The DART imaging and CaT survey of the Fornax dwarf spheroidal galaxy

Abstract

Aims. As part of the DART project we have used the ESO/2.2m Wide Field Imager in conjunction with the VLT/FLAMES GIRAFFE spectrograph to study the detailed properties of the resolved stellar population of the Fornax dwarf spheroidal galaxy out to and beyond its tidal radius. Fornax dwarf spheroidal galaxy has had a complicated evolution and contains significant numbers of young, intermediate age and old stars. We investigate the relation between these different components by studying their photometric, kinematic and abundance distributions. Methods. We re-derived the structural parameters of the Fornax dwarf spheroidal using our wide field imaging covering the galaxy out to its tidal radius, and analysed the spatial distribution of the Fornax stars of different ages as selected from colour-magnitude diagram analysis. We have obtained accurate velocities and metallicities from spectra in the Ca II triplet wavelength region for 562 Red Giant Branch stars which have velocities consistent with membership of the Fornax dwarf spheroidal. Results. We have found evidence for the presence of at least three distinct stellar components: a young population (few 100 Myr old) concentrated in the centre of the galaxy, visible as a Main Sequence in the colour-magnitude diagram; an intermediate age population (2-8 Gyr old); and an ancient population (>10 Gyr), which are distinguishable from each other kinematically, from the metallicity distribution and in the spatial distribution of stars found in the colour-magnitude diagram. Conclusions. From our spectroscopic analysis we find that the "metal rich" stars ([Fe/H] > −1.3) show a less extended and more concentrated spatial distribution, and display colder kinematics than the "metal poor" stars ([Fe/H] < −1.3). There is tentative evidence that the ancient stellar population in the centre of Fornax does not exhibit equilibrium kinematics. This could be a sign of a relatively recent accretion of external material, such as the merger of another galaxy or other means of gas accretion at some point in the fairly recent past, consistent with other recent evidence of substructure (Coleman et al.

Mergers and the outside-in formation of dwarf spheroidals

“…These gradients are particularly clear in some galaxies and, when accompanied by correlated differences in kinematics and metallicity, are often interpreted as evidence for distinct stellar components. The Sculptor dSph makes a compelling example of a system with at least two distinct components (Tolstoy et al 2004;Battaglia et al 2008), but so do other dwarfs such as Sextans (Battaglia et al 2011); the And II companion to M31 (McConnachie et al 2007;Amorisco et al 2014); and Fornax (Battaglia et al 2006;Walker & Peñarrubia 2011). We note, however, that clear-cut evidence for distinct components is often weak, and that these may also be interpreted as simply cases where the radial gradients are particularly pronounced.…”

“…Top arrows indicate the projected half-mass radius of each component; the rest of the arrows show the same, but for the other simulated dwarfs (profiles not shown). The arrows labelled "Fornax" correspond to the half-number radii of the intermediate-age and older populations derived by Battaglia et al (2006). Note that the old/metal-poor component is always less centrally concentrated than their young/metal-rich counterpart.…”

“…They are not currently forming stars and show prominent old stellar populations, but their star formation histories (SFHs) are not uniformly old (see the recent reviews of Tolstoy et al 2009;Weisz et al 2014). Indeed, dSphs show a wide variety of SFHs; from systems that essentially completed forming their stars ∼ 10 Gyr ago, like Cetus and Draco (Monelli et al 2010;Aparicio et al 2001) to others where star formation was sustained over a long time and ceased only as recently as 6 Gyr ago (e.g., Leo II, Dolphin 2002) or even < 1 Gyr ago (e.g., Fornax, Stetson et al 1998;Saviane et al 2000;Battaglia et al 2006;de Boer et al 2012, and references therein).…”

The imprint of reionization on the star formation histories of dwarf galaxies

“…Battaglia et al 2006Battaglia et al , 2011, and nearby field dwarfs such as KKH37 and DDO6 (Weisz et al 2011). The best studied examples indicate that the two components differ not only in age, but also in their metallicity, spatial distribution, and kinematics (Walker & Peñarrubia 2011).…”

About the Chemical Evolution of dSphs (and the peculiar Globular Cluster ω Cen)

“…We finally stress that our model envisages a central depression in the radial distribution of the stellar velocity dispersion (Marcolini et al 2008). This naturally entails two different stellar populations with an anti-correlation between [Fe/H] and velocity dispersion, which has been observed in the case of the Sculptor dSph (Tolstoy et al 2004) and the Fornax dSph (Battaglia et al 2006). …”