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

Carlin, Jeffrey L., Sand, David J., Price, Paul, Willman, Beth, Karunakaran, Ananthan, Spekkens, Kristine, Bell, Eric F., Brodie, Jean P., Crnojević, Denija, Forbes, Duncan A., Hargis, Jonathan, Kirby, Evan, Lupton, Robert, Peter, Annika H. G., Romanowsky, Aaron J., Strader, Jay

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.