infrared selection of obscured agn - 京都大学iwamuro/lecture/agn/ir_agn...infrared excess...
TRANSCRIPT
Infrared Selection of Obscured AGN
Astronomy 278
Outline
Motivation: Why search for AGN in the IR?
Infrared Selection techniques (Donley et al. 2008)
Compton Thick AGN
How can we get a complete selection of AGN?
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Other selection techniques (Radio, optical, X-ray) are known to be incomplete
What motivates the search for AGN at infrared wavelengths?
Want to be able to separately identify the non-stellar emission from a galaxy.
It is thought there is a population of AGN which are not being detected with any of the above techniques- the obscured or Compton-thick AGN.
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
AGN Spectrum in the MIR
Normal galaxy spectra
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Infrared selection with Spitzer
GOODS-S
Herschel+Spitzer
The GOODS field is ideal for the characterization of IR selection techniques:
-Deep MIPS and IRAC -Overlap with deep X-ray and optical surveys
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Donley et al. 2008 review three types of infrared selection:
Power law selection (Alonso-Herrero 2006, Donley 2007)
Color-Color Selection (Lacy et al. 2004, Stern et al. 2005)
IR-Excess Selection (Daddi et al. 2007)
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
IRAC power law selection
Select galaxies consistent with F α ν-0.5
Donley 2008 identify 55 power law galaxies (PLGs) in their sample of ~700 24-micron selected galaxies.
PLGs had a high fraction of X-ray counterparts: 45% ( 64% including marginal detections).
The detected & non-X-ray detected PLGs had consistent properties, suggesting they are all AGN.
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Contamination
Power law selection appears to exclude most of the predicted contamination from star forming galaxies
Models of SED evolution for star forming galaxies
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Color-Color selection--- The WedgesLacy et al. 2004Used models from ISO spectra to predict a color space dominated by AGN emission.
Lacy et al. 2007 tested this model for a sample of 24-micron selected galaxies, and find only 9% contamination.
Stern et al. 2005Empirically identify a Spitzer-IRAC color-space that isolates AGN, with estimated 80% completeness and less than 20% contamination
Lacy 2007
Stern 2005
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Donley 2008 test both Stern & Lacy Wedges
Power-law selected galaxies fall almost entirely inside both wedges.
Donley et al. 2008
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
The Lacy 2004 color space includes essentially all sources at z > 1.75, irrespective of their nature
Potential for significant contamination from z ~2 star forming galaxies
Donley et al. 2008
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
20-30% of K-bright galaxies with 1.7 < z < 2.5 show a mid-infrared excess.
Daddi et al. 2007 analyze this population, and infer a hard X-ray spectrum from a stacking analysis.
They estimate that 50% of their sample are Compton-thick AGN.
Infrared Excess selection
Daddi et al. 2007
Donley et al. 2008 cannot rule this out, but suggest that the majority of these sources could be consistent with star formation alone.
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
What does it mean to be Compton thick?
NH > 1024 molecules per square centimeter
Sgr A*
Reflection-dominated spectra with wide Iron K α lines
Gilli 2007
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Why do we think there is a large fraction of Compton-thick AGN?
Measurements of the column density in hard X-rays and soft gamma rays show that20-25% of the local AGN are Compton-thick.
Risaliti et al. 1999
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Why care about Compton-thick AGN?
A significant population of Compton-thick AGN are required to fit the observed X-ray Background spectrum
Gilli et al. 2007
ComptonThick
Poor fit
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Feruglio et al. 2011 find 2 Compton-thick AGN in the 4 Ms CDFS (z=2.5, 2.9). The implied volume density of Compton-thick sources is in agreement with predictions from XRB models.
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Gilli et al. 2011 find a Compton-thick AGN in the 4 Ms CDFS at z=4.76, which was previously classified by optical spectroscopy.
Comastri et al. 2011 find 2 Compton-thick AGN in the CDFS with a 3.3 Ms XMM survey (z=1.53,3.70) that would have been missed by several IR-excess selection techniques. One of these sources is an analog to NGC 1068.
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Color-color selection revisited (Donley et al. 2012)
Revise selection criteria to emphasize power-law selection, and a high x-ray detection fraction.
More efficiently exclude z~2 star forming galaxies
However, increase their bias toward detecting only the more luminous AGN
+ =
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
4 Ms CDF observations of a sample of BzK selected galaxies.
Find that only 25% of these galaxies have X-ray spectra consistent with being Compton-thick.The rest are unobscured AGN and starbursts.
IR excess selection revisited (Alexander et al. 2011)
These results are inconsistent with fraction of Compton-thick AGN inferred from IR selection of Daddi and Fiore.
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Currently, the most reliable way to identify Compton-thick AGN at high redshift appears to be with deep X-ray observations.
IR-selection techniques:
• Are biased against low-luminosity AGN and AGN with substantial host contribution
• Can suffer from contamination by star forming galaxies, especially at high redshift
• Can efficiently identify luminous, obscured AGN
Why select in the IR? IR Selection techniques Compton-Thick Completeness?
Future prospects for detecting and characterizing Compton-thick AGN
Schleicher, Spaans & Klessen 2010
Just as the excitation of the ionized gas provides some of the most robust identification of AGN, the excitation of the molecular gas may also discriminate between the presence of an AGN and star formation.