Riccardo Scarpa

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

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 (� )

A Survey of Extended Radio Jets in Active Galactic Nuclei with Chandra and the Hubble Space Telescope: First Results

We present the first results from an X-ray and optical survey of a sample of radio jets in active galactic nuclei with Chandra and the Hubble Space Telescope (HST). We focus here on the first six sources observed in X-ray, in four of which a bright X-ray jet was detected for the first time. In three out of the four cases, optical emission from the jet is also detected in our HST images. We compare the X-ray morphology with the radio as derived from improved processing of archival Very Large Array data, and we construct spectral energy distributions (SEDs) for the most conspicuous emission knots. In most cases, the SEDs, together with the similarity of the X-ray and radio morphologies, favor an inverse Compton origin for the X-rays. The most likely origin of the seed photons is the cosmic microwave background, implying the jets are still relativistic on kiloparsec scales. However, in the first knot of the PKS 1136-135 jet, X-rays are likely produced via the synchrotron process. In all four cases, bulk Lorentz factors of a few are required. The radio maps of the two jets not detected by either Chandra or HST suggest that they are less beamed at large scales than the other four detected sources. Our results demonstrate that, at the sensitivity and resolution of Chandra, X-ray emission from extragalactic jets is common, yielding essential information on their physical properties.We present the first results from an X-ray and optical survey of a sample of AGN radio jets with Chandra and HST. We focus here on the first six sources observed at X-rays, in four of which a bright X-ray jet was detected for the first time. In three out of four cases optical emission from the jet is also detected in our HST images. We compare the X-ray morphology with the radio as derived from improved processing of archival VLA data and we construct spectral energy distributions (SED) for the most conspicuous emission knots. In most cases the SEDs, together with the similarity of the X-ray and radio morphologies, favor an inverse Compton origin of the X-rays. The most likely origin of the seed photons is the Cosmic Microwave Background, implying the jets are still relativistic on kiloparsec scales. However, in the first knot of the PKS 1136-135 jet, X-rays are likely produced via the synchrotron process. In all four cases bulk Lorentz factors of a few are required. The radio maps of the two jets not detected by either Chandra or HST suggest that they are less beamed at large scales than the other four detected sources. Our results demonstrate that, at the sensitivity and resolution of Chandra, X-ray emission from extragalactic jets is common, yielding essential information on their physical properties.

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.

The fundamental plane of radio galaxies

We collected photometrical and dynamical data for 73 low red-shift ( z< 0:2) Radio Galaxies (LzRG) in order to study their Fundamental Plane (FP). For 22 sources we also present new velocity dispersion data that complement the photometric data given in our previous study of LzRG (Govoni et al. 2000a). It is found that the FP of LzRG is similar to the one dened by non-active elliptical galaxies, with LzRG representing the brightest end of the population of early type galaxies. Since the FP mainly reflects the virial equilibrium condition, our result implies that the global properties of early{type galaxies (dening the FP) are not influenced by the presence of gas accretion in the central black hole. This is fully in agreement with the recent results in black hole demography, showing that virtually all luminous spheroidal galaxies host a massive black hole and therefore may potentially become active. We conrm and extend to giant ellipticals the systematic increase of the mass-to-light ratio with galaxy luminosity.

ESO Very Large Telescope Optical Spectroscopy of BL Lacertae Objects. I. New Redshifts

We report redshift measurements for 12 BL Lacertae objects from a program aimed at obtaining high signal-to-noise ratio (up to ~500) optical spectroscopy of a mixed sample of objects. The new observations, gathered with the 8 m ESO Very Large Telescope, allowed us to detect weak spectral features down to a line equivalent width as small as ~1 A. The new redshifts fall in the 0.2–1.3 interval. For nine objects we observe emission lines from the active nucleus. In the remaining three cases absorption lines from the host galaxy are found. For two objects we also detect absorption lines from intervening systems.

Detection of an X-Ray Jet in 3C 371 with Chandra

We report the detection at X-rays of the radio/optical jet of 3C 371 from a short (10 ks) Chandra exposure in 2000 March. We also present a new MERLIN observation at 1.4 GHz together with a reanalysis of the archival Hubble Space Telescope Wide Field Planetary Camera 2 F555W image. Despite the limited signal-to-noise ratio of the Chandra data, the X-ray morphology is clearly different from that of the radio/optical emission, with the brightest X-ray knot at 17 from the nucleus and little X-ray emission from the brightest radio/optical knot at 31. We construct the spectral energy distributions for the two emission regions at 17 and 31. Both show that the X-ray flux is below the extrapolation from the radio-to-optical continuum, suggesting a moderately beamed synchrotron from an electron population with a decreasing high-energy cutoff as a plausible emission mechanism.

