AGN-host galaxy connection: morphology and colours of X-ray selected AGN at z ≤ 2
Context. The connection between active galactic nuclei (AGN) and their
host galaxies has been widely studied and found to be of great
importance for providing answers to some fundamental questions related
to AGN fuelling mechanisms, and both their formation and evolution. <BR
/> Aims: Using X-ray data and one of the deepest broad-band optical data
sets available, we study how morphology and colours are related to X-ray
properties for sources at redshifts z ≤ 2.0, using a sample of 262
AGN in the Subaru/XMM-Newton Deep Survey (SXDS). <BR /> Methods: We
performed our morphological classification using the galSVM code, which
is a new method that is particularly suited to dealing with
high-redshift sources. Colour-magnitude diagrams were studied in
relationship to redshift, morphology, X-ray obscuration, and
X-ray-to-optical flux ratio. We analysed the different regions in the
colour-magnitude diagrams, and searched for correlations with the
observed properties of AGN populations using models of their formation
and evolution. <BR /> Results: We confirm that a robust and reliable
morphological classification of a general galaxy population at high
redshift should be based on a multi-parametric approach. At least 50% of
X-ray detected AGN at z ≤ 2.0 analysed in this work reside in
spheroidal and bulge-dominated galaxies, while at least 18% have
disk-dominated hosts. This suggests that different mechanisms may be
responsible for triggering the nuclear activity. When analysing
populations of X-ray detected AGN in both colour-magnitude and
colour-stellar mass diagrams, the highest number of sources is found to
reside in the green valley at redshifts ≈0.5-1.5. However, a larger
number of low-luminosity AGN have been detected than in previous works
owing to the substantial depth of the SXDS optical data. Whether AGN are
hosted by early- or late-type galaxies, no clear relationship has been
found with the optical colours (independently of redshift), as is
typical of normal galaxies. Both early- and late-type AGN cover similar
ranges of X-ray obscuration, for both unobscured and obscured sources.
<BR /> Conclusions: Our findings appear to confirm some previous
suggestions that X-ray selected AGN residing in the green valley
represent a transitional population, quenching star formation by means
of different AGN feedback mechanisms and evolving to red-sequence
galaxies. They might be hosted by similar sources (the majority of
sources being late-type elliptical and lenticular galaxies, and
early-type spirals) with similar stellar populations, which are
triggered mainly by major and/or minor mergers, and in some cases by
means of secular mechanism, as shown in previous numerical simulations.
In the aforementioned transition we observe different phases of AGN
activity, with some AGN being in the "QSO-mode" detected as compact,
blue, and unobscured in X-rays, and with others passing through
different phases before and after the "QSO-mode", being obscured and
unobscured in X-rays, respectively.
Full Table A.1 is only available at the CDS via anonymous ftp to
cdsarc.u-strasbg.fr (130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/541/A118">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/541/A118</A>