arXiv subjects

Carme Gallart

Publications and source records attributed to Carme Gallart.

82 records · Page 5Linked to original sources

arXiv2005

Spectroscopy of QUEST RR Lyrae Variables: the new Virgo Stellar Stream

Eighteen RR Lyrae variables (RRLs) that lie in the "$12\fh 4$ clump" identified by the QUEST survey have been observed spectroscopically to measure their radial velocities and metal abundances. Ten blue horizontal branch (BHB) stars identified by the Sloan Digital Sky Survey (SDSS) were added to this sample. Six of the 9 stars in the densest region of the clump have a mean radial velocity in the galactic rest frame ($V_{\rm gsr}$) of 99.8 and $σ$ = 17.3 ${\rm km s}^{-1}$, which is slightly smaller than the average error of the measurements. The whole sample contains 8 RRLs and 5 BHB stars that have values of $V_{\rm gsr}$ suggesting membership in this stream. For 7 of these RRLs, the measurements of [Fe/H], which have an internal precision of 0.08 dex, yield $<{\rm [Fe/H]}> = -1.86$ and $σ$ = 0.40. These values suggest that the stream is a tidally disrupted dwarf spheroidal galaxy of low luminosity. Photometry from the database of the SDSS indicates that this stream covers at least 106 deg$^2$ of the sky in the constellation Virgo. The name Virgo Stellar Stream is suggested.

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arXiv2005

SMC: Stellar Populations through deep CMDs

We present deep color-magnitud diagrams (CMDs) reaching the oldest main-sequence turnoffs for 12 fields in the SMC. The {\it B}-band and {\it R}-band observations were performed using the 100-inch Irénée du Pont telescope at Las Campanas Observatory, Chile, during four different campaigns (2001-2004). Our fields cover a wide range of galactocentric distance ranging from $\sim1°$ to $\sim4°$ from the center of the galaxy and are located a different position angles. Photometry was carried out using DAOPHOT II/ALLSTAR/ALLFRAME. Teramo isochrones have been overlapped. All our unprecedented deep ground-based CMDs reach the old MS turnoffs with very good photometric accuracy. They clearly show stellar population gradients as a function of both galactocentric distance and position angle. The most conspicuous difference involves the young population (age$<$1 Gyr): the young MS is much more populated on the eastern fields, located on the SMC wing area, than on the western fields located at similar galactocentric radius. In addition, the main stellar population gets progresively older on average as we go to larger galactocentric radius.

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arXiv2004

VLT Spectroscopy of RR Lyrae Stars in the Sagittarius Tidal Stream

Sixteen RR Lyrae variables from the QUEST survey that lie in the leading arm of the tidal stream from the Sagittarius dSph galaxy have been observed spectroscopically to measure their radial velocities and metal abundances. The systemic velocities of 14 stars, which were determined by fitting a standard velocity curve to the individual measurements, have a sharply peaked distribution with a mean of 33 km/s and a standard deviation of only 25 km/s. The [Fe/H] distribution of these stars has a mean of -1.76 and a standard deviation of 0.22. These measurements are in good agreement with previous ones from smaller samples of stars. The mean metallicity is consistent with the age-metallicity relation that is observed in the main body of the Sgr dSph galaxy. The radial velocities and the distances from the Sun of these stars are compared with recent numerical simulations of the Sgr streams that assume different shapes for the dark matter halo. Models that assume a oblate halo do not fit the data as well as ones that assume a spherical or a prolate distribution. However, none of the fits are completely satisfactory. Every model fails to reproduce the long extent of the stream in right ascension (36 degr) that is seen in the region covered by the QUEST survey. Further modeling is required to see if this and the other mismatches between theory and observation can be removed by judicial choices for the model parameters or instead rule out a class of models.

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arXiv2003

The Chemical Enrichment History of the Fornax Dwarf Spheroidal Galaxy from the Infrared Calcium Triplet

Near infrared spectra were obtained for 117 red giants in the Fornax dwarf spheroidal galaxy with the FORS1 spectrograph on the VLT, in order to study the metallicity distribution of the stars and to lift the age-metallicity degeneracy of the red giant branch (RGB) in the color-magnitude diagram (CMD). Metallicities are derived from the equivalent widths of the infrared Calcium triplet lines at 8498, 8542, and 8662 A, calibrated with data from globular clusters, the open cluster M67 and the LMC. For a substantial portion of the sample, the strength of the Calcium triplet is unexpectedly high, clearly indicating that the main stellar population of Fornax is significantly more metal-rich than could be inferred from the position of its RGB in the CMD. We show that the relative narrowness of the RGB in Fornax is caused by the superposition of stars of very different ages and metallicities. The metallicity distribution in Fornax is centered at [Fe/H]= -0.9, with a metal-poor tail extending to [Fe/H] ~= -2. While the distribution to higher metallicities is less well determined by our observations, the comparison with LMC data indicates that it extends to [Fe/H] ~ -0.4. By comparing the metallicities of the stars with their positions in the CMD, we have derived the complex age-metallicity relation of Fornax. In the first few Gyr, the metal abundance rose to [Fe/H] ~ -1.0 dex. The enrichment accelerated in the past ~ 1-4 Gyr to reach [Fe/H] ~ -0.4 dex. More than half the sample is constituted of star younger than ~ 4 Gyr, thus indicating sustained recent star formation in Fornax. These results indicate that the capacity of dwarf spheroidal galaxies to retain the heavy elements that they produce is larger than expected. (Abridged)

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arXiv2003

Halo Substructure in the QUEST RR Lyrae Survey

A survey of 380 sq. deg. of the sky with the 1m Schmidt telescope at the Observatorio Nacional de Llano del Halo and the QUEST camera has found 498 RR Lyrae variables lying from 4 to 60 kpc from the Sun. We describe the halo substructure revealed by these data and the results of measuring some of the stars' radial velocities and metal abundances.

