Alberto Buzzoni

Global Properties of Stellar Populations and the Spectral Evolution of Galaxies

Several relevant properties of evolving stellar populations are discussed from a theoretical viewpoint. These include the absolute and specific evolutionary flux of a stellar population, the relative contributions of stars in the various evolutionary stages, the bolometric evolution, and the total and specific rates of mass return. We then introduce the concept of phase transition in the stellar content of a population, an event which is produced by the first appearance of stars with either C-O or He degenerate cores. The spectral energy distribution of a population is then predicted to suffer a (major) change at each of these phase transitions. Some uncertainties in both current stellar and population evolutionary models, and in population synthesis techniques are then briefly discussed, and the need is emphasized for appropriate checks and calibrations using template stellar populations. Magellanic Cloud Clusters offer the best tool for these purposes, and we then discuss in some detail current problems in dating MC clusters, as well as the origin of the blue to red transition in these clusters and its possible connection with the mentioned phase transitions. Finally, as examples of how some of these concepts could be applied to real galaxies, we discuss the possible occurrence and detection of population phase transition in high redshift elliptical galaxies, as well as the origin of the ultraviolet light of ellipticals and its predicted behaviour with look back time.

Evolutionary population synthesis in stellar systems. I: A global approach

A new grid of 450 theoretical models for old simple stellar populations is presented. Evolutionary synthesis explores the influence of relevant parameters such as age, chemical composition, initial mass function, and stellar mass loss, on the integrated spectral energy distribution. Ages range between 4 and 18 Gyr, and the metallicity Z ranges from 0.0001 to 0.03, with helium content Y = 0.23 and 0.25, respectively. Three values are considered for the initial mass function, assumed as a power law, N(M) proportional to M exp-s; s = 1.35, 2.35, and 3.35. The computational code takes into account in a quantitative way the contributions from all the relevant stellar evolutionary phases according to the theory of the stellar evolution. Thus, late stage in the life of low- mass stars, such as the horizontal branch and the asymptotic and postasymptotic giant branches are accounted for. Furthermore, a simplified treatment for the evolution of the horizontal branch is developed, and the influence of different morphologies are investigated. 109 refs.

Broad-band colours and overall photometric properties of template galaxy models from stellar population synthesis

We present here a new set of evolutionary population synthesis models for template galaxies along the Hubble morphological sequence. The models, which account for the individual evolution of the bulge, disc, and halo components, provide basic morphological features, along with bolometric luminosity and colour evolution (including Johnson/Cousins, Gunn g, r, i, and Washington C, M, T1, T2 photometric systems) between 1 and 15 Gyr. The luminosity contribution from residual gas is also evaluated, both in terms of nebular continuum and Balmer-line enhancement. Our theoretical framework relies on the observed colours of present-day galaxies, coupled with a minimal set of physical assumptions related to simple stellar population (SSP) evolution theory, to constrain the overall distinctive properties of galaxies at earlier epochs. A comparison with more elaborate photometric models, and with empirical sets of reference spectral energy distributions (SEDs) for early- and late-type galaxies is accomplished, in order to test output reliability and investigate the internal uncertainty of the models. The match with observed colours of present-day galaxies tightly constrain the stellar birth rate, b, which smoothly increases from E to Im types. The comparison with the observed supernova (SN) rate in low-redshift galaxies shows, as well, a pretty good agreement, and allows us to tune up the inferred star formation activity and the SN and hypernova rates among the different galaxy morphological types. Among others, these results could find useful application also in cosmological studies, given for instance the claimed relationship between hypernova events and gamma-ray bursts. One outstanding feature of the back-in-time evolution model is the prevailing luminosity contribution of the bulge at early epochs. As a consequence, the current morphological look of galaxies might drastically change when moving to larger distances, and we discuss here how sensibly this bias could affect the observation (and the interpretation) of high-redshift surveys. In addition to broad-band colours, the modelling of Balmer line emission in disc-dominated systems shows that striking emission lines, like Hα, can very effectively track stellar birth rate in a galaxy. For these features to be useful age tracers as well, however, one should first assess the real change of b versus time on the basis of supplementary (and physically independent) arguments.

An updated survey of globular clusters in M 31 - II. Newly discovered bright and remote clusters

Aims. We present the first results of a large spectroscopic survey of candidate globular clusters located in the extreme outskirts of the nearby M 31 galaxy. The survey is aimed at ascertaining the nature of the selected candidates to increase the sample of confirmed M 31 clusters lying more that 2 ◦ away from the center of the galaxy. Methods. We obtained low resolution spectra (λ/∆λ � 800–1300) of 48 targets selected from the Extended Source Catalogue of 2MASS, as in Galleti et al. (2005, AA the other four are completely new discoveries: B516, B517, B518, B519. The newly discovered clusters lie at projected distance 40 kpc Rp 100 kpc from the center of M 31, and have absolute integrated magnitude −9.5 MV −7.5. For all the observed clusters we have measured the strongest Lick indices and we have obtained spectroscopic metallicity estimates. Mackey-GC1, Martin-GC1, B517 and B518 have spectra typical of old and metal poor globular clusters ([Fe/H] −1.3); B519 appears old but quite metal-rich ([Fe/H] �− 0.5); B516 presents very strong Balmer absorption lines: if this is indeed a cluster it should have a relatively young age (likely < 2 Gyr). Conclusions. The present analysis nearly doubles the number of M 31 globulars at Rp ≥ 40 kpc. At odds with the Milky Way, M 31 appears to have a significant population of very bright globular clusters in its extreme outskirts.

