Statistical Predictions of the Accreted Stellar Halos around Milky Way-Like Galaxies

TL;DR

This study presents a SatGen-based semi-analytic framework to statistically model ex-situ stellar halos (ASH) around Milky Way–like galaxies by leveraging large ensembles of merger trees under ΛCDM. It shows that ASHs are typically built from a small number of dominant progenitors, with final halo mass budgets highly sensitive to the fate of the most massive satellite, producing order-unity variations in ${M_{ m ASH}}$ at fixed host mass. The authors quantify how central galaxy mass, ASH mass, and surviving satellites trace different assembly epochs and demonstrate that Random Forest Regression can infer halo assembly histories from observable halo components, albeit with limitations due to halo-mass mixing. The results provide a framework for interpreting upcoming low-surface-brightness observations of stellar halos and for constraining the low-mass end of the stellar-halo mass relation.

Abstract

In the $Λ$CDM paradigm, stellar halos form through the accretion and disruption of satellite galaxies. We introduce new semi-analytic modeling within the SatGen framework to track the ex-situ stellar components of Milky Way--like galaxies across large ensembles of merger trees, enabling a statistical study of the stochastic nature of galaxy assembly. We find that accreted stellar halos are typically built by only a few progenitors and are highly sensitive to the fate of the most massive satellite, producing order-of-magnitude variations in accreted stellar halo mass even at fixed host halo mass. Different stellar components trace distinct phases of host halo growth: central and accreted stellar mass correlate most strongly with early assembly, while surviving satellites trace more recent accretion. Finally, using Random Forest Regression, we quantify how well observable galaxy properties can recover halo assembly histories, providing a framework for interpreting upcoming low-surface-brightness observations of stellar halos.

Figures (16)

• Figure 1: Left panel: The SHMR from Behroozi.etal.19 used to assign stellar masses to all the subhalos in our merger trees at accretion. We show the relation across several decades in dark matter mass despite only populating subhalos between $10^{9} \>{\rm M_{\odot}} \leq m_{\rm acc} \lesssim 10^{12} \>{\rm M_{\odot}}$. For clarity we don't show the constant 0.2 dex scatter that is assumed when drawing stellar masses. For reference we show the SHMR from RodriguezPuebla.etal.17 and the fiducial SHMR from Monzon.etal.24. Right panel: The deterministic relation between star formation rate (SFR) and its maximum circular halo velocity $V_{\rm max}$ from the UniverseMachine Behroozi.etal.19. We use this relation to build up the "in-situ" component of central galaxies in our model by integrating SFRs across time snapshots.
• Figure 2: Left panel: The distribution of our three merger tree samples ($S_0, S_{15}, S_{30}$) in the SHMR plane assuming central galaxies grow according to the $\psi_{\rm SF} - V_{\rm max}$ relation defined in Behroozi.etal.19. For reference we show the present-day SHMR (thick blue line) along with its 0.2 dex scatter (blue shaded region). Right panel: The distribution of halo formation times (z$_{50}$) against present-day central stellar mass. Notice how the $S_0$ sample spans a similar range to the other two samples despite being restricted to present-day DM mass of $10^{12} \>{\rm M_{\odot}}$. This illustrates what we refer to as halo-to-halo variance.
• Figure 3: The stellar tidal tracks defined in Errani.etal.18 that we use to evolve the satellites. Top panel: The effective radius of the satellite as a function of the bound fraction of dark matter mass ($f_b$). The blue lines correspond to an initially cuspy (NFW) density profile for the dark matter. The solid vs. dashed lines indicate the initial sizes of the satellite galaxies relative to their dark matter subhalos as indicated. Bottom panel: The stellar mass of the satellites as a function of the $f_b$. They grey dotted line in both panels indicates our disruption criteria. Notice that, according to these models, a significant fraction of dark matter mass must be lost before any stars can be stripped.
• Figure 4: The fiducial distributions of the main mass components in our model for all three merger tree samples: $S_0, S_{15}, S_{30}$. Left panel: shows the PDFs of the $M_{\rm sat}$ component which are all centered on $\sim 10^8 \>{\rm M_{\odot}}$. Middle panel: the PDFs of the $M_{\rm ASH}$ component which are all centered on $\sim 10^9 \>{\rm M_{\odot}}$. Right panel: shows the PDFs of the $M_{\rm cen}$ component which are all centered on $\sim 10^{10} \>{\rm M_{\odot}}$. Notice that all distributions widen as the amount of host halo mass-mixing increases.
• Figure 5: Top Panel: The stellar masses ($m_{\rm *}$) at accretion vs accretion redshift ($z_{\rm acc}$) for all 1st order satellites in the $S_0$ sample. The grid indicates a simple 15 x 15 binning scheme in which survival probabilities were measured. Here, the lighter yellow cells denote higher probabilities of survival as indicated. Notice that, for a fixed $z_{\rm acc}$, increasing stellar mass coincides with lower survival probability. Bottom Panel: The distribution of accretion redshifts binned by stellar mass. Here each point marks the median and 16-84 percentile, and the color indicates the satellite population. This clearly shows that for a fixed stellar mass, systems that survive are those which were accreted more recently.