Chemical and Kinematical Evolution in Nearby Dwarf Spheroidal GalaxiesAuthor(s): Andreas KochSource: Publications of the Astronomical Society of the Pacific, Vol. 119, No. 853 (March2007), pp. 347-348Published by: The University of Chicago Press on behalf of the Astronomical Society of the PacificStable URL: http://www.jstor.org/stable/10.1086/512865 .

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Dissertation Summary

Chemical and Kinematical Evolution in Nearby Dwarf Spheroidal Galaxies1

Andreas Koch

Current address: Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, CA; akoch@astro.ucla.eduThesis work conducted at Astronomical Institute of the University of Basel, Binningen, Switzerland

Ph.D. thesis directed by Eva K. Grebel; Ph.D. degree awarded 2006 September 25

Received 2007 January 15; accepted 2007 January 18; published 2007 February 9

Dwarf spheroidal (dSph) galaxies are the least massive andluminous objects known to exist. These galaxies are often con-sidered the earliest building blocks of massive systems pre-dicted by some cosmological models. Nonetheless, evidencehas been mounting that this idea of hierarchical assembly maybe too simplistic a picture, since a number of dSph character-istics, such as the a-element abundance patterns, stand in con-tradiction to the properties of stars in the Galactic halo. Furtheryet unsolved puzzles include the missing satellite problem, theinfluence of feedback and reionization, and the nature of darkmatter. How then do the dSphs form, to what extent do theycontribute to the build-up of massive galaxies, what can we sayabout their dark matter content, and how can one characterizetheir role in cosmology and galactic evolution? These systems’intriguing properties, such as the omnipresence of old stellarpopulations, their gas deficiency, their high-velocity dispersion,and their flat dispersion profiles provide stringent tests of theparadigm of galaxy formation and render these systems importantbenchmarks for studying galactic evolution from the earliest ep-ochs on. In particular, the proximity of the dSphs in the LocalGroup (LG) allows us to resolve their stellar populations and topursue near-field cosmology on the smallest scales.

In this thesis, I address several evolutionary aspects of thesegalaxies by concentrating on three Galactic satellites and ad-ditionally investigating the global satellite galaxy system of theAndromeda galaxy, M31.

In this context, I spectroscopically analyzed the Carina dSph,which stands out among the LG dSphs because of its unusual,episodic star formation (SF) history. Carina bears evidence ofat least three prominent stellar populations. Hence, I aimed atstudying the metallicity spread of such systems, investigatingpotential age-metallicity relations, searching for spatial gradi-ents, and exploring its evolutionary history, accounting forchemical enrichment. This was achieved by obtaining medium-resolution spectroscopy of ∼1200 targets in Carina. Based onthe near-infrared calcium triplet as a well-established metallic-ity indicator, I was able to compile the metallicity distribution

1 The complete thesis is available at http://pages.unibas.ch/diss/2006/DissB_7665.htm.

function (MDF) of this galaxy from a large sample of stars.Despite the wide spread in stellar ages present in Carina, orig-inating from its episodic SF, it exhibits a remarkably narrowred giant branch (RGB). On the other hand, I found a widespread in the metallicities, reaching from �3 dex to near-solar(Koch et al. 2006, AJ, 133, 895). Hence, I could show thatage and metallicity conspire to produce old, metal-poor starsat the same locus on the RGB as young, metal-rich ones. Thismanifestation of an age-metallicity degeneracy generically ex-plains the observed narrowness of Carina’s RGB. In addition,I could also derive the age of each single red giant from iso-chrone fits (Koch et al. 2005, IAU Colloq. 198, 134). Theresulting age distribution indicates the presence of three majorpeaks, with a prevailing intermediate-age population. Thesepopulations may in fact be related to Carina’s three well-es-tablished SF episodes. By correlating metallicities and spatialinformation, I could confirm a radial population gradient in themetallicities, in the sense that more metal-rich stars are foundtoward the center of the galaxy. This phenomenon points to adeep central potential well in the dSphs, where gas is longerretained for SF and enrichment. In order to analyze the shapeof the observed MDF, several simple models of chemical evo-lution were calculated, which support the view that the galaxy’searly SF must have occurred from preenriched gas. Moreover,all the models in use tend to overestimate the number of metal-poor stars (i.e., there is a persistent G-dwarf problem).

