Ivo Saviane

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.

HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands ⋆

We present the complete photometric database and the color-magnitude diagrams for 74 Galactic globular clusters observed with the HST/WFPC2 camera in the F439W and F555W bands. A detailed discussion of the various reduction steps is also presented, and of the procedures to transform instrumental magnitudes into both the HST F439W and F555W flight system and the standard Johnson B and V systems. We also describe the artificial star experiments which have been performed to derive the star count completeness in all the relevant branches of the color magnitude diagram. The entire photometric database and the completeness function will be made available on the Web immediately after the publication of the present paper.

Homogeneous Metallicities and Radial Velocities for Galactic Globular Clusters ⋆ First CaT metallicities for twenty clusters

Well determined radial velocities and abundances are essential for analyzing the properties of the globular cluster system of the Milky Way. However more than 50% of these clusters have no spectroscopic measure of their metallicity. In this context, this work provides new radial velocities and abundances for twenty Milky Way globular clusters which lack or have poorly known values for these quantities. The radial velocities and abundances are derived from spectra obtained at the Ca ii triplet using the FORS2 imager and spectrograph at the VLT, calibrated with spectra of red giants in a number of clusters with well determined abundances. For about half of the clusters in our sample we present significant revisions of the existing velocities or abundances, or both. We also confirm the existence of a sizable abundance spread in the globular cluster M54, which lies at the center of the Sagittarius dwarf galaxy. In addition evidence is provided for the existence of a small intrinsic internal abundance spread (σ[Fe/H]int ≈ 0.11−0.14 dex, similar to that of M54) in the luminous distant globular cluster NGC5824. This cluster thus joins the small number of Galactic globular clusters known to possess internal metallicity ([Fe/H]) spreads.

The Lower Main Sequence and Mass Function of the Globular Cluster Messier 4

The deepest optical image ever in a globular star cluster, a Hubble Space Telescope 123 orbit exposure in a single field of Messier 4, was obtained in two filters (F606W, F814W) over a 10 week period in early 2001. A somewhat shallower image obtained in 1995 allowed us to select out cluster and field objects via their proper-motion displacement, resulting in remarkably clean color-magnitude diagrams that reach to V = 30, I = 28. The cluster main-sequence luminosity function contains very few stars fainter than MV = 15.0, MI = 11.8, which, in both filters, is more than 2 mag brighter than our limit. This is about the faintest luminosity seen among field Population II subdwarfs of the same metallicity. However, there remains a sprinkling of potential cluster stars to lower luminosity all the way down to our limiting magnitudes. These latter objects are significantly redder than any known metal-poor field subdwarf. Comparison with the current generation of theoretical stellar models implies that the masses of the lowest luminosity cluster stars observed are near 0.09 M☉. We derive the mass function of the cluster in our field and find that it is very slowly rising toward the lowest masses with no convincing evidence of a turnover even below 0.1 M☉. The formal slope between 0.65 and 0.09 M☉ is α = 0.75 (Salpeter of 2.35) with a 99% confidence interval of 0.55-1.05. A consistency check between these slopes and the number of observed cluster white dwarfs yields a range of possible conclusions, one of which is that we have indeed seen the termination of the white dwarf cooling sequence in M4.

The distance to the Fornax dwarf spheroidal galaxy

ABSTRACT A large multicolour, wide-field photometric database of the Fornax dwarf spheroidalgalaxy has been analysed using three different methods to provide revised distanceestimates based on stellarpopulations in different ageintervals. The distance to Fornaxwas obtained from the Tip of the Red Giant Branch measured by a new method, andusing the luminosity of Horizontal Branch stars and Red Clump stars corrected forstellar population effects. Assuming a reddening E(B − V) = 0.02, the followingdistance moduli were derived: (m− M) 0 = 20.71± 0.07 based on the Tip of the RedGiant Branch, (m − M) 0 = 20.72 ± 0.06 from the level of the Horizontal Branch,and (m − M) 0 = 20.73 ± 0.09 using the Red Clump method. The weighted meandistance modulus to Fornax is (m − M) 0 = 20.72 ± 0.04. All these measurementsagree within the errors, and are fully consistent with previous determinations andwith the distance measurements obtained in a companion paper from near-infraredcolour-magnitude diagrams.Key words: Galaxies: fundamental parameters – Galaxies: distances – Galaxies: in-dividual: Fornax dwarf spheroidal

The blue plume population in dwarf spheroidal galaxies - Genuine blue stragglers or young stellar population?

