arXiv subjects

Alejo Martinez-Sansigre

Publications and source records attributed to Alejo Martinez-Sansigre.

21 records · Page 2Linked to original sources

arXiv2006

A population of high-redshift type-2 quasars-I. Selection Criteria and Optical Spectra

We discuss the relative merits of mid-infrared and X-ray selection of type-2 quasars. We describe the mid-infrared, near-infrared and radio selection criteria used to find a population of redshift z~2 type-2 quasars which we previously argued suggests that most supermassive black hole growth in the Universe is obscured (Martinez-Sansigre et al., 2005). We present the optical spectra obtained from the William Herschel Telescope, and we compare the narrow emission line luminosity, radio luminosity and maximum size of jets to those of objects from radio-selected samples. This analysis suggests that these are genuine radio-quiet type-2 quasars, albeit the radio-bright end of this population. We also discuss the possibility of two different types of quasar obscuration, which could explain how the ~2-3:1 ratio of type-2 to type-1 quasars preferred by modelling our population can be reconciled with the ~1:1 ratio predicted by unified schemes.

astro-ph↗

arXiv2005

Most supermassive black hole growth is obscured by dust

We present an alternative method to X-ray surveys for hunting down the high-redshift type-2 quasar population, using Spitzer and VLA data on the Spitzer First Look Survey. By demanding objects to be bright at 24 microns but faint at 3.6 microns, and combining this with a radio criterion, we find 21 type-2 radio-quiet quasar candidates at the epoch at which the quasar activity peaked. Optical spectroscopy with the WHT confirmed 10 of these objects to be type-2s with 1.4 < z < 4.2 while the rest are blank. There is no evidence for contamination in our sample, and we postulate that our 11 blank-spectrum candidates are obscured by kpc-scale dust as opposed to dust from a torus around the accretion disk. By carefully modelling our selection criteria, we conclude that, at high redshift, 50-80 % of the supermassive black hole growth is obscured by dust.

astro-ph↗

arXiv2005

The obscuration by dust of most of the growth of supermassive black holes

Supermassive black holes underwent periods of exponential growth during which we seem them as quasars in the distant Universe. The summed emission from these quasars generates the cosmic X-ray background, the spectrum of which has been used to argue that most black-hole growth is obscured. There are clear examples of obscured black-hole growth in the form of `type-2' quasars, but their numbers are fewer than expected from modelling of the X-ray background. Here we report on the direct detection of a population of distant type-2 quasars which is at least comparable in size to the well-known unobscured type-1 population. We selected objects that have mid-infrared and radio emissions characteristic of quasars, but which are faint at near-infrared and optical wavelengths. This population is responsible for most of the black hole growth in the young Universe and, throughout cosmic history, black-hole growth occurs in the dusty, gas-rich centres of active galaxies.

astro-ph↗