Laura Greggio

Age and metallicity distribution of the Galactic bulge from extensive optical and near-IR stellar photometry

We present a new determination of the metallicity distribution, age, and luminosity function of the Galactic bulge stellar population. By combining near-IR data from the 2MASS survey, from the SOFI imager at ESO NTT and the NICMOS camera on board HST we were able to construct color-magnitude diagrams (CMD) and luminosity functions (LF) with large statistics and small photometric errors from the Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) tip down to ∼0.15 M� . This is the most extended and complete LF so far obtained for the galactic bulge. Similar near-IR data for a disk control field were used to decontaminate the bulge CMDs from foreground disk stars, and hence to set a stronger constraint on the bulge age, which we found to be as large as that of Galactic globular clusters, or >10 Gyr. No trace is found for any younger stellar population. Synthetic CMDs have been constructed to simulate the effect of photometric errors, blending, differential reddening, metallicity dispersion and depth effect in the comparison with the observational data. By combining the near-IR data with optical ones, from the Wide Field Imager at the ESO/MPG 2.2 m telescope, a disk-decontaminated (MK,V-K )C MD has been constructed and used to derive the bulge metallicity distribution, by comparison with empirical RGB templates. The bulge metallicity is found to peak at near solar value, with a sharp cutoff just above solar, and a tail towards lower metallicity that does not appreciably extend below (M/H) ∼− 1.5.

OmegaCAM: wide-field imaging with fine spatial resolution

OmegaCAM is the wide-field camera for the VLT Survey Telescope being completed for ESOs Paranal observatory. The instrument, as well as the telescope, have been designed for very good, natural seeing-limited image quality over a 1 degree field. At the heart of the project are a square-foot photometric shutter, a 12-filter storage/exchange mechanism, a 16k x 16k CCD detector mosaic, and plenty of software for instrument control and data handling, analysis and archiving.

Constraints on galaxy formation from α‐enhancement in luminous elliptical galaxies

We explore the formation of α-enhanced and metal-rich stellar populations in the nuclei of luminous ellipticals under the assumption of two extreme galaxy formation scenarios based on hierarchical clustering, namely a fast clumpy collapse and the merger of two spirals. We investigate the parameter space of the star formation time-scale, the slope of the initial mass function (IMF) and stellar yields. In particular, the latter add a huge uncertainty in constraining time-scales and IMF slopes. We find that—for Thielemann, Nomoto & Hashimoto nucleosynthesis—in a fast clumpy collapse scenario an [α/Fe] overabundance of ∼ 0.2 dex in the high-metallicity stars can be achieved with a Salpeter IMF and star formation time-scales of the order 109 yr. The scenario of two merging spirals that are similar to our Galaxy, instead, fails to reproduce the α-enhanced abundance ratios in the metal-rich stars, unless the IMF is flattened during the burst ignited by the merger. This result is independent of the burst time-scale. We suggest that abundance gradients give hints to distinguish between the two extreme formation scenarios considered in this paper.

The Resolved Stellar Population of the Poststarburst Galaxy NGC 1569

We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) photometry of the resolved stellar population in the poststarburst galaxy NGC 1569. The color-magnitude diagram (CMD) derived in the F439W and F555W photometric bands contains ~2800 stars with a photometric error of ≤0.2 mag down to mF439, mF555 26 and is complete for mF555 23. Adopting the literature-distance modulus and reddening, our CMD samples stars more massive than ~4 M☉, allowing us to study the star formation (SF) history over the last ~0.15 Gyr. The data are interpreted using theoretical simulations based on stellar evolutionary models. The synthetic diagrams include photometric errors and incompleteness factors. Testing various sets of tracks, we find that the ability of the models to reproduce the observed features in the CMD is strictly related to the shape of the blue loops of the sequences with masses around 5 M☉. The field of NGC 1569 experienced a global SF burst of 0.1 Gyr duration, ending ~5-10 Myr ago. During the burst, the SF rate was approximately constant, and, if quiescent periods occurred, they lasted less than ~10 Myr. The level of the SF rate was very high; for a single-slope initial mass function (IMF) ranging from 0.1 to 120 M☉, we find values of 3, 1, and 0.5 M☉ yr-1 for α = 3, 2.6, and 2.35 (Salpeter), respectively. When scaled for the surveyed area, these rates are approximately 100 times larger than found in the most active dwarf irregulars in the Local Group. The data are consistent with a Salpeter IMF, though our best models indicate slightly steeper exponents. We discuss the implications of our results in the general context of the evolution of dwarf galaxies.

