Single-burst Stellar Population Research Articles

The Evolution of AGN Activity in Brightest Cluster Galaxies

Abstract We present the results of an analysis of Wide-field Infrared Survey Explorer (WISE) observations of the full 2500 deg2 South Pole Telescope (SPT)-Sunyaev–Zel’dovich cluster sample. We describe a process for identifying active galactic nuclei (AGN) in brightest cluster galaxies (BCGs) based on WISE mid-IR color and redshift. Applying this technique to the BCGs of the SPT-SZ sample, we calculate the AGN-hosting BCG fraction, which is defined as the fraction of BCGs hosting bright central AGNs over all possible BCGs. Assuming an evolving single-burst stellar population model, we find statistically significant evidence (>99.9%) for a mid-IR excess at high redshift compared to low redshift, suggesting that the fraction of AGN-hosting BCGs increases with redshift over the range of 0 < z < 1.3. The best-fit redshift trend of the AGN-hosting BCG fraction has the form (1 + z)4.1±1.0. These results are consistent with previous studies in galaxy clusters as well as as in field galaxies. One way to explain this result is that member galaxies at high redshift tend to have more cold gas. While BCGs in nearby galaxy clusters grow mostly by dry mergers with cluster members, leading to no increase in AGN activity, BCGs at high redshift could primarily merge with gas-rich satellites, providing fuel for feeding AGNs. If this observed increase in AGN activity is linked to gas-rich mergers rather than ICM cooling, we would expect to see an increase in scatter in the P cav versus L cool relation at z > 1. Last, this work confirms that the runaway cooling phase, as predicted by the classical cooling-flow model, in the Phoenix cluster is extremely rare and most BCGs have low (relative to Eddington) black hole accretion rates.

Firefly (Fitting IteRativEly For Likelihood analYsis): a full spectral fitting code

We present a new spectral fitting code, Firefly, for deriving the stellar population properties of stellar systems. Firefly is a chi-squared minimisation fitting code that fits combinations of single-burst stellar population models to spectroscopic data, following an iterative best-fitting process controlled by the Bayesian Information Criterion. No priors are applied, rather all solutions within a statistical cut are retained with their weight. Moreover, no additive or multiplicative polynomia are employed to adjust the spectral shape. This fitting freedom is envisaged in order to map out the effect of intrinsic spectral energy distribution (SED) degeneracies, such as age, metallicity, dust reddening on galaxy properties, and to quantify the effect of varying input model components on such properties. Dust attenuation is included using a new procedure, which was tested on Integral Field Spectroscopic (IFS) data in a previous paper. The fitting method is extensively tested with a comprehensive suite of mock galaxies, real galaxies from the Sloan Digital Sky Survey and Milky Way globular clusters. We also assess the robustness of the derived properties as a function of signal-to-noise ratio and adopted wavelength range. We show that \FF\ is able to recover age, metallicity, stellar mass and even the star formation history remarkably well down to a $S/N\sim5$, for moderately dusty systems. Code and results are publicly available at www.icg.port.ac.uk/firefly.

Stellar content of extremely red quiescent galaxies atz> 2

CONTEXT. A set of 20 extremely red galaxies at 2.5<z<3.8 with photometric features of old passive-evolving galaxies without dust, with stellar masses of ~10^{11} M_sun, have colors that could be related to passive-evolving galaxies with mean ages larger than 1 Gyr. This suggests they have been formed, on average, when the Universe was very young (<1 Gyr). AIMS. We provide new estimates for the stellar content of these 20 galaxies, with a deeper analysis for two of them that includes spectroscopy. METHODS. We obtained, with the GRANTECAN-10.4 m, ultraviolet rest-frame spectra of two galaxies and analyzed them together with photometric data. The remaining 18 galaxies are analyzed only with photometry. We fit the data with models of a single-burst stellar population (SSP), combinations of two SSPs, as well as with extended star formation. RESULTS. Fits based on one SSP do not provide consistent results for the blue and red wavelengths. Moreover, the absence in the spectra of a break at ~2,000 angstroms indicates that a rather young component is necessary. Using two SSPs we can match the photometric and spectroscopic data, with the bulk of the stellar population being very old (several Gyr) and the remaining contribution (<5% of stellar mass fraction) from a young, likely residual star formation component with age <~0.1 Gyr. Exponentially decaying extended star formation models improve slightly the fits with respect to the single burst model, but they are considerably worse than the two SSP based fits, further supporting the residual star formation scenario. CONCLUSIONS. The fact that one SSP cannot match these early-type galaxies highlights the limitations for the use of age estimators based on single lines or breaks, such as the Balmer break used in cosmic chronometers, thus questioning this approach for cosmological purposes.

