Hyper Wide Field Imaging of the Local Group Dwarf Irregular Galaxy IC
  1613: An Extended Component of Metal-poor Stars
  Stellar halos offer fossil evidence for hierarchical structure formation.
Since halo assembly is predicted to be scale-free, stellar halos around
low-mass galaxies constrain properties such as star formation in the accreted
subhalos and the formation of dwarf galaxies. However, few observational
searches for stellar halos in dwarfs exist. Here we present gi photometry of
resolved stars in isolated Local Group dwarf irregular galaxy IC 1613
($M_{\star} \sim 10^8 M_{\odot})$. These Subaru/Hyper Suprime-Cam observations
are the widest and deepest of IC 1613 to date. We measure surface density
profiles of young main-sequence, intermediate to old red giant branch, and
ancient horizontal branch stars outside of 12' ($\sim 2.6$ kpc; 2.5 half-light
radii) from the IC 1613 center. All of the populations extend to ~24' ($\sim
5.2$ kpc; 5 half-light radii), with the older populations best fit by a broken
exponential in these outer regions. Comparison with earlier studies sensitive
to IC 1613's inner regions shows that the density of old stellar populations
steepens substantially with distance from the center; we trace the $g$-band
effective surface brightness to an extremely faint limit of $\sim 33.7$ mag
arcsec$^{-2}$. Conversely, the distribution of younger stars follows a single,
shallow exponential profile in the outer regions, demonstrating different
formation channels for the younger and older components of IC 1613. The
outermost, intermediate-age and old stars have properties consistent with those
expected for accreted stellar halos, though future observational and
theoretical work is needed to definitively distinguish this scenario from other
possibilities.
