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Relating the Diverse Merger Histories and Satellite Populations of Nearby Galaxies

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Relating the Diverse Merger Histories and Satellite Populations of Nearby Galaxies Draft version July 12, 2021 Typeset using LATEX twocolumn style in AASTeX63 Relating the Diverse Merger Histories and Satellite Populations of Nearby Galaxies Adam Smercina ,1 Eric F. Bell ,2 Jenna Samuel ,3 and Richard D'Souza 2, 4 1Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195-1580, USA 2Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA 3Department of Physics and Astronomy, University of California, Davis, CA 95616, USA 4Vatican Observatory, Specola Vaticana, V-00120, Vatican City State ABSTRACT 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?;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 MV < 9 satellites within − 150 kpc (NSat) and M?;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?;Dom{NSat 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?;Dom and NSat in FIRE, and a universally large scatter in NSat with M?;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. 1. INTRODUCTION these processes, are now able to reproduce many of the A hallmark of the current Λ–Cold Dark Matter most well-constrained large-scale observables, such as (ΛCDM) paradigm is that the halos of dark matter the bright end of the galaxy luminosity function (e.g., in which galaxies reside assemble hierarchically (e.g., Bell et al. 2003) and the cosmic star formation rate White & Rees 1978). As a consequence of this hierar- (SFR; Madau et al. 1998). Perhaps the most impor- chical growth, dark matter and galaxy formation models tant benchmarks of these simulations are the canonical alike predict that galaxies should accrue extensive pop- galaxy scaling relations | for example, the Kennicutt- ulations of lower-mass dwarf `satellite' galaxies, tidally Schmidt star formation relation (e.g., Kennicutt 1998; interacting with many of them (e.g., Somerville & Ko- Orr et al. 2018), the Tully-Fisher relation (e.g., Tully & latt 1999; Bullock et al. 2001; van den Bosch 2002; Fisher 1977; Dalcanton et al. 1997; Bell & de Jong 2001; arXiv:2107.04591v1 [astro-ph.GA] 9 Jul 2021 Wechsler et al. 2002). The largest of these accreted Schaye et al. 2015), and the galaxy star-forming main galaxies experience strong dynamical friction and can sequence (Noeske et al. 2007; Schaye et al. 2015; Jar- merge with the central galaxy before they tidally dis- rett et al. 2017) | which operate across the large range rupt, strongly impacting the properties and evolution of scales that govern galaxies' properties. Yet, despite of the central galaxy (e.g., Barnes & Hernquist 1991; this impressive progress in explaining the formation and Hopkins et al. 2006). Modern cosmological simula- evolution of relatively massive galaxies, the regime of tions, built on a ΛCDM framework and incorporating small-scale galaxy formation has remained problematic for simulations. Corresponding author: Adam Smercina This difficulty in modeling small-scale galaxy for- [email protected] mation is due, in large part, to the sensitivity of low- mass galaxy properties to stellar feedback and other 2 Smercina et al. baryonic processes. Consequently, this results in a mag- 2017; D'Souza & Bell 2018a; Smercina et al. 2020). nification of small differences in physical prescriptions on Meanwhile, recent evidence suggests that the `classical' predicted galaxy properties in the low-mass regime (e.g., satellite populations of nearby MW-analogs are equally Bullock & Boylan-Kolchin 2017). As processes such as diverse (Geha et al. 2017; Smercina et al. 2018; Bennet stellar feedback can operate on both extremely small et al. 2019; Carlsten et al. 2021). spatial and short time scales, simultaneously achiev- Recent evidence from the Local Group (LG) sug- ing the required spatial and temporal resolution for the gests that the diversity in both of these fundamental complex associated physics is extremely difficult (e.g., properties of galactic systems may be related. Stud- Hopkins et al. 2014). Thus, modeling galaxy formation ies of the MW's satellites using Gaia data and detailed at the smallest scales, and therefore investigating the orbital modeling postulate that a number of its satel- model-based links between small- and large-scale galaxy lites may have been brought in during the infall of the formation, has proved challenging to date. Large Magellanic Cloud system (e.g., Gaia Collabora- The persistent difficulty of modeling small-scale tion et al. 2018; Kallivayalil et al. 2018; Jahn et al. 2019; galaxy formation has motivated two decades of careful Pardy et al. 2020; Erkal & Belokurov 2020; Patel et al. theoretical and observational study of dwarf galaxies. 