Black hole masses and the Fundamental Plane of BL Lac objects

We report on measurements of the stellar velocity dispersion (sigma) from the optical spectra of the host galaxies of four BL Lac objects. Together with our earlier results on seven BL Lac objects (Falomo et al. 2002), and with the previously derived photometrical and structural properties, these data are used to construct the Fundamental Plane (FP) of the BL Lac hosts. We find that the BL Lacs follow the same FP as low redshift radio galaxies and inactive luminous ellipticals, in agreement with similar results presented by Barth et al. (2003). This indicates that the photometrical, structural and kinematical properties of the host galaxies of BL Lacs are indistinguishable from those of inactive massive ellipticals. Using the correlation between black hole mass (M_BH) and sigma in nearby elliptical galaxies, we derive the masses of the central black hole in BL Lacs. These masses, in the range of 6 x 10^7 to 9 x 10^8 M_o, are consistent with the values derived from the bulge luminosity and appear to be linearly correlated with the mass of the galaxies (M_BH} ~0.001 x M_bulge).Comment: ApJ, accepted, 17 page

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

Fermi Observations of the Very Hard Gamma-ray Blazar PG 1553+113

We report the observations of PG 1553+113 during the first ~200 days of Fermi Gamma-ray Space Telescope science operations, from 4 August 2008 to 22 February 2009 (MJD 54682.7-54884.2). This is the first detailed study of PG 1553+113 in the GeV gamma-ray regime and it allows us to fill a gap of three decades in energy in its spectral energy distribution. We find PG 1553+113 to be a steady source with a hard spectrum that is best fit by a simple power-law in the Fermi energy band. We combine the Fermi data with archival radio, optical, X-ray and very high energy (VHE) gamma-ray data to model its broadband spectral energy distribution and find that a simple, one-zone synchrotron self-Compton model provides a reasonable fit. PG 1553+113 has the softest VHE spectrum of all sources detected in that regime and, out of those with significant detections across the Fermi energy bandpass so far, the hardest spectrum in that energy regime. Thus, it has the largest spectral break of any gamma-ray source studied to date, which could be due to the absorption of the intrinsic gamma-ray spectrum by the extragalactic background light (EBL). Assuming this to be the case, we selected a model with a low level of EBL and used it to absorb the power-law spectrum from PG 1553+113 measured with Fermi (200 MeV - 157 GeV) to find the redshift which gave the best fit to the measured VHE data (90 GeV - 1.1 TeV) for this parameterisation of the EBL. We show that this redshift can be considered an upper limit on the distance to PG 1553+113.Comment: Accepted for publication in the Astrophysical Journal (28 pages, 5 figures

Optical spectroscopy of BL Lacertae objects. Broad lines, companion galaxies and redshift lower limits

Aims: We present optical spectroscopy of a sample of BL Lac objects, to determine their redshift, to study their broad emission line properties and to characterize their close environment. Methods: Twelve objects were observed using the ESO 3.6m and the NOT 2.5m telescopes, obtaining spectra for the BL Lacs and for nearby sources. Results: For seven objects, nuclear emission lines and/or absorption lines from the host galaxy were detected. In all the four cases where absorption lines were revealed, the host galaxy has been resolved with HST or ground-based imaging. The broad H_alpha luminosities (or their upper limits) of the BL Lacs are similar to those of radio-loud quasars. For two BL Lacs, spectroscopy of close companions indicates that they are at the redshift of the BL Lacs, and therefore physically associated and likely interacting. Five BL Lacs have a featureless spectrum. In these cases, we apply a new technique to derive lower limits for their redshift. which are consistent with lower limits deduced from imaging.Comment: Accepted for publication on Astronomy & Astrophysics, 15 pages, 8 figure