The magnesium Mg2 index as an indicator of metallicity in elliptical galaxies

A quantitative calibration of the Mg 2 index [Faber et al., AJ, 82, 941 (1977)] is attempted deriving a metallicity scale for elliptical galaxies. The dependence of the index on stellar temperature, gravity, and metallicity has been studied through spectroscopic observations of 87 standards applying the derived calibration to models for stellar population synthesis. Buzzonis [ApJS, 71, 817 (1989)] computational code has been used to explore the behavior of the index versus age, IMF, and metallicity of simple stellar populations (SSPs) inferring galactic metallicity for the Davies et al. [ApJS, 69, 581 (1987)] extensive observational database

An updated survey of globular clusters in M 31 - III. A spectroscopic metallicity scale for the Revised Bologna Catalog

Aims. We present a new homogeneous set of metallicity estimates based on Lick indices for the old globular clusters of the M 31 galaxy. The final aim is to add homogeneous spectroscopic metallicities to as many entries as possible of the Revised Bologna Catalog of M 31 clusters ��� , by reporting Lick index measurements from any source (literature, new observations, etc.) on the same scale. Methods. New empirical relations of [Fe/H] as a function of [MgFe] and Mg2 indices are based on the well-studied galactic globular clusters, complemented with theoretical model predictions for −0.2 ≤ [Fe/H] ≤ +0.5. Lick indices for M 31 clusters from various literature sources (225 clusters) and from new observations by our team (71 clusters) have been transformed into the Trager et al. system, yielding new metallicity estimates for 245 globular clusters of M 31. Results. Our values are in good agreement with recent estimates based on detailed spectral fitting and with those obtained from color magnitude diagrams of clusters imaged with the Hubble Space Telescope. The typical uncertainty on individual estimates is �± 0.25 dex, as resulted from the comparison with metallicities derived from color magnitude diagrams of individual clusters. Conclusions. The metallicity distribution of M 31 globular cluster is briefly discussed and compared with that of the Milky Way. Simple parametric statistical tests suggest that the distribution is probably not unimodal. The strong correlation between metallicity and kinematics found in previous studies is confirmed. The most metal-rich GCs tend to be packed into the center of the system and to cluster tightly around the galactic rotation curve defined by the HI disk, while the velocity dispersion about the curve increases with decreasing metallicity. However, also the clusters with [Fe/H] < −1.0 display a clear rotation pattern, at odds with their Milky Way counterparts.

An updated survey of globular clusters in M 31 I. Classification and radial velocity for 76 candidate clusters

Aims. We present the first results of a large spectroscopic survey of globular clusters and candidate globular clusters in the nearby M 31 galaxy. The survey is aimed at the classification of known candidate M 31 clusters and at the study of their kinematic properties. Methods. We obtained low-resolution spectroscopy (λ/∆λ � 800 − 1300) for 133 targets, including 76 yet-to-be-confirmed candidate clusters (i.e. with no previous spectroscopic information), 55 already-confirmed genuine M 31 clusters, and 2 uncertain candidates. Our observations allowed a reliable estimate of the target radial velocity, within a typical accuracy of ∼±20 km s −1 . The observed candidates have been robustly classified according to their radial velocity and shape parameters that allowed us to confidently discriminate between point sources and extended objects even from low-spatial-resolution imagery. Results. In our set of 76 candidate clusters we found: 42 newly-confirmed bona-fide M 31 clusters, 12 background galaxies, 17 foreground Galactic stars, 2 Hii regions belonging to M 31 and 3 unclassified (possibly M 31 clusters or foreground stars) objects. The classification of a few other candidates not included in our survey has been also reassessed on various observational bases. All the sources of radial velocity estimates for M 31 known globular clusters available in the literature have been compared and checked, and a homogeneous general list has been obtained for 349 confirmed clusters with radial velocity. Conclusions. Our results suggest that a significant number of genuine clusters (100) is still hidden among the plethora of known candidates proposed by various authors. Hence our knowledge of the globular cluster system of the M 31 galaxy is still far from complete even in terms of simple membership.

