Title: Globular Clusters in M104: Tracing Kinematics and Metallicities from The Centre to The Halo
Authors: Katja Fahrion, Michael A. Beasley, Eric Emsellem, Anastasia Gvozdenko, Oliver Müller, Marina Rejkuba, and Ana L. Chies-Santos
First Author’s Institution: Department of Astrophysics, University of Vienna, Vienna, Austria
Status: Accepted to A&A, available on arXiv
Globular clusters – ancient, compact systems of tens of thousands of stars – are one of the most powerful tools we can use to study the galaxies which host them. This is not only because globular clusters (GCs) are bright enough to be observed at very large distances, but because they are also old and dense enough to have survived the turbulent merger histories of their host galaxies, making them excellent tracers of galaxy evolution. So, if you want to do a deep dive into the assembly history of a galaxy you’ll need to get as much data on as many of its GCs as possible. The authors in today’s paper set out to do just that for M104, also known as the Sombrero Galaxy.
The Target
M104 is located about 10 Mpc away from us and is the most massive galaxy in its group, coming in at about 2 x 1011 solar masses, which is an order of magnitude more massive than the Milky Way. It also has a very rich GC system, hosting an estimated 1610 GCs, based on previous photometric observations. Photometry is particularly good at finding and identifying GCs, and at distances like these it can also yield their positions, magnitudes, and also average colours if you have multiple filters available.
However, more detailed information about GCs, like their velocities, ages, or metallicites, are also needed to uncover a galaxy’s dynamic history. To get at that, you’ll need spectroscopy too. The trade-off though is that spectroscopic instruments cannot observe as many objects in one go, cannot go as faint as photometry, and tend to have smaller fields of view. The result is that previous spectroscopic catalogs of M104’s GC system have just 360 confirmed GCs, only about 20% of the total population. The authors of this paper have put together data from three different spectrographs (MUSE, FLAMES, and OSIRIS) to expand M104’s spectroscopic GC catalog to 499 GCs.
Measuring Globular Cluster Properties
When spectroscopy is applied to unresolved systems like the GCs in M104, the resulting spectra need to be compared to models, with their ages, metallicites, and velocities varied until the simulated spectra best match the observed spectra for each GC. The authors did this for every candidate GC in their observed fields, removing contaminants such as foreground stars and background galaxies along the way.
Once they had rotational velocities, velocity dispersions, and metallicities for all of the GCs, they could divide them into radial bins to study the properties of M104’s GC system as a whole. They found that GCs which were more centrally concentrated in the galaxy were also more likely to be metal-rich (red in colour), while metal-poor GCs (blue in colour) were more more spread out in the galaxy and dominated at higher radii. They also found that kinematically the blue GCs were more dispersion-dominated, meaning their movements were more random and less coherent with one another, while the red GCs better traced out the kinematics of the other stars in the galaxy.
Both of these are indications that the majority of the red GCs were likely formed in M104 (in-situ), while the blue GCs are more likely to have been originally formed in lower mass, less chemically enriched galaxies that were since accreted onto M104 via galaxy mergers (ex-situ). This is somewhat expected, as similar GC system properties have been observed for other galaxies around the same mass as M104.
On the other hand, M104 has significant scatter in GC metallicities, even at large radii where we would expect to see the blue GCs more exclusively. These red GCs out in M104’s halo could have also been accreted, just from a more massive, metal-rich galaxy, implying M104 may have undergone a major merger in its past. However, more detailed observations would be needed to confirm this.
Outlook
Looking ahead, the authors of this paper plan to use the velocity measurements of their GCs to build a more detailed dynamical model of M104, which can help make more accurate estimates of the galaxy’s total dynamical mass. With their increased catalog size, growing to nearly 30% of the full GC system of M104, this is a significant improvement on previous studies of M104’s stellar halo, and yields new insights into the evolutionary history of the Sombrero galaxy.
Edited by: Skylar Grayson
Featured image credit: Wikimedia Commons
