Counting the Unseen
Measuring massive black hole (MBH) demographics in low-mass galaxies provides a promising route toward constraining the different theoretical formation mechanisms for MBHs. The advent of the Rubin Observatory (LSST) will allow tidal disruption events (TDEs) to be used in measuring MBH demographics, given the anticipated observation rate of 10 events per night. In addition to a statistically significant sample of TDEs, this measurement requires accurate TDE rate estimations. Specifically, this requires comparing observed TDE rates to theoretical TDE rate models for model galaxy samples. TDE rates are dominated by the distribution of stars near the MBH sphere of influence (the region where the MBH dominates the gravitational potential, pc scales). Therefore, it is crucial that model galaxies used for TDE rate estimates reflect the true distribution of stellar densities on these small scales. We use Hubble-based surface brightness measurements for a large sample of galaxies to measure 3D stellar densities and develop new scaling relations between the pc-scale density structures and host galaxy stellar mass. These relations are used to inform the model galaxy samples with present-day MBH mass distributions based on different theoretical formation mechanisms. The resulting intrinsic TDE rates are then forward-modelled into detection rates for comparison with observations.
Spatially Resolving the Star Formation Histories of Three Nearby Nuclear Star Clusters
The formation of nuclear star clusters (NSCs) remains an open question. In this work, we use spatially-resolved HST/STIS spectroscopic observations of three nearby NSCs (hosted by NGC 5102, NGC 5206, and NGC 205) to constrain their formation histories by exploring radial variations of the stellar populations within each cluster. Utilizing full-spectrum fitting, we find substantial age and metallicity gradients within the central 0.″9 (16 pc) of the NSC in NGC 5102 where populations near the center are young/metal-rich (age ~400 Myr and [M/H] ~ -0.4) and become older/metal-poor at larger radii (mean age ~1 Gyr and mean [M/H] ~ -1.6 in the radial range [0.″3, 0.″9]). This behavior suggests that the young/metal-rich population at the center was formed from a period of in situ formation, while the older/metal-poor populations were likely formed by inspiraled globular clusters. The two broad populations observed in the NGC 5102 NSC (young/metal-rich and old/metal-poor) appear to be linked to the transition between the two morphological components of the NSC derived from the surface-brightness profile in Nguyen et al. (2018). The radial ranges explored in NGC 5206 and NGC 205 were much smaller due to poor data quality; in NGC 5206 we find a similar metallicity gradient to NGC 5102 (but with much lower significance), while the data for NGC 205 is too poor to reach any conclusions. Overall, this data highlights the links between the morphological and stellar population complexity of NSCs and their formation mechanisms.