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In the current JWST era, rest-frame UV spectra play a crucial role in enhancing our understanding of the interstellar medium (ISM) and stellar properties of the first galaxies in the epoch of reionization (EoR, $z>6$). Here, we compare well-known and reliable optical diagrams sensitive to the main ionization source (i.e., star formation, SF; active galactic nuclei, AGN; shocks) to UV counterparts proposed in the literature - the so-called ``UV-BPT diagrams'' - using the HST COS Legacy Archive Spectroscopic SurveY (CLASSY), the largest high-quality, high-resolution and broad-wavelength range atlas of far-UV spectra for 45 local star-forming galaxies. In particular, we explore where CLASSY UV line ratios are located in the different UV diagnostic plots, taking into account state-of-the-art photoionization and shock models and, for the first time, the measured ISM and stellar properties (e.g., gas-phase metallicity, ionization parameter, carbon abundance, stellar age). We find that the combination of C III] $\lambda\lambda$1907,9 He II $\lambda1640$ and O III] $\lambda$1666 can be a powerful tool to separate between SF, shocks and AGN at sub-solar metallicities. We also confirm that alternative diagrams without O III] $\lambda$1666 still allow us to define a SF-locus with some caveats. Diagrams including C IV $\lambda\lambda$1548,51 should be taken with caution given the complexity of this doublet profile. Finally, we present a discussion detailing the ISM conditions required to detect UV emission lines, visible only in low gas-phase metallicity (12+log(O/H) $\lesssim8.3$) and high ionization parameter (log($U$) $\gtrsim-2.5$) environments. Overall, CLASSY and our UV toolkit will be crucial in interpreting the spectra of the earliest galaxies that JWST is currently revealing., Comment: 31 pages, submitted to ApJ, comments welcome

Observations of low-ionization state (LIS) metal lines provide crucial insights into the interstellar medium of galaxies, yet, disentangling the physical processes responsible for the emerging line profiles is difficult. This work investigates how mock spectra generated using a single galaxy in a radiation-hydrodynamical simulation can help us interpret observations of a real galaxy. We create 22,500 C II and Si II spectra from the virtual galaxy at different times and through multiple lines of sight and compare them with the 45 observations of low-redshift star-forming galaxies from the COS Legacy Spectroscopic SurveY (CLASSY). We find that the mock profiles provide accurate replicates to the observations of 38 galaxies with a broad range of stellar masses ($10^6$ to $10^9$ $M_\odot$) and metallicities (0.02 to 0.55 $Z_\odot$). Additionally, we highlight that aperture losses explain the weakness of the fluorescent emission in several CLASSY spectra and must be accounted for when comparing simulations to observations. Overall, we show that the evolution of a single simulated galaxy can produce a large diversity of spectra whose properties are representative of galaxies of comparable or smaller masses. Building upon these results, we explore the origin of the continuum, residual flux, and fluorescent emission in the simulation. We find that these different spectral features all emerge from distinct regions in the galaxy's ISM, and their characteristics can vary as a function of the viewing angle. While these outcomes challenge simplified interpretations of down-the-barrel spectra, our results indicate that high-resolution simulations provide an optimal framework to interpret these observations., Accepted for publication in Apj

We provide one of the most comprehensive metallicity studies at z ~ 4 by analyzing the UV/optical HST photometry, and rest-frame VLT-FORS2 ultraviolet and VLT-XSHOOTER optical spectra of J0332-3557, a gravitationally lensed galaxy magnified by a factor of 20. With a 5{\sigma} detection of the auroral O III] {\lambda}1666 line, we are able to derive a direct gas metallicity estimate for our target. We find Zgas = 12 + log(O/H) = 8.26 +/- 0.06, which is compatible with an increasing of both the gas fraction and the outflow metal loading factor from z ~ 0 to z ~ 4. J0332 is the most metal-rich individual galaxy at z > 3.6 for which the C/O ratio has been measured. We derive a low log(C/O)= -1.02 +/- 0.2, which suggests that J0332 is in the early stages of ISM carbon enrichment driven mostly by massive stars. The low C/O also indicates that J0332 is characterized by a low star formation efficiency, higher yields of oxygen and longer burst duration. We find that the EW[C III]1906,9 is as low as ~ 3 {\AA}. The main drivers of the low EW[C III]1906,9 are the higher gas metallicity and the low C/O. J0332 is characterized by one diffuse and two more compact regions ~ 1 kpc in size. We find that the carbon emission mostly originates in the compact knots., Comment: 26 pages, 16 figures

We present the JWST Resolved Stellar Populations Early Release Science (ERS) science program. We obtained 27.5 hours of NIRCam and NIRISS imaging of three targets in the Local Group (Milky Way globular cluster M92, ultra-faint dwarf galaxy Draco II, star-forming dwarf galaxy WLM), which span factors of $\sim10^5$ in luminosity, $\sim10^4$ in distance, and $\sim10^5$ 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 ($, 23 pages, 7 Figures, 3 Tables. Submitted to AAS Journals. Comments welcome

We analyze the rest-frame near-UV and optical nebular spectra of three $z > 7$ galaxies from the Early Release Observations taken with the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST). These three high-z galaxies show the detection of several strong-emission nebular lines, including the temperature-sensitive [O III] $λ$4363 line, allowing us to directly determine the nebular conditions and abundances for O/H, C/O, and Ne/O. We derive O/H abundances and ionization parameters that are generally consistent with other recent analyses. We analyze the mass-metallicity relationship (i.e., slope) and its redshift evolution by comparing between the three z > 7 galaxies and local star-forming galaxies. We also detect the C III] $λλ$1907,1909 emission in a z > 8 galaxy from which we determine the most distant C/O abundance to date. This valuable detection of log(C/O) = $-0.83\pm0.38$ provides the first test of C/O redshift evolution out to high-redshift. For neon, we use the high-ionization [Ne III] $λ$3869 line to measure the first Ne/O abundances at z>7, finding no evolution in this $α$-element ratio. We explore the tentative detection of [Fe II] and [Fe III] lines in a z>8 galaxy, which would indicate a rapid build up of metals. Importantly, we demonstrate that properly flux-calibrated and higher S/N spectra are crucial to robustly determine the abundance pattern in z>7 galaxies with NIRSpec/JWST., Accepted for publication on ApJL

