Rosa M. González Delgado

Starburst99: Synthesis Models for Galaxies with Active Star Formation

Starburst99 is a comprehensive set of model predictions for spectrophotometric and related properties of galaxies with active star formation. The models are an improved and extended version of the data set previously published by Leitherer & Heckman. We have upgraded our code by implementing the latest set of stellar evolution models of the Geneva group and the model atmosphere grid compiled by Lejeune et al. Several predictions which were not included in the previous publication are shown here for the first time. The models are presented in a homogeneous way for five metallicities between Z = 0.040 and 0.001 and three choices of the initial mass function. The age coverage is 106—109 yr. We also show the spectral energy distributions which are used to compute colors and other quantities. The full data set is available for retrieval at a Web site, which allows users to run specific models with nonstandard parameters as well. We also make the source code available to the community.

Violent star formation in NGC 2363

This is an electronic version of an article published in The Astrophysical Journal. Gonzalez-Delgado, R.M. et al. Violent star formation in NGC 2363. The Astrophysical Journal 437 (1994): 239-261

A high-resolution stellar library for evolutionary population synthesis

We present a library of 1654 high-resolution stellar spectra, with a sampling of 0.3 A and covering the wavelength range from 3000 to 7000 A. The library was computed with the latest improvements in stellar atmospheres, incorporating non-local thermodynamic equilibrium (non-LTE) line-blanketed models for hot, massive (T eff ≥ 27 500 K) and line-blanketed models for cool (3000 ≤ T eff ≤ 4500 K) stars. The total coverage of the grid is 3000 K ≤ T eff ≤ 55 000 K and -0.5 ≤ log g ≤ 5.5, for four chemical abundance values: twice solar, solar, half solar and 1/10 solar. Evolutionary synthesis models using this library are presented in a companion paper. We tested the general behaviour of the library by calculating and comparing equivalent widths of numerous H and He I lines, and some of the commonly used metallic indices. We also compared the library with the empirical libraries STELIB and Indo-US. The full set of the synthetic stellar spectra is available from our websites (http://www.iaa.csic.es/∼rosa and http://www.astro.iag.usp.br/∼lucimara/library.htm).

THE STELLAR POPULATIONS OF LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI. I. GROUND-BASED OBSERVATIONS

We present a spectroscopic study of the stellar populations of low-luminosity active galactic nuclei (LLAGNs). Our main goal is to determine whether the stars that live in the innermost (100 pc scale) regions of these galaxies are in some way related to the emission-line properties, which would imply a link between the stellar population and the ionization mechanism. High signal-to-noise ratio, ground-based long-slit spectra in the 3500-5500 A interval were collected for 60 galaxies: 51 LINERs and LINER/H II transition objects, two starburst galaxies, and seven nonactive galaxies. In this paper, the first of a series, we (1) describe the sample; (2) present the nuclear spectra; (3) characterize the stellar populations of LLAGNs by means of an empirical comparison with normal galaxies; (4) measure a set of spectral indices, including several absorption-line equivalent widths and colors indicative of stellar populations; and (5) correlate the stellar indices with emission-line ratios that may distinguish between possible excitation sources for the gas. Our main findings are as follows: (1) Few LLAGNs have a detectable young (107 yr) starburst component, indicating that very massive stars do not contribute significantly to the optical continuum. In particular, no features due to Wolf-Rayet stars were convincingly detected. (2) High-order Balmer absorption lines of H I (HOBLs), on the other hand, are detected in ~40% of LLAGNs. These features, which are strongest in 108-109 yr intermediate-age stellar populations, are accompanied by diluted metal absorption lines and bluer colors than other objects in the sample. (3) These intermediate-age populations are very common (~50%) in LLAGNs with relatively weak [O I] emission ([O ]/Hα ≤ 0.25) but rare (~10%) in LLAGNs with stronger [O I]. This is intriguing since LLAGNs with weak [O I] have been previously hypothesized to be transition objects in which both an AGN and young stars contribute to the emission-line excitation. Massive stars, if present, are completely outshone by intermediate-age and old stars in the optical. This happens in at least a couple of objects where independent UV spectroscopy detects young starbursts not seen in the optical. (4) Objects with predominantly old stars span the whole range of [O I]/Hα values, but (5) sources with significant young and/or intermediate-age populations are nearly all (~90%) weak-[O I] emitters. These new findings suggest a link between the stellar populations and the gas ionization mechanism. The strong-[O I] objects are most likely true LLAGNs, with stellar processes being insignificant. However, the weak-[O I] objects may comprise two populations, one where the ionization is dominated by stellar processes and another where it is governed by either an AGN or a more even mixture of stellar and AGN processes. Possible stellar sources for the ionization include weak starbursts, supernova remnants, and evolved poststarburst populations. These scenarios are examined and constrained by means of complementary observations and detailed modeling of the stellar populations in forthcoming communications.

