Cole, A.

Fraternali, F.

Irwin, M.

Marconi, G.

Szeifert, T.

Tolstoy, E.

English

Proceedings - University of Groningen

   University of GroningenStellar Spectroscopy of Individual Stars in Local Group Galaxies with the VLTTolstoy, E.; Irwin, M.; Cole, A.; Fraternali, F.; Szeifert, T.; Marconi, G.Published in:The MessengerIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Tolstoy, E., Irwin, M., Cole, A., Fraternali, F., Szeifert, T., & Marconi, G. (2004). Stellar Spectroscopy ofIndividual Stars in Local Group Galaxies with the VLT. The Messenger, 115, 32-34.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 03-06-2022THE HIGH RESOLUTION SPECTRO-graph, UVES, has providedoutstanding data for the studyof stellar abundances in extra-galactic environments. It ispossible to observe red giant branch (RGB)stars at high resolution in galaxies as faraway as Leo I (250 kpc) with UVES(Shetrone et al.  2002; Tolstoy et al. 2002),and to look at super-giants up to 1.5 Mpcaway at the boundary of the Local Group(e.g., Venn et al. 2003; Kaufer et al. 2004).These studies have been, of necessity,painstakingly long integrations of individualstars down to the faint limit of the instrument(I ~ 19.5).The multi-fibre instrument, FLAMES, isproviding a dramatic increase in multi-plex-ing with a 25 arcmin diameter field of view,and 130 fibres in Medusa mode (mag. limitI ~ 18.5), over selectable wavelength ranges(per setting), at resolutions useful for stellarspectroscopy (R ~ 20000), and 8 fibres feed-ing UVES over a longer wavelength range,with a typical UVES resolution (R ~ 40000).This field of view and density of fibres is agood match to the size of the central regionsof nearby dwarf spheroidal galaxies, and thenumber of RGB stars at magnitudes brightenough to obtain good quality high resolu-tion spectra.However, FLAMES is not always thebest instrument for every stellar project.Due to light loss from fibre transmission,and restrictions on choice of resolution andon placing fibres close to each other,FLAMES is well suited for abundance stud-ies of stars in uncrowded regions of ournearest neighbour galaxies (within~220 kpc).  If we want to look at morecrowded regions (e.g., the bar of the LMC),or if we want to use the VLT to push beyondthe galaxies in and around the halo of ourGalaxy to look at a completely separate andindependent environments, we have to usethe slit spectrographs, FORS1 and FORS2.These instruments also have enhanced mul-tiplexing capabilities such as slit masks, andwith a blocking filter, FORS2 can match oreven exceed the capabilities of FLAMES.Of course this means fewer lines from a sin-gle element at lower resolution, but the con-verse is that we can look at significantlyfainter stars, probing a more distant and var-ied set of environ-ments.A commonlyused abundanceindicator at inter-mediate resolutionis the Ca II triplet(CaT), a set ofthree absorption lines near 8500 Å.  Theequivalent widths of these lines (ie. the lineflux with respect to the continuum luminosi-ty of the star) have been shown to correlatevery well with high resolution [Fe/H] abun-dance (see Armandroff & Da Costa 1991;Rutledge 1997). We have also made a com-parison of our own, not matched on individ-ual stars, but using preliminary results fromthe HR FLAMES data we are collecting for100+ stars in the Sculptor galaxy (Hill et al.,in prep.).  We compare the distribution of[Fe/H] abundances found with FLAMESwith those measured on the basis of the CaTfrom a previous FORS1 study of Sculptor(Tolstoy et al. 2001).  As can be seen inFigure 1, there is good agreement betweenthe two samples, although we note that theFLAMES results are preliminary.The FORS2 spectrograph, combinedwith the red-optimised MIT/LL CCDs, is auniquely powerful instrument for measuringthe metallicities of RGB stars as faint as I =The Messenger 11532STELLAR SPECTROSCOPY OF INDIVIDUAL STARS IN LOCALGROUP GALAXIES WITH THE VLT: KINEMATICS ANDCALCIUM TRIPLET ABUNDANCESE. TOLSTOY1, M. IRWIN2, A. COLE1, F. FRATERNALI3,4, T. SZEIFERT5, G. MARCONI51KAPTEYN ASTRONOMICAL INSTITUTE, UNIVERSITY OF GRONINGEN, THE NETHERLANDS2INSTITUTE OF ASTRONOMY, UNIVERSITY OF CAMBRIDGE, UNITED KINGDOM3ASTRON, DWINGELOO, THE NETHERLANDS4DEPT OF THEORETICAL PHYSICS, UNIVERSITY OF OXFORD, UNITED KINGDOM5EUROPEAN SOUTHERN OBSERVATORYTHE LARGE COLLECTING AREA AND HIGH-THROUGHPUT MULTI-OBJECT INSTRUMENTS ON THE VLT MAKE IT POS-SIBLE TO CARRY OUT DETAILED STUDIES OF STELLAR PROPERTIES AND DISTRIBUTIONS IN ENVIRONMENTS WELLBEYOND OUR GALAXY.Figure 1: The solid line in the (normalised) histogram of [Fe/H] is fora sample of RGB stars observed in the Sculptor dwarf spheroidal inthe CaT region (by Tolstoy et al. 2001) with FORS1.  