R. Falomo

The Hubble Space Telescope Survey of BL Lacertae Objects. II. Host Galaxies

We have used the Hubble Space Telescope (HST ) WFPC2 camera to survey 132 BL Lac objects com- prising seven complete radio-, X-ray¨, and optically selected samples. We obtained useful images for 110 targets spanning the redshift range These represent an unbiased subsample of the original 0 ( z ( 1.3. 132 since they were snapshots selected to —ll random holes in the HST schedule. The exposure times ranged from a few hundred to D1000 s, increasing with redshift. Most images were taken in the F702W —lter; those already observed in F814W during Cycle 5 were reobserved in F606W to give broader wave- length coverage. The data were analyzed uniformly, and both statistical and systematic errors were esti- mated (the latter dominate). In of the BL Lac images, host galaxies are detected, including nearly all 2 for z \ 0.5 (58 of 63). In contrast, only one-quarter of the BL Lac objects with z ( 0.5 (six of 22) were resolved because of the relatively short exposure times, and these tend to be very luminous host galaxies. The highest redshift host galaxy detected is in a BL Lac object at z \ 0.664. HST data add critical mor- phological information in the range a few tenths to a few arcseconds. In 58 of the 72 resolved host galaxies, a de Vaucouleurs pro—le is signi—cantly preferred, at con—dence, over a pure exponential Z99% disk; the two —ts are comparable in the remaining 14 cases because of their generally lower signal-to- noise ratios. These results limit the number of disk systems to at most 8% of BL Lac objects (at 99% con—dence) and are consistent with all BL Lac host galaxies being ellipticals. The detected host galaxies are luminous ellipticals with a median absolute K-corrected magnitude of mag (rms M R D (23.7 ^ 0.6 dispersion), at least 1 mag brighter than M* and comparable to brightest cluster galaxies. The galaxy morphologies are generally smooth and undisturbed, with small or negligible ellipticities

Multiwavelength Observations of a Dramatic High-Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (IUE) and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of 3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ~1 day. The optical-UV light curves, which are not well sampled during the high-energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron self-Compton (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a few days. This occurs naturally in the mirror model, wherein the flaring region in the jet photoionizes nearby broad emission line clouds, which, in turn, provide soft external photons that are Comptonized to γ-ray energies.

Imaging redshifts of BL lacertae objects

The HST snapshot imaging survey of 110 BL Lac objects (Urry et al.) has clearly shown that the host galaxies are massive and luminous ellipticals. The dispersion of the absolute magnitudes is sufficiently small that the measurement of the galaxy brightness becomes a valuable way of estimating their distance. This is illustrated by constructing a Hubble diagram of the 64 resolved objects with known redshift. By means of this relationship, we estimate the redshift of five resolved BL Lac objects of the survey that have no spectroscopic z. The adopted method also allows us to evaluate lower limits to the redshift for 13 objects of still unknown z using the lower limit on the host galaxy magnitude. This technique can be applied to any BL Lac object for which an estimate or a lower limit of the host galaxy magnitude is available. Finally, we show that the distribution of the nuclear luminosity of all the BL Lac objects of the survey indicates that the objects for which both the redshift and the host galaxy are undetected are among the most luminous, and possibly the most highly beamed.

The Hubble Space Telescope Survey of BL Lacertae Objects. I. Surface Brightness Profiles, Magnitudes, and Radii of Host Galaxies

We report on a large Hubble Space Telescope imaging survey of BL Lac objects, at spatial resolution 10 times better than previous ground-based surveys. We focus on data reduction and analysis, describing the procedures used to model the host galaxy surface brightness radial profiles. A total of 69 host galaxies were resolved out of 110 objects observed, including almost all sources at z 0.5. We classify them morphologically by fitting with either an exponential disk or a de Vaucouleurs profile; when one fit is preferred over the other, in 58 of 69 cases, it is invariably the elliptical morphology. This is a very strong result given the large number of BL Lac objects, the unprecedented spatial resolution, and the homogeneity of the data set. With the present reclassification of the host galaxy of 1418+546 as an elliptical, there remain no undisputed examples of a disk galaxy hosting a BL Lac nucleus. This implies that, at 99% confidence, fewer than 7% of BL Lac objects can be in disk galaxies. The apparent magnitude of the host galaxies varies with distance as expected if the absolute magnitudes are approximately the same, with a spread of ±1 mag, out to redshift z ~ 0.5. At larger redshifts, only six of 23 BL Lac objects are resolved so the present data do not constrain possible luminosity evolution of the host galaxies. The collective Hubble diagram for BL Lac host galaxies and radio galaxies strongly supports their unification.

