What Shapes Dwarf Spheroidal Galaxies? the Case of Andromeda II and Fornax 1 1 2 1 Andrés Del Pino , Ewa L
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What shapes dwarf spheroidal galaxies? The case of Andromeda II and Fornax 1 1 2 1 Andrés del Pino , Ewa L. Lokas , Sebastian L. Hidalgo , Sylvain Fouquet . 1Nicolaus Copernicus Astronomical Center (CAMK), 2Instituto de Astrofísica de Canarias (IAC) We present a comprehensive photometric study of the Fornax and the Andromeda II dSph galaxies. It is based on the up-to-date deepest photometric data for both galaxies. We have derived their detailed star formation histories (SFHs) as a function of galactocentric radius. This allowed us to analyze in detail the spatial distribution of their different stellar populations in wider field-of-view photometry (WFOVP) lists for both galaxies. Our results show that these galaxies may be the remants of mergers between gas-rich dwarf galaxies at z ~ 1-2. FORNAX Star Formation Histories ANDROMEDA II Detailed SFHs are the key ingredient in our method. They require deep and precise photometry, reaching at least 1 magnitude below the oldest main sequence turn-off (oMSTO). We used the deepest photometry available today (from VLT and HST) to derive the SFHs (Fig. 1, 2) using the CMD- fitting techniques (del Pino et al. 2013, Hidalgo et al. in prep). Sampling the CMD The study of possible spatial gradients requires the WFOVP covering the whole body of the galaxy. We assigned ages and metallicities to stars present in our WFOVP Fig. 1: The star formation rate (top panel), metallicity using their position in the CMD and the Fig. 2: Same as Fig. 1 for Andromeda II (Hidalgo et al. in (middle panel) and the cumulative stellar mass fraction prep.). (bottom panel) for Fornax as a function of time (del Pino et derived SFH (Fig. 3). al. 2013, 2015). Maps and Radial Profiles We selected different stellar populations in the WFOVP using the age and metallicity of their stars. Fig. 4 shows surface density maps for 5 distinct stellar populations in Fornax. Fig. 6 shows the radial density profiles for the old, young and total population in Andromeda II. In the case of Fornax, strong asymmetries were found in its younger populations. Andromeda II shows two stellar populations. One old (> 10 Gyr), more extended, and a younger one, clearly more concentrated (See also Fig. 5). Fig. 3: CMD from the WFOVP of Fornax and the 6 regions defined for the study of the spatial distribution of stars (Fig. 4). From left to right: observed stars, expected age and metallicity from the SFH (del Pino et al. 2015). Fig. 5: Same as Fig. 4 for Andromeda II. 1 2 3 4 5 Fig. 6: Radial surface density profiles for the two stellar populations found in Andromeda II. Sersic profiles have been fitted to the data. Residuals from the fitting are shown in the bottom panel. Fig. 4: Spatial distribution maps for the 6 populations defined in the CMD (Fig. 3). Panels are ordered from old to young populations and labelled with the approximate age range. Colour scale is the same for all panels and indicates the concentration of stars. Black narrow-lined ellipses indicate the best fits to isopleths. The blue dashed-lined ellipse References Lokas, E. et al. 2014, MNRAS, 445,6 represents the tidal radius of Fornax, while its core radius is shown by a blue solid-lined ellipse. Small blue open Coleman, M. et al. 2004, AJ, 127, 832 del Pino, A. et al. 2015, MNRAS, 454, 3996 circles mark the positions of globular clusters. The position of the shell found by Coleman et al. (2004) is marked at Ho, N. et al. 2012 ApJ, 758, 124 Fouquet, S. et al. 2016 (In preparation) the south-east of the core of the galaxy (del Pino et al. 2015). del Pino, A. et al. 2013, MNRAS, 413,1505 Hidalgo, S. L. et al. 2016 (In preparation) Both galaxies are complex and rare. The strong asymmetries found in the young stellar populations of Fornax suggest an interaction with another system. This event would have triggered the star formation in the galaxy and could also explain the formation of shell-like structures (del Pino et al. 2015). Andromeda II, on the other hand, shows two distinct populations, differentiated both spatially and by age, and in addition an overdensity of stars in the centre (Fig. 6). This, together with the anomalous rotation (Ho et al. 2012), lead us to propose a merger scenario to explain its properties (Lokas et al. 2014, Fouquet et al. 2016, in prep. del Pino et al. 2016, in prep.)..