The ACS LCID project. V. The Star Formation History of the Dwarf Galaxy \objectname[]{LGS-3}: Clues for Cosmic Reionization and Feedback

We present an analysis of the star formation history (SFH) of the transition-type (dIrr/dSph) Local Group galaxy \objectname[]{LGS-3} (Pisces) based on deep photometry obtained with the {\it Advanced Camera for Surveys} onboard the {\it Hubble Space Telescope}. Our analysis shows that the SFH of \objectname[]{LGS-3} is dominated by a main episode $\sim 11.7$ Gyr ago with a duration of $\sim$ 1.4 Gyr which formed $\sim 90$ of the stars. Subsequently, \objectname[]{LGS-3} continued forming stars until the present, although at a much lower rate. The lack of early chemical enrichment is in contrast to that observed in the isolated dSph galaxies of comparable luminosity, implying that the dSphs were more massive and subjected to more tidal stripping. We compare the SFH of \objectname[]{LGS-3} with expectations from cosmological models. Most or all the star formation was produced in \objectname[]{LGS-3} after the reionization epoch, assumed to be completed at $z\sim6$ or $\sim 12.7$ Gyr ago. The total mass of the galaxy is estimated to be between 2 and $4\times 10^8$ M$_\odot$, corresponding to circular velocities between 28 km\ s$^{-1}$ to 36 km\ s$^{-1}$. These values are close to but somewhat above the limit of 30 km\ s$^{-1}$ below which the UV background is expected to prevent any star formation after reionization. Feedback from SNe associated with the initial episode of star formation (mechanical luminosity from SNe $L_w=5.3\times 10^{38}$ erg s$^{-1}$) is probably inadequate to completely blow away the gas. However, the combined effects of SN feedback and UV background heating might be expected to completely halt star formation at the reionization epoch for the low mass of \objectname[]{LGS-3}; this suggests that self-shielding is important to the early evolution of galaxies in this mass range.Comment: accepted to be published at Ap

The Hubble Space Telescope Survey of M31 Satellite Galaxies. II. The Star Formation Histories of Ultrafaint Dwarf Galaxies

We present the lifetime star formation histories (SFHs) for six ultrafaint dwarf (UFD; M V > − 7.0, 4.9<log10(M*(z=0)/M⊙)<5.5 ) satellite galaxies of M31 based on deep color–magnitude diagrams constructed from Hubble Space Telescope imaging. These are the first SFHs obtained from the oldest main-sequence turnoff of UFDs outside the halo of the Milky Way (MW). We find that five UFDs formed at least 50% of their stellar mass by z = 5 (12.6 Gyr ago), similar to known UFDs around the MW, but that 10%–40% of their stellar mass formed at later times. We uncover one remarkable UFD, And xiii, which formed only 10% of its stellar mass by z = 5, and 75% in a rapid burst at z ∼ 2–3, a result that is robust to choices of underlying stellar model and is consistent with its predominantly red horizontal branch. This “young” UFD is the first of its kind and indicates that not all UFDs are necessarily quenched by reionization, which is consistent with predictions from several cosmological simulations of faint dwarf galaxies. SFHs of the combined MW and M31 samples suggest reionization did not homogeneously quench UFDs. We find that the least-massive MW UFDs (M *(z = 5) ≲ 5 × 104 M ⊙) are likely quenched by reionization, whereas more-massive M31 UFDs (M *(z = 5) ≳ 105 M ⊙) may only have their star formation suppressed by reionization and quench at a later time. We discuss these findings in the context of the evolution and quenching of UFDs

