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-Luminosity functions for observations and model predictions. The long-dashed line shows the observed Milky Way satellite luminosity function corrected for sky coverage and depth effects, while the cyan swath represents the statistical error. The red dotted, green dashed, and blue dotdashed lines represent reionization models of varying z reion = 5, 8, and 12, respectively. Mt is set using the virial temperature, T vir (Mt) = 8 × 10 3 K. The thicker set of lines shows predicted luminosity functions using an extrapolated abundance matching method to assign luminosities to the galaxies. The thinner set of lines use a stellar population synthesis model to predict the luminosities.
Source publication
We use the publicly available subhalo catalogs from the Via Lactea simulation along with a Gpc-scale N-body simulation to understand the impact of inhomogeneous reionization on the satellite galaxy population of the Milky Way. The large-volume simulation is combined with a model for reionization that allows us to predict the distribution of reioniz...
Contexts in source publication
Context 1
... addition to form- ing new satellite galaxies, pushing z reion to later epochs also causes the existing satellites to brighten. Figure 5 compares the luminosity functions from our model with observations. The thicker lines show magnitudes set by the abundance matching method, and the thinner lines by the SPS model. ...
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... high level of agreement between our SPS model and observations comes from tuning the efficiency param- eter, , to match observations independently for all values of z reion . Thus, the three curves in Figure 5 have values = 0.1, 10, and 300M /yr, which were selected for no phys- ical reason other than to match the luminosity function. It is, however, interesting to note that the faint-end slope of this model almost perfectly matches the observations. ...
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... next consider the mass distribution of the satellite galax- ies hosting halos. Figure 7 shows the changes in the v max dis- tribution for satellites as z reion is varied, given a threshold mass of T vir (M t ) = 8 × 10 3 K as in Figure 5. Here, the solid black line shows the distribution for all via Lactea II subhalos, while the red dotted, green dashed, and blue dot-dashed show the distri- butions from our model for three values of z reion = 5, 8, and 12. ...
Context 4
... that these errors assume that the dark matter substructure of the Milky Way is identical to that of the via Lactea II halo; if the Milky Way is more typical for its mass it may have a larger total number of observable satellites. A further success of this model is that, when we constrain z reion using the v max function, we naturally reproduce both the ob- served luminosity function and radial distribution, as shown by the green dashed lines in Figures 5 and 8. We should, however, caution again that much of this depends on the par- ticular realization that is the via Lactea II halo, and that more statistics will be necessary for a more robust prediction. ...
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... red dotted, green dashed, and blue dot-dashed show the effect of changing the reionization time, z reion = 5, 8, 12. We should note that, unlike in Figure 5, we did not tune the parameters α and of equation 2 to reproduce the observational sample at all values of z reion , but just set the pa- rameters to fit the z reion = 9.6 model. Additionally, this plot does not include any observational magnitude cuts, equation 3. ...
Context 6
... functions from our SPS model for both the case of an instantaneous gas heating (thin lines) and a more gradual turnoff as given by equation 6 (thick lines). Colors and line-styles represent different epochs of reionization as in Figure 5 agreement as to the impact the time of reionization has on the satellite galaxy population of a halo, the rate at which star for- mation is squelched can also have a significant impact on the population. ...
Context 7
... the parameter α directly allows us to adjust the slope of the luminosity function and merge the two populations into a single, continuous distribu-tion. In particular, the value α = 2, measured from high mass objects (Conroy & Wechsler 2009;Drory & Alvarez 2008) provides a close fit to the data, Figure 5. Thus, we conclude that a sharp cutoff of star formation can closely match the ob- servational data. ...
