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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, 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 mm)/(11.3 mm), (7.7 mm)/(11.3 mm), (8.6 mm)/(11.3 mm), (7.7 mm)/(6.2 mm), and (8.6 mm)/(6.2 mm) 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 mm) 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 mm)/(6.2 mm) and (8.6 mm)/(6.2 mm) to the mixed (CC/CH) mode PAH ratio (7.7 mm)/(11.3 mm). 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.

The starburst phenomenon can shape the evolution of the host galaxy and the surrounding intergalactic medium. The extent of the evolutionary impact is partly determined by the duration of the starburst, which has a direct correlation with both the amount of stellar feedback and the development of galactic winds, particularly for smaller mass dwarf systems. We measure the duration of starbursts in twenty nearby, ongoing, and "fossil" starbursts in dwarf galaxies based on the recent star formation histories derived from resolved stellar population data obtained with the Hubble Space Telescope. Contrary to the shorter times of 3-10 Myr often cited, the starburst durations we measure range from 450to650 Myr in fifteen of the dwarf galaxies and up to 1.3 Gyr in four galaxies; these longer durations are comparable to or longer than the dynamical timescales for each system. The same feedback from massive stars that may quench the flickering star formation does not disrupt the overall burst event in our sample of galaxies. While five galaxies present fossil bursts, fifteen galaxies show ongoing bursts and thus the final durations may be longer than we report here for these systems. One galaxy shows a burst that has been ongoing for only 20 Myr; we are likely seeing the beginning of a burst event in this system. Using the duration of the starbursts, we calculate that the bursts deposited 1053.9-1057.2 erg of energy into the interstellar medium through stellar winds and supernovae, and produced 3%-26% of the host galaxy's mass.

We use archival Hubble Space Telescope observations of resolved stellar populations to derive the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In this first paper, we present the observations, color-magnitude diagrams (CMDs), and the SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data reduction, differential extinction, and treatment of photometric completeness. We adopt a star formation rate (SFR) threshold normalized to the average SFR of the individual system as a metric for classifying starbursts in SFHs derived from resolved stellar populations. This choice facilitates finding not only the currently bursting galaxies but also "fossil" bursts increasing the sample size of starburst galaxies in the nearby (D < 8 Mpc) universe. Thirteen of the eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a burst are sustained for hundreds of Myr with variations on small timescales. A long >100 Myr temporal baseline is thus fundamental to any starburst definition or identification method. The longer lived bursts rule out rapid "self-quenching" of starbursts on global scales. The bursting galaxies' gas consumption timescales are shorter than the Hubble time for all but one galaxy confirming the short-lived nature of starbursts based on fuel limitations. Additionally, we find that the strength of the Ha emission usually correlates with the CMD-based SFR during the last 4-10 Myr. However, in four cases, the Ha emission is significantly less than what is expected for models of starbursts; the discrepancy is due to the SFR changing on timescales of a few Myr. The inherently short timescale of the Ha emission limits identifying galaxies as starbursts based on the current characteristics which may or may not be representative of the recent SFH of a galaxy.

We present the results of a new search for variable stars in the Local Group (LG) isolated dwarf galaxy IC 1613, based on 24 orbits of F475W and F814W photometry from the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. We detected 259 candidate variables in this field, of which only 13 (all of them bright Cepheids) were previously known. Out of the confirmed variables, we found 90 RR Lyrae stars, 49 classical Cepheids (including 36 new discoveries), and 38 eclipsing binary stars for which we could determine a period. The RR Lyrae include 61 fundamental (RRab) and 24 first-overtone (FO, RRc) pulsators, and five pulsating in both modes simultaneously (RRd). As for the majority of LG dwarfs, the mean periods of the RRab and RRc (0.611 and 0.334 days, respectively) as well as the fraction of overtone pulsators (f c = 0.28) place this galaxy in the intermediate regime between the Oosterhoff types. From their position on the period-luminosity diagram and light-curve morphology, we can unambiguously classify 25 and 14 Cepheids as fundamental and FO mode pulsators, respectively. Another two are clearly second-overtone Cepheids, the first ones to be discovered beyond the Magellanic Clouds. Among the remaining candidate variables, five were classified as d-Scuti and five as long-period variables. Most of the others are located on the main sequence, the majority of them likely eclipsing binary systems, although some present variations similar to pulsating stars. We estimate the distance to IC 1613 using various methods based on the photometric and pulsational properties of the Cepheids and RR Lyrae stars. The values we find are in very good agreement with each other and with previous estimates based on independent methods. When corrected to a common reddening of E(B - V) = 0.025 and true Large Magellanic Cloud (LMC) distance modulus of (m-M)LMC,0 = 18.515 +- 0.085, we find that all the distance determinations from the literature converge to a common value of (m-M)0 = 24.400 +- 0.014 (statistical) or 760 kpc. The parallel WFPC2 field, which lies within three core radii, was also searched for variable stars. We discovered nine RR Lyrae stars (four RRab, four RRc, and one RRd) and two Cepheids, even though the lower signal-to-noise ratio of the observations did not allow us to measure their periods as accurately as for the variables in the ACS field of view. We provide their coordinates and approximate properties for completeness.