THE STELLAR POPULATIONS OF LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI. II. SPACE TELESCOPE IMAGING SPECTROGRAPH OBSERVATIONS

We present a study of the stellar population in Low Luminosity AGN (LLAGN). Our goal is to search for spectroscopic signatures of young and intermediate age stars, and to investigate their relationship with the ionization mechanism in LLAGN. The method used is based on the stellar population synthesis of the optical continuum of the innermost (20-100 pc) regions in these galaxies. For this purpose, we have collected high spatial resolution optical (2900-5700 A) STIS spectra of 28 nearby LLAGN that are available in the Hubble Space Telescope archive. The analysis of these data is compared with a similar analysis also presented here for 51 ground-based spectra of LLAGN. Our main findings are: (1) No features due to Wolf-Rayet stars were convincingly detected in the STIS spectra. (2) Young stars contribute very little to the optical continuum in the ground-based aperture. However, the fraction of light provided by these stars is higher than 10% in most of the weak-[OI] LLAGN STIS spectra. (3) Intermediate age stars contribute significantly to the optical continuum of these nuclei. This population is more frequent in objects with weak than with strong [OI]. Weak-[OI] LLAGN that have young stars stand out for their intermediate age population. (4) Most of the strong-[OI] LLAGN have predominantly old stellar population. A few of these objects also show a feature-less continuum that contributes significantly to the optical continuum. These results suggest that young and intermediate age stars do not play a significant role in the ionization of LLAGN with strong [OI]. However, the ionization in weak-[OI] LLAGN with young and/or intermediate age population could be due to stellar processes.We present a study of the stellar populations of low-luminosity active galactic nuclei (LLAGNs). Our goal is to search for spectroscopic signatures of young and intermediate-age stars and to investigate their relationship with the ionization mechanism in LLAGNs. The method used is based on the stellar population synthesis of the optical continuum of the innermost (20-100 pc) regions in these galaxies. For this purpose, we have collected high spatial resolution optical (2900-5700 ?) STIS spectra of 28 nearby LLAGNs that are available in the Hubble Space Telescope archive. The analysis of these data is compared with a similar analysis also presented here for 51 ground-based spectra of LLAGNs. Our main findings are as follows: (1) No features due to Wolf-Rayet stars were convincingly detected in the STIS spectra. (2) Young stars contribute very little to the optical continuum in the ground-based aperture. However, the fraction of light provided by these stars is higher than 10% in most of the weak-[O I] ([O ]/H? ? 0.25) LLAGN STIS spectra. (3) Intermediate-age stars contribute significantly to the optical continuum of these nuclei. This population is more frequent in objects with weak than with strong [O I]. Weak-[O I] LLAGNs that have young stars stand out for their intermediate-age population. (4) Most of the strong-[O I] LLAGNs have predominantly old stellar population. A few of these objects also show a featureless continuum that contributes significantly to the optical continuum. These results suggest that young and intermediate-age stars do not play a significant role in the ionization of LLAGNs with strong [O I]. However, the ionization in weak-[O I] LLAGNs with young and/or intermediate-age populations could be due to stellar processes. A comparison of the properties of these objects with Seyfert 2 galaxies that harbor a nuclear starburst suggests that weak-[O I] LLAGNs are the lower luminosity counterparts of the Seyfert 2 composite nuclei.

