The surface brightness profile of the remote cluster NGC 2419

It is well known that the bright and remote Galactic globular cluster NGC2419 has a very peculiar structure. In particular its half-light radius is significantly larger than that of ordinary globular clusters of similar luminosity, being as large as that of the brightest nuclei of dwarf elliptical galaxies. In this context it is particularly worth to check the reliability of the existing surface brightness profiles for this cluster and of the available estimates of its structural parameters. Combining different datasets I derive the surface brightness profile going from the cluster center out to ~ 480 arcsec, i.e. ~25 core radii. (Abridged). The newly obtained surface brightness profile is in excellent agreement with that provided by Trager, King & Djorgovski for r>= 4 arcsec; it is best fitted by a King model having r_c=0.32 arcmin, mu_V(0)=19.55 and C=1.35. Also new independent estimates of the total integrated V magnitude (V_t=10.47 +/- 0.07) and of the half-light radius (r_h=0.96 arcmin +/- 0.2 arcmin) have been obtained. (Abridged). The structure of NGC2419 is now reliably constrained by (at least) two fully independent observational profiles that are in good agreement one with the other. Also the overall agreement between structural parameters independently obtained by different authors is quite satisfying.Comment: Research Note, accepted for publication by A&A. 6 pages with 4 figures + 3 pages of Online Material (table

The Sagittarius Dwarf spheroidal Galaxy Survey (SDGS) II: The stellar content and constraints on the star formation history

A detailed study of the Star Formation History of the Sgr dSph galaxy is performed through the analysis of the data from the Sagittarius Dwarf Galaxy Survey (SDGS; Bellazzini, Ferraro & Buonanno 1999). Accurate statistical decontamination of the SDGS Color - Magnitude diagrams allow us to obtain many useful constraints on the age and metal content of the Sgr stellar populations in three different region of the galaxy. A coarse metallicity distribution of Sgr stars is derived, ranging from [Fe/H]~ -2.0 to [Fe/H]~ -0.7, the upper limit being somewhat higher in the central region of the galaxy. A qualitative global fit to all the observed CMD features is attempted, and a general scheme for the Star Formation History of the Sgr is derived. According to this scheme, star formation began at very early time from a low metal content Inter Stellar Medium and lasted for several Gyr, coupled with progressive chemical enrichment. The Star Formation Rate (SFR) had a peak from 8 to 10 gyr ago when the mean metallicity was in the range -1.3<= [Fe/H] <= -0.7. After that maximum, the SFR rapidly decreased and very low rate star formation took place until ~1-0.5 Gyr ago.Comment: 21 pages, 12 figures, figg. 1,2,3,5,6,10 and 11 provided in lower resolution format. For full resolution versions see http://www.bo.astro.it/bap/BAPhome.html Accepted by MNRA

Chemical enrichment in very low-metallicity environments: Bootes I

We present different chemical evolution models for the ultrafaint dwarf galaxy Bootes I. We either assume that the galaxy accretes its mass through smooth infall of gas of primordial chemical composition (classical models) or adopt mass accretion histories derived from the combination of merger trees with semi-analytical modelling (cosmologically-motivated models). Furthermore, we consider models with and without taking into account inhomogeneous mixing in the ISM within the galaxy. The theoretical predictions are then compared to each other and to the body of the available data. From this analysis, we confirm previous findings that Bootes I has formed stars with very low efficiency but, at variance with previous studies, we do not find a clear-cut indication that supernova explosions have sustained long-lasting galactic-scale outflows in this galaxy. Therefore, we suggest that external mechanisms such as ram pressure stripping and tidal stripping are needed to explain the absence of neutral gas in Bootes I today.Comment: 13 pages, 11 figures, accepted for publication in MNRA