References of "Letawe, Yannick"
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See detailNew Insights into the Quasar Type 1/Type 2 Dichotomy from Correlations between Quasar Host Orientation and Polarization
Borguet, Benoît ULg; Hutsemekers, Damien ULg; Letawe, Géraldine ULg et al

in Bastien, Pierre (Ed.) Astronomical Polarimetry 2008: Science from Small to Large Telescopes. ASPC 449 (2011, November 01)

We investigate correlations between the direction of the optical linear polarization and the orientation of the host galaxy/extended emission for type 1 and type 2 radio-loud and radio-quiet quasars. We ... [more ▼]

We investigate correlations between the direction of the optical linear polarization and the orientation of the host galaxy/extended emission for type 1 and type 2 radio-loud and radio-quiet quasars. We have used high resolution Hubble Space Telescope data and a deconvolution process to obtain a good determination of the host galaxy/extended emission (EE) position angle. With these new measurements and a compilation of data from the literature, we find a significant correlation, different for type 1 and type 2 objects, between the linear polarization position angle and the orientation of the EE, suggesting scattering by an extended UV/blue region in both types of objects. Our observations support the extension of the Unification Model to the higher luminosity AGNs like the quasars, assuming a two component scattering model. [less ▲]

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See detailNear-intrared observations of the HE 0450-2958 system: discovery of a second active galactic nucleus?
Letawe, Géraldine ULg; Magain, Pierre ULg; Chantry, Virginie ULg et al

in Monthly Notices of the Royal Astronomical Society (2009), 396(1), 78-84

The quasi-stellar object (QSO) HE 0450−2958 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 ... [more ▼]

The quasi-stellar object (QSO) HE 0450−2958 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 Near Infrared Camera and Multi-Object Spectrometer (NICMOS) onboard Hubble Space Telescope, Infrared Spectrometer and Array Camera (ISAAC) and Very Large Telescope Imager and Spectrometer in the Infrared (VISIR) on the European Southern Observatory Very Large Telescope. 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 analysed and it is found that the only plausible one is that it is a strongly reddened active galactic nucleus 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 three-body black holes interaction is also reviewed, on the basis of this new insight on a definitely complex system. [less ▲]

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See detailQuasar Host Orientation and Polarization: Insights into the Type 1/Type 2 Dichotomy
Borguet, Benoît ULg; Hutsemekers, Damien ULg; Letawe, Géraldine ULg et al

in Revista Mexicana de Astronomía y Astrofísica (2008), 32

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 ... [more ▼]

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. [less ▲]

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See detailEvidence of a type 1/type 2 dichotomy in the correlation between quasar optical polarization and host-galaxy/extended emission position angles
Borguet, Benoît ULg; Hutsemekers, Damien ULg; Letawe, Géraldine ULg et al

in Astronomy and Astrophysics (2008), 478

Aims.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 ... [more ▼]

Aims.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. [less ▲]

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See detailStudy of the QSO HE0354-5500 with combined HST imaging and VLT spectroscopy . An example of a deconvolution-based method for probing the QSOs host galaxies characteristics
Letawe, Yannick ULg; Magain, Pierre ULg; Letawe, Géraldine ULg et al

in Memorie della Società Astronomica Italiana : Journal of the Italian Astronomical Society (2008), 79

The host galaxy of the QSO HE0354-5500 (M_B=-24.7, z=0.2674) is studied on the basis of high resolution HST optical images and spatially resolved VLT slit spectra. The morphology and dynamics of the host ... [more ▼]

The host galaxy of the QSO HE0354-5500 (M_B=-24.7, z=0.2674) is studied on the basis of high resolution HST optical images and spatially resolved VLT slit spectra. The morphology and dynamics of the host are described. The gas ionization and velocity are mapped as a function of the distance to the central QSO. Reflection or scattering of the QSO Halpha line from remote regions of the galaxy is detected. The line shifts show that the matter responsible for the light reflection moves away from the QSO, likely accelerated by its radiation pressure. Moreover, different resolved emission regions are found in the central kpc, both in the images and the spectra. [less ▲]

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See detailUnderstanding the relations between QSOs and their host galaxies from combined HST imaging and VLT spectroscopy
Letawe, Yannick ULg; Magain, Pierre ULg; Letawe, Géraldine ULg et al

in Astrophysical Journal (2008), 679(2), 967-983

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 ... [more ▼]

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 H alpha 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). [less ▲]

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See detailA deconvolution-based algorithm for crowded field photometry with unknown point spread function
Magain, Pierre ULg; Courbin, F.; Gillon, Michaël ULg et al

in Astronomy and Astrophysics (2007), 461

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 image deconvolution algorithm. It ... [more ▼]

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 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 us to construct an accurate PSF. The possibility to simultaneously consider several exposures makes it 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 NGC 5128 (Cen A). [less ▲]

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See detailThe Host Galaxies of the Brightest Quasars: Gas-Rich Galaxies, Mergers, and Young Stars
Courbin, Frédéric; Letawe, Géraldine ULg; Meylan, Georges et al

in The Messenger (2006), 124

Because they are faint and hidden in the glare of a much brighter unresolved source, quasar host galaxies still challenge the most powerful telescopes, instrumentation and processing techniques ... [more ▼]

Because they are faint and hidden in the glare of a much brighter unresolved source, quasar host galaxies still challenge the most powerful telescopes, instrumentation and processing techniques. Determining their basic morphological parameters and their integrated colours is feasible, but difficult, from imaging alone. However, detailed information on their stellar and gas contents and on their dynamics is achievable with deep spectroscopy. [less ▲]

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