The dotted his-togram comes from the preliminary results of the FLAMES LP andGTO time which are based on high resolution spectra for ~100 RGBstars in Sculptor. The samples are not overlapping, but both are ran-domly chosen on the RGB. The FLAMES samples are necessarilynearer the tip of the RGB. The FORS1 sample was chosen lowerdown the RGB, at somewhat fainter magnitudes.22.  This brings every Local Group galaxyoutside the Galactic zone of avoidance intorange for kinematic and metallicity studiesof intermediate-age and old stars.  FORS1/2can thus be used to efficiently look at sam-ples of stars in galaxies which are too distantto allow high resolution abundances or evenfibre spectroscopy (e.g., Tucana, Fraternaliet al. 2004, in prep.).  The field of view ofthe FORS1/2 spectrograph and the numberof slits are a good match to the density ofRGB stars in low surface brightness dwarfgalaxies out to the edge of the Local Group.In Figure 2 we can see the raw dataobtained for the Tucana dwarf spheroidalgalaxy, and the field of view covered byFORS2/MXU on the galaxy. Out of 47 tar-gets placed on the slits 25 could be success-fully extracted. The rest were too faint for asuccessful extraction. Of these successfullyextracted stars 18 were considered of goodenough quality to determine velocities, andof these 3 were quite distinct from the aver-age velocity of the majority of stars.  Thesecluster around the systemic velocity ofTucana, determined now for the first time tobe 182 km/s.  In Figure 3 we show the his-togram of these velocities.With this reasonably large sample ofstars across the Tucana galaxy we can see ifthere are any obvious kinematic patterns.Figure 4 shows the velocities of the starsobserved in Tucana with their positions inthe galaxy.  The blue crosses have velocitiesless than systemic, and red crosses havegreater than systemic velocities.  The twogreen crosses are at systemic velocity. Thereis intriguing evidence for one side of thegalaxy appearing to recede and the other tobe approaching. This could be interpreted asrotation, or possibly some indication of atidal disturbance, although Tucana’s isolatedposition in the Local Group mitigates againstthe latter possibility.   Although the number© ESO - March 2004 33Figure 2: A FORS1 R-band image of Tucana taken in August 1999.  The green outline shows the region covered by the MXU slit mask, and thusthe region where spectra were obtained. The CCD image on the right is the raw spectra on one of the two FORS2 CCDs (the “master” CCD) forour spectra of stars in Tucana.Figure 3: Histogramof the radial veloci-ties measured fromthe CaT region for 18stars in the Tucanadwaf spheroidalgalaxy. The gaussianfit gives a central,systemic, opticalvelocity of Tucana of182 km/s, with a dis-persion on 12 km/s.statistics are still very small, interestingly, ifthis is rotation it would be, as expected, rota-tion about the minor axis.In Figure 5 we show preliminary resultsfrom the CaT metallicities of these stars inTucana. The spread in abundances looksquite similar to Sculptor (from Tolstoy et al.2001). From this plot, and taking intoaccount the velocity and position of Tucanain the Local Group it is clear that the proper-ties of Tucana are consistent with those ofnearer Galactic satellite dwarf spheroidalgalaxies, such as Sculptor.The efficient use of FORS1/2 is notrestricted to the faintest stars; the same tech-niques can be successfully applied to muchnearer galaxies.  Here, the efficiency withwhich it is possible to change the slit config-uration, the ease of target acquisition basedon pre-images and slit configurations createdwith the FIMS software, together with see-ing of 0.7 arc-seconds have allowed majornew studies of crowded regions like the cen-ter of the LMC bar (e.g., Cole et al. 2004, inprep.).  With a surface brightness of20.6 mag/arcsec2, RGB stars in the bar werebeyond the capabilities of spectrographs on4-meter class telescopes.  