optical bands, GCs appear to have colours that are only slightly different from stars. The inclusion of archival IRAC data yields dramatic improvements in colour separation, as the long wavelength baseline aids somewhat in the separation from stars and clearly separates GCs from many compact background galaxies. We show that some previously spectroscopically-identified GCs in the M81 group are instead foreground stars or background galaxies. GCs close to M82 have radial velocities suggesting that they fell into the M81 group along with M82. The overall M81 GC luminosity function is similar to the Milky Way and M31. M81's outer halo GCs are similar to the Milky Way in their metallicities and numbers, and much less numerous than M31's more metal-rich outer halo GC population. These properties reflect differences in the three galaxies' merger histories, highlighting the possibility of using outer halo GCs to trace merger history in larger samples of galaxies.

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arXiv2022

Relating the Diverse Merger Histories and Satellite Populations of Nearby Galaxies

We investigate whether the considerable diversity in the satellite populations of nearby Milky Way (MW)-mass galaxies is connected with the diversity in their host's merger histories. Analyzing 8 nearby galaxies with extensive observations of their satellite populations and stellar halos, we characterize each galaxy's merger history using the metric of its most dominant merger, $M_{\rm \star,Dom}$, defined as the greater of either its total accreted stellar mass or most massive current satellite. We find an unexpectedly tight relationship between these galaxies' number of $M_{V}\,{<}\,{-}9$ satellites within 150 kpc ($N_{\rm Sat}$) and $M_{\rm \star,Dom}$. This relationship remains even after accounting for differences in galaxy mass. Using the star formation and orbital histories of satellites around the MW and M81, we demonstrate that both likely evolved along the $M_{\rm\star,Dom}{-}N_{\rm Sat}$ relation during their current dominant mergers with the LMC and M82, respectively. We investigate the presence of this relation in galaxy formation models, including using the FIRE simulations to directly compare to the observations. We find no relation between $M_{\rm\star,Dom}$ and $N_{\rm Sat}$ in FIRE, and a universally large scatter in $N_{\rm Sat}$ with $M_{\rm \star,Dom}$ across models $-$ in direct contrast with the tightness of the empirical relation. This acute difference in the observed and predicted scaling relation between two fundamental galaxy properties signals that current simulations do not sufficiently reproduce diverse merger histories and their effects on satellite populations. Explaining the emergence of this relation is therefore essential for obtaining a complete understanding of galaxy formation.

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arXiv2022

The Global Dynamical Atlas of the Milky Way mergers: Constraints from Gaia EDR3 based orbits of globular clusters, stellar streams and satellite galaxies

The Milky Way halo was predominantly formed by the merging of numerous progenitor galaxies. However, our knowledge of this process is still incomplete, especially in regard to the total number of mergers, their global dynamical properties and their contribution to the stellar population of the Galactic halo. Here, we uncover the Milky Way mergers by detecting groupings of globular clusters, stellar streams and satellite galaxies in action ($\mathbf{J}$) space. While actions fully characterize the orbits, we additionally use the redundant information on their energy ($\textit{E}$) to enhance the contrast between groupings. For this endeavour, we use $\textit{Gaia}$ EDR3 based measurements of $170$ globular clusters, $41$ streams and $46$ satellites to derive their $\mathbf{J}$ and $\textit{E}$. To detect groups, we use the $\texttt{ENLINK}$ software, coupled with a statistical procedure that accounts for the observed phase-space uncertainties of these objects. We detect a total of $N=6$ groups, including the previously known mergers $\textit{Sagittarius}$, $\textit{Cetus}$, $\textit{Gaia-Sausage/Enceladus}$, $\textit{LMS-1/Wukong}$, $\textit{Arjuna/Sequoia/I'itoi}$ and one new merger that we call $\textit{Pontus}$. All of these mergers, together, comprise $62$ objects ($\approx 25\%$ of our sample). We discuss their members, orbital properties and metallicity distributions. We find that the three most metal-poor streams of our Galaxy -- "C-19" ([Fe/H]$=-3.4$ dex), "Sylgr" ([Fe/H]$=-2.9$ dex) and "Phoenix" ([Fe/H]$=-2.7$ dex) -- are associated with $\textit{LMS-1/Wukong}$; showing it to be the most metal-poor merger. The global dynamical atlas of Milky Way mergers that we present here provides a present-day reference for galaxy formation models.

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