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Citations
... This assumption likely does not capture the complexity of real dwarf galaxy populations, in which galaxy formation and evolution are environmentally dependent (e.g., Christensen et al. 2024; also see Danieli et al. 2023 for discussion in the context of Local Volume satellite populations), particularly for low-mass galaxies affected by photoionization (e.g., Benson et al. 2003). Modeling environmental effects is particularly relevant because the large-scale environment of the Local Volume may be unusual (e.g., Neuzil et al. 2020;McAlpine et al. 2022), thereby impacting the expected number of nearby dwarf galaxies and the effects of photoionization on these systems (e.g., Benson et al. 2002;Busha et al. 2010;Li et al. 2014;Dixon et al. 2018). ...
The abundance of faint dwarf galaxies is determined by the underlying population of low-mass dark matter (DM) halos and the efficiency of galaxy formation in these systems. Here, we quantify potential galaxy formation and DM constraints from future dwarf satellite galaxy surveys. We generate satellite populations using a suite of Milky Way (MW)–mass cosmological zoom-in simulations and an empirical galaxy–halo connection model, and assess sensitivity to galaxy formation and DM signals when marginalizing over galaxy–halo connection uncertainties. We find that a survey of all satellites around one MW-mass host can constrain a galaxy formation cutoff at peak virial masses of M 50 = 10 8 M ⊙ at the 1 σ level; however, a tail toward low M 50 prevents a 2 σ measurement. In this scenario, combining hosts with differing bright satellite abundances significantly reduces uncertainties on M 50 at the 1 σ level, but the 2 σ tail toward low M 50 persists. We project that observations of one (two) complete satellite populations can constrain warm DM models with m WDM ≈ 10 keV (20 keV). Subhalo mass function (SHMF) suppression can be constrained to ≈70%, 60%, and 50% that in cold dark matter (CDM) at peak virial masses of 10 ⁸ , 10 ⁹ , and 10 ¹⁰ M ⊙ , respectively; SHMF enhancement constraints are weaker (≈20, 4, and 2 times that in CDM, respectively) due to galaxy–halo connection degeneracies. These results motivate searches for faint dwarf galaxies beyond the MW and indicate that ongoing missions like Euclid and upcoming facilities including the Vera C. Rubin Observatory and Nancy Grace Roman Space Telescope will probe new galaxy formation and DM physics.
... Ultrafaint dwarf (UFD) galaxies around the Milky Way (MW) represent our strongest observational link between cosmic reionization and low-mass galaxy formation. Long-standing theoretical models posit that the ultraviolet (UV) background in the early Universe should prevent or suppress star formation in the lowest-mass dark matter halos (e.g., Efstathiou 1992;Bullock et al. 2000;Benson et al. 2002Benson et al. , 2003Somerville 2002;Bovill & Ricotti 2009;Muñoz et al. 2009;Salvadori & Ferrara 2009;Busha et al. 2010;Tumlinson 2010;Simpson et al. 2013;Wheeler et al. 2015;Jeon et al. 2017). Stellar populations of these "fossil" galaxies were predicted to be largely ancient (z > 6), but colormagnitude diagram (CMD) analysis of classical Local Group dwarfs found virtually all of them to have substantial star formation at younger ages (e.g., Tolstoy et al. 1998;Gallart et al. 1999;Grebel & Gallagher 2004;Monelli et al. 2010bMonelli et al. , 2010ade Boer et al. 2012de Boer et al. , 2014Weisz et al. 2014a;Skillman et al. 2017;Savino et al. 2019). ...
... Studies consistently show that, below a certain halo mass threshold, the post-reionization UV background is able to prevent the accretion of new gas into the cold interstellar medium (ISM) reservoir (e.g., Wheeler et al. 2015;Dayal & Ferrara 2018;Hutter et al. 2021). It has also been proposed that the cold gas already present in the halo might be photoevaporated, thus halting star formation, although the effectiveness of this mechanism is debated (e.g., Barkana & Loeb 1999;Haiman et al. 2001;Hoeft et al. 2006;Busha et al. 2010;Sawala et al. 2010;Simpson et al. 2013;Gutcke et al. 2022). The cosmic UV feedback has been therefore suggested to be the primary quenching mechanism for galaxies with stellar masses as high as 10 6 M e (e.g., Bovill & Ricotti 2009;Salvadori & Ferrara 2009). ...
