47 research outputs found
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
The HELLAS2XMM survey: IV. Optical identifications and the evolution of the accretion luminosity in the Universe
We present results from the photometric and spectroscopic identification of
122 X-ray sources recently discovered by XMM-Newton in the 2-10 keV band (the
HELLAS2XMM 1dF sample). Their flux cover the range 8E-15-4E-13 cgs and the
total area surveyed is 0.9 deg2. About 20% of the hard X-ray selected sources
have an X-ray to optical flux ratio (X/O) ten times or more higher than that of
optically selected AGN. Unlike the faint sources found in the ultra-deep
Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes more
than one order of magnitude lower than the HELLAS2XMM survey sources, many of
the extreme X/O sources in our sample have R<=25 and are therefore accessible
to optical spectroscopy. We report the identification of 13 sources with
X/O>10: 8 are narrow line QSO (i.e. QSO2), four are broad line QSO. We use a
combined sample of 317 hard X-ray selected sources (HELLAS2XMM 1dF, CDFN 1Msec,
SSA13 and Lockman Hole flux limited samples), 221 with measured z, to evaluate
the cosmological evolution of the hard X-ray source's number and luminosity
densities. Looking backward in time, the low luminosity sources (logL(2-10keV)
= 43-44 erg/s) increase in number at a rate different than the high luminosity
sources (logL(2-10keV)>44.5 erg/s), reaching a maximum around z=1 and then
levelling off beyond z=2. This translates into an accretion driven luminosity
density which is dominated by sources with logL(2-10keV) < 44.5 erg/s up to at
least z=1, while the contribution of the same sources and of those with
logL(2-10keV)>44.5 erg/s appear to be comparable between z=2 and 4.Comment: v2, minor changes, A&A in pres
The HELLAS2XMM survey. VII. The hard X-ray luminosity function of AGN up to z=4: more absorbed AGN at low luminosities and high redshifts
We have determined the cosmological evolution of the density of AGN and of
their Nh distribution as a function of the un-absorbed 2-10 keV luminosity up
to redshift 4. We used the HELLAS2XMM sample combined with other published
catalogs, yielding a total of 508 AGN. Our best fit is obtained with a
luminosity-dependent density evolution (LDDE) model where low luminosity
(Lx~10^43 erg s^-1) AGN peak at z~0.7, while high luminosity AGN (Lx>10^45 erg
s^-1) peak at z~2.0. A pure luminosity evolution model (PLE) can instead be
rejected. There is evidence that the fraction of absorbed (Nh>10^22 cm^-2) AGN
decreases with the intrinsic X-ray luminosity, and increases with the redshift.
Our best fit solution provides a good fit to the observed counts, the cosmic
X-ray background, and to the observed fraction of absorbed AGN as a function of
the flux in the 10^-15<S(2-10)<10^-10 erg s^-1 cm^-2 range. We find that the
absorbed, high luminosity (Lx>10^44 erg s^-1) AGN have a density of 267 deg^-2
at fluxes S(2-10)>10^-15 erg s^-1 cm^-2. Using these results, we estimate a
density of supermassive black holes in the local Universe of 3.2 (h^2_70) x
10^5 M_sol Mpc^-3, which is consistent with the recent measurements of the
black hole mass function in the local galaxies.Comment: 16 pages, 20 figures. Scheduled on ApJ, Vol. 635, 2005 December 2
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
The HELLAS2XMM survey: VI. X-ray absorption in the 1dfAGN sample through a spectral analysis
The spectroscopic analysis of 117 serendipitous sources in the HELLAS2XMM 1df
(1 degree field) survey is described. Of these, 106 sources, of which 86% have
a spectroscopic redshift, are used to evaluate the fraction of X-ray absorbed
(log NH>22) Active Galactic Nuclei (AGN) in the 2--10 keV flux range 0.8-20E-14
erg/cm2/s. This fraction turns out lower than what is predicted by two well
known Cosmic X-Ray Background synthesis models, and the discrepancy is
significant at the 99.999% level. This result consolidates the findings
recently obtained by other authors. In the flux interval explored, the data are
consistent with an intrinsic distribution of the absorbing columns (flat per
decade above logNH>21) independent of luminosity and redshift, together with an
AGN luminosity function evolving purely in luminosity. It is shown that, on the
other hand, extrapolation to lower fluxes fails to reproduce the results
inferred from the Chandra Deep Field North survey. It is found that about 40%
of the high luminosity sources in our sample have best fit logNH>22, and the
surface density of these X-ray obscured QSOs can then be estimated at about 48
per square degree, at the flux limit of ~1E-14} erg/cm2/s of the HELLAS2XMM 1df
survey. As a side issue, 5 or 6 out of 60 sources, that is about 10%,
identified with broad line AGN, turn out to be affected by logNH>22 absorption.Comment: A&A in pres