2020) | possibly including some of the MW's `classi- Difficult to detect observationally owing to their intrin- cal' satellites, such as Carina and Fornax. Additionally, sic faintness, dwarf galaxies are both the lowest-mass deep resolved star formation histories of the M31's dwarf and most dark matter-dominated galaxies in the Uni- satellites with the Hubble Space Telescope (HST) have verse (e.g., see the recent review by Wechsler & Tinker recently revealed that nearly 50% share a remarkably 2018). Consequently, their properties are critical bench- similar timescale for the `shutdown' of their star forma- marks for our models of galaxy formation. Dwarf galax- tion, 3{6 Gyr ago (Weisz et al. 2019). This is similar to ies are the observational bedrock of our understanding the predicted∼ first infall time of the massive progenitor of small-scale cosmology (e.g., see the recent review by galaxy whose merger with M31 likely formed M31's mas- Bullock & Boylan-Kolchin 2017). sive and structured stellar halo (M32p; Hammer et al. While they are predicted to also exist in isolation, 2018; D'Souza & Bell 2018b). This suggests that the until recently nearly all of the low-mass dwarf galax- merger histories of the MW and M31 may need to be ies readily accessible for observational study were `satel- accounted for in order to explain the properties of their lites' of Milky Way (MW)-mass galaxies (e.g., see Mc- present-day satellite populations. Connachie 2012, for an overview of the Local Group This represents an important new lens through satellite populations) | existing within an environment which to view and test predictions for MW-like systems which is dominated by a much larger central galaxy. from galaxy formation models. Is the infall of satel- This is an important consideration, as the large-scale lites during mergers with galaxies such as the LMC or environments of galaxies like the MW are not static, M32p surprising? In a dark matter only (DMO) view, `closed' boxes | they are complex ecosystems that fre- the overall number of surviving subhalos within a cen- quently experience substantial mergers with other sys- tral halo will primarily be a function of the mass of the tems throughout their lives. central halo (e.g., Kravtsov et al. 2004), similar to what Are galaxies like the MW expected to have expe- has long been seen in the idealized case of rich galaxy rienced similar ensemble merger histories over the course clusters (e.g., Bahcall & Cen 1993). However, due to the of their lives? Quite the opposite! Theoretical modeling self-similarity of CDM, dwarf galaxies are predicted to has long predicted that MW-mass galaxies should have host their own populations of satellites prior to group in- a wide range in their merger histories, and particularly fall (e.g., Deason et al. 2015b; Dooley et al. 2017a,b) and in the largest mergers that they have experienced (e.g., there has been direct confirmation of these `satellites-of- Purcell et al. 2010; Deason et al. 2015a), with many satellites' in both the Local and nearby groups (e.g., galaxies experiencing mergers with progenitors more Deason et al. 2014, 2015a; Smercina et al. 2017). There- massive than the Large Magellanic Cloud (LMC; Stew- fore, these large satellites are entirely expected to bring art et al. 2008). Study of their stellar halos has largely in their own populations of low-mass satellites. Indeed, confirmed this predicted diversity by observing a wide it is expected that the distribution of satellite infall range of stellar halo masses and metallicities around times in MW-like halos should cluster around massive galaxies with masses comparable to the MW, with cor- accretion events (e.g., Deason et al. 2015b; D'Souza & relations between them showing that stars accreted from Bell 2021). Yet, because of the strong halo mass de- the largest past merger dominate the properties of the pendence on the number of preexisting low-mass satel- halo stellar populations (Harmsen et al. 2017; Bell et al. lites, and the fact that all galaxies experience accretions, 3 the default DMO prediction is that the satellite popu- have remained a substantial focus of the field over the lations of MW-mass galaxies should not, overall, corre- last two decades.
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