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The Dwarf Galaxy Abundances and Radial-Velocities Team (DART) Large Programme – a Close Look at Nearby Galaxies

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The Dwarf Galaxy Abundances and Radial-Velocities Team (DART) Large Programme – a Close Look at Nearby Galaxies Reports from Observers The Dwarf galaxy Abundances and Radial-velocities Team (DART) Large Programme – A Close Look at Nearby Galaxies Eline Tolstoy1 The dwarf galaxies we have studied are nearby galaxies. The modes of operation, Vanessa Hill 2 the lowest-luminosity (and mass) galax- the sensitivity and the field of view are Mike Irwin ies that have ever been found. It is likely an almost perfect match to requirements Amina Helmi1 that these low-mass dwarfs are the most for the study of Galactic dSph galaxies. Giuseppina Battaglia1 common type of galaxy in the Universe, For example, it is now possible to meas- Bruno Letarte1 but because of their extremely low ure the abundance of numerous elements Kim Venn surface brightness our ability to detect in nearby galaxies for more than 100 stars Pascale Jablonka 5,6 them diminishes rapidly with increasing over a 25;-diameter field of view in one Matthew Shetrone 7 distance. The only place where we can shot. A vast improvement on previous la- Nobuo Arimoto 8 be reasonably sure to detect a large frac- borious (but valiant) efforts with single-slit Tom Abel 9 tion of these objects is in the Local spectrographs to observe a handful of Francesca Primas10 Group, and even here, ‘complete sam- stars per galaxy (e.g., Tolstoy et al. 200; Andreas Kaufer10 ples’ are added to each year. Within Shetrone et al. 200; Geisler et al. 2005). Thomas Szeifert10 250 kpc of our Galaxy there are nine low- Patrick Francois 2 mass galaxies (seven observable from The DART Large Programme has meas- Kozo Sadakane11 the southern hemisphere), including Sag- ured abundances and velocities for sev- ittarius which is in the process of merging eral hundred individual stars in a sample with our Galaxy. We will show that only of four nearby dSph galaxies: Sculptor, 1 Kapteyn Institute, University of Gronin- the closest galaxies can be observed in Fornax, Sextans and Carina. We have gen, the Netherlands detail, even with an 8-m telescope. used the VLT/FLAMES facility in the low- 2 Observatoire de Paris, France resolution mode (LR 8, R ~ 8 000) to 3 Institute of Astronomy, University of The lowest-mass galaxies are dwarf irreg- obtain Ca II triplet metallicity estimates as Cambridge, United Kingdom ular (dI) and dwarf spheroidal (dSph) type well as accurate radial-velocity measure- 4 Department of Physics and Astronomy, galaxies. The only difference between ments over large areas in Sculptor, For- University of Victoria, Canada these low-mass dSphs and dIs seems to nax and Sextans out to the tidal radius 5 Observatoire de Genève, Laboratoire be the presence of gas and current star (Tolstoy et al. 200; Battaglia et al. 2006, d’Astrophysique, Ecole Polytechnique formation in dIs. It has already been not- in prep.), see Figure 1 for the example Fédérale de Lausanne, Switzerland ed that dSphs predominantly lie close of Sculptor. In Figure 1 we show the area 6 on leave from Observatoire de Paris, to our Galaxy (< 250 kpc away), and dIs on the sky around the Sculptor dSph France predominantly further away (> 400 kpc galaxy. The large ellipse is the tidal radius 7 University of Texas, McDonald Observ- away). This suggests that the proximity of of Sculptor as determined by Irwin and atory, Fort Davis, Texas, USA dSph to our Galaxy played a role in the Hatzidimitriou (1995). The positions ob- 8 National Astronomical Observatory, removal of gas from these systems. How- served with VLT/FLAMES are marked as Tokyo, Japan ever, the range of properties found in circles, where the solid circles repre- 9 KIPAC, Stanford University, Menlo dSph and dI galaxies does not allow a sent the 10 individual FLAMES point- Park, California, USA straightforward explanation, particular- ings analysed so far, and the dashed-line 10 ESO ly not the large variations in star-formation circles are the positions of five addition- 11 Astronomical Institute, Osaka Kyoiku histories and chemical-evolution paths al fields that were observed in November University, Japan that have now been observed in different 2005. Also plotted for the analysed fields systems (e.g., Dolphin et al. 2005). are the positions of the stars that are probable members (small red squares) We review the progress of ESO/WFI Im- It is perhaps not a surprise that there is and the non-members (black crosses). aging and VLT/FLAMES spectroscopy apparently a lower limit to the mass of an Each of the four galaxies has been ob- of large numbers of individual stars in object which is able to form more than served at high resolution (Giraffe settings nearby dwarf spheroidal galaxies by