Publications (105)70.46 Total impact
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Article: Reclassification of the nearest quasar pair candidate: SDSS J15244+3032 - RXS J15244+3032
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ABSTRACT: We present optical spectroscopy of the nearest quasar pair listed in the 13th edition of the Veron-Cetty & Veron catalogue, i.e. the two quasars SDSS J15244+3032 and RXS J15244+3032 (redshift z~0.27, angular separation ~7 arcsec, and line-of-sight velocity difference ~1900 km/s). This system would be an optimal candidate to investigate the mutual interaction of the host galaxies with ground based optical imaging and spectroscopy. However, new optical data demonstrate that RXS J15244+3032 is indeed a star of spectral type G. This paper includes data gathered with the Asiago 1.82m telescope (Cima Ekar Observatory, Asiago, Italy).02/2013; -
Article: On the radio and NIR jet of PKS 2155-304 and its close environment
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ABSTRACT: PKS 2155-304 is one of the brightest BL Lac object in the sky and a very well studied target from radio to TeV bands. We report on high-resolution (~ 0.12 arcsec) direct imaging of the field of PKS 2155-304 using adaptive optics near-IR observations in J and Ks bands obtained with the ESO multi-conjugate adaptive optic demonstrator (MAD) at the Very Large Telescope. These data are complemented with archival VLA images at various frequencies to investigate the properties of the close environment of the source. We characterized the faint galaxies that form the poor group associated to the target. No radio emission is present for these galaxies, while an old radio jet at ~ 20 kpc from the nucleus of PKS 2155-304 and a jet-like structure of ~ 2 kpc (~ 1 arcsec) in the eastern direction are revealed. No counterparts of these radio jets are found in the NIR or in archival Chandra observations.01/2013; -
Article: On the cool gaseous haloes of quasars
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ABSTRACT: We present optical spectroscopy of projected QSO pairs to investigate the MgII and the CIV absorption features imprinted on the spectrum of the background object by the gaseous halo surrounding the foreground QSO. We observed 13 projected pairs in the redshift range 0.7<z<2.2 spanning projected separations between 60 kpc and 120 kpc. In the spectra of the background QSOs, we identify MgII intervening absorption systems associated to the foreground QSOs in 7 out of 10 pairs, and 1 absorption system out of 3 is found for CIV. The distribution of the equivalent width as a function of the impact parameter shows that, unlike the case of normal galaxies, some strong absorption systems (EWr > 1 Ang) are present also beyond a projected radius of ~70 kpc. If we take into account the mass of the galaxies as an additional parameter that influence the extent of the gaseous haloes, the distribution of the absorptions connected to the QSOs is consistent to that of galaxies. In the spectra of the foreground QSOs we do not detect any MgII absorption lines originated by the gas surrounding the QSO itself, but in 2 cases these features are present for CIV. The comparison between the absorption features observed in the transverse direction and those along the line of sight allows us to comment on the distribution of the absorbing gas and on the emission properties of the QSOs. Based on observations undertaken at the European Southern Observatory (ESO) Very Large Telescope (VLT) under Programmes 085.B-0210(A) and 086.B-0028(A).11/2012; -
Article: Resolved stellar population of distant galaxies in the ELT era
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ABSTRACT: The expected imaging capabilities of future Extremely Large Telescopes (ELTs) will offer the unique possibility to investigate the stellar population of distant galaxies from the photometry of the stars in very crowded fields. Using simulated images and photometric analysis we explore here two representative science cases aimed at recovering the characteristics of the stellar populations in the inner regions of distant galaxies. Specifically: case A) at the center of the disk of a giant spiral in the Centaurus Group, (mu B~21, distance of 4.6 Mpc); and, case B) at half of the effective radius of a giant elliptical in the Virgo Cluster (mu~19.5, distance of 18 Mpc). We generate synthetic frames by distributing model stellar populations and adopting a representative instrumental set up, i.e. a 42 m Telescope operating close to the diffraction limit. The effect of crowding is discussed in detail showing how stars are measured preferentially brighter than they are as the confusion limit is approached. We find that (i) accurate photometry (sigma~0.1, completeness >90%) can be obtained for case B) down to I~28.5, J~27.5 allowing us to recover the stellar metallicity distribution in the inner regions of ellipticals in Virgo to within ~0.1 dex; (ii) the same photometric accuracy holds for the science case A) down to J~28.0, K~27.0, enabling to reconstruct of the star formation history up to the Hubble time via simple star counts in diagnostic boxes. For this latter case we discuss the possibility of deriving more detailed information on the star formation history from the analysis of their Horizontal Branch stars. We show that the combined features of high sensitivity and angular resolution of ELTs may open a new era for our knowledge of the stellar content of galaxies of different morphological type up to the distance of the Virgo cluster.06/2012; -
