We have obtained the best t to the light pro les of a luminosity limited sample of elliptical and S0 galaxies with a power law r 1=n , letting the exponent remain free rather than keeping it xed at 1=n = 1=4 as in the well known de Vaucouleurs formula. The introduction of a free parameter in the tting formula (ranging from n = 0:5 for hr e i = 0:3 kpc to n = 16 for hr e i = 25 kpc) is justi ed by the existence of a good correlation between n and the global galaxian parameters, such as total luminosity and scale-radius. This result seems to be in line with the segregation of properties between the`ordinary' and`bright' families of early-type galaxies, and has consequence for the claimed independence of the shape of galaxy pro les with respect to the Fundamental Plane parameters.

We present evidence for a strong correlation between the concentration of bulges and the mass of their central supermassive black hole (M bh ) -more concentrated bulges have more massive black holes. Using C re (1/3) from Trujillo, Graham, & Caon (2001b) as a measure of bulge concentration, we find that log(M bh /M ⊙ ) = 6.81(±0.95)C re (1/3) + 5.03 ± 0.41. This correlation is shown to be marginally stronger (Spearman's r s = 0.91) than the relationship between the logarithm of the stellar velocity dispersion and log M bh (Spearman's r s = 0.86), and has comparable, or less, scatter (0.31 dex in log M bh , which decreases to 0.19 dex when we use only those galaxies whose supermassive black hole's radius of influence is resolved and remove one well understood outlying data point). It would appear that the central black hole mass can be estimated from surface photometry alone, without the expensive addition of velocity dispersion determinations.

Abstract. -We present results of a CCD optical imaging survey of the ionized gas in 73 luminous elliptical and lenticular galaxies, selected from the RC3 catalog to represent a broad variety of X-ray, radio, infrared and kinematical properties. For each galaxy we have used broad-band R images and narrow-band images centered at the Hα and [NII] emission lines to derive the luminosity and distribution of the ionized gas. We found that a large fraction of E (72%) and SO (85%) galaxies in our sample contain ionized gas. The gas morphology appears to be rather smooth for most galaxies; however ~ 12% of the sample galaxies show a very extended filamentary structure. According to the morphology and size of the gas distribution, the galaxies have been classified into three broad groups, named small disk (SD), regular extended (RE) and filamentary structure (F). The mean diameter of the emitting region ranges between 1 and 10 kpc; the derived mass of the ionized gas ranges between 103 and 105 solar masses. A sig nificant correlation between Hα +[NII] and X-ray luminosities is found for those galaxies (27% of the sample) for which we have detected ionized gas and are also listed as X-ray sources. However, there are relatively strong X-ray emitting galaxies for which we have not detected Ha+[NII] emission and objects which show emission-lines but are not listed either in the EINSTEIN or in the ROSAT databases. The distribution of datapoint and upper limits in this diagram suggests that galaxies with warm gas are also X-ray emitters, while there are X-ray emitters without measur able Hα+[NII] emission. Similar characteristics are present in the correlation between the infrared luminosity in the 12 p m band and Lhα+ [nii]; correlations with other infrared wavelengths are weaker. A strong correlation was also found between the Ha+[NIIj luminosity and the luminosity in the B band inside the region occupied by the lineemitting gas. We use these correlations to discuss the possible mechanisms responsible for the gas ionization and excitation, analyzing in particular the role of the post-AGB stars and the thermal conduction from the X-ray halo in providing the necessary source of ionization.

This paper addresses some questions which have arisen from the use of the Sérsic r1/n law in modelling the luminosity profiles of early‐type galaxies. The first issue deals with the trend between the half‐light radius and the structural parameter n. We show that the correlation between these two parameters is not only real, but also a natural consequence from the previous relations found to exist between the model‐independent parameters: total luminosity, effective radius and effective surface brightness. We also define a new galaxy concentration index which is largely independent of the image exposure depth, and is shown to be monotonically related with n. The second question concerns the curious coincidence between the form of the Fundamental Plane and the coupling between 〈I〉e and re when modelling a light profile. We explain, through a mathematical analysis of the Sérsic law, why the quantity re〈I〉e0.7 appears almost constant for an individual galaxy, regardless of the value of n (over a large range) adopted in the fit to the light profile. Consequently, Fundamental Planes of the form re〈I〉e0.7∝σ0x (for any x, and where σ0 is the central galaxy velocity dispersion) are insensitive to galaxy structure. Finally, we address the problematic issue of the use of model‐dependent galaxy light‐profile parameters versus model‐independent quantities for the half‐light radii, mean surface brightness and total galaxy magnitude. The former implicitly assume that the light‐profile model can be extrapolated to infinity, while the latter quantities, in general, are derived from a signal‐to‐noise ratio truncated profile. We quantify (mathematically) how these parameters change as one reduces the outer radius of an r1/n profile, and reveal how these can vary substantially when n≥4.

We report new HST WFPC2 and NICMOS observations of the center of the nearest radio galaxy Centaurus A (NGC 5128) and discuss their implications for our understanding of the active nucleus and jet. We detect the active nucleus in the near-IR (K and H) and, for the first time, in the optical (I and V), deriving the spectral energy distribution of the nucleus from the radio to X-rays. The optical and part of the near-IR emission can be explained by the extrapolation of the X-ray power law reddened by A_V~14mag, a value consistent with other independent estimates. The 20pc-scale nuclear disk discovered by Schreier et al. (1998) is detected in the [FeII] 1.64mic line and presents a morphology similar to that observed in Pa alpha with a [FeII]/Pa alpha ratio typical of low ionization Seyfert galaxies and LINERs. NICMOS 3 Pa alpha observations in a 50"x50" circumnuclear region suggest enhanced star formation (~0.3Msun/yr) at the edges of the putative bar seen with ISO, perhaps due to shocks driven into the gas. The light profile, reconstructed from V, H and K observations, shows that Centaurus A has a core profile with a resolved break at ~4" and suggests a black--hole mass of ~10^9 Msun. A linear blue structure aligned with the radio/X-ray jet may indicate a channel of relatively low reddening in which dust has been swept away by the jet.Comment: 19 pages, 13 figures, Astrophysical Journal, in press. High quality figures available at http://www.arcetri.astro.it/~marconi/colpic.htm