The COS Legacy Archive Spectroscopic SurveY (CLASSY) is designed to provide the community with a spectral atlas of 45 nearby star-forming galaxies which were chosen to cover similar properties as those seen at high-z (z>6). The prime high level science product of CLASSY is accurately coadded UV spectra, ranging from ~1000-2000A, derived from a combination of archival and new data obtained with HST's Cosmic Origins Spectrograph (COS). This paper details the multi-stage technical processes of creating this prime data product, and the methodologies involved in extracting, reducing, aligning, and coadding far-ultraviolet (FUV) and near-ultraviolet (NUV) spectra. We provide guidelines on how to successfully utilize COS observations of extended sources, despite COS being optimized for point sources, and best-practice recommendations for the coaddition of UV spectra in general. Moreover, we discuss the effects of our reduction and coaddition techniques in the scientific application of the CLASSY data. In particular, we find that accurately accounting for flux calibration offsets can affect the derived properties of the stellar populations, while customized extractions of NUV spectra for extended sources are essential for correctly diagnosing the metallicity of galaxies via CIII] nebular emission. Despite changes in spectral resolution of up to ~25% between individual datasets (due to changes in the COS line spread function), no adverse affects were observed on the difference in velocity width and outflow velocities of isolated absorption lines when measured in the final combined data products, owing in-part to our signal-to-noise regime of S/N, 31 pages, 17 figures, accepted for publication in ApJS

We report results of a novel high-energy follow-up observation of a potential Fast Radio Burst. The radio burst was detected by VLA/realfast and followed-up by the Neil Gehrels Swift Observatory in very low latency utilizing new operational capabilities of Swift (arXiv:2005.01751), with pointed soft X-ray and UV observations beginning at T0+32 minutes, and hard X-ray/gamma-ray event data saved around T0. These observations are $>10$x faster than previous X-ray/UV follow-up of any radio transient to date. No emission is seen coincident with the FRB candidate at T0, with a 0.2s fluence $5��$ upper limit of $1.35\times10^{-8}$ erg cm$^{-2}$ (14-195 keV) for a SGR 1935+2154-like flare, nor at T0+32 minutes down to $3��$ upper limits of 22.18 AB mag in UVOT u band, and $3.33\times10^{-13}$ erg cm$^{-2}$ s$^{-1}$ from 0.3-10 keV for the 2 ks observation. The candidate FRB alone is not significant enough to be considered astrophysical, so this note serves as a technical demonstration. These new Swift operational capabilities will allow future FRB detections to be followed up with Swift at even lower latencies than demonstrated here: 15-20 minutes should be regularly achievable, and 5-10 minutes occasionally achievable. We encourage FRB detecting facilities to release alerts in low latency to enable this science., Technical note and capability update for the community. We encourage low latency FRB alerts from relevant facilities to enable this science

Accurate distances are fundamental to interpreting many measured properties of galaxies. Surprisingly, many of the best-studied spiral galaxies in the Local Volume have distance uncertainties that are much larger than can be achieved with modern observations. Using Hubble Space Telescope optical imaging, we use the tip of the red giant branch method to measure the distances to six galaxies that are included in the Spitzer Infrared Nearby Galaxies Survey (SINGS) program and its offspring surveys. The sample includes M63, M74, NGC 1291, NGC 4559, NGC 4625, and NGC 5398. We compare our results with distances reported to these galaxies based on a variety of methods. Depending on the technique, there can be a wide range in published distances, particularly from the Tully-Fisher relation. In addition, differences between the Planetary Nebula Luminosity Function and Surface Brightness Fluctuation techniques can vary between galaxies suggesting inaccuracies that cannot be explained by systematics in the calibrations. Our distances improve upon previous results as we use a well-calibrated, stable distance indicator, precision photometry in an optimally selected field of view, and a Bayesian Maximum Likelihood technique that reduces measurement uncertainties., Comment: 14 pages, 8 figures, 8 tables

Understanding star formation in resolved low mass systems requires the integration of information obtained from observations at different wavelengths. We have combined new and archival multi-wavelength observations on a set of 20 nearby starburst and post-starburst dwarf galaxies to create a data archive of calibrated, homogeneously reduced images. Named the panchromatic "STARBurst IRregular Dwarf Survey" (STARBIRDS) archive, the data are publicly accessible through the Mikulski Archive for Space Telescopes (MAST). This first release of the archive includes images from the Galaxy Evolution Explorer Telescope (GALEX), the Hubble Space Telescope (HST), and the Spitzer Space Telescope (Spitzer) MIPS instrument. The datasets include flux calibrated, background subtracted images, that are registered to the same world coordinate system. Additionally, a set of images are available which are all cropped to match the HST field of view. The GALEX and Spitzer images are available with foreground and background contamination masked. Larger GALEX images extending to 4 times the optical extent of the galaxies are also available. Finally, HST images convolved with a 5 arcsec point spread function and rebinned to the larger pixel scale of the GALEX and Spitzer 24 micron images are provided. Future additions are planned that will include data at other wavelengths such as Spitzer IRAC, ground based Halpha, Chandra X-ray, and Green Bank Telescope HI imaging., 13 pages, 7 figures, 4 tables

