Investigating M94’s Stellar Halo

Galaxy evolution is driven by both secular processes and merging, both of which contribute to a galaxy’s internal morphology and structure. Yet we still do not understand how structures such as bulges and pseudobulges relate to these processes. The spiral galaxy M94 (NGC 4736) has the largest pseudobulge in our Local Universe (~50% its stellar mass) and is relatively close-by at ~4.2 Mpc, making it an interesting candidate for investigating the role of merging in the formation of its pseudobulge. We present a first-ever look at the stellar halo of M94, which we expect to contain the fossil record of M94’s past mergers. Using Subaru’s Hyper Suprime-Cam (HSC) imager, we resolved stellar populations in M94’s halo in three bands in a ~3 square degree area. We identified red giant branch stars in a color-magnitude diagram of halo stars and found two distinct stellar populations. Using star/galaxy separation techniques and correcting for photometric completeness by using artificial star tests, we produced a stellar mass density and surface brightness map to show that these two populations belong to M94’s accreted halo and its disk, respectively. We find that M94’s stellar halo implies a total accreted mass of ~5×10⁸ M_sun, with a median metallicity of [M/H] = -1.4, and we detect the disk out to 30 kpc. This indicates that M94’s most massive past merger was with a galaxy less massive than the Small Magellanic Cloud. Notwithstanding hosting the largest pseudobulge in the Local Universe, the properties of M94’s stellar halo suggest that it also likely experienced one of the quietest merger histories among galaxies of comparable mass. Therefore, we conclude that M94’s pseudobulge likely does not have a merger origin.