Getting warmer: Detecting AGNs through infrared radiation
Abstract
This thesis addresses using infrared bands to find obscured active galactic nuclei (AGNs). The end goal is to create more complete and reliable samples of AGNs to investigate their general properties, as well as to advance our understanding of topics such as accretion processes and galaxy formation. A multi-wavelength approach is needed to find the numbers of AGNs predicted by theories, and infrared observations provide a particularly useful way of searching for AGNs hidden by a dusty torus. Here, we select AGNs in the Extended Groth strip (EGS) using several infrared techniques with data from the Spitzer Space Telescope, and analyze their properties across radio to X-ray wavelengths.
The first part of this thesis analyzes 72 AGN candidates found using two mid- infrared and radio selection techniques. The first selects objects with a radio flux density excess compared to values expected for star-forming systems. The second searches for AGNs with high mid-infrared, low near-infrared, and intermediate radio flux densities. Thirty percent of these have X-ray counterparts and moderate obscuration with column densities of N H [Special characters omitted.] 10^22 cm -2 . A stacking of the sources without X-ray catalog counterparts reveals low but significant X-ray emission suggesting that the sources are faint or obscured AGNs. In the second part of this work, we examine 489 luminous sources with mid-IR SEDs fitting a red power-law shape. One-third of these are strongly detected in deep X-ray observations. A stacking analysis of the X-ray undetected sources demonstrates significant X-ray emission with a hard, moderately obscured signal. The red power-law sources populate a luminosity range that is distinct from the blue power-law sample with a division at L X ~ 10^44 ergs s -1 , which is near the break luminosity for sources at our observed redshifts. We also examine the SEDs of the sources and investigate the reliability of previously defined IRAC color-color AGN selection methods. With the selection methods addressed here, nearly 60% of the samples are unidentified as AGNs via a variety of other selection techniques. Thus, these selection methods provide a powerful way to identify previously missed AGNs.- Publication:
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Ph.D. Thesis
- Pub Date:
- 2009
- Bibcode:
- 2009PhDT.........9P
- Keywords:
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- AGN;
- Infrared;
- Active galactic nuclei;
- Black holes