METALLICITY AND AGE OF THE STELLAR STREAM AROUND THE DISK GALAXY NGC 5907

Stellar streams have become central to studies of the interaction histories of nearby galaxies. To characterize the most prominent parts of the stellar stream around the well-known nearby (d = 17 Mpc) edge-on disk galaxy NGC 5907, we have obtained and analyzed new, deep gri Subaru/Suprime-Cam and 3.6 micron Spitzer/Infrared Array Camera (IRAC) observations. Combining the near-infrared 3.6 micron data with visible-light images allows us to use a long wavelength baseline to estimate the metallicity and age of the stellar population along a ~60 kpc long segment of the stream. We have fitted the stellar spectral energy distribution (SED) with a single-burst stellar population synthesis model and we use it to distinguish between the proposed satellite accretion and minor/major merger formation models of the stellar stream around this galaxy. We conclude that a massive minor merger (stellar mass ratio of at least 1:8) can best account for the metallicity of -0.3 inferred along the brightest parts of the stream.

MILES extended: Stellar population synthesis models from the optical to the infrared

We present the first single-burst stellar population models which covers the optical and the infrared wavelength range between 3500 and 50000 Angstrom and which are exclusively based on empirical stellar spectra. To obtain these joint models, we combined the extended MILES models in the optical with our new infrared models that are based on the IRTF (Infrared Telescope Facility) library. The latter are available only for a limited range in terms of both age and metallicity. Our combined single-burst stellar population models were calculated for ages larger than 1 Gyr, for metallicities between [Fe/H] = -0.40 and 0.26, for initial mass functions of various types and slopes, and on the basis of two different sets of isochrones. They are available to the scientific community on the MILES web page. We checked the internal consistency of our models and compared their colour predictions to those of other models that are available in the literature. Optical and near infrared colours that are measured from our models are found to reproduce the colours well that were observed for various samples of early-type galaxies. Our models will enable a detailed analysis of the stellar populations of observed galaxies.

Stellar population synthesis models between 2.5 and 5 μm based on the empirical IRTF stellar library

We present the first single-burst stellar population models in the infrared wavelength range between 2.5 and 5 {\mu}m which are exclusively based on empirical stellar spectra. Our models take as input 180 spectra from the stellar IRTF (Infrared Telescope Facility) library. Our final single-burst stellar population models are calculated based on two different sets of isochrones and various types of initial mass functions of different slopes, ages larger than 1 Gyr and metallicities between [Fe/H] = -0.70 and 0.26. They are made available online to the scientific community on the MILES web page. We analyse the behaviour of the Spitzer [3.6]-[4.5] colour calculated from our single stellar population models and find only slight dependences on both metallicity and age. When comparing to the colours of observed early-type galaxies, we find a good agreement for older, more massive galaxies that resemble a single-burst population. Younger, less massive and more metal-poor galaxies show redder colours with respect to our models. This mismatch can be explained by a more extended star formation history of these galaxies which includes a metal-poor or/and young population. Moreover, the colours derived from our models agree very well with most other models available in this wavelength range. We confirm that the mass-to-light ratio determined in the Spitzer [3.6] {\mu}m band changes much less as a function of both age and metallicity than in the optical bands.

