63 research outputs found
Understanding the relations between QSOs and their host galaxies from combined HST imaging and VLT spectroscopy
The host galaxies of six nearby QSOs are studied on the basis of high
resolution HST optical images and spatially resolved VLT slit spectra. The gas
ionization and velocity are mapped as a function of the distance to the central
QSO. In the majority of the cases, the QSO significantly contributes to the gas
ionization in its whole host galaxy, and sometimes even outside.
Reflection or scattering of the QSO \ha line from remote regions of the
galaxy is detected in several instances. The line shifts show that, in all
cases, the matter responsible for the light reflection moves away from the QSO,
likely accelerated by its radiation pressure.
The two faintest QSOs reside in spirals, with some signs of a past
gravitational perturbation. One of the intermediate luminosity QSOs resides in
a massive elliptical containing gas ionized (and probably pushed away) by the
QSO radiation. The other medium-power object is found in a spiral galaxy
displaying complex velocity structure, with the central QSO moving with respect
to the bulge, probably as a result of a galactic collision. The two most
powerful objects are involved in violent gravitational interactions and one of
them has no detected host.
These results suggest that (1) large-scale phenomena, such as galactic
collisions, are closely related to the triggering and the feeding of the QSO
and (2) once ignited, the QSO has significant influence on its large-scale
neighborhood (often the whole host and sometimes further away).Comment: Accepted for publication in ApJ. 31 pages, 17 figures, 3 table
Quasar Host Orientation and Polarization: Insights into the Type 1/Type 2 Dichotomy
We investigate correlations between the optical linear polarization position
angle and the orientation of the host galaxy/extended emission of Type 1 and
Type 2 Radio-Loud (RL) and Radio-Quiet (RQ) quasars. We have used high
resolution Hubble Space Telescope (HST) data and deconvolution process to
obtain a good determination of the host galaxy orientation. With these new
measurements and a compilation of data from the literature, we find a
significant correlation between the polarization position angle and the
position angle of the major axis of the host galaxy/extended emission. The
correlation appears different for Type 1 and Type 2 objects and depends on the
redshift of the source. Interpretations in the framework of the unification
model are discussed.Comment: 3 pages, 1 figure, to appear in "The Nuclear Region, Host Galaxy and
Environment of Active Galaxies", eds. E. Benitez, I. Cruz-Gonzalez and Y.
Krongold, RevMexAA (Serie de Conferencias
Deep near-infrared imaging of the HE0450-2958 system
The QSO HE0450-2958 and the companion galaxy with which it is interacting,
both ultra luminous in the infrared, have been the subject of much attention in
recent years, as the quasar host galaxy remained undetected. This led to
various interpretations on QSO and galaxy formation and co-evolution, such as
black hole ejection, jet induced star formation, dust obscured galaxy, or
normal host below the detection limit. We carried out deep observations in the
near-IR in order to solve the puzzle concerning the existence of any host. The
object was observed with the ESO VLT and HAWK-I in the near-IR J-band for 8
hours. The images have been processed with the MCS deconvolution method
(Magain, Courbin & Sohy, 1998), permitting accurate subtraction of the QSO
light from the observations. The compact emission region situated close to the
QSO, called the blob, which previously showed only gas emission lines in the
optical spectra, is now detected in our near-IR images. Its high brightness
implies that stars likely contribute to the near-IR emission. The blob might
thus be interpreted as an off-centre, bright and very compact host galaxy,
involved in a violent collision with its companion.Comment: 4 pages, 3 figures, accepted for publication in A&
Slit and integral-field optical spectroscopy of the enigmatic quasar HE0450-2958
Interest in the quasar HE0450-2958 arose following the publication of the
non-detection of its expected massive host, leading to various interpretations.
This article investigates the gaseous and stellar contents of the system
through additional VLT/FORS slit spectra and integral field spectroscopy from
VLT/VIMOS. We apply our MCS deconvolution algorithm on slit spectra for the
separation of the QSO and diffuse components, and develop a new method to
remove the point sources in Integral Field Spectra, allowing extraction of
velocity maps, narrow-line images, spatially resolved spectra or ionization
diagrams of the surroundings of HE0450-2958. The whole system is embedded in
gas, mostly ionized by the QSO radiation field and shocks associated with radio
jets. The observed gas and star dynamics are unrelated, revealing a strongly
perturbed system. Despite longer spectroscopic observations, the host galaxy
remains undetected.Comment: 9 pages, 13 figures, in press in A&
On-axis spectroscopy of the host galaxies of 20 optically luminous quasars at z~0.3
We present the analysis of a sample of 20 bright low-redshift quasars
(M_B<-23 and z < 0.35) observed spectroscopically with the VLT. The FORS1
spectra, obtained in Multi Object Spectroscopy (MOS) mode, allow to observe
simultaneously the quasars and several reference stars used to spatially
deconvolve the data. Applying the MCS deconvolution method, we are able to
separate the individual spectra of the quasar and of the underlying host
galaxy. Contrary to some previous claims, we find that luminous quasars are not
exclusively hosted by massive ellipticals. Most quasar host galaxies harbour
large amounts of gas, irrespective of their morphological type. Moreover, the
stellar content of half of the hosts is a young Sc-like population, associated
with a rather low metallicity interstellar medium. A significant fraction of
the galaxies contain gas ionized at large distances by the quasar radiation.
