The Strikingly Metal-rich Halo of the Sombrero Galaxy Suggests a Turbulent PastP. Goudfrooij (goudfroo[at]stsci.edu) and R. E. Cohen (rcohen[at]stsci.edu)
One of the main goals of extragalactic astronomy is constraining the star formation and assembly histories of galaxies. The oldest populations are typically found in outer galactic halos (located tens of kpc from the galaxy centers). Hence, their properties have long been thought to hold important clues to early galaxy assembly. Recent cosmological hydrodynamical simulations have yielded several relevant insights in this context: Cooper et al. (2015) showed that for galaxies with Milky Way-like masses, halos outside of ~20 kpc are dominated by stars that were accreted from other galaxies. Furthermore, similar simulations revealed a correlation between the mean metallicity of spiral galaxy halos and the mass of the most massive galaxy that was accreted in the past (e.g., Deason et al. 2016; D’Souza & Bell 2018).
Hubble plays a crucial role in this field due to its uniquely sharp eye and the high sensitivity of the ACS and WFC3 cameras. Individual red giant stars in galactic halos can be resolved using Hubble out to distances of 15–18 Mpc. Since the colors of these stars are predominantly sensitive to metallicity rather than age, this allows astronomers to determine the metallicity distribution function (MDF) of halo stars. This is a major improvement over the more common use of colors of the diffuse light of distant galaxies, for which the effects of age and metallicity are degenerate, especially in the optical part of the electromagnetic spectrum.
Recent Hubble observations of outer halos of nearby massive galaxies have revealed significant differences between early-type galaxies (elliptical and lenticular galaxies; hereafter ETGs) and spiral galaxies. Starting with the latter, the GHOSTS survey (Harmsen et al. 2017) revealed that Milky Way-size spiral galaxies generally host metal-poor outer halos, with logarithmic metallicities –2 < [Z/H] < –1 (i.e., between 1/100 and 1/10 of solar metallicity). In contrast, the MDFs of ETG halos peak at 3–10 times higher metallicities than those of spiral galaxies of similar total mass (Harris et al. 2007; Rejkuba et al. 2014; Peacock et al. 2015). Using the observed halo mass-metallicity relation in the simulations mentioned above, these metallicities suggest that the most massive galaxies accreted by massive spiral galaxies were small dwarf galaxies with masses in the range of 108–3 x 109 solar masses (for comparison, the mass of the Small Magellanic Cloud is ~7 x 109 solar masses), while the bulk of the accreted halo stars in ETGs came from relatively massive satellite galaxies.
However, while metal-rich stars do dominate ETG halos, they also contain a significant fraction of metal-poor stars (with [Z/H] < –1) at galactocentric radii beyond ~10 effective radii (Reff) of their spheroid (Rejkuba et al. 2014). The presence of this metal-poor component in ETG halos was eagerly anticipated, because populations of globular clusters (GCs) in the outskirts of massive ETGs are typically dominated by metal-poor GCs (e.g., Brodie & Strader 2006). Since GCs are believed to lose a significant fraction of their constituent stars (~10–100%, depending on their initial mass) over a Hubble time due to dynamical evolution in the tidal field of their parent galaxy, metal-poor stars ought to be fairly common in the outer halos as well.
Enter our Hubble observations of the halo of the well-known Sombrero galaxy (M104), located at a distance of 9.5 Mpc. This very massive galaxy (a baryonic mass of ~2 x 1011 solar masses, about twice as massive as our Milky Way) is formally classified as an early-type spiral galaxy, but it really does not fit any standard mold. In addition to its large, dusty spiral disk whose edge-on configuration gives it its nickname, the Sombrero also exhibits a spheroidal structure that is remarkably large relative to those in other spiral galaxies. Usually, such "bulge" structures in spiral galaxies have elliptical shapes. However, a deep Spitzer Space Telescope image revealed that while the inner parts of the Sombrero's spheroid are indeed highly elliptical, the outer parts become increasingly round, strongly suggesting a separate, and significant, halo component (Gadotti & Sánchez-Janssen 2012).
Our Hubble observations of the Sombrero galaxy, published in Cohen et al. (2020) and highlighted recently in HST Press Release 2020-08, Astronomy.com, and AAS Nova, consist of deep images taken with the ACS and WFC3 cameras operated in parallel. The footprints of the images are centered in the halo at radii of 16 and 33 kpc, corresponding to 8 and 17 Reff of the Sombrero's bulge (see Figure 1). At these large radii, the MDF of the GC system of the Sombrero peaks at low metallicities ([Z/H] < –1; see Dowell et al. 2014), as commonly seen in other massive galaxies. However, surprisingly, we find stellar halo MDFs with a very high median metallicity ([Z/H] ~ +0.06 and –0.15 in the inner and outer fields, respectively), and in which the fraction of metal-poor stars with [Z/H] < –1 is negligible (<1%). Figure 2 illustrates this unexpected result by comparing the MDFs of the halo stars in the Sombrero with those of the globular clusters within the same ranges of galactocentric radius.
The main conclusions of our study of the halo of the Sombrero are twofold:
- The strikingly high median metallicity of the Sombrero's halo has an interesting implication. Inserting this metallicity in the aforementioned relation between the metallicity of the outer halo and the mass of the most massive progenitor galaxy, we find a mass of ~1011 solar masses, which is consistent with the mass of Sombrero's halo, and ~half its current total mass. This suggests that we have indirectly witnessed a major (~1:1 ratio) merger between two galaxies in the distant past, producing the Sombrero galaxy as it appears today!
- The extreme scarcity of metal-poor stars in our halo fields in the Sombrero is very puzzling, given the large number of metal-poor GCs at those radii. In the local universe, such a large number of GCs per unit stellar mass is unprecedented among massive galaxies, and only seen in some dwarf spheroidal galaxies (Georgiev et al. 2010; Larsen et al. 2014). However, the latter galaxies are significantly more metal-poor ([Z/H] ~ –2) than the bulk of GCs in the Sombrero. One interesting possibility is that the metal-poor stellar component in the Sombrero is located at even larger radii, or perhaps at different position angles than those covered by our images. Hopefully, that possibility will be studied using future observations with Hubble.
In the more distant future, the Wide Field Infrared Survey Telescope (WFIRST), with a currently estimated launch in 2025, will be able to image a field of view 100 times that of Hubble, at a similarly high spatial resolution. This significant boost in field coverage can be expected to yield major leaps forward in our understanding of galactic halos and galaxy assembly.
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