Great investments of observing time have been dedicated to the study of nearby spiral galaxies with diverse goals ranging from understanding the star formation process to characterizing their dark matter distributions. Accurate distances are fundamental to interpreting observations of these galaxies, yet many of the best studied nearby galaxies have distances based on methods with relatively large uncertainties. We have started a program to derive accurate distances to these galaxies. Here we measure the distance to M51 - the Whirlpool galaxy - from newly obtained Hubble Space Telescope optical imaging using the tip of the red giant branch method. We measure the distance modulus to be 8.58+/-0.10 Mpc (statistical), corresponding to a distance modulus of 29.67+/-0.02 mag. Our distance is an improvement over previous results as we use a well-calibrated, stable distance indicator, precision photometry in a optimally selected field of view, and a Bayesian Maximum Likelihood technique that reduces measurement uncertainties., 10 pages, 3 figures, 2 tables

M104 (NGC 4594; the Sombrero galaxy) is a nearby, well-studied elliptical galaxy included in scores of surveys focused on understanding the details of galaxy evolution. Despite the importance of observations of M104, a consensus distance has not yet been established. Here, we use newly obtained Hubble Space Telescope optical imaging to measure the distance to M104 based on the tip of the red giant branch method. Our measurement yields the distance to M104 to be 9.55 +/- 0.13 +/- 0.31 Mpc equivalent to a distance modulus of 29.90 +/- 0.03 +/- 0.07 mag. Our distance is an improvement over previous results as we use a well-calibrated, stable distance indicator, precision photometry in a optimally selected field of view, and a Bayesian Maximum Likelihood technique that reduces measurement uncertainties. The most discrepant previous results are due to Tully-Fisher method distances, which are likely inappropriate for M104 given its peculiar morphology and structure. Our results are part of a larger program to measure accurate distances to a sample of well-known spiral galaxies (including M51, M74, and M63) using the tip of the red giant branch method., 8 pages, 4 figures, 2 tables. Second paper in series. Paper I: McQuinn et al. 2016, ApJ, 826, 21 [arXiv:1606.04120]

The CHemical Abundances of Spirals (CHAOS) project leverages the combined power of the Large Binocular Telescope (LBT) with the broad spectral range and sensitivity of the Multi Object Double Spectrograph (MODS) to measure "direct" abundances in large samples of HII regions in spiral galaxies. We present LBT MODS observations of 62 HII regions in the nearby NGC628. We measure one or more auroral lines ([OIII] 4363, [NII] 5755, [SIII] 6312, or [OII] 7320,7330) in a large number of HII regions (40). Comparing derived temperatures from multiple auroral line measurements, we find: a strong correlation between temperatures based on [SIII] and [NII]; and large discrepancies for some temperatures based on [OII] and [OIII]. These trends are consistent with other observations in the literature, yet, given the widespread use and acceptance of [OIII] as a temperature determinant, the magnitude of the T[OIII] discrepancies still came as a surprise. Based on these results, we conduct a uniform abundance analysis using the temperatures derived from [SIII] and [NII], and report the gas-phase abundance gradients for NGC628. Relative abundances of S/O, Ne/O, and Ar/O are constant across the galaxy, consistent with no systematic change in the upper IMF over the sampled range in metallicity. These alpha-element ratios, along with N/O, all show small dispersions consistent with no intrinsic dispersion (0.05) over 70% of the azimuthally averaged radius. We interpret these results as an indication that, at a given radius, the interstellar medium in NGC628 is chemically well-mixed. Unlike the nearly temperature-independent gradients, O/H abundances have a larger intrinsic dispersion of ~0.13 dex. We posit that this dispersion represents an upper limit to the true dispersion in O/H at a given radius and that some of that dispersion is due to systematic uncertainties arising from temperature measurements., 24 pages, 13 Figures, accepted to ApJ

We present sensitive CO (J = 1 - 0) emission line observations of three metal-poor dwarf irregular galaxies Leo P (Z ~ 3% Z_Solar), Sextans A (Z ~ 7.5% Z_Solar), and Sextans B (Z ~ 7.5% Z_Solar), all obtained with the Combined Array for Millimeter-wave Astronomy (CARMA) interferometer. While no CO emission was detected, the proximity of the three systems allows us to place very stringent (4 sigma) upper limits on the CO luminosity (L_CO) in these metal-poor galaxies. We find the CO luminosities to be L_CO < 2900 K km/s pc^2 for Leo P, L_CO < 12400 K km/s pc^2 for Sextans A, and L_CO < 9700 K km/s pc^2 for Sextans B. Comparison of our results with recent observational estimates of the factor for converting between L_CO and the mass of molecular hydrogen, as well as theoretical models, provides further evidence that either the CO-to-H_2 conversion factor increases sharply as metallicity decreases, or that stars are forming in these three galaxies very efficiently, requiring little molecular hydrogen., Accepted for publication in ApJ

We have undertaken the largest systematic study of the high-mass stellar initial mass function (IMF) to date using the optical color-magnitude diagrams (CMDs) of 85 resolved, young (4 Myr < t < 25 Myr), intermediate mass star clusters (10^3-10^4 Msun), observed as part of the Panchromatic Hubble Andromeda Treasury (PHAT) program. We fit each cluster's CMD to measure its mass function (MF) slope for stars >2 Msun. For the ensemble of clusters, the distribution of stellar MF slopes is best described by $\Gamma=+1.45^{+0.03}_{-0.06}$ with a very small intrinsic scatter. The data also imply no significant dependencies of the MF slope on cluster age, mass, and size, providing direct observational evidence that the measured MF represents the IMF. This analysis implies that the high-mass IMF slope in M31 clusters is universal with a slope ($\Gamma=+1.45^{+0.03}_{-0.06}$) that is steeper than the canonical Kroupa (+1.30) and Salpeter (+1.35) values. Using our inference model on select Milky Way (MW) and LMC high-mass IMF studies from the literature, we find $\Gamma_{\rm MW} \sim+1.15\pm0.1$ and $\Gamma_{\rm LMC} \sim+1.3\pm0.1$, both with intrinsic scatter of ~0.3-0.4 dex. Thus, while the high-mass IMF in the Local Group may be universal, systematics in literature IMF studies preclude any definitive conclusions; homogenous investigations of the high-mass IMF in the local universe are needed to overcome this limitation. Consequently, the present study represents the most robust measurement of the high-mass IMF slope to date. We have grafted the M31 high-mass IMF slope onto widely used sub-solar mass Kroupa and Chabrier IMFs and show that commonly used UV- and Halpha-based star formation rates should be increased by a factor of ~1.3-1.5 and the number of stars with masses >8 Msun are ~25% fewer than expected for a Salpeter/Kroupa IMF. [abridged], Comment: 11 pages, 7 Figures, submitted to ApJ. Comments welcome

