The Spitzer/Herschel Active Galaxies Survey
IntroductionWe have selected 170 AGN comprising matched samples of radio-quiet and radio-loud QSOs and radio galaxies. The AGN, all at z=1, span a factor of 100 in optical luminosity allowing us to study a wide range of black hole activity at a single cosmologically important epoch devoid of concerns about redshift-luminosity degeneracy. Complete datasets are in-hand from Herschel (PACS/SPIRE - PI Stevens (Herts)), Spitzer (IRAC/MIPS- PI Jarvis (Oxford)) while we have data from XMM-Newton and Swift (PI Page (MSSL)) for the radio quiet objects (PI Page (MSSL)) and now for most of the radio-loud objects as well (PI Hardcastle (Herts)). Additional optical data are available from SDSS and from the XMM Optical Monitor (OM). The major goals of the survey are split into 3 themes:
- The first theme tackles AGN feedback and the co-evolution of galaxies and black holes. With its high-sensitivity and broad wavelength coverage the Herschel Space Observatory offers the best way of measuring the instantaneous star-formation rates (SFR) of AGN. We can compare the measured SFR to the rate needed to bring the host galaxy and black-hole masses on to the present-day black hole mass-bulge relation. We have measured the black-hole masses using the virial estimate and the bulge luminosities from deep K-band imaging for all of our targets which will allow us to determine the fundamental link between the fuelling of the AGN and star-formation in the host galaxy and how this scales with accretion rate, providing crucial information on their co-evolution.
- The second theme is a detailed picture of how the full SEDs of the AGN change as a function of luminosity, orientation, radio-loudness and redshift. For many years the mean QSO SED presented by Elvis et al. (1994) has been the standard for bolometric correction of QSO spectra. However, it is now widely suspected that Elvis et al. (1994) is not appropriate for the purpose, given that its fairly extreme selection criteria (observation and detection with the Einstein observatory) produces a significant bias towards blue and X-ray bright objects. Furthermore, the more recent studies of composite QSO SEDs which address some of the Elvis et al. selection biases are based on quasars covering a wide range of redshifts (0.1 - 2.0). Our sample is at a single redshift and so redshift evolution can be neglected.
- The third theme is a study of the environments of the AGN. From numerical simulations and observations, one would expect the most powerful AGN, which host the most massive black holes, to lie in the most over-dense environments at high redshifts. However, these studies at high-redshift have mostly been carried out around the most powerful radio galaxies and quasars. Our study will push this work to more typical mass scales, and will allow investigation of the environmental density in a systematic manner over a large range of optical/radio luminosity.
One of our fields centred on a QSO at z=1 showing evidence for a surrounding cluster. Colour scale shows the IRAC 3.6 micron image, the contours are Herschel SPIRE data at 250 microns, and the image is about 4.5 x 4.5 arcmin.
Over-density of galaxies at 3.6 microns relative to the background calculated in annuli centred on the AGN. We find evidence for larger over-densities around the radio-loud AGN possibly indicating that the dense environment is enhancing the radio luminosity. Alternatively the effect might be caused by an increased merger rate in dense environments which leads to higher black hole spin and thus higher radio luminosity (from Falder et al. 2010, MNRAS, 405, 347).