Single-metallicity Stellar Populations Research Articles

No Evidence of Chemical Abundance Variations in the Intermediate-age Cluster NGC 1783

Abstract We have analyzed multi-passband photometric observations, obtained with the Hubble Space Telescope, of the massive (1.8 × 105 M ⊙), intermediate-age (1.8 Gyr-old) Large Magellanic Cloud star cluster NGC 1783. The morphology of the cluster’s red giant branch does not exhibit a clear broadening beyond its intrinsic width; the observed width is consistent with that owing to photometric uncertainties alone and independent of the photometric selection boundaries we applied to obtain our sample of red giant stars. The color dispersion of the cluster’s red giant stars around the best-fitting ridgeline is 0.062 ± 0.009 mag, which is equivalent to the width of 0.080 ± 0.001 mag derived from artificial simple stellar population tests, that is, tests based on single-age, single-metallicity stellar populations. NGC 1783 is comparably as massive as other star clusters that show clear evidence of multiple stellar populations. After incorporating mass-loss recipes from its current age of 1.8 Gyr to an age of 6 Gyr, NGC 1783 is expected to remain as massive as some other clusters that host clear multiple populations at these intermediate ages. If we were to assume that mass is an important driver of multiple population formation, then NGC 1783 should have exhibited clear evidence of chemical abundance variations. However, our results support the absence of any chemical abundance variations in NGC 1783.

UV-extended E-MILES stellar population models: young components in massive early-type galaxies

We present UV-extended E-MILES stellar population synthesis models covering the spectral range 1680-50000A at moderately high resolution. We employ the NGSL space-based stellar library to compute spectra of single-age, single-metallicity stellar populations in the wavelength range from 1680 to 3540A. These models represent a significant improvement in resolution and age/metallicity coverage over previous studies based on earlier space-based libraries. These model spectra were joined with those we computed in the visible using MILES, and other empirical libraries for redder wavelengths. The models span the metallicity range -1.79<[M/H]<+0.26 and ages above 30 Myr, for a suite of IMF types with varying slopes. We focus on the behaviour of colours, spectra and line-strength indices in the UV range as a function of relevant stellar population parameters. Whereas some indices strengthen with increasing age and metallicity, as most metallicity indicators in the visible, other indices peak around 3 Gyr for metal-rich stellar populations, such as Mg at 2800A. Our models provide reasonably good fits to the integrated colours and most line-strengths of the stellar clusters of the Milky-Way and LMC. Our full-spectrum fits in the UV range for a representative set of ETGs of varying mass yield age and metallicity estimates in very good agreement with those obtained in the optical range. The comparison of UV colours and line-strengths of massive ETGs with our models reveals the presence of young stellar components, with ages in the range 0.1-0.5 Gyr and mass fractions 0.1-0.5%, on the top of an old stellar population.

The morphology of the sub-giant branch and red clump reveal no sign of age spreads in intermediate-age clusters

A recent surprise in stellar cluster research, made possible through the precision of Hubble Space Telescope photometry, was that some intermediate age (1-2 Gyr) clusters in the Large and Small Magellanic Clouds have main sequence turn-off (MSTO) widths that are significantly broader than would be expected for a simple stellar population (SSP). One interpretation of these extended MSTOs (eMSTOs) is that age spreads of the order of ~500 Myr exist within the clusters, radically redefining our view of stellar clusters, which are traditionally thought of as single age, single metallicity stellar populations. Here we test this interpretation by studying other regions of the CMD that should also be affected by such large age spreads, namely the width of the sub-giant branch (SGB) and the red clump (RC). We study two massive clusters in the LMC that display the eMSTO phenomenon (NGC 1806 & NGC 1846) and show that both have SGB and RC morphologies that are in conflict with expectations if large age spreads exist within the clusters. We conclude that the SGB and RC widths are inconsistent with extended star-formation histories within these clusters, hence age spreads are not likely to be the cause of the eMSTO phenomenon. Our results are in agreement with recent studies that also have cast doubt on whether large age spreads can exist in massive clusters; namely the failure to find age spreads in young massive clusters, a lack of gas/dust detected within massive clusters, and homogeneous abundances within clusters that exhibit the eMSTO phenomenon.