The Survey of HI in Extremely Low-mass Dwarf galaxies (SHIELD) is an on-going multi-wavelength program to characterize the gas, star formation, and evolution in gas-rich, very low-mass galaxies. The galaxies were selected from the first ~10% of the HI ALFALFA survey based on their inferred low HI mass and low baryonic mass, and all systems have recent star formation. Thus, the SHIELD sample probes the faint end of the galaxy luminosity function for star-forming galaxies. Here, we measure the distances to the 12 SHIELD galaxies to be between 5-12 Mpc by applying the tip of the red giant method to the resolved stellar populations imaged by the Hubble Space Telescope. Based on these distances, the HI masses in the sample range from $4\times10^6$ to $6\times10^7$ M$_{\odot}$, with a median HI mass of $1\times10^7$ M$_{\odot}$. The TRGB distances are up to 73% farther than flow-model estimates in the ALFALFA catalog. Because of the relatively large uncertainties of flow model distances, we are biased towards selecting galaxies from the ALFALFA catalog where the flow model underestimates the true distances. The measured distances allow for an assessment of the native environments around the sample members. Five of the galaxies are part of the NGC 672 and NGC 784 groups, which together constitute a single structure. One galaxy is part of a larger linear ensemble of 9 systems that stretches 1.6 Mpc from end to end. Two galaxies reside in regions with 1-4 neighbors, and four galaxies are truly isolated with no known system identified within a radius of 1 Mpc., Comment: 15 pages, 6 figures, 2 tables

We present ages and masses for 601 star clusters in M31 from the analysis of the six filter integrated light measurements from near ultraviolet to near infrared wavelengths, made as part of the Panchromatic Hubble Andromeda Treasury (PHAT). We derive the ages and masses using a probabilistic technique, which accounts for the effects of stochastic sampling of the stellar initial mass function. Tests on synthetic data show that this method, in conjunction with the exquisite sensitivity of the PHAT observations and their broad wavelength baseline, provides robust age and mass recovery for clusters ranging from $\sim 10^2 - 2 \times 10^6 M_\odot$. We find that the cluster age distribution is consistent with being uniform over the past $100$ Myr, which suggests a weak effect of cluster disruption within M31. The age distribution of older ($>100$ Myr) clusters fall towards old ages, consistent with a power-law decline of index $-1$, likely from a combination of fading and disruption of the clusters. We find that the mass distribution of the whole sample can be well-described by a single power-law with a spectral index of $-1.9 \pm 0.1$ over the range of $10^3-3 \times 10^5 M_\odot$. However, if we subdivide the sample by galactocentric radius, we find that the age distributions remain unchanged. However, the mass spectral index varies significantly, showing best fit values between $-2.2$ and $-1.8$, with the shallower slope in the highest star formation intensity regions. We explore the robustness of our study to potential systematics and conclude that the cluster mass function may vary with respect to environment., 19 pages, 13 figures, catalog available as online material. Accepted for publication in ApJ

We compare the stellar populations and complex neutral gas dynamics of the M81 group dIrr galaxy DDO 165 using data from the HST and the VLA. Paper I identified two kinematically distinct HI components, multiple localized high velocity gas features, and eight HI holes and shells (the largest of which spans ~2.2x1.1 kpc). Using the spatial and temporal information from the stellar populations in DDO 165, we compare the patterns of star formation over the past 500 Myr with the HI dynamics. We extract localized star formation histories within 6 of the 8 HI holes identified in Paper I, as well as 23 other regions that sample a range of stellar densities and neutral gas properties. From population synthesis modeling, we derive the energy outputs (from stellar winds and supernovae) of the stellar populations within these regions over the last 100 Myr, and compare with refined estimates of the energies required to create the HI holes. In all cases, we find that "feedback" is energetically capable of creating the observed structures in the ISM. Numerous regions with significant energy inputs from feedback lack coherent HI structures but show prominent localized high velocity gas features; this feedback signature is a natural product of temporally and spatially distributed star formation. In DDO 165, the extended period of heightened star formation activity (lasting more than 1 Gyr) is energetically capable of creating the observed holes and high velocity gas features in the neutral ISM., The Astrophysical Journal, in press. Full-resolution version available on request from the first author

We present new MMT spectroscopic observations of four dwarf galaxies representative of a larger sample observed by the Sloan Digital Sky Survey (SDSS) and identified by Peeples et al. (2008) as low-mass, high oxygen abundance outliers from the mass-metallicity relation. Peeples et al. (2008) showed that these four objects (with metallicity estimates of 8.5 =< 12 + log(O/H) =< 8.8) have oxygen abundance offsets of 0.4-0.6 dex from the M_B luminosity-metallicity relation. Our new observations extend the wavelength coverage to include the [OII] 3726,3729 doublet, which adds leverage in oxygen abundance estimates and allows measurements of N/O ratios. All four spectra are low excitation, with relatively high N/O ratios (N/O >~ 0.10), each of which tend to bias estimates based on strong emission lines toward high oxygen abundances. These spectra all fall in a regime where the "standard" strong line methods for metallicity determinations are not well calibrated either empirically or by photoionization modeling. By comparing our spectra directly to photoionization models, we estimate oxygen abundances in the range of 7.9 =< 12 + log(O/H) =< 8.4, consistent with the scatter of the mass-metallicity relation. We discuss the physical nature of these galaxies that leads to their unusual spectra (and previous classification as outliers), finding their low excitation, elevated N/O, and strong Balmer absorption are consistent with the properties expected from galaxies evolving past the "Wolf-Rayet galaxy" phase. We compare our results to the "main" sample of Peeples et al. (2008) and conclude that they are outliers primarily due to enrichment of nitrogen relative to oxygen, and not due to unusually high oxygen abundances for their masses or luminosities., 38 pages, 10 figures, accepted to ApJ

