EDP Sciences
Free access
Issue
A&A
Volume 507, Number 2, November IV 2009
Page(s) 781 - 793
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361/200912730
Published online 01 October 2009
A&A 507, 781-793 (2009)
DOI: 10.1051/0004-6361/200912730

Low redshift AGN in the Hamburg/ESO survey

I. The local AGN luminosity function
A. Schulze, L. Wisotzki, and B. Husemann

Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
    e-mail: aschulze@aip.de

Received 19 June 2009 / Accepted 17 September 2009

Abstract
We present a determination of the local ($z\approx0$) luminosity function of optically selected type 1 (broad-line) active galactic nuclei. Our primary resource is the Hamburg/ESO survey (HES), which provides a well-defined sample of more than 300 optically bright AGN with redshifts z<0.3 and blue magnitudes $B \la 17.5$. AGN luminosities were estimated in two ways, always taking care to minimise photometric biases due to host galaxy light contamination. Firstly, we measured broad-band BJ (blue) magnitudes of the objects over small apertures of the size of the seeing disk. Secondly, we extracted H$\alpha$ and H$\beta$ broad emission line luminosities from the spectra which should be entirely free of any starlight contribution. Within the luminosity range covered by the HES ( $-19 \ga M_{B_J} \ga - 26$), the two measures are tightly correlated. The resulting AGN luminosity function (AGNLF) is consistent with a single power law, also when considering the effects of number density evolution within the narrow redshift range. We compared our AGNLF with the H$\alpha$ luminosity function of lower luminosity Seyfert 1 galaxies by Hao et al. (2005a, AJ, 129, 1795) and found a smooth transition between both, with excellent agreement in the overlapping region. From the combination of HES and SDSS samples we constructed a single local AGNLF spanning more than 4 orders of magnitude in luminosity. It shows only mild curvature which can be well described as a double power law with slope indices of -2.0 for the faint end and -2.8 for the bright end. We predicted the local AGNLF in the soft X-ray domain and compared this to recent literature data. The quality of the match depends strongly on the adopted translation of optical to X-ray luminosities and is best for an approximately constant optical/X-ray ratio. We also compared the local AGNLF with results obtained at higher redshifts and find strong evidence for luminosity-dependent evolution, in the sense that AGN with luminosities around $M_B \simeq -19$ are as common in the local universe as they were at z=1.5. This supports the “AGN downsizing” picture first found from X-ray selected AGN samples.


Key words: galaxies: active -- galaxies: nuclei -- quasars: general



© ESO 2009

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