Unravelling the mass spectrum of destroyed dwarf galaxies with the metallicity distribution function

Abstract

Accreted stellar populations are comprised of the remnants of destroyed galaxies, and often dominate the ‘stellar haloes’ of galaxies such as the Milky Way (MW). This ensemble of external contributors is a key indicator of the past assembly history of a galaxy. We introduce a novel statistical method that uses the unbinned metallicity distribution function (MDF) of a stellar population to estimate the mass spectrum of its progenitors. Our model makes use of the well-known mass-metallicity relation of galaxies and assumes Gaussian MDF distributions for individual progenitors: the overall MDF is thus a mixture of MDFs from smaller galaxies. We apply the method to the stellar halo of the MW, as well as the classical MW satellite galaxies. The stellar components of the satellite galaxies have relatively small sample sizes, but we do not find any evidence for accreted populations with L > Lhost/100. We find that the MW stellar halo has N ∼ 1 − 3 massive progenitors (L ≳ 108L⊙) within 10 kpc, and likely several hundred progenitors in total. We also test our method on simulations of MW-mass haloes, and find that our method is able to recover the true accreted population within a factor of two. Future datasets will provide MDFs with orders of magnitude more stars, and this method could be a powerful technique to quantify the accreted populations down to the ultra-faint dwarf mass-scale for both the MW and its satellites.

Abundance Analysis of Stars at Large Radius in the Sextans Dwarf Spheroidal Galaxy*

“…Another possible scenario is that these star-forming regions could have been actual UFD galaxies. This idea is supported by the recent work of Deason et al (2023), who found that the stellar metallicity distribution of Sextans could allow for the accretion of multiple UFD-like systems. Much larger samples of stars at large radius will be necessary to distinguish among these scenarios.…”

“…Studies by Chiti et al (2021Chiti et al ( , 2023, Filion & Wyse (2021), Longeard et al (2022Longeard et al ( , 2023, Qi et al (2022), Yang et al (2022), and Sestito et al (2023aand Sestito et al ( , 2023b have shown that several dSph and UFD galaxies contain stars near their tidal radii. These extended stellar halos may have formed through dwarf galaxy mergers (Rey et al 2019;Tarumi et al 2021), and multiple mergers may have occurred within individual dSph galaxies around the Milky Way (Griffen et al 2018;Deason et al 2023). These stars frequently exhibit low metallicities, [Fe/H] < −2.…”

Figuring Out Gas and Galaxies in Enzo (FOGGIE). VII. The (Dis)assembly of Stellar Halos

“…Because of their proximity, the stellar halos of the Milky Way and M31 have thus far been our primary sources of data. Both halos are predominantly old and metal-poor (e.g., Unavane et al 1996;Chiba & Beers 2000;Kalirai et al 2006;Carollo et al 2007) but have been found to contain a substantial amount of substructure (Ibata et al 1994(Ibata et al , 2001a(Ibata et al , 2007Majewski et al 1996;Chiba & Beers 2000;Ivezić et al 2000;Yanny et al 2000;Newberg et al 2002;Ferguson et al 2002;Jurić et al 2008;Bell et al 2008;Gilbert et al 2012;Naidu et al 2020), some of which is chemically distinct from the bulk of the halo (e.g., Cohen et al 2018;Deason et al 2023). Although stars that likely originated in the central disks have been found in the stellar halos of both the Milky Way and M31 (e.g., Carollo et al 2007;Bonaca et al 2017), the general consensus is that most of their stars formed in satellites that were eventually tidally disrupted (e.g., Searle & Zinn 1978;Majewski et al 1996;Bullock et al 2001;Purcell et al 2007;Bell et al 2008), and differences between the halos can therefore generally be attributed to differences in their accretion histories.…”