the one generation of stars, which is related HR 10, 1 and 1) in the central region to Dwarf galaxy Abundances and Radial- to the limit below which one supernova obtain detailed abundances for a range of velocities Team (DART). These observa- will completely destroy a galaxy. Numeri- interesting elements such as Mg, Ca, O, tions have allowed us to show that cal and analytic models tell us that this Ti, Na, Eu to name a few (Hill et al. 2006 6 neither the kinematics nor the abundan- must be around a few × 10 MA. Globular in prep.; Letarte et al. 2006a in prep.) ces nor the spatial distributions are easy clusters typically have much lower masses for about 100 stars. During the Giraffe HR to explain in a straightforward manner than this limit. observations we were also able to use for these smallest galaxies. The main the fibre feed to the UVES spectrograph result is that dwarf galaxies show com- to obtain greater wavelength coverage plex and highly specific evolutionary Observations and higher resolution for a sample of sev- and metal-enrichment processes, espe- en to 1 stars per galaxy (e.g., Venn et cially at ancient times. This conclusively VLT/FLAMES with fibre-feeds to the Gi- al. 2006 in prep.; Shetrone et al. 2006 in proves that these small galaxies are raffe and UVES spectrographs has made prep.). The comparison we can make not the building blocks of the larger gal- a revolution possible in spectroscopic between results from UVES spectroscopy axies in the Local Group. studies of resolved stellar populations in (R ~ 40 000) and Giraffe spectroscopy The Messenger 123 – March 2006 33 Reports from Observers Tolstoy E. et al., The DART Large Programme – A Close Look at Nearby Galaxies Sculptor dSph Figure 1: The area on the sky around (R ~ 20 000) for the same stars is very 1.5 useful in convincing ourselves, and oth- the Sculptor dSph galaxy. The posi- tions observed with VLT/FLAMES are ers, that we are able to obtain reliable marked as circles. Also plotted are results with lower resolution spectra than 1.0 the positions of the stars that are prob- was previously thought possible or advis- able members (red squares) and the able. This lower-resolution is less of a non-members (black crosses). 0.5 problem at lower metallicity (e.g., Sculp- tor) than at higher metallicity (e.g., Fornax). ] ees 0.0 egr [d η Colour-magnitude diagrams –0.5 The first step in a detailed analysis of the resolved stellar population of a galaxy –1.0 is an accurate colour-magnitude diagram (e.g., Figure 2), ideally down to the old- –1.5 est main-sequence turnoffs (MV ~ + .5). 1.5 1.0 0.5 0.0 –0.5 –1.0 –1.5 In Figure 2 is plotted the colour-magni- ξ [degrees] tude diagram of the Sculptor dSph from ESO/WFI imaging out to the tidal radius. The coloured symbols are the stars we –3 Figure 2: Colour-magnitude diagram observed with VLT/FLAMES. The prob- WFI/FLAMES Sculptor dSph of the Sculptor dSph from ESO/WFI possible members imaging. The coloured symbols are able members of Sculptor are shown as probable non-members the stars observed with VLT/FLAMES. purple circles and the non-members are The probable members of Sculptor shown as green stars. A careful analysis –2 are shown as purple circles and the leads to the star-formation history all the non-members are shown as green stars. Also shown outlined in blue are way back to the first star formation in the regions used to define the Blue FG the Galaxy. This approach is the most ac- –1 Horizontal Branch (BHB), the Red Hor- curate for intermediate-age populations, izontal Branch (RHB) and foreground comparison (FG) stellar populations. but for stars older than about 10 Gyr V M the time resolution gets quite poor (and the stars are getting very faint), and it 0 BHB becomes hard to distinguish a 12-Gyr- RHB old star from a 10-Gyr-old star. Here it becomes useful to consider the Horizon- 1 tal Branch stars (MV ~ 0) which are the bright He-burning phase of low-mass stars > 10 Gyr old. The ratio of red to blue horizontal-branch stars (see Figure 2) 2 tells us about the age and metallicity vari- 0 0.5 1 1.5 V–I ation. In Figure 2 regions used to define the Blue Horizontal Branch (BHB), the by Cole et al. 2005; their Figure 8). The pockets of interstellar medium of differ- Red Horizontal Branch (RHB) and fore- observed magnitude and colour (e.g., ent ages (enriched by different num- ground comparison (FG) stellar popula- MV, V−I) of a star combined with a meas- bers of processes) conveniently covering tions in Sculptor are outlined in blue ured [Fe/H], allows us to effectively the nuclear burning core of stars. This boxes. The spatial distribution of red and remove the age-metallicity degeneracy hot stellar core provides a useful bright blue horizontal-branch stars in Sculp- and determine the age of an RGB star background source to be absorbed in the tor is shown in Figure .
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