Publications (52)89.13 Total impact
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Article: ESO VLT Optical Spectroscopy of BL Lac Objects IV. New spectra and properties of the full sample
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ABSTRACT: We present the last chapter of a spectroscopy program aimed at deriving the redshift or a lower limit to the redshift of BL Lac objects using medium resolution spectroscopy. Here we report new spectra for 33 BL Lac object candidates obtained in 2008-2009 confirming the BL Lac nature of 25 sources and for 5 objects we obtained new redshifts. These new observations are combined with our previous data in order to construct a homogeneous sample of \sim 70 BL Lacs with high quality spectroscopy. All these spectra can be accessed at the website http://www.oapd.inaf.it/zbllac/. The average spectrum, beaming properties of the full sample, discussion on intervening systems and future perspectives are addressed.02/2013; -
Article: Quasar host galaxies in the SDSS Stripe 82
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ABSTRACT: We present first results from our study of the properties of ~400 low redshift (z < 0.5) quasars, based on a large homogeneous dataset derived from the Stripe 82 area of the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7). For this sky region, deep (r~22.4) u,g,r,i,z images are available, up to ~2 mag deeper than standard SDSS images, allowing us to study both the host galaxies and the Mpc-scale environments of the quasars. This sample greatly outnumbers previous studies of low redshift quasar hosts, from the ground or from space. Here we report the preliminary results for the quasar host galaxies. We are able to resolve the host galaxy in ~80 % of the quasars. The quasar hosts are luminous and large, the majority of them in the range between M*-1 and M*-2, and with ~10 kpc galaxy scale-lengths. Almost half of the host galaxies are best fit with an exponential disk, while the rest are spheroid-dominated. There is a reasonable relation between the central black hole mass and the host galaxy luminosity.02/2013; -
Article: On the redshift of the bright BL Lac object PKS 0048-097
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ABSTRACT: Aims: The determination of elusive redshifts of bright BL Lac objects Methods: We use the capabilities of newly available spectrograph X-Shooter at European Southern Observatory (ESO) Very Large Telescope, that combines high resolution and a large wavelength range, to obtain UVB to near-IR spectra of BL Lacs. Results: Our observations of PKS 0048-097 detect three emission lines that permit to derive a redshift z = 0.635. Moreover, a Mg II absorption system at z = 0.154 that is associated with a foreground spiral galaxy at 50 Kpc projected distance is found. Conclusions: The obtained redshift allows us to comment about the optical beaming factor and the absorption of the high energy spectrum by the Extragalactic Background Light.05/2012; -
Article: The Gray Needle: Large Grains in the HD 15115 Debris Disk from LBT/PISCES/Ks and LBTI/LMIRcam/L' Adaptive Optics Imaging
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ABSTRACT: We present diffraction-limited \ks band and \lprime adaptive optics images of the edge-on debris disk around the nearby F2 star HD 15115, obtained with a single 8.4 m primary mirror at the Large Binocular Telescope. At \ks band the disk is detected at signal-to-noise per resolution element (SNRE) \about 3-8 from \about 1-2\fasec 5 (45-113 AU) on the western side, and from \about 1.2-2\fasec 1 (63-90 AU) on the east. At \lprime the disk is detected at SNRE \about 2.5 from \about 1-1\fasec 45 (45-90 AU) on both sides, implying more symmetric disk structure at 3.8 \microns . At both wavelengths the disk has a bow-like shape and is offset from the star to the north by a few AU. A surface brightness asymmetry exists between the two sides of the disk at \ks band, but not at \lprime . The surface brightness at \ks band declines inside 1\asec (\about 45 AU), which may be indicative of a gap in the disk near 1\asec. The \ks - \lprime disk color, after removal of the stellar color, is mostly grey for both sides of the disk. This suggests that scattered light is coming from large dust grains, with 3-10 \microns -sized grains on the east side and 1-10 \microns dust grains on the west. This may suggest that the west side is composed of smaller dust grains than the east side, which would support the interpretation that the disk is being dynamically affected by interactions with the local interstellar medium.03/2012; -
Article: The low-mass end of the Mbh/Mhost relation in quasars