Detection of a Super-Star Cluster as the Ionizing Source in the Low-Luminosity Active Galactic Nucleus NGC 4303

Hubble Space Telescope (HST) ultraviolet Space Telescope Imaging Spectrograph (STIS) imaging and spectroscopy of the low-luminosity active galactic nucleus (LLAGN) NGC 4303 have identified the previously detected UV-bright nucleus of this galaxy as a compact, massive, and luminous stellar cluster. The cluster with a size (FWHM) of 3.1 pc and an ultraviolet luminosity log L1500 A(ergs s-1 A-1) = 38.33 is identified as a nuclear super-star cluster (SSC) like those detected in the circumnuclear regions of spirals and starburst galaxies. The UV spectrum showing the characteristic broad P Cygni lines produced by the winds of massive young stars is best fitted by the spectral energy distribution of a massive cluster of 105 M☉ (for a Salpeter initial mass function law with lower mass cutoff of 1 M☉) generated in an instantaneous burst 4 Myr ago. The ionizing energy produced by this cluster exceeds the flux needed to explain the nuclear Hα luminosity. No evidence for an additional nonthermal ionizing source associated with an accreting black hole is detected in the ultraviolet. These new HST/STIS results unambiguously show the presence of a compact SSC in the nucleus of a low-luminosity AGN, which is also its dominant ionizing source. We hypothesize that at least some LLAGNs in spirals could be understood as the result of the combined ionizing radiation emitted by an evolving SSC (i.e., determined by the mass and age) and a black hole accreting with low radiative efficiency (i.e., radiating at low sub-Eddington luminosities) coexisting in the inner few parsecs region. Complementary multifrequency studies give the first hints of the very complex structure of the central 10 pc of NGC 4303, where a young SSC apparently coexists with a low-efficiency accreting black hole and with an intermediate/old compact star cluster and where, in addition, an evolved starburst could also be present. If structures such as those detected in NGC 4303 are common in the nuclei of spirals, the modeling of the different stellar components and their contribution to the dynamical mass has to be established accurately before deriving any firm conclusion about the mass of central black holes of few to several million solar masses.

SYNTHETIC SPECTRA OF H BALMER AND He I ABSORPTION LINES. I. STELLAR LIBRARY

We present a grid of synthetic profiles of stellar H Balmer and HeI lines at optical wavelengths with a sampling of 0.3 A. The grid spans a range of effective temperature 4000 K 25000 K, NLTE stellar atmosphere models are computed using the code TLUSTY (Hubeny 1988). For cooler stars, Kurucz (1993) LTE models are used to compute thesynthetic spectra. The grid includes the profiles of the high-order hydrogen Balmer series and HeI lines for effective temperatures and gravities that have not been previously synthesized. The behavior of H8 to H13 and HeI 3819 with effective temperature and gravity is very similar to that of the lower terms of the series (e.g. Hb) and the other HeI lines at longer wavelengths; therefore, they are suited for the determination of the atmospheric parameters of stars. These lines are potentially important to make predictions for these stellar absorption features in galaxies with active star formation. Evolutionary synthesis models of these lines for starburst and post-starburst galaxies are presented in a companion paper. The full set of the synthetic stellar spectra is available for retrieval at our website this http URL and this http URL or on request from the authors at rosa@iaa.es.

An Atlas of the Circumnuclear Regions of 75 Seyfert Galaxies in the Near-Ultraviolet with the Hubble Space Telescope Advanced Camera for Surveys