MIUSCAT: extended MILES spectral coverage - I. Stellar population synthesis models

We extend the spectral range of our stellar population synthesis models based on the MILES and CaT empirical stellar spectral libraries. For this purpose, we combine these two libraries with the Indo-U.S. to construct composite stellar spectra to feed our models. The spectral energy distributions (SEDs) computed with these models and the originally published models are combined to construct composite SEDs for single-age, single-metallicity stellar populations (SSPs) covering the range λλ3465–9469 Å at moderately high and uniform resolution (full width at half-maximum = 2.51 Å). The colours derived from these SSP SEDs provide good fits to Galactic globular cluster data. We find that the colours involving redder filters are very sensitive to the initial mass function (IMF), as well as a number of features and molecular bands throughout the spectra. To illustrate the potential use of these models, we focus on the Na i doublet at 8200 Å and with the aid of the newly synthesized SSP model SEDs, we define a new IMF-sensitive index that is based on this feature, which overcomes various limitations from previous index definitions for low-velocity dispersion stellar systems. We propose an index–index diagram based on this feature and the neighbouring Ca ii triplet at 8600 Å, to constrain the IMF if the age and [Na/Fe] abundance are known. Finally we also show a survey-oriented spectrophotometric application which evidences the accurate flux calibration of these models for carrying out reliable spectral fitting techniques. These models are available through our user-friendly website.

Evolutionary stellar population synthesis with MILES - I. The base models and a new line index system

[Abridged]. We present SEDs for single-age, single-metallicity stellar populations (SSPs) covering the optical range at resolution 2.3A (FWHM). These SEDs constitute our base models, as they combine scaled-solar isochrones with MILES empirical stellar library, which follows the chemical evolution pattern of the solar neighbourhood. The models rely as much as possible on empirical ingredients, not just on the stellar spectra, but also on extensive photometric libraries. The unprecedented stellar parameter coverage of MILES allowed us to safely extend our optical SSP SED predictions from intermediate- to very-old age regimes, and the metallicity coverage of the SSPs from super-solar to [M/H]=-2.3. SSPs with such low metallicities are particularly useful for globular cluster studies. Observed spectra can be studied by means of full spectrum fitting or line-strengths. For the latter we propose a new Line Index System (LIS) to avoid the intrinsic uncertainties associated with the popular Lick/IDS system and provide more appropriate, uniform, spectral resolution. Apart from constant resolution as function of wavelength the system is also based on flux-calibrated spectra. Data can be analyzed at three different resolutions: 5A, 8.4A and 14A (FWHM), which are appropriate for studying globular cluster, low and intermediate-mass galaxies, and massive galaxies, respectively. Polynomials to transform current Lick/IDS line index measurements to the new system are provided. A web-page with a suite of on-line tools to facilitate the handling and transformation of the spectra is available at http://miles.iac.es.

MILES SSP Models

AbstractWe present SEDs for single-age, single-metallicity stellar populations (SSPs) covering the full optical spectral range at resolution (FWHM = 2.3Å). These SEDs can be regarded as our base models, as we combine scaled-solar isochrones with an empirical stellar spectral library (MILES), which follows the chemical evolution pattern of the solar neighbourhood. The models rely as much as possible on empirical ingredients as also employ extensive photometric libraries. Thanks to the unprecedented parameter coverage of the MILES library we synthesize SSP SEDs from intermediate- to very-old age regimes, and the metallicity from super-solar to [M/H] = −2.3, all for a suite of IMF shapes and slopes. We propose a new Line Index System (LIS), based on flux-calibrated spectra, to avoid the intrinsic uncertainties associated with the Lick/IDS system and provide more appropriate, uniform, spectral resolution.