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ABSTRACT: The Mbh-Mhost relation in quasars has been probed only in a limited parameter space, namely at Mbh~10^9 Msun and Mhost~10^12 Msun. Here we present a study of 26 quasars lying in the low-mass end of the relation, down to Mbh~10^7 Msun. We selected quasars from the SDSS and HST-FOS archives, requiring modest Mbh (as derived through the virial paradigm). We imaged our sources in H band from the Nordic Optical Telescope. The quasar host galaxies have been resolved in 25 out of 26 observed targets. Host galaxy luminosities and stellar masses are computed, under reasonable assumptions on their star formation histories. Combining these results with those from our previous studies, we manage to extend the sampled parameter space of the Mbh-Mhost relation in quasars. The relation holds over 2 dex in both the parameters, similarly to what observed in low-luminosity AGN and in quiescent galaxies. For the first time, we are able to measure the slope of the Mbh-Mhost relation in quasars. We find that it is consistent with the linear case (similarly to what observed in quiescent galaxies). We do not find any evidence of a population of massive black holes lying below the relation.10/2011; -
Article: An update of the on-sky performance of the Layer-Oriented wave-front sensor for MAD
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ABSTRACT: The Multi-conjugate Adaptive optics Demonstrator, MAD, successfully demonstrated on sky the MCAO technique both in Layer Oriented and Star Oriented modes. As results of the Guaranteed Time Observations in Layer Oriented mode quality astronomy papers have been published. In this paper we concentrate on the instrumentation issues and technical aspects which stay behind this success. Comment: 12 pages, 10 figures, Proceedings of the SPIE conference "Adaptive Optics Systems II", 27 June 2010, San Diego, California, USA09/2010; -
Article: The Properties of Quasar Hosts at the Peak of the Quasar Activity
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ABSTRACT: We present near-infrared imaging obtained with ESO VLT/ISAAC of a sample of 16 low luminosity radio-quiet quasars (RQQs) at the epoch around the peak of the quasar activity (2 < z < 3), aimed at investigating their host galaxies. For 11 quasars, we are able to detect the host galaxies and derive their properties, while for the other 5 quasars, upper limits to the host luminosity are estimated. The luminosities of the host galaxies of RQQs at high redshift are in the range of those of massive inactive elliptical galaxies. This work complements our previous systematic study of quasar hosts aimed to trace the cosmological luminosity evolution of the host galaxies up to z ~ 2 and extends our pilot study of a few luminous quasars at z > 2. The luminosity trend with a cosmic epoch resembles that observed for massive inactive galaxies, suggesting a similar star formation history. In particular, both quasar host galaxies and massive inactive galaxies appear mostly assembled already at the peak age of the quasar activity. This result is of key importance for testing the models of joint formation and evolution of galaxies and their active nuclei.The Astrophysical Journal 09/2009; 703(2):1663. · 6.02 Impact Factor -
Article: Downsizing of supermassive black holes from the SDSS quasar survey (II). Extension to z~4
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ABSTRACT: Starting from the quasar sample of the Sloan Digital Sky Survey (SDSS) for which the CIV line is observed, we use an analysis scheme to derive the z-dependence of the maximum mass of active black holes, which overcomes the problems related to the Malmquist bias. The same procedure is applied to the low redshift sample of SDSS quasars for which Hbeta measurements are available. Combining with the results from the previously studied MgII sample, we find that the maximum mass of the quasar population increases as (1+z)^(1.64+/-0.04) in the redshift range 0.1<z<4, which includes the epoch of maximum quasar activity. Comment: 9 pages, 8 figures. To appear in MNRAS07/2009; -
Article: Downsizing of supermassive black holes from the SDSS quasar survey
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ABSTRACT: Starting from the ~50000 quasars of the Sloan Digital Sky Survey for which MgII line width and 3000 A monochromatic flux are available, we aim to study the dependence of the mass of active black holes on redshift. We focus on the observed distribution in the FWHM - nuclear luminosity plane, which can be reproduced at all redshifts assuming a limiting BH mass, a maximum Eddington ratio and a minimum luminosity (due to the survey flux limit). We study the z-dependence of the best fit parameters of assumed distributions at increasing redshift and find that the maximum mass of the quasar population evolves as log[M_BH(max)/M_Sun]~0.3z+9, while the maximum Eddington ratio (~0.45) is practically independent of cosmic time. These results are unaffected by the Malmquist bias.04/2009; -