This large distance ionization is always associated with signs of gravitational
interactions (as detected from images or disturbed rotation curves). The
spectra of the quasars themselves provide evidence that gravitational
interactions bring dust and gas in the immediate surrounding of the super
massive black hole, allowing to feed it. The quasar activity might thus be
triggered (1) in young gas-rich spiral galaxies by local events and (2) in more
evolved galaxies by gravitational interactions or collisions. The latter
mechanism gives rises to the most powerful quasars. Finally, we derive mass
models for the isolated spiral host galaxies and we show that the most reliable
estimators of the systemic redshift in the quasar spectrum are the tips of the
Ha and Hb lines.Comment: 30 pages, 19 figures, 9 tables, accepted for publication in MNRAS,
major revisio
A deconvolution-based algorithm for crowded field photometry with unknown Point Spread Function
A new method is presented for determining the Point Spread Function (PSF) of
images that lack bright and isolated stars. It is based on the same principles
as the MCS (Magain, Courbin, Sohy, 1998) image deconvolution algorithm. It uses
the information contained in all stellar images to achieve the double task of
reconstructing the PSFs for single or multiple exposures of the same field and
to extract the photometry of all point sources in the field of view. The use of
the full information available allows to construct an accurate PSF. The
possibility to simultaneously consider several exposures makes it very well
suited to the measurement of the light curves of blended point sources from
data that would be very difficult or even impossible to analyse with
traditional PSF fitting techniques. The potential of the method for the
analysis of ground-based and space-based data is tested on artificial images
and illustrated by several examples, including HST/NICMOS images of a lensed
quasar and VLT/ISAAC images of a faint blended Mira star in the halo of the
giant elliptical galaxy NGC5128 (Cen A).Comment: Institutes: (1) Institut d'Astrophysique et de Geophysique,
Universite de Liege, allee du 6 Aout 17, B-4000 Liege, Belgium; (2) Ecole
Polytechnique Federale de Lausanne (EPFL), Laboratoire d'Astrophysique,
Observatoire, CH-1290 Sauverny, Switzerland; (3) Observatoire de Geneve, 51
Chemin des Maillettes, CH-1290 Sauverny, Switzerland. 8 pages, 8 figures.
Accepted for publication in A&
Near-IR observations of the HE0450-2958 system: discovery of a second AGN?
The QSO HE0450-2958 was brought to the front scene by the non-detection of
its host galaxy and strong upper limits on the latter's luminosity. The QSO is
also a powerful infrared emitter, in gravitational interaction with a strongly
distorted UltraLuminous InfraRed companion galaxy. We investigate the
properties of the companion galaxy, through new near- and mid-infrared
observations of the system obtained with NICMOS onboard HST, ISAAC and VISIR on
the ESO VLT. The companion galaxy is found to harbour a point source revealed
only in the infrared, in what appears as a hole or dark patch in the optical
images. Various hypotheses on the nature of this point source are analyzed and
it is found that the only plausible one is that it is a strongly reddened AGN
hidden behind a thick dust cloud. The hypothesis that the QSO supermassive
black hole might have been ejected from the companion galaxy in the course of a
galactic collision involving 3-body black holes interaction is also reviewed,
on the basis of this new insight on a definitely complex system.Comment: 8 pages, 8 figures, accepted for publication in MNRA
Spatial decomposition of on-nucleus spectra of quasar host galaxies
In order to study the host galaxies of type 1 (broad-line) quasars, we present a semi-analytic modelling method to decompose the on-nucleus spectra of quasars into nuclear and host galaxy channels. The method uses the spatial information contained in long-slit or slitlet spectra. A routine determines the best-fitting combination of the spatial distribution of the point like nucleus and extended host galaxy. This is fully complementary to a numerical spatial deconvolution technique that we applied to the same data in a previous analysis, which allows a cross-calibration of the two methods. Inputs are a simultaneously observed point spread function, and external constraints on galaxy morphology from imaging. We demonstrate the capabilities of the method to two samples of a total of 18 quasars observed with EFOSC at the ESO 3.6-m telescope and FORS1 at the ESO VLT. ∼50 per cent of the host galaxies with successful decomposition show distortions in their rotation curves or peculiar gas velocities above