Issue |
A&A
Volume 383, Number 3, MarchI 2002
|
|
---|---|---|
Page(s) | 838 - 853 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20011842 | |
Published online | 15 March 2002 |
The AGN contribution to mid-infrared surveys*
X-ray counterparts of the mid-IR sources in the Lockman Hole and HDF-N
1
Instituto de Astrofísica de Canarias (IAC), Via Lactea S/N, 38205 La Laguna, Tenerife, Spain
2
CEA Saclay – Service d'Astrophysique, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
3
Observatoire de Paris Meudon, DAEC, 92195 Meudon Principal Cedex, France
4
AIP, An der Sternwarte 16, 14482 Potsdam, Germany
5
Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
6
XMM-Newton Operations Centre, ESA Vilspa, Apartado 50727, 28080 Madrid, Spain
7
ESO, Karl-Schwarzschild Straße 2, 85748 Garching bei München, Germany
Corresponding author: D. Fadda, fadda@ll.iac.es
Received:
12
June
2001
Accepted:
20
December
2001
We provide constraints on the AGN contribution to the
mid-IR extragalactic background light from a correlation
analysis of deep X-ray and mid-IR observations in two regions
centred on the Lockman Hole and Hubble Deep Field North
(HDF-N). The Lockman region, of more than 200 square
arcminutes, was observed by ISOCAM and XMM–Newton to a depth
of 0.3 mJy at 15 μm (resolving more than 30% of the
mid-IR background). In the same area XMM–Newton reached flux
limits of erg cm-2 s-1 in the
2–10 keV energy band and
erg cm-2
s-1 in the 5–10 keV energy band, resolving about 80% of
the 2–10 keV and 60% of the 5–10 keV backgrounds (the
deepest observation in this hard band to date). Among the 76
galaxies detected by XMM–Newton, 24 show mid-IR emission, but
the relative percentage of X-ray sources with mid-IR
counterparts increases with the band energy: from 30% of the
0.5–2 keV sources up to 63% of the 5–10 keV sources. In
contrast, only a small fraction of the mid-IR sources (around
10%) show X-ray emission within the sensitivity limits of
XMM–Newton observations. The region centred on the HDF-N has
been observed by ISOCAM (24 square arcminutes) to a depth of
0.05 mJy (more than 60% of the mid-IR background is resolved
at this limit) and covered with a 1 Msec exposure by Chandra.
In this case, 25% of the mid-IR sources are detected in the
X-ray, while 30–40% of the X-ray sources show mid-IR
emission. Under the assumption that all XMM sources except
stars or galaxy clusters are AGN–dominated, AGNs contribute
of the total mid-IR flux in the Lockman Hole. For
the HDF-N we have assumed that AGN–dominated sources are
luminous X-ray sources and sources with SEDs from radio to
X-ray wavelengths typical of local AGNs, in which case we find
that
of the mid-IR flux are due to AGN emission.
If we put together all the existing information from the
deepest HDF-N data to the bright large-area sample in the
ELAIS S1 region observed with BeppoSAX (for a total of 50
X-ray–mid-IR matched sources) using the median mid-IR to
X-ray spectral indices as a function of the X-ray flux, we
find an AGN contribution to the 15 μm background of
. Since the IR spectra of typical AGNs peak
around 20 μm while starburst spectra peak at significantly
longer infrared wavelengths, this figure may be considered as an
upper limit to the AGN contribution to the CIRB energy
density. We conclude that the population of IR luminous
galaxies detected in the ISOCAM deep surveys, and the CIRB
sources themselves, are mainly constituted by dust-obscured
starbursts.
Key words: cosmology: observations / infrared: galaxies / X-ray: galaxies: quasars: general
© ESO, 2002
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