Article: HST Observations of Host Galaxies in Three Radio-selected BL Lacertae Objects
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ABSTRACT: We present high-resolution images of three BL Lac objects (0814+425, 1823+568, 2254+074) taken with the WFPC-2 camera on Hubble Space Telescope (HST) as part of a study of BL Lacs from a complete radio-selected sample. In two cases we clearly detect the host galaxies while the third BL Lac (0814+425) is unresolved. The host galaxy of 2254+074 is a moderately bright elliptical, while the inferred luminosity of the host galaxy for 1823+568 is at the high end of the range for previously known BL Lac hosts. The lack of detection of a host galaxy for 0814+425 calls into question the validity of its low reported redshift. Comparison of the HST data with subarcsecond ground-based R-band images shows excellent agreement on the largest scales, while on smaller scales only HST detects the galaxy significantly above the nuclear light. In the case of 2254+074, the high resolution of HST allows us to establish that the host galaxy is an elliptical.The Astrophysical Journal 01/2009; 476(1):113. · 6.02 Impact Factor -
Article: Multiwavelength Monitoring of the BL Lacertae Object PKS 2155–304 in 1994 May. II. The IUE Campaign
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ABSTRACT: PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was monitored with the International Ultraviolet Explorer satellite at ~1 hr time resolution for 10 nearly uninterrupted days in 1994 May. The campaign, which was coordinated with Extreme Ultraviolet Explorer, ROSAT, and ASCA monitoring, along with optical and radio observations from the ground, yielded the largest set of spectra and the richest short-timescale variability information ever gathered for a blazar at UV wavelengths. The source flared dramatically during the first day, with an increase by a factor of ~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly constant level for ~5 days, then flared with ~35% amplitude for 2 days. The same variability was seen in both short- and long-wavelength IUE light curves, with zero formal lag (2 hr), except during the rapid initial flare, when the variations were not resolved. Spectral index variations were small and not clearly correlated with flux. The flux variability observed in the present monitoring is so rapid that, for the first time, based on the UV emission alone, the traditional ΔL/Δt limit indicating relativistic beaming is exceeded. The most rapid variations, under the likely assumption of synchrotron radiation, lead to a lower limit of 1 G on the magnetic field strength in the UV-emitting region. These results are compared with earlier intensive monitoring of PKS 2155-304 with IUE in 1991 November, when the UV flux variations had completely different characteristics.The Astrophysical Journal 01/2009; 486(2):784. · 6.02 Impact Factor -
Article: The Hubble Space Telescope Survey of BL Lacertae Objects: Gravitational Lens Candidates and Other Unusual Sources
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ABSTRACT: We present Hubble Space Telescope (HST) observations of seven unusual objects from the HST "snapshot survey" of BL Lacertae objects, of which four are gravitational lens candidates. In three cases a double point source is observed: 0033+595, with 158 separation, and 0502+675 and 1440+122, each with ~03 separation. The last two also show one or more galaxies, which could be either host or lensing galaxies. If any are confirmed as lenses, these BL Lac objects are excellent candidates for measuring H0 via gravitational time delay because of their characteristic rapid, high-amplitude variability. An 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 24. If this is indeed an Einstein ring, it is most likely a background source gravitationally lensed by the BL Lac object host galaxy and possibly a surrounding group or cluster. In the extreme case that all four candidates are true lenses, the derived frequency 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 contains an uncommon arclike structure; and 1959+650, which is hosted by a gas-rich elliptical galaxy with a prominent dust lane of ~5 × 105 M☉.The Astrophysical Journal 01/2009; 521(1):134. · 6.02 Impact Factor -
Article: Hubble Space Telescope Observations of the Optical Jets of PKS 0521–365, 3C 371, and PKS 2201+044
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ABSTRACT: 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.The Astrophysical Journal 01/2009; 526(2):643. · 6.02 Impact Factor -
Article: Hubble Space Telescope Observations of the Host Galaxies of BL Lacertae Objects
