Hubble Achieves 50% Completeness Mapping Stellar Populations in M96’s Halo

Scientists are now probing the furthest reaches of spiral galaxy M96 to understand how such galaxies assemble their stellar halos. J. Christopher Mihos (Case Western Reserve University), Patrick R. Durrell (Youngstown State University), and Brian Malkan (Case Western Reserve University), alongside Aaron E. Watkins et al., present deep Hubble Space Telescope imaging revealing the properties of stars in M96’s extreme outer halo, some 50 kiloparsecs from the galactic centre. This research is significant because it establishes a surprisingly metal-poor halo for a galaxy of its mass, potentially indicating a unique accretion history differing from other spirals in our local universe and challenging current models of galactic formation.

Their imaging campaign focused on two fields positioned 50 kiloparsecs from the galaxy’s centre, achieving a photometric completeness limit of F814W = 28.0, surpassing the red giant branch tip by nearly two magnitudes. In both observed fields, the research team definitively detected red giant stars within M96’s halo, with a density corresponding to a broadband surface brightness of 31.7 mag arcsec−2. Notably, the study revealed minimal variation in the spatial density or colour of these red giant branch stars between the two fields examined.

Utilising isochrone matching techniques, researchers derived a median metallicity of [M/H] = -1.36 for the red giants, with an interquartile spread of ±0.75 dex, providing insights into the chemical composition of the halo stars. Adopting a power-law radial density profile, the team calculated a total halo mass of 7.8+17.4−4.9 × 109 M⊙ for M96, implying a stellar halo mass fraction of 15+33−9 %, placing it on the higher end of the range observed for spiral galaxies, although with considerable uncertainty. This suggests a substantial contribution from accreted stellar material to the galaxy’s halo. The study further reveals that M96 deviates from the established stellar halo mass-metallicity relationship typically observed in spiral galaxies, exhibiting a halo that is notably metal-poor given its overall halo mass.
While acknowledging the potential influence of systematic effects, the team proposes that this offset may indicate a significantly different accretion history for M96 compared to other spirals in the nearby universe. This finding opens avenues for further investigation into the unique evolutionary path of M96 and its interaction within the Leo I group. This work establishes a detailed characterisation of the outer halo of M96, providing crucial data for understanding the build-up of stellar halos through galactic accretion. The team’s precise measurements of stellar populations and halo mass contribute to a growing body of evidence linking a galaxy’s accretion history to the properties of its stellar halo, offering a new perspective on galaxy evolution. The research targeted two distinct fields positioned 50 kpc from the galactic centre, achieving a photometric completeness limit of F814W = 28.0, representing a sensitivity nearly two magnitudes fainter than the red giant branch tip. Within both fields, the study unequivocally detected red giant stars belonging to M96’s halo, exhibiting a density corresponding to a broadband surface brightness of 31.7 mag arcsec⁻². Careful analysis revealed minimal variation in the spatial density or colour of the red giant stars between the two observed fields.

Researchers utilised isochrone matching techniques to determine a median metallicity of [M/H] = -1.36 for the red giants, with an interquartile spread of 0.75 dex, providing a precise measurement of their chemical composition. Adopting a power-law radial density profile, the team derived a total halo mass of M , enabling an estimation of the stellar halo mass fraction at %. This value places M96 on the higher end of the range observed for spiral galaxies, although the calculation carries inherent uncertainty. The study pioneered a detailed comparison of M96’s halo mass and metallicity with established relationships for spiral galaxies, revealing a significant offset.

M96 exhibits a distinctly metal-poor halo given its substantial halo mass, challenging conventional expectations. This innovative approach involved meticulous photometric calibration and background subtraction to ensure accurate stellar counts and colour measurements. While acknowledging potential systematic effects, the findings suggest a potentially unique accretion history for M96, differing markedly from other spirals in the local universe. The team’s precise measurements and careful modelling provide a robust foundation for future investigations into the formation and evolution of galactic halos.

M96 Halo Red Giants Show Uniform Distribution

Scientists utilising deep Hubble imaging have investigated stellar populations within the outer halo of the spiral galaxy M96, situated in the dynamically active Leo I galaxy group. Their imaging targeted two fields at a projected distance of 50 kiloparsecs from the galaxy’s centre, achieving a 50% photometric completeness limit of F814W = 28.0, which is nearly two magnitudes fainter than the tip of the red giant branch. In both observed fields, the team detected a clear population of red giant stars in M96’s halo, corresponding to an equivalent broadband surface brightness of 31.7 magnitudes per square arcsecond. Experiments revealed minimal variation in the spatial density or colour of the red giant branch stars between the two fields studied.

Through isochrone matching, researchers derived a median metallicity of [M/H] = -1.36 for the red giants, with an interquartile spread of 0.75 dex, indicating a relatively narrow range of metallicities within the observed stellar population. Adopting a power-law radial density profile, the study calculated a total halo mass of 7.8+17.4 −4.9 × 109 solar masses for M96. Measurements confirm a stellar halo mass fraction of 15+33 −9 %, placing M96 on the high end of the range observed for spiral galaxies, although this value carries significant uncertainty. The breakthrough delivers a finding that M96 deviates from the established stellar halo mass-metallicity relationship for spirals, exhibiting a halo that is notably metal-poor given its halo mass. While systematic effects could contribute to this offset, the results suggest a potentially distinct accretion history for M96 compared to other spirals in the nearby universe. Further research may confirm whether M96’s unique properties stem from an unusual past of galactic mergers and interactions.

M96 Halo Stars Show Consistent Metallicity gradients

Scientists have investigated the stellar populations within the outer halo of the spiral galaxy M96, a member of the Leo I galaxy group, utilising deep Hubble imaging. Observations targeted two fields situated 50 kiloparsecs from the galactic centre, achieving a photometric completeness limit of F814W = 28.0, significantly fainter than the red giant branch tip. Red giant stars were clearly detected in M96’s halo in both fields, corresponding to a surface brightness of 31.7 mag arcsec⁻². No substantial differences were found in the spatial density or colour of the red giant stars between the two observed fields.

Isochrone matching yielded a median metallicity of [M/H] = -1.36 for the red giants, with an interquartile spread of 0.75 dex. A power-law radial density profile suggests a total halo mass of M, resulting in a stellar halo mass fraction of %, which is relatively high for spiral galaxies, although this value carries considerable uncertainty. The research indicates that M96 deviates from the established stellar halo mass-metallicity relationship for spirals, exhibiting a halo that is notably metal-poor given its halo mass. While systematic effects could account for this offset, the findings potentially suggest a unique accretion history for M96 compared to other nearby spiral galaxies.

This study establishes a detailed characterisation of the outer stellar halo of M96, providing constraints on its mass and metallicity. The unexpectedly low metallicity for its halo mass is a significant result, challenging current understanding of halo formation and evolution in spiral galaxies. The authors acknowledge uncertainties in the derived halo mass and metallicity, stemming from the assumptions inherent in the modelling and the limited spatial extent of the observations. Future research could focus on extending the observations to larger radii and utilising different methods to determine the halo’s mass and metallicity, potentially confirming or refining the current findings and further elucidating the accretion history of M96.