The ACS Fornax Cluster Survey. III. Globular Cluster Specific Frequencies of Early-Type Galaxies

The globular cluster (GC) specific frequency ($S_N$), defined as the number of GCs per unit galactic luminosity, represents the efficiency of GC formation (and survival) compared to field stars. Despite the naive expectation that star cluster formation should scale directly with star formation, this efficiency varies widely across galaxies. To explore this variation we measure the z-band GC specific frequency ($S_{N,z}$) for 43 early-type galaxies (ETGs) from the Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) Fornax Cluster Survey. Combined with the homogenous measurements of $S_{N,z}$ in 100 ETGs from the HST/ACS Virgo Cluster Survey from Peng et al. (2008), we investigate the dependence of $S_{N,z}$ on mass and environment over a range of galaxy properties. We find that $S_{N,z}$ behaves similarly in the two galaxy clusters, despite the clusters' order-of-magnitude difference in mass density. The $S_{N,z}$ is low in intermediate-mass ETGs ($-20<M_z<-23$), and increases with galaxy luminosity. It is elevated at low masses, on average, but with a large scatter driven by galaxies in dense environments. The densest environments with the strongest tidal forces appear to strip the GC systems of low-mass galaxies. However, in low-mass galaxies that are not in strong tidal fields, denser environments correlate with enhanced GC formation efficiencies. Normalizing by inferred halo masses, the GC mass fraction, $\eta=(3.36\pm0.2)\times10^{-5}$, is constant for ETGs with stellar masses $\mathcal{M}_\star \lesssim 3\times10^{10}M_\odot$, in agreement with previous studies. The lack of correlation between the fraction of GCs and the nuclear light implies only a weak link between the infall of GCs and the formation of nuclei.Comment: 16 pages, 7 figures, 6 tables; accepted by Ap

From Supermassive Black Holes to Dwarf Elliptical Nuclei: a Mass Continuum

Considerable evidence suggests that supermassive black holes reside at the centers of massive galactic bulges. At a lower galactic mass range, many dwarf galaxies contain extremely compact nuclei that structurally resemble massive globular clusters. We show that both these types of central massive objects (CMO's) define a single unbroken relation between CMO mass and the luminosity of their host galaxy spheroid. Equivalently, M_CMO is directly proportional to the host spheroid mass over 4 orders of magnitude. We note that this result has been simultaneously and independently identified by Cote et al. (2006), see also Ferrarese et al. (2006). We therefore suggest that the dE,N nuclei may be the low-mass analogs of supermassive black holes, and that these two types of CMO's may have both developed starting from similar initial formation processes. The overlap mass interval between the two types of CMO's is small, and suggests that for M_CMO > 10^7 M_sun, the formation of a black hole was strongly favored, perhaps because the initial gas infall to the center was too rapid and violent for star formation to occur efficiently.Comment: 4 pages, 2 figures, submitted to ApJ

Induced Nested Galactic Bars Inside Assembling Dark Matter Halos

We investigate the formation and evolution of nested bar systems in disk galaxies in a cosmological setting by following the development of an isolated dark matter (DM) and baryon density perturbation. The disks form within the assembling triaxial DM halos and the feedback from the stellar evolution is accounted for in terms of supernovae and OB stellar winds. Focusing on a representative model, we show the formation of an oval disk and of a first generation of nested bars with characteristic sub-kpc and a few kpc sizes. The system evolves through successive dynamical couplings and decouplings, forcing the gas inwards and settles in a state of resonant coupling. The inflow rate can support a broad range of activity within the central kpc, from quasar- to Seyfert-types, supplemented by a vigorous star formation as a by-product. The initial bar formation is triggered in response to the tidal torques from the triaxial DM halo, which acts as a finite perturbation. This first generation of bars does not survive for more than 4--5 Gyr: by that time the secondary bar has totally dissolved, while the primary one has very substantially weakened, reduced to a fat oval. This evolution is largely due to chaos introduced by the interaction of the multiple non-axisymmetric components.Comment: 4 pages, 4 figures, 1 mpeg animation. To be published by the Astrophysical Journal Letters. The animation can be found at http://www.pa.uky.edu/~shlosman/research/galdyn/movies.html Replaced with an updated version (small text corrections

Radial stability of a family of anisotropic Hernquist models with and without a supermassive black hole

