The Centers of Early-Type Galaxies withHubble Space Telescope. V. New WFPC2 Photometry

Abstract

We present observations of 77 early-type galaxies imaged with the PC1 CCD of the Hubble Space Telescope (HST ) WFPC2. ''Nuker-law'' parametric fits to the surface brightness profiles are used to classify the central structure into ''core'' or ''power-law'' forms. Core galaxies are typically rounder than power-law galaxies. Nearly all power-law galaxies with central ellipticities ! 0:3 have stellar disks, implying that disks are present in powerlaw galaxies with < 0:3 but are not visible because of unfavorable geometry. A few low-luminosity flattened core galaxies also have disks; these may be transition forms from power-law galaxies to more luminous core galaxies, which lack disks. Several core galaxies have strong isophote twists interior to their break radii, although power-law galaxies have interior twists of similar physical significance when the photometric perturbations implied by the twists are evaluated. Central color gradients are typically consistent with the envelope gradients; core galaxies have somewhat weaker color gradients than power-law galaxies. Nuclei are found in 29% of the core galaxies and 60% of the power-law galaxies. Nuclei are typically bluer than the surrounding galaxy. While some nuclei are associated with active galactic nuclei (AGNs), just as many are not; conversely, not all galaxies known to have a low-level AGN exhibit detectable nuclei in the broadband filters. NGC 4073 and 4382 are found to have central minima in their intrinsic starlight distributions; NGC 4382 resembles the double nucleus of M31. In general, the peak brightness location is coincident with the photocenter of the core to a typical physical scale of <1 pc. Five galaxies, however, have centers significantly displaced from their surrounding cores; these may be unresolved asymmetric double nuclei. Finally, as noted by previous authors, central dust is visible in about half of the galaxies. The presence and strength of dust correlates with nuclear emission; thus, dust may outline gas that is falling into the central black hole. The prevalence of dust and its morphology suggest that dust clouds form, settle to the center, and disappear repeatedly on $10 8 yr timescales. We discuss the hypothesis that cores are created by the decay of a massive black hole binary formed in a merger. Apart from their brightness profiles, there are no strong differences between core galaxies and power-law galaxies that demand this scenario; however, the rounder shapes of core, their lack of disks, and their reduced color gradients may be consistent with it.

HST/ACS observations of shell galaxies: inner shells, shell colours and dust

“…Our observations may help to learn more about dust origin and formation theories (Lauer et al 2005).…”

“…Number counts give a ratio of 3:5 for the two types of dust features. They are almost never both seen in one galaxy (Lauer et al 2005), which has been used as evidence for an (episodic) dust settling sequence scenario (Tran et al 2001;Lauer et al 2005;Verdoes Kleijn & de Zeeuw 2005). Here, first the irregular dust patches and lanes form in some way, while some time later these dust features move to the centre and form a disk in dynamic equilibrium.…”

“…Using HST archival data, about half of all elliptical galaxies exhibit visible dust features, which are equally present in power-law and core galaxies (Lauer et al 2005: 47% of 177 in field galaxies). Assuming that a similar dust detection rate of 50% is representative for our galaxy sample and given the fact that dust is visible in all our galaxies, we can reject the statement that our sample belongs to the parent set of normal early type galaxies with visible dust at the 97.5% level.…”

The Kinematics of Core and Cusp Galaxies: Comparing HST Imaging and Integral-Field Observations

“…There are 33 galaxies in the SAURON sample of 48 elliptical and lenticular galaxies with known values for their inner profile slope, which separates them in "core" and "power-law" galaxies [6,14,13,11]. From the point of view of their stellar kinematics (see [4]) early-type galaxies appear in two broad flavours, depending on whether they exhibit clear large-scale rotation or not.…”

“…An illustration of a core and a cuspy nuclear profiles of two galaxies in the SAURON sample from HST V-band observations. Data from [11].…”