Aims. Blue stragglers (BSS) in the Milky Way field and globular/open clusters are thought to be the product of either primordial or collisional binary systems. In the context of dwarf spheroidal galaxies it is hard to firmly disentangle a genuine BSS population from young main sequence (MS) stars tracing a ∼1-2 Gyr old star forming episode. Methods. Assuming that their blue plume populations are made of BSS, we estimate the BSS frequency (F BSS HB ; as normalized to the horizontal branch star counts) for 8 Local Group non star-forming dwarf galaxies, using a compilation of ground and space based photometry. Results. (i) The BSS frequency in dwarf galaxies, at any given M v , is always higher than that in globular clusters of similar luminosities; (ii) the BSS frequency for the lowest luminosity dwarf galaxies is in excellent agreement with that observed in the Milky Way halo and open clusters; and most interestingly (iii) derive a statistically significant F BSS HB - M v anti-correlation for dwarf galaxies, similar to that observed in globular clusters. Conclusions. The low density, almost collision-less, environments of our dwarf galaxy sample allow us to infer (i) their very low dynamical evolution; (ii) a negligible production of collisional BSS; and consequently (iii) that their blue plumes are mainly made of primordial binaries. The dwarf galaxies F BSS HB -M v anti-correlation can be used as a discriminator: galaxies obeying the anti-correlation are more likely to possess genuine primordial BSS rather than young main sequence stars.

Groups of Galaxies in the Nearby Universe

For every galaxy in the field or in clusters, there are about three galaxies in groups. Therefore, the evolution of most galaxies actually happens in groups. The Milky Way resides in a group, and groups can be found at high redshift. The current generation of 10-m-class telescopes and space facilities allows us to study members of nearby groups with exquisite detail, and their properties can be correlated with the global properties of their host group. Finally, groups are relevant for cosmology, since they trace largescale structures better than clusters, and the evolution of groups and clusters may be related.

A New Red Giant-based Distance Modulus of 13.3 Mpc to the Antennae Galaxies and Its Consequences*

The Antennae galaxies are the closest example of an ongoing major galaxy merger and, as such, represent a unique laboratory for furthering the understanding of the formation of exotic objects (e.g., tidal dwarf galaxies, ultraluminous X-ray sources, super stellar clusters). In a previous paper HST WFPC2 observations were used to demonstrate that the Antennae system might be at a distance considerably less than that conventionally assumed in the literature. Here we report new, much deeper HST ACS imaging that resolves the composite stellar populations and, most importantly, reveals a well-defined red giant branch. The tip of this red giant branch (TRGB) is unambiguously detected at -->I0TRGB = 26.65 ? 0.09 mag. Adopting the most recent calibration of the luminosity of the TRGB then yields a distance modulus for the Antennae of -->(m ? M)0 = 30.62 ? 0.17 corresponding to a distance of -->13.3 ? 1.0 Mpc. This is consistent with our earlier result, once the different calibrations for the standard candle are considered. We briefly discuss the implications of this now well-determined shorter distance.

Clues to the Evolution of the Carina Dwarf Spheroidal Galaxy from the Color Distribution of its Red Giant Stars

The thin red giant branch (RGB) of the Carina dwarf spheroidal galaxy appears at first sight quite puzzling and seemingly in contrast with the presence of several distinct bursts of star formation. In this Letter, we provide a measurement of the color spread of red giant stars in Carina based on new BVI wide-field observations, and model the width of the RGB by means of synthetic color-magnitude diagrams. The measured color spread, Sigma{V-I}=0.021 +/- 0.005, is quite naturally accounted for by the star-formation history of the galaxy. The thin RGB appears to be essentially related to the limited age range of its dominant stellar populations, with no need for a metallicity dispersion at a given age. This result is relatively robust with respect to changes in the assumed age-metallicity relation, as long as the mean metallicity over the galaxy lifetime matches the observed value ([Fe/H] = -1.91 +/- 0.12 after correction for the age effects). This analysis of photometric data also sets some constraints on the chemical evolution of Carina by indicating that the chemical abundance of the interstellar medium in Carina remained low throughout each episode of star formation even though these episodes occurred over many Gyr.

Discovery of new Milky Way star cluster candidates in the 2MASS Point Source Catalog. II. Physical properties of the star cluster CC 01

Three new obscured Milky Way clusters were detected as surface density peaks in the 2MASS point source catalog during our on-going search for hidden globular clusters and massive Arches-like star clusters. One more cluster was discovered serendipitously during a visual inspection of the candidates. The first deep J, H ,a ndKs imaging of the cluster (IBP 2002) CC 01 is presented. We estimated a cluster age of ∼1-3 Myr, distance modulus of (m − M)0 = 12.56 ± 0.08 mag (D = 3.5 kpc), and extinction of AV ∼ 7.7 mag. We also derived the initial mass function slope for the cluster: Γ= −2.23 ± 0.16 (ΓSalpeter = −2.35). The integration over the initial mass function yielded a total cluster mass Mtotal ≤ 1800 ± 200 M� . CC 01 appears to be a regular, not very massive star cluster, whose formation has probably been induced by the shock front from the nearby H region Sh 2-228.