Surface-brightness fluctuations in stellar populations

Resumen en: A new theoretical calibration of surface-brightness fluctuations (SBF) for single age, single metallicity stellar populations is presented for the optic...

Stellar Population SEDs at 2.3Å

We present a new spectral library for old and intermediate-aged single-age, single-metallicity stellar populations for a wide metallicity range. The major ingredient of these models is a new empirical stellar library composed of 985 stars, whose main characteristic is its unprecedented stellar atmospheric parameters coverage. The model SEDs cover the spectral range 3540-7410A at 2.3A (FWHM). We present some advantages and applications of these models.

Surface-brightness fluctuations in stellar populations

Aims. A new theoretical calibration of surface-brightness fluctuations (SBF) for single age, single metallicity stellar populations is presented for the optical and near-IR broad-band filters, as well as for the HST WFPC2 and ACS filters. Methods. The IAC-star code is used. Two Padua and the Teramo stellar evolution libraries have been considered. A set of single-burst stellar populations (SSP) with a wide range of ages (3 Gy−15 Gy) and metallicities (Z = 0.0001−0.03) have been computed using each one of the three considered stellar evolution libraries. For each SSP, color indexes and SBF magnitudes are given for the filters U, B, V, R, I, J, H, K, F218W, F336W, F439W, F450W, F555W and F814W, and for the first time, an uncertainty has been estimated for the SBF theoretical calibration. Results. Although some differences might be addressed, the Padua and Teramo stellar evolution libraries provide comparable SBF results. A detailed comparison of the present SBF calibrations with both previous calibrations and observational data is also presented. Comparing the different models with observational data, Padua based models reproduce fairly well the optical data for globular clusters, while Teramo based models fits both optical galaxies and globular clusters data, as well. In the near-IR wavelengths, the Teramo based models provide the only SBF theoretical calibration to date able to properly reproduce the observational data for superclusters, with intermediate-to-low metallicity. As a conclusion, Teramo based models work better than any other calibration reproducing observational data for the near-IR wavelengths. Furthermore, the age-metallicity degeneracy is broken for low metallicity (Z ≤ 0.0037) stellar populations. Finally, a clear relation between the B SBF absolute magnitude of a stellar population and its metallicity is found for intermediate to old populations, so the B-band fluctuation magnitude is proposed as a metallicity tracer. The present theoretical calibration shows that the analysis of SBF provides a very powerful tool in the study and characterization of unresolved stellar populations.

Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories

We introduce and discuss the properties of a theoretical (B−K)(J−K) integrated colour diagram for single-age, single-metallicity stellar populations. We show how this combination of integrated colours is able to largely disentangle the well-known age–metallicity degeneracy when the age of the population is greater than ∼300 Myr, and thus provides valuable estimates of both age and metallicity of unresolved stellar systems. We discuss in detail the effect on this colour–colour diagram of α-enhanced metal abundance ratios (typical of the oldest populations in the Galaxy), the presence of blue horizontal branch stars unaccounted for in the theoretical calibration and of statistical colour fluctuations in low-mass stellar systems. In the case of populations with multiple stellar generations, the luminosity-weighted mean age obtained from this diagram is shown to be heavily biased towards the youngest stellar components. We then apply this method to several data sets for which optical and near-infrared photometry are available in the literature. We find that Large Magellanic Cloud and M31 clusters have colours which are consistent with the predictions of the models, but these do not provide a sensitive test due to the fluctuations which are predicted by our modelling of the Poisson statistics in such low-mass systems. For the two Local Group dwarf galaxies NGC 185 and 6822, the mean ages derived from the integrated colours are consistent with the star formation histories inferred independently from photometric observations of their resolved stellar populations. The methods developed here are applied to samples of nearby early-type galaxies with high-quality aperture photometry in the literature. A sample of bright field and Virgo cluster elliptical galaxies is found to exhibit a range of luminosity-weighted mean ages from 3 to 14 Gyr, with a mean of ∼8 Gyr, independent of environment, and mean metallicities at or just above the solar value. Colour gradients are found in all of the galaxies studied, in the sense that central regions are redder. Apart from two radio galaxies, where the extreme central colours are clearly driven by the active galactic nucleus, and one galaxy which also shows a radial age gradient, these colour changes appear consistent with metallicity changes at a constant mean age. Finally, aperture data for five Virgo early-type dwarf galaxies show that these galaxies appear to be shifted to lower mean metallicities and lower mean ages (range 1–6 Gyr) than their higher luminosity counterparts.

