The black hole mass of BL Lacs from stellar velocity dispersion of the host galaxy

We present the first results from our on-going program to estimate black hole masses [M(BH)] of nearby BL Lac objects. The estimates are based on stellar velocity dispersion (sigma) of the BL Lac host galaxies from optical spectroscopy, and the recently found tight correlation between M{BH} and sigma in nearby early-type galaxies. For the first three BL Lacs, we find log M(BH) = 7.5 - 8.7 and M(BH)/M(host) = 0.03 - 0.1.Comment: 2 pages, to appear in ESO Astrophysics Symposium "The Mass of Galaxies at Low and High Redshift", eds. R. Bender & A. Renzin

Studying stellar halos with future facilities

Stellar halos around galaxies retain fundamental evidence of the processes which lead to their build up. Sophisticated models of galaxy formation in a cosmological context yield quantitative predictions about various observable characteristics, including the amount of substructure, the slope of radial mass profiles and three dimensional shapes, and the properties of the stellar populations in the halos. The comparison of such models with the observations provides constraints on the general picture of galaxy formation in the hierarchical Universe, as well as on the physical processes taking place in the halos formation. With the current observing facilities, stellar halos can be effectively probed only for a limited number of nearby galaxies. In this paper we illustrate the progress that we expect in this field with the future ground based large aperture telescopes (E-ELT) and with space based facilities as JWST.Comment: 6 pages, 4 figures, to appear in Proc. IAU Symposium 317 "The General Assembly of Stellar Halos: Structure, Origin and Evolution.

Spectroscopy of BL Lac Objects: new redshifts and mis-identified sources

We are carrying out a program of high signal to noise optical spectroscopy of BL Lacs with unknown or tentative redshift. Here we report some preliminary results. New redshifts are measured for PKS0754+100 (z=0.266) and 1ES0715-259 (z=0.464) . From lineless spectra of PG1553+113 and PKS1722+119 we set a lower limit of z>0.3 for both sources. In two cases (UM493 and 1620+103) stellar spectra indicate a wrong classification.Comment: 4 pages; Conference proceeding "High Energy Blazar Astronomy", Tuorla Observatory, Finland, 17-21 June 2002; to be published in the PASP conference serie

On the distance of PG 1553+11. A lineless BL Lac object active in the TeV band

Context: The redshift of PG 1553+11, a bright BL Lac object (V~14), is still unknown. It has been recently observed in the TeV band, a fact that offers an upper limit for the redshift z<0.4. Aims: We intend to provide a lower limit for the distance of the object. Methods: We used a chi^2 procedure to constrain the apparent magnitude of the host galaxy in archived HST images. Supposing that the host galaxy is typical of BL Lac objects (M_{R} -22.8), a lower limit to the distance can be obtained from the limit on the apparent magnitude of the host galaxy. Results: Using the 3 sigma limit on the host galaxy magnitude, the redshift is found to be greater or equal to 0.25. Conlusions: The redshift of PG 1553+11 is probably in the range z=0.3-0.4, making this object the most distant extragalactic source so far detected in the TeV energies. We suggest that other bright BL Lac objects of unknown redshift and similar spectroscopic characteristics may be interesting targets for TeV observations.Comment: Accepted for publication in A&A letters, 4 pages, 5 figure

The nature of close companions of the BL Lac Objects 1ES 0502+675 and 1ES 1440+122

We report on deep radio images and optical spectroscopy of two BL Lac objects that have very close compact companions. The two targets, 1ES 0502+675 and 1ES 1440+122, were selected from the HST imaging survey of 110 BL Lacs as candidates for possible gravitational lensing. The new observations clearly demonstrate that the companion objects are not secondary images of the active nuclei but, in spite of the relatively low chance projection probability, foreground Galactic stars. Gravitational lensing appears to be unimportant to the BL Lac phenomenon. We discuss the radio properties of the BL Lac objects in the context of standard beaming models, and show they are as expected for beamed FRI radio galaxies.Comment: 13 pages, 5 figures, ApJ in pres

Optical surface photometry of radio galaxies - II. Observations and data analysis

Optical imaging observations for 50 radio galaxies are presented. For each object isophotal contours, photometric profiles, structural parameters (position angle, ellipticity, Fourier coefficients), and total magnitudes are given. These observations, obtained in the Cousins R band, complement the data presented in a previous paper and are part of a larger project aimed at studying the optical properties of low redshift (z<0.12) radio galaxies (Govoni et al. 1999). Comments for each individual source are reported.Comment: 9 pages, plus 17 .gif figures, accepted by Astronomy and Astrophysics, Supplement Serie

The BH mass of nearby QSOs: a comparison of the bulge luminosity and virial methods

We report on the analysis of the photometric and spectroscopic properties of a sample of 29 low redshift (z<0.6) QSOs for which both HST WFPC2 images and ultraviolet HST FOS spectra are available. For each object we measure the R band absolute magnitude of the host galaxy, the CIV (1550A) line width and the 1350A continuum luminosity. From these quantities we can estimate the black hole (BH) mass through the M(BH)-L(bulge) relation for inactive galaxies, and from the virial method based on the kinematics of the regions emitting the broad lines. The comparison of the masses derived from the two methods yields information on the geometry of the gas emitting regions bound to the massive BH. The cumulative distribution of the line widths is consistent with that produced by matter laying in planes with inclinations uniformly distributed between 10 and 50 deg, which corresponds to a geometrical factor f=1.3. Our results are compared with those of the literature and discussed within the unified model of AGN.Comment: 10 pages, 6 figures. To appear in MNRA