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ABSTRACT: Six BL Lac objects from the complete 1 Jy radio-selected sample of 34 objects were observed in cycle 5 with the Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 (WFPC2) to an equivalent limiting flux of μ~26 mag arcsec-2. Here we report results for the second half of this sample, as well as new results for the first three objects, discussed previously by Falomo and coworkers. In addition, we have analyzed in the same way HST images of three X-ray-selected BL Lac objects observed by Jannuzi and coworkers. The ensemble of nine BL Lac objects spans the redshift range from z=0.19 to ~1. Host galaxies are clearly detected in seven cases, while the other two, at z~0.258 (redshift highly uncertain) and z=0.997, are not resolved. The HST images make up a homogeneous data set with unprecedented morphological information between a few tenths of an arcsecond and several arcseconds from the nucleus, allowing us to rule out definitively a pure disk light profile in six of the seven detected host galaxies. The host galaxies are luminous ellipticals with an average absolute magnitude of M~-24.6 mag (with dispersion 0.7 mag), more than a magnitude brighter than L and comparable to the brightest cluster galaxies. The morphologies are generally smooth and have small ellipticities (0.2). Given such roundness, there is no obvious alignment with the more linear radio structures. In the six cases for which we have HST WFPC2 images in two filters, the derived color profiles show no strong spatial gradients and are as expected for K-corrected passively evolving elliptical galaxies. The host galaxies of the radio-selected and X-ray-selected BL Lac objects for this very limited sample are comparable in both morphology and luminosity.The Astrophysical Journal 01/2009; 512(1):88. · 6.02 Impact Factor -
Chapter: A Few Degrees Very Wide Field of View Camera for VLT as a Finder for ELT
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ABSTRACT: The quest for wide field imaging, with some added multi-object spectroscopic capability, immediately evokes the Prime Focus option. On the existing 8 m class telescopes the Subaru [Y. Komiyama et al. in Proc. SPIE 5492, 525–532 (2004)] and the LBT [R. Ragazzoni et al. in Proc. SPIE 5492, 507–512 (2004)] owns these kind of instruments, while for Gemini some has been planned although none has been actually built.12/2008: pages 385-388; -
Article: The Nuclear to Host Galaxy Relation of High-Redshift Quasars
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ABSTRACT: We present near-infrared imaging of the host galaxies of low-luminosity quasars at 1 < z < 2, aimed at investigating the relationship between the nuclear and host-galaxy luminosities at high redshift. This work complements our previous study to trace the cosmological evolution of the host galaxies of high-luminosity quasars. The sample consists of nine radio-loud (RLQ) and six radio-quiet (RQQ) low-luminosity quasars. They have similar redshift and optical luminosity distributions, and together with the high-luminosity quasars, cover a large range of the quasar luminosity function. For all but two of the quasars, we have been able to derive the global properties of the surrounding nebulosity. The host galaxies of both types of quasars are massive inactive ellipticals between L* and 10L*, with RLQ hosts being significantly more luminous than RQQ hosts. This luminosity gap is independent of the rest-frame U-band luminosity but correlated with the rest-frame R-band luminosity. The color difference between the RQQs and the RLQs is likely a combination of an intrinsic difference in the strength of the thermal and nonthermal components in their SEDs, and a selection effect due to internal dust extinction. For the combined set of quasars, we find a reasonable correlation between the nuclear and the host luminosities. This correlation is less apparent for RQQs than for RLQs. If the R-band luminosity represents the bolometric luminosity, and the host luminosity is proportional to the black hole mass, as in nearby massive spheroids, quasars emit in a relatively narrow range with respect to their Eddington luminosity and with the same distribution for RLQs and RQQs.The Astrophysical Journal 12/2008; 660(2):1039. · 6.02 Impact Factor -
Article: The Cosmic Evolution of Quasar Host Galaxies
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ABSTRACT: 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 Astrophysical Journal 12/2008; 604(2):495. · 6.02 Impact Factor -
Article: The Hubble Space Telescope Survey of BL Lacertae Objects. III. Morphological Properties of Low-Redshift Host Galaxies