We present the lifetime star formation histories (SFHs) for six ultrafaint dwarf (UFD; M V > − 7.0, 4.9 < log 10 ( M * ( z = 0 ) / M ⊙ ) < 5.5 ) satellite galaxies of M31 based on deep color–magnitude diagrams constructed from Hubble Space Telescope imaging. These are the first SFHs obtained from the oldest main-sequence turnoff of UFDs outside the halo of the Milky Way (MW). We find that five UFDs formed at least 50% of their stellar mass by z = 5 (12.6 Gyr ago), similar to known UFDs around the MW, but that 10%–40% of their stellar mass formed at later times. We uncover one remarkable UFD, And xiii , which formed only 10% of its stellar mass by z = 5, and 75% in a rapid burst at z ∼ 2–3, a result that is robust to choices of underlying stellar model and is consistent with its predominantly red horizontal branch. This “young” UFD is the first of its kind and indicates that not all UFDs are necessarily quenched by reionization, which is consistent with predictions from several cosmological simulations of faint dwarf galaxies. SFHs of the combined MW and M31 samples suggest reionization did not homogeneously quench UFDs. We find that the least-massive MW UFDs ( M * ( z = 5) ≲ 5 × 10 ⁴ M ⊙ ) are likely quenched by reionization, whereas more-massive M31 UFDs ( M * ( z = 5) ≳ 10 ⁵ M ⊙ ) may only have their star formation suppressed by reionization and quench at a later time. We discuss these findings in the context of the evolution and quenching of UFDs.
... One of the N-body simulations used is the Via Lactea II simulation, which was adopted by Busha et al. (2010) to explore the effects of inhomogeneous reionization on the population of MW satellites. The availability of larger and smaller volume (lower and higher resolutions) realizations of the simulation allowed these authors to assess spatial variations in the epoch of reionization. ...
In this study, we modify the semi-analytic model Galacticus in order to accurately reproduce the observed properties of dwarf galaxies in the Milky Way. We find that reproducing observational determinations of the halo occupation fraction and mass-metallicity relation for dwarf galaxies requires us to include H$_2$ cooling, an updated UV background radiation model, and to introduce a model for the metal content of the intergalactic medium. By fine-tuning various model parameters and incorporating empirical constraints, we have tailored the model to match the statistical properties of Milky Way dwarf galaxies, such as their luminosity function and size$-$mass relation. We have validated our modified semi-analytic framework by undertaking a comparative analysis of the resulting galaxy-halo connection. We predict a total of $300 ^{+75} _{-99}$ satellites with an absolute $V$-band magnitude (M$_{V}$) less than $0$ within $300$ kpc from our Milky Way-analogs. The fraction of subhalos that host a galaxy at least this bright drops to $50\%$ by a halo peak mass of $\sim 8.9 \times 10^{7}$ M$_{\odot}$, consistent with the occupation fraction inferred from the latest observations of Milky Way satellite population.
... Ultra-faint dwarf (UFD) galaxies around the Milky Way (MW) represent our strongest observational link between cosmic reionization and low-mass galaxy formation. Long-standing theoretical models posit that the ultra-violet (UV) background in the early Universe should prevent or suppress star formation in the lowest mass dark matter halos (e.g., Efstathiou 1992;Bullock et al. 2000;Somerville 2002;Benson et al. 2002Benson et al. , 2003Bovill & Ricotti 2009;Muñoz et al. 2009;Salvadori & Ferrara 2009;Busha et al. 2010;Tumlinson 2010;Simpson et al. 2013;Wheeler et al. 2015;Jeon et al. 2017). Stellar populations of these "fossil" galaxies were predicted to be largely ancient (z > 6), but color-magnitude diagram (CMD) analysis of classical Local Group dwarfs found virtually all of them to have substantial star formation at younger ages (e.g., Tolstoy et al. 1998;Gallart et al. 1999;Grebel & Gallagher 2004;Monelli et al. 2010a,b;de Boer et al. 2012;Weisz et al. 2014a;de Boer et al. 2014;Skillman et al. 2017;Savino et al. 2019). ...