OBJECTIVE IDENTIFICATION OF INFORMATIVE WAVELENGTH REGIONS IN GALAXY SPECTRA

Understanding the diversity in spectra is the key to determining the physical parameters of galaxies. The optical spectra of galaxies are highly convoluted with continuum and lines which are potentially sensitive to different physical parameters. Defining the wavelength regions of interest is therefore an important question. In this work, we identify informative wavelength regions in a single-burst stellar populations model by using the CUR Matrix Decomposition. Simulating the Lick/IDS spectrograph configuration, we recover the widely used Dn(4000), Hbeta, and HdeltaA to be most informative. Simulating the SDSS spectrograph configuration with a wavelength range 3450-8350 Angstrom and a model-limited spectral resolution of 3 Angstrom, the most informative regions are: first region-the 4000 Angstrom break and the Hdelta line; second region-the Fe-like indices; third region-the Hbeta line; fourth region-the G band and the Hgamma line. A Principal Component Analysis on the first region shows that the first eigenspectrum tells primarily the stellar age, the second eigenspectrum is related to the age-metallicity degeneracy, and the third eigenspectrum shows an anti-correlation between the strengths of the Balmer and the Ca K and H absorptions. The regions can be used to determine the stellar age and metallicity in early-type galaxies which have solar abundance ratios, no dust, and a single-burst star formation history. The region identification method can be applied to any set of spectra of the user's interest, so that we eliminate the need for a common, fixed-resolution index system. We discuss future directions in extending the current analysis to late-type galaxies.

ULTRAVIOLET/OPTICAL/INFRARED COLOR SEQUENCES ALONG THE TIDAL RING/ARM OF Arp 107

We construct UV/optical/IR spectral energy distributions for 29 star forming regions in the interacting galaxy Arp 107, using GALEX UV, Sloan Digitized Sky Survey optical, and Spitzer infrared images. In an earlier study utilizing only the Spitzer data, we found a sequence in the mid-infrared colors of star-forming knots along the strong tidal arm in this system. In the current study, we find sequences in the UV/optical colors along the tidal arm that mirror those in the mid-infrared, with blue UV/optical colors found for regions that are red in the mid-infrared, and vice versa. With single-burst stellar population synthesis models, we find a sequence in the average stellar age along this arm, with younger stars preferentially located further out in the arm. Models that allow two populations of different ages and dust attenuations suggest that there may be both a young component and an older population present in these regions. Thus the observed color sequences may be better interpreted as a sequence in the relative proportion of young and old stars along the arm, with a larger fraction of young stars near the end. Comparison with star forming regions in other interacting galaxies shows that the Arp 107 regions are relatively quiescent, with less intense star formation than in many other systems.

Age and metallicity of compact stellar systems in Virgo and Fornax

For high signal-to-noise ratio GMOS-N and AAOmega spectra, we have measured line-strength indices in the Lick/IDS system for several luminous and spatially dispersed compact stellar systems (CSSs) located in the Virgo and Fornax galaxy clusters. We estimate their [alpha/Fe] ratios, ages and metallicities using a simple (single-burst) stellar population model We confirm that the Virgo core region luminous CSSs (M(bj) <= -10.5) contain ancient stellar populations with subsolar total metallicities, suggesting that they comprise the bright tail of M87's GC distribution. The two Virgo intracluster globular clusters have ages and metallicities consistent with the cluster core CSSs. Two Fornax luminous CSSs also have ancient stellar populations but are at the upper end of the Virgo CSS metallicity range, while the third (UCD3) appears to be relatively young, metal-rich and with a core + halo radial profile. Our results suggest that Fornax may contain an extra population of luminous CSSs formed more recently than the ancient GC-like systems found in both clusters.

