We use a combination of the XMM-Newton serendipitous X-ray survey with the optical SDSS, and the infrared WISE all-sky survey in order to check the efficiency of the low X-ray to infrared luminosity selection method in finding heavily obscured AGN. W
e select sources in the 2-8 keV X-ray band which have a redshift determination in the SDSS catalogue. We match this sample with the WISE catalogue, and fit the SEDs of the 2844 sources which have three, or more, photometric data-points in the infrared. We then select the heavily obscured AGN candidates by comparing their 12 micron AGN luminosity to the observed 2-10 keV X-ray luminosity and their expected intrinsic relation. With this approach we find 20 candidates, and we examine their X-ray and optical spectra. Of the 20 initial candidates, we find nine (64%; out of the 14, for which X-ray spectra could be fit) based on the X-ray spectra, and seven (78%; out of the nine detected spectroscopically in the SDSS) based on the [OIII] line fluxes. Combining all criteria, we determine the final number of heavily obscured AGN to be 12-19, and the number of Compton-thick AGN to be 2-5, showing that the method is reliable in finding obscured AGN, but not Compton-thick. However those numbers are smaller than what would be expected from X-ray background population synthesis models, which demonstrates how the optical-infrared selection and the scatter of the L_x-L_MIR relation introduced by observational constraints limit the efficiency of the method. Finally, we test popular obscured AGN selection methods based on mid-infrared colours, and find that the probability of an AGN to be selected by its mid-infrared colours increases with the X-ray luminosity. However, a selection scheme based on a relatively low X-ray luminosity and mid-infrared colours characteristic of QSOs would not select ~25% of the heavily obscured AGN of our sample. (abridged)
Models of galaxy evolution assume some connection between the AGN and star formation activity in galaxies. We use the multi-wavelength information of the CDFS to assess this issue. We select the AGNs from the 3Ms XMM-Newton survey and measure the sta
r-formation rates of their hosts using data that probe rest-frame wavelengths longward of 20 um. Star-formation rates are obtained from spectral energy distribution fits, identifying and subtracting an AGN component. We divide the star-formation rates by the stellar masses of the hosts to derive specific star-formation rates (sSFR) and find evidence for a positive correlation between the AGN activity (proxied by the X-ray luminosity) and the sSFR for the most active systems with X-ray luminosities exceeding Lx=10^43 erg/s and redshifts z~1. We do not find evidence for such a correlation for lower luminosity systems or those at lower redshifts. We do not find any correlation between the SFR (or the sSFR) and the X-ray absorption derived from high-quality XMM-Newton spectra either, showing that the absorption is likely to be linked to the nuclear region rather than the host, while the star-formation is not nuclear. Comparing the sSFR of the hosts to the characteristic sSFR of star-forming galaxies at the same redshift we find that the AGNs reside mostly in main-sequence and starburst hosts, reflecting the AGN - sSFR connection. Limiting our analysis to the highest X-ray luminosity AGNs (X-ray QSOs with Lx>10^44 erg/s), we find that the highest-redshift QSOs (with z>2) reside predominantly in starburst hosts, with an average sSFR more than double that of the main sequence, and we find a few cases of QSOs at z~1.5 with specific star-formation rates compatible with the main-sequence, or even in the quiescent region. (abridged)
We present the observed-frame optical, near- and mid-infrared properties of X-ray selected AGN in the Lockman Hole. Using a likelihood ratio method on optical, near-infrared or mid-infrared catalogues, we assigned counterparts to 401 out of the 409 X
-ray sources of the XMM-Newton catalogue. Accurate photometry was collected for all the sources from U to 24um. We used X-ray and optical criteria to remove any normal galaxies, galactic stars, or X-ray clusters among them and studied the multi-wavelength properties of the remaining 377 AGN. We used a mid-IR colour-colour selection to understand the AGN contribution to the optical and infrared emission. Using this selection, we identified different behaviours of AGN-dominated and host-dominated sources in X-ray-optical-infrared colour-colour diagrams. More specifically, the AGN dominated sources show a clear trend in the f_x/f_R vs. R-K and f_24um/f_R vs. R-K diagrams, while the hosts follow the behaviour of non X-ray detected galaxies. In the optical-near-infrared colour-magnitude diagram we see that the known trend of redder objects being more obscured in X-rays is stronger for AGN-dominated than for host-dominated systems. This is an indication that the trend is more related to the AGN contaminating the overall colours than any evolutionary effects. Finally, we find that a significant fraction (~30%) of the reddest AGN are not obscured in X-rays.
We used the large binocular camera (LBC) mounted on the large binocular telescope (LBT) to observe the Lockman Hole in the U, B, and V bands. Our observations cover an area of 925 sq.arcmin. We reached depths of 26.7, 26.3, and 26.3 mag(AB) in the th
ree bands, respectively, in terms of 50% source detection efficiency, making this survey the deepest U-band survey and one of the deepest B and V band surveys with respect to its covered area. We extracted a large number of sources (~89000), detected in all three bands and examined their surface density, comparing it with models of galaxy evolution. We find good agreement with previous claims of a steep faint-end slope of the luminosity functions, caused by late-type and irregular galaxies at z>1.5. A population of dwarf star-forming galaxies at 1.5<z<2.5 is needed to explain the U-band number counts. We also find evidence of strong supernova feedback at high redshift. This survey is complementary to the r, i, and z Lockman Hole survey conducted with the Subaru telescope and provides the essential wavelength coverage to derive photometric redshifts and select different types of sources from the Lockman Hole for further study.
Context: We use the first XMM serendipitous source catalogue (1XMM) to compile a sample of normal X-ray galaxies Aims: We seek to expand the database of X-ray selected normal galaxies at intermediate redshifts and examine the relation between X-ray
emission and star formation for late-type systems Methods: The candidates are selected based on their X-ray (soft spectra), X-ray to optical [log(fx/fo)<-2] and optical (extended sources) properties. 44 candidates are found and 35 are spectroscopically observed with the Australian National Universitys 2.3m telescope to examine their nature. Results: Of the 35 sources observed, 2 are AGN, 11 emission line galaxies, 12 absorption line galaxies, 6 have featureless spectra while 4 are associated with Galactic stars. We combine our emission line sample with earlier works forming the most comprehensive X-ray selected galaxy sample for the study of the X-ray luminosity to the Halpha luminosity - a well-calibrated star-formation indicator - relation. Conclusions: We find that the X-ray luminosity strongly correlates with the Halpha luminosity, suggesting that the X-rays efficiently trace the star-formation.
