Lithium-rich giants in the Sagittarius dSph Tidal Streams
L. Monaco, P. Bonifacio
备注：
5 pages, 4 figures, 2 tables. Proceedings of the workshop "XXI Century challenges for stellar evolution", held in Cefalu' (Sicily, Italy), August 29 - September 2, 2007; S. Cassisi and M. Salaris Eds. To be published in Mem. SAIt Vol. 79 No. 2
The nature and origin of Lithium rich giant stars is still matter of debate. In the contest of our spectroscopic survey of giants in the tidal streams of the Sagittarius dwarf spheroidal (dSph) galaxy, we present the serendipitous discovery of 2 super Li-rich stars (A(Li)$>$3.5-4.0). Besides D461 in Draco, these are the only Li-rich stars known in a Local Group dSph galaxy. The high Li abundance and the low mass of these stars support their origin as due to fresh Li production in the stars associated with some kind of extra-mixing process.
The Sagittarius dwarf spheroidal (Sgr) and a few paculiar Galactic Globular Clusters (GGC) can be considered as building blocks of theGalactic Halo. We present a series of results based on a wide field photometry of the Sgr system and the GGC M22. In particular for Sgr we present: (1) the detection of the RGB-bump, a feature that together with the shape of the RGB can be used to constrain the allowed range of age and metallicity and (2) the detection of a clear blue horizontal branch which probes the existance of a metal poor population. In the case of M22 we present clear evidence that differ…
ROSAT observations found no convincing evidence for X-ray sources located in local dwarf spheroidal galaxies (dSph). Now with more sensitive instruments on board Chandra and XMM-Newton we can reach fainter luminosity levels. We report on an observation of the Sagittarius (Sgr) dSph made using Chandra and an observation of the Carina (Car) dSph made using XMM-Newton. Our observations are sensitive to sources with X-ray luminosities in the 0.1-10 keV band of \~1x10^{32 ergs/s and 3x10^34 ergs/s for the Sgr and Car fields respectively. We have identified a total of 80 sources in the Sgr field and…
We present V, I photometry of the Sagittarius Dwarf Spheroidal galaxy (Sgr) for a region of ~ 1^{circ} times 1^{circ}, 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 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 (Po…
The main characteristics of a wide photometric survey of the Sgr dwarf spheroidal galaxy are shortly presented. V and I photometry has been obtained for ~ 90000 stars toward Sgr and for ~ 9000 stars in a region devoid of Sgr stars (for decontamination purposes). The full potential of this large database is far from being completely explored. Here we present only preliminary results from the analysis of statistically decontaminated Color Magnitude Diagrams, trying to set a scheme of the Star Formation History of the Sgr Galaxy. A scenario is proposed in which star formation in Sgr began very ea…
We outline the results of a study of the chemical composition of 14 stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph). For the Sgr dSph stars with [Fe/H]>-1 the abundances are highly unusual, showing a striking enhancement in heavy s-process elements, increasing with [Fe/H], deficiencies of the alpha- elements (O, Si, Ca, and Ti), deficiencies of Al and Na, and deficiencies of the iron-peak elements Mn and Cu. Our abundances suggest that the composition of the metal-rich Sgr dSph stars is dominated by the ejecta of an old, metal-poor population, including products of AGB stars and ty…
