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Probing the Dark Matter Content of Local Group Dwarf Spheroidal Galaxies with FLAMES

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Probing the Dark Matter Content of Local Group Dwarf Spheroidal Galaxies with FLAMES Reports from Observers Probing the Dark Matter Content of Local Group Dwarf Spheroidal Galaxies with FLAMES Mark I. Wilkinson1 radial length scale) which are inferred to field stars (Shetrone et al. 2001; Tolstoy Jan T. Kleyna 2 contain dynamically significant quantities et al. 2006). Thus, the Galactic halo was Gerard F. Gilmore1 of dark matter. not predominantly formed by disrupt- N. Wyn Evans1 ing galaxies like the present-day dSphs. Andreas Koch 3 The proximity of the Milky Way dSph sat- Nevertheless, the chemical properties Eva K. Grebel 3 ellites makes it feasible to obtain spectra of dSphs provide valuable insights into Rosemary F. G. Wyse 4 for large numbers of individual stars in the processes of galaxy formation and Daniel R. Harbeck 5 them, facilitating more detailed studies of evolution on the smallest scales. their properties than are possible for galaxies beyond the Local Group. These It is the large apparent mass-to-light ra- 1 Institute of Astronomy, Cambridge spectra provide radial velocities and tios of dSph galaxies, and the conclu- University, Cambridge, United Kingdom chemical abundance determinations sion that they contain significant amounts 2 Institute for Astronomy, Honolulu, USA which, respectively, enable us to to probe of dark matter, that has caused them 3 Astronomical Institute of the University the stellar velocity distribution function to attract much attention in recent years. of Basel, Department of Physics and and halo potential within a dSph and pro- The first evidence of this emerged in Astronomy, Switzerland vide detailed information about the star- 1983, when Aaronson published velocity 4 The John Hopkins University, Baltimore, formation history of these systems. measurements of three carbon stars in USA Two recent Messenger articles have dis- the Draco dSph. These data implied a 5 University of Wisconsin, Madison, USA cussed in detail the insights into the velocity dispersion for Draco of 6.5 km/s, chemical evolution of the stellar popula- from which Aaronson cautiously inferred tions in dSphs which have been gained a mass-to-slight ratio of 30 solar units. We present preliminary kinematic re- by recent observations with the FLAMES As this was significantly higher than the sults from our VLT programme of spec- spectrograph on the VLT (Koch et al. values typical of globular star clusters, troscopic observations in the Carina 2006a; Tolstoy et al. 2006). In this article this provided the first hint that the dSphs dwarf spheroidal galaxy using the we will focus on the implications of the were a class of stellar system distinct FLAMES multi-object spectrograph. kinematic data in the Carina dSph for our from the globular clusters, despite the These new data suggest that the dark understanding of the dark matter. fact that in many cases their stellar mass matter halo of this galaxy has a uniform was similar. Subsequent observations density core. The implications for our The dSphs are the lowest-luminosity sat- have borne out these early estimates and understanding of the nature of the dark ellites of the Milky Way and M31, charac- all dSphs observed to date have velocity matter are discussed. terised both by their low stellar luminosi- dispersions in excess of 6.5 km/s. ties (up to 2 × 107 solar luminosities, with the majority in the range 105–106 solar By 1998, due to the dedicated efforts of Dark matter in dSphs luminosities) and the apparent absence of several teams, velocity dispersions had gas and on-going star formation. The been measured for eight dSphs (Mateo According to the current cosmological recent discovery of the Ursa Major dSph 1998). However, dSph velocity distribu- paradigm, non-luminous, non-baryonic (Willman et al. 2005) and the Canes tions were still represented solely by the matter contributes approximately 20 % of Venatici (Zucker et al. 2006) and Bootes central velocity dispersion, which con- the overall mass-energy budget of the (Belokurov et al. 2006) dSph candidates tains only limited information about the Universe. Locally, on the scale of galaxies brings to twelve the number of dSphs nature of the system. The situation like the Milky Way, it constitutes about around the Milky Way. A similar number changed in 1997, with the publication of 90 % of the gravitating mass. Determining have been identified orbiting M31. At the first velocity dispersion profile for the nature of this dark matter is a key least one dSph is clearly being disrupted a dSph (Mateo 1997). Using 215 individual goal of contemporary astronomy. The by the tidal field of the Milky Way – the stellar velocities in Fornax, Mateo showed dwarf spheroidal galaxies (dSphs) of the Sagittarius dSph has extensive debris that the velocity dispersion remains ap- Local Group provide a particularly valua- trails which have been observed over proximately flat almost to the edge of the ble window on the properties of dark much of the sky. The stellar distributions light distribution. This profile is inconsist- matter on small (~ 1 kpc) scales. Several of most dSphs exhibit some level of dis- ent with the simplest model of Fornax of these low-luminosity galaxies have turbance in their outer regions, although in which the mass is distributed in the been found to have extremely large whether this is due to their proximity same way as the light and the stars have mass-to-light (M/L) ratios (up to 1000 in to their parent galaxies remains contro- an isotropic velocity distribution. solar units) which suggests that these versial. Although the dSphs were once are the most dark-matter-dominated stel- thought to be the primordial building The past decade has seen a rapid in- lar systems known in the Universe (e.g. blocks whose disruption contributed to crease in the size of dSph kinematic data Kleyna et al. 2001, Wilkinson et al. 2004, the hierarchical build-up of the stellar sets, driven by the availability of multi- Mateo et al. 1998). Given the apparent halo of the Milky Way, recent studies object spectrographs on 4-m to 10-m- absence of dark matter in globular star have found that the chemical composi- class telescopes which allow the simulta- clusters, they are also the smallest stellar tion of the stars in the present-day neous acquisition of spectra for large systems (in terms of stellar mass and dSphs differs substantially from that of numbers of stars. The first dSph disper- The Messenger 124 – June 2006 25 Reports from Observers Wilkinson M. I. et al., Probing the Dark Matter Content of Local Group dSphs sion profile based on multi-fibre observa- 25 Figure 1: Spatial distribution of Carina tions was that of Draco, based on veloc- members observed in our survey. 20 Stars approaching and receding (re- ities for 159 stars obtained using the 15 lative to Carina) are shown as blue WYFFOS spectrograph on the William and red symbols, respectively. The Herschel Telescope (Kleyna et al. 2001). 10 size of the symbol is proportional to the magnitude of the radial velocity. Since that time, dispersion profiles 5 n) The ellipse indicates the nominal tidal have been measured for all the Milky Way mi rc 0 limit of Carina with semi-major axis dSphs. For most Milky Way dSphs, the of 28.8 arcmin. large (25 arcmin diameter) field of view of Y (a −5 the FLAMES spectrograph on the VLT −10 is ideally matched to their angular size −15 facilitating the efficient coverage of the full −20 area of each object. −25 −30 −20 −10 0 10 20 30 The properties of dSph haloes are largely X (arcmin) determined by the physical properties of the dark matter. For example, the num- ber of low-mass dark-matter haloes in global stellar kinematics of all the dSphs errors are 1.2 km/s for the entire data the vicinity of a Milky Way-type galaxy is have not yet been defined sufficiently set and 1.5 km/s for the faintest stars a strong function of the nature of the dark well to yield definitive results on the inner (V = 20–20.5). matter – cold dark matter (CDM) simu- slope of their dark matter distributions. lations typically predict hundreds of low- Our Carina data set will be sufficiently A total of 1257 targets in Carina were ob- mass haloes around the Milky Way while large to place robust constraints on the served. Of these, 535 are probable mem- warm dark-matter models contain sig- profile of a dSph dark-matter halo, while bers of Carina (based on their radial nificantly fewer. However, it is currently our modelling will further the analyses velocities). The remainder are foreground not clear which objects in the simulations of the data from all the dSphs. Milky Way stars which can be used to correspond to the observed dSphs. A investigate the properties of the velocity number of authors have claimed that the distribution of our own Galaxy (see properties of dSphs are consistent with Observations of the Carina dSph below). Of the probable members, some their residing in haloes of mass > 109 437 stars have spectra with sufficient solar masses. In this case, the numbers The Carina dSph has previously attracted signal-to-noise to enable chemical abun- of dSphs might be consistent with attention due to its unusual, bursty star- dances to be determined using the Cal- CDM simulations. One of the key goals of formation history, with the most recent cium triplet estimator (Koch et al. 2006b). our observations in Carina is to test in peak occurring around 3 Gyr ago (e.g. The spatial distribution of Carina mem- detail whether its properties are consist- Monelli et al. 2003). Its central velocity bers from our survey is shown in Figure 1.
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