Level Of Dust Obscuration Research Articles

The invisible AGN catalogue: a mid-infrared–radio selection method for optically faint active galactic nuclei

A large fraction of active galactic nuclei (AGN) are "invisible" in extant optical surveys due to either distance or dust-obscuration. The existence of this large population of dust-obscured, infrared-bright AGN is predicted by models of galaxy - supermassive black hole coevolution and is required to explain the observed X-ray and infrared backgrounds. Recently, infrared colour-cuts with WISE have identified a portion of this missing population. However, as the host galaxy brightness relative to that of the AGN increases, it becomes increasingly difficult to differentiate between infrared emission originating from the AGN and from its host galaxy. As a solution, we have developed a new method to select obscured AGN using their 20 cm continuum emission to identify the objects as AGN. We created the resulting Invisible AGN Catalogue by selecting objects that are detected in AllWISE (mid-IR) and FIRST (20 cm), but are not detected in SDSS (optical) or 2MASS (near-IR), producing a final catalogue of 46,258 objects. 30 per cent of the objects are selected by existing selection methods, while the remaining 70 per cent represent a potential previously-unidentified population of candidate AGN that are missed by mid-infrared colour cuts. Additionally, by relying on a radio continuum detection, this technique is efficient at detecting radio-loud AGN at z > 0.29, regardless of their level of dust obscuration or their host galaxy's relative brightness.

Radio-selected Galaxies in Very Rich Clusters at [CLC][ITAL]z[/ITAL][/CLC] ≤ 0.25. II. Radio Properties and Analysis

We report on the properties of radio-selected galaxies within 30 very-rich Abell clusters with z . 0.25. The radio, optical, and x-ray data for these clusters were presented in Paper I (Morrison et al. 2002). These radio data sample the ultra-faint (L1.4 ≥ 2 × 10 22 W Hz −1 ) radio galaxy population with MR ≤ −21 using the well-known FIR/radio correlation to link the radio with ongoing star formation within individual cluster galaxies. Spectroscopic redshifts exist for ∼ 96% of the optical identifications. These radio-selected galaxies reveal the ‘active’ galaxy population (starburst and active galactic nuclei) within these rich cluster environments that can be identified regardless of their level of dust obscuration. These new radio data provide the largest sample to date of low-luminosity radio galaxies within rich cluster environments allowing an unbiased search for dusty starbursting galaxies. For all clusters in our sample, we are sensitive to star formation rates (M ≥ 5M⊙) & 5 M⊙yr −1 . We have found that the excess number of low-luminosity ‘starburst’ radio-selected galaxies (SBRG) found by Owen et al. (1999) in Abell 2125 is not indicative of other rich clusters in our sample. The average fraction of SBRG is h fSBRG i = 0.022 ± 0.003. The A2125 fraction is fSBRG = 0.09 ± 0.03 which is significantly different from the sample average at a > 99.99% confidence level. Both A1278 and A1689 are slightly different from the rest of the sample at ∼ 90% confidence level. The bimodal structure of both the x-ray brightness distribution and optical adaptively smoothed images of A1278 and A2125 suggests that ongoing cluster-cluster mergers may be enhancing this SBRG population. The A1689 excess low-luminosity (and high-luminosity) radio galaxy population may be due to interaction with the ICM. The mid-infrared ISOCAM results for A1689’s radio galaxy population suggests that the radio emission for both low- and high-luminosity radio galaxies is AGN in origin except for one radio galaxy. There is a significant spatial distribution difference between the low and high-luminosity (HLRG) radio-selected populations. The SBRG have a core radius of 0.40 ± 0.08 Mpc which is > 3× larger than the HLRG core radius. In addition, 48% of the SBRGs have colors that are bluer than a typical Sab galaxy compared to 4% for the HLRGs. The average absolute magnitude for the SBRG’s is h MR i = −21.93 ± 0.05, while for the HLRG’s it is h MR i = −22.33 ± 0.07, indicating that the SBRG are less optically luminous than their HLRG counterparts. The HLRGs seem to be a subclass of the cluster’s massive red elliptical population, while the SBRGs have a projected radial distribution more like the blue spiral population. Our results indicate that most of the SBRGs are probably gas-rich disk galaxies undergoing & 5 M⊙ yr −1 of star-formation. Subject headings: galaxies: evolution — galaxies: clusters: Abell — galaxies: starburst — radio continuum: galaxies