Active Galactic Nuclei
Science projects Publications

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:


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 Wisotzki (2000).

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 ≤ 22.5 . 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, see respectively 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 galaxies (XBONGS).

For a detailed description of the X-ray catalog and its follow-up observations see Krumpe et al. (2007).


Overview table :

Survey Selection technique Nb obj. Limiting flux Redshift range References
COMBO-17 SED fitting
17 filters, UV to NIR
192 R < 24 1.2 < z < 5 Wolf et al. (2003)
HES slitless spectrocopy (continuum + EL) 415 BJ < 17.5 0 < z < 3.2 Wisotzki et al. (2000)
Wisotzki (2000)
ROSAT Bright Survey X-ray 669*
fX∼2.4×10-12 erg cm-2s-1 [0.5-2.0 keV] 0 < z < 2.7 Schwope et al. (2000)
VVDS I-band flux limit
broad EL in spectrum
R < 22.5
R < 24
0 < z < 5 Gavignaud et al. (2006)
Bongiorno et al. (2007, sub.)
XMM Marano Field Survey X-ray 125*
fX∼5×10-15 erg cm-2s-1 [0.2-10 keV] 0 < z < 3 Krumpe et al. (2007)
*Spectroscopic AGN (Type-1 and type-2); **Total number of X-ray sources.

Surveys Science projects Publications

Last change 2007 April 11 by Isabelle Gavignaud