In this work we present the first spectroscopic results obtained with the Southern African Large Telescope (SALT) telescope during its perfomance-verification phase. We find that the Sagittarius dwarf spheroidal galaxy (Sgr) Sgr contains a youngest stellar population with [O/H] -0.2 and age t>1 Gyr, and an oldest population with [O/H]=-2.0. The values are based on spectra of two planetary nebulae (PNe), using empirical abundance determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He. We confirm the high abundances of PN StWr2-21 with 12+log(O/H) = 8.57+/-0.02 dex. The o…
We present the first (V,V-I) Color -- Magnitude Diagram (CMD) for the poorly studied globular cluster NGC 5634. The CMD shows a steep Red Giant Branch (RGB) and a predominantly blue Horizontal Branch (HB): both these characteristics suggest a low metal content for this cluster. From the position of the RGB in the CMD we estimate [Fe/H] = -1.94 +/- 0.10 and E(B-V)=0.06 +/- 0.01. The CMD presented here reaches V\simeq 23, allowing us to obtain the first measure of the Main Sequence Turn-Off (TO) V_{TO}= 21.22 +/- 0.15 for this cluster. By combining this figure with the apparent luminosity of the…
We present first results of a large photometric survey devoted to study the star formation history of the Sagittarius dwarf spheroidal galaxy (Sgr dSph). Three wide strips (9 X 35 arcmin^2) located at l~5 and b=-16, -14, -12 have been observed. Each strip is roughly EW oriented, nearly along the major axis of the galaxy. A control field (~9 X 24 arcmin^2), located outside the body of Sgr dSph has also been observed for statistic decontamination purposes. Accurate and well calibrated V,I photometry down to V~22 have been obtained for \~90000 stars toward the Sgr dSph, and ~8000 stars in the con…
We demonstrate that there is a clear statistical correlation between the (X,Y,Z,V_r) phase-space distribution of the outer halo Galactic globular clusters (having 10 kpc <= R_GC <= 40 kpc) and the orbital path of the Sagittarius dwarf spheroidal galaxy (Sgr dSph), as derived by Ibata & Lewis. At least 4 of the sample of 35 globular clusters in this distance range were formerly members of the Sgr galaxy (at the 95 % confidence level), and are now distributed along the Sgr Stream, a giant tidal structure that surrounds the Milky Way. This is the first instance that a statistically significant st…
To study the effect of metallicity on the mass-loss rate of asymptotic giant branch (AGB) stars, we have conducted mid-infrared photometric measurements of such stars in the Sagittarius (Sgr dSph) and Fornax dwarf spheroidal galaxies with the 10-$μ$m camera VISIR at the VLT. We derive mass-loss rates for 29 AGB stars in Sgr dSph and 2 in Fornax. The dust mass-loss rates are estimated from the $K-[9]$ and $K-[11]$ colours. Radiative transfer models are used to check the consistency of the method. Published IRAS and Spitzer data confirm that the same tight correlation between $K-[12]$ colour and…
We report on the chemical abundances derived from high-dispersion spectra of 14 red giant stars in the Sagittarius dwarf spheroidal (Sgr dSph) galaxy. The stars span a wide range of metallicities, -1.6 < [Fe/H] < -0.1 dex, and exhibit very unusual abundance variations. For metal-poor stars with [Fe/H] < -1, [alpha/Fe] \approx +0.3 similar to Galactic halo stars, but for more metal-rich stars the relationship of [alpha/Fe] as a function of [Fe/H] is lower than that of the Galactic disk by 0.1 dex. The light elements [Al/Fe] and [Na/Fe] are sub-solar by an even larger amount, 0.4 dex. The patter…
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 center 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 center.
