The star formation history (SFH) of Local Group galaxies is a powerful tool for studying their evolution, including chemical enrichment histories and stellar population gradients, which in turn may shed light on the role of reionization or Supernovae feedback in galaxy formation and evolution. In particular, isolated dwarfs are ideal laboratories since their evolution has not been complicated by the vicinity of giant galaxies. In this paper, we present the project Local Cosmology from Isolated Dwarfs (LCID), aimed at deriving detailed SFHs for a sample of Local Group isolated dwarf galaxies. To accomplish this goal we have collected, using the ACS on board the HST, color-magnitude diagrams (CMD) reaching their oldest main sequence turnoff (V≃ 28 or MV ≃ + 3.5) with good photometric accuracy. Some preliminary results from the CMDs are shown and briefly discussed.

We have obtained deep images of the Local Group dwarf irregular galaxy Leo A using the Hubble Space Telescope Advanced Camera for Surveys. From these images we have constructed the deepest ever color-magnitude diagram (CMD) for an irregular galaxy beyond the Magellanic Clouds. We derive the star-formation rate (SFR) as a function of time over the entire history of the galaxy. We find that over 90% of Leo A's star formation has occurred more recently than 8 Gyr ago. Our measurements are not inconsistent with a scenario in which all of the ancient star formation occurred prior to the end of the era of reionization, with subsequent suppression until lookback times corresponding to redshift z ≈ 1. However, given Leo A's strongly isolated state and its dynamical mass of ≳ 108 M⊙, its large gas supply was more likely to have been merely heated than driven out of the galaxy altogether.

The Ks-band magnitude distribution of carbon-rich and oxygen-rich asymptotic giant branch stars within Local Group galaxies like the Magellanic Clouds, NGC 6822, M33 and SagDIG is easily obtained from ground-based observations. Appropriate stellar evolutionary models covering a range of metallicities and star formation rates are used to produce theoretical distributions that allow us to derive the history of star formation across these galaxies. I will show the result of these studies and discuss the application of this technique to more distant systems as well as deeper observations, like those that VISTA will provide, to improve our understanding of, in particular, the Magellanic Clouds.

We present deep ground-based B and R observations of 12 fields in the Small Magellanic Cloud (SMC). The resulting color-magnitude diagrams (CMDs) reach the oldest main-sequence (MS) turnoff at MR∼3.5 and reveal the stellar population differences between the part of the galaxy facing the Large Magellanic Cloud (LMC) and an area on the opposite side. In the Southern part of the galaxy, we found that there are still intermediate-age stars as far as 4 kpc from the SMC center. The Chemical Enrichment History (CEH) in one of our SMC fields is also presented.

We present results from new deep HST/ACS photometry of I Zw 18, the most metal-poor blue compact dwarf galaxy in the nearby universe. It has been previously argued that this is a very young system that started forming stars only ≲500 Gyr ago, but other work has hinted that older (≳1 Gyr) red giant branch (RGB) stars may exist in this galaxy. Our deeper data indeed reveal evidence for an RGB. Underlying old (≳1 Gyr) populations are therefore present in even the most metal-poor systems, implying that star formation started at z ≳ 0.1. The RGB tip (TRGB) magnitude and the properties of Cepheid variables identified from our program indicate that I Zw 18 is farther away (D = 19.0 ± 1.8 Mpc) than previously believed.

The stellar populations of a galaxy are a fossil record of its formation and evolution and the various physical processes involved. Therefore, studying the resolved stellar populations of nearby galaxies can provide important constraints on their structure, formation, and evolution. To that end, we have obtained VI photometry with the Advanced Camera for Surveys on the Hubble Space Telescope for three fields at deprojected radii Rdp ∼ 9 − 13 kpc (∼ 4 − 6 visual scale lengths) along M33's southeast minor axis. We present results for the star formation history (SFH) of these fields based on the technique of synthetic CMD fitting.

We present preliminary results concerning the search for short-period variable stars in Tucana and LGS3 based on very deep HST/ACS imaging. In the one chip per galaxy we studied so far, a total of 230 and 80 candidates variables were found, respectively. For Tucana, we identified 134 of them as RR Lyrae stars (RRL) pulsating in the fundamental mode (RRab), 51 in the first-overtone mode (RRc), and 37 in both modes simultaneoulsy (RRd), as well as four candidate anomalous Cepheids (AC). In the case of LGS3, we found 45 RRab and 5 RRc, plus three candidates RRd and five candidate AC. The metallicities obtained from the mean period of the RRab are [Fe/H]Tuc = − 1.7 and [Fe/H]LGS3 = − 1.8.