New near-infrared surface brightness fluctuation models

We present new theoretical models for surface brightness fluctuations (SBFs) in the nearinfrared (near-IR). We show the time evolution of near-IR brightness fluctuation properties over large age and metallicity ranges, i.e. from 12 Myr to 16 Gyr and from Z/Z� = 1/50 to 2.5, for single age, single metallicity stellar populations. All the stellar models are followed from the zero-age main-sequence to the central carbon ignition for massive stars, or to the end of the thermally pulsing regime of the asymptotic giant branch (AGB) phase for low- and intermediate-mass stars. The new models are compared with observed near-IR fluctuation absolute magnitudes and colours for a sample of Magellanic Cloud (MC) star clusters and Fornax cluster galaxies. For star clusters younger than ∼3 Gyr, the predicted near-IR fluctuation properties are in satisfactory agreement with observed ones over a wide range of stellar population metallicities. However, for older star clusters, the agreement between the observed and predicted nearIR brightness fluctuations depends on how the surface brightness absolute magnitudes are estimated. The computed set of models are not able to match the observed near-IR fluctuation absolute magnitudes and colours simultaneously. We argue that the observed discrepancies between the predicted and observed properties of old MC superclusters are more likely due to observational reasons.

The Integrated Spectrum of M67 and the Spectroscopic Age of M32

We construct an integrated spectrum of the intermediate-age, solar-metallicity Galactic cluster M67, from individual spectroscopic observations of bona fide cluster members. The spectrum so obtained is used as a template to test our stellar population synthesis models, in an age and metallicity regime where such models remain largely untested. As a result, we demonstrate that our models predict a spectroscopic age of 3.5 ± 0.5 Gyr for M67, which is the same age we obtain from fitting isochrones to the color-magnitude diagram of the cluster. Full consistency is reached when using either Hβ, Hγ, or Hδ as the age indicator. We also check if the models, when applied to the cluster integrated spectrum, predict elemental abundances in agreement with the known detailed abundance pattern of the cluster. The models also pass the latter test, by predicting the abundances of iron, magnesium, carbon, and nitrogen in agreement with detailed abundance analyses of cluster stars to within 0.1 dex. Encouraged by the high degree of consistency of our models, we apply them to the study of the integrated spectrum of the central 3'' of the compact elliptical galaxy M32. The resulting luminosity-weighted age of the galaxy ranges between 2 and 3.5 Gyr, depending on the age indicator adopted. According to our models, the center of M32 seems to have a supersolar iron abundance, ranging between [Fe/H] ~ +0.1 and +0.3, depending on the spectral index adopted. The light element magnesium seems to be underabundant in the center of M32 relative to iron by about ~0.1–0.2 dex, whereas the data are consistent with nearly solar carbon and nitrogen abundances relative to iron. We find that single-age, single-metallicity stellar population models with solar-scaled abundance patterns cannot fit all the Balmer and metal lines in the integrated spectrum of M32. In particular, there is a systematic trend in the sense that bluer absorption lines indicate a younger age and a higher metallicity. This slight inconsistency can be due either to (unaccounted for) abundance ratio effects on blue iron and Balmer line indices or to a spread of the ages of the stellar populations in M32. Current stellar population models cannot break this degeneracy at the level of accuracy required to address this problem.