We have derived nebular abundances for 10 dwarf galaxies belonging to the M81 Group, including several galaxies which do not have abundances previously reported in the literature. For each galaxy, multiple H II regions were observed with GMOS-N at the Gemini Observatory in order to determine abundances of several elements (oxygen, nitrogen, sulfur, neon, and argon). For seven galaxies, at least one H II region had a detection of the temperature sensitive [O III] l4363 line, allowing a "direct" determination of the oxygen abundance. No abundance gradients were detected in the targeted galaxies, and the observed oxygen abundances are typically in agreement with the well-known metallicity-luminosity relation. However, three candidate "tidal dwarf" galaxies lie well off this relation: UGC 5336, Garland, and KDG 61. The nature of these systems suggests that UGC 5336 and Garland are indeed recently formed systems, whereas KDG 61 is most likely a dwarf spheroidal galaxy which lies along the same line of sight as the M81 tidal debris field. We propose that these H II regions formed from previously enriched gas which was stripped from nearby massive galaxies (e.g., NGC 3077 and M81) during a recent tidal interaction.

We use deep Hubble Space Telescope (HST)/Advanced Camera for Surveys (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 interstellar medium (H I) 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 H I holes from two independent holes catalogs, as well as select control fields, i.e., similar sized regions that span a range of H I column densities. The CMDs reveal young (< 200 Myr) stellar populations inside all H I holes, which contain very few bright OB stars with ages less than 10 Myr, indicating they are not reliable tracers of H I hole locations while the recent SFHs confirm multiple episodes of star formation within most holes. 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 timescale that is less than the estimated kinematic age of the hole. A similar analysis of stars in the control fields finds that the stellar populations of the control fields and H I holes are statistically indistinguishable. However, because we are only sensitive to holes [?]100 pc in diameter, we cannot tell if there are smaller holes inside the control fields. The combination of the CMDs, recent SFHs, and locations of young stars shows that the stellar populations inside H I 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 H I holes, we propose a potential new model: a viable mechanism for creating the observed H I 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 H I hole is intrinsically ambiguous. For H I holes in the outer parts of Ho II, located beyond the HST/ACS coverage, we use Monte Carlo simulations of expected stellar populations to show that low level SF could provide the energy necessary to form these holes. Applying the same method to the SMC, we find that the holes that appear to be void of stars could have formed via stellar feedback from low level SF. We further find that Ha and 24 um emission, tracers of the most recent star formation, do not correlate well with the positions of the H I holes. However, UV emission, which traces star formation over roughly the last 100 Myr, shows a much better correlation with the locations of the H I holes.

We present the first study of the variable star populations in the isolated dwarf spheroidal galaxies (dSphs) 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 (ACs), four candidate binaries and one candidate long-period variable (LPV), while six ACs 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. The parallel WFPC2 fields were also searched for variables, as they lie well within the tidal radius of Cetus, and at its limit in the case of Tucana. No variables were found in the latter, while 15 were discovered in the outer field of Cetus (11 RRab, three RRc, and one RRd), even though the lower signal-to-noise ratio of the observations did not allow us to measure their periods accurately. We provide their coordinates and approximate properties for completeness.

We present the first determination of the radial velocities and metallicities of 78 red giant stars in the isolated dwarf irregular galaxy WLM. Observations of the calcium II triplet in these stars were made with FORS2 at the VLT-UT2 in two separated fields of view in WLM, and the [Fe/H] values were conformed to the Carretta & Gratton ([Fe/H]CG97) metallicity scale. The mean metallicity is [?][Fe/H][?] = -1.27 +- 0.04 dex, with a standard deviation of s = 0.37. We find that the stars in the inner field are more metal-rich by D[Fe/H]=0.30 +- 0.06 dex. These results are in agreement with previous photometric studies that found a radial population gradient, as well as the expectation of higher metallicities in the central star-forming regions. Age estimates using Victoria-Regina stellar models show that the youngest stars in the sample (less than 6 Gyr) are more metal-rich by D[Fe/H]=0.32 +- 0.08 dex. These stars also show a lower velocity dispersion at all elliptical radii compared to the metal-poor stars. Kinematics for the whole red giant sample suggest a velocity gradient approximately half that of the gas rotation curve, with the stellar component occupying a thicker disk decoupled from the H I rotation plane. Taken together, the kinematics, metallicities, and ages in our sample suggest a young metal-rich, and kinematically cold stellar population in the central gas-rich regions of WLM, surrounded by a separate dynamically hot halo of older, metal-poor stars.

We present the third and final part of a census of asymptotic giant branch (AGB) stars in Local Group dwarf irregular (dIrr) galaxies. Papers I and II presented the results for WLM and IC 1613. Included here are Phoenix, LGS 3, DDO 210, Leo A, Pegasus dIrr, and Sextans A. Spitzer photometry at 3.6, 4.5, 5.8, and 8 mm are presented, along with a more thorough treatment of background galaxy contamination than was presented in Papers I and II. We find that at least a small population of completely optically obscured AGB stars exists in each galaxy, regardless of the galaxy's metallicity, but that higher metallicity galaxies tend to harbor more stars with slight infrared excesses. The optical incompleteness increases for the redder AGB stars, in line with the expectation that some AGB stars are not detected in the optical due to large amounts of extinction associated with in situ dust production. Overall, there is an underrepresentation of 30%-40% in the optical AGB within the 1s errors for all of the galaxies in our sample. This undetected population is large enough to affect star formation histories derived from optical color-magnitude diagrams. As measured from the [3.6] - [4.5] color excesses, we find average stellar mass-loss rates (MLRs) ranging from 3.1 x 10-7to6.6 x 10-6 M yr-1, and integrated galaxy MLRs ranging from 4.4 x 10-5to1.4 x 10-3 M yr-1. The integrated MLR is sufficient to sustain the current star formation rate in only LGS 3 and DDO 210, requiring either significant nondusty mass loss or gas accretion in Phoenix, Leo A, Pegasus dIrr, Sextans A, WLM, and IC 1613 if they are to maintain their status as gas-rich galaxies.

