Lorenzo Monaco

The Ital-FLAMES survey of the Sagittarius dwarf spheroidal galaxy - I. Chemical abundances of bright RGB stars

We present iron and α element (Mg, Ca, Ti) abundances for a sample of 15 Red Giant Branch stars belonging to the main body of the Sagittarius dwarf Spheroidal galaxy. Abundances have been obtained from spectra collected using the high resolution spectrograph FLAMES-UVES mounted at the VLT. Stars of our sample have a mean metallicity of [Fe/H] = -0.41 ± 0.20 with a metal-poor tail extending to [Fe/H] = -1.52. The α element abundance ratios are slightly subsolar for metallicities higher than [Fe/H]? -1, suggesting a slow star formation rate. The [α/Fe] of stars having [Fe/H] < -1 are compatible to what observed in Milky Way stars of comparable metallicity.

The Draco and Ursa Minor Dwarf Spheroidal Galaxies: A Comparative Study*

We present (V,I) photometry of two wide (~25 X 25 arcmin^2) fields centered on the low surface brightness dwarf spheroidal galaxies Draco and Ursa Minor. New estimates of the distance to these galaxies are provided ((m-M)_0(UMi)=19.41 pm 0.12 and (m-M)_0(Dra)=19.84 pm 0.14) and a comparative study of their evolved stellar population is presented. We detect for the first time the RGB-bump in the Luminosity Function of UMi (V_{RGB}^{Bump}=19.40pm 0.06) while the feature is not detected in Draco. Photometric metallicity distributions are obtained for the two galaxies and an accurate analysis to determine the intrinsic metallicity spread is performed by means of artificial stars experiments. (Abridged) We demonstrate that the inner region of UMi is significantly structured, at odds with what expected for a system in dynamical equilibrium. In particular we show that the main density peak of UMi is off-centered with respect to the center of symmetry of the whole galaxy and it shows a much lower ellipticity with respect to the rest of the galaxy. Moreover, UMi stars are shown to be clustered according to two different characteristic clustering scales, as opposite to Draco, which instead has a very symmetric and smooth density profile. The possible consequences of this striking structural difference on our ideas about galaxy formation are briefly discussed. Combining our distance modulus with the more recent estimates of the total luminosity of UMi, we find that the mass to light (M/L) ratio of this galaxy may be as low as M/L ~ 7, a factor 5-10 lower than current estimates.

Deep Hubble Space Telescope WFPC2 Photometry of NGC 288. I. Binary Systems and Blue Stragglers

We present the first results of a deep WFPC2 photometric survey of the loose galactic globular cluster NGC 288. The fraction of binary systems is estimated from the color distribution of objects near the main sequence (MS) with a method analogous to that introduced by Rubenstein & Bailyn. We have unequivocally detected a significant population of binary systems with a radial distribution that has been significantly influenced by mass segregation. In the inner region of the cluster (r < 1rh 1.6rc) the binary fraction (fb) lies in the range 0.08–0.38 regardless of the assumed distribution of mass ratios, F(q). The most probable fb lies between 0.10 and 0.20 depending on the adopted F(q). On the other hand, in the outer region (r ≥ 1rh), fb must be less than 0.10, and the most likely value is 0.0, independently of the adopted F(q). The detected population of binaries is dominated by primordial systems. The specific frequency of blue stragglers (BSs) is exceptionally high, suggesting that the BS production mechanism via binary evolution can be very efficient. A large population of BSs is possible even in low-density environments if a sufficient reservoir of primordial binaries is available. The observed distribution of BSs in the color-magnitude diagram is not compatible with a rate of BS production that has been constant in time, if it is assumed that all the BSs are formed by the merging of two stars.

The Draco and Ursa Minor dwarf spheroidals. A comparative study

We present (V,I) photometry of two wide (~25 X 25 arcmin^2) fields centered on the low surface brightness dwarf spheroidal galaxies Draco and Ursa Minor. New estimates of the distance to these galaxies are provided ((m-M)_0(UMi)=19.41 pm 0.12 and (m-M)_0(Dra)=19.84 pm 0.14) and a comparative study of their evolved stellar population is presented. We detect for the first time the RGB-bump in the Luminosity Function of UMi (V_{RGB}^{Bump}=19.40pm 0.06) while the feature is not detected in Draco. Photometric metallicity distributions are obtained for the two galaxies and an accurate analysis to determine the intrinsic metallicity spread is performed by means of artificial stars experiments. (Abridged) We demonstrate that the inner region of UMi is significantly structured, at odds with what expected for a system in dynamical equilibrium. In particular we show that the main density peak of UMi is off-centered with respect to the center of symmetry of the whole galaxy and it shows a much lower ellipticity with respect to the rest of the galaxy. Moreover, UMi stars are shown to be clustered according to two different characteristic clustering scales, as opposite to Draco, which instead has a very symmetric and smooth density profile. The possible consequences of this striking structural difference on our ideas about galaxy formation are briefly discussed. Combining our distance modulus with the more recent estimates of the total luminosity of UMi, we find that the mass to light (M/L) ratio of this galaxy may be as low as M/L ~ 7, a factor 5-10 lower than current estimates.