Nitrogen Overabundance: Globular Cluster and Halo Formation

Halo globular clusters pose four succinct issues that must be solved in any scenario of their formation: single-age, single metallicity stellar populations; a lower limit ([Fe/H] ~ -2.3) to their average metallicity; comprising only 1% of the stellar halo mass, and being among the oldest stars in our Galaxy. New spectra are presented of Galactic stars and integrated spectra of Galactic globular clusters which extend to 3250 angstroms. These spectra show show that the most metal-poor and among the best-studied Galactic globular clusters show strong NH3360 absorption, even though their spectral energy distributions in the near-UV are dominated by blue horizontal branch, AF-type stars. These strong NH features must be coming from the main sequence stars in these clusters. These new data are combined with existing data on the wide range of carbon and nitrogen abundance in very metal-poor ([Fe/H] < -3.5) halo giant and dwarf stars, together with recent models of zero-metal star formation, to make a strawman scenario for globular cluster formation that can reproduce three of the above four issues, and well as related two of the three issues pertaining to nitrogen overabundance. This strawman proposal makes observational and theoretical predictions that are testable, needing specific help from the modelers to understand all of the elemental constraints on globular cluster and halo formation.

An empirical test of the theoretical population corrections to the red clump absolute magnitude

The mean absolute magnitude of the local red clump (RC), MRCλ, is a very well-determined quantity owing to the availability of accurate Hipparcos parallaxes for several hundred RC stars, potentially allowing it to be used as an accurate extragalactic distance indicator. Theoretical models predict that the RC mean magnitude is dependent on both age and metallicity and, furthermore, that these dependences are non-linear. This suggests that a population correction, ΔMRCλ, based on the star formation rate (SFR) and age–metallicity relation (AMR) of the system in question, should be applied to the local RC magnitude before it can be compared to the RC in any other system in order to make a meaningful distance determination. Using a sample of eight Galactic open clusters and the Galactic globular cluster 47 Tuc, we determine the cluster distances, and hence the RC absolute magnitude in V, I and K, by applying our empirical main-sequence (MS) fitting method, which utilizes a large sample of local field dwarfs with accurate Hipparcos parallaxes. The nine clusters have metallicities in the range −0.7 ≤[Fe/H]≤+0.02 and ages from 1 to 11 Gyr, enabling us to make a quantitative assessment of the age and metallicity dependences of ΔMRCλ predicted by the recent theoretical models of Girardi & Salaris and Salaris & Girardi. We find excellent agreement between the empirical data and the models in all three passbands, with no statistically significant trends or offsets, thus fully confirming the applicability of the models to single-age, single-metallicity stellar populations. Since, from the models, ΔMRCλ is a complicated function of both metallicity and age, if this method is used to derive distances to composite populations, it is essential to have an accurate assessment of the SFR and AMR of the system in question, if errors of several tenths of a magnitude are to be avoided. Using recent determinations of the SFR and AMR for four systems — the Large and Small Magellanic Clouds, Carina and the solar neighbourhood — we examine the quantity IRCobs−KRCobs, which is the difference between the mean magnitude of the RC in the I band and the K band. Comparing the theoretical predictions with the most recent observational data, we find complete agreement between the observations and the models, thus confirming even further the applicability of the population corrections predicted from theory.

Empirical calibration of the near-infrared Ca II triplet -- IV. The stellar population synthesis models