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 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 to [?]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.

We present observations and analysis of nine dwarf irregular galaxies (dIs) in the M81 Group taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The nine galaxies (the Garland, M81 Dwarf A, DDO 53, Ho IX, Ho I, DDO 165, NGC 2366, Ho II, and IC 2574) span 6 mag in luminosity, a factor of 1000 in current star formation rate, and 0.5 dex in metallicity. We use color-magnitude diagrams of resolved stellar populations to study the star formation histories (SFHs) of these galaxies. Dividing the sample at MB = [?] 15, we analyze the similarities and differences in the SFHs, birthrate parameters (b), fraction of stars formed per time interval (f), and spatial distribution of stellar components. As function of luminosity, we find only minor differences in SF characteristics among the M81 Group dIs despite a wide range of physical properties. We extend our comparison to select dIs in the Local Group (LG) and, again, find little variation in SF parameters. Our average f parameters are consistent with a constant SFH; however, individual galaxies can show significant departures from this. We find this result underlines the importance of stochastic SF in dIs. We also compare possible formation scenarios of the fainter and candidate tidal dwarfs (TDGs) in the M81 Group. The SFHs and the spatial distribution of associated red stars suggest that the Garland and Ho IX are not dIs and are potentially TDGs. A noteworthy difference between the LG and the M81 Group is the lack of TDGs in the LG.

The paucity of observed dwarf galaxies in the Local Group relative to the abundance of predicted dark matter halos remains one of the greatest puzzles of the LCDM paradigm. Solving this puzzle now requires not only matching the numbers of objects but also understanding the details of their star formation histories. We present a summary of such histories derived from the HST data using the color-magnitude diagram fitting method. To reduce observational uncertainties, we condense the data into five cumulative parameters: the fractions of stellar mass formed in the last 1, 2, 5, and 10 Gyr, and the mean stellar age. We interpret the new data with a phenomenological model based on the mass assembly histories of dark matter halos and the Schmidt law of star formation. The model correctly predicts the radial distribution of the dwarfs and the fractions of stars formed in the last 5 and 10 Gyr. However, in order to be consistent with the observations, the model requires a significant amount of recent star formation in the last 2 Gyr. Within the framework of our model, this prolonged star formation can be achieved by adding a stochastic variation in the density threshold of the star formation law. The model results are not sensitive to late gas accretion, the slope of the Schmidt law, or the details of cosmic reionization. A few discrepancies still remain: our model typically predicts too large stellar masses, only a modest population of ultrafaint dwarfs, and a small number of dwarfs with anomalously young stellar populations. Nevertheless, the observed star formation histories of Local Group dwarfs are generally consistent the expected star formation in cold dark matter halos.

We present Spitzer IRAC photometry of the Local Group dwarf irregular galaxy IC 1613. We compare our 3.6, 4.5, 5.8, and 8.0 mm photometry with broadband optical photometry and find that from the optical data we cannot detect 43% and that an additional 11% of the total AGB population are misidentified, likely because of extinction caused by circumstellar material. Furthermore, we find that a narrowband optical carbon star study of IC 1613 detects 50% of the total AGB population and only considers 18% of this population in calculating the carbon to M-type AGB ratio. We derive an integrated mass-loss rate from the AGB stars of (0.2-1.0) x 10-3 M yr-1 and find that the distribution of bolometric luminosities and mass-loss rates are consistent with those for other nearby metal-poor galaxies. Both the optical completeness fractions and mass-loss rates in IC 1613 are very similar to those in the Local Group dwarf irregular galaxy WLM, which is expected, given their similar characteristics and evolutionary histories.

The Local Group galaxy Phoenix has the properties of a dwarf spheroidal galaxy, but an adjacent H I cloud has been recently found to be at the same radial velocity as the stars. The proximity suggests that this cloud is associated with the most recent ([?]100 Myr) star formation in Phoenix. We have obtained relatively high sensitivity and high resolution H I imaging with the VLA with the goal of distinguishing between different processes for displacing the gas from the galaxy. Due to the outer curvature of the H I cloud, it appears that expulsion from the galaxy by winds from supernovae is more likely than ram pressure stripping. The isolation of the galaxy makes tidal stripping highly unlikely. Using a star formation history constructed from HST imaging, we construct a simple kinematic model that implies that the H I cloud is still gravitationally bound to the galaxy. Gas that is expelled from the centers of dwarf galaxies but remains gravitationally bound may explain the episodic star formation observed in several dwarfs. In the specific case of Phoenix, there may be future star formation in this currently dSph-like galaxy.

