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

TL;DR

This study probes the outer stellar halo of the spiral galaxy M96 (NGC 3368) at ~50 kpc using deep HST ACS and WFC3 imaging to resolve red giant branch stars. By constructing CMDs, applying a robust background subtraction, and fitting 10 Gyr isochrones, the authors derive a metallicity distribution with median [M/H] ≈ -1.36 and a broad spread, and estimate a halo mass of ≈ 7.8 × 10^9 M_sun with a 10-40 kpc mass of ≈ 2.6 × 10^9 M_sun, leading to a halo mass fraction of ≈ 0.15. The results place M96 as an outlier in the halo mass–metallicity relation, being distinctly metal-poor for its halo mass, which could indicate a unique accretion history such as an early massive, metal-poor merger or extensive accretion of low-mass satellites. The authors highlight potential systematic uncertainties (metallicity gradient, halo profile, and substructure) and emphasize the need for additional halo fields to test radial trends and substructure, particularly within the Leo I group context. Overall, the work suggests that spiral halo assembly histories may exhibit substantial scatter beyond what is captured by existing relations, motivating broader spatial sampling of halo populations.

Abstract

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.

Figures (5)

• Figure 1: Placement of the WFC3 and ACS3 imaging fields. The left panel shows the ACS (green) and WFC3 (orange) footprints overlaid on the B-band imaging of M96 from watkins14. The WFC3 and ACS3 images are shown in the center and right panel, respectively. The dotted ellipse in the ACS image shows the location of the ultradiffuse galaxy BST1047+1156 mihos24. The ACS field lies at a projected distance of 15.2 (48.6 kpc) from the center of M96, while the WFC3 field lies at a distance of 16.3 (52.2 kpc). In all images north is up and east is to the left.
• Figure 2: Color-magnitude diagrams for our ACS (left) and WFC3 (center) fields, along with a similarly extracted CMD for the Abell 2744 Flanking Field (right). The latter dataset has been randomly subsampled down by a factor of 0.6 to account for its larger area (see text). The dashed box in each CMD shows the region expected to be populated by metal-poor red giant stars at the distance of M96.
• Figure 3: Spatial distribution of the stellar populations in the ACS and WFC3 fields. The light grey points show all detected point sources, while red dots show red giant stars selected from the dashed RGB boxes in Figure \ref{['cmds']}. In the ACS field, the dotted oval shows the region containing BST1047, and the shaded oval shows the (slightly expanded) mask to exclude sources in this region. However, for clarity in showing the overall distribution of RGB stars in the field, the black dots show RGB stars within the masked region not included in our analysis. In this figure, north is up and east is to the left.
• Figure 4: Left: The color-magnitude diagram for the combined ACS and WFC3 fields, overlaid with 10 Gyr old Parsec 1.2S isochrones bressan12marigo17 of varying metallicity. Right: the inferred M96 halo metallicity distribution function (MDF) for the RGB population selected from stars in the dashed box of the CMD. The halo MDF has been corrected for background contamination based on an analysis of similarly-selected sources in the Abell2744 Flanking Field photometry, shown in red. See text for details.
• Figure 5: The halo mass -- halo metallicity relationship from gozman23, with our measurement of M96 added. Note that the metallicity measurements for the gozman23 points are measured at 30 kpc, while our fields in M96 are at a larger projected distance of 50 kpc.