Optical Surface Brightness Fluctuations of Shell Galaxies toward 100 Mpc

We measure F814W surface brightness fluctuations (SBFs) for a sample of distant shell galaxies with radial velocities ranging from 4000 to 8000 km s−1. The distance at galaxies is then evaluated by using the SBF method. For this purpose, theoretical SBF magnitudes for the HST ACS filters are computed for single-burst stellar populations covering a wide range of ages (t = 1.5–14 Gyr) and metallicities (Z = 0.008–0.04). Using these stellar population models, we provide the first F814W versus (F 475W − F 814W)0 calibration and we extend the previous M1 versus (B − I)0 color relation to colors (B − I)0 ≤ 2.0 mag. Coupling our SBF measurements with the theoretical calibration we derive distances with a statistical uncertainty of ~8%, and systematic error of ~6%. The procedure developed to analyze data ensures that the indetermination due to possible unmasked residual shells is well below ~12%. The results suggest that optical SBFs can be measured at d ≥ 100 Mpc with HST ACS imaging. SBF-based distances coupled with recession velocities corrected for peculiar motion, allow us obtain H0 = 76 ± 6 (statistical) ±5 (systematic) km s−1 Mpc−1.

A deep AAOmega survey of low-luminosity galaxies in the Shapley supercluster: stellar population trends

We present new optical spectroscopy for 342 R<18 galaxies in the Shapley Supercluster obtained with the AAOmega facility at the Anglo-Australian Telescope. We describe the observations and measurements of central velocity dispersion, emission line equivalent widths and absorption line indices. The distinguishing characteristic of the survey is its coverage of a very wide baseline in velocity dispersion (30-300 km/s), while achieving high signal-to-noise ratio throughout (median 60 per Ang). Significant emission at H-alpha was detected in ~20 per cent of red-sequence Shapley members. Using line-ratio diagnostics, we find that the emission is LINER-like at high luminosity, but driven by star-formation in low-luminosity galaxies. We use Lick indices to characterise the absorption spectra. We define a subset of galaxies with very low emission contamination, and fit the index-sigma relations for this subset. Comparing the index-sigma slopes against predictions from single-burst stellar population models, we infer the scaling relations of age, total metallicity, [Z/H], and alpha-element abundance ratio, [a/Fe]. To reproduce the observed index-sigma slopes, all three parameters must increase with increasing velocity dispersion. Specifically, we recover: Age \propto sigma^0.52+/-0.10, Z/H \propto sigma^0.34+/-0.07, and a/Fe \propto sigma^0.23+/-0.06 (error reflects systematic effects), derived over a decade baseline in velocity dispersion. The recovered age-sigma relation is shown to be consistent with the observed evolution in the giant-to-dwarf galaxy ratio in clusters at redshifts z=0.4-0.8. A companion paper will analyse the distribution of age, [Z/H] and [a/Fe] for individual galaxies. (Abridged.)

The Ages, Metallicities, and Star Formation Histories of Early‐Type Galaxies in the SDSS

We use the spectra of ~22,000 early-type galaxies, selected from the Sloan Digital Sky Survey, to infer the ages, metallicities, and star formation histories of these galaxies. We find clear evidence of downsizing, i.e., that galaxies with larger velocity dispersion have older stellar populations. In particular, we find that most early-type galaxies with velocity dispersion exceeding 200 km s-1 formed more than 90% of their current stellar mass at redshift z > 2.5. Therefore, star formation was suppressed around this redshift. We also show that chemical enrichment was rapid, lasting 1-2 Gyr, and find evidence that [Fe/H] is subsolar. We study the robustness of these results by comparing three different approaches: (1) using Lick absorption line indices, (2) fitting a single-burst stellar population model to the whole spectrum (lines and continuum), and (3) reconstructing the star formation and metallicity histories in multiple age bins, providing a method to measure mass-weighted ages and metallicities. We find good agreement between the luminosity-weighted ages and metallicities computed with these three methods.