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ABSTRACT: We report on the optical properties of a sample of 30 BL Lac host galaxies in the redshift range 0.03 < z < 0.2, as derived from Hubble Space Telescope observations. All galaxies are fully resolved in the WFPC2 (F702W filter) images, allowing a quantitative analysis in two dimensions. Most and possibly all of these galaxies have characteristics very similar to those of "normal" giant ellipticals. The luminosity, ellipticity, isophote twisting, and amount of disky or boxy isophotes are consistent with those found in nonactive ellipticals and in radio galaxies. In all cases the BL Lac nucleus is well centered in the main body of its host galaxy, a result that argues strongly against the microlensing hypothesis for any significant fraction of the population. A search for faint substructures in the host galaxies has not revealed notable signatures of tidal distortions or subcomponents (faint disks, bars, X features, etc.), and with only one exception, there are no prominent dusty features in the central regions. Instead, the BL Lac host galaxies are smooth and unperturbed, suggesting that strong external gravitational interactions are not important to ongoing activity. A careful examination of the environment around the nucleus, however, shows a high incidence of close companion objects, whose nature remains unclear pending spectroscopic observations.The Astrophysical Journal 12/2008; 542(2):731. · 6.02 Impact Factor -
Article: The Hubble Space Telescope Survey of BL Lacertae Objects. II. Host Galaxies
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ABSTRACT: We have used the Hubble Space Telescope (HST) WFPC2 camera to survey 132 BL Lac objects comprising seven complete radio-, X-ray-, and optically selected samples. We obtained useful images for 110 targets spanning the redshift range 0 z 1.3. These represent an unbiased subsample of the original 132 since they were snapshots selected to fill random holes in the HST schedule. The exposure times ranged from a few hundred to ~1000 s, increasing with redshift. Most images were taken in the F702W filter; 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 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 profile is significantly preferred, at 99% confidence, over a pure exponential disk; the two fits 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% confidence) 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 MR ~ -23.7 ± 0.6 mag (rms 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 ( 0.2). The 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-ray-selected BL Lac objects are comparable in both morphology and luminosity, strongly suggesting that nuclear properties do not have a dramatic effect 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 II's, which instead are more similar to quasars. Thus the present data strongly support the unification picture with FR I galaxies constituting the bulk of the parent population of BL Lac objects.The Astrophysical Journal 12/2008; 532(2):816. · 6.02 Impact Factor -
Article: Near-Infrared Adaptive Optics Imaging of High-Redshift Quasars
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ABSTRACT: The properties of high-redshift quasar host galaxies are studied in order to investigate the connection between galaxy evolution, nuclear activity, and the formation of supermassive black holes. We combine new near-IR observations of three high-redshift quasars (2 < z < 3), obtained at the ESO Very Large Telescope equipped with adaptive optics, with selected data from the literature. For the three new objects we were able to detect and characterize the properties of the host galaxy, found to be consistent with those of massive elliptical galaxies of MR ~ − 24.7 for the one radio-loud quasar, and MR ~ − 23.8 for the two radio-quiet quasars. When combined with existing data at lower redshift, these new observations depict a scenario where the host galaxies of radio-loud quasars are seen to follow the expected trend of luminous (~5L*) elliptical galaxies undergoing passive evolution. This trend is remarkably similar to that followed by radio galaxies at z > 1.5. Radio-quiet quasar hosts also follow a similar trend but at a lower average luminosity (~0.5 mag dimmer). The data indicate that quasar host galaxies are already fully formed at epochs as early as ~2 Gyr after the big bang and then passively fade in luminosity to the present epoch.The Astrophysical Journal 12/2008; 673(2):694. · 6.02 Impact Factor
Top co-authors
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Institutions
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2009
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Astronomical Observatory of Trieste
Trieste, Friuli Venezia Giulia, Italy
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2005
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INAF-Istituto Nazionale di Astrofisica
- Institute of Space Astrophysics and Cosmic Physics IASF - Rome
Roma, Latium, Italy
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