Hubble Space Telescope Observations of Two Faint Dwarf Satellites of Nearby LMC Analogs from MADCASH

We present a deep Hubble Space Telescope (HST) imaging study of two dwarf galaxies in the halos of Local Volume Large Magellanic Cloud (LMC) analogs. These dwarfs were discovered as part of our Subaru+Hyper Suprime-Cam MADCASH survey: MADCASH-1 is a satellite of NGC 2403 (D similar to 3.2 Mpc), and MADCASH-2 is a previously unknown dwarf galaxy near NGC 4214 (D similar to 3 Mpc). Our HST data reach >3.5 mag below the tip of the red giant branch (TRGB) of each dwarf, allowing us to derive their structural parameters and assess their stellar populations. We measure TRGB distances (DMADCASH-1 = 3.41(-0.2)(-0.24) Mpc, = - DMADCASH-2 = 3.00(-0.15+)(0.13) Mpc), and confirm the dwarfs' associations with their host galaxies. MADCASH-1 is a predominantly old, metal-poor stellar system (age similar to 13.5 Gyr, [M/H]similar to -2.0), similar to many Local Group dwarfs. Modelling of MADCASH-2's color-magnitude diagram suggests that it contains mostly ancient, metal-poor stars (age similar to 13.5 Gyr, [M/H]similar to-2.0), but that similar to 10% of its stellar mass was formed 1.1-1.5 Gyr ago and similar to 1% was formed 400-500Myr ago. Given its recent star formation, we search MADCASH-2 for neutral hydrogen using the Green Bank Telescope, but find no emission and estimate an upper limit on the H I mass of <4.8 x 10(4) M-circle dot. These are the faintest dwarf satellites known around host galaxies of LMC mass outside the Local Group (M-V,M-MADCASH-1 = -7.81 +/- 0.18, M-V,M-MADCASH-2=-9.15 +/- 0.12), and one of them shows signs of recent environmental quenching by its host. Once the MADCASH survey for faint dwarf satellites is complete, our census will enable us to test predictions from cold dark matter models for hierarchical structure formation and discover the physical mechanisms by which low-mass hosts influence the evolution of their satellites.

Automated summary

In this study, two dwarf galaxies in the halos of Large Magellanic Cloud (LMC) analogs were examined using deep Hubble Space Telescope (HST) imaging, revealing their structural parameters and stellar populations. One of the dwarfs showed recent star formation while the other contained mostly ancient, metal-poor stars. These faint dwarf satellites are among the faintest known around host galaxies of LMC mass, and their census will contribute to testing predictions from cold dark matter models and understanding the influence of low-mass hosts on the evolution of satellites.