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D. Pelat

Publications and source records attributed to D. Pelat.

8 recordsLinked to original sources
arXiv2016

Understanding Active Galactic Nuclei using near-infrared high angular resolution polarimetry II: Preliminary results

In this second research note of a series of two, we present the first near-infrared results we obtained when modeling Active Galactic Nuclei (AGN). Our first proceedings showed the comparison between the MontAGN and STOKES Monte Carlo codes. Now we use our radiative transfer codes to simulate the polarization maps of a prototypical, NGC 1068-like, type-2 radio-quiet AGN. We produced high angular resolution infrared (1 micron) polarization images to be compared with recent observations in this wavelength range. Our preliminary results already show a good agreement between the models and observations but cannot account for the peculiar linear polarization angle of the torus such as observed. Gratadour et al. 2015 found a polarization position angle being perpendicular to the bipolar outflows axis. Further work is needed to improve the models by adding physical phenomena such as dichroism and clumpiness.

astro-ph.GA↗
arXiv2007

Generation of potential/surface density pairs in flat disks Power law distributions

We report a simple method to generate potential/surface density pairs in flat axially symmetric finite size disks. Potential/surface density pairs consist of a ``homogeneous'' pair (a closed form expression) corresponding to a uniform disk, and a ``residual'' pair. This residual component is converted into an infinite series of integrals over the radial extent of the disk. For a certain class of surface density distributions (like power laws of the radius), this series is fully analytical. The extraction of the homogeneous pair is equivalent to a convergence acceleration technique, in a matematical sense. In the case of power law distributions, the convergence rate of the residual series is shown to be cubic inside the source. As a consequence, very accurate potential values are obtained by low order truncation of the series. At zero order, relative errors on potential values do not exceed a few percent typically, and scale with the order N of truncation as 1/N**3. This method is superior to the classical multipole expansion whose very slow convergence is often critical for most practical applications.

astro-ph↗
arXiv2004

HST/WFPC2 morphologies and color maps of distant luminous infrared galaxies

Using HST/WFPC2 imaging in F606W (or F450W) and F814W filters, we obtained the color maps in observed frame for 36 distant (0.4<z<1.2) luminous infrared galaxies (LIRGs), with average star formation rates of ~100 M_sun/yr. Stars and compact sources are taken as references to align images after correction of geometric distortion. This leads to an alignment accuracy of 0.15 pixel, which is a prerequisite for studying the detailed color properties of galaxies with complex morphologies. A new method is developed to quantify the reliability of each pixel in the color map without any bias against very red or blue color regions.Based on analyses of two-dimensional structure and spatially resolved color distribution, we carried out morphological classification for LIRGs. About 36% of the LIRGs were classified as disk galaxies and 22% as irregulars. Only 6 (17%) systems are obvious ongoing major mergers. An upper limit of 58% was found for the fraction of mergers in LIRGs with all the possible merging/interacting systems included. Strikingly, the fraction of compact sources is as high as 25%, similar to that found in optically selected samples. From their K band luminosities, LIRGs are relatively massive systems, with an average stellar mass of about 1.1x10^11 solar mass. They are related to the formation of massive and large disks, from their morphologies and also from the fact that they represent a significant fraction of distant disks selected by their sizes. The compact LIRGs show blue cores, which could be associated with the formation of the central region of these galaxies. We suggest that there are many massive disks still forming a large fraction of their stellar mass since z=1. For most of them, their central parts (bulge?) were formed prior to the formation of their disks.