Monte Carlo simulations of metal-poor star clusters

Context. Metal-poor globular clusters (GCs) can provide a probe of the earliest epoch of star formation in the Universe, being the oldest observable stellar systems. In addition, young and intermediate-age low-metallicity GCs are present in external galaxies. Nevertheless, inferring their evolutionary status by using integrated properties may suffer from large intrinsic uncertainty caused by the discrete nature of stars in stellar systems, especially in the case of faint objects. Aims. In this paper, we evaluate the intrinsic uncertainty (due to statistical effects) affecting the integrated colours and mass-to-light ratios as a function of the cluster's integrated visual magnitude ($M_V^{\rm tot}$), which represents a directly measured quantity. We investigate the case of metal-poor, single-burst stellar populations with age from a few million years to a likely upper value for the Galactic globular cluster ages (~15 Gyr). Methods. Our approach is based on Monte Carlo techniques for randomly generating stars distributed according to the cluster's mass function. Results. Integrated colours and mass-to-light ratios in different photometric bands are checked for good agreement with the observational values of low-metallicity Galactic clusters; the effect of different assumptions on the horizontal branch (HB) morphology is shown to be irrelevant, at least for the photometric bands explored here. We present integrated colours and mass-to-light ratios as a function of age for different assumptions on the cluster total V magnitude. We find that the intrinsic uncertainty cannot be neglected. In particular, in models with $M_V^{\rm tot}=-4$ the broad-band colours show an intrinsic uncertainty high enough to prevent the precise age of the cluster from being evaluated. The effects of different assumptions on the initial mass function and on the minimum mass for which carbon burning is ignited for both integrated colours and mass-to-light ratios are also analysed. Finally, the present predictions are compared with recent results available in the literature, showing non-negligible differences in some cases.

Population Synthesis Models for Late Buildup of the Red Sequence

We present population synthesis models designed to represent the star formation histories of L* red sequence galaxies (RSGs). Earlier work has shown that single-burst stellar populations (SSPs) are unable to match Balmer line strengths simultaneously at high and low redshift. We therefore consider alternative star formation histories in which RSGs contain intermediate-aged stars even at late epochs. The models are compared to Balmer Hdelta absorption strengths, U-B color data, and the number density of red sequence galaxies from z=1 to z=0. We find that quenched models (which consist of constant star formation histories truncated at regularly-spaced intervals) average to an RSG population that matches the data well, showing slow evolution in color and Balmer line strength and a rise in number density by a factor of a few after z=1. The data are best fit by a turn-on of quenching at redshifts z=1.5-2.

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.

Evolution and Environment of Early-Type Galaxies

Chemical abundance indicators are studied using composite spectra, which we provide in tabular form. Tables of line strengths measured from these spectra and parameters derived from these line strengths are also provided. From these we find that at fixed luminosity, early-type galaxies in low-density environments are slightly bluer, with stronger O II emission and stronger Hδ and Hγ Balmer absorption lines, indicative of star formation in the not very distant past. These galaxies also tend to have systematically weaker D4000 indices. The Lick indices and α-element abundance indicators correlate weakly but significantly with environment. For example, at fixed velocity dispersion, Mg is weaker in early-type galaxies in low-density environments by 30% of the rms scatter across the full sample, whereas most Fe indicators show no significant environmental dependence. The galaxies in our sample span a redshift range that corresponds to look-back times of ∼1 Gyr. We see clear evidence for evolution of line-index strengths over this time. Since the low-redshift population is almost certainly a passively aged version of the more distant population, age is likely the main driver for any observed evolution. We use the observed redshift evolution as a model-independent clock to identify indicators that are more sensitive to age than to other effects such as metallicity. In principle, for a passively evolving population, comparison of the trends with redshift and environment constrain how strongly the luminosity-weighted ages and metallicities depend on environment. We develop a method for doing this that does not depend on the details of stellar population synthesis models. Our analysis suggests that the galaxies that populate the densest regions in our sample are older by ∼1 Gyr than objects of the same luminosity in the least dense regions, and that metallicity differences are negligible. We also use single-burst stellar population synthesis models, which allow for nonsolar α-element abundance ratios, to interpret our data. The combination of Hβ, Mg b, and ⟨Fe⟩ lines suggests that age, metallicity, and α-enhancement all increase with velocity dispersion. The objects at lower redshifts are older but have the same metallicities and α-enhancements as their counterparts of the same σ at higher redshifts, as expected if the low-redshift sample is a passively aged version of the sample at higher redshifts. In addition, objects in dense environments are less than 1 Gyr older and α-enhanced by ∼0.02 relative to their counterparts of the same velocity dispersion in less dense regions, but the metallicities show no dependence on environment. This suggests that in dense regions, the stars in early-type galaxies formed at slightly earlier times and on a slightly shorter timescale than in less dense regions. Using HγF instead of Hβ leads to slightly younger ages but the same qualitative differences between environments. In particular, we find no evidence that objects in low-density regions are more metal-rich.