... Studies consistently show that, below a certain halo mass threshold, the postreionization UV background is able to prevent the accretion of new gas into the cold inter-stellar medium (ISM) reservoir (e.g., Wheeler et al. 2015;Dayal & Ferrara 2018;Hutter et al. 2021). It has also been proposed that the cold gas already present in the halo might be photoevaporated, thus halting star formation, although the effectiveness of this mechanism is debated (e.g., Barkana & Loeb 1999;Haiman et al. 2001;Hoeft et al. 2006;Busha et al. 2010;Sawala et al. 2010;Simpson et al. 2013;Gutcke et al. 2022). The cosmic UV feedback has been therefore suggested to be the primary quenching mechanism for galaxies with stellar masses as high as 10 6 M ⊙ (e.g., Bovill & Ricotti 2009;Salvadori & Ferrara 2009). ...
We present the lifetime star formation histories (SFHs) for six ultra-faint dwarf (UFD; $M_V>-7.0$, $ 4.9<\log_{10}({M_*(z=0)}/{M_{\odot}})<5.5$) satellite galaxies of M31 based on deep color-magnitude diagrams constructed from Hubble Space Telescope imaging. These are the first SFHs obtained from the oldest main sequence turn-off of UFDs outside the halo of the Milky Way (MW). We find that five UFDs formed at least 50% of their stellar mass by $z=5$ (12.6 Gyr ago), similar to known UFDs around the MW, but that 10-40% of their stellar mass formed at later times. We uncover one remarkable UFD, And XIII, which formed only 10% of its stellar mass by $z=5$, and 75% in a rapid burst at $z\sim2-3$, a result that is robust to choices of underlying stellar model and is consistent with its predominantly red horizontal branch. This ''young'' UFD is the first of its kind and indicates that not all UFDs are necessarily quenched by reionization, which is consistent with predictions from several cosmological simulations of faint dwarf galaxies. SFHs of the combined MW and M31 samples suggest reionization did not homogeneously quench UFDs. We find that the least massive MW UFDs ($M_*(z=5) \lesssim 5\cdot10^4 M_{\odot}$) are likely quenched by reionization, whereas more massive M31 UFDs ($M_*(z=5) \gtrsim 10^5 M_{\odot}$) may only have their star formation suppressed by reionization and quench at a later time. We discuss these findings in the context of the evolution and quenching of UFDs.
... Reionization redshift: the redshift of reionization for the Milky Way and its local environment is set at z reion = 9. This value falls within the range of reionization redshifts calculated by previous works (Bullock et al. 2000;Gnedin 2000;Alvarez et al. 2009;Busha et al. 2010;Iliev et al. 2011;Spitler et al. 2012;Ocvirk et al. 2013;Li et al. 2014;Aubert et al. 2018). ...
Dwarf galaxies are ubiquitous throughout the universe and are extremely sensitive to various forms of internal and external feedback. Over the last two decades, the census of dwarf galaxies in the Local Group and beyond has increased markedly. While hydrodynamic simulations (e.g., FIRE II, Mint Justice League) have reproduced the observed dwarf properties down to the ultrafaints, such simulations require extensive computational resources to run. In this work, we constrain the standard physical implementations in the semianalytic model Galacticus to reproduce the observed properties of the Milky Way satellites down to the ultrafaint dwarfs found in the Sloan Digital Sky Survey. We run Galacticus on merger trees from our high-resolution N –body simulation of a Milky Way analog. We determine the best-fit parameters by matching the cumulative luminosity function and luminosity–metallicity relation from both observations and hydrodynamic simulations. With the correct parameters, the standard physics in Galacticus can reproduce the observed luminosity function and luminosity–metallicity relation of the Milky Way dwarfs. In addition, we find a multidimensional match with half-light radii, velocity dispersions, and mass to light ratios at z = 0 down to M V ≤ −6 ( L ≥ 10 ⁴ L ⊙ ). In addition to successfully reproducing the properties of the z = 0 Milky Way satellite population, our modeled dwarfs have star formation histories that are consistent with those of the Local Group dwarfs.