We present Spitzer IRAC observations at 3.6 and 4.5 mm along with optical data from the Local Group Galaxies Survey to investigate the evolved stellar population of the Local Group dwarf irregular galaxy WLM. These observations provide a nearly complete census of the AGB stars. We find that 39% of the infrared-detected AGB stars are not detected in the optical data, even though our 50% completeness limit is 3 mag fainter than the red giant branch tip. An additional 4% of the infrared-detected AGBs are misidentified in the optical, presumably due to reddening by circumstellar dust. We also compare our results with those of a narrowband optical carbon star survey of WLM and find the latter study sensitive to only 18% of the total AGB population. We detect objects with infrared fluxes consistent with them being mass-losing AGB stars and derive a present-day total mass-loss rate from the AGB stars of M = (0.7-2.4) x 10-3 M yr-1. The distributions of mass-loss rates and bolometric luminosities of AGBs and red supergiants are very similar to those in the LMC and SMC, and the empirical maximum mass-loss rate observed in the LMC and SMC is in excellent agreement with our WLM data.

We report 4.5 mm luminosities for 27 nearby (D [?] 5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 mm luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 mm L-Z relation is 12 + log(O/H) = (5.78 +- 0.21) + (-0.122 +- 0.012)M[4.5], where M[4.5] is the absolute magnitude at 4.5 mm. The dispersion in the near-infrared L-Z relation is smaller than the corresponding dispersion in the optical L-Z relation. The subsequently derived stellar mass-metallicity (M*-Z) relation is 12 + log(O/H) = (5.65 +- 0.23) + (0.298 +- 0.030) log M*, and extends the SDSS M*- Z relation to lower mass by about 2.5 dex. We find that the dispersion in the M*-Z relation is similar over 5 orders of magnitude in stellar mass, and that the relationship between stellar mass and interstellar medium metallicity is similarly tight from high-mass to low-mass systems. We find a larger scatter at low mass in the relation between effective yield and total baryonic mass. In fact, there are a few dwarf galaxies with large yields, which is difficult to explain if galactic winds are ubiquitous in dwarf galaxies. The low scatter in the L-Z and M*-Z relationships are difficult to understand if galactic superwinds or blowout are responsible for the low metallicities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed.

We present Spitzer 4.5 and 8.0 mm imaging of 15 Local Group and nearby dwarf galaxies. We find that the diffuse 8 mm emission is spatially correlated with regions of active star formation. Our sample spans a range of >1 dex in nebular metallicity and 3 orders of magnitude in current star formation rate, allowing us to examine the dependence of emission from hot dust and PAHs on these parameters. We detect prominent diffuse 8 mm emission from the four most luminous galaxies in the sample (IC 1613, IC 5152, NGC 55, and NGC 3109) and only very low surface brightness emission from four others (DDO 216, Sextans A, Sextans B, and WLM). These are the first spatially resolved images of diffuse 8 mm emission from such low-metallicity objects [12 + log(O/H) ~ 7.5]. We observe correlations of this emission with the current star formation rate and the nebular metallicity of these galaxies. However, we also see evidence suggesting that other processes may also have a significant effect on the generation of this emission. These systems all have evidence for old and intermediate-age star formation; thus, the lack of diffuse 8 mm emission cannot be attributed to low galaxy ages. Also, winds cannot explain the paucity of this emission, since high-resolution imaging of the neutral gas in these objects shows no evidence of blowout. We propose that the lack of diffuse 8 mm emission in low-metallicity systems may be due to the destruction of dust grains by supernova shocks, assuming a long timescale to regrow dust. It is likely that the observed weak emission is at least partly due to a general absence of dust (including PAHs), in agreement with their low metallicities.

To test the existence of a possible radial gradient in oxygen abundances within the Local Group dwarf irregular galaxy NGC 6822, we have obtained optical spectra of 19 nebulae with the EFOSC2 spectrograph on the 3.6 m telescope at ESO La Silla. The extent of the measured nebulae spans galactocentric radii in the range between 0.05 and 2 kpc (over 4 exponential scale lengths). In five H II regions (Hubble I, Hubble V, Ka, Kb, KD 28e), the temperature-sensitive [O III] l4363 emission line was detected, and direct oxygen abundances were derived. Oxygen abundances for the remaining H II regions were derived using bright-line methods. The oxygen abundances for three A-type supergiant stars are slightly higher than nebular values at comparable radii. Linear least-square fits to various subsets of abundance data were obtained. When all of the measured nebulae are included, no clear signature is found for an abundance gradient. A fit to only newly observed H II regions with [O III] l4363 detections yields an oxygen abundance gradient of -0.14 +- 0.07 dex kpc-1. The gradient becomes slightly more significant (-0.16 +- 0.05 dex kpc-1) when three additional H II regions with [O III] l4363 measurements from the literature are added. Assuming no abundance gradient, we derive a mean nebular oxygen abundance 12 + log(O/H) = 8.11 +- 0.10 from [O III] l4363 detections in the five H II regions from our present data; this mean value corresponds to [O/H] = -0.55.