New Optical and Near-Infrared Surface Brightness Fluctuation Models. II. Young and Intermediate-Age Stellar Populations

We present theoretical surface brightness fluctuation (SBF) amplitudes for single-burst stellar populations of young and intermediate age (25 Myr ≤ t ≤ 5 Gyr) and metallicities Z = 0.0003, 0.001, 0.004, 0.008, 0.01, 0.02, and 0.04. The fluctuation magnitudes and colors as expected in the Johnson-Cousins (UBVRIJHK) photometric system are provided. We pay attention to the contribution of thermally pulsating asymptotic giant branch (TP-AGB) stars. The sensitivity of the predicted SBF to changes in the mass-loss scenario along the TP-AGB phase is examined. Below 0.6–1 Gyr both optical and near-IR SBF models exhibit a strong dependence on age and mass loss. We also evaluate SBF amplitudes using Monte Carlo techniques to reproduce the random variation in the number of stars experiencing bright and fast evolutionary phases (red giant branch, AGB, TP-AGB). On these grounds we provide constraints on the faintest integrated flux of real stellar populations required to derive reliable and meaningful SBF measurements. We analyze a technique for deriving SBF amplitudes of star clusters from the photometry of individual stars and estimate the uncertainty due to statistical effects, which may impinge on the procedure. The first optical SBF measurements for 11 Large Magellanic Cloud (LMC) star-rich clusters—with ages ranging from a few megayears to several gigayears—are derived using Hubble Space Telescope observations. The measurements are compared to our SBF predictions, providing a good agreement with models of metallicity Z = 0.0001–0.01. Our results suggest that, for TP-AGB stars, a mass loss as a power-law function of the star luminosity is required in order to properly reproduce the optical SBF data of the LMC clusters. Finally, near-IR models have been compared to available data, thus showing that the general trend is well fitted. We suggest how to overcome the general problem of SBF models in reproducing the details of the near-IR SBF measurements of the Magellanic Cloud star clusters.

The rates of type Ia supernovae

The aim of this paper is to provide a handy tool to compute the impact of type Ia SN (SNIa) events on the evolution of stellar systems. An effective formalism to couple the rate of SNIa explosions from a single burst of star formation and the star formation history is presented, which rests upon the definition of the realization probability of the SNIa event () and the distribution function of the delay times (()). It is shown that the current SNIa rate in late type galaxies constrains to be on the order of 10-3 (i.e. 1 SNIa every 1000 of gas turned into stars), while the comparison of the current rates in early and late type galaxies implies that ought to be more populated at short delays. The paper presents analytical formulations for the description of the function for the most popular models of SNIa progenitors, namely Single Degenerates (Chandrasekhar and Sub-Chandrasekhar exploders), and Double Degenerates. These formulations follow entirely from general considerations on the evolutionary behavior of stars in binary systems, modulo a schematization of the outcome of the phases of mass exchange, and compare well with the results of population synthesis codes, for the same choice of parameters. The derivation presented here offers an immediate astrophysical interpretation of the shape of the functions, and have a built in parametrization of the key properties of the alternative candidates. The important parameters appear to be the minimum and maximum masses of the components of the binary systems giving rise to a SNIa explosions, the distribution of the primary mass and of the mass ratios in these systems, the distribution of the separations of the DD systems at their birth. The various models for the progenitors correspond to markedly different impact on the large scales; correspondingly, the model for the progenitor can be constrained by examining the relevant observations. Among these, the paper concentrates on the trend of the current SNIa rate with parent galaxy type. The recent data by Mannucci et al. (2005, A&A, 433, 807) favor the DD channel over the SD one, which tends to predict a too steep distribution function of the delay times. The SD scenario can be reconciled with the observations only if the distribution of the mass ratios in the primordial binaries is flat and the accretion efficiency onto the WD is close to 100%. The various models are characterized by different timescales for the Fe release from a single burst stellar population. In particular the delay time within which half of the SNIa events from such a population have occurred, ranges between 0.3 and 3 Gyr, for a wide variety of hypothesis on the progenitors.