... The luminosity function (LF) is a fundamental property of galaxy population in general, with the faint-end of LF in the dwarf regime of particular importance as it is expected to be shaped by feedback processes accompanying galaxy formation (e.g. Bullock et al. 2000 ;Benson et al. 2002a ;Somerville 2002 ;Busha et al. 2010 ;Macci ò et al. 2010 ;Font et al. 2011 ;Bose, Deason & Frenk 2018 ;Rey et al. 2019 ;Katz et al. 2020 ). The UFD tail of the LF, which is expected to be particularly sensitive to both outflows and suppression of gas accretion due to UV heating after reionization, is particularly interesting. ...
We use GRUMPY, a simple regulator-type model for dwarf galaxy formation and evolution, to forward model the dwarf galaxy satellite population of the Milky Way (MW) using the Caterpillar zoom-in simulation suite. We show that luminosity and distance distributions of the model satellites are consistent with the distributions measured in the DES, PS1 and SDSS surveys, even without including a model for the orphan galaxies. We also show that our model for dwarf galaxy sizes can simultaneously reproduce the observed distribution of stellar half-mass radii, r1/2, of the MW satellites and the overall r1/2 − M⋆ relation exhibited by observed dwarf galaxies. The model predicts that some of the observed faint stellar systems with r1/2 < 10 pc are ultra-faint dwarf galaxies. Scaling of the stellar mass M⋆ and peak halo mass Mpeak for the model satellites is not described by a power law, but has a clear flattening of M⋆ − Mpeak scaling at Mpeak < 108 M⊙ imprinted by reionization. As a result, the fraction of low mass haloes (Mpeak < 108 M⊙) hosting galaxies with MV < 0 is predicted to be 50 per cent at Mpeak ∼ 3.6 × 107 M⊙. We find that such high fraction at that halo mass helps to reproduce the number of dwarf galaxies discovered recently in the HSC-SSP survey. Using the model we forecast that there should be the total of $440^{+201}_{-147}$ (68 per cent confidence interval) MW satellites with MV < 0 and r1/2 > 10 pc within 300 kpc and make specific predictions for the HSC-SSP, DELVE-WIDE and LSST surveys.
... This scenario helps reproducing the satellite population of the Local Group (e.g. Koposov et al. 2009 ;Mu ˜ noz et al. 2009 ;Busha et al. 2010 ). Observations seem to confirm that low-mass satellites of the Local Group have star formation histories compatible with an early suppression (e.g. Brown et al. 2014 ). ...
Patchy cosmic reionization resulted in the ionizing UV background asynchronous rise across the Universe. The latter might have left imprints visible in present day observations. Several numerical simulation-based studies show correlations between reionization time and overdensities and object masses today. To remove the mass from the study, as it may not be the sole important parameter, this paper focuses solely on the properties of paired haloes within the same mass range as the Milky Way. For this purpose, it uses CoDaII, a fully-coupled radiation hydrodynamics reionization simulation of the local Universe. This simulation holds a halo pair representing the Local Group, in addition to other pairs, sharing similar mass, mass ratio, distance separation and isolation criteria but in other environments, alongside isolated haloes within the same mass range. Investigations of the paired halo reionization histories reveal a wide diversity although always inside-out given our reionization model. Within this model, haloes in a close pair tend to be reionized at the same time but being in a pair does not bring to an earlier time their mean reionization. The only significant trend is found between the total energy at z = 0 of the pairs and their mean reionization time: pairs with the smallest total energy (bound) are reionized up to 50 Myr earlier than others (unbound). Above all, this study reveals the variety of reionization histories undergone by halo pairs similar to the Local Group, that of the Local Group being far from an average one. In our model, its reionization time is ∼625 Myr against 660±4 Myr (z∼8.25 against 7.87±0.02) on average.