The central density cusp of the Sagittarius dwarf spheroidal galaxy

We present an analysis of the density profile in the central region of the Sagittarius dwarf spheroidal galaxy. A strong density enhancement of Sgr stars is observed. The position of the peak of the detected cusp is indistinguishable from the centre of M54. The photometric properties of the cusp are fully compatible with those observed in the nuclei of dwarf elliptical galaxies, indicating that the Sgr dSph would appear as a nucleated galaxy independently of the presence of M54 at its centre.

The distance to the Sagittarius dwarf spheroidal galaxy from the red giant branch tip: The distance to the Sgr dwarf spheroidal galaxy

We derived the distance to the central region of the Sagittarius (Sgr) dwarf spheroidal galaxy from the red giant branch tip. The obtained distance modulus is (m - M) 0 = 17.10 ± 0.15, corresponding to a heliocentric distance D = 26.30 ± 1.8 Kpc. This estimate is in good agreement with the distance obtained from RR Lyrae (RR Ly) stars of the globular cluster M 54, located in the core of the Sgr galaxy, once the most accurate estimate of the cluster metallicity and the most recent calibration of the M V (RR Ly) versus [Fe/H] relation are adopted.

First Detection of the Red Giant Branch Bump in the Sagittarius Dwarf Spheroidal Galaxy

We present V, I photometry of the Sagittarius Dwarf Spheroidal galaxy (Sgr) for a region of ∼ 1 • × 1 • , centered on the globular cluster M 54. This catalog is the largest database of stars (∼500,000) ever obtained for this galaxy. The wide area covered allows us to measure for the first time the position of the RGB-bump, a feature that has been identified in most Galactic globular clusters and 1 Based on observations made with the European Southern Observatory telescopes, using the Wide Field Imager, as part of the observing program 65.L-0463. Also based on data obtained from the ESO/ST-ECF Science Archive Facility. 2 This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation – 2 – only recently in a few galaxies of the Local Group. The presence of a single-peaked bump in the RGB differential Luminosity Function confirms that there is a dominant population in Sgr (Pop A). The photometric properties of the Pop A RGB and the position of the RGB bump have been used to constrain the range of possible ages and metallicities of this population. The most likely solution lies in the range −0.6 < [M/H] ≤ −0.4 and 4 Gyr ≤ age ≤ 8 Gyr.We present V, I photometry of the Sagittarius (Sgr) dwarf spheroidal galaxy for a region of ~1? ? 1?, centered on the globular cluster M54. This catalog is the largest database of stars (~500,000) ever obtained for this galaxy. The wide area covered allows us to measure for the first time the position of the red giant branch (RGB) bump, a feature that has been identified in most Galactic globular clusters and only recently in a few galaxies of the Local Group. The presence of a single-peaked bump in the RGB differential luminosity function confirms that there is a dominant population in Sgr (Pop A). The photometric properties of the Pop A RGB and the position of the RGB bump have been used to constrain the range of possible ages and metallicities of this population. The most likely solution lies in the range -0.6 < [M/H] ? -0.4 and 4 Gyr ? age ? 8 Gyr.

Blue Horizontal-Branch Stars in the Sagittarius Dwarf Spheroidal Galaxy

We report on the discovery of a blue horizontal-branch (BHB) population belonging to the Sagittarius dwarf spheroidal galaxy. The sequence is clearly identified in the (V, V-I) color-magnitude diagram obtained for about 500,000 stars in the region of the globular cluster M54. The BHB morphology is similar to the analogous sequence in M54, but it is unambiguously associated with Sgr since (1) it is detected well outside the main body of the cluster, up to more than five tidal radii from the cluster center and (2) the BHB stars follow the radial distribution of the other stellar populations of Sgr. This finding finally demonstrates that the Sgr galaxy hosts a significant (of the order of ~10%) old and metal-poor stellar population ([Fe/H] -1.3; age 10 Gyr), similar to that of its oldest clusters (M54, Ter 8). We also show that the Sgr BHB sequence found here is the counterpart of the analogous feature observed by Newberg et al. in the Sgr stream, in a field more than 80° away from the center of the galaxy.

Deep Hubble Space Telescope WFPC2 Photometry of NGC 288. II. The Main-Sequence Luminosity Function*

The main-sequence luminosity function (LF) of the Galactic globular cluster NGC 288 has been obtained using deep Wide Field Planetary Camera 2 photometry. We have employed a new method to correct for completeness and fully account for bin-to-bin migration due to blending and/or observational scatter. The effect of the presence of binary systems in the final LF is quantified and is found to be negligible. There is a strong indication of the mass segregation of unevolved single stars and clear signs of a depletion of low-mass stars in NGC 288 with respect to other clusters. The results are in good agreement with the prediction of theoretical models of the dynamical evolution of NGC 288 that take into account the extreme orbital properties of this cluster.

Chemical abundances of RGB-Tip stars in the Sagittarius dwarf spheroidal galaxy

We present preliminary iron abundances and