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 two magnitudes below the tip of the red giant branch. In both fields we find a clear detection of red giant stars in M96’s halo, with a space density that corresponds to an equivalent broadband surface brightness of $μ_V \approx $ 31.7 mag arcsec$^{-2}$. We find little evidence for any difference 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 $\pm$0.75 dex. Adopting a power-law radial density profile, we also derive a total halo mass of $M_h = 7.8^{+17.4}{-4.9}\times10^9$ M$\odot$, implying a stellar halo mass fraction of $M_{,halo}/M_{,tot} = 15^{+33}_{-9}$%, 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.

💡 Research Summary

This paper presents a deep Hubble Space Telescope (HST) study of the outer stellar halo of the spiral galaxy M96 (NGC 3368), which resides in the dynamically active Leo I group. Two fields were observed with the ACS and WFC3 cameras at projected distances of ~50 kpc from the galaxy centre, using the F606W and F814W filters for a total of 15 HST orbits per field. The data were reduced with the standard drizzlepac pipeline, and point‑source photometry was performed on the individual CTE‑corrected flc images using DOLPHOT. Extensive artificial‑star tests (10⁵ stars per field) quantified completeness and systematic photometric biases; the 50 % completeness limit reaches F814W≈28 mag, roughly two magnitudes below the tip of the red giant branch (TRGB).

Stringent quality cuts (TYPE = 1, crowding < 0.25, S/N > 3.5, CHI < 2.4, and a magnitude‑dependent SHARP criterion) were applied, together with masks for bright foreground stars, background galaxies, chip gaps, and the ultra‑diffuse galaxy BST1047+1156 that lies in the ACS field. After these selections, color‑magnitude diagrams (CMDs) show a clear excess of stars in the region expected for metal‑poor red giant branch (RGB) stars at the distance of M96. A control field (the Abell 2744 Flanking Field) observed with identical depth and filters provides a background estimate; the M96 fields contain ~10 times more sources in the RGB box than the control, while Galactic halo contamination from Milky Way stars is negligible (<0.3 arcmin⁻²).

Metallicities were derived by matching the observed RGB colors to Dartmouth α‑enhanced isochrones. The median metallicity of the halo RGB population is