John Kormendy

The centers of early-type galaxies with HST. IV. Central parameter relations

We analyze Hubble Space Telescope surface-brightness profiles of 61 elliptical galaxies and spiral bulges (hot galaxies). Luminous hot galaxies have cuspy cores with steep outer power-law profiles that break at r ~ r_b to shallow inner profiles with logslope less than 0.3. Faint hot galaxies show steep, largely featureless power-law profiles at all radii and lack cores. The centers of power-law galaxies are up to 1000 times denser in mass and luminosity than the cores of large galaxies at a limiting radius of 10 pc. At intermediate magnitudes (-22.0 < M_V < -20.5), core and power-law galaxies coexist, and there is a range in r_b at a given luminosity of at least two orders of magnitude. Central properties correlate with global rotation and shape: core galaxies tend to be boxy and slowly rotating, whereas power-law galaxies tend to be disky and rapidly rotating. The dense power-law centers of disky, rotating galaxies are consistent with their formation in gas-rich mergers. The parallel proposition that cores are simply the by-products of gas-free stellar mergers is less compelling. For example, core galaxies accrete small, dense, gas-free galaxies at a rate sufficient to fill in low-density cores if the satellites survived and sank to the center. An alternative model for core formation involves the orbital decay of massive black holes (BHs): the BH may heat and eject stars from the center, eroding a power law if any exists and scouring out a core. An average BH mass per spheroid of 0.002 times the stellar mass yields reasonably good agreement with the masses and radii of observed cores and in addition is consistent with the energetics of AGNs and kinematic detections of BHs in nearby galaxies.

A family of models for spherical stellar systems

We describe a one-parameter family of models of stable sperical stellar systems in which the phase-space distribution function depends only on energy. The models have similar density profiles in their outer parts (rho propotional to r(exp -4)) and central power-law density cusps, rho proportional to r(exp 3-eta), 0 less than eta less than or = 3. The family contains the Jaffe (1983) and Hernquist (1990) models as special cases. We evaluate the surface brightness profile, the line-of-sight velocity dispersion profile, and the distribution function, and discuss analogs of Kings core-fitting formula for determining mass-to-light ratio. We also generalize the models to a two-parameter family, in which the galaxy contains a central black hole; the second parameter is the mass of the black hole. Our models can be used to estimate the detectability of central black holes and the velocity-dispersion profiles of galaxies that contain central cusps, with or without a central black hole.

The centers of early-type galaxies with HST. II. Empirical models and structural parameters

We present a set of structural parameters for the central parts of 57 early-type galaxies observed with the Planetary Camera of the Hubble Space Telescope. These parameters are based on a new empirical law that successfully characterizes the centers of early type galaxies. This empirical law assumes that the surface brightness profile is a combination of two power laws with different slopes gamma and beta for the inner and outer regions. Conventional structural parameters such as core radius and central surface brightness are replaced by break radius r_b, where the transition between power-law slopes takes place, and surface brightness mu_b at that radius. An additional parameter alpha describes the sharpness of the break. The structural parameters are derived using a chi-squared minimization process applied to the mean surface brightness profiles. The resulting model profiles generally give very good agreement to the observed profiles out to the radius of 10 arcseconds imaged by the Planetary Camera. Exceptions include galaxies which depart from pure power-laws at large radius, those with strong nuclear components, and galaxies partly obscured by dust. The uncertainties in the derived parameters are estimated using Monte-Carlo simulations which test the stability of solutions in the face of photon noise and the effects of the deconvolution process. The covariance of the structural parameters is examined by computing contours of constant chi squared in multi-dimensional parameter space.

Evidence for a supermassive black hole in NGC 3115

Stellar rotation velocities and velocity dispersions along the major and minor axes of the edge-on S0 galaxy NGC 3115 have been measured in effective seeing σ * =0″.42-0″.69 with the Canada-France-Hawaii Telescope. Also, V-band surface photometry has been obtained in seeing σ * =0″.33-0″.48. The kinematic data show an unresolved rise in rotation velocity to V=165±4 km s −1 at 2″ and a 62% increase in velocity dispersion at r<2″. The central dispersion, σ=295±9 km s −1 , is well above the scatter in the Faber-Jackson correlation between σ and bulge luminosity

The dark matter halos of Draco and Ursa Minor

Published density profiles and central velocity dispersions place important constraints on the stellar velocity ellipsoid and on the distribution of dark matter (DM) in the dwarf spheroidal galaxies Draco and Ursa Minor. Central velocity dispersions of 9 km/s are adopted for Draco and 11 km/s for Ursa Minor. Then, for an isotropic stellar velocity distribution, the central DM densities are 0.8 and 1.0 solar mass/cu pc, respectively, if visible and dark matter have the same core radius. If DM has a much larger core radius than visible matter but nevertheless dominates the potential, these densities are reduced by a factor of 2. Central DM densities can be lower than this only if the stellar velocity distribution is anisotropic. Simple two-component King models are used to investigate this and to look for the smallest DM densities that are consistent with the observations. 36 refs.