Article: The optical spectrum of PKS 1222+216 and its black hole mass
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ABSTRACT: We investigate the optical spectral properties of the blazar PKS 1222+216 during a period of 3 years. While the continuum is highly variable the broad line emission is practically constant. This supports a scenario in which the broad line region is not affected by jet continuum variations. We thus infer the thermal component of the continuum from the line luminosity and we show that it is comparable with the continuum level observed during the phases of minimum optical activity. The mass of the black hole is estimated through the virial method from the FWHM of MgII, Hbeta, and Halpha broad lines and from the thermal continuum luminosity. This yields a consistent black hole mass value of 6x10^8 solar masses.04/2012; -
Article: On the cosmological evolution of the black hole - host galaxy relation in quasars
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ABSTRACT: Quasars are useful tracers of the cosmological evolution of the black hole mass - galaxy relation. We compare the expectations of Semi-Analytical Models (SAM) of galaxy evolution, to the largest available datasets of quasar host galaxies out to z=3. Observed quasar hosts are consistent with no evolution from the local M(BH) - L(host) relation, and suggest a significant increase of the mass ratio Gamma = M(BH)/M(host) from z=0 to z=3. Taken at face value, this is totally at odds with the predictions of SAM, where the intrinsic Gamma shows little evolution and quasar host galaxies at high redshift are systematically overluminous (and/or have undermassive BH). However, since quasars preferentially trace very massive black holes (10^9-10^10 Msun) at the steep end of the luminosity and mass function, the ensuing selection biases can reconcile the present SAM with the observations. A proper interpretation of quasar host data thus requires the global approach of SAM so as to account for statistical biases.10/2011; -
Article: The host galaxy of the BL Lacertae object 1ES 0647+250 and its imaging redshift
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ABSTRACT: Since no spectroscopic redshift is available for the remarkable BL Lac object 1ES 0647+250, we aim to derive an estimate of its distance from the properties of its host galaxy.We obtained a deep, high-resolution near-infrared H-band image of the BL Lacertae object 1ES 0647+250. We are able to detect the underlying host galaxy in the near-infrared. The host galaxy has an H-band magnitude of 16.9+-0.2 and an effective radius of 1.6+-0.3 arcsec. Using the imaging redshift method by Sbarufatti et al. (2005), we estimate a redshift z = 0.41+-0.06. This redshift is consistent with the previously reported imaging redshift estimate from the optical i'-band, z = 0.45+-0.08 by Meisner & Romani (2010), and with previously reported lower limits for the redshift. It is also in agreement with constraints from its gamma-ray emission. Imaging searches in the near-infrared, even with moderately sized telescopes, for the host galaxies of BL Lac objects at unknown redshift, are encouraged, as well as optical spectroscopy of 1ES 0647+250 with large telescopes to determine its spectroscopic redshift.09/2011; -
Article: A study of six low‐redshift quasar pairs
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ABSTRACT: The dynamical properties of six Sloan Digital Sky Survey quasar pairs at z≤ 0.8 are investigated. The pairs have proper transverse separation R⊥≤ 500 kpc, and velocity difference along the line of sight ΔVr≤ 500 km s−1. If they are bound systems their dynamical mass can be evaluated and compared with that of host galaxies. Evidence is found of an excess of the former mass with respect to the latter. This suggests that these quasar pairs are hosted by galaxies with massive dark haloes or that they reside in a group/cluster of galaxies.Monthly Notices of the Royal Astronomical Society 06/2011; 415(4):3163 - 3167. · 4.90 Impact Factor -
Article: BL Lac objects with optical jets: PKS 2201+044, 3C 371 and PKS 0521-365
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ABSTRACT: We investigate the properties of the three BL Lac objects, PKS 2201+044, 3C 371 and PKS 0521-365, that exhibit prominent optical jets. We present high resolution near-IR images of the jet of the first two, obtained with an innovative adaptive-optics system (MAD) at ESO VLT telescope. Comparison of the jet in the optical, radio, NIR and X-ray bands reveals strong similarities in the morphology. A common property of these sources is the presence of broad emission lines in their optical spectra at variance with the typical featureless spectrum of the nearby BL Lac objects. Despite some resemblances (e.g. in the radio type), significant differences (e.g. in the central black hole masses and radio structures) with radio-loud NLS1s are found.05/2011; -
Article: The jet of the BL Lacertae object PKS 2201+044: MAD near-IR adaptive optics observations and comparison with optical, radio and X-ray data