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arXiv2003

Velocities of RR Lyrae Stars in the Sagittarius Tidal Stream

We have measured radial velocities and metallicities of 16 RR Lyrae stars, from the QUEST survey, in the Sagittarius tidal stream at 50 kpc from the galactic center. The distribution of velocities is quite narrow (std dev=25 km/s) indicating that the structure is coherent also in velocity space. The mean heliocentric velocity in this part of the stream is 32 km/s. The mean metallicity of the RR Lyrae stars is [Fe/H]=-1.7. Both results are consistent with previous studies of red giant stars in this part of the stream. The velocities also agree with a theoretical model of the disruption of the Sagittarius galaxy.

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arXiv2003

The Stellar Content of M31's Bulge

In this paper we analyze the stellar populations present in M31 using nine sets of adjacent HST-NICMOS Camera 1 and 2 fields with galactocentric distances ranging from 2' to 20'. These infrared observations provide some of the highest spatial resolution measurements of M31 to date; our data place tight constraints on the maximum luminosities of stars in the bulge of M31. The tip of the red giant branch is clearly visible at Mbol ~ -3.8, and the tip of the asymptotic giant branch (AGB) extends to Mbol ~ -5. This AGB peak luminosity is significantly fainter than previously claimed; through direct comparisons and simulations we show that previous measurements were affected by image blending. We do observe field-to-field variations in the luminosity functions, but simulations show that these differences can be produced by blending in the higher surface brightness fields. We conclude that the red giant branch of the bulge of M31 is not measurably different from that of the Milky Way's bulge. We also find an unusually high number of bright blueish stars (7.3/arcmin^2) which appear to be Galactic foreground stars.

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arXiv2000

The star formation history of resolved galaxies: Composite H-R diagram

I discuss the information provided by color-magnitude diagrams of different absolute magnitude limits in the study of the star formation histories of nearby galaxies. I review the recent progress made in the field, with emphasis on the results for Local Group dwarf galaxies.

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arXiv1999

The Star Formation History of the Local Group dwarf galaxy Leo I

We present a quantitative analysis of the star formation history (SFH) of the Local Group dSph galaxy Leo I, from the information in its HST [(V-I),I] color-magnitude diagram (CMD). The method we use is based in comparing, via synthetic CMDs, the expected distribution of stars in the CMD for different evolutionary scenarios, with the observed distribution. We consider the SFH to be composed by the SFR(t), the Z(t), the IMF, and a function $β(f,q)$, controlling the fraction $f$ and mass ratio distribution $q$ of binary stars. The comparison between the observed CMD and the model CMDs is done through chi-square minimization of the differences in the number of stars in a set of regions of the CMD. Our solution for the SFH of Leo I defines a minimum of chi-square in a well defined position of the parameter space, and the derived SFR(t) is robust, in the sense that its main characteristics are unchanged for different combinations of the remaining parameters. However, only a narrow range of assumptions for Z(t), IMF and $β(f,q)$ result in a good agreement between the data and the models, namely: Z=0.0004, a Kroupa et al. (1993) IMF or slightly steeper, and a relatively large fraction of binary stars. Most star formation activity (70% to 80%) occurred between 7 and 1 Gyr ago. At 1 Gyr ago, it abruptly dropped to a negligible value, but seems to have been active until at least ~ 300 Myr ago. Our results don't unambiguously answer the question of whether Leo I began forming stars around 15 Gyr ago, but it appears that the amount of this star formation, if existing at all, would be small.

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arXiv1997

Observational discovery of the AGB-bump in densely populated color-magnitude diagrams of galaxies and star clusters

The huge photometric databases that are being created for stars in the Magellanic Clouds and in Local Group galaxies not only allow detailed study of the star formation history of these systems, but also provide important information on stellar evolutionary theory. Two low-level features above the red-clump in the LMC color-magnitude diagram (CMD) have been discussed by Zaritsky & Lin (1997) and Alcock et al. (1997) as the possible signature of an intervening population. I conclude that one of the features, which has also been associated with the red giant branch bump (RGB-bump) predicted by theory and observed in globular clusters, is instead produced during asymtotic giant branch (AGB) evolution. I will call it the AGB-bump. In this paper, it will be shown that the stellar evolution predictions about the position and strength of the AGB-bump are in very good agreement with the observed structures in the LMC, M31 and globular clusters. The position in the CMD of the other feature is consistent with the location of the blue-loops of a few Myr old stars and it has also been observed in the CMD of other galaxies with a young stellar population.

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