We present elemental abundances for multiple H II regions in Leo A and GR 8 obtained from long-slit optical spectroscopy of these two nearby low-luminosity dwarf irregular galaxies. As expected from their luminosities, and in agreement with previous observations, the derived oxygen abundances are extremely low in both galaxies. High signal-to-noise ratio (S/N) observations of a planetary nebula in Leo A yield 12 + log(O/H) = 7.30 +- 0.05; semiempirical calculations of the oxygen abundance in four H II regions in Leo A indicate 12 + log(O/H) = 7.38 +- 0.10. These results confirm that Leo A has one of the lowest ISM metal abundances of known nearby galaxies. Based on results from two H II regions with high S/N measurements of the weak [O III] l4363 line, the mean oxygen abundance of GR 8 is 12 + log(O/H) = 7.65 +- 0.06; using "empirical" and "semiempirical" methods, similar abundances are derived for six other GR 8 H II regions. Similar to previous results in other low-metallicity galaxies, the mean log(N/O) = -1.53 +- 0.09 for Leo A and -1.51 +- 0.07 for GR 8. There is no evidence of significant variations in either O/H or N/O in the H II regions. The metallicity-luminosity relation for nearby (D < 5 Mpc) dwarf irregular galaxies with measured oxygen abundances has a mean correlation of 12 + log(O/H) = 5.67MB-0.151MB, with a dispersion in oxygen about the relationship of s = 0.21. These observations confirm that gas-rich, low-luminosity galaxies have extremely low elemental abundances in the ionized gas phase of their interstellar media. Although Leo A has one of the lowest metal abundances of known nearby galaxies, detection of tracers of an older stellar population (RR Lyrae variable stars, horizontal branch stars, and a well-populated red giant branch) indicate that it is not a newly formed galaxy, as has been proposed for some other similar low-metallicity star-forming galaxies.

We obtained new optical spectra of 13 H II regions in WLM with EFOSC2; oxygen abundances are derived for nine H II regions. The temperature-sensitive [O III] l4363 emission line was measured in two bright H II regions, HM 7 and HM 9. The direct oxygen abundances for HM 7 and HM 9 are 12 + log(O/H) = 7.72 +- 0.04 and 7.91 +- 0.04, respectively. We adopt a mean oxygen abundance of 12 + log(O/H) = 7.83 +- 0.06. This corresponds to [O/H] = -0.83 dex, or 15% of the solar value. In H II regions where [O III] l4363 was not measured, oxygen abundances derived with bright-line methods are in general agreement with direct values of the oxygen abundance to an accuracy of about 0.2 dex. In general, the present measurements show that the H II region oxygen abundances agree with previous values in the literature. The nebular oxygen abundances are marginally consistent with the mean stellar magnesium abundance ([Mg/H] = -0.62). However, there is still a 0.62 dex discrepancy in oxygen abundance between the nebular result and the A-type supergiant star WLM 15 ([O/H] = -0.21). Nonzero reddening values derived from Balmer line ratios were found in H II regions near a second H I peak. There may be a connection between the location of the second H I peak, regions of higher extinction, and the position of WLM 15 on the eastern side of the galaxy.

We present new Far Ultraviolet Spectroscopic Explorer (FUSE) spectroscopy of the dwarf starburst galaxy NGC 625. These observations probe multiple phases of the interstellar medium (ISM), 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 O VI l1032 absorption. The centroid of the O VI profile is blueshifted with respect to the galaxy systemic velocity by ~30 km s-1, suggesting a low-velocity outflow. The implied O VI velocity extent is found to be 100 +- 20 km s-1, which is fully consistent with the detected H I 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 O I line, and higher signal-to-noise ratio (S/N) observations are required to resolve this issue.

Using the WMAP determination of the baryon density, the standard model of big bang nucleosynthesis yields relatively precise predictions of the primordial light-element abundances. Currently there are two significantly different observational determinations of the primordial helium abundance, and if only statistical errors in 4He abundance determinations are considered, the discrepancies between the observational determinations and the value favored by the WMAP results are significant. Here we examine in detail some likely sources of systematic uncertainties that may resolve the differences between the two determinations. We conclude that the observational determination of the primordial helium abundance is completely limited by systematic errors and that these systematic errors have not been fully accounted for in any published observational determination of the primordial helium abundance. In principle, the observed metal-poor H II region spectra should be analyzed in a nonparametric way, such that the H II region physical conditions and the helium abundance are derived solely from the relative flux ratios of the helium and hydrogen emission lines. In practice, there are very few H II region spectra with the quality that allows this, so that most analyses depend on assumed ranges or relationships between physical parameters, resulting in parametric solutions with underestimated error bars. A representative result of our analysis yields Yp = 0.249 +- 0.009. We stress that the main result of the present work is the increase in the size of the uncertainty rather than the shift in the primordial value. Furthermore, given that most of the spectra analyzed to date do not significantly constrain the primordial helium abundance, we argue in favor of a range of allowed values of 0.232 [?] Yp [?] 0.258. This easily allows for concordance between measurements of the baryon-to-photon ratio (e) from WMAP, deuterium abundances, and helium abundance (although the discrepancy with lithium remains).

