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 formul\ae defining the set of acceptable solutions. All solutions within this set are compatible, at a given confidence level, with the observations.Comment: 11 pages, LaTeX, 4 figures, 1 table. M.N.R.A.S.(2000) in pres

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.Comment: Accepted in A&A. 15 pages, 5 figures and 6 table

Subaru High-Dispersion Spectroscopy of Narrow-Line Region in the Seyfert Galaxy NGC 4151

We report on a study of forbidden emission-line spectrum of nearby Seyfert 1.5 galaxy NGC 4151 based on the high-resolution (R ~ 45,000) optical spectrum obtained by using the High Dispersion Spectrograph boarded on the Subaru Telescope. The profile parameters such as the emission-line widths, the velocity shifts from the recession velocity of the host galaxy, and the asymmetry indices, for emission lines including very faint ones such as [Ar IV]4712,4740 and [Fe VI]5631,5677 are investigated. Statistically significant correlations between the measured profile parameters and the critical densities of transitions are found while there are no meaningful correlations between the profile parameters and the ionization potentials of ions. By comparing the results with photoionization model calculations, we remark that a simple power-law distribution of the gas density which is independent of the radius from the nucleus cannot explain the observed correlation between the emission-line widths and the critical densities of the transitions. Taking the additional dense gas component expected to exist at the innermost of the narrow-line region into account, the observed correlations between the emission-line width and the critical density of the transitions can be understood since high-critical-density emission lines can arise at such relatively inner regions even if their ionization potentials are low. The observed correlation between the blueshift amounts of emission lines and the critical densities of the ions is also explained if such dense gas clouds located closer to the nucleus have larger outflowing velocities.Comment: 19 pages and 1 separate jpeg figure. Accepted for publication in A

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.Comment: Accepted for publication in Astronomy & Astrophysics 7 pages, 8 figures, F90-code available at http://www.obs.u-bordeaux1.fr/radio/JMHure/intro2applawd.htm

ISO-SWS spectroscopy of NGC 1068

We present ISO-SWS spectroscopy of NGC 1068 for the wavelength range 2.4 to 45um, detecting a total of 36 emission lines. Most of the observed transitions are fine structure and recombination lines originating in the narrow line region. We compare the line profiles of optical lines and reddening-insensitive infrared lines to constrain the dynamical structure and extinction properties of the NLR. The considerable differences found are most likely explained by two effects. (1) The spatial structure of the NLR is a combination of a highly ionized outflow cone and lower excitation extended emission. (2) Parts of the NLR, mainly in the receding part at velocities above systemic, are subject to extinction that is significantly suppressing optical emission. Line asymmetries and net blueshifts remain, however, even for infrared fine structure lines suffering very little obscuration. This may be either due to an intrinsic asymmetry of the NLR, or due to a very high column density obscuring component which is hiding part of the NLR even from infrared view. Mid-infrared emission of molecular hydrogen in NGC 1068 arises in a dense molecular medium at temperatures of a few hundred Kelvin that is most likely closely related to the warm and dense components seen in the near-infrared H2 transitions, and in millimeter wave tracers of molecular gas. Any emission of the putative pc-scale molecular torus is likely overwhelmed by this larger scale emission.Comment: aastex (V4), 9 eps figures. Accepted by Ap

Active Galactic Nuclei with Candidate Intermediate-Mass Black Holes

We present an initial sample of 19 intermediate-mass black hole candidates in active galactic nuclei culled from the First Data Release of the Sloan Digital Sky Survey. Using the linewidth-luminosity-mass scaling relation established for broad-line active nuclei, we estimate black hole masses in the range of 8 x 10^4 - 10^6 solar masses, a regime in which only two objects are currently known. The absolute magnitudes are faint for active galactic nuclei, ranging from M_g of -15 to -18 mag, while the bolometric luminosities are all close to the Eddington limit. The entire sample formally satisfies the linewidth criterion for so-called narrow-line Seyfert 1 galaxies; however, they display a wider range of FeII and [OIII] (5007) line strengths than is typically observed in this class of objects. Although the available imaging data are of insufficient quality to ascertain the detailed morphologies of the host galaxies, it is likely that the majority of the hosts are relatively late-type systems. The host galaxies have estimated g-band luminosities ~ 1 mag fainter than M* for the general galaxy population at z of 0.1. Beyond simply extending the known mass range of central black holes in galactic nuclei, these objects provide unique observational constraints on the progenitors of supermassive black holes. They are also expected to contribute significantly to the integrated signal for future gravitational wave experiments.Comment: ApJ Accepted, 13 pages, 9 figures, uses emulateapj.cl

Polarized Narrow-Line Emission from the Nucleus of NGC 4258

The detection of polarized continuum and line emission from the nucleus of NGC 4258 by Wilkes et al. (1995) provides an intriguing application of the unified model of Seyfert nuclei to a galaxy in which there is known to be an edge-on, rotating disk of molecular gas surrounding the nucleus. Unlike most Seyfert nuclei, however, NGC 4258 has strongly polarized narrow emission lines. To further investigate the origin of the polarized emission, we have obtained spectropolarimetric observations of the NGC 4258 nucleus at the Keck-II telescope. The narrow-line polarizations range from 1.0% for [S II] 6716 to 13.9% for the [O II] 7319,7331 blend, and the position angle of polarization is oriented nearly parallel to the projected plane of the masing disk. A correlation between critical density and degree of polarization is detected for the forbidden lines, indicating that the polarized emission arises from relatively dense (n_e > 10^4 cm^-3) gas. An archival Hubble Space Telescope narrow-band [O III] image shows that the narrow-line region has a compact, nearly unresolved core, implying a FWHM size of <2.5 pc. We discuss the possibility that the polarized emission might arise from the accretion disk itself and become polarized by scattering within the disk atmosphere. A more likely scenario is an obscuring torus or strongly warped disk surrounding the inner portion of a narrow-line region which is strongly stratified in density. The compact size of the narrow-line region implies that the obscuring structure must be smaller than ~2.5 pc in diameter.Comment: To appear in the Astronomical Journal. 13 pages, including 1 table and 4 figures. Uses emulateapj.st

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.Comment: 20 pages, 14 figures, accepted for publication in A&

Ensemble Learning Independent Component Analysis of Normal Galaxy Spectra

In this paper, we employe a new statistical analysis technique, Ensemble Learning for Independent Component Analysis (EL-ICA), on the synthetic galaxy spectra from a newly released high resolution evolutionary model by Bruzual & Charlot. We find that EL-ICA can sufficiently compress the synthetic galaxy spectral library to 6 non-negative Independent Components (ICs), which are good templates to model huge amount of normal galaxy spectra, such as the galaxy spectra in the Sloan Digital Sky Survey (SDSS). Important spectral parameters, such as starlight reddening, stellar velocity dispersion, stellar mass and star formation histories, can be given simultaneously by the fit. Extensive tests show that the fit and the derived parameters are reliable for galaxy spectra with the typical quality of the SDSS.Comment: 41 pages, 23 figures, to be published in A