The determination of the redshift of extragalactic gamma-ray sources is of fundamental importance for the interpretation of their emission models and extragalactic population studies, active galactic ...nuclei classification schemes, and to study the interaction with the extragalactic background light. Here we focus on the BL Lac S2 0109+22, recently detected at very high energies. We obtained a high signal-to-noise optical spectrum at Gran Telescopio Canarias for the source over the spectral range 4000–10 000 Å, to search for spectral features of the host galaxy and/or of the nuclear source and it was compared with previous observations in order to confirm and/or dispute its redshift determination. We find the S2 0109+22 optical spectrum is well represented by a power-law continuum without any absorption or emission feature of equivalent width >0.1 Å. Based on the new optical spectrum, we are able to set a redshift lower limit z > 0.35, assuming the source is hosted by a massive elliptical galaxy typical for this class of sources. The redshift z = 0.265 proposed in the literature for this object is most probably referred to a r = 18.3 galaxy at 15 arcsec from S2 0109. This galaxy could belong to a group of faint galaxies located at z ∼ 0.26.
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.
We study the geometry of the Hβ broad emission region by comparing the MBH 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 of f values we found, together with the strong dependence of f 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.
Abstract
We report on a study of the galaxy environments of low redshift physical quasars pairs. We selected 20 pairs having projected separation < 0.5 Mpc and difference of systemic velocity < 800 ...km s−1. Using Sloan Digital Sky Survey images, we evaluated the galaxy overdensity around these quasars in pairs and then compare it with that of a sample of isolated quasars with same redshift and luminosity. It is found that on average there is a systematic larger overdensity of galaxies around quasars in pairs with respect to that of isolated quasars. This may represent a significant link between nuclear activity and galaxy environment. However, at odds with that, the closest quasar pairs seem to inhabit poorer environments. Implications of present results and perspectives for future work are briefly discussed.
The environment of low-redshift quasar pairs Sandrinelli, A; Falomo, R; Treves, A ...
Monthly notices of the Royal Astronomical Society,
10/2014, Letnik:
444, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We investigate the properties of the galaxy environment of a sample of 14 low-redshift (z < 0.85) quasar (QSO) physical pairs extracted from Sloan Digital Sky Survey Data Release 10 archives. The ...pairs have a systemic radial velocity difference ΔV
∥ ≤ 600 km s−1 (based on O iii5007 Å line) and projected distance R
⊥ ≤ 600 kpc. The physical association of the pairs is statistically confirmed at a level of ∼90 per cent. For most of the images of these QSOs we are able to resolve their host galaxies that turn out to be on average similar to those of QSOs not in pairs. We also found that QSOs in a pair are on average in region of modest galaxy overdensity extending up 0.5 Mpc from the QSO. This galaxy overdensity is indistinguishable from that of a homogeneous sample of isolated QSOs at the same redshift and with similar host galaxy luminosity. These results, albeit derived from a small (but homogeneous) sample of objects, suggest that the rare activation of two QSOs with small physical separation does not require any extraordinary environment.
Context. Blazars are astrophysical sources whose emission is dominated by non-thermal processes, i.e. synchrotron and inverse Compton emission. Although the general picture is rather robust and ...consistent with observations, many aspects are still unexplored. Aims. Polarimetric monitoring can offer a wealth of information about the physical processes in blazars. Models with largely different physical ingredients can provide almost indistinguishable predictions for the total flux, but usually are characterized by different polarization properties. We explore the possibility to derive structural information about the emitting regions of blazars by means of a joint analysis of rapid variability of the total and polarized flux at optical wavelengths. Methods. Short timescale (from tens of seconds to a couple of minutes) optical linear polarimetry and photometry for two blazars, BL Lacertae and PKS 1424+240, was carried out with the PAOLO polarimeter at the 3.6 m Telescopio Nazionale Galileo. Several hours of almost continuous observations were obtained for both sources. Results. Our intense monitoring allowed us to draw different scenarios for BL Lacertae and PKS 1424+240, with the former characterized by intense variability and the latter practically constant in total flux. Essentially the same behavior is observed for the polarized flux and the position angle. The variability time-scales turned out to be as short as a few minutes, although involving only a few percent variation of the flux. The polarization variability time-scale is generally consistent with the total flux variability. Total and polarized flux appear to be essentially uncorrelated. However, even during our relatively short monitoring, different regimes can be singled out. Conclusions. No simple scenario is able to satisfactorily model the very rich phenomenology exhibited in our data. Detailed numerical simulations show that the emitting region should be characterized by some symmetry, and the inclusion of turbulence for the magnetic field may constitute the missing ingredient for a more complete interpretation of the data.
Quasar projected pairs (QPPs) can be used for investigating the circumgalactic medium of quasars through the study of intervening absorption lines in the spectrum of the background quasar (QSO
B
) ...that are at the same redshift of the foreground quasar (QSO
F
). Here we report on optical spectroscopy, gathered at Gran Telescopio Canarias, of 14 QPPs. In 7 cases we find MgII absorption lines associated with the foreground quasar. Only for two cases line-of-sight absorptions (LOS) are revealed. These new observations complement our previous study performed on other 30 QPPs. A brief discussion of the properties of the intervening absorption lines associated with the foreground quasar for the full dataset is reported.
We investigate the properties of circumgalactic gas in the halo of quasar (QSO) host galaxies from C iv absorption-line systems. We use the optical spectroscopy of closely aligned pairs of quasars ...(projected distance ≤ 200 kpc) obtained at the Gran Telescopio Canarias to investigate the distribution of the absorbing gas for a sample of 18 quasars at z ∼ 2. We have found that the detected absorption systems of EW ≥ 0.3 Å associated with the foreground QSOs are revealed up to 200 kpc from the centre of the host galaxy. The structure of the absorbing gas is patchy with a covering fraction of the gas that quickly decreases beyond 100 kpc. These results are in qualitative agreement with those found for the lower ionization metal Mg ii λ2800 Å.
On the cool gaseous haloes of quasars Farina, E. P; Falomo, R; Decarli, R ...
Monthly notices of the Royal Astronomical Society,
02/2013, Letnik:
429, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present the optical spectroscopy of projected quasar (QSO) pairs in order to investigate the Mg ii and C iv absorption features imprinted on the spectrum of the background object by the gaseous ...halo surrounding the foreground QSO. We have observed 13 projected pairs in the redshift range 0.7 z
F 2.2, spanning projected separations between 60 and 120 kpc. In the spectra of the background QSOs, we have identified Mg ii intervening absorption systems associated with the foreground QSOs in seven out of 10 pairs, and one absorption system out of three is found for C iv. The distribution of the equivalent width (EW) as a function of the impact parameter shows that, unlike the case of normal galaxies, some strong absorption systems (EWr > 1 Å) are also present beyond a projected radius of ∼70 kpc. If we take into account the mass of the galaxies as an additional parameter that influences the extent of the gaseous haloes, the distribution of the absorptions connected to the QSOs is consistent with that of galaxies. In the spectra of the foreground QSOs, we do not detect any Mg ii absorption lines originated by the gas surrounding the QSO itself, but in two cases, these features are present for C iv. A 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.
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.
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