We report optical spectroscopy of 16 H II regions in NGC 1705 and [O III] l4363 detections for the first time in five H II regions. The resulting mean oxygen abundance derived directly from measured electron temperatures is 12 + log(O/H) = 8.21 +- 0.05, which corresponds to [O/H] = -0.45, or 35% of the solar value. There are no significant spatial inhomogeneities in [O III] l4363 oxygen abundances from H II regions at a radius approximately 10'' from the super star cluster. In H II regions where [O III] l4363 was not measured, oxygen abundances derived with bright-line methods (accurate only to 0.2 dex) are in agreement with direct values of the oxygen abundance. Faint, narrow He II l4686 emission is found in two H II regions, but the implied contribution from O+3 to the total oxygen abundance is only 0.01 dex. The mean argon-, neon-, and nitrogen-to-oxygen abundance ratios are consistent with mean values for other dwarf irregular galaxies, blue compact dwarf galaxies, and H II galaxies at comparable oxygen abundances. Interestingly, the nitrogen-to-oxygen abundance ratio in the ionized H II gas agrees with the value for the neutral H I, even though the metallicity of the neutral gas may be a factor of 6 lower than that of the ionized gas. This may be indicative of low-metallicity gas in the halo of the galaxy. Extinction values, AV, derived from observed Balmer line ratios along lines of sight to H II regions are in the range between 0 and 0.9 mag. Significant and variable extinction may have important effects on the interpretation of resolved stellar populations and derived star formation histories. With respect to the metallicity-luminosity and metallicity-gas fraction diagnostics, the measured oxygen abundance for NGC 1705 is comparable to those of Local Group dwarf irregular galaxies at a given luminosity and gas fraction. Simple chemical evolution models suggest that the galaxy is quickly evolving into a gas-poor dwarf galaxy.

We present new multifrequency radio continuum imaging of the dwarf starburst galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm reveal global continuum emission dominated by free-free emission, with only mild synchrotron components. Each of the major H II regions is detected; the individual spectral indices are thermal for the youngest regions (showing largest Ha emission) and nonthermal for the oldest. We do not detect any sources that appear to be associated with deeply embedded, dense, young clusters, although we have discovered one low-luminosity, obscured source that has no luminous optical counterpart and resides in the region of highest optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent star formation, these radio properties suggest that the youngest star formation complexes have not yet evolved to the point where their thermal spectra are significantly contaminated by synchrotron emission. The nonthermal components are associated with regions of older star formation that have smaller ionized gas components. These results imply a range of ages for the H II regions and radio components that agrees with our previous resolved stellar population analysis, where an extended burst of star formation has pervaded the disk of NGC 625 over the last ~50 Myr. We compare the nature of radio continuum emission in selected nearby dwarf starburst and Wolf-Rayet galaxies, demonstrating that thermal radio continuum emission appears to be more common in these systems than in typical H II galaxies with less recent star formation and more evolved stellar clusters.

We present new Very Large Array (VLA) C-configuration H I imaging of the Lya-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 = 72 kpc minimum separation), and IRAS 08339+6517 with 2MASX J08380769+6508579 (2.'4 = 56 kpc minimum separation). The H I emission is extended in these systems, with tidal tails and debris between the target galaxies and their companions. Since Lya emission has been detected from both of these primary systems, these observations suggest that the geometry of the interstellar medium is one of the factors affecting the escape fraction of Lya emission from starburst environments. Furthermore, these observations argue for the importance of interactions in triggering massive star formation events.

We present new multiconfiguration H I aperture synthesis imaging of the nearby dwarf starburst galaxy NGC 625 obtained with the Australia Telescope Compact Array. Total H I column density images show gas well aligned with the optical major axis and low column density H I extending to greater than 6 optical scale lengths. The H I velocity field, on the other hand, is highly disturbed, with neutral gas at nearly all detected velocities within the central region. After considering various interpretations, we find that a blowout scenario most accurately describes the data. Since at our resolution we do not detect any large evacuated holes in the H I disk, we interpret this blowout to be the result of the extended (both spatially and temporally) star formation event that NGC 625 has undergone in the last 100 Myr. This is one of the clearest examples of H I outflow detected in a dwarf galaxy. We find no obvious external trigger for this extended star formation event. We detect strong radio continuum emission from the largest H II regions; comparing to our Hubble Space Telescope and ground-based Ha fluxes suggests either appreciable amounts of extinction toward the star formation regions or the contribution of nonthermal sources to the radio continuum luminosity.

We have taken deep images of an outlying field in the Local Group dwarf irregular galaxy IC 1613 with the WFPC2 aboard the Hubble Space Telescope in the standard broadband F555W (V, 8 orbits) and F814W (I, 16 orbits) filters. The photometry reaches to V = 27.7 (MV = +3.4) and I = 27.1 (MI = +2.8) at the 50% completeness level, the deepest to date for an isolated dwarf irregular galaxy. We analyze the resulting color-magnitude diagram (CMD) and compare it with CMDs created from theoretical stellar models using three different methods to derive a star formation history (SFH) as well as constrain the chemical evolution for IC 1613. All three methods find an enhanced star formation rate (SFR), at roughly the same magnitude (factor of 3), over roughly the same period (from 3 to 6 Gyr ago). Additionally, all three methods were driven to similar age-metallicity relationships (AMR) that show an increase from [Fe/H] [?] -1.3 at earliest times to [Fe/H] [?] -0.7 at present. Good agreement is found between the AMR which is derived from the CMD analysis and that which can be inferred from the derived SFH at all but the earliest ages. The agreement between the three models and the self-consistency of the derived chemical enrichment history support the reality of the derived SFH of IC 1613 and, more generally, are supportive of the practice of constructing galaxy SFHs from CMDs. A comparison of the newly observed outer field with an earlier studied central field of IC 1613 shows that the SFR in the outer field has been significantly depressed during the last Gyr. This implies that the optical scale length of the galaxy has been decreasing with time and that comparison of galaxies at intermediate redshift with present-day galaxies should take this effect into account. Comparing the CMD of the outer field of IC 1613 with CMDs of Milky Way dSph companions, we find strong similarities between IC 1613 and the more distant dSph companions (Carina, Fornax, Leo I, and Leo II) in that all are dominated by star formation at intermediate ages. In particular, the SFH and AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr galaxies cannot be distinguished from dSphs by their intermediate-age stellar populations. This type of a SFH may also be evidence for slower or suppressed early star formation in dwarf galaxies due to photoionization after the reionization of the universe by background radiation. Assuming that IC 1613 is typical of a dIrr evolving in isolation, since most of the star formation occurs at intermediate ages, these dwarf systems cannot be responsible for the fast chemical enrichment of the intergalactic medium that is seen at high redshift. There is no evidence for any large-amplitude bursts of star formation in IC 1613, and we find it highly unlikely that analogs of IC 1613 have contributed to the excess of faint blue galaxies in existing galaxy redshift surveys.