We present an atlas of the central regions of 75 Seyfert galaxies imaged in the near-UV with the Advanced Camera for Surveys of the Hubble Space Telescope at an average resolution of ~10 pc. These data complement archival high-resolution data from the Hubble Space Telescope at optical and near-IR wavelengths, creating an extremely valuable data set for astronomers with a broad range of scientific interests. Our goal is to investigate the nature of the near-UV light in these objects, its relation to the circumnuclear starburst phenomenon, and the connection of this to the evolution and growth of the galaxy bulge and central black hole. In this paper we describe the near-UV morphology of the objects and characterize the near-UV emission. We estimate the size and the luminosity of the emitting regions and extract the luminosity profile. We also determine the presence of unresolved compact nuclei. In addition, the circumnuclear stellar cluster population is identified, and the contribution of the stellar clusters to the total light, at this wavelength, is estimated. The size of the sample allows us to draw robust statistical conclusions. We find that Seyfert 1 galaxies (Sy1s) are completely dominated by their bright and compact nuclei, which remains pointlike at this resolution, while we find almost no unresolved nuclei in Seyfert 2 galaxies (Sy2s). The Seyfert types 1 and 2 are quite segregated in an asymmetry versus compactness plot. Stellar clusters are found somewhat more frequently in Sy2s (in ~70% of the galaxies) than in Sy1s (~57%), and contribute more to the total light in Sy2s, but these two differences seem to be mostly due to the large contribution of the compact nuclei in Sy1s, as the luminosity distribution of the clusters is similar in both Seyfert types.

Nuclear Activity and Massive Star Formation in the Low-Luminosity Active Galactic Nucleus NGC 4303: Chandra X-Ray Observations

We present evidence of the coexistence of either an AGN or an ultraluminous X-ray source, together with a young superstellar cluster, in the central 3 pc of NGC 4303. The galaxy contains a low-luminosity AGN and hosts a number of starburst regions in a circumnuclear spiral, as well as in the nucleus itself. A high spatial resolution Chandra image of this source reveals that the soft X-ray emission traces the ultraviolet nuclear spiral down to a core, which is unresolved in both soft and hard X-rays. The astrometry of the X-ray core coincides with the UV core within the Chandra positioning accuracy. The total X-ray luminosity of the core, ~1.5 × 1039 ergs s-1, is similar to that from some LINERs or from the weakest Seyfert galaxies detected so far. The soft X-rays in both the core and the extended structure surrounding it can be well reproduced by evolutionary synthesis models (which include the emission expected from single stars, the hot diffuse gas, supernova remnants, and binary systems), consistent with the properties of the young stellar clusters identified in the UV. The hard X-ray tail detected in the core spectrum, however, most likely requires the presence of an additional source. This additional source could be either a weak active nucleus black hole or an ultraluminous X-ray object. The implications of these results are discussed.

Multiwavelength Study of the Starburst Galaxy NGC 7714. I. Ultraviolet-Optical Spectroscopy*

We have studied the physical conditions in the central 300 pc of the prototypical starburst galaxy NGC 7714. Our analysis is based on ultraviolet spectroscopy with the HST/GHRS and ground-based optical observations, and it also covers X-ray and radio data taken from the literature. The data are interpreted using evolutionary models optimized for young starburst regions. The massive stellar population is derived in a self-consistent way using the continuum and stellar absorption lines in the ultraviolet and the nebular emission-line optical spectrum. The central starburst has an age of about 4.5 Myr, with little evidence for an age spread. Wolf-Rayet features at the ultraviolet indicate a stellar population of ~2000 Wolf-Rayet stars. The overall properties of the newly formed stars are quite similar to those derived, e.g., in 30 Doradus. A standard Salpeter initial mass function is consistent with all observational constraints. The nucleus of NGC 7714 has a bolometric luminosity of (0.5-1)×1010 L☉ and a mass of (5-10)×106 M☉ (if low-mass stars have formed). We find evidence for spatial structure within the central 300 pc sampled. Therefore it is unlikely that the nucleus of NGC 7714 hosts a single star cluster exceeding the properties of other known clusters. Contrary to previous suggestions, we find no evidence for a nuclear supernova rate that would significantly exceed the total disk-integrated rate. About one supernova event per century is predicted. Most of these events are associated with the core collapse of a hydrogen-free or -poor progenitor. An older stellar generation, with ages of tens of Myr and older, is suggested as well. This population is less concentrated toward the nucleus and extends over kiloparsec scales.