Low-resolution, mid-infrared Spitzer/IRS spectral maps are presented for three nearby, low-metallicity dwarf galaxies (NGC 55, NGC 3109 and IC 5152) for the purpose of examining the spatial distribution and variation of polycyclic aromatic hydrocarbon (PAH) emission. The sample straddles a metallicity of 12+log(O/H)~8.0, a transition point below which PAH intensity empirically drops and the character of the interstellar medium changes. We derive quantitative radiances of PAH features and atomic lines on both global and spatially-resolved scales. The Spitzer spectra, combined with extensive ancillary data from the UV through the mid-infrared, allow us to examine changes in the physical environments and in PAH feature radiances down to a physical scale of 50 pc. We discuss correlations between various PAH emission feature and atomic line radiances. The (6.2 micron)/(11.3 micron), (7.7 micron)/(11.3 micron), (8.6 micron)/(11.3 micron), (7.7 micron)/(6.2 micron), and (8.6 micron)/(6.2 micron) PAH radiance ratios are found to be independent of position across all three galaxies, although the ratios do vary from galaxy to galaxy. As seen in other galaxies, we find no variation in the grain size distribution as a function of local radiation field strength. Absolute PAH feature intensities as measured by a ratio of PAH/(24 micron) radiances are seen to vary both positionally within a given galaxy, and from one galaxy to another when integrated over the full observed extent of each system. We examine direct comparisons of CC mode PAH ratios (7.7 micron)/(6.2 micron) and (8.6 micron)/(6.2 micron) to the mixed (CC/CH) mode PAH ratio (7.7 micron)/(11.3 micron). We find little variation in either mode, and no difference in trends between modes. While the local conditions change markedly over the observed regions of these galaxies, the properties of PAH emission show a remarkable degree of uniformity., Astrophysical Journal, in press

The duration of a starburst is a fundamental parameter affecting the evolution of galaxies yet, to date, observational constraints on the durations of starbursts are not well established. Here we study the recent star formation histories (SFHs) of three nearby dwarf galaxies to rigorously quantify the duration of their starburst events using a uniform and consistent approach. We find that the bursts range from ~200 - ~400 Myr in duration resolving the tension between the shorter timescales often derived observationally with the longer timescales derived from dynamical arguments. If these three starbursts are typical of starbursts in dwarf galaxies, then the short timescales (3 - 10 Myr) associated with starbursts in previous studies are best understood as "flickering" events which are simply small components of the larger starburst. In this sample of three nearby dwarfs, the bursts are not localized events. All three systems show bursting levels of star formation in regions of both high and low stellar density. The enhanced star formation moves around the galaxy during the bursts and covers a large fraction of the area of the galaxy. These massive, long duration bursts can significantly affect the structure, dynamics, and chemical evolution of the host galaxy and can be the progenitors of "superwinds" that drive much of the recently chemically enriched material from the galaxy into the intergalactic medium., 41 pages, 14 figures, ApJ, Accepted

We use deep HST/ACS F555W and F814W photometry of resolved stars in the M81 Group dwarf irregular galaxy Ho II to study the hypothesis that the holes identified in the neutral ISM (HI) are created by stellar feedback. From the deep photometry, we construct color-magnitude diagrams (CMDs) and measure the star formation histories (SFHs) for stars contained in HI holes from two independent holes catalogs, as well as select control fields, i.e., similar sized regions that span a range of HI column densities. Converting the recent SFHs into stellar feedback energies, we find that enough energy has been generated to have created all holes. However, the required energy is not always produced over a time scale that is less than the estimated kinematic age of the hole. The combination of the CMDs, recent SFHs, and locations of young stars shows that the stellar populations inside HI holes are not coherent, single-aged, stellar clusters, as previously suggested, but rather multi-age populations distributed across each hole. From a comparison of the modeled and observed integrated magnitudes, and the locations and energetics of stars inside of HI holes, we propose a potential new model: a viable mechanism for creating the observed HI holes in Ho II is stellar feedback from multiple generations of SF spread out over tens or hundreds of Myr, and thus, the concept of an age for an HI hole is intrinsically ambiguous. We further find that \halpha and 24 micron emission, tracers of the most recent star formation, do not correlate well with the positions of the HI holes. However, UV emission, which traces star formation over roughly the last 100 Myr, shows a much better correlation with the locations of the HI holes., Comment: Accepted for Publication in The Astrophysical Journal; 28 Pages, 31 Figures, a version of this paper with full resolution figures is available at http://homepages.spa.umn.edu/~dweisz/ho_ii_weisz.pdf