THE COSMIC EvolUTION OF QUASAR HOST GALAXIES

We present the results of a near-infrared imaging study of the host galaxies of 17 quasars in the redshift range 1 < z < 2. The observations were carried out at the ESO VLT UT1 8 m telescope under excellent seeing conditions (~04). The sample includes radio-loud (RLQs) and radio-quiet (RQQs) quasars with similar distribution of redshift and optical luminosity. For all the observed objects but one we have been able to derive the global properties of the surrounding nebulosity. The host galaxies of both types of quasars appear to follow the expected trend in luminosity of massive ellipticals undergoing simple passive evolution. However, we find a systematic difference by a factor ~2 in the host luminosity between RLQs and RQQs [RLQ (host) = -27.55 ± 0.12 and RLQ (host) = -26.83 ± 0.25]. Comparison with other samples of quasar hosts at similar and lower redshift indicates that the difference in the host luminosity between RLQs and RQQs remains the same from z = 2 to the present epoch. No significant correlation is found between the nuclear and the host luminosities. Assuming that the host luminosity is proportional to the black hole mass, as observed in nearby massive spheroids, these quasars emit at very different levels (spread ~1.5 dex) with respect to their Eddington luminosity and with the same distribution for RLQs and RQQs. Apart from a factor of ~2 difference in luminosity, the hosts of RLQs and RQQs of comparable nuclear luminosity appear to follow the same cosmic evolution as massive inactive spheroids. Taken together, our results support a view where nuclear activity can occur in all luminous ellipticals without producing a significant change in their global properties and evolution. Quasar hosts appear to be already well formed at z ~ 2, in disagreement with the predictions of models for the joint formation and evolution of galaxies and active nuclei based on the hierarchical structure formation scenario.

The Hubble Space Telescope Survey of BL Lacertae Objects: Gravitational Lens Candidates and Other Unusual Sources

We present Hubble Space Telescope (HST ) observations of seven unusual objects from the HST ii snapshot survey ˇˇ of BL Lacertae objects, of which four are gravitational lens candidates. In three cases a double point source is observed: 0033)595, with separation, and 0502)675 and 1440)122, each 1A with separation. The last two also show one or more galaxies, which could be either host or D0A.3 lensing galaxies. If any are con—rmed as lenses, these BL Lac objects are excellent candidates for measur- ing via gravitational time delay because of their characteristic rapid, high-amplitude variability. An H 0 additional advantage is that, like other blazars, they are likely superluminal radio sources, in which case the source plane is mapped out over a period of years, providing strong additional constraints on the lensing mass distribution. The fourth gravitational lens candidate is 1517)656, which is surrounded by three arclets forming an almost perfect ring of radius If this is indeed an Einstein ring, it is most 2A.4. likely a background source gravitationally lensed by the BL Lac object host galaxy and possibly a sur- rounding group or cluster. In the extreme case that all four candidates are true lenses, the derived fre- quency of gravitational lensing in this BL Lac sample would be an order of magnitude higher than in comparable quasar samples. We also report on three other remarkable BL Lac objects: 0138(097, which is surrounded by a large number of close companion galaxies; 0806)524, whose host galaxy con- tains an uncommon arclike structure; and 1959)650, which is hosted by a gas-rich elliptical galaxy with a prominent dust lane of D5 ) 105 M _ . Subject headings: BL Lacertae objects: generalgalaxies: structuregravitational lensing

Hubble Space Telescope Observations of the Optical Jets of PKS 0521–365, 3C 371, and PKS 2201+044

Hubble Space Telescope (HST) observations have led to the discovery of the optical counterpart of the radio jet of PKS 2201+044 and to a detailed analysis of the optical jets of PKS 0521-365 and 3C 371. At HST spatial resolution these jets are well resolved, displaying knotty morphologies. When compared with radio maps of appropriate resolution, a clear one-to-one correspondence between optical and radio structures is found, showing that all detected optical structures are indeed related to the radio synchrotron emission. Photometry of the brightest knots shows that the radio-to-optical spectral index and the derived intensity of the equipartition magnetic field are approximately constant along the jet. Thus, present observations suggest that the electron energy distribution does not change significantly all along the jet.