241 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&
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. Inputs are a simultaneously observed PSF, 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 3.6m telescope and FORS1 at the ESO VLT.
~50% of the host galaxies with sucessful decomposition show distortions in
their rotation curves or peculiar gas velocities above normal maximum
velocities for disks. This is consistent with the fraction from optical
imaging. All host galaxies have quite young stellar populations, typically 1-2
Gyr. For the disk 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 AGN
activity for half of the sample, this is not clear for the other half: These
are often undistorted disk 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 is consistent with
velocity fields created by galaxy interaction.Comment: Accepted for publication in MNRAS; 19 pages, 12 figure
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&
Evidence of a type 1/type 2 dichotomy in the correlation between quasar optical polarization and host-galaxy/extended emission position angles
peer reviewedAims.For Seyfert galaxies, the AGN unification model provides a simple and well-established explanation of the type 1/type 2 dichotomy through orientation-based effects. The generalization of this unification model to the higher luminosity AGNs that quasars are remains a key question. The recent detection of type 2 radio-quiet quasars seems to support such an extension. We propose a further test of this scenario. Methods: On the basis of a compilation of quasar host-galaxy position angles consisting of previously published data and of new measurements performed using HST Archive images, we investigate the possible existence of a correlation between the linear polarization position angle and the host-galaxy/extended emission position angle of quasars. Results: We find that the orientation of the rest-frame UV/blue extended emission is correlated to the direction of the quasar polarization. For type 1 quasars, the polarization is aligned with the extended UV/blue emission, while these two quantities are perpendicular in type 2 objects. This result is independent of the quasar radio loudness. We interpret this (anti-)alignment effect in terms of scattering in a two-component polar+equatorial model that applies to both type 1 and type 2 objects. Moreover, the orientation of the polarization -and then of the UV/blue scattered light- does not appear correlated to the major axis of the stellar component of the host-galaxy measured from near-IR images
The role of quasars in galaxy formation
We discuss evidence that quasars, and more generally radio jets, may have
played an active role in the formation stage of galaxies by inducing star
formation, i.e. through positive feedback. This mechanism first proposed in the
70's has been considered as anecdotic until now, contrary to the opposite
effect that is generally put forward, the quenching of star formation in
massive galaxies to explain the galaxy bimodality, downsizing and the universal
black hole mass over bulge stellar mass ratio. This suggestion is based on the
recent discovery of an ultra-luminous infrared galaxies, i.e. an extreme
starburst, which appears to be triggered by a radio jet from the QSO
HE0450-2958 at z=0.2863, together with the finding in several systems of an
offset between molecular gas and quasars, which may be explained by the
positive feedback effect of radio jets on their local environment.Comment: Invited talk, to appear in the Proceedings of the IAU Symposium 267,
"Co-Evolution of Central Black Holes and Galaxies", B.M. Peterson, R.S.
Somerville, T. Storchi-Bergmann, eds., in press (8 pages, 3 figures
Study of a homogeneous QSO sample: relations between the QSO and its host galaxy
We analyse a sample of 69 QSOs which have been randomly selected in a
complete sample of 104 QSOs (R<18, 0.142 < z < 0.198). 60 have been observed
with the NTT/SUSI2 at La Silla, through two filters in the optical band (WB#655
and V#812), and the remaining 9 are taken from archive databases. The filter
V#812 contains the redshifted Hbeta and forbidden [OIII] emission lines, while
WB#655 covers a spectral region devoid of emission lines, thus measuring the
QSO and stellar continua. The contributions of the QSO and the host are
separated thanks to the MCS deconvolution algorithm, allowing a morphological
classification of the host, and the computation of several parameters such as
the host and nucleus absolute V-magnitude, distance between the luminosity
center of the host and the QSO, and colour of the host and nucleus. We define a
new asymmetry coefficient, independent of any galaxy models and well suited for
QSO host studies. The main results from this study are: (i) 25% of the total
number of QSO hosts are spirals, 51% are ellipticals and 60% show signs of
interaction; (ii) Highly asymmetric systems tend to have a higher gas
ionization level (iii) Elliptical hosts contain a substantial amount of ionized
gas, and some show off-nuclear activity. These results agree with hierarchical
models merger driven evolution.Comment: accepted for publication in MNRAS, 19 pages, 22 figures, 8 table
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