Forward-modelling the Luminosity, Distance, and Size distributions of the Milky Way Satellites

Abstract

We use 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 distribution of stellar half-mass radii, 𝑟 1/2 , of the MW satellites and the overall 𝑟 1/2 − 𝑀 ★ 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 ★ and peak halo mass 𝑀 peak for the model satellites is not described by a power law, but has a clear flattening of 𝑀 ★ − 𝑀 peak scaling at 𝑀 peak < 10 8 𝑀 imprinted by reionization. As a result, the fraction of low mass haloes (𝑀 peak < 10 8 𝑀 ) hosting galaxies with 𝑀 𝑉 < 0 is predicted to be 50% at 𝑀 peak ∼ 3.6 × 10 7 𝑀 . 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.

Pegasus IV: Discovery and Spectroscopic Confirmation of an Ultra-Faint Dwarf Galaxy in the Constellation Pegasus

“…−13 ultra-faint dwarf galaxies with M V < 0 and r 1/2 > 10 pc across its nominal footprint (δ 2000 < 0 • ; |b| > 10 • ), assuming that DELVE will achieve comparable sensitivity to searches over third-year Dark Energy Survey data (DES Y3; Drlica-Wagner et al 2020). Furthermore, the upcoming Vera C. Rubin Observatory Legacy Survey of Space and Time (Ivezić et al 2019) is expected to discover hundreds of ultra-faint dwarf galaxies both around the Milky Way and beyond (e.g., Hargis et al 2014;Mutlu-Pakdil et al 2021;Trujillo et al 2021;Manwadkar & Kravtsov 2021). This growing sample of ultra-faint dwarf galaxies will undoubtedly provide new constraints on the properties of dark matter and will offer key insight into the process of galaxy formation on the smallest scales.…”

“…Survey efforts including DELVE-WIDE will likely continue to play an important role in this ongoing satellite census. Illustratively, Manwadkar & Kravtsov (2021) recently forecasted that DELVE-WIDE may discover 34 +17…”

“…Despite the explosion of discoveries in the last two decades, cold dark matter simulations predict that numerous ultra-faint Milky Way satellites remain to be discovered, even in regions of sky covered by previous sky surveys (e.g., Hargis et al 2014;Newton et al 2018;Nadler et al 2020;Manwadkar & Kravtsov 2021). This prediction has recently been affirmed by the discovery of three new Milky Way satellite galaxies by the Hyper Suprime-Cam Subaru Strategic Program (Homma et al 2016(Homma et al , 2018(Homma et al , 2019 and four additional satellites (including both dwarf galaxy candidates and globular clusters) by the DECam Local Volume Exploration (DELVE; Drlica-Wagner et al 2021;Mau et al 2020;Cerny et al 2021a,b).…”

Snowmass2021 Cosmic Frontier White Paper: Cosmological Simulations for Dark Matter Physics

“…Simulations that capture the unique assembly history of the Milky Way have also proven crucial to interpret observations of its substructure on larger scales. For example, the Milky Way's largest satellite galaxy, the Large Magellanic Cloud (LMC), is predicted to fall into the Milky Way with its own population of satellites, which has been characterized using zoom-in simulations and is required to explain the anisotropy in the observed satellite population after accounting for observational selection effects [204,237]. The LMC has also significantly perturbed several of the Milky Way's stellar streams, and idealized simulations including the dynamical impact of the LMC have been used to facilitate simultaneous inference of streams' orbital properties and the mass of the LMC [238,239].…”

Detailed chemical abundances of stars in the outskirts of the Tucana II ultra-faint dwarf galaxy

“…The Milky Way's ultra-faint dwarf satellite galaxies (UFDs; L 10 5 L , Bullock & Boylan-Kolchin 2017; Simon 2019) are nearby systems that formed at high redshifts (z 6; Brown et al 2014;Gallart et al 2021;Simon et al 2021), making them local laboratories for studies of early galaxy formation. Characteristics of UFDs, including their frequency, size, and mean metallicity, are linked to our understanding of early feedback (Agertz et al 2020), early galactic assembly (Rey et al 2019;Tarumi et al 2021), reionization and Milky Way assembly (e.g., Manwadkar & Kravtsov 2021), and the nature of dark matter (e.g., Nadler et al 2021). UFDs are also of broader astrophysical importance, as they are the most dark matter dominated stellar systems known (M/L 100 M /L ; Simon 2019) and their most metal-poor stars likely reflect yields from the supernovae of the first stars (Jeon et al 2021).…”