Fitting the integrated Spectral Energy Distributions of Galaxies

Fitting the spectral energy distributions (SEDs) of galaxies is an almost universally used technique that has matured significantly in the last decade. Model predictions and fitting procedures have improved significantly over this time, attempting to keep up with the vastly increased volume and quality of available data. We review here the field of SED fitting, describing the modelling of ultraviolet to infrared galaxy SEDs, the creation of multiwavelength data sets, and the methods used to fit model SEDs to observed galaxy data sets. We touch upon the achievements and challenges in the major ingredients of SED fitting, with a special emphasis on describing the interplay between the quality of the available data, the quality of the available models, and the best fitting technique to use in order to obtain a realistic measurement as well as realistic uncertainties. We conclude that SED fitting can be used effectively to derive a range of physical properties of galaxies, such as redshift, stellar masses, star formation rates, dust masses, and metallicities, with care taken not to over-interpret the available data. Yet there still exist many issues such as estimating the age of the oldest stars in a galaxy, finer details ofdust properties and dust-star geometry, and the influences of poorly understood, luminous stellar types and phases. The challenge for the coming years will be to improve both the models and the observational data sets to resolve these uncertainties. The present review will be made available on an interactive, moderated web page (sedfitting.org), where the community can access and change the text. The intention is to expand the text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics & Space Scienc

The JWST Resolved Stellar Populations Early Release Science Program. II. Survey Overview

We present the JWST Resolved Stellar Populations Early Release Science (ERS) program. We obtained 27.5 hr of NIRCam and NIRISS imaging of three targets in the Local Group (Milky Way globular cluster M92, ultrafaint dwarf galaxy Draco II, and star-forming dwarf galaxy WLM), which span factors of similar to 10(5) in luminosity, similar to 10(4) in distance, and similar to 10(5) in surface brightness. We describe the survey strategy, scientific and technical goals, implementation details, present select NIRCam color-magnitude diagrams (CMDs), and validate the NIRCam exposure time calculator (ETC). Our CMDs are among the deepest in existence for each class of target. They touch the theoretical hydrogen-burning limit in M92 (&lt;0.08 M-circle dot; M-F090W similar to +13.6), include the lowest-mass stars observed outside the Milky Way in Draco II (0.09M(circle dot); M-F090W similar to +12.1), and reach similar to 1.5 mag below the oldest main-sequence turnoff in WLM (M-F090W similar to +4.6). The PARSEC stellar models provide a good qualitative match to the NIRCam CMDs, though they are similar to 0.05 mag too blue compared to M92 F090W - F150W data. Our CMDs show detector-dependent color offsets ranging from similar to 0.02 mag in F090W - F150W to similar to 0.1 mag in F277W - F444W; these appear to be due to differences in the zero-point calibrations among the detectors. The NIRCam ETC (v2.0) matches the signal-to-noise ratios based on photon noise in uncrowded fields, but the ETC may not be accurate in more crowded fields, similar to what is known for the Hubble Space Telescope. We release the point-source photometry package DOLPHOT, optimized for NIRCam and NIRISS, for the community

Kepler-1649b: : An Exo-Venus in the Solar Neighborhood

Angelo, et al, 'Kepler-1649b: An Exo-Venus in the Solar Neighborhood', The Astronomical Journal, 153:162 (8pp), 2017 April. The version of record is availalbe online at doi: https://doi.org/10.3847/1538-3881/aa615f. © 2017. The American Astronomical Society. All rights reservedThe Kepler mission has revealed that Earth-sized planets are common, and dozens have been discovered to orbit in or near their host star's habitable zone. A major focus in astronomy is to determine which of these exoplanets are likely to have Earth-like properties that are amenable to follow-up with both ground- and future space-based surveys, with an ultimate goal of probing their atmospheres to look for signs of life. Venus-like atmospheres will be of particular interest in these surveys. While Earth and Venus evolved to have similar sizes and densities, it remains unclear what factors led to the dramatic divergence of their atmospheres. Studying analogs to both Earth and Venus can thus shed light on the limits of habitability and the potential for life on known exoplanets. Here we present the discovery and confirmation of Kepler-1649b, an Earth-sized planet orbiting a nearby M5V star that receives incident flux at a level similar to that of Venus. We present our methods for characterizing the star, using a combination of PSF photometry, ground-based spectroscopy and imaging, to confirm the planetary nature of Kepler-1649b. Planets like Kepler-1649b will be prime candidates for atmospheric and habitability studies in the next generation of space missions.Peer reviewedFinal Published versio