Final Report Summary - CTAGN (Detecting Compton thick AGN with the European missions XMM and Herschel)
The last years were very prolific for X-ray Astronomy because of the launch of two major missions, the European XMM and the US Chandra. The number density of sources found in deep X-ray observations reaches about 20, 000 per sq. degree. This is to be compared with about a few hundred which comes from optical AGN surveys. Thus X-ray wavelengths have proved to be the most efficient way of detecting Active Galactic Nuclei i. e. supermassive black holes with black hole masses about 100 million solar masses which lurk in the center of almost all galaxies. The XMM and Chandra surveys provided vital information on these black holes, their evolution with cosmic time as well. However, even the X-ray surveys have difficulties in identifying the most obscured AGN, namely sources with obscuring column densities around 10^24 cm-2 (or equivalent optical obscuration of AV~400 mag). These extreme sources are nick-named Compton-thick AGN because the attenuation of the X-ray radiation is caused by Compton scattering on electrons rather than photoelectric absorption. The identification of this'hidden'part of the Universe was the scope of the current project, where the most comprehensive methodology so far developed
for the detection of Compton-thick AGN is presented.
We employed not only deep X-ray observations but also IR observations. The advantage of the IR observations is that the obscured radiation is re-emitted at these wavelengths making these sources copious IR emitters. Our analysis takes place mainly in the Chandra Deep Field South (CDFS). This is the region of the sky with the deepest X-ray observations ever obtained (3Ms of XMM and 4 Ms of Chandra) but also with a variety of multi-wavelength observations (e. g. Spitzer, Herschel). In specific we used a plethora of tools to reveal this elusive class of objects, including: a) X-ray spectroscopy. This is the most direct way and is based on the detection of either a flat spectrum or a strong iron line (both hints of reflecting light on the obscuring screen; Comastri et l. 2001) b) mid-IR spectroscopy. The presence of optically-thick material at mid-IR wavelengths (9. 7 microns) in Spitzer mission spectra. (Georgantopoulos et al. 2011c) c) iR photometry. Selection of mid-IR excess sources (24 micron) relative to their optical emission (Georgantopoulos et al. 2011a). These heavily obscured sources in the optical have high probability in being obscured in X-rays as well as is revaled from X-ray spectroscopy d) It has been proposed that low ratios of X-ray relative to the mid-IR luminosity can select Compton-thick sources. The key for this diagnostic is that the X-ray emission is obscured while the IR emission remains unattenuated. It has been shown for the first time, that this method is not as efficient as expected. In particular there are many Compton-thick sources which present high X-ray to mid-IR luminosity ratios and hence remain unidentified by this technique (Georgantopoulos et al. 2011d).
The research project findings had a relevant impact on the design of the observing strategy of the forthcoming X-ray mission NuSTAR (due for launch early 2012). The discovery of hidden, accretion powered, Super-massive Black Holes is among the key science goal of the mission.
Important synergies have been forged with institutes which are key players in the analysis of Herschel data and in particular with Saclay in France. These collaborations revolved around two axes. First, the Spitzer spectral analysis of sources in the CDFS. Second, the photometric analysis of Herschel sources in the CDFS. Expertise for the development of SED fitting tools has also been acquired through these collaborations. In particular there have been close collaborations with Dr. Elbaz, Dr. Dasyra and Dr. Mullaney.
An extended description of the various project work packages and training activities is described in the attached document. The ultra deep (3Ms) XMM image of the CDFS is also attached
for the detection of Compton-thick AGN is presented.
We employed not only deep X-ray observations but also IR observations. The advantage of the IR observations is that the obscured radiation is re-emitted at these wavelengths making these sources copious IR emitters. Our analysis takes place mainly in the Chandra Deep Field South (CDFS). This is the region of the sky with the deepest X-ray observations ever obtained (3Ms of XMM and 4 Ms of Chandra) but also with a variety of multi-wavelength observations (e. g. Spitzer, Herschel). In specific we used a plethora of tools to reveal this elusive class of objects, including: a) X-ray spectroscopy. This is the most direct way and is based on the detection of either a flat spectrum or a strong iron line (both hints of reflecting light on the obscuring screen; Comastri et l. 2001) b) mid-IR spectroscopy. The presence of optically-thick material at mid-IR wavelengths (9. 7 microns) in Spitzer mission spectra. (Georgantopoulos et al. 2011c) c) iR photometry. Selection of mid-IR excess sources (24 micron) relative to their optical emission (Georgantopoulos et al. 2011a). These heavily obscured sources in the optical have high probability in being obscured in X-rays as well as is revaled from X-ray spectroscopy d) It has been proposed that low ratios of X-ray relative to the mid-IR luminosity can select Compton-thick sources. The key for this diagnostic is that the X-ray emission is obscured while the IR emission remains unattenuated. It has been shown for the first time, that this method is not as efficient as expected. In particular there are many Compton-thick sources which present high X-ray to mid-IR luminosity ratios and hence remain unidentified by this technique (Georgantopoulos et al. 2011d).
The research project findings had a relevant impact on the design of the observing strategy of the forthcoming X-ray mission NuSTAR (due for launch early 2012). The discovery of hidden, accretion powered, Super-massive Black Holes is among the key science goal of the mission.
Important synergies have been forged with institutes which are key players in the analysis of Herschel data and in particular with Saclay in France. These collaborations revolved around two axes. First, the Spitzer spectral analysis of sources in the CDFS. Second, the photometric analysis of Herschel sources in the CDFS. Expertise for the development of SED fitting tools has also been acquired through these collaborations. In particular there have been close collaborations with Dr. Elbaz, Dr. Dasyra and Dr. Mullaney.
An extended description of the various project work packages and training activities is described in the attached document. The ultra deep (3Ms) XMM image of the CDFS is also attached
