No abstract

We fit the surface-brightness profiles of 21 elliptical galaxies using both the Sérsic function and a new empirical model which combines an inner power law with an outer Sérsic function. The profiles are combinations of deconvolved HST profiles from the literature and ellipse fits to the full WFPC2 mosaic images, and thus span a radial range from ∼ 0. ′′ 02 to ∼ twice the half-light radius. We are able to accurately fit the entire profiles using either the Sérsic function or our new model. In doing so, we demonstrate that most, if not all, so-called "power-law" galaxies are better described as "Sérsic galaxies" -they are well modeled by the three-parameter Sérsic profile into the limits of HST resolution -and that "core" galaxies are best understood as consisting of an outer Sérsic profile with an inner power-law cusp, which is a downward deviation from the inward extrapolation of the Sérsic profile. This definition of cores resolves ambiguities that result when the popular "Nuker law" is fitted to the profiles of ellipticals and bulges, particularly at lower luminosities. We also find that using the Nuker law to model core-galaxy nuclear profiles systematically overestimates the core radii by factors of 1.5-4.5 and underestimates the inner power-law slope by ∼ 20-40% or more.

We used the CLUMPY torus models and a Bayesian approach to fit the infrared spectral energy distributions (SEDs) and ground-based high-angular resolution mid-infrared spectroscopy of 13 nearby Seyfert galaxies. This allowed us to put tight constraints on torus model parameters such as the viewing angle i, the radial thickness of the torus Y , the angular size of the cloud distribution σ torus , and the average number of clouds along radial equatorial rays N 0 . We found that the viewing angle i is not the only parameter controlling the classification of a galaxy into a type 1 or a type 2. In principle type 2s could be viewed at any viewing angle i as long as there is one cloud along the line of sight. A more relevant quantity for clumpy media is the probability for an AGN photon to escape unabsorbed. In our sample, type 1s have relatively high escape probabilities, P esc ∼ 12 − 44%, while type 2s, as expected, tend to have very low escape probabilities. Our fits also confirmed that the tori of Seyfert galaxies are compact with torus model radii in the range 1-6 pc. The scaling of the models to the data also provided the AGN bolometric luminosities L bol (AGN), which were found to be in good agreement with estimates from the literature. When we combined our sample of Seyfert galaxies with a sample of PG quasars from the literature to span a range of L bol (AGN) ∼ 10 43 − 10 47 erg s −1 , we found plausible evidence of the receding torus. That is, there is a tendency for the torus geometrical covering factor to be lower (f 2 ∼ 0.1 − 0.3) at high AGN luminosities than at low AGN luminosities (f 2 ∼ 0.9 − 1 at ∼ 10 43−44 erg s −1 ). This is because at low AGN luminosities the tori appear to have wider angular sizes (larger σ torus ) and more clouds along radial equatorial rays. We cannot, however rule out the possibility that this is due to contamination by extended dust structures not associated with the dusty torus at low AGN luminosities, since most of these in our sample are hosted in highly inclined galaxies.

The Nuker law was designed to match the inner few ($3-10) arcseconds of predominantly nearby (d30 Mpc) early-type galaxy light profiles; it was never intended to describe an entire profile. The Sérsic model, on the other hand, was developed to fit the entire profile; however, because of the presence of partially depleted galaxy cores, the Sérsic model cannot always describe the very inner region. We have therefore developed a new empirical model consisting of an inner power law, a transition region, and an outer Sérsic model to connect the inner and outer structure of elliptical galaxies. We have additionally explored the stability of the Nuker model parameters. Surprisingly, none are found to be stable quantities; all are shown to vary systematically with a profile's fitted radial extent, and often by more than 100%. Considering elliptical galaxies spanning a range of 7.5 mag, we reveal that the central stellar densities of the underlying host galaxies increase with galaxy luminosity until the onset of core formation, detected only in the brightest elliptical galaxies. We suggest that the so-called power-law galaxies may actually be described by the Sérsic model over their entire radial range.