Hubble Space Telescope STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble Type

By J. Rossa, R. P. van der Marel, T. Böker, J. Gerssen, Luis C. Ho, Hans-Walter Rix, J. Shields, C. Walcher. 2006

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We study the nuclear star clusters (NCs) in spiral galaxies of various Hubble types using spectra obtained with the STIS on board the Hubble Space Telescope (HST). We observed the nuclear clusters in 40 galaxies, selected from two previous HST WFPC2 imaging surveys. At a spatial resolution of ∼0.″2 the spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra. Approximately half of the spectra have a sufficiently high signal-to-noise ratio for detailed stellar population analysis. For the other half we only measure the continuum slope, as quantified by the B - V color. To infer the star formation history, metallicity, and dust extinction, we fit weighted superpositions of single-age stellar population templates to the high signal-to-noise ratio spectra. We use the results to determine the luminosity-weighted age, mass-to-light ratio, and masses of the clusters. Approximately half of the sample clusters contain a population younger than 1 Gyr. The luminosity-weighted ages range from 10 Myr to 10 Gyr. The stellar populations of NCs are generally best fit as a mixture of populations of different ages. This indicates that NCs did not form in a single event, but that instead they had additional star formation long after the oldest stars formed. On average, the sample clusters in late-type spirals have a younger luminosity-weighted mean age than those in early-type spirals ( = 8.37 ± 0.25 vs. 9.23 ± 0.21). The average mass-weighted ages are older by ∼0.7 dex, indicating that there often is an underlying older population that does not contribute much light but does contain most of the mass. The average cluster masses are smaller in late-type spirals than in early-type spirals ( = 6.25 ± 0.21 vs. 7.63 ± 0.24) and exceed the masses typical of globular clusters. The cluster mass correlates loosely with total galaxy luminosity. It correlates more strongly with both the Hubble type of the host galaxy and the luminosity of its bulge. The latter correlation has the same slope as the well-known correlation between supermassive black hole mass and bulge luminosity. The properties of both nuclear clusters and black holes in the centers of spiral galaxies are therefore intimately connected to the properties of the host galaxy, and in particular its bulge component. Plausible formation scenarios have to account for this. We discuss various possible selection biases in our results, but conclude that none of them can explain the differences seen between clusters in early- and late-type spirals. The inability to infer spectroscopically the populations of faint clusters does introduce a bias toward younger ages, but not necessarily toward higher masses.