We present a new evolutionary stellar population synthesis model, which predicts spectral energy distributions for single-age single-metallicity stellar populations (SSPs) at resolution 1.5 Angstrom (FWHM) in the spectral region of the near-infrared Ca II triplet feature. The main ingredient of the model is a new extensive empirical stellar spectral library that has been recently presented by Cenarro et al., which is composed of more than 600 stars with an unprecedented coverage of the stellar atmospheric parameters. Two main products of interest for stellar population analysis are presented. The first is a spectral library for SSPs with metallicities -1.7<[Fe/H]<+0.2, a large range of ages (0.1-18 Gyr) and initial mass function (IMF) types. They are well suited to modelling galaxy data, since the SSP spectra, with flux-calibrated response curves, can be smoothed to the resolution of the observational data, taking into account the internal velocity dispersion of the galaxy, allowing the user to analyse the observed spectrum in its own system. We also produce integrated absorption-line indices (namely CaT*, CaT and PaT) for the same SSPs in the form of equivalent widths. We find the following behaviour for the Ca II triplet feature in old-aged SSPs: (i) the strength of the CaT* index does not change much with time for all metallicities for ages larger than similar to3 Gyr; (ii) this index shows a strong dependence on metallicity for values below [M/H]similar to-0.5; and (iii) for larger metallicities this feature does not show a significant dependence either on age or on the metallicity, being more sensitive to changes in the slope of power-like IMF shapes. The SSP spectra have been calibrated with measurements for globular clusters by Armandroff & Zinn, which are well reproduced, probing the validity of using the integrated Ca II triplet feature for determining the metallicities of these systems. Fitting the models to two early-type galaxies of different luminosities (NGC 4478 and 4365), we find that the Ca II triplet measurements cannot be fitted unless a very dwarf-dominated IMF is imposed, or if the Ca abundance is even lower than the Fe abundance. More details can be found in work by Cenarro et al.

Spectral and photometric evolution of simple stellar populations at various metallicities

A new set of evolutionary synthesis spectra are presented for Simple Stellar Populations (SSPs) covering ranges in metallicity from 0.02 � Z/Z⊙ � 2.5 and ages from 4 · 10 6 yrt � 16 Gyr. They are based on the most recent isochrones from the Padova group that extend earlier models by the inclusion of the thermal pulsing AGB phase for stars in the mass range 2 M⊙ � m � 7 M⊙ in accordance with the fuel consumption theorem. We show that with respect to earlier models, inclusion of the TP-AGB phase leads to significant changes in the (V I) and (V K) colors of SSPs in the age range from 10 8 to � 10 9 yr. Using model atmosphere spectra from Lejeune et al. (1997, 1998), we calculate the spectral evolution of single burst populations of various metallicities covering the wavelength range from 90 u through 160 � m. Isochrone spectra are convolved with filter response functions to describe the time evolution of luminosities and colors in Johnson, Thuan & Gunn, Koo, HST, Washington and Stromgren filters. The models and their results are not only intended for use in the interpretation of star clusters but also for combination with any kind of dynamical galaxy formation and/or evolution model that contains a star formation criterion. Moreover, the evolution of these single burst single metallicity stellar populations is readily folded with any kind of star formation - and eventually chemical enrichment - history to describe the evolutionary spectral synthesis of composite stellar populations like galaxies of any type with continuous or discontinuous star formation. For these latter purposes we also present the time evolution of ejection rates for gas and metals for two different Initial Mass Functions (IMFs) as well as cosmological and evolutionary corrections for all the filters as a function of redshift for 0 � z � 5 and two different cosmologies. Extensive data files are provided in the electronic version, at CDS, and at our above www-address.