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ABSTRACT: Relativistic jets are a common feature of radio loud active galactic nuclei. Multifrequency observations are a unique tool to constrain their physics. We report on a detailed study of the properties of the jet of the nearby BL Lac object PKS 2201+044, one of the rare cases where the jet is detected from radio to X-rays. We use new adaptive optics near-IR observations of the source, obtained with the ESO multi-conjugated adaptive optics demonstrator (MAD) at the Very Large Telescope. These observations acquired in Ground-Layer Adaptive Optics mode are combined with images previously achieved by HST, VLA and Chandra to perform a morphological and photometric study of the jet. We find a noticeable similarity in the morphology of the jet at radio, near-IR and optical wavelengths. We construct the spectral shape of the main knot of jet that appears dominated by synchrotron radiation. On the basis of the jet morphology and the weak lines spectrum we suggest that PKS 2201+044 belongs to the class of radio sources intermediate between FRIs and FRIIs. Comment: 11 pages, 8 figures, accepted for pubblication in A&A12/2010; -
Article: The quasar relation through cosmic time – II. Evidence for evolution from z = 3 to the present age
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ABSTRACT: We study the dependence of the 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 relation along 85 per cent of the Universe age. We show that, in the sampled redshift range, the relation remains nearly unchanged. Once we take into account the ageing of the stellar population, we find that the / ratio (Γ) increases by a factor of ∼7 from z= 0 to z= 3. We show that Γ 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.Monthly Notices of the Royal Astronomical Society 02/2010; 402(4):2453 - 2461. · 4.90 Impact Factor -
Article: A quasar companion to the puzzling quasar SDSS J0927+2943
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ABSTRACT: We report the discovery of a quasar close to SDSS J0927+2943 (z = 0.713), which is a massive binary / recoiling black hole candidate. The companion quasar is at a projected distance of 125 h_70^{-1} kpc and exhibits a radial velocity difference of ~1400 km/s with respect to the known quasar. We discuss the nature of this peculiar quasar pair and the properties of its environment. We propose that the overall system is caught in the process of ongoing structure formation. Comment: 4 pages, 3 figures. Accepted for publication in A&A01/2010; -
Article: The quasar M_bh - M_host relation through Cosmic Time I - Dataset and black hole masses
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ABSTRACT: We study the M_bh - M_host relation as a function of Cosmic Time in a sample of 96 quasars from z=3 to the present epoch. In this paper we describe the sample, the data sources and the new spectroscopic observations. We then illustrate how we derive M_bh from single-epoch spectra, pointing out the uncertainties in the procedure. In a companion paper, we address the dependence of the ratio between the black hole mass and the host galaxy luminosity and mass on Cosmic Time. Comment: 16 pages, 6 figures, 5 tables. Accepted for publication in MNRAS11/2009; -
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 C iv 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 Hβ measurements are available. Combining with the results from the previously studied Mg ii 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.Monthly Notices of the Royal Astronomical Society 11/2009; 399(4):2099 - 2106. · 4.90 Impact Factor -
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 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 five quasars, upper limits to the host luminosity are estimated. The luminosities of the host galaxies of radio-quiet quasars 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 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. Comment: 27 pages, 8 figures, 2 tables. Accepted for publication in ApJ08/2009; -
Article: Co-Evolution of Supermassive Black Holes and Their Host Galaxies
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ABSTRACT: We study the evolution of the MBH/Mhost relation up to z = 3 for a sample of 96 quasars with known host galaxy luminosities. Black hole masses are estimated assuming virial equilibrium in the broad-line regions, while the host galaxy masses are inferred from their luminosities. With this data, we are able to pin down the evolution of the MBH/Mhost relation over 85% of the age of the universe. While the MBH/Lhost relation remains nearly unchanged, taking into account the aging of the stellar population, we find that the MBH/Mhost ratio (Γ) increases by a factor ~ 7 from z = 0 to z = 3. We show that the evolution of Γ is independent of radio loudness and quasar luminosity. We propose that the most massive black holes, in their quasar phase at high-redshift, become extremely rare objects in host galaxies of similar mass in the local universe.Proceedings of the International Astronomical Union 07/2009; 5:34 - 39. -
Article: Probing the nature of the massive black hole binary candidate SDSS J1536+0441