We present a method to investigate the radial stability of a spherical anisotropic system that hosts a central supermassive black hole (SBH). Such systems have never been tested before for stability, although high anisotropies have been considered in the dynamical models that were used to estimate the masses of the central putative supermassive black holes. A family of analytical anisotropic spherical Hernquist models with and without a black hole were investigated by means of N-body simulations. A clear trend emerges that the supermassive black hole has a significant effect on the overall stability of the system, i.e. an SBH with a mass of a few percent of the total mass of the galaxy can prevent or reduce the bar instabilities in anisotropic systems. Its mass not only determines the strength of the instability reduction, but also the time in which this occurs. These effects are most significant for models with strong radial anisotropies. Furthermore, our analysis shows that unstable systems with similar SBH but with different anisotropy radii evolve differently: highly radial systems become oblate, while more isotropic models tend to form into prolate structures. In addition to this study, we also present a Monte-Carlo algorithm to generate particles in spherical anisotropic systems.Comment: 16 pages, 12 figures, accepted for publication in MNRAS (some figures have a lowered resolution

The Existence of Sterile Neutrino Halos in Galactic Centers as an Explanation of the Black Hole mass - Velocity Dispersion Relation

If sterile neutrinos exist and form halos in galactic centers, they can give rise to observational consequences. In particular, the sterile neutrinos decay radiatively and heat up the gas in the protogalaxy to achieve hydrostatic equilibrium, and they provide the mass to form supermassive blackholes. A natural correlation between the blackhole mass and velocity dispersion thus arises $\log(M_{BH,f}/M_{\odot})=\alpha \log (\sigma /200 {\rm km s^{-1}})+ \beta$ with $\alpha \approx 4$ and $\beta \approx 8$.Comment: Accepted in Ap

AGN heating, thermal conduction and Sunyaev-Zeldovich effect in galaxy groups and clusters

(abridged) We investigate in detail the role of active galactic nuclei on the physical state of the gas in galaxy groups and clusters, and the implications for anisotropy in the CMB from Sunyaev-Zeldovich effect. We include the effect of thermal conduction, and find that the resulting profiles of temperature and entropy are consistent with observations. Unlike previously proposed models, our model predicts that isentropic cores are not an inevitable consequence of preheating. The model also reproduces the observational trend for the density profiles to flatten in lower mass systems. We deduce the energy E_agn required to explain the entropy observations as a function of mass of groups and clusters M_cl and show that E_agn is proportional to M_cl^alpha with alpha~1.5. We demonstrate that the entropy measurements, in conjunction with our model, can be translated into constraints on the cluster--black hole mass relation. The inferred relation is nonlinear and has the form M_bh\propto M_cl^alpha. This scaling is an analog and extension of a similar relation between the black hole mass and the galactic halo mass that holds on smaller scales. We show that the central decrement of the CMB temperature is reduced due to the enhanced entropy of the ICM, and that the decrement predicted from the plausible range of energy input from the AGN is consistent with available data of SZ decrement. We show that AGN heating, combined with the observational constraints on entropy, leads to suppression of higher multipole moments in the angular power spectrum and we find that this effect is stronger than previously thought.Comment: accepted for publication in The Astrophysical Journa

Discovery of an active supermassive black hole in the bulge-less galaxy NGC 4561

We present XMM-Newton observations of the Chandra-detected nuclear X-ray source in NGC 4561. The hard X-ray spectrum can be described by a model composed of an absorbed power-law with Gamma= 2.5^{+0.4}_{-0.3}, and column density N_H=1.9^{+0.1}_{-0.2} times 10^{22} atoms cm^{-2}. The absorption corrected luminosity of the source is L(0.2 - 10.0 keV) = 2.5 times 10^{41} ergs s^{-1}, with bolometric luminosity over 3 \times 10^{42} ergs s^{-1}. Based on the spectrum and the luminosity, we identify the nuclear X-ray source in NGC 4561 to be an AGN, with a black hole of mass M_BH > 20,000 solar masses. The presence of a supermassive black hole at the center of this bulge-less galaxy shows that black hole masses are not necessarily related to bulge properties, contrary to the general belief. Observations such as these call into question several theoretical models of BH--galaxy co-evolution that are based on merger-driven BH growth; secular processes clearly play an important role. Several emission lines are detected in the soft X-ray spectrum of the source which can be well parametrized by an absorbed diffuse thermal plasma with non-solar abundances of some heavy elements. Similar soft X-ray emission is observed in spectra of Seyfert 2 galaxies and low luminosity AGNs, suggesting an origin in the circumnuclear plasma.Comment: To appear in Ap

Supermassive Black Holes and Galaxy Formation

The formation of supermassive black holes (SMBH) is intimately related to galaxy formation, although precisely how remains a mystery. I speculate that formation of, and feedback from, SMBH may alleviate problems that have arisen in our understanding of the cores of dark halos of galaxies.Comment: Talk at conference on Matter in the Universe, March 2001, ISSI Ber