This medium-resolution study was complemented by mea-surements of iron- and a-element abundance ratios from high-resolution spectra of 10 stars in Carina. These elements areimportant tracers of SF and thus reflect the evolutionary statusof any stellar system. It could be shown that the calibration ofthe metallicity [Fe/H] via the calcium triplet reproduces the“true” stellar iron abundance well for moderately metal-poorstars, but the calibration tends to fail toward the most metal-poor populations. Carina’s [a/Fe] ratios are very consistentwith those measured in other dSphs of the LG and confirmthat they are systematically lower than those in Galactic halostars of comparable metallicities. The overall abundance pat-terns are not inconsistent with an episodic SF, but the accuracyand small-number statistics of such measurements impede

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2007 PASP, 119:347–348

quantification of the underlying evolution. However, from thepresent data, it is safe to say that Carina inherits the typicalcharacteristics of other dSphs in terms of a low SF efficiencyand the occurrence of strong galactic winds.

In an analogous manner, I determined metallicities in theremote Galactic satellite Leo II. The resulting MDF also showsa deficiency in very metal-poor stars. Furthermore, it turns outto be rather asymmetric, with a rapid decline toward highermetallicities. By comparing my measurements with model pre-dictions of LG dSphs, I illustrated that Leo II’s MDF bearsresemblance to the UMi and Scl dSphs, albeit none of themodels succeed in reproducing all features of the MDF si-multaneously. By additionally determining the ages of the RGBstars in Leo II, I showed that the age-metallicity relation inthis galaxy is essentially flat over a long time interval, whilethere is evidence for an enrichment during the last 2–4 Gyr.The overall wide spread in ages that is present in this dSphsupport earlier views that Leo II is in fact a galaxy with aprominent old and a prevalent intermediate-age population.Contrary to Carina, I could not detect any radial metallicity orage gradient in Leo II (Koch et al. 2007, AJ, 133, 270).

Another important aspect of the nature and evolution ofdSphs is the study of their kinematical properties. These low-mass galaxies are believed to be the smallest cosmologicalstructures containing dark matter. By measuring radial veloc-ities in the remote dSph Leo I, I showed that the resultingvelocity dispersion profile is essentially flat out to the nominaltidal radius. The nondetection of any apparent velocity gradientacross the galaxy supports the negligible role of Galactic tides

in the course of its whole evolution. The application of dy-namical modeling under the assumption of an isotropic ve-locity distribution then yielded mass and density profiles.Moreover, the behavior of the velocity anisotropy was ana-lyzed. The resulting high mass-to-light ratio of Leo I is sup-portive of the idea that all dSphs share a common dark halomass scale of ∼4 # 107 M,, so that the pure velocity (dis-persion) information of such a system is a direct proxy formass. All this argues in favor of a general dark matter dom-inance in the dSphs and renders the hypothesis that thesesystems are of tidal origin less likely (Koch et al. 2007, ApJ,657, in press [astro-ph/0611372]).

Finally, information about the origin and evolution of thedSphs can be gleaned by examining their spatial distributionaround their host galaxies. It has often been reported that inthe Milky Way system, the dSphs are aligned along one ormore great circles or polar planes. Hence, I reconstructed thethree-dimensional distribution of the entire M31 satellite sam-ple. By applying detailed statistical methods, I could demon-strate that seven out of 16 satellites are located within a thinpolar sheet. One reason for this planar alignment could be thebreakup of a common progenitor that was orbiting M31. Alsoplausible is that the dSphs fell in along the filamentary darkmatter structure of the cosmic web, which is underscored bythe fact that the plane extends in the direction of nearby galaxygroups (Koch & Grebel 2006, AJ, 131, 1405).

This work was supported by grants from the Swiss NationalScience Foundation.

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