We present the deepest, widest color-magnitude diagrams at mid infrared wavelengths ever produced for the Large Magellanic Cloud (LMC). The SAGE (Surveying the Agents of a Galaxy's Evolution) survey of the LMC comprises a seven by seven degree mapping of the entire LMC in each of the Spitzer IRAC and MIPS bands. The present work includes mid and near infrared photometry between 1.25 and 24 microns. These data are used to identify 100's of thousands of red giants and 10's of thousands of asymptotic giant branch stars and supergiants which are classified as oxygen rich, carbon rich, or “extreme” mass losing stars, among others. SAGE will revolutionize the study of evolved stars and mass input into the ISM in the LMC because its deep photometry is sensitive to all significant mass losing sources in the galaxy and provides the context of the overall stellar populations which give rise to the mass losing AGB stars. The SAGE data can be used to help synthesize and assess models of star forming galaxies at high redshift for a range of ages and chemical compositions.

We are using Subaru/Suprime-Cam to survey the luminous resolved stellar populations in the outskirts of galaxies lying beyond the Local Group. We report here on first results from our analysis of the low mass edge-on galaxy, NGC 4244, lying at 4.4 Mpc. Red giant branch stars are clearly resolved in the outer disk and extraplanar regions and our preliminary stellar density maps suggest the presence of an extended and asymmetric stellar component reaching significant distances above the disk plane.

We report the results of our project devoted to study the chemical enrichment history of the field population in the Magellanic Clouds using Ca II triplet spectroscopy.

A new method to extract information from color-magnitude diagrams (CMDs) is presented. We employ a Richardson-Lucy (RL) algorithm to the analysis of the observational uncertainties. The resulting reconstruction is a cleaned data set with which to perform analyses of the star formation history. For this task, synthetic stellar populations are built and compared with the data CMD. As an application, the star formation rate in the solar vicinity is recovered.

Integrated spectra of 17 blue Large Magellanic Cloud (LMC) clusters were obtained in the (3600-6800 Å) range using the CASLEO (Argentina) 2.15 m telescope. The typical resolution and dispersion were 12 Å and 3.5 Å/pixel, respectively. Cluster ages were derived by means of two methods: the template matching, in which the observed spectra are compared and matched to template spectra with well-known determined properties, and the equivalent width (EW) method, in which diagnostic diagrams involving the sum of EWs of selected spectral lines were employed together with their calibrations with age and metallicity given by Santos & Piatti (2004), hereafter SP. The spectra were normalized to Fλ = 1 at ∼ 5870 Å. The EWs of H Balmer, KCaII, G band and MgI were measured within the spectral windows defined by Bica & Alloin (1986). We then obtained the sum of EWs of the 3 metallic lines (Sm) and of the 3 Balmer lines Hβ, Hγ and Hδ (Sh). As a first approach to get cluster ages, we used the diagnostic diagrams defined by SP. The clusters were then age-ranked according to the SP's calibrations. We used Sm to get a first age estimate using: log t(Gyr) = a0 + a1Sm + a2Sm2, where a0 = −2.18 ± 0.38, a1 = 0.188 ± 0.080 and a2 = −0.0030 ± 0.0032. We then used Sh to get a second age estimate guided by the previous Sm estimate, since from Sh two solutions are possible: log t(Gyr) = { − b ± [b2 − 4a(c − Sh)]1/2}/2a, where a = −6.35 ± 0.18, b = −8.56 ± 0.35 and c = 23.32 ± 0.20. The average of these two estimates is listed in column 7 of Table 1.

We present deep B,V,I photometry of three wide fields within the core of the Sagittarius dwarf spheroidal galaxy (Sgr dSph). We provide accurate templates for some well identified sequences of the CMD of the galaxy (Blue HB, Red HB, etc.) as tools to recognize portion of the tidal Stream detected by various surveys, to compare the stellar populations in the main body of Sgr and in its Stream, and to derive accurate distances to Stream stars.