We study the chemical composition of 5 giant stars in the globular cluster Terzan 7 associated with the Sagittarius dwarf spheroidal galaxy (Sgr dSph), based on high resolution UVES-VLT spectra. We confirm the metallicity found by previous high resolution investigations: [Fe/H] ~ -0.6. We also show that this cluster displays the same low alpha-element to iron ratio found in Sgr dSph field stars of similar metallicity, as well as the same low Ni/Fe ratio. These chemical signatures are characteristic of the Sgr dSph system, and appear to be shared both by the globular cluster Pal 12, which was m…
We present reliable measurements of the metallicity distribution function (MDF) at different points along the tidal stream of the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy, based on high resolution, echelle spectroscopy of candidate M giant members of the Sgr system. The Sgr MDF is found to evolve significantly from a median [Fe/H] ~-0.4 in the core to ~-1.1 dex over a Sgr leading arm length representing ~2.5-3.0 Gyr of dynamical (i.e. tidal stripping) age. This is direct evidence that there can be significant chemical differences between current dSph satellites and the bulk of the star…
We present abundances for 12 giants in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) obtained from VLT - UVES spectra. Moving on a short period, polar orbit around the Milky Way, the Sgr dSph is undergoing tidal disruption and will eventually dissolve in the Halo.Our sample is dominated by a metal rich, alpha-element-poor population, indicative of long chemical processing of the dSph gas during a slow, probably bursting star formation history. This population is the most metal-rich ever observed in a Dwarf Galaxy of the Local Group, and has the lowest alpha-element content. Placing the kn…
We present the results of a thorough study of the nucleus of the Sgr dwarf spheroidal galaxy and of the bright globular cluster M54 that resides within the same nucleus (Sgr,N). We have obtained accurate radial velocities and metallicity estimates for 1152 candidate Red Giant Branch stars of Sgr and M54 lying within ~ 9 arcmin from the center of the galaxy, from Keck/DEIMOS and VLT/FLAMES spectra of the infrared Calcium II triplet. Using both velocity and metallicity information we selected two samples of 425 and 321 very-likely members of M54 and of Sgr,N, respectively. The two considered sys…
In order to verify the effects of the most recent data on the evolution of Carina and Sagittarius Dwarf Spheroidal Galaxies (dSph) and to set tight constraints on the main parameters of chemical evolution models, we study in detail the chemical evolution of these galaxies through comparisons between the new data and the predictions of a model, already tested to reproduce the main observational constraints in dSphs. Several abundance ratios, such as [$α$/Fe], [Ba/Fe] and [Eu/Fe], and the metallicity distribution of stars are compared to the predictions of our models adopting the observationally…
We present a statistically decontaminated Color Magnitude Diagram of a 1 deg X 1 deg field in the core of the Sagittarius dSph galaxy. Coupling this CMD with the most recent metallicity distributions obtained from high resolution spectroscopy we derive robust constraints on the mean age of the stellar population that dominates the galaxy (Pop A). Using three different sets of theoretical isochrones in the metallicity range -0.4<= [M/H]<= -0.7 and taking into consideration distance moduli in the range 16.90<= (m-M)_0<= 17.20 we find that the mean age of Pop A is larger than 5 Gyr, and the best-…
We present detailed Chemical abundances for 12 stars in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) and 3 in the Canis Mayor Overdensity (CMa), based on UVES spectra. Sgr dSph stars present high metallicity ([Fe/H] between -1 and 0) low [alpha/Fe] ratios, overabundant La, Ce and Nd, underabundant Na, Al,Ni and Zn. Zn underabundance in particular is interesting, since it may undermine the parallel drawn between dSph and DLAs, where Zn is used as a proxy for Fe. Of the three CMa stars, one is likely a Milky Way (MW) interloper, the other two present solar metallicity, underabundant alpha …
Dwarf Spheroidal galaxies are amongst the best targets to search for a Dark Matter annihilation signal. The annihilation of WIMPs in the center of Sagittarius dwarf spheroidal (Sgr dSph) galaxy would produce high energy gamma-rays in the final state. Observations carried out with the H.E.S.S. array of Imaging Atmospheric Cherenkov telescopes are presented. A careful modelling of the Dark Matter halo profile of Sgr dwarf was performed using latest measurements on its structural parameters. Constraints on the velocity-weighted cross section of Dark Matter particles are derived in the framework o…
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 …
We present preliminary results of a long-term project to investigate the process of accretion and tidal disruption of dSph satellites in the Galactic halo and, in particular, to search for new tidal tails in a sample of nearby dSph satellites of the Milky Way. Here we present our finding of extra-tidal debris in the Ursa Minor dS ph and discuss the detection by the Sloan Digitized Sky Survey team of what could be a tidal stream associated to the Sagittarius dSph.