The quasar relation through cosmic time – II. Evidence for evolution from z = 3 to the present age

We study the dependence of the MBH–Mhost relation on the redshift up to z = 3 for a sample of 96 quasars, the host galaxy luminosities of which are known. Black hole masses were estimated assuming virial equilibrium in the broad-line regions, while the host galaxy masses were inferred from their luminosities. With these data, we are able to pin down the redshift dependence of the MBH–Mhost relation along 85 per cent of the Universe age. We show that, in the sampled redshift range, the MBH–Lhost relation remains nearly unchanged. Once we take into account the ageing of the stellar population, we find that the MBH/Mhost ratio (� )

The black hole mass of low redshift radiogalaxies

We make use of two empirical relations between the black hole mass and the global properties (bulge luminosity and stellar velocity dispersion) of nearby elliptical galaxies, to infer the mass of the central black hole (MBH) in low redshift radiogalaxies. Using the most recent determinations of black hole masses for inactive early type galaxies we show that the bulge luminosity and the central velocity dispersion are almost equally correlated (similar scatter) with the central black-hole mass. Applying these relations to two large and homogeneous datasets of radiogalaxies we find that they host black-holes whose mass ranges between � 5×10 7 to � 6×10 9 M⊙ (average �8.9). MBH is found to be proportional to the mass of the bulge (Mbulge). The distribution of the ratio MBH/Mbulge has a mean value of 8×10 −4 and shows a scatter that is consistent with that expected from the associated errors. At variance with previous claims no significant correlation is instead found between MBH (or Mbulge) and the radio power at 5 GHz.

Detection of polarization in the afterglow of GRB 990510 with the ESO Very Large Telescope

Following a BeppoSAX alert and the discovery of the OT at SAAO, we observed GRB 990510 with the FORS instrument on ESOs VLT Unit 1 (Antu). The burst is unremarkable in gamma rays, but in optical is the first one to show good evidence for jetlike outflow. We report the detection of significant linear polarization in the afterglow: it is 1.6 ± 0.2% 0.86 days after trigger, and after 1.81 days is consistent with that same value, but much more uncertain. The polarization angle is constant on a timescale of hours and may be constant over one day. We conclude that the polarization is intrinsic to the source and due to the synchrotron nature of the emission, and discuss the random and ordered field geometries that may be responsible for it.

A Sample of Low-Redshift BL Lacertae Objects. I. The Radio Data

ABSTRACTWe present a new sample of 30 nearby (z < 0.2) BL Lacs, selected to studythe nuclear as well as the large scale properties of low power radio sources. In thisfirst paper, we show and discuss new radio data taken with the VLA (19 objectsat 1.4 GHz, either in A or C configuration, or both) as well as with the VLBA(15 sources at 5 GHz). On the kiloparsec scale, all objects exhibit a compact coreand a variety of radio morphologies (jets, halos, secondary compact components).On the parsec scale, we find weak cores and a few short, one-sided, jets. From thejet/counter-jet ratio, core dominance, and synchrotron self Compton model weestimate the intrinsic orientation and velocity ofthe jets. The resulting propertiesof BL Lacs are similar to those of a parent population composed of FR I radiogalaxies. Subject headings: galaxies: active — galaxies: nuclei — galaxies: jets — BLLacertae objects: general — radio continuum: galaxies1. INTRODUCTIONEver since the first suggestion of a possible unification of radio sources (e.g., Orr &Browne 1982), the viability for a comprehensive view of Active Galactic Nuclei (AGN) hasbeen actively debated. A major achievement is the formulation of a scheme in which BL

ESO VERY LARGE TELESCOPE OPTICAL SPECTROSCOPY OF BL LACERTAE OBJECTS. II. NEW REDSHIFTS, FEATURELESS OBJECTS, AND CLASSIFICATION ASSESSMENTS

We report on ESO Very Large Telescope optical spectroscopy of 42 BL Lacertae objects of unknown redshift. Nuclear emission lines were observed in 12 objects, while for another six we detected absorption features due to the host galaxy. The new high signal-to-noise ratio spectra therefore allow us to measure the redshift of 18 sources. Five of the observed objects were reclassified as either stars or quasars, and one is of uncertain nature. For the remaining 18 the optical spectra appear without intrinsic features in spite of our ability to measure rather faint (equivalent width ~0.1 A) spectral lines. For the latter sources a lower limit to the redshift was set, exploiting the very fact that the absorption lines of the host galaxy are undetected in the observed spectra.

On the geometry of broad emission region in quasars

We study the geometry of the Hβ broad emission region by comparing the M BH values derived from Hβ through the virial relation with those obtained from the host galaxy luminosity in a sample of 36 low-redshift (z ∼ 0.3) quasars. This comparison lets us infer the geometrical factor f needed to deproject the line-of-sight velocity component of the emitting gas. The wide range off values we found, together with the strong dependence off on the observed linewidth, suggests that a disc-like model for the broad-line region is preferable to an isotropic model, both for radio-loud and radio-quiet quasars. We examined similar observations of the C iv line and found no correlation in the width of the two lines. Our results indicate that an inflated disc broad-line region, in which the Carbon line is emitted in a flat disc while Hβ is produced in a geometrically thick region, can account for the observed differences in the width and shape of the two emission lines.