(abridged) We present the first study of the variable star populations in the isolated dwarf spheroidal galaxies (dSph) Cetus and Tucana. Based on Hubble Space Telescope images obtained with the Advanced Camera for Surveys in the F475W and F814W bands, we identified 180 and 371 variables in Cetus and Tucana, respectively. The vast majority are RR Lyrae stars. In Cetus we also found three anomalous Cepheids, four candidate binaries and one candidate long-period variable (LPV), while six anomalous Cepheids and seven LPV candidates were found in Tucana. Of the RR Lyrae stars, 147 were identified as fundamental mode (RRab) and only eight as first-overtone mode (RRc) in Cetus, with mean periods of 0.614 and 0.363 day, respectively. In Tucana we found 216 RRab and 82 RRc giving mean periods of 0.604 and 0.353 day. These values place both galaxies in the so-called Oosterhoff Gap, as is generally the case for dSph. We calculated the distance modulus to both galaxies using different approaches based on the properties of RRab and RRc, namely the luminosity-metallicity and period-luminosity-metallicity relations, and found values in excellent agreement with previous estimates using independent methods: (m-M)_{0,Cet}=24.46+-0.12 and (m-M)_{0,Tuc}=24.74+-0.12, corresponding to 780+-40 kpc and 890+-50 kpc. We also found numerous RR Lyrae variables pulsating in both modes simultaneously (RRd): 17 in Cetus and 60 in Tucana. Tucana is, after Fornax, the second dSph in which such a large fraction of RRd (~17%) has been observed. We provide the photometry and pulsation parameters for all the variables, and compare the latter with values from the literature for well-studied dSph of the Local Group and Galactic globular clusters., 26 pages, 24 figures, in emulateapj format. To be published in ApJ. Some figures heavily degraded; See http://www.iac.es/project/LCID/?p=publications for a version with full resolution figures

We present Spitzer Space Telescope 4.5 and 8.0 micron imaging of 15 Local Group and nearby dwarf galaxies. Our sample spans a range of more than one dex in nebular metallicity and over three orders of magnitude in current star formation rate, allowing us to examine the dependence of the diffuse 8 micron emission, originating from hot dust and PAHs, on these parameters. We detect prominent diffuse 8 micron emission in four of the most luminous galaxies in the sample (IC 1613, IC 5152, NGC 55, and NGC 3109), low surface brightness emission from four others (DDO 216, Sextans A, Sextans B, WLM), and no diffuse emission from the remaining objects. We observe general correlations of the diffuse 8 micron emission with both the current star formation rate and the nebular metallicity of the galaxies in our sample. However, we also see exceptions to these correlations that suggest other processes may also have a significant effect on the generation of hot dust/PAH emission. These systems all have evidence for old and intermediate age star formation, thus the lack of diffuse 8 micron emission cannot be attributed to young galaxy ages. Also, we find that winds are unlikely to explain the paucity of diffuse 8 micron emission, since high resolution imaging of the neutral gas in these objects show no evidence of blowout. Additionally, we propose the lack of diffuse 8 micron emission in low-metallicity systems may be due to the destruction of dust grains by supernova shocks, assuming the timescale to regrow dust grains and PAH molecules is long compared to the destruction timescale. The most likely explanation for the observed weak diffuse 8 micron emission is at least partly due to a general absence of dust (including PAHs), in agreement with their low metallicities., Accepted by ApJ, 26 pages, 8 figures, version with high-resolution figures available at: http://webusers.astro.umn.edu/~djackson/

We report preliminary results of a project to homogeneously measure star formation histories for the majority of Local Group dwarf galaxies, based on archival WFPC2 observations. For the dwarf spheroidal and elliptical galaxies, we find that the more luminous systems all show extended star formation episodes, while the less luminous systems typically formed most of their stars over ten Gyr ago. Irregular galaxies show no comparable trend. We find that a galaxy's classification as an irregular or spheroidal is entirely explained by its star formation within the past Gyr or less. Several elliptical/spheroidal systems actively formed stars within the past two Gyr, and would have been indistinguishable from present-day irregulars if observed two Gyr ago., Comment: 30 pages; Invited Review at the meeting "Resolved Stellar Populations", held in Cancun, Mexico, 18-22 April 2005; paper with full resolution figures available at http://purcell.as.arizona.edu/pubs/cancun.ps.gz

We present new FUSE spectroscopy of the dwarf starburst galaxy NGC 625. These observations probe multiple phases of the interstellar medium, including the coronal, ionized, neutral and molecular gas. This nearby (D = 3.9 +/- 0.2 Mpc) system shows a clear detection of outflowing coronal gas as traced by OVI 1032 Angstrom absorption. The centroid of the OVI profile is blueshifted with respect to the galaxy systemic velocity by ~ 30 km/sec, suggesting a low-velocity outflow. The implied OVI velocity extent is found to be 100 +/- 20 km/sec, which is fully consistent with the detected HI outflow velocity found in radio synthesis observations. We detect multiple lines of diffuse H2 absorption from the ISM of NGC 625; this is one of only a few extragalactic systems with FUSE detections of H2 lines in the Lyman and Werner bands. We find a potential abundance offset between the neutral and nebular gas that exceeds the errors on the derived column densities. Since such an offset has been found in multiple dwarf galaxies, we discuss the implications of a lower-metallicity halo surrounding the central star forming regions of dwarf galaxies. The apparent offset may be due to saturation of the observed OI line, and higher S/N observations are required to resolve this issue., ApJ, in press; full-resolution version may be obtained at http://www.astro.umn.edu/~cannon/n625.fuse.ps

We present new VLA C-configuration HI imaging of the Lyman Alpha-emitting starburst galaxies Tol 1924-416 and IRAS 08339+6517. The effective resolution probes neutral gas structures larger than 4.7 kpc in Tol 1924-416, and larger than 8.1 kpc in IRAS 08339+6517. Both systems are revealed to be tidally interacting: Tol 1924-416 with ESO 338-IG04B (6.6 arcminutes = 72 kpc minimum separation), and IRAS 08339+6517 with 2MASX J08380769+6508579 (2.4 arcminutes = 56 kpc minimum separation). The HI emission is extended in these systems, with tidal tails and debris between the target galaxies and their companions. Since Lyman Alpha emission has been detected from both of these primary systems, these observations suggest that the geometry of the ISM is one of the factors affecting the escape fraction of Lyman Alpha emission from starburst environments. Furthermore, these observations argue for the importance of interactions in triggering massive star formation events., Comment: ApJ, in press; 11 pages, 2 color figures