Two giants of the Sagittarius dwarf spheroidal have been observed with the UVES spectrograph on the ESO 8.2m Kueyen telescope, during the commissioning of the instrument. Sgr 139 has [Fe/H]=-0.28 and Sgr 143 [Fe/H]=-0.21, these values are considerably higher than photometric estimates of the metallicity of the main population of Sgr. We derived abundances for O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Y, Ba, La, Ce, Nd and Eu; the abundance ratios found are essentially solar with a few exceptions: Na shows a strong overdeficiency, the heavy elements Ba to Eu, are overabundant, while…
In the first part of this contribution the observed velocity dispersions in Galactic halo dwarf spheroidal (dSph) galaxies are reviewed, and the consequences for dark matter content outlined. The results are suggestive of a dSph dark matter mass of ~2x10^7 solar masses, independent of the luminosity of the dSph. Alternatives to the dark matter interpretation are also briefly discussed. In the second part of the contribution the emphasis is on the stellar populations of Galactic dSphs. Recent results for the ages of the oldest populations are presented. These data, together with similar recent …
We use infrared Color Magnitude Diagrams from the 2-Micron All-Sky Survey (2MASS) to search for stars belonging to the tidal stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) around selected Galactic globular clusters. Statistically significant detections are presented for the cases of Pal 12 and NGC 4147, strongly supporting the idea that these clusters are associated with the Sgr Stream and that they were previous members of the Sgr dSph galaxy.
Observations of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy were carried out with the H.E.S.S. array of four imaging air Cherenkov telescopes in June 2006. A total of 11 hours of high quality data are available after data selection. There is no evidence for a very high energy gamma-ray signal above the energy threshold at the target position. A 95% C.L. flux limit of 3.6 x 10-12 cm-2s-1 above 250 GeV has been derived. Constraints on the velocity-weighted cross section <sigma v> are calculated in the framework of Dark Matter particle annihilation using realistic models for the Dark Matte…
We predict the metallicity distribution of stars and the age-metallicity relation for 6 Dwarf Spheroidal (dSph) galaxies of the Local Group by means of a chemical evolution model which is able to reproduce several observed abundance ratios and the present day total mass and gas content of these galaxies. The model adopts up to date nucleosynthesis and takes into account the role played by supernovae of different types (II, Ia) allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca, and Fe). Each galaxy model is specified by the prescription…
We summarize the results of our ongoing investigation of the chemical abundances in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) and in the Canis Major Overdensity (CMa). 12 RGB stars were analyzed in the Sgr dSph, plus 5 in the associated globular Terzan 7, together with three CMa candidate members. Detailed abundances have been derived for up to 23 elements from Oxygen to Europium.
We have obtained deep photometric data in 24 fields along the southeast extension of the major axis of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy, and in four fields along the northwest extension. Using star counts at the expected position of the Sgr upper main-sequence within the resulting color-magnitude diagrams (CMDs), we unambiguously detect Sgr stars in the southeast over the range 10--34 degreesfrom the galaxy's center. If Sgr is symmetric, this implies a true major-axis diameter of at least 68 degrees, or nearly 30 kpc if all portions of Sgr are equally distant from the Sun. St…
Photometry of a large field around the young globular cluster Palomar 12 has revealed the main-sequence of a low surface-brightness stellar system. This main-sequence is indicative of a stellar population that varies significantly in metallicity and/or age, but in the mean is more metal poor than Pal 12. Under different assumptions for the properties of this population, we find distances from the Sun in the range 17-24 kpc, which encompasses the distance to Pal 12, $19.0\pm0.9$ kpc. The stellar system is also detected in a field 2$\arcdeg$ North of Pal 12, which indicates it has a minimum diam…
(Abridged) We use N-body simulations to study the evolution of dwarf spheroidal galaxies (dSphs) driven by galactic tides. We adopt a cosmologically-motivated model where dSphs are approximated by a King model embedded within an NFW halo. We find that these NFW-embedded King models are extraordinarily resilient to tides; the stellar density profile still resembles a King model even after losing more than 99% of the stars. As tides strip the galaxy, the stellar luminosity, velocity dispersion, central surface brightness, and core radius decrease monotonically. Remarkably, we find that the evolu…
