We estimate the potential contribution of M < 10^9 Msun dwarf galaxies to the reionization and early metal-enrichment of the Milky Way environment, or circum-Galactic Medium. Our approach is to use the observed properties of ancient stars (> 12 Gyr o
ld) measured in nearby dwarf galaxies to characterize the star-formation at high-z. We use a merger-tree model for the build-up of the Milky Way, which self-consistently accounts for feedback processes, and which is calibrated to match the present-day properties of the Galaxy and its dwarf satellites. We show that the high-z analogues of nearby dwarf galaxies can produce the bulk of ionizing radiation (>80%) required to reionize the Milky Way environment. Our fiducial model shows that the gaseous environment can be 50% reionized at z ~ 8 by galaxies with 10^7 Msun < M < 10^8 Msun. At later times, radiative feedback stops the star-formation in these small systems, and reionization is completed by more massive dwarf galaxies by z_rei = 6.4pm 0.5. The metals ejected by supernova-driven outflows from M < 10^9 Msun dwarf galaxies almost uniformly fill the Milky Way environment by z ~ 5, enriching it to Z ~ 2 10^-2 Zsun. At z ~ 2 these early metals are still found to represent ~ 50% of the total mass of heavy elements in the circum-Galactic Medium.
The ages of individual Red Giant Branch stars (RGB) can range from 1 Gyr old to the age of the Universe, and it is believed that the abundances of most chemical elements in their photospheres remain unchanged with time (those that are not affected by
the 1st dredge-up). This means that they trace the ISM in the galaxy at the time the star formed, and hence the chemical enrichment history of the galaxy. CMD analysis has shown the Carina dwarf spheroidal (dSph) to have had an unusually episodic star formation history (SFH) which is expected to be reflected in the abundances of different chemical elements. We use the VLT-FLAMES spectrograph in HR mode (R~20000) to measure the abundances of several chemical elements in a sample of 35 RGB stars in Carina. We also combine these abundances with photometry to derive age estimates for these stars. This allows us to determine which of two distinct star formation (SF) episodes the stars in our sample belong to, and thus to define the relationship between SF and chemical enrichment during these two episodes. As is expected from the SFH, Carina contains two distinct populations of RGB stars: one old (>10 Gyr), which we have found to be metal-poor ([Fe/H]<-1.5), and alpha-rich ([Mg/Fe]>0.0); the other intermediate age (~2-6 Gyr), which we have found to have a metallicity range (-1.8<[Fe/H]<-1.2) with a large spread in [alpha/Fe] abundance, going from extremely low values ([Mg/Fe]<-0.3) to the same mean values as the older population (<[Mg/Fe]>~0.3). We show that the chemical enrichment history of the Carina dSph was different for each SF episode. The earliest was short (~2-3 Gyr) and resulted in the rapid chemical enrichment of the whole galaxy to [Fe/H] ~ -1.5 with both SNe II and SNe Ia contributions. The subsequent episode occured after a gap of ~3-4 Gyr and appears to have resulted in relatively little evolution in either [Fe/H] or [alpha/Fe].
One of the key science goals for a diffraction limited imager on an Extremely Large Telescope (ELT) is the resolution of individual stars down to faint limits in distant galaxies. The aim of this study is to test the proposed capabilities of a multi-
conjugate adaptive optics (MCAO) assisted imager working at the diffraction limit, in IJHK$_s$ filters, on a 42m diameter ELT to carry out accurate stellar photometry in crowded images in an Elliptical-like galaxy at the distance of the Virgo cluster. As the basis for realistic simulations we have used the phase A studies of the European-ELT project, including the MICADO imager (Davies & Genzel 2010) and the MAORY MCAO module (Diolaiti 2010). We convolved a complex resolved stellar population with the telescope and instrument performance expectations to create realistic images. We then tested the ability of the currently available photometric packages STARFINDER and DAOPHOT to handle the simulated images. Our results show that deep Colour-Magnitude Diagrams (photometric error, $pm$0.25 at I$ge$27.2; H$ge$25. and K$_sge$24.6) of old stellar populations in galaxies, at the distance of Virgo, are feasible at a maximum surface brightness, $mu_V sim$ 17 mag/arcsec$^2$ (down to M$_I > -4$ and M$_H sim$ M$_K > -6$), and significantly deeper (photometric error, $pm$0.25 at I$ge$29.3; H$ge$26.6 and K$_sge$26.2) for $mu_V sim$ 21 mag/arcsec$^2$ (down to M$_I ge -2$ and M$_H sim$ M$_K ge -4.5$). The photometric errors, and thus also the depth of the photometry should be improved with photometry packages specifically designed to adapt to an ELT MCAO Point Spread Function. We also make a simple comparison between these simulations and what can be expected from a Single Conjugate Adaptive Optics feed to MICADO and also the James Webb Space Telescope.
