First results from the MADCASH Survey: A Faint Dwarf Galaxy Companion to the Low Mass Spiral Galaxy NGC 2403 at 3.2 Mpc

TL;DR

This paper reports the discovery of the faintest known dwarf galaxy satellite of an LMC-like galaxy beyond the Local Group, using deep Subaru imaging, revealing an old, metal-poor dwarf with no signs of recent star formation.

Contribution

First detection of a very faint dwarf satellite around a low-mass spiral galaxy beyond the Local Group, demonstrating the effectiveness of deep imaging surveys like MADCASH.

Findings

Discovered a dwarf galaxy at 3.39 Mpc with $M_g = -7.4$

The dwarf shows no evidence of young stars or HI gas

Models predict 4-10 similar satellites around NGC 2403

Abstract

We report the discovery of the faintest known dwarf galaxy satellite of an LMC stellar-mass host beyond the Local Group, based on deep imaging with Subaru/Hyper Suprime-Cam. 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 $168\pm70$ pc, at the calculated distance of $3.39\pm0.41$ 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 Local Group. The lack of either detected HI gas ($M_{\rm HI}/L_{V} < 0.69 M_\odot/L_\odot$, 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 (Magellanic Analog Dwarf Companions And Stellar Halos) 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 $\sim$4-10 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.