The Low-Mass Stellar Initial Mass Function in Nearby Ultra-Faint Dwarf Galaxies
arxiv(2024)
摘要
The stellar initial mass function (IMF) describes the distribution of stellar
masses that form in a given star-formation event. The long main-sequence
lifetimes of low-mass stars mean that the IMF in this regime (below
∼ 1 M_⊙) can be investigated through star counts. Ultra-faint
dwarf galaxies are low luminosity systems with ancient, metal poor stellar
populations. We investigate the low-mass IMF in four such systems (Reticulum
II, Ursa Major II, Triangulum II, and Segue 1), using Hubble Space Telescope
imaging data that reaches to ≲ 0.2 M_⊙ in each galaxy. The
analysis techniques that we adopt depend on the number of low-mass stars in
each sample. We use Kolmogorov-Smirnov tests for all four galaxies to determine
whether their observed apparent magnitude distributions can reject a given
combination of IMF parameters and binary fraction for the underlying
population. We forward model a thousand synthetic populations for each
combination of parameters, and reject those parameters only if each of the
thousand realizations reject the null hypothesis. We find that all four
galaxies reject a variety of IMFs, and the IMFs that they cannot reject include
those that are identical, or similar, to that of the stellar populations of the
Milky Way. We determine the best-fit parameter values for the IMF in Reticulum
II and Ursa Major II and find that the IMF in Reticulum II is generally
consistent with that of the Milky Way, while the IMF in Ursa Major II is more
bottom-heavy. The interpretation of the results for Ursa Major II is
complicated by possible contamination from two known background galaxy
clusters.
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