We present a measurement of the star formation history of Sextans A, based on WFPC2 photometry that is 50% complete to V=27.5 (M_V ~+ 1.9) and I=27.0. The star formation history and chemical enrichment history have been measured through modeling of the CMD. We find evidence for increased reddening in the youngest stellar populations and an intrinsic metallicity spread at all ages. Sextans A has been actively forming stars at a high rate for ~ 2.5 Gyr ago, with an increased rate beginning ~ 0.1 Gyr ago. We find a non-zero number of stars older than 2.5 Gyr, due to the limited depth of the photometry, a detailed star formation history at intermediate and older ages has considerable uncertainties. The mean metallicity was found to be [M/H] ~- 1.4 over the measured history of the galaxy, with most of the enrichment happening at ages of at least 10 Gyr. We also find that an rms metallicity spread of 0.15 dex at all ages allows the best fits to the observed CMD. We revisit our determination of the recent star formation history (age, 24 pages, 10 figures accepted for publication in Astronomical Journal

Investigation of spectra from the Sloan Digital Sky Survey reveals the presence of a region of ionized gas of >2 degrees diameter centered approximately at alpha = 10^h 37^m delta = -00^o 18' (J2000) (Galactic coordinates l=248, b=+48). [OIII] 4959,5007 emission is particularly strong and emission from H-alpha and [NII] 6548,6583 is also detectable over a substantial area on the sky. The combination of emission line ratios, the close to zero heliocentric radial velocity and the morphology of the structure are consistent with an identification as a very nearby planetary nebula. The proximity of the hot, DO white dwarf PG1034+001 further strengthens this interpretation. The object is: i) the largest planetary nebula on the sky, ii) certainly closer than any planetary nebula other than Sh 2--216, iii) the first to be unambiguously associated with a DO white dwarf. A parallax distance for PG1034+001 would establish whether the structure is in fact the closest, and one of the physically largest, planetary nebula known., 12 pages including 4 figures. ApJ Letters in press

We have identified 82 short-period variable stars in Sextans A from deep WFPC2 observations. All of the periodic variables appear to be short-period Cepheids, with periods as small as 0.8 days for fundamental-mode Cepheids and 0.5 days for first-overtone Cepheids. These objects have been used, along with measurements of the RGB tip and red clump, to measure a true distance modulus to Sextans A of (m-M)_0 = 25.61 +/- 0.07, corresponding to a distance of d = 1.32 +/- 0.04 Mpc. Comparing distances calculated by these techniques, we find that short-period Cepheids (P < 2 days) are accurate distance indicators for objects at or below the metallicity of the SMC. As these objects are quite numerous in low-metallicity star-forming galaxies, they have the potential for providing extremely precise distances throughout the Local Group. We have also compared the relative distances produced by other distance indicators. We conclude that calibrations of RR Lyraes, the RGB tip, and the red clump are self-consistent, but that there appears to be a small dependence of long-period Cepheid distances on metallicity. Finally, we present relative distances of Sextans A, Leo A, IC 1613, and the Magellanic Clouds., 59 pages, 24 figures Scheduled for publication in March 2003 AJ

Using the CTIO 4-m telescope, we have obtained optical spectra of HII regions in five Sculptor Group dwarf irregular galaxies. We derive oxygen, nitrogen, and sulfur abundances from the HII region spectra. Oxygen abundances are derived via three different methods (the ``direct'' method, the empirical method guided by photoionization modeling of McGaugh (1991), and the purely empirical method of Pilyugin (2000)) and are compared. Significant systematic differences are found between the three methods, and we suggest that a recalibration of the empirical abundance scale is required. The N/O ratio for the metal-poor dI ESO 473-G24 of log (N/O) = -1.43 +/- 0.03 lies well above the plateau of log (N/O) = -1.60 +/- 0.02 found by Izotov & Thuan (1999) for a collection of metal-poor blue compact galaxies. This shows that not all galaxies with 12 + log (O/H) < 7.6 have identical elemental abundance ratios, and this implies that the Izotov & Thuan scenario for low metallicity galaxies is not universal. Measurements of the HII regions in NGC 625 yield log (N/O) = -1.25. Assuming N production by intermediate mass stars, this relatively high N/O ratio may be indicative of a long quiescent period prior to the recent active burst of star formation. The oxygen abundances in the Sculptor Group dIs are in good agreement with the relationship between metallicity and luminosity observed in the Local Group dIs. The Sculptor Group dIs, in general, lie closer to the simple closed box model evolutionary path than the Local Group dIs. The higher gas contents, lower average star formation rates, and closer resemblance to closed box evolution could all be indicative of evolution in a relatively low density environment., 40 pages, 9 figures, accepted for Feb 2003 AJ

We report ISO SWS infrared spectroscopy of the H II region Hubble V in NGC 6822 and the blue compact dwarf galaxy I Zw 36. Observations of Br alpha, [S III] at 18.7 and 33.5 microns, and [S IV] at 10.5 microns are used to determine ionic sulfur abundances in these H II regions. There is relatively good agreement between our observations and predictions of S^+3 abundances based on photoionization calculations, although there is an offset in the sense that the models overpredict the S^+3 abundances. We emphasize a need for more observations of this type in order to place nebular sulfur abundance determinations on firmer ground. The S/O ratios derived using the ISO observations in combination with optical data are consistent with values of S/O, derived from optical measurements of other metal-poor galaxies. We present a new formalism for the simultaneous determination of the temperature, temperature fluctuations, and abundances in a nebula, given a mix of optical and infrared observed line ratios. The uncertainties in our ISO measurements and the lack of observations of [S III] lambda 9532 or lambda 9069 do not allow an accurate determination of the amplitude of temperature fluctuations for Hubble V and I Zw 36. Finally, using synthetic data, we illustrate the diagnostic power and limitations of our new method., Comment: 32 Pages total, including 6 encapsulated postscript figures (one with two parts). Accepted for Publication in the 20 Dec 2002 ApJ

