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Register a new userThe properties of quasar hosts at the peak of the quasar activity
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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.
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Several analytic and numerical studies have indicated that the interstellar medium of a quasar host galaxy heated by feedback can contribute to a substantial secondary signal in the cosmic microwave background (CMB) through the thermal Sunyaev-Zeldovich (SZ) effect. Recently, many groups have tried to detect this signal by cross-correlating CMB maps with quasar catalogs. Using a self-similar model for the gas in the intra-cluster medium and a realistic halo occupation distribution (HOD) prescription for quasars we estimate the level of SZ signal from gravitational heating of quasar hosts. The bias in the host halo signal estimation due to unconstrained high mass HOD tail and yet unknown redshift dependence of the quasar HOD restricts us from drawing any robust conclusions at low redshift (z<1.5) from our analysis. However, at higher redshifts (z>2.5), we find an excess signal in recent observations than what is predicted from our model. The excess signal could be potentially generated from additional heating due to quasar feedback.
We present models representing the scattering of quasar radiation off free electrons and dust grains in geometries that approximate the structure of quasar host galaxies. We show that, for reasonable assumptions, scattering alone can easily produce ratios of nuclear (point source) to extended fluxes comparable to those determined in studies of quasar hosts. This result suggests that scattered quasar light, as well as stellar emission from the host galaxy, contributes significantly to the detected extended flux, leading to uncertainty in the inferred properties of quasar host. A significant contribution from scattered quasar light will lead to overestimates of the luminosity and hence mass of the host galaxy, and may also distort its morphology. Scattering of quasar light within the host galaxy may provide alternative explanations for the apparent peak in host luminosity at z = 2-3; possibly the overall average higher luminosity of radio-loud host galaxies relative to those of radio-quiet quasars (RQQs), and the apparent preference of high-luminosity RQQs for spheroidal rather than disk galaxies.
We perform a statistical analysis of strong gravitational lensing by quasar hosts of background galaxies, in the two competing models of dark matter halos of quasars, HOD and CS models. Utilizing the BolshoiP Simulation we demonstrate that strong gravitational lensing provides a potentially very powerful test of models of quasar hosting halos. For quasars at $z=0.5$, the lensing probability by quasars of background galaxies in the HOD model is higher than that of the CS model by two orders of magnitude or more for lensing image separations in the range of $thetasim 1.2-12~$arcsec. To observationally test this, we show that, as an example, at the depth of the CANDELS wide field survey and with a quasar sample of $1000$ at $z=0.5$, the two models can be differentiated at $3-4sigma$ confidence level.
Gravitational lensing assists in the detection of quasar hosts by amplifying and distorting the host light away from the unresolved quasar core images. We present the results of HST observations of 30 quasar hosts at redshifts 1 < z < 4.5. The hosts are small in size (r_e <~ 6 kpc), and span a range of morphologies consistent with early-types (though smaller in mass) to disky/late-type. The ratio of the black hole mass (MBH, from the virial technique) to the bulge mass (M_bulge, from the stellar luminosity) at 1<z<1.7 is broadly consistent with the local value; while MBH/M_bulge at z>1.7 is a factor of 3--6 higher than the local value. But, depending on the stellar content the ratio may decline at z>4 (if E/S0-like), flatten off to 6--10 times the local value (if Sbc-like), or continue to rise (if Im-like). We infer that galaxy bulge masses must have grown by a factor of 3--6 over the redshift range 3>z>1, and then changed little since z~1. This suggests that the peak epoch of galaxy formation for massive galaxies is above z~1. We also estimate the duty cycle of luminous AGNs at z>1 to be ~1%, or 10^7 yrs, with sizable scatter.
In the present work the part of the quasar UV-optical bump within the wavelength range 1210-1450AA was studied with the help of composite spectra compiled from the samples of SDSS DR7 spectra with the similar spectral index alpha_{lambda} within 1270-1480 AA. This division allowed to see weak emission lines, which were not detected in previous studies of the quasar composite spectra, but were known from individual optical or composite UV spectra from the Hubble Space Telescope. Although the physical explanation of the difference in spectral indices between quasars and their dependence on quasar parameters is still not clear, it is obvious that this difference has to be taken into account when generating composite spectra, e.g. for redshift measurements. It was also shown that the equivalent width of the emission lines does not depend on the spectral index.
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