Crowding prob-lems can be minimized by target selectionfrom pre-images taken in good seeing, andthe difficulties of sky fiber placement can becompletely avoided by using slits that spanmany arc-seconds around each target.  Thesecapabilities ensure a valuable place forFORS even after the advent of large fibresystems such as FLAMES.In nearby galaxies, where targets arebright, the speed with which high signal-to-noise spectra can be acquired becomes atremendous advantage of FORS1/2.Comparing CaT spectra of RGB stars in theLMC obtained in a large survey of fieldstars, UT4/FORS2 yields higher signal-to-noise in 1200 second exposures (Cole et al.2004) than CTIO-Hydra in exposures 10times as long (Smecker-Hane et al. 2004, inprep).  Because the metallicity distributionfunctions of galaxies are not well con-strained, sample size is a crucial aspect ofany chemical evolution study.  Sparse sam-ples, measuring only a few stars, can all tooeasily miss minority populations which maybe critical for understanding the chemical(and dynamical) evolution of the hostgalaxy.There are several galaxies in the LocalGroup for which we do not have even themost basic information, such as the opticalradial velocity.  This is especially importantfor dwarf spheroidal type galaxies whichhave no (observable) gas. Once the stellarvelocity of the system is known it is possibleto look more carefully for associated gas andto analyse possible orbital trajectories to itscurrent location.We have used the FORS spectrographsto determine the optical velocities, previous-ly unknown, of Antlia (Tolstoy & Irwin2000), and Tucana (Fraternali et al. 2004)and a reassessment of the optical velocity ofPhoenix (Irwin & Tolstoy 2002).  In the caseof the Tucana dwarf spheroidal, it has alwaysbeen held to be an unusual object because ofits extreme distance from the Galaxy andM31, unlike all other Local Group dwarfspheroidals (at least prior to the discovery ofCetus).In the case of the Phoenix and Antliadwarf galaxies it is possible for the first timeto make clear the association of the opticalgalaxy with the HI gas seen near theseobjects.  Without a reliable optical velocitythis association remained uncertain.  Thesekinematic studies therefore have far reachingimplications for our understanding of evolu-tionary processes in nearby dwarf galaxies.There are currently two large pro-grammes at ESO to use FLAMES toincrease the number of stars with velocityand abundance measurements to improvedetailed modelling in several nearby galax-ies (e.g., 171.B-0588, PI Tolstoy), for whichthe first results will be available soon.  Thisability to look at individual stars in differentgalaxies with such accuracy over such alarge range in distance means we can probethe properties of stars in many differentenvironments - metal poor, metal rich,dense, sparse etc.  This is an exciting era forunderstanding the detailed properties ofresolved stellar populations across the LocalGroup, which naturally will have implica-tions for studies of the most distant galaxies.REFERENCESArmandroff & Da Costa 1991, AJ, 101, 1329Irwin & Tolstoy 2002, MNRAS, 336, 643Kaufer et al. 2004, AJ, in pressRutledge et al. 1997, PASP, 109, 907Shetrone et al. 2003, AJ, 125, 684Tolstoy & Irwin 2000, MNRAS, 318, 1241Tolstoy et al. 2001, MNRAS, 327, 918Tolstoy et al. 2003, AJ, 125, 707Venn et al. 2003, AJ, 126, 1326The Messenger 11534Figure 4: A 20( by14( field of view cen-tred on Tucana,taken from DSSplates. The bluecrosses representthose stars withvelocities less thanvsys (182 km/s) andthe red crossesgreater than sys-temic.  The greencrosses are thosestars at systemicvelocity. North is upand East to the left.Figure 5: The blackcrosses are the measure-ments of the CaT widthsof the two strongest linesin our FORS2/MXUspectra of stars inTucana plotted againstthe difference in theirmagnitude with that ofthe horizontal branch inTucana (VH). The dashedlines are the relationscalibrated by Tolstoy etal. 2001 correspondingto where stars of differ-ent metallicity should lie.The red crosses are theCaT measurementstaken with the sameinstrumental set up andprocessed in the sameway for stars of knownmetallicty in Galacticglobular cluster, M15.

Stellar Spectroscopy of Individual Stars in Local Group Galaxies with the VLT

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Stellar Spectroscopy of Individual Stars in Local Group Galaxies with the VLT

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