A Robust Age Indicator for Old Stellar Populations

We derive new spectral Hγ index definitions that are robust age indicators for old and relatively old stellar populations and therefore have great potential for solving the age-metallicity degeneracy of galaxy spectra. To study this feature as a function of age, metallicity, and resolution, we have used a new spectral synthesis model that predicts spectral energy distributions of single-age, single-metallicity stellar populations at resolution FWHM~1.8 Å (which can be smoothed to different resolutions), allowing direct measurements of the equivalent widths of particular absorption features. We show that Hγ's strong age-disentangling power is due to a compensating effect: at a specified age, Hγ strengthens with metallicity owing to an adjacent metallic absorption, but on the other hand, the adopted pseudocontinua are depressed by the effects of strong neighboring Fe I lines on both sides of Hγ. Despite the fact that this effect depends strongly on the adopted resolution and galaxy velocity dispersion σ, we propose a system of indicators that are completely insensitive to metallicity and stable against resolution, allowing the study of galaxies up to σ ~ 300 km s-1. An extensive analysis of the characteristics of these indices indicates that observational spectra of very high signal-to-noise ratio and relatively high dispersion are required to gain this unprecedented age-discriminating power. Once such spectra are obtained, accurate and reliable estimates for the luminosity-weighted average stellar ages of these galaxies will become possible for the first time, without assessing their metallicities. We measured this index for two globular clusters, a number of low-luminosity elliptical galaxies, and a standard S0 galaxy. We find a large spread in the average stellar ages of a sample of low-luminosity ellipticals. In particular, these indices yield 4 Gyr for M32. This value is in excellent agreement with the age provided by the extraordinary fit to the full spectrum of this galaxy that we achieve in this paper.

Synthetic Spectra of H Balmer and He i Absorption Lines. II. Evolutionary Synthesis Models for Starburst and Poststarburst Galaxies

We present evolutionary stellar population synthesis models to predict the spectrum of a single-metallicity stellar population, with a spectral sampling of 0.3 A in five spectral regions between 3700 and 5000 A. The models, which are optimized for galaxies with active star formation, synthesize the profiles of the hydrogen Balmer series (Hβ, Hγ, Hδ, H8, H9, H10, H11, H12, and H13) and the neutral helium absorption lines (He I λ4922, He I λ4471, He I λ4388, He I λ4144, He I λ4121, He I λ4026, He I λ4009, and He I λ3819) for a burst with an age ranging from 106 to 109 yr, and different assumptions about the stellar initial mass function (IMF). Continuous star formation models lasting for 1 Gyr are also presented. The input stellar library includes non-LTE absorption profiles for stars hotter than 25,000 K and LTE profiles for lower temperatures. The temperature and gravity coverage is 4000 K ≤ Teff ≤ 50,000 K and 0.0 ≤ log g ≤ 5.0, respectively. The metallicity is solar. It is found that the Balmer and He I line profiles are sensitive to the age, except during the first 4 Myr of the evolution, when the equivalent widths of these lines are constant. At these early stages of the evolution, the profiles of the lines are also sensitive to the IMF. However, strong H Balmer and He I lines are predicted even when the low-mass cutoff of the IMF is as high as 10 M☉. The equivalent widths of the Balmer lines range from 2 to 16 A and those of the He I lines from 0.2 to 1.2 A. During the nebular phase (cluster younger than about 10 Myr), Hβ ranges from 2 to 5 A and He I λ4471 ranges between 0.5 and 1.2 A. The strength of the lines is maximum when the cluster is a few hundred (for the Balmer lines) and a few tens (for the He I lines) of Myr old. In the continuous star formation scenario, the strength of the Balmer and He I lines increases monotonically with time until 500 and 100 Myr, respectively. However, the lines are weaker than in the burst models owing to the dilution of the Balmer and He I lines by the contribution from very massive stars. The high spectral resolution of the profiles is useful to reproduce the absorption wings observed in regions of recent star formation and to estimate the effect of the underlying absorption on the nebular emission lines. The strength of the nebular emission Balmer and He I lines compared with the stellar absorption components indicates that Hδ and the higher order terms of the Balmer series and He I are dominated by the stellar absorption component if an instantaneous burst is older than 5 Myr. Some of the He I lines (e.g., He I λ3819 and He I λ4922) are more favorable than others (e.g., He I λ4471) for the detection of stellar features in the presence of nebular emission. We estimate that the correction to the He I λ4471 nebular emission line due to the stellar absorption is between 5% and 25%, if the nebular emission has equivalent width between 10 and 2.5 A (corresponding to a burst age between 1 and 3 Myr). The models can be used to date starburst and poststarburst galaxies until 1 Gyr. They have been tested on data for clusters in the LMC, the super-star cluster B in the starburst galaxy NGC 1569, the nucleus of the dwarf elliptical NGC 205 and a luminous E+A galaxy. The full data set is available for retrieval at our websites or on request from the authors.

