Stellar Populations in the Extreme Outer Halo of the Spiral Galaxy M96

We use deep Hubble Space Telescope imaging to study stellar populations in the outer halo of the spiral galaxy M96, located in the dynamically active Leo I galaxy group. Our imaging targets two fields at a projected distance of 50 kpc from the galaxy’s center, with a 50% photometric completeness limit of F814W = 28.0—nearly 2 magnitudes below the tip of the red giant branch (RGB). In both fields, we clearly detect red giant stars in M96’s halo, with a space density that corresponds to an equivalent broadband surface brightness of μV ≈ 31.7 mag arcsec−2. We find little evidence for any differences in the spatial density or color of the RGB stars in the two fields. Using isochrone matching, we derive a median metallicity for the red giants of [M/H] = −1.36, with an interquartile spread of ± 0.75 dex. Adopting a power-law radial density profile, we also derive a total halo mass of Mh=7.8−4.9+17.4×109 M⊙, implying a stellar halo mass fraction of M*, halo/M*, tot≈15−9+33 %—on the high end for spiral galaxies, but with significant uncertainty. Finally, we find that M96 appears offset from the stellar halo mass–metallicity relationship for spirals, with a halo that is distinctly metal-poor for its halo mass. While a variety of systematic effects could have conspired to drive M96 off this relationship, if confirmed our results may argue for a markedly different accretion history for M96 compared to other spirals in the nearby Universe.