We have used the Hubble Space T elescope (HST ) WFPC2 camera to survey 132 BL Lac objects comprising seven complete radio-, X-rayÈ, 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 Ðll 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 Ðlter ; those already observed in F814W during Cycle 5 were reobserved in F606W to give broader wavelength coverage. The data were analyzed uniformly, and both statistical and systematic errors were estimated (the latter dominate). In of the BL Lac images, host galaxies are detected, including nearly all 2 3 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 morphological information in the range a few tenths to a few arcseconds. In 58 of the 72 resolved host galaxies, a de Vaucouleurs proÐle is signiÐcantly preferred, at conÐdence, over a pure exponential Z99% disk ; the two Ðts are comparable in the remaining 14 cases because of their generally lower signal-tonoise ratios. These results limit the number of disk systems to at most 8% of BL Lac objects (at 99% conÐdence) 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 (v [ 0.2). half-light surface brightness is anticorrelated with half-light radius in quantitatively the same way as other elliptical galaxies, indicating that apart from their highly active nuclei, BL Lac objects appear to be absolutely normal ellipticals. There is no correlation between host galaxy and observed nuclear magnitude or estimated jet power corrected for beaming. If black hole mass is correlated linearly with bulge mass in general, this implies a large range in Eddington ratio. The host galaxies of the radio-selected and X-rayÈselected BL Lac objects are comparable in both morphology and luminosity, strongly suggesting that nuclear properties do not have a dramatic e †ect on large-scale host galaxy properties, or vice versa. BL Lac objects have extended radio powers and host galaxy magnitudes very much like those of FR I galaxies, and quite distinct from FR IIÏs, which instead are more similar to quasars. Thus the present data strongly support the uniÐcation picture with FR I galaxies constituting the bulk of the parent population of BL Lac objects.

We study the dependence of the M BH -M host 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 M BH -M host relation along 85 per cent of the Universe age. We show that, in the sampled redshift range, the M BH -L host relation remains nearly unchanged. Once we take into account the ageing of the stellar population, we find that the M BH /M host ratio ( ) increases by a factor of ∼7 from z = 0 to z = 3. We show that evolves with z regardless of the radio loudness and of the quasar luminosity. We propose that the most massive black holes, living their quasar phase at high redshift, become extremely rare objects in host galaxies of similar mass in the Local Universe.

We report on a large HST 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 Lacs 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 6 of 23 BL Lacs 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.

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We report results from Chandra observations of the X-ray jet of 3C 273 during the calibration phase in 2000 January. The zeroth-order images and spectra from two 40 ks exposures with the High-Energy Transmission Grating and Low-Energy Transmission Grating ϩ Advanced CCD Imaging Spectrometer-S show a complex Xray structure. The brightest optical knots are detected and resolved in the 0.2-8 keV energy band. The X-ray morphology tracks well the optical. However, while the X-ray brightness decreases along the jet, the outer parts of the jet tend to be increasingly bright with increasing wavelength. The spectral energy distributions of four selected regions can best be explained by the inverse Compton scattering of (beamed) cosmic microwave background photons. The model parameters are compatible with equipartition and a moderate Doppler factor, which is consistent with the one-sidedness of the jet. Alternative models either imply implausible physical conditions and energetics (the synchrotron self-Compton model) or are sufficiently ad hoc to be unconstrained by the present data (synchrotron radiation from a spatially or temporally distinct particle population).