HSTSTIS Spectroscopy of the Triple Nucleus of M31: Two Nested Disks in Keplerian Rotation around a Supermassive Black Hole

We present Hubble Space Telescope (HST) spectroscopy of the nucleus of M31 obtained with the Space TelescopeImagingSpectrograph(STIS).SpectrathatincludetheCaiiinfraredtriplet(k ’ 85008)seeonlythered giant stars in the double brightness peaks P1 and P2. In contrast, spectra taken atk ’ 3600 51008 are sensitive to thetinybluenucleusembeddedinP2,thelowersurfacebrightnessnucleusofthegalaxy.P2 hasaK-typespectrum, but we find that the blue nucleus has an A-type spectrum: it shows strong Balmer absorption lines. Hence, the blue nucleus is blue not because of AGN light but rather because it is dominated by hot stars. We show that the spectrum is well described by A0 giant stars, A0 dwarf stars, or a 200 Myr old, single-burst stellar population. White dwarfs, in contrast, cannot fit the blue nucleus spectrum. Given the small likelihood for stellar collisions, recent star formation appears to be the most plausible origin of the blue nucleus. In stellar population, size, and velocity dispersion, the blue nucleus is so different from P1 and P2 that we call it P3 and refer to the nucleus of M31 as triple. Because P2 and P3 have very different spectra, we can make a clean decomposition of the red and blue stars and hence measure the light distribution and kinematics of eachuncontaminated by the other. The line-of-sight velocity distributions of the red stars near P2 strengthen the support for Tremaine’s eccentric disk model. Their wings indicate the presence of stars with velocities of up to 1000 km s � 1 on the anti-P1 side of P2. The kinematics of P3 are consistent with a circular stellar disk in Keplerian rotation around a supermassive black hole.If the P3 diskis perfectlythin,thentheinclination anglei ’ 55 � isidentical withinthe errorsto theinclination of the eccentric disk models for P1+P2 by Peiris & Tremaine and by Salow & Statler. Both disks rotate in the same sense and are almost coplanar. The observed velocity dispersion of P3 is largely caused by blurred rotation and has a maximum value of � ¼ 1183 � 201 km s � 1 . This is much larger than the dispersion � ’ 250 km s � 1 of the red stars along the same line of sight and is the largest integrated velocity dispersion observed in any galaxy. The rotation curve of P3 is symmetric around its center. It reaches an observed velocity of V ¼ 618 � 81 km s � 1 at radius 0B05 ¼ 0:19 pc, where the observed velocity dispersion is � ¼ 674 � 95 km s � 1 . The corresponding circular rotation velocity at this radius is � 1700 km s � 1 . We therefore confirm earlier suggestions that the central dark object

Intracluster Stars and the Chemical Enrichment of the Intracluster Medium

We explore the contribution of intracluster stars (ICS) to the chemical enrichment history of the intracluster medium (ICM). In contrast to scenarios in which all the metals originate in cluster galaxies and are then transported into the ICM, intracluster stars enrich the ICM in situ, thereby contributing 100% of their supernovae ejecta directly into the ICM. Modeling the ICS as an ancient, single burst stellar population with a normal initial mass function, we generate ICM iron abundances in the range of the observed values of several tenths solar. Large observational and theoretical uncertainties preclude us from concluding that the intracluster stars are the primary contributor of metals to the ICM in general. However, for the two clusters in our sample, and one from the literature, for which all of the required observational constraints exist, we are able to reproduce between half and all of the ICM iron. Due to the ubiquity of intracluster stars in clusters and their direct connection with the ICM, we conclude that all models of the chemical enrichment history and energy budget of the ICM should account for the impact of the ICS.