Supernova rates from the Southern inTermediate Redshift ESO Supernova Search (STRESS)

Aims. To measure the supernova (SN) rates at intermediate redshift we performed the Southern inTermediate Redshift ESO Supernova Search (STRESS). Unlike most of the current high redshift SN searches, this survey was specifically designed to estimate the rate for both type Ia and core collapse (CC) SNe. Methods. We counted the SNe discovered in a selected galaxy sample measuring SN rate per unit blue band luminosity. Our analysis is based on a sample of∼ 43000 galaxies and on 25 spectroscopically confirmed SNe plu s 64 selected SN candidates. Our approach is aimed at obtaining a direct comparison of the high redshift and local rates and at investigating the dependence of the rat es on specific galaxy properties, most notably their colour. Results. The type Ia SN rate, at mean redshift z = 0.3, amounts to 0.22 +0.10+0.16 −0.08−0.14 h 2 70 SNu, while the CC SN rate, at z = 0.21, is 0.82 +0.31+0.30 −0.24−0.26 h 2 70 SNu. The quoted errors are the statistical and systematic un certainties. Conclusions. With respect to local value, the CC SN rate at z = 0.2 is higher by a factor of∼ 2 already at redshift , whereas the type Ia SN rate remains almost constant. This implies that a significant fraction of SN Ia progenitors has a lifetime longer tha n 2− 3 Gyr. We also measured the SN rates in the red and blue galaxies and found that the SN Ia rate seems to be constant in galaxies of different colour, whereas the CC SN rate seems to peak in blue galaxies, as in the local Universe. SN rates per unit volume were found to be consistent with other measurements showing a steeper evolution with redshift for CC SNe with respect to SNe Ia. Finally we have exploited the link between SFH and SN rates to predict the evolutionary behaviour of the SN rates and compare it with the path indicated by observations. We conclude that in order to constrain the mass range of CC SN progenitors and SN Ia progenitor models it is necessary to reduce the uncertainti es in the cosmic SFH. In addition it is important to apply a consistent dust extinction correction both to SF and to CC SN rate and to measure SN Ia rate in star forming and in passive evolving galaxies in a wide redshift range.

DEEP ACS IMAGING OF THE HALO OF NGC 5128: REACHING THE HORIZONTAL BRANCH

Using the Hubble Space Telescope (HST) Wide Field Camera (WFC) of the Advanced Camera for Surveys (ACS), we have obtained deep (V, I) photometry of an outer halo field in NGC 5128, to a limiting magnitude of I 29. Our photometry directly reveals the core helium burning stellar population (the red clump or horizontal branch) in a giant E/S0 galaxy for the first time. The color-magnitude diagram displays a very wide red giant branch (RGB), an asymptotic giant branch (AGB) bump, and the red clump; no noticeable population of blue horizontal branch stars is present, confirming previous suggestions that old, very metal-poor population is not ubiquitous in the halo of this galaxy. From the upper RGB we derive the metallicity distribution, which we find to be very broad and moderately metal-rich, with average [M/H] = -0.64 and dispersion 0.49 dex. The metallicity distribution function is virtually identical to that found in other halo fields observed previously with HST, but with an enhanced metal-rich population that was partially missed in the previous surveys due to V-band incompleteness for these very red stars. Combining the metallicity-sensitive colors of the RGB stars with the metallicity- and age-sensitive features of the AGB bump and the red clump, we infer the average age of the halo stars to be ~8 Gyr. As part of our study, we present an empirical calibration of the ACS F606W and F814W filters to the standard V and I bands, achieved with ground-based observations of the same field made from the EMMI camera of the New Technology Telescope of the ESO La Silla Observatory.

Exploring Cluster Elliptical Galaxies as Cosmological Standard Rods

We explore the possibility of calibrating massive cluster elliptical galaxies as cosmological standard rods using the fundamental plane relation combined with a correction for luminosity evolution. Although cluster ellipticals certainly formed in a complex way, their passive evolution out to redshifts of about 1 indicates that basically all major merging and accretion events took place at higher redshifts. Therefore, a calibration of their luminosity evolution can be attempted. We propose to use the Mg-? relation for that purpose because it is independent of distance and cosmology. We discuss a variety of possible caveats, ranging from dynamical evolution to uncertainties in stellar population models and evolution corrections to the presence of age spread. Sources of major random and systematic errors are analyzed as well. We apply the described procedure to nine elliptical galaxies in two clusters at z = 0.375 and derive constraints on the cosmological model. For the best-fitting ?-free cosmological model we obtain q0 ? 0.1, with 90% confidence limits being 0 < q0 < 0.7 (the lower limit being due to the presence of matter in the universe). If the inflationary scenario applies (i.e., the universe has flat geometry), then, for the best-fitting model, matter and ? contribute about equally to the critical cosmic density (i.e., ?m ? ?? ? 0.5). With 90% confidence, ?? should be smaller than 0.9.

