The Stellar Halo and Tidal Streams of Messier 63

Abstract: We present new near-infrared (NIR) observations of M63 from the Extended Disk Galaxy Exploration Science (EDGES) Survey. The extremely deep 3.6 $\mu$m mosaic reaches 29 AB mag arcsec$^{-2}$ at the outer reaches of the azimuthally-averaged surface brightness profile. At this depth the consequences of galactic accretion are found within a nearby tidal stream and an up-bending break in the slope of the surface brightness profile. This break occurs at a semi-major axis length of $\sim$8', and is evidence of either an enhanced outer disc or an inner stellar halo. Simulations of galaxy evolution, along with our observations, support an inner halo as the explanation for the up-bending break. The mass of this halo component is the largest found in an individual galaxy thus far. Additionally, our observations detect a nearby tidal stream. The mass of the stream suggests that a handful of such accretion events are necessary to populate the inner stellar halo. We also find that the accretion rate of the galaxy from the stream alone underestimates the accretion rate required to build M63's inner stellar halo.