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ABSTRACT: We present an imaging study of the black hole binary candidate SDSS J1536+0441 (z=0.3893), based on deep, high resolution VzK images collected at the ESO/VLT. The images clearly show an asymmetric elongation, indicating the presence of a companion source at ~1" (~5 kpc projected distance) East from the quasar. The host galaxy of the quasar is marginally resolved. We find that the companion source is a luminous galaxy, the light profile of which suggests the presence of an unresolved, faint nucleus (either an obscured AGN or a compact stellar bulge). The study of the environment around the quasar indicates the occurrence of a significant over-density of galaxies with a redshift compatible with z~0.4. This suggests that it resides in a moderately rich cluster of galaxies. Comment: 4 figures, 1 table. Accepted for publication in ApJ Letters07/2009; -
Article: Downsizing of supermassive black holes from the SDSS quasar survey
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ABSTRACT: Starting from the ∼50 000 quasars of the Sloan Digital Sky Survey for which Mg ii line width and 3000 Å 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 full width at half-maximum–nuclear luminosity plane, which can be reproduced at all redshifts assuming a limiting MBH, a maximum Eddington ratio and a minimum luminosity (due to the survey flux limit). We study the z-dependence of the best-fitting parameters of assumed distributions at increasing redshift and find that the maximum mass of the quasar population evolves as log (MBH(max)/M⊙) ∼ 0.3z+ 9, while the maximum Eddington ratio (∼0.45) is practically independent of cosmic time. These results are unaffected by the Malmquist bias.Monthly Notices of the Royal Astronomical Society 06/2009; 396(3):1537 - 1544. · 4.90 Impact Factor -
Article: The jet of the BL Lac object PKS 0521 -365 in the near-IR : MAD adaptive optics observations
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ABSTRACT: BL Lac objects are low--power active nuclei exhibiting a variety of peculiar properties that are caused by the presence of a relativistic jet and orientation effects. We present here adaptive optics near-IR images at high spatial resolution of the nearby BL Lac object PKS 0521-365, which is known to display a prominent jet both at radio and optical frequencies. The observations were obtained in Ks--band using the ESO multi-conjugated adaptive optics demonstrator at the Very Large Telescope. This allowed us to obtain images with 0.1 arcsec effective resolution. We performed a detailed analysis of the jet and its related features from the near-IR images, and combined them with images previously obtained with HST in the R band and by a re-analysis of VLA radio maps. We find a remarkable similarity in the structure of the jet at radio, near-IR, and optical wavelengths. The broad--band emission of the jet knots is dominated by synchrotron radiation, while the nucleus also exhibits a significant inverse Compton component. We discovered the near-IR counterpart of the radio hotspot and found that the near-IR flux is consistent with being a synchrotron emission from radio to X-ray. The bright red object (red-tip), detached but well aligned with the jet, is well resolved in the near-IR and has a linear light profile. Since it has no radio counterpart, we propose that it is a background galaxy not associated with the jet. The new adaptive optics near-IR images and previous observations at other frequencies allow us to study the complex environment around the remarkable BL Lac object PKS 0521-365. These data exemplify the capabilities of multi conjugate adaptive optics observations of extragalactic extended sources. Comment: accepted for publication in Astronomy and Astrophysics 9 pages. A & A 2009, in press06/2009; -
Article: A new apparent quasar pair: Q2225-403A,B
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ABSTRACT: We report the serendipitous discovery of a previously unknown quasar at 10.5" from Q2225-403 (z=2.410). The observation of the broad emission line of MgII_2798 and of the surrounding FeII multiplets indicates that the companion quasar is at z=0.932. The spectrum of Q2225-403 shows a number of absorption lines, the most noteworthy is the MgII line at the same redshift of the companion, suggesting that we are probing the gas within the halo ~80 kpc from the closer quasar. From high resolution NIR images we were able to resolve the host galaxies of the two quasars. Basing on the known surface density of quasars in the 2dF survey we estimate that the probability of finding such a close pair is < ~1 per cent. Comment: 5 pages, 3 figures. Accepted for publication in MNRAS Letters03/2009;
Top co-authors
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Institutions
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2006–2009
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Astronomical Observatory of Trieste
Trieste, Friuli Venezia Giulia, Italy
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1999
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George Mason University
Fairfax, VA, USA
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1994
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Scuola Internazionale Superiore di Studi Avanzati di Trieste
Trieste, Friuli Venezia Giulia, Italy
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