Properties of Nuclear Star Clusters

With densities of ∼ 106−7Mpc−3(Walcher et al.,2005;Misgeld & Hilker,2011;Norris et al.,

2014), NSCs are the densest stellar systems in the universe. They have masses from 105 to 108M, and half-light radii of about 1-10 pc (Georgiev & B¨oker,2014;Georgiev et al.,2016).

These characteristics clearly distinguish them from galaxy bulges. Figure 1.1 shows examples of galaxies hosting NSCs that are easily spotted by eye as highly compact and bright objects. NSCs reside at the photometric center of their host (B¨oker et al., 2002), and coincide with their kinematic center (e.g., in bulgeless galaxies, Neumayer et al., 2011). In the center of the Milky Way, at ∼ 8 kpc from the Sun, resides the closest NSC with a dynamical mass of (2.1 ± 0.7) × 107M (Feldmeier-Krause et al.,2017b). The proximity of the Milky Way NSC

enables resolved studies of its stellar populations and their characterization, including dynamical modeling for the kinematics of this NSC (e.g.,Sch¨odel et al.,2007,2009,2014a;Chatzopoulos et al.,2015;Feldmeier et al.,2014;Feldmeier-Krause et al.,2015,2017a,b).

Figure 1.1 From left to right: NGC 4395, NGC 1042, NGC 3621, NGC 4178. Examples of bulgeless galaxies hosting an NSC in their center. Figure fromB¨oker(2010).

High-resolution Hubble Space Telescope (HST) imaging of nearby galaxies has revealed that NSCs are present in&70% of galaxies across the Hubble sequence (Phillips et al.,1996;Carollo et al.,1998;B¨oker et al.,2002,2004;Cˆot´e et al.,2006;Turner et al.,2012;Georgiev et al.,2009;

Georgiev & B¨oker,2014). Moreover, the nucleation fraction of galaxies (i.e. fraction of galaxies of a certain mass that host an NSC) seems to be similar for different galaxy cluster environments, e.g., Fornax (Turner et al.,2012;Mu˜noz et al.,2015), Virgo (Cˆot´e et al.,2006;S´anchez-Janssen et al.,2018), and Coma (den Brok et al.,2014b). Figure 1.2 shows the nucleation fraction for galaxies in the stellar mass range of 105 to 1012M. The nucleation fraction peaks at galaxy

masses in the range 108−10M, reaching a fraction over 90% for galaxies with masses of 109M

(see alsoGeorgiev et al.,2009;Ordenes-Brice˜no et al.,2018). The nucleation fraction declines toward lower and higher masses from the peak value. In the low-mass regime, the nucleation

Introduction: Nuclear Star Clusters 9

Figure 1.2 Galaxy nucleation fraction versus galaxy stellar mass in different cluster environ- ments: Virgo (Cˆot´e et al.,2006;S´anchez-Janssen et al.,2018), Fornax (Mu˜noz et al.,2015) and Coma (den Brok et al.,2014b) clusters as circles, squares, and triangles, respectively. Figure fromS´anchez-Janssen et al.(2018)

fraction is well described as fn∝ logM∗1/4(S´anchez-Janssen et al.,2018). At masses of 105M

and lower, galaxies do not seem to host an NSC. However, the currently known NSC fractions could be considered as lower limits, since due to limits on the observing capabilities there might be a number of NSCs we are not able to detect.

The size of NSCs correlates with their mass, as well as with the stellar mass of the host galaxy (see left and middle panel of Figure 1.3). Observations of a wide sample of NSCs hosted by early- and late-type galaxies show a difference of ∼ 1.5σ between the size and NSC mass corre- lation for early- and late-type galaxies. This suggests that the correlation is galaxy-type depen- dent, hence at a certain NSC mass, the NSCs hosted by late-type galaxies appear more compact than those found in early-type galaxies. This can be attributed to the still ongoing evolution of NSCs in late-type galaxies (Georgiev & B¨oker,2014;Georgiev et al.,2016).

The masses of NSCs are found to be ∼ 0.1 ± 0.2% of the total stellar mass of the host galaxy (Georgiev et al.,2016), and seems to be correlated with the properties of the host galaxy, e.g., mass, luminosity, velocity dispersion (B¨oker et al., 2004;Cˆot´e et al., 2006; Ferrarese et al.,

2006;Seth et al.,2008;Georgiev et al.,2016). The correlation between the NSC and host mass is also found to be dependent on the galaxy type (see right panel of Figure 1.3). For early-type

Figure 1.3 Left panel: NSC size vs. host stellar mass relation. Middle panel: NSC size vs. NSC mass relation. Right panel: NSC mass vs. host stellar mass relation. For the three panels red circles and black squares represent late- and early-type galaxies, respectively. Gray circles are for NSCs with uncertainties > 100%. Figures fromGeorgiev et al.(2016).

galaxies the correlation is steeper than for late-type galaxies. It is unknown if this difference is caused by a bias in the structural decomposition of the observations or due to real physical effects (Georgiev et al.,2016). The relation between the NSC mass and the stellar mass of the host galaxy is non-linear but seems universal for galaxies in different environments, suggesting that instead of the density of the cluster, the mass of the NSC would depend on the mass of their host (S´anchez-Janssen et al.,2018). In addition, NSC masses show a correlation with the luminosity and mass of the bulge of the host galaxies (Ferrarese et al.,2006;Wehner & Harris,

2006;Erwin & Gadotti,2012;Savorgnan et al.,2016). All these correlations between the host galaxy and its NSC suggest that the NSC properties are not random and might be directly linked to the evolution of the host.

The inability of fitting single stellar population models to the observed spectrum of NSCs sug- gests that they actually host multiple stellar populations with differences in metallicity and age of a few Gyr (e.g.,Walcher et al.,2006;Rossa et al.,2006;Seth et al.,2006;Lyubenova et al.,

2013;Carson et al.,2015;Kacharov et al.,2018). The existence of a young population explains the size of the NSC increasing with the wavelength (Georgiev & B¨oker, 2014;Carson et al.,

2015), as young stellar populations are found to be more centrally concentrated than the old ones (e.g., Seth et al., 2008), as observed in the Milky Way (Do et al.,2013;Lu et al., 2013;

Feldmeier-Krause et al.,2015).