normal maximum velocities for discs. This is consistent with the fraction from optical imaging. All host galaxies have quite young stellar populations, typically 1-2 Gyr. For the disc dominated hosts these are consistent with their inactive counterparts, the luminosity-weighted stellar ages are much younger for the bulge dominated hosts, compared to inactive early-type galaxies. While this presents further evidence for a connection of galaxy interaction and active galactic nucleus (AGN) activity for half of the sample, this is not clear for the other half. These are often undistorted disc dominated host galaxies, and interaction on a smaller level might be detected in deeper high-resolution images or deeper spectroscopic data. The velocity information does not show obvious signs for large scale outflows triggered by AGN feedback - the data are consistent with velocity fields created by galaxy interactio
On-axis spectroscopy of the host galaxies of 20 optically luminous quasars at z ∼ 0.3
We present the analysis of a sample of 20 bright low-redshift quasars (MB < −23 and z < 0.35) observed spectroscopically with the Very Large Telescope (VLT). The Focal Reducer/low dispersion Spectrograph (FORS1) spectra, obtained in Multi-Object Spectroscopy (MOS) mode, allow to observe simultaneously the quasars and several reference stars used to spatially deconvolve the data. Applying the Magain, Courbin & Sohy (MCS) deconvolution method, we are able to separate the individual spectra of the quasar and of the underlying host galaxy. Contrary to some previous claims, we find that luminous quasars are not exclusively hosted by massive ellipticals. Most quasar host galaxies harbour large amounts of gas, irrespective of their morphological type. Moreover, the stellar content of half of the hosts is a young Sc-like population, associated with a rather low-metallicity interstellar medium. A significant fraction of the galaxies contain gas ionized at large distances by the quasar radiation. This large distance ionization is always associated with signs of gravitational interactions (as detected from images or disturbed rotation curves). The spectra of the quasars themselves provide evidence that gravitational interactions bring dust and gas in the immediate surrounding of the super massive black hole, allowing to feed it. The quasar activity might thus be triggered (1) in young gas-rich spiral galaxies by local events and (2) in more evolved galaxies by gravitational interactions or collisions. The latter mechanism gives rises to the most powerful quasars. Finally, we derive mass models for the isolated spiral host galaxies and we show that the most reliable estimators of the systemic redshift in the quasar spectrum are the tips of the Hα and Hβ line
Quasar induced galaxy formation: a new paradigm ?
We discuss observational evidence that quasars play a key role in the
formation of galaxies starting from the detailed study of the quasar
HE0450-2958 and extending the discussion to a series of converging evidence
that radio jets may trigger galaxy formation. The direct detection with VISIR
at the ESO-VLT of the 7 kpc distant companion galaxy of HE0450-2958 allows us
to spatially separate the sites of quasar and star formation activity in this
composite system made of two ultra-luminous infrared galaxies (ULIRGs). No host
galaxy has yet been detected for this quasar, but the companion galaxy stellar
mass would bring HE0450-2958 in the local M(BH)-M(stellar bulge) relation if it
were to merge with the QSO. This is bound to happen because of their close
distance (7 kpc) and small relative velocity (~60-200 km/s). We conclude that
we may be witnessing the building of the M(BH)-M(stellar bulge) relation, or at
least of a major event in that process. The star formation rate (~340 Msun/yr),
age (40-200 Myr) and stellar mass ([5-6]x10^10 Msun) are consistent with
jet-induced formation of the companion galaxy. We suggest that HE0450-2958 may
be fueled in fresh material by cold gas accretion from intergalactic filaments.
We map the projected galaxy density surrounding the QSO as a potential tracer
of intergalactic filaments and discuss a putative detection. Comparison to
other systems suggests that inside-out formation of quasar host galaxies and
jet-induced galaxy formation may be a common process. Two tests are proposed
for this new paradigm: (1) the detection of offset molecular gas or dust
emission with respect to the position of distant QSOs, (2) the delayed
formation of host galaxies as a result of QSO activity, hence the two step
building of the M(BH)/M(stellar bulge) ratio.Comment: 15 pages, 8 figures, accepted for publication in Astronomy and
Astrophysics (with minor corrections
- …