Optical and IR IDs to Chandra AEGIS-X Catalogue, v2

Full details of the optical and infra-red identification method can be
found in Laird et al. 2008, submitted to ApJS.

The  optical identification  uses the  Maximum Likelihood  (LR) method
(Ciliegi et al.  2003, A&A, 398, 901; 2005, A&A, 441 879). This is the
ratio between the probability that  a given source at a given distance
from the X-ray centroid and with a given optical magnitude is the true
ID and the  probability that this source is  a spurious alignment. The
LR  ratio takes  into account  the X-ray  positional  uncertainty, the
background density of optical  galaxies and the apriori probability of
an optical counterpart with a given optical magnitude.

We adopt a search radius  of 4" and identify all possible counterparts
within that  distance. The table  lists only the counterpart  with the
higher LR. For  the cross-correlation with the DEEP2  catalogue we use
the R-magnitude to estimate the surface density of background optical
sources. For the CFHTLS we use the i-band selected catalogue. For the
cross-correlation with the IRAC catalogue we use the 3.6 micron to
estimate the surface density of background IR sources.

NOTE  THAT  ***NOT*** ALL  THE  LISTED SOURCES  IN  THE  TABLE HAVE  A
RELIABLE  COUNTERPART EVEN  IF  THERE  IS AN  ENTRY.  WHEN USING  THIS
CATALOGUE YOU NEED TO APPLY A CUTOFF IN LR AS LISTED BELOW. WE SUGGEST
THAT YOU USE ONLY THOSE IDS WITH:

 *** LH>0.5 FOR DEEP2 

 *** LH>0.5 FOR THE CFHTLS. 
 
 *** LH>0.5 FOR IRAC

THESE LR VALUES CORRESPOND TO A 6% SPURIOUS CONTAMINATION RATE FOR THE
OPTICAL IDS AND A 1% CONTAMINATION RATE FOR THE IRAC IDS.

The table columns are:

1. AEGIS X-ray source unique ID 

2. Unique DEEP2 ID. This is  the counterpart with the higher LR within
a search radius of 4arcsec. 1000  means no ID to the DEEP2 photometric
survey limit. -999 means source is outside of the DEEP2 area.

3. offset between X-ray and DEEP2 position in arcsec 

4. DEEP2   ID  likelihood   ratio.   Should   be  >0.5   for  reliable
counterparts. For  this threshold the spurious  identification rate is
6%. 

5. DEEP2 R band magnitude (AB).

6. Unique CFHTLS ID. This is the counterpart with the higher LR within
a search  radius of 4arcsec .  -999 means that the  sources is outside
the  CFHTLS or in a masked  region (i.e.  nearby bright  stars). 1000
means no optical ID to the CFHTLS limit. 

7. offset between X-ray and CFHTLS position in arcsec 

8. CFHTLS likelihood ratio. Should be >0.5 for reliable counterparts.
For this threshold there are ~6%  spurious identifications. 

9. CFHTLS i' band magnitude (AB).

10. CFHTLS r band magnitude (AB).

11. Unique IRAC 3.6 ID, from Barmby et al. 2008, ApJS in press.  This is
the counterpart with the higher LR within a search  radius of 4arcsec.
-999 means that the  sources is outside the IRAC survey.

12. offset between X-ray and IRAC position in arcsec 

13. IRAC likelihood ratio. Should be >0.5 for
reliablecounterparts. For this threshold there are ~1% spurious
identifications.  

14. IRAC 3.6 micron magnitude (AB).

Questions   or   problems?   Please   contact   Antonis   Georgakakis,
age@astro.noa.gr or Elise Laird, e.laird@imperial.ac.uk.