Of the light element abundances capable of testing standard big bang nucleosynthesis, only He4 is measured with an accuracy of a few percent. Thus, it is imperative to establish a comprehensive technique for determining He4 abundances and reliable estimates of the true systematic uncertainties. Helium abundance determinations in H II regions are made from the observations of several distinct He I emission lines and their strengths relative to H I emission lines. With the general availability of large format, linear CCD detectors, the accuracy of relative emission line ratios has improved to the point where several terms in the error budget which were assumed to be negligible may now be important. Here we investigate the estimation of errors in deriving and reporting nebular helium abundances from optical emission line spectra., 38 pages, latex, 11 ps figures

We present WFPC2 VI photometry of a field in the halo of IC 1613, finding 13 RR Lyraes and 11 Cepheids. Our photometry of the red giant branch tip and red clump is used to derive distances to IC 1613, which are consistent with each other and with distances based on the variable stars. We compare these values with similarly-measured distances for the Magellanic Clouds, and are able to measure metallicity dependencies of the RR Lyrae and Cepheid distances by requiring consistent relative distance measurements from the four techniques. For metallicities of [Fe/H] = -1.3 (RR Lyraes) and -1.0 (Cepheids), we find a relatively steep slope of 0.34 +/- 0.20 magnitudes per dex for the RR Lyraes and a shallow slope of -0.07 +/- 0.16 mag/dex for the Cepheids, both values within the range of theoretical and empirical results in the literature. We find that a dependence of the red clump absolute magnitude on age, in addition to metallicity, is required to produce self-consistent relative distances between IC 1613 and the Magellanic Clouds. Adopting such a red clump calibration and self-consistent calibrations for the other three distance indicators, we find that the distances to all three objects are in excellent agreement. Our best distance modulus to IC 1613 is mu_0 = 24.31 +/- 0.06, corresponding to a distance of 730 +/- 20 kpc. This distance produces an RR Lyrae absolute magnitude of MV = 0.61 +/- 0.08., Comment: To appear in March ApJ 36 pages, 10 figures

We present 12CO 1-0 observations of eleven low luminosity M_B > -18), HI--rich dwarf galaxies. Only the three most metal-rich galaxies, with 12+log(O/H) ~ 8.2, are detected. Very deep CO spectra of six extremely metal-poor systems (12+log(O/H) < 7.5) yield only low upper limits on the CO surface brightness, I_CO < 0.1 K km/s. Three of these six have never before been observed in a CO line, while the others now have much more stringent upper limits. For the very low metallicity galaxy Leo A, we do not confirm a previously reported detection in CO, and the limits are consistent with another recent nondetection. We combine these new observations with data from the literature to form a sample of dwarf galaxies which all have CO observations and measured oxygen abundances. No known galaxies with 12+log(O/H) < 7.9 (Z < 0.1 solar) have been detected in CO. Most of the star-forming galaxies with higher (12+log(O/H) > 8.1) metallicities are detected at similar or higher I_CO surface brightnesses. The data are consistent with a strong dependence of the I_CO/M_H_2 = X_CO conversion factor on ambient metallicity. The strikingly low upper limits on some metal-poor galaxies lead us to predict that the conversion factor is non-linear, increasing sharply below approximately 1/10 of the solar metallicity (12+log(O/H) < 7.9)., 25 pages, 4 figures, 3 tables. Accepted for publication in AJ Tables replaced -- now formated for landscape orientation

Using Hubble Space Telescope ultraviolet and ground-based optical spectroscopy, we measure the C/O and N/O ratios of three metal-poor galaxies with similar metallicity but differing N/O. The C/O ratios estimated from C III] lambda1909 and [O III] lambda5007 lines are consistent with those measured from C II] lambda2326 and [O II] lambda3727 lines. We develop the use of the C^+/O^+ ratio as a reasonable substitute diagnostic of the carbon abundance in HII regions. The proximity of the C II] lambda2326 multiplet to the [O II] lambda3726,3729 lines makes these transitions a potential tool for measuring the carbon abundance in high-redshift objects. Derived chemical properties are consistent with a significant correlation between N and C abundances in metal-poor extragalactic HII regions. This result is unexpected if the dispersion in N/O among galaxies of similar metallicity is caused by localized, temporary chemical enrichments from massive stars. The presence of a correlation suggests that the majority of N and C production is coupled, as expected from chemical evolution models where C is produced predominantly by low mass stars and N is produced predominantly by intermediate mass stars. Since the occurance of localized chemical "pollution" in star-forming galaxies appears to be low, the relative overabundance of N in some galaxies compared to others at similar metallicity is most plausibly interpreted as an indicator of the global, secular chemical enrichment history. The N/O and perhaps the C/O ratios can be used as a ``clock'' to estimate the time since the last major episode of star formation., 35 pages, 11 figures, 6 tables; uses aaspp4.sty, psfig.sty; to appear in the Astrophysical

We present a longslit optical spectrophotometric survey in the irregular "post-starburst" galaxy NGC 1569 to search for chemical gradients and inhomogeneities in the interstellar medium. Despite the presence of two massive evolved starclusters and numerous HII regions, we find no evidence for chemical gradients or inhomogeneities that may be attributed to enrichment from the recent star formation activity. The chemical properties are consistent with 12+log(O/H)=8.19+/-0.04 and log(N/O)=-1.40+/-0.05 at all locations. No localized chemical self-enrichment ("pollution") from massive star evolution is found, even though the data are sensitive to the chemical yields from as few as two or three massive stars. Flat chemical abundance profiles appear to be the rule rather than the exception in low-mass galaxies. Strong chemical signatures in the surrounding interstellar material should be detected unless one or more of the following are true: 1) Different star forming regions throughout the studied galaxies ``conspire'' to keep star formation rates and global abundances uniform at all times, 2) ejecta from stellar winds and supernovae are transported to all corners of the galaxy on timescales of, 43 pages including figures; uses aaspp and psfig; 5 tables use apjpt4; Accepted (11 June 1997) for publication in the Astrophysical Journal