We report Infrared Space Observatory (ISO) Short Wavelength Spectrometer infrared spectroscopy of the H II region Hubble V in NGC 6822 and the blue compact dwarf galaxy I Zw 36. Observations of Bra, [S III] at 18.7 and 33.5 mm, and [S IV] at 10.5 mm 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] l9532 or l9069 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.

We consider the effect of optical depth of the 2 3S level on the nebular recombination spectrum of He I for a spherically symmetric nebula with no systematic velocity gradients. These calculations, using many improvements in atomic data, can be used in place of the earlier calculations of Robbins. We give representative Case B line fluxes for UV, optical, and IR emission lines over a range of physical conditions: T = 5000-20,000 K, ne = 1-108 cm-3, and t3889 = 0-100. A FORTRAN program for calculating emissivities for all lines arising from quantum levels with n [?] 10 is also available from the authors. We present a special set of fitting formulae for the physical conditions relevant to low-metallicity extragalactic H II regions: T = 12,000-20,000 K, ne = 1-300 cm-3, and t3889 < 2.0). For this range of physical conditions, the Case B line fluxes of the bright optical lines 4471 A, 5876 A, and 6678 A, are changed less than 1%, in agreement with previous studies. However, the 7065 A corrections are much smaller than those calculated by Izotov & Thuan based on the earlier calculations by Robbins. This means that the 7065 A line is a better density diagnostic than previously thought. Two corrections to the fitting functions calculated in our previous work are also given.

We present new WFPC2 narrowband imaging of the blue compact dwarf galaxy I Zw 18, which is host to the lowest metallicity H II regions known. Images at Ha and Hb are combined with archival broadband images to allow the study of the ionized gas distribution and morphology. Analysis of the Ha/Hb flux ratio reveals significant enhancements in some areas of both the northwest and southeast regions of the galaxy, with ratios elevated to levels as high as 3.4. The Ha/Hb ratio varies considerably with position throughout the galaxy. Comparing this distribution with the stellar distribution indicates that the regions of enhanced Ha/Hb ratio are not due to the effects of either collisional excitation or underlying stellar absorption and therefore are most likely interpreted as the presence of dust. This dust has an estimated mass of (2-5) x 103 M, which is consistent with the IRAS far-IR nondetection. Under the assumption that dust traces the presence of molecular gas, these results suggest that the molecular component of the interstellar medium of I Zw 18, which is needed to fuel its active star formation, is also very clumpy. Such a distribution would be consistent with the recent Far-Ultraviolet Spectroscopic Explorer nondetections of diffuse H2.

We present WFPC2 VI photometry of a field in the halo of IC 1613, finding 13 RR Lyrae stars 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 Lyrae stars) and -1.0 (Cepheids), we find a relatively steep slope of 0.34 +- 0.20 mag per dex for the RR Lyrae stars and a shallow slope of -0.07 +- 0.16 mag per 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 m0 = 24.31 +- 0.06, corresponding to a distance of 730 +- 20 kpc. This distance produces an RR Lyrae absolute magnitude of 0.61 +- 0.08.

We have combined the detailed He I recombination model of Smits with the collisional transitions of Sawey & Berrington in order to produce new accurate helium emissivities that include the effects of collisional excitation from both the 2 3S and 2 1S levels. We present a grid of emissivities for a range of temperature and densities along with analytical fits and error estimates. These grids eliminate the necessity of making corrections for collisional enhancements as in the work of Clegg or Kingdon & Ferland for lines with upper levels below n=5. For densities greater than ne[?]106 cm-3, inclusion of collisional excitation from the 2 1S level is also necessary if accuracies of greater than a few percent are required. Atomic data for a model atom with 29 levels (nmax=5) are presented that match the recombination model of Smits to within 5% over the temperature range T=5000-20,000 K. Collisional effects are calculated self-consistently using the algorithm of Almog & Netzer. This model atom will be useful in models of radiative transfer. A notable feature of this technique is an algorithm that calculates the "indirect" recombination rates, the recombination to individual levels that go through n>nmax first. Fits accurate to within 1% are given for the emissivities of the brightest lines over a restricted range for estimates of primordial helium abundance. We characterize the analysis uncertainties associated with uncertainties in temperature, density, fitting functions, and input atomic data. We estimate that atomic data uncertainties alone may limit abundance estimates to an accuracy of ~1.5%; systematic errors may be greater than this. This analysis uncertainty must be incorporated when attempting to make high-accuracy estimates of the helium abundance. For example, in recent determinations of the primordial helium abundance, uncertainties in the input atomic data have been neglected. Finally, we compare our theoretical calculations to the measured strengths of a few dozen helium emission lines in three nebulae, Orion (NGC 1976) and the planetary nebulae NGC 6572 and IC 4997. Incorporation of collisional effects yields noticeable improvements for some lines, but some notable discrepancies remain.

