Surveys for Quasars and AGN
Quasars or active galactic nuclei (AGN) are faint and rare objects. We give here a short description of the
AGN surveys on which our scientific projects are based:
GEMS / STAGES
The COMBO-17 survey (Classifying Objects by Medium-
Band Observations in 17 filters) has imaged 1 square degree of
sky in 17 optical filters using the
Wide Field Imager at the MPG/ESO 2.2-m telescope at La Silla, Chile.
These multi-band data allowed photometric redshift determinations with
accuracies of dz(1+z)~0.02 for several 10.000 galaxies down to R<24.
Two COMBO fields were also observed with the Hubble Space Telescope,
these are the second and third largest mosaics ever taken with HST:
the GEMS (Galaxy Evolution from Morphology and SEDs)
and STAGES (Space Telescope A901/902 Galaxy Evolution Survey) surveys.
GEMS is also the largest HST mosaic ever taken in more than one filter
(V- and z-band).
On the basis of COMBO/GEMS data, we found that distant AGN host galaxies
have bluer colors than their quiescent counterparts, possibly indicating
recent or ongoing star formation in connection with the nuclear active phase.
One of the other scientific highlights is that the stellar mass in galaxies
with ancient stellar populations has increased over the last 8 billion years,
indicating a fill-up of the 'red sequence' by a galaxies that were initially
part of the blue, star forming population.
For more details on the COMBO-17 survey, see Wolf et al. (2003)).
An introduction to the GEMS survey is given in Rix et al. (2004). Our AGN host analyses can be found in Sanchez et al. (2004) and Jahnke et al. (2004);
for our study of the red sequence build-up we refer to Bell et al. (2004).
Hamburg ESO Survey
The Hamburg/ESO survey (HES) was initiated in 1990 as an ESO key
programme, to search in the southern sky for new gravitational lenses and
for bright QSOs suited for high-resolution spectroscopy.
The strategy adopted to cover the large area necessary to find these
extremely rare objects was to uses slitless spectroscopy. Objective prism
plates were taken with the ESO Schmidt telescope and digitised with the
Hamburg PDS microdensitometer.
The selection of QSO candidates from the database of digital spectra
involves a multitude of selection criteria. Extensive follow-up spectroscopy
conducted at ESO has allowed to construct a new flux-limited sample of 415
objects within the redshift range 0 < z < 3.2, with optical magnitudes
BJ < 17.5.
Analysis based on HES QSO showed that the bright tail of the luminosity was
increasing significantly towards higher z.
This phenomenon is opposite to what would be expected from
gravitational lensing, showing that magnification bias does not significantly
distort the QSO luminosity function within the redshift range covered.
For more details on the survey see
Reimers & Wisotzki (1997) and
Wisotzki et al. (2000).
The analysis of the luminosity function is presented in
ROSAT Bright Survey
The ROSAT Bright Survey (RBS) is an identification program of the
more than 2000 X-ray sources detected during the ROSAT All-Sky Survey at high
galactic latitude, |b| > 30°, with countrate above 0.2 s-1. This
program is to more than 99.5% complete.
A sub-sample of 931 sources with countrate above 0.2 s-1 in the hard spectral
band between 0.5 and 2.0 keV is to 100% identified. The total survey area
comprises 20391 deg2 at a flux limit of 2.4×10-12
erg cm-2 s-1 in the 0.5 - 2.0 keV band.
About 1500 sources of the complete sample could be identified by
correlating the RBS with SIMBAD and the NED. The remaining 500 sources were
identified by low-resolution optical spectroscopy and CCD imaging utilizing
telescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea.
As a result it constitutes one of the most massive complete sample of X-ray
selected AGNs with a total of 669 members.
Further detail about the ROSAT All-Sky Survey Bright Source Catalogue (RASS-BSC, revision 1RXS) can be found in
Voges et al. (1999).
For the spectroscopical identification campain of the high galactic latitude
sources see Schwope et al. (2000).
VIRMOS VLT Deep Survey
The VVDS is a purely I-band flux limited spectroscopic survey
designed to study the evolution of galaxy, large scale structure and AGN.
It aims to be the complementary deep and high redshift counterpart of the
2dF and Sloan surveys.
It comprises two subsets: a deep survey with a limit of IAB
≤ 24 and a wide and somewhat shallower survey with IAB ≤
Both surveys utilize the VIMOS multi-object spectrograph on the ESO-VLT to take spectra
of all objects above the flux limit, irrespective of their morphological
properties or colours.
On this basis, a sample of broad line AGN was constructed with a very simple selection
criterion, which requires only the presence of a broad emission line (FWHM
≥ 1000 km -1) in any given spectrum.
In the paper describing the first epoch AGN sample, , it is demonstrated
that, since the sample is unaffected by morphological or colour
pre-selection biases, it is also much less prone to incompleteness due to
host galaxy contamination. The sample has already been used to investigate
the AGN luminosity function and the evolution of the UV luminosity density.
For a complete description of the general VVDS-deep and VVDS-wide samples,
Le Fèvre et al. (2005)
and Garilli et al. (2007, sub.). The broad line AGN sample is described
in Gavignaud et al. (2006) and the analysis of the
luminosity function in
Bongiorno et al. (2007, sub.).
XMM-Newton Marano field survey
This X-ray survey cover a contiguous area of 0.6 deg2 with a
sensivity of fX∼5×10-15erg
cm-2s-1 in the 0.2-10 keV band.
The field was name by an optical quasar survey by
Marano et al. (1988).
This early work, completed by Zitelli et al. (1992) lead to spectroscopically complete
sample of 54 quasars down to a magnitude of J ≤ 22.0, selected from various optical techniques (colors, grism
plate and variability).
In 1992-93 ROSAT observed the central part of this field and could detect
66% of the optically selected quasars with a flux limit of (Zamorani et al. 1999).
The XMM-Newton Marano field survey covers
almost the size of the optical quasar survey. 328 X-ray sources are
detected in total. The core sample where detailed optical follow-up
observations were obtained contains 170 X-ray sources of which 110 are
spectroscopically classified, leading thus to a completeness of
∼65%. About one third of the XMM sources are classified as type II AGN
with redshift mostly below 1.0. Furthermore, it contain as optical
counterparts five high redshift type II QSOs with
2.2 ≤ z ≤ 2.8 and three X-ray bright optically normal
For a detailed description of the X-ray catalog and its follow-up
observations see Krumpe et al. (2007).
Overview table :
*Spectroscopic AGN (Type-1 and type-2);
**Total number of X-ray sources.
Last change 2007 April 11 by Isabelle Gavignaud