THE STAR FORMATION HISTORY OF I Zw 18

The star formation history in I Zw 18 has been inferred from Hubble Space Telescope Wide Field Planetary Camera 2 archival data. This is done by comparing the derived V, B-V and V, V-I color-magnitude diagrams and luminosity functions with synthetic ones, based on various sets of stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the field of I Zw 18 allow for a look-back time up to 1 Gyr. We find that the main body is not experiencing its first episode of star formation. Instead, it has been forming stars over the last 0.5?1 Gyr, at a rate of ~(1?2) ? 10-2 M? yr-1 kpc-2. A more intense activity of (6?16) ? 10-2 M? yr-1 kpc-2 has taken place between 15 and 20 Myr ago. For the secondary body, the look-back time is 0.2 Gyr at most and the uncertainty is much higher because of the shallower diagrams and the small number of resolved stars. The derived range of star formation rate is (3?10) ? 10-3 M? yr-1 kpc-2. The IMF providing the best fit to the observed stellar populations in the main body has a slope 1.5, much flatter than in any similar galaxy analyzed with the same method. In the secondary body, it is peaked at ? 2.2, closer to Salpeters slope (? = 2.35).

The Star Formation History of IZw18

The star formation history in I Zw 18 has been inferred from Hubble Space Telescope Wide Field Planetary Camera 2 archival data. This is done by comparing the derived V, B-V and V, V-I color-magnitude diagrams and luminosity functions with synthetic ones, based on various sets of stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the field of I Zw 18 allow for a look-back time up to 1 Gyr. We find that the main body is not experiencing its first episode of star formation. Instead, it has been forming stars over the last 0.5?1 Gyr, at a rate of ~(1?2) ? 10-2 M? yr-1 kpc-2. A more intense activity of (6?16) ? 10-2 M? yr-1 kpc-2 has taken place between 15 and 20 Myr ago. For the secondary body, the look-back time is 0.2 Gyr at most and the uncertainty is much higher because of the shallower diagrams and the small number of resolved stars. The derived range of star formation rate is (3?10) ? 10-3 M? yr-1 kpc-2. The IMF providing the best fit to the observed stellar populations in the main body has a slope 1.5, much flatter than in any similar galaxy analyzed with the same method. In the secondary body, it is peaked at ? 2.2, closer to Salpeters slope (? = 2.35).

Integrated spectroscopy of bulge globular clusters and fields. II. Implications for population synthesis models and elliptical galaxies

An empirical calibration is presented for the synthetic Lick indices (e.g. Mg2, ‹Fe›, H s, etc.) of Simple Stellar Population (SSP) models that for the first time extends up to solar metallicity. This is accomplished by means of a sample of Milky Way globular clusters (GCs) whose metallicities range from ~ Zʘ /30 to Z ~ Zʘ, thanks to the inclusion of several metal rich clusters belonging to the Galactic bulge (e.g., NGC 6553 and NGC 6528). This metallicity range approaches the regime that is relevant for the interpretation of the integrated spectra of elliptical galaxies. It is shown that the spectra of both the globular clusters and the Galactic bulge follow the same correlation between magnesium and iron indices that extends to elliptical galaxies, showing weaker iron indices at given magnesium indices with respect to the predictions of models that assume solar-scaled abundances. This similarity provides robust empirical evidence for enhanced [ α/Fe] ratios in the stellar populations of elliptical galaxies, since the globular clusters are independently known to have enhanced [ α/Fe] ratios from spectroscopy of individual stars. We check the uniqueness of this α-overabundance solution by exploring the whole range of model ingredients and parameters, i.e. fitting functions, stellar tracks, and the initial mass function (IMF). We argue that the standard models (meant for solar abundance ratios) succeed in reproducing the Mg-Fe correlation at low metallicities ( [Z/H] ≲ -0.7) because the stellar templates used in the synthesis are Galactic halo stars that actually are