Subfields: Astronomy and Astrophysics, Instrumentation

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Stellar Populations in the Nuclei of Late-Type Spiral Galaxies (2006, background): “This is lower than for the nuclear regions in earlier-type galaxies (see e.g., Sarzi et al. 2005 and Rossa et al. 2006).”
Masses and scaling relations for nuclear star clusters, and their co-existence with central black holes (2016, unknown): “…obtained from spectroscopic 4 We found no significant differences in the best-fit parameters when including these NSCs in the fits, using weights that are inversely proportional to their uncertainty. analysis (from stellar population fitting, Rossa et al. 2006, or line widths, Walcher et al. 2006).”
The Coincidence of Nuclear Star Clusters and Active Galactic Nuclei (2008, methodology): “…NC and galaxy luminosities or masses have been shown for samples of galaxies of a single type (Carollo et al. 1998; Böker et al. 2004; Côté et al. 2006; Rossa et al. 2006; Ferrarese et al. 2006; Wehner & Harris 2006), this is the first time they have been compared across all Hubble types.”
Surface brightness profile of the Milky Way’s nuclear star cluster (2014, background): “The majority of NSCs have mixed old and young stellar populations and show frequently signs of star formation within the past 100 Ma (e.g., Rossa et al. 2006; Seth et al. 2006; Walcher et al. 2006).”
Multiple populations in globular clusters (2012, background): “…central black hole in galactic nuclei, and might explain the origin of the nuclear star clusters observed in almost all late type galaxies including the Milky Way (Böker 2010) where there seems to be repeated episodes of star formation (Krabbe et al. 1995; Rossa et al. 2006; Walcher et al. 2006).”
A Log-Quadratic Relation for Predicting Supermassive Black Hole Masses from the Host Bulge Sérsic Index (2006, result): “Similar results using host bulge mass, rather than Sérsic index, have recently been reported in Côté et al. (2006), Ferrarese et al. (2006b), Rossa et al. (2006), and Wehner & Harris (2006).”
Families of dynamically hot stellar systems over 10 orders of magnitude in mass (2011, methodology): “These objects might have already formed more compact than isolated star clusters via recurrent gas inflow, which causes repeated star formation events (e.g. Walcher et al. 2005; Rossa et al. 2006).”
CO-EVOLUTION OF GALACTIC NUCLEI AND GLOBULAR CLUSTER SYSTEMS (2013, background): “In spiral galaxies, nuclei tend to be smaller and younger in latetype galaxies relative to early-type spirals (Rossa et al. 2006).”
THE NGC 404 NUCLEUS: STAR CLUSTER AND POSSIBLE INTERMEDIATE-MASS BLACK HOLE (2010, background): “Kinematically, both NSCs and extra-light can be disky and rotating (Krajnović et al. 2008; Seth et al. 2008b; Trippe et al. 2008) and in early-type galaxies, the stellar populations of many NSCs are not significantly different from their underlying galaxy (Côté et al. 2006; Rossa et al. 2006).”
MERGING BLACK HOLE BINARIES IN GALACTIC NUCLEI: IMPLICATIONS FOR ADVANCED-LIGO DETECTIONS (2016, background): “…NSCs are characterized by complex star formation histories with a mixture of morphological components and different stellar populations spanning a wide range of characteristic ages (from 10 Myr to 10 Gyr) and metallicities (Figer et al. 2004; Rossa et al. 2006; Walcher et al. 2006; Do et al. 2015).”
The Mass Function of Active Black Holes in the Local Universe (2007, background): “…POX 52 may show evidence of nucleation fromHST observations (C. J. Thornton et al., in preparation), but inge - eral it is not possible to predict nuclear cluster luminosity from galaxy luminosity (although the two may be correlated; Ferrarese et al. 2006; Rossa et al. 2006; Wehner & Harris 2006).”
THE NEXT GENERATION VIRGO CLUSTER SURVEY (NGVS). I. INTRODUCTION TO THE SURVEY (2012, background): “As the best approximations found in nature to simple stellar populations (SSPs), such star clusters provide both important probes of galaxy formation and test beds for stellar dynamical and evolutionary models (see, e.g., the reviews of Harris 2001; West et al. 2004; Brodie & Strader 2006; Portegies”
Greatly Enhanced Merger Rates of Compact-object Binaries in Non-spherical Nuclear Star Clusters (2017, background): “Young stellar populations are ubiquitous in NCs that are found in external galaxies (e.g., Carson et al. 2015), suggesting continuous/episodic star formation as a generic mode for NC formation and growth (Rossa et al. 2006; Antonini et al. 2015a,b).”
Nuclear star clusters (2009, methodology): “…relations, subsequent papers examined NSC–galaxy scaling relations using both luminosity and mass (Ferrarese et al. 2006a; Wehner and Harris 2006; Rossa et al. 2006; Balcells et al. 2007; Seth et al. 2008b; Erwin and Gadotti 2012; Scott and Graham 2013; den Brok et al. 2014; Sánchez-Janssen et…”
THE NUCLEUS OF THE SAGITTARIUS DSPH GALAXY AND M54: A WINDOW ON THE PROCESS OF GALAXY NUCLEATION (2008, result): “…episodes of star formation (not observed in extra-nuclear fields, Bellazzini et al. 2006a), pointing to a history of subsequent phases of re-accumulation of gas followed by star formation bursts, very similar to what is observed by Rossa et al. (2006) in a large sample of distant galactic nuclei.”