ATLAS Versus NextGen Model Atmospheres: A Combined Analysis of Synthetic Spectral Energy Distributions

We carried out a critical appraisal of the two theoretical models, Kurucz’ ATLAS9 and PHOENIX/NextGen, for stellar atmosphere synthesis. Our tests relied on the theoretical fit of spectral energy distributions (SED) for a sample of 334 target stars along the whole spectraltype sequence, from the classical optical catalogs of Gunn & Stryker (1983) and Jacoby et al. (1984). The best-fitting physical parameters (Teff, log g) of stars allowed an independent calibration of the temperature and bolometric scale vs. empirical classification parameters (i.e. spectral type and MK luminosity class); in addition, the comparison of the synthetic templates from the ATLAS and NextGen grids allowed us to probe the capability of the models to match spectrophotometric properties of real stars and assess the impact of the different input physics. We can sketch the following main conclusions of our analysis: i) fitting accuracy of both theoretical libraries drastically degrades at low Teff, where both ATLAS and NextGen models still fail to properly account for the contribution of molecular features in the observed SED of K-M stars. ii) Comparing with empirical calibrations, both ATLAS and NextGen fits tend, in average, to predict slightly warmer (by 4–8%) Teff for both giant and dwarf stars of fixed spectral type, but ATLAS provides in general a sensibly better fit (a factor of two lower σ of flux residuals) than NextGen. iii) There is a striking tendency of NextGen to label target stars with an effective temperature and surface gravity in excess with respect to ATLAS. The effect is especially evident for MK I-III objects, where a fraction of stars of about one in four is clearly misclassified by NextGen in log g. This is a consequence of some “degeneracy” in the solution space, partly induced by the different input physics and geometry constraints in the computation of the integrated emerging flux (ATLAS model atmospheres assume standard plane-parallel layers, while NextGen adopts, for low-gravity stars, a spherical-shell geometry). A different T(τ) vertical structure of stellar atmosphere seems also required for NextGen synthetic SEDs in order to better account for limbdarkening effects in cool stars, as supported by the recent observations of the EROS BLG2000-5 microlensing event. Subject headings: stars: atmospheres - fundamental parameters

Probing Atlas model atmospheres at high spectral resolution - Stellar synthesis and reference template validation

Aims. The fast improvement of spectroscopic observations makes mandatory a strong effort on the theoretical side to better reproduce the spectral energy distribution (SED) of stars at high spectral resolution. In this regard, relying on the Kurucz Atlas/Synthe original codes we computed the Bluered library, consisting of 832 synthetic SED of stars, that cover a large parameter space at very high spectral resolution (R = 500 000) along the 3500−7000 A wavelength range. Methods. Bluered synthetic spectra have been used to assess in finer detail the intrinsic reliability and the performance limits of the Atlas theoretical framework. The continuum-normalized spectra of the Sun, Arcturus, and Vega, plus a selected list of 45 bright stars with high-quality SEDs from the Prugniel & Soubiran Elodie catalog, form our sample designed to probe the global properties of synthetic spectra across the entire range of H-R parameters. Results. Atlas models display a better fitting performance with increasing stellar temperature. High-resolution spectra of Vega, the Sun, and Arcturus have been reproduced at R = 100 000, respectively, within a 0.7%, 4.5%, and 8.8% relative scatter in residual flux. In all the three cases, the residual flux distribution shows a significant asymmetry (skewness parameter γ = −2.21, −0.98, −0.67, respectively), which neatly confirms an overall “excess” of theoretical line blanketing. For the Sun, this apparent discrepancy is alleviated, but not recovered, by a systematic decrease (−40%) of the line oscillator strengths, log(gf ), especially referring to iron transitions. Definitely, a straight “astrophysical” determination of log(gf ) for each individual atomic transition has to be devised to overcome the problem. By neglecting overblanketing effects in theoretical models when fitting high-resolution continuum-normalized spectra of real stars, we lead to a systematically warmer effective temperature (between +80 and +300 K for the solar fit) and a slightly poorer metal content.

Massive Young Clusters in the Disk of M31

We have studied the properties of a sample of 67 very blue and likely young massive clusters in M31 extracted from the Bologna Revised Catalog of globular clusters, selected according to their color [(B - V)0 ≤ 0.45] and/or the strength of their Hβ spectral index (Hβ ≥ 3.5 A). Their existence in M31 has been noted by several authors in the past; we show here that these blue luminous compact clusters (BLCCs) are a significant fraction (15%) of the whole globular cluster system of M31. Compared to the global properties of the M31 globular cluster system, they appear to be intrinsically fainter and morphologically less concentrated, with a shallower Balmer jump and enhanced Hβ absorption in their spectra. Empirical comparison with integrated properties of clusters with known ages, as well as with theoretical simple stellar population models, consistently indicates that their typical age is less than ~2 Gyr, so they are probably not as metal-poor as would be deduced if they were older. When selecting BLCCs by either their (B - V)0 colors or the strength of their Hβ index, the cluster sample turns out to be distributed on the outskirts of the M31 disk, sharing the kinematic properties of the thin, rapidly rotating disk component. If confirmed to be young and not metal-poor, these clusters indicate the occurrence of significant recent star formation in the thin disk of M31, although they do not set constraints on the epoch of its early formation.