Issue |
A&A
Volume 468, Number 3, June IV 2007
Extended baselines for the IRAM Plateau de Bure interferometer: First results
|
|
---|---|---|
Page(s) | 979 - 992 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20077252 | |
Published online | 11 April 2007 |
Dust covering factor, silicate emission, and star formation in luminous QSOs
1
INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy e-mail: maiolino@oa-roma.inaf.it
2
Department of Astronomy and Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802, USA
3
National Astronomical Observatory, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
4
School of Physics and Astronomy and the Wise Observatory , Tel-Aviv University, Tel-Aviv 69978, Israel
5
INAF - Telescopio Nazionale Galileo, PO Box 565, 38700 Santa Cruz de La Palma, Tenerife, Spain
6
Max-Planck-Institut für Extraterrestrische Physik, 85741 Garching, Germany
Received:
7
February
2007
Accepted:
5
April
2007
We present Spitzer IRS low resolution, mid-IR spectra of
a sample of 25 high luminosity QSOs at . When combined with
archival IRS observations of local, low luminosity type-I active galactic nuclei (AGNs),
the sample spans five orders of magnitude in
luminosity.
We find that the continuum dust thermal emission at
is correlated
with the optical luminosity, following the non-linear relation
.
We also find an anti correlation between
and
the [Oiii]λ5007 line luminosity. These effects are
interpreted as a decreasing covering factor of the circumnuclear
dust as a function of luminosity. Such a result is in agreement
with the decreasing fraction
of absorbed AGNs as a function of luminosity recently found in
various surveys. In particular, while X-ray surveys find a decreasing
covering factor of the absorbing gas as a function of luminosity,
our data provides an independent and
complementary confirmation by finding a decreasing covering factor of dust.
We clearly detect the silicate emission feature in
the average spectrum, but also in four individual objects. These
are the silicate emission in the most luminous
objects obtained so far. When combined with the silicate emission
observed in local, low luminosity type-I AGNs, we find that the
silicate emission strength
is correlated with luminosity.
The silicate strength of all type-I AGNs
also follows a positive
correlation with the black hole mass and with the accretion rate.
The Polycyclic Aromatic Hydrocarbon (PAH) emission
features, expected from starburst activity, are not detected in the
average spectrum of luminous, high-z QSOs. The
upper limit inferred
from the average spectrum points to a ratio between PAH luminosity
and QSO optical luminosity significantly lower than observed in
lower luminosity AGNs, implying that the correlation between star
formation rate and AGN power saturates at high
luminosities.
Key words: infrared: galaxies / galaxies: nuclei / galaxies: active / galaxies: Seyfert / galaxies: starburst / quasars: general
© ESO, 2007
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