astro-ph↗
arXiv2004

Constraining the solutions of an inverse method of stellar population synthesis

In three previous papers (Pelat 1997, 1998 and Moultaka & Pelat 2000), we set out an inverse stellar population synthesis method which uses a database of stellar spectra. Unlike other methods, this one provides a full knowledge of all possible solutions as well as a good estimation of their stability; moreover, it provides the unique approximate solution, when the problem is overdetermined, using a rigorous minimization procedure. In Boisson et al. (2000), this method has been applied to 10 active and 2 normal galaxies. In this paper we analyse the results of the method after constraining the solutions. Adding {\it a priori} physical conditions on the solutions constitutes a good way to regularize the synthesis problem. As an illustration we introduce physical constraints on the relative number of stars taking into account our present knowledge of the initial mass function in galaxies. In order to avoid biases on the solutions due to such constraints, we use constraints involving only inequalities between the number of stars, after dividing the H-R diagram into various groups of stellar masses. We discuss the results for a well-known globular cluster of the galaxy M31 and discuss some of the galaxies studied in Boisson et al. (2000). We find that, given the spectral resolution and the spectral domain, the method is very stable according to such constraints (i.e. the constrained solutions are almost the same as the unconstrained one). However, an additional information can be derived about the evolutionary stage of the last burst of star formation, but the precise age of this particular burst seems to be questionable.

astro-ph↗
arXiv2003

Escape probability methods versus "exact" transfer for modelling the X-ray spectrum of Active Galactic Nuclei and X-ray binaries

In the era of XMM-Newton and Chandra missions, it is crucial to use codes able to compute correctly the line spectrum of X-ray irradiated thick media (Thomson thickness of the order of unity) to build models for the structure and the emission of the central regions of AGN or X-ray binaries. In all photoionized codes except in our code Titan, the line intensities are computed with the "escape probability approximation". In its last version, Titan solves the transfer of a thousand lines and of the continuum with the ``Accelerated Lambda Iteration" method, which is one of the most efficient and most secure for line transfer. We find that for conditions typical of the AGN or X-ray binary emission medium, all escape approximations commonly used lead to an overestimation of the soft X-ray lines which can reach one order of magnitude for intense lines.

astro-ph↗
arXiv2000

Error analysis for stellar population synthesis as an inverse problem

Stellar population synthesis can be approached as an inverse problem. The physical information is extracted from the observations through an inverse model. The process requires the transformation of the observational errors into model errors. A description is given for the error analysis to obtain objectively the errors in the model. Finding a solution for overdetermined and under-determined case was the purpose of two preceding papers. This new one completes the problem of stellar populations synthesis by means of a data base, by providing practical formulædefining the set of acceptable solutions. All solutions within this set are compatible, at a given confidence level, with the observations.

astro-ph↗
arXiv1995

Multi-Frequency Monitoring of the Seyfert 1 Galaxy NGC~4593 II: a Small, Compact Nucleus ?

We discuss the results of a campaign to monitor spectral variations in the low-luminosity Seyfert~1 galaxy NGC~4593, at X-rays, ultraviolet, optical and near IR frequencies. The observations and data analysis have been described in a companion paper (Santos-Lle\'o et al. 1994; Paper~1). The active nucleus in this galaxy is strongly and rapidly variable in all wavebands, implying that the continuum source is unusually compact. Its energy distribution from 1.2~$\mu$m to 1200~\AA\/ obeys a power-law whose index is significantly steeper than is usual in Seyfert's or QSO's; the ``big bump'' is either absent or shifted to wavelengths shorter than 1200~\AA\/. The variations of the soft-X~ray {\em excess\/} do not correlate with those of the UV or hard X-ray continuum. The far UV and optical fluxes are well correlated, while the correlation between the hard X-rays and 1447 \AA\ continuum is only marginally significant. Moreover, the optical flux cannot lag behind the UV by more than 6 days. These results cannot be accommodated in the framework of the standard geometrically thin accretion disk model. Rather, they suggest that the bulk of the UV and optical flux originates from thermal reprocessing of X-rays irradiating the disk. The soft X-ray excess is probably the only spectral component which originates from viscous dissipation inside the disk and the near infrared is probably emitted by hot dust heated by the UV radiation. Such a model is consistent with NGC~4593 having a relatively small black-hole mass of the order of $2\times10^{6}{\rm M_{\odot}}$ as inferred from the line variability study. The high ionization/excitation emission lines are very broad and strongly variable and their variations correlate with those of the continuum. The low excitation lines are significantly narrower and remain constant within the accuracy of our measurements. These results suggest a stratified BLR, where the degree of ionization and the velocity dispersion of the gas increase toward small radii. The \lya\ line responds to the variations of the continuum with a delay $\leq 4$ days. To a first order approximation, the BLR in NGC~4593 is well modelled with two different zones at distances of $\sim$~15 and 3 lt-ds from the ionizing source respectively.

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