... Although reionization is typically imposed in models as a relatively uniform "event," due in large part to the complex subgrid physics involved with the production and escape of ionizing photons from early galaxies (e.g., Ma et al. 2015), it is generally agreed that reionization was likely a localized process, with variations based on localized star formation conditions. These could have affected the formation of very low-mass galaxies (Benson et al. 2003;Busha et al. 2010;Lunnan et al. 2012;Ocvirk et al. 2013). ...
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 eight 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 M V < − 9 satellites within 150 kpc ( N Sat ) 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 – N Sat relation during their current dominant mergers with the Large Magellanic Cloud and M82, respectively. We investigate the presence of this relation in galaxy formation models, including using the Feedback In Realistic Environments (FIRE) simulations to directly compare to the observations. We find no relation between M ⋆,Dom and N Sat in FIRE, and a universally large scatter in N Sat with M ⋆,Dom across simulations—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.
... The luminosity function (LF) is a fundamental property of galaxy population in general, with the faint-end of LF in the dwarf regime of particular importance as it is expected to be shaped by feedback processes accompanying galaxy formation (e.g., Bullock et al. 2000;Benson et al. 2002a;Somerville 2002;Busha et al. 2010;Macciò et al. 2010;Font et al. 2011;Bose et al. 2018;Rey et al. 2019;Katz et al. 2020). The UFD tail of the LF, which is expected to be particularly sensitive to both outflows and suppression of gas accretion due to UV heating after reionization is particularly interesting. ...
We use \texttt{GRUMPY}, a simple regulator-type model for dwarf galaxy formation and evolution, to forward model dwarf galaxy satellite population of the Milky Way (MW) using the Caterpillar zoom-in simulation suite. We show that luminosity and distance distributions of the model satellites are consistent with the distributions measured in the DES, PS1 and SDSS surveys, even without including a model for the orphan galaxies. We also show for the first time that our model for dwarf galaxy sizes can reproduce both the observed {\it distribution} of stellar half-mass radii, $r_{1/2}$, of the MW satellites and the overall $r_{1/2}-M_\star$ relation exhibited by observed dwarf galaxies. The model predicts that some of the observed faint stellar systems with $r_{1/2}<10$ pc are ultra-faint dwarf galaxies. Scaling of the stellar mass $M_\star$ and peak halo mass $M_{\rm peak}$ for the model satellites is not described by a power law, but has a clear flattening of $M_\star-M_{\rm peak}$ scaling at $M_{\rm peak}<10^8\,M_\odot$ imprinted by reionization. As a result, the fraction of low mass haloes ($M_{\rm peak} < 10^8 M_\odot$) hosting galaxies with $M_V<0$ is predicted to be 50\% at $M_{\rm peak} \sim 3.6 \times 10^7\,M_\odot$. We find that such high fraction at that halo mass is in fact required to explain the number of dwarf galaxies discovered recently in the HSC-SSP survey. Using the model we forecast that there should be the total of $440^{+201}_{-147}$ MW satellites with $M_V < 0$ and $r_{1/2} > 10$ pc within 300 kpc and make specific predictions for the HSC-SSP, DELVE-WIDE and LSST surveys.
... This idea appears to be confirmed by the uniformly ancient stellar populations in ultra-faint dwarf galaxies around the MW (e.g., Brown et al. 2014). Although reionization is typically imposed in models as a relatively uniform 'event', due in large part to the complex sub-grid physics involved with the production and escape of ionizing photons from early galaxies (e.g., Ma et al. 2015), it is generally agreed that reionization was likely a localized process, with variations based on localized star formation conditions, that could have affected the formation of very low-mass galaxies (Benson et al. 2003;Busha et al. 2010;Lunnan et al. 2012;Ocvirk et al. 2013). ...
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.