optical imaging has revealed an H II region in UGC 4483 with an H? flux of ~2 X 10-13^ erg cm-2^ A-1^ s-1^. Optical, ultraviolet, and near-infrared spectroscopy of this H II region yield He, C, N, O, Ne, and S abundances for the ISM in this galaxy. With an oxygen abundance of 2.1 X 10-5^(12 + log(O/H) = 7.3), this galaxy is among the most metal- poor dwarf irregulars known to date. A comparison of the S/O abundance ratio in this galaxy with that in I Zw 18 supports the claim by Garnett 1990 that the S/O ratio is consistent with the solar ratio for low- metallicity dwarf irregulars. The C/O ratio in UGC 4483 is lower than that derived for I Zw 18 and, therefore, more in line with the trend seen in higher metallicity H II regions. The derived helium abundance is He/H = 0.075, which converts to a helium mass fraction of 0.23, consistent with earlier determinations of the primordial helium abundance.

Optical and ultraviolet spectroscopy obtained with the Hubble Space Telescope Faint Object Spectrograph at three locations in the central H II complex of the amorphous starburst galaxy NGC 5253 confirms an apparent area of enhanced nitrogen abundance seen in ground-based studies. At two positions, N appears enhanced by a factor of 3 [log (N/O) = -0.85] compared to a third location 50 pc away where the measured N abundance is typical of metal-poor galaxies [log (N/O) = -1.30]. No other elemental species shows spatial abundance fluctuations, including C as measured from the C III] ll1907, 1909 A emission lines. Extinction to the emission-line regions is quite low (AV = 0.25), and we rule out reddening uncertainties as the cause of the apparent N enhancement. Comparison of the observed ionic fractions to predictions of photoionization models shows that ionization uncertainties cannot be responsible for the anomalously high N/O ratio. These findings require the existence of a N production mechanism that is decoupled from C and O production. Although the high N/He enrichment ratio is consistent with that expected from non-Type I planetary nebulae (PNs), the localized nature of the enrichment, the large number of PNs (150-500), and the long timescales (109 yr) required make this explanation highly unlikely. Given the proximity of the N overabundance to a very young, heavily obscured star cluster, we attribute the N excess to recent "pollution" from massive stars. Plausible N enrichment scenarios involve O star winds, He-deficient W-R star winds, and/or ejection events from luminous blue variables. If the N enrichment is due to localized pollution from the winds of W-R stars observed in the burst, then an accompanying He enrichment of 40% ought to be observed, but we are able to rule out He enhancements at the 2 s level at both locations showing N enhancement. We propose that if the N-rich wind or ejecta is incorporated into self-gravitating clumps of molecular gas, and if the 106 yr old clusters of massive stars in starburst galaxies are precursors of globular clusters, then N-overabundant halo and globular cluster stars in the Galaxy may owe their chemical peculiarity to similar N-enrichment episodes early in the history of the Milky Way.

We report H? imaging and optical spectrophotometry of a faint H IIregion in the Pegasus dwarf irregular galaxy, a low-luminosity, possible Local Group member. We find that the emission-line spectrum is consistent with excitation by a single star with a temperature of less than 35,000 K, in agreement with Aparicio & Gallart (1995). Although we cannot measure the electron temperature of the ionized gas directly, we can infer a narrow range in electron temperature by comparing the emission-line spectrum with photoionization models. Then we derive a relatively low oxygen abundance of approximately 10% of the solar value, in accordance with the well-known metallicity-luminosity relationship for dwarf irregular galaxies. We also find an N/O ratio of 6%, which is higher than the typical value of 3% found in more actively star-forming dwarf irregular galaxies. By comparing this high N/O ratio with the recent star formation history of the Pegasus dwarf irregular derived by Aparicio & Gallart, we hypothesize that this may be evidence of delayed N production.

This spectroscopic study of NGC 4214 is part of a project to determine the extent to which the winds and supernovae from massive stars contribute to the short-term, localizedchemical enrichment of the interstellar medium (ISM) in low-metallicity galaxies. Long-slit optical spectroscopy at 82 distinct spatial locations covering multiple starburst knots reveals no significant localizedO, N, or He abundance differences that might be attributed to the winds of massive stars. We do find large-scale(200 pc) variations in the oxygen abundance. The southernmost, and probably youngest, starburst region exhibits higher O abundances by 0.095±0.019 dex and correspondingly lower N/O (0.108±0.038 dex) than the rest of the bright emission-line regions. This difference is consistent with O pollution at the locations of the young starburst, possibly from recent supernovae. The surveyed regions exhibit an anticorrelation between N/O and O/H consistent with O pollution, but no significant correlations between Ne/O and O/H. If, as expected from nucleosynthesis models, O and Ne are produced predominantly in the same short-lived massive stars, and N is produced predominantly in less massive, longer lived stars, then these trends are both consistent with the O pollution hypothesis. We show that observed internal N/O and O variations within NGC 4214, NGC 5253, and NGC 3125 are consistent with theoretical chemical evolution predictions during a phase of O production associated with massive stars early in a starburst. While uncertainties in the electron temperature, T(O++) are the dominant source of error, we show that the observed abundance