Issue |
A&A
Volume 490, Number 3, November II 2008
|
|
---|---|---|
Page(s) | 893 - 904 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20079299 | |
Published online | 01 July 2008 |
Radio-loud AGN in the XMM-LSS field
II. A dichotomy in environment and accretion mode?
1
Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: tasse@strw.leidenuniv.nl
2
SUPA, Institute for Astronomy, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
3
CEA/DSM/DAPNIA, Service d'Astrophysique, Saclay, 91191 Gif-sur-Yvette, France
Received:
20
December
2007
Accepted:
9
May
2008
In recent years, several authors have argued that low
luminosity radio-loud active galactic nuclei (AGN) have
a different mode of accretion, triggered by different physical
mechanisms, than “normal” optically- or X-ray-selected AGN. The
latter have a radiatively efficient nucleus (sometimes called “Quasar-mode”),
which according to the unified scheme may be obscured from direct
view at optical wavelengths, whereas essentially all of the
energetic output of the low-luminosity radio-loud AGN is in their
radio jets (“Radio-mode”).
In this paper, we independently study the internal and
environmental properties of the optical hosts of the sample of
∼110 radio sources with redshifts in the
XMM-LSS Survey region. We do
this by building a comoving-scale-dependent overdensity
parameter, based on the photometric redshift probability
functions, to constrain the small (~75 kpc) and large
(~450 kpc) scale environments of radio sources independently
from their stellar mass estimates. Our results support the
picture in which the comoving evolution of radio sources in the
redshift range
1 is caused by two distinct galaxy
populations, whose radio source activity is triggered by two
different mechanisms. The first population, which dominates at
high stellar masses (
) is that of
massive elliptical galaxies, lying in galaxy groups or clusters,
where the radio source is triggered by the cooling of the hot gas
in their atmosphere. At these stellar masses, we find that the
fraction of galaxies that host radio-loud AGN is essentially the same
as that in the local Universe. The second population of radio
sources have lower stellar masses, lie in large scale
underdensities, and show excess mid-IR emission consistent with a
hidden radiatively efficient active nucleus. The radio-loud
fraction at these masses is increased relative to the local
Universe. We argue that galaxy mergers and interactions may play
an important role in triggering the AGN activity of this second population.
Key words: surveys / galaxies: active / galaxies: fundamental parameters / radio continuum: galaxies / infrared: galaxies / cosmology: large-scale structure of Universe
© ESO, 2008
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