Properties of Dwarf Ellipticals in the Coma Cluster Color gradients in cluster dwarfs Photometric and Structural Properties of Dwarf Ellipticals in the Coma Cluster M. den Brok1, R.F. Peletier1, M. Balcells2, D. Carter3, G. Verdoes-Kleijn1, E.A. Valentijn1 1Kapteyn Astronomical Institute, Groningen, The Netherlands, 2IAC, La Laguna, Tenerife, Spain 3Liverpool John Moores University, Birkenhead, Wirral, UK Introduction Internal colour gradients in galaxies probe the variation of stellar populations with radius. Theoretical models predict that galaxies which have formed through monolithic collapse exhibit steep metallicity gradients (Carlberg, 1984), whereas formation by hierarchical merging yields less steep gradients (White, 1980). Studies of elliptical galaxies have shown that metallicity gradients are primarily negative (Peletier e.a. 1990, Goudfrooij e.a. 1994) The situation in dwarf ellipticals is however not so clear. Van Zee e.a. (2004) study a sample of Virgo cluster dwarf ellipticals and find both positive and negative gradients. A detailed study of gradients in dwarf ellipticals is therefore interesting and may shed light, not only on the formation mechanisms, but also on environmental processes which may regulate the later evolution of dwarf galaxies, such as ram pressure stripping or harassment. On this poster we present results on colour gradients of dwarf ellipticals in the Coma cluster. We compare our results with those of larger elliptical galaxies to see if the formation and evoltion of dwarf galaxies is different from a stellar populations point of view. In addition to this, we compare our results with other structural and photometric properties such as the presence and populations of nuclear star clusters, and also the environment. The coma cluster, being the nearest (m-M=35.0) very rich cluster, is an excellent environment for studying galaxy evolution and important as a zeropoint for high-redshift studies. Data and Sample We use very deep high-resolution data from the HST Coma ACS Survey (Carter e.a. 2008). This survey covers the core of the Coma cluster in two passbands (F475W, F814W) and also has some observations near the outskirts of the cluster. Our sample consists of bright dwarf galaxies and higher mass early type galaxies (F814W(AB) < -15). Most of these are spectroscopically confirmed members, but some low surface brightness dwarfs were selected by eye (N. Trentham and H.C. Ferguson, priv. comm.) Spectroscopic follow-up of a subset of these sources has shown that the dwarf in our sample are likely all members of Coma (Chiboucas e.a. in prep). Results The Figure below shows our colour gradient results as a function of host galaxy magnitude. Fainter galaxies have less steep gradients, which even become positive for dwarf galaxies. However, most dwarf galaxies have central nuclear star clusters, which can have a different colour than the host galaxy. In the same Figure we also show colour gradients in the outer parts of galaxies as a function of host galaxy magnitude. Still, we see that fainter galaxies have less steep gradients than brighter galaxies, but now they are essentially all negative. Compact galaxies (in red, identified by Price et al.) are clearly outliers. We also show the metallicity gradients from Sanchez-Blazquez e.a. (blue) and Spolaor e.a. (green), converted to colour gradients. Our outer gradients disagree with the mass-gradient relation from Spolaor and also our scatter is much higher. This plot shows that the superior resolution of HST is really necessary for separating different structural components, and furthermore that we can probe population gradients in much fainter galaxies. Structural Parameters The results our colour gradients as a function of other structural parameters are shown below. A priori, one would expect a strong correlation of colour gradient with mass, and hence with magnitude.all of these correlate with magnitude. The compact galaxies (in red) stand out again. Environment We have tried to search for possible dependence of colour gradients on environment. The Figure below shows that there is not much evidence for dependence of colour gradient on clustercentric distance. Because dwarf ellipticals and large ellipticals have different clustering properties, we have tried to divide out the global trend in the colour gradient-magnitude relation. Still, we do not see any influence of the environment on colour gradients. Conclusions - Colour gradients (and hence metallicity gradients) are primarily negative in early type galaxies, but become flatter for dwarf galaxies. - Almost all dwarf galaxies have nuclear star clusters, which can affect the fit of the colour gradient. There are however no differences in colour gradients between galaxies with and without nuclear point source. - Our colour gradients are consistent with galaxies built up through an early monolithic collapse fase, followed later by a central starburst, although simulations still have problems reproducing the observations. - We do not see any evidence for the influence of environment on colour gradients, except for compact galaxies, which have steeper gradients than is expected on basis of their magnitude. References Carlberg, R.G., 1984, ApJ 286, 416 Carter, D. et al., 2008, ApJS 176, 424 Ferrarese, L. et al. 2006, ApJ 644, L21 Goudfrooij, P. et al., 1994, A&AS 104, 79 Kawata, D. & Gibson, B.K. 2003 MNRAS 340, 908 Peletier, R.F. et al., 1990, AJ 100, 1091 Pipino, A. et al. 2010, MNRAS accepted Price, J. et al. 2009, MNRAS, 397, 181 Sanchez-Blazques, P. et al. 2006, A&A 457, 823 Spolaor, M. et al. 2009, ApJ 691, 138 Van Zee, L. et al., 2004, AJ 128, 2797 White, S.D.M, 1980, MNRAS 191, 1 Simulations In the Figure below we compare our observations with simulations of metallicity gradients from Kawata et al. (dashed line) and Pipino et al. which are essentially revised monolithic collapse models of elliptical galaxies (cyan, red and blue correspond to large ellipticals, small ellipticals and bulges). Both simulations fail to match the observations, although the simulations from Pipino do get similar scatter, which is obtained in the simulations by changing the star formation efficiency, and the simulations from Kawata reproduce better the global trend that gradients become less steep for low mass galaxies. Nuclear properties Nuclei are places where violent physical processes play an important role and are therefore interesting to study. While perhaps all large early-type galaxies contain supermassive black holes, dwarf ellipticals often contain central star clusters. The formation of these nuclear star clusters is not understood - they may be survivers of star clusters in harassed spirals, or formed during primordial star formation, or inspiralling globular clusters. Although there is strong evidence for correlations with host galaxy properties (see e.g. Ferrarese e.a.), the local density and pressure due to the intergalactic medium may play a role in triggering star formation and retaining gas. Below, we confirm this by plotting the magnitude of the central nuclear star cluster (if detected) against the host galaxy magnitude. Analysis For each dwarf galaxy, we measure the azimuthally averaged surface brightness profile in each of the to a common resolution convolved passbands. We fit a scale free logarithmic gradient to the colour profile. Additionally, we fit structural parameters to each galaxy with a Bayesian code, which helps us distinguish between models with and without central point source. with center without center Carter et al. 2008 Surface brightness profile of a galaxy with nuclear point Colour profile of a dwarf galaxy: the center is much bluer
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