We present wide-field photometry of resolved stars in the nearby Sculptor dwarf spheroidal galaxy using CTIO/MOSAIC, going down to the oldest Main Sequence Turn-Off. The accurately flux calibrated wide field Colour-Magnitude Diagrams can be used to c
onstrain the ages of different stellar populations, and also their spatial distribution. The Sculptor dSph contains a predominantly ancient stellar population (>10 Gyr old) which can be easily resolved into individual stars. A galaxy dominated by an old population provides a clear view of ancient processes of galaxy formation unimpeded by overlying younger populations. By using spectroscopic metallicities of RGB stars in combination with our deep Main Sequence Turn-Off photometry we can constrain the ages of different stellar populations with particular accuracy. We find that the known metallicity gradient in Sculptor is well matched to an age gradient. This is the first time that this link with age has been directly quantified. This gradient has been previously observed as a variation in Horizontal Branch properties and is now confirmed to exist for Main Sequence Turn-Offs as well. It is likely the Sculptor dSph first formed an extended metal-poor population at the oldest times, and subsequent more metal-rich, younger stars were formed more towards the centre until the gas was depleted or lost roughly 7 Gyr ago. The fact that these clear radial gradients have been preserved up to the present day is consistent with the apparent lack of signs of recent tidal interactions.
For the first time we show the detailed late-stage chemical evolution history of small nearby dwarf spheroidal galaxy in the Local Group. We present the results of a high resolution (R$sim$20000) FLAMES/GIRAFFE abundance study at ESO/VLT of 81 photom
etrically selected red giant branch stars in the central 25$$ of the Fornax dwarf spheroidal galaxy. We present abundances of alfe (Mg, Si, Ca and Ti), iron-peak elements (Fe, Ni and Cr) and heavy elements (Y, Ba, La, Nd and Eu). Our sample was randomly selected, and is clearly dominated by the younger and more metal rich component of Fornax which represents the major fraction of stars in the central region. This means that the majority of our stars are 1$-$4 Gyr old, and thus represent the end phase of chemical evolution in this system. Our sample of stars has unusually low [$alpha$/Fe], [Ni/Fe] and [Na/Fe] compared to the Milky Way stellar populations at the same [Fe/H]. The particularly important role of stellar winds from low metallicity AGB stars in the creation of s-process elements is clearly seen from the high [Ba/Y]. Furthermore, we present evidence for an s-process contribution to Eu.
Dwarf spheroidal galaxies in the Local Group are usually located close to the Milky Way or M31. Currently, there are two clear exceptions to this rule, and the Tucana dwarf galaxy is the most distant at almost 1 Mpc from the Milky Way. Using the VLT/
FORS2 spectrograph in multi-object mode we were able to measure the velocities of 23 individual Red Giant Branch stars in and around Tucana using the Ca triplet absorption lines. From this sample 17 reliable members have been identified. We measured the systemic velocity and dispersion of Tucana to be v_hel = +194.0+-4.3 km/s and sigma_l.o.s. = 15.8+4.1-3.1 km/s respectively. These measures are derived after removing the signature of rotation using a linear gradient of 6.5 x R/R_core+-2.9 km/s, which corresponds to a rotation of 16 km/s at the reliable limit of our data. Our systemic velocity corresponds to a receding velocity from the barycentre of the Local Group of v_LG = +73.3 km/s. We also determined the mean metallicity of Tucana to be [Fe/H] = -1.95+-0.15 with a dispersion of 0.32+-0.06 dex. Our study firmly excludes any obvious association of Tucana with the HI emission in the vicinity and shows that Tucana is a genuine dwarf spheroidal, with low metallicity stars, no gaseous ISM and no recent star formation. The present location and relatively high recession velocity are consistent with Tucana having been an isolated Local Group galaxy for the majority of its existence.
Blue straggler star (BSS) candidates have been observed in all old dwarf spheroidal galaxies (dSphs), however whether or not they are authentic BSSs or young stars has been a point of debate. To both address this issue and obtain a better understandi
ng of the formation of BSSs in different environments we have analysed a sample of BSS candidates in two nearby Galactic dSphs, Draco and Ursa Minor. We have determined their radial and luminosity distributions from wide field multicolour imaging data extending beyond the tidal radii of both galaxies. BSS candidates are uniformly distributed through the host galaxy, whereas a young population is expected to show a more clumpy distribution. Furthermore, the observed radial distribution of BSSs, normalized to both red giant branch (RGB) and horizontal branch (HB) stars, is almost flat, with a slight decrease towards the centre. Such a distribution is at odds with the predictions for a young stellar population, which should be more concentrated. Instead, it is consistent with model predictions for BSS formation by mass transfer in binaries (MT-BSSs). Such results, although not decisive, suggest that these candidates are indeed BSSs and that MT-BSSs form in the same way in Draco and Ursa Minor as in globular clusters. This favours the conclusion that Draco and Ursa Minor are truly fossil galaxies, where star formation ceased completely more than 8 billion years ago.
