Inset: HSC image (in the g-band, ∼2.8×1.8 arcmin in size) of the candidate dwarf galaxy MADCASH J074238+652501-dw. The center of NGC 2403 is ∼38′ away, or ∼35 kpc in projection. Background image: color composite from SDSS-III of NGC 2403.

First Results from the MADCASH Survey: A Faint Dwarf Galaxy Companion to the Low-mass Spiral Galaxy NGC 2403 at 3.2 Mpc

Abstract

We report the discovery of the faintest known dwarf galaxy satellite of a Large Magellanic Cloud (LMC) stellar-mass host beyond the Local Group (LG), based on deep imaging with Subaru/Hyper Suprime-Cam. Magellanic Analog Dwarf Companions And Stellar Halos (MADCASH) J074238+652501-dw lies $\sim$35 kpc in projection from NGC 2403, a dwarf spiral galaxy at D$\approx$3.2 Mpc. This new dwarf has $M_g=-7.4\pm0.4$ and a half-light radius of 168$\pm$70 pc, at the calculated distance of 3.39$\pm$0.41 Mpc. The color-magnitude diagram reveals no evidence of young stellar populations, suggesting that MADCASH J074238+652501-dw is an old, metal-poor dwarf similar to low-luminosity dwarfs in the LG. The lack of either detected HI gas ($M_{\rm HI} / L_{\rm V} < 0.69 M_\odot / L_\odot$, based on Green Bank Telescope observations) or GALEX NUV/FUV flux enhancement is consistent with a lack of young stars. This is the first result from the MADCASH survey, which is conducting a census of the stellar substructure and faint satellites in the halos of Local Volume LMC analogs via resolved stellar populations. Models predict a total of $\sim4-10$ satellites at least as massive as MADCASH J074238+652501-dw around a host with the mass of NGC 2403, with 2-3 within our field of view, slightly more than the one such satellite observed in our footprint.

Publication
In The Astrophysical Journal Letters
Date