V, I time-series photometry of the new Milky Way satellite recently discovered in the Bootes constellation (Belokurov et al. 2006) was collected at the TNG, WIRO and Loiano telescopes and used to search for variable stars. We have identified a non-negligible population of RR Lyrae variables, which confirms the presence of an old population in this newly discovered structure. The distribution of periods coupled with information on the evolutionary status of the system derived fromthe color-magnitude diagram provide important hints on the nature and history of this intriguing new companion of the Milky Way.

We present the first results of our study of stellar populations in the Large and Small Magellanic Clouds based on multi-band WFPC2 observations of “random” fields taken as part of the “pure parallel” programme carried out with the HST as a service to the community.

We present a brief summary of our current results on the stellar distribution and population gradients of the resolved stars in the surroundings of ∼50 nearby disk galaxies, observed with space- (Hubble & Spitzer) and ground-based telescopes (Subaru, VLT, BTA, Palomar, CFHT, & INT). We examine the radial (in-plane) and vertical (extraplanar) distributions of resolved stars as a function of stellar age and metallicity by tracking changes in the color-magnitude diagram of face-on and edge-on galaxies. Our data show, that the scale length and height of a stellar population increases with age, with the oldest detected stellar populations identified at a large galactocentric radius or extraplanar height, out to typically a few kpc. In the most massive of the studied galaxies there is evidence for a break in number density and color gradients of evolved stars, which plausibly correspond to the thick disk and halo components of the galaxies. The ratio of intermediate-age to old stars in the outermost fields correlate with the gas fraction, while relative sizes of the thick-to-thin disks anticorrelate with galactic circular velocity.

We present VLT spectroscopy of 13 RR Lyrae variables from the QUEST survey that lie in the leading arm of the tidal stream from Sagittarius dSph galaxy. We observed these stars to measure their radial velocities and metal abundances and to complete a sample of stars from the overdensity detected by QUEST in this area of the sky. The full sample contains 42 stars, 29 observed with VLT and 13 observed with GEMINI, we report here the results from the VLT observations so far. We conclude that the new stars and stars reported before by Vivas, Zinn and Gallart (2005) are consistent with belonging to Sgr. We discuss the importance of the final third of the sample, the GEMINI targets, which will provide important constrains on models of the Sgr streams that assume different dark matter halo shapes.

We present a preliminary reconstruction of the star formation history of the Sagittarius dwarf irregular galaxy (SagDIG), a dIrr galaxy at the border of the Local Group. SagDIG is a lively star-forming dwarf galaxy with very low metallicity and an abundant gas reservoir, for which deep HST ACS observations have been obtained (Momany et al. 2005). We have built synthetic color-magnitude diagrams in the intrinsic ACS bands F606W and F814W and compared them with our HST/ACS observations to derive the global star formation history of the galaxy. We find a broad episode of star formation between 3 and 8 Gyr ago, with a low-intensity tail of star formation at older ages, and a SF enhancement in the last Gyr. An upper limit to the old populations is set by modeling the old HB. Our modeling of the color-magnitude diagram also provides some constraints on the chemical enrichment history of SagDIG.

In the framework of an ongoing ACS@HST project (HST program #10586, PI: Aloisi) we have obtained deep multi-color imaging of the very metal-poor Blue Compact Dwarf galaxy IZw18. The data were acquired in time-series fashion to allow the identification of Classical Cepheids (CCs). The main aim of this project is to constrain both the distance and the Star Formation History of the galaxy. However, as a byproduct these data also provide new insights into the properties of CCs at very low metallicities. We have identified 24 candidate CCs in IZw18. New theoretical pulsation models of CCs specifically for the low metallicity of this primordial galaxy (Z=0.0004, Y=0.24) have been computed to interpret our results.

We have applied the Image Subtraction to B,V time-series photometry of four globular clusters of the Fornax dwarf spheoridal galaxy, which were observed with the 2.2m ESO-MPI, the 6.5m Magellan/Clay, the 4m CTIO/Blanco telescopes and the WFPC2 on board of HST. We have identified RR Lyrae stars and obtained well sampled light curves for the clusters' variables. From the periods and pulsation properties of their RR Lyrae stars Fornax globular clusters seem to be of Oosterhoff intermediate type.