Evolutionary Stellar Population Synthesis at 2 A Spectral Resolution

In this paper we develop an evolutionary stellar population synthesis model to predict spectral energy distributions (SEDs) for single-age, single-metallicity stellar populations (SSPs) at resolution ~1.8 A in two reduced but very important spectral regions around 4000 and 5000 A. The input stellar database is composed of a subsample of ~500 stars selected from the original Jones spectral library This is the first time that such an evolutionary model has employed such an extensive empirical stellar spectral library, at such high resolution, for supporting its SED predictions. A spectral library corresponding to simple old stellar populations with metallicities in the range -0.7≤[Fe/H]≤+0.2 is presented here, as well as an extensive discussion about the most popular system of absorption indices at intermediate resolution, the Lick system, showing the advantages of using the new model predictions. Also, we show for the first time the behavior of the Rose system of indices, at higher resolution, as a function of the age and the metallicity of the stellar population. The newly synthesized model spectra can be used to analyze the observed galaxy spectrum in a very easy and flexible way, allowing us to adapt the theoretical predictions to the characteristics of the data instead of proceeding in the opposite direction as, for example, we must do when transforming the observational data for using model predictions based on a particular instrument-dependent system of indices at a specific resolution. The synthetic SSP spectra, with flux-calibrated spectral response, can be smoothed to the same resolution as the observations or to the measured galaxy internal velocity dispersion, allowing us to analyze the observed spectrum in its own system. Therefore, we are able to utilize all the information contained in the data at their spectral resolution. After performing this step, the entire observational spectrum can be compared at one time, or the analysis can be done by measuring a particular set of features in the synthesized and the observational spectra rather than by trying to correct the latter from broadening or instrumental effects. The SSP model spectra were calibrated at relatively high resolution with two well-studied metal-rich globular clusters in our Galaxy, 47 Tuc and NGC 6624, providing very good fits and being able to detect well-known spectral peculiarities such as the CN anomaly of 47 Tuc. The model was also applied to an early-type galaxy, NGC 3379, revealing its well-known abundance of magnesium relative to iron and showing the appropriateness of the new model predictions as well as the way in which they can be used for studying the elemental ratios of these stellar systems. In fact, different models of different metallicities provide equal approaches to the galaxy spectrum: once Hβ is properly constrained, we are able to fit either the iron features (with a metallicity somewhat in the range -0.4≤[Fe/H]≤0) or the magnesium features (with a metallicity in the range 0≤[Fe/H]≤+0.2), but not both simultaneously.

On the metallicity distribution in the nuclei of elliptical galaxies

Using current models of spectrophotometric properties of single age, single metallicity stellar populations I have computed the Mg2, Hbeta, Fe52 and Fe53 line strengths for stellar populations with a metallicity spread. The comparison of these models with the nuclear indices of early type galaxies yield the following major conclusions. The metallicity distribution of the closed box, simple model for the chemical evolution of galaxies is not able to account for Mg2 and Fe52, Fe53 values in excess of 0.27,3 and 2.7, respectively, which are observed in the nuclei of a large fraction of Ellipticals. To reproduce the line strengths in these galaxies high average metallicities, small metallicity dispersion and old ages are required. In particular, Mg2 values of 0.3 are reproduced only with a metallicity distribution ranging from 0.5\Zsun to 3\Zsun, and 15 Gyr old stellar populations. I interpret the data as indicating that the gas out of which the nuclei of ellipticals formed was pre-enriched, to larger metallicities for increasing \Mgtw.The presence of a metallicity dispersion does not alter the relation between Mg2 and Iron indices with respect to the SSP models. Thus, the need for a Mg/Fe overabundance in the strongest lined galaxies is confirmed, and I present a simple way to estimate the [Mg/Fe] ratio on the basis of existing models with solar abundance ratios.