Correlations between r1/4-Law Parameters for Bulges and Elliptical Galaxies

The correlation between the effective radius r e and surface brightness μ e for elliptical galaxies is a fundamental scaling law that theories of galaxy formation must explain. When r e and μ e are derived by fitting two-parameter fitting functions such as the de Vaucouleurs r 1/4 law to brightness profiles, the errors in the parameters are strongly coupled. The purpose of this paper is to rederive the μ e (log r e) relations for ellipticals and bulges, taking account of the coupling in the errors and using only high-accuracy CCD data. Our preliminary conclusions are: (1) The coupled errors are too small to affect significantly the correlation derived ior elliptical galaxies. (2) The correlation for bulges is not very different from that for ellipticals, but the galaxy sample is small and the errors in the parameters are large due to the inherent uncertainty in bulge-disk decomposition.

Supermassive black holes in nuclei of galaxies

We review the motivation and search for supermassive black holes (BHs) in galaxies. Energetic nuclear activity provides indirect but compelling evidence for BH engines. Ground-based dynamical searches for central dark objects are reviewed in Kormendy & Richstone (1995, ARA&A, 33, 581). Here we provide an update of results from the Hubble Space Telescope (HST). This has greatly accelerated the detection rate. As of 2001 March, dynamical BH detections are available for at least 37 galaxies. The demographics of these objects lead to the following conclusions: (1) BH mass correlates with the luminosity of the bulge component of the host galaxy, albeit with considerable scatter. The median BH mass fraction is 0.13% of the mass of the bulge. (2) BH mass correlates with the mean velocity dispersion of the bulge inside its effective radius, i.e., with how strongly the bulge stars are gravitationally bound to each other. For the best mass determinations, the scatter is consistent with the measurement errors. (3) B...

An HST Survey of Cores of Early-Type Galaxies 1

Photometry of the central parts of bulges and elliptical galaxies with the Hubble Space Telescope (HST) confirms and extends ground-based results. Most giant ellipticals have cuspy cores: at the “break radius”r b (formerly the core radius r c), the steep outer surface brightness profile turns down to a shallow inner power law \(I(r)\propto r^{-\gamma}\), \(0\leq \gamma \lesssim 0.25\). The corresponding slope of the deprojected profile is derived; the flattest cores allow box orbits to survive. Cores continue to satisfy fundamental plane parameter correlations like those found from the ground. In particular, HST confirms that the luminosity sequence of elliptical galaxies (from cDs to M 32) is physically unrelated to spheroidal galaxies like Fornax. The latter are closely related to late-type dwarfs. Low-luminosity ellipticals do not show cores: \(0.5 \lesssim \gamma \lesssim 1.3\). The most important new result is that global and core properties both show signs of a dichotomy between (i) low-luminosity ellipticals that rotate rapidly, that are nearly isotropic and oblate-spheroidal, that have disky-distorted isophotes, and that are coreless and (ii) giant ellipticals that are essentially nonrotating, anisotropic, and moderately triaxial, that are boxy-distorted, and that have cuspy cores.

Kinematics of ten early-type galaxies from HST and ground-based spectroscopy

We present stellar kinematics for a sample of 10 early-type galaxies observed using the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope, and the Modular Spectrograph on the MDM Observatory 2.4-m telescope. These observations are a part of an ongoing program to understand the co-evolution of supermassive black holes and their host galaxies. Our spectral ranges include either the calcium triplet absorption lines at 8498, 8542, and 8662 A, or the Mg b absorption at 5175 A. The lines are used to derive line-of-sight velocity distributions (LOSVDs) of the stars using a Maximum Penalized Likelihood method. We use Gauss-Hermite polynomials to parameterize the LOSVDs and find predominant ly negative h4 values (boxy distributions) in the central regions of our galaxies. One galaxy, NGC 4697, has significantly positive central h4 (high tail weight). The majority of galaxies have a central velocity dispersion excess in the STIS kinematics over ground-based velocity dispersions. The galaxies with the strongest rotational su pport, as quantified with vMAX/σST IS, have the smallest dispersion excess at STIS resolution. The best-fitting, general, axisymmetric dynamical models ( described in a companion paper) require black holes in all cases, with masses ranging from 10 6.5 to 10 9.3 M⊙. We replot these updated masses on the Mbh - σ relation, and show that the fit to only these 10 galaxies has a slope consi stent with the fits to larger samples. The greatest outlier is NGC 2778, a dwarf elliptical with relatively poor ly constrained black hole mass. The two best candidates for pseudobulges, NGC 3384 and 7457, do not deviate significa ntly from the established relation between Mbh and σ. Neither do the three galaxies which show the most evidence of a recent merger, NGC 3608, 4473, and 4697. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: kinema tics and dynamics

The Core of the M 87 Globular Cluster System

We have observed the central distribution of globular clusters in M 87. The core radius of the cluster system is an order of magnitude larger than that of the underlying galaxy.