Enrico V. Held

GRB 050904 at redshift 6.3: observations of the oldest cosmic explosion after the Big Bang ⋆

We present optical and near-infrared observations of the afterglow of the gamma-ray burst GRB 050904. We derive a photometric redshift z = 6.3, estimated from the presence of the Lyman break falling between the I and J filters. This is by far the most distant GRB known to date. Its isotropic-equivalent energy is 3.4 × 10 53 erg in the rest-frame 110−1100 keV energy band. Despite the high redshift, both the prompt and the afterglow emission are not peculiar with respect to other GRBs. We find a break in the J-band light curve at tb = 2.6 ± 1.0 d (observer frame). If we assume this is the jet break, we derive a beaming-corrected energy Eγ ∼ (4 ÷ 12) × 10 51 erg. This limit shows that GRB 050904 is consistent with the Amati and Ghirlanda relations. This detection is consistent with the expected number of GRBs at z > 6 and shows that GRBs are a powerful tool to study the star formation history up to very high redshift.

Age distribution of young clusters and field stars in the Small Magellanic Cloud

Aims. In this paper we discuss the cluster and field star formation in the central part of the Small Magellanic Cloud. The main goal is to study the correlation between young objects and their interstellar environment. Methods. The ages of about 164 associations and 311 clusters younger than 1 Gyr are determined using isochrone fitting. The spatial distribution of the clusters is compared with the HI maps, with the HI velocity dispersion field, with the location of the CO clouds and with the distribution of young field stars. Results. The cluster age distribution supports the idea that clusters formed in the last 1 Gyr of the SMC history in a roughly continuous way with periods of enhancements. The two super-shells 37A and 304A detected in the HI distribution are clearly visible in the age distribution of the clusters: an enhancement in the cluster formation rate has taken place from the epoch of the shell formation. A tight correlation between young clusters and the HI intensity is found. The degree of correlation is decreasing with the age of the clusters. Clusters older than 300 Myr are located away from the HI peaks. Clusters and associations younger than 10 Myr are related to the CO clouds in the SW region of the SMC disk. A positive correlation between the location of the young clusters and the velocity dispersion field of the atomic gas is derived only for the shell 304A, suggesting that the cloud-cloud collision is probably not the most important mechanism of cluster formation. Evidence of gravitational triggered episode due to the most recent close interaction between SMC and LMC is found both in the cluster and field star distribution.

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.

Deep near-infrared photometry of the globular cluster 47Tucanae. Reconciling theory and observations

Context. The Galactic globular cluster 47 Tucanae is central to studies of Galaxy formation, and a test-bed for theoretical models, distance determination and extragalactic age-dating techniques. Independent parallax-based distance determinations in the optical spectral range provide discrepant results; also, star counts along the Red Giant Branch from optical data have disclosed a worrying disagreement with theoretical predictions, that impacts not only the theory of red giant stars, but also the calibration of the age scale of extragalactic systems. Aims. Our new near-infrared data for 47 Tuc set constraints on its distance and test the reliability of theoretical red giant branch star counts, independently of previous conclusions from optical work. Methods. We have obtained deep near-infrared imaging of 47 Tuc using SOFI at the ESO New Technology Telescope. Colour−magnitude diagrams, isochrones and synthetic horizontal branch modelling have been used to determine the distance of 47 Tuc and constrain its age. We have also constructed a luminosity function of red giant stars, which has been compared with theoretical predictions of stellar evolution models. Results. We obtain a distance (m − M)0 = 13.18 ± 0.03 (random) ± 0.04 (systematic), for [Fe/H] = −0.7 ± 0. 1a ndE(B − V) = 0.04±0.02. This supports the shorter end of the range of distances obtained from optical studies. The mean horizontal branch star mass is between 0.65 and 0.66 M� , and its 1σ Gaussian dispersion is between 0.010 and 0.012 M� . The cluster age can only be approximately estimated from the data, and is between ∼10 and ∼13 Gyr. The luminosity function of red giant branch (and early-asymptotic giant branch) stars does not show a statistically significant discrepancy with theory. The brightness of the red giant branch bump in the near-infrared is possibly fainter than the models, although the uncertainty on the spectroscopic metallicity and age prevents to reach a firm conclusion on this issue.

RR Lyrae and Short-Period Variable Stars in the Dwarf Irregular Galaxy NGC 6822

We report the discovery of a large number of short-period variable stars in the dwarf irregular galaxy NGC 6822, based on deep time-series imaging carried out with the ESO Very Large Telescope. In particular, we found a modest population of RR Lyrae stars tracing the presence of an old stellar component in NGC 6822. Measurements of the average luminosity of RR Lyrae stars provide a new independent estimate of the distance to this galaxy based on a Population II indicator, (m - M)0 = 23.36 ± 0.17. In addition, our new data show a significant population of small-amplitude, short-period variable stars filling the instability strip starting at luminosities only a few tenths of a magnitude brighter than the RR Lyrae stars. Given the presence of an extended star formation in NGC 6822, the faint end of this distribution of short-period variable stars is likely to originate from a population of intermediate-age, metal-poor He-burning stars, younger and more massive than RR Lyrae stars.

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.

Spectroscopy of dwarf elliptical galaxies in the Fornax cluster

We present the results of spectroscopic observations of 10 nucleated dwarf elliptical galaxies (dEs) in the Fornax cluster. The blue spectra of Fornax dE galaxies indicate a wide range of metallicities at a given luminosity, similar to those of intermediate to metal-rich globular clusters. Metal abundances derived in this paper are well correlated with optical colors and agree with previous spectroscopic results. A discrepancy with metallicities inferred from infrared colors is evident; possible causes include an intermediate age population and dilution of spectral features by a blue light excess. Dwarf ellipticals exhibit a wide variation of hydrogen line strength which points to a complex star formation history. Prominent Balmer absorption lines are the signature of a young stellar population in the nuclei of some (but not all) dEs, while moderately strong Balmer lines in relatively metal-rich dEs are more consistent with an extended main sequence. In a few metal-poor dE galaxies, the hydrogen lines are consisent with, or perhaps weaker than, those found in Galactic globulars of similar metallicity. In the limited magnitude range of this sample, there is no apparent correlation of metallicity either with effective and central surface brightness, or with total and nuclear magnitudes. The velocity distribution of the Fornax dwarfs is flatter than that of brighter galaxies at the 75% confidence level, possibly indicating a difference in the kinematics of the two samples.

Variable Stars in the Fornax dSph Galaxy. II. Pulsating Stars below the Horizontal Branch

We have carried out an intensive survey of the northern region of the Fornax dwarf spheroidal galaxy with the aim of detecting the galaxys short-period pulsating stars (P < 0.25 days). Observations collected over three consecutive nights with the Wide Field Imager of the 2.2 m MPI telescope at ESO allowed us to detect 85 high-amplitude (0.20-1.00 mag in B light) variable stars with periods in the range from 0.046 to 0.126 days, similar to SX Phoenicis stars in Galactic metal-poor stellar populations. The plots of the observed periods vs. the B and V magnitudes show a dispersion largely exceeding the observational errors. To disentangle the matter, we separated the first-overtone from the fundamental-mode pulsators and tentatively identified a group of subluminous variables, about 0.35 mag fainter than the others. Their nature as either metal-poor intermediate-age stars or stars formed by the merging of close binary systems is discussed. The rich sample of the Fornax variables also led us to reconstruct the period-luminosity relation for short-period pulsating stars. An excellent linear fit, MV = − 1.83(± 0.08) − 3.65(± 0.07) log PF, was obtained using 153 δ Scuti and SX Phoenicis stars in a number of different stellar systems.

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.