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…
We present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy as part of the ACS Survey of Galactic Globular Clusters. Our deep (F606W~26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. The distance and reddening to M54 are revised usingboth isochrone and main-sequence fitting to (m-M)_0=17.27 and E(B-V)=0.15. Preliminary assessment finds the M54+Sg…
(Abridged) We carried out a period analysis on ~3700 RR Lyrae stars spread over ~50 square degrees towards the Sagittarius dwarf galaxy (Sgr). The RR Lyraes are distributed almost evenly between Sgr and the Milky Way. For Sgr members, the average periods are 0.574 day and 0.322 day for RRab and RRc stars respectively. This places Sgr in the long-period tail of the Oosterhoff I group. We report the detection of 53 double-mode RR Lyrae stars (RRd) within our sample. The magnitude of 40 of these stars is consistent with membership in Sgr whereas 13 RRds are located within our Galaxy. We also foun…
We present the first results of our ongoing chemical study of carbon stars in the Local Group of galaxies. We used spectra obtained with UVES at the 8.2 m Kueyen-VLT telescope and a new grid of spherical model atmospheres for cool carbon-rich stars which include polyatomic opacities, to perform a full chemical analysis of one carbon star, BMB-B~30, in the Small Magellanic Cloud (SMC) and two, IGI95-C1 and IGI95-C3, in the Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy. Our main goal is to test the dependence on the stellar metallicity of the s-process nucleosynthesis and mixing mechanism occur…
We present here the first measurements of sulphur abundances in extragalactic stars. We make use of high resolution spectra, obtained with UVES at the ESO 8.2 m Kueyen telescope, of three giants of the Globular Cluster Terzan 7, which belongs to the Sagittarius dwarf galaxy. We measure the sulphur abundances using the lines of S I multiplet 1. The S/Fe ratios for all three stars are nearly solar, thus considerably lower than what is found in Galactic stars of comparable iron content ([Fe/H] ~ -0.50). This finding is in keeping with the abundances of other alpha-chain elements in this cluster a…
The Sagittarius galaxy (Sgr), the closest satellite galaxy of the Milky Way, has survived for many orbits about the Galaxy. Extant numerical calculations modeled this galaxy as a system with a centrally-concentrated mass profile, following the light, and found that it should lose more than one-half of its mass every 2--4 orbits and be completely disrupted long before now. Apparently Sgr, and by implication other dSph galaxies, do not have a centrally-concentrated profile for their dark matter. We develop a model in which the stars of the Sgr dwarf are embedded in a constant-density dark matter…
(Abridged) We use N-body simulations to explore the effects of tidal stripping on the structure of dwarf spheroidal galaxies (dSphs). We model dSphs as King spheres embedded in NFW dark halos that orbit the Galactic potential on eccentric orbits. We find that episodes of stellar mass loss remove the initial cutoff of the bound stellar core. Once equilibrium has been re-established the outer mass profile approaches a power-law in projection that is well described by a simple Plummer model. Tides also result in transient features in the outer density profile. As the system re-equilibrates, an ou…
Neutral hydrogen observations towards the Bootes dwarf spheroidal galaxy, a very low luminosity metal-poor Galactic satellite, were obtained using the Parkes Radio Telescope. We do not detect any HI in or around Bootes to a 3sigma upper limit of 180 Msun within the optical half light radius and 8000 Msun within 1.6 kpc. Its HI mass-to-light ratio is less than 0.002 Msun/Lsun, making Bootes one of the most gas-poor galaxies known. Either reionisation severely inhibited gas infall onto the proto-Bootes, or large amounts of gas have been removed by ram pressure and/or tidal stripping. Since Boote…
Our work focuses on the understanding of the origin of CNO-anomalies, which have been detected in several Galactic globular clusters (GCs). This study is based on a homogeneous data set of hundreds of medium resolution spectra of stars in eight GCs. Two of the clusters are believed to be former members of the Sagittarius dSph galaxy. Our sample comprises stars in different evolutionary states, namely the main sequence turn-off (MSTO) region, the subgiant branch (SGB), and the base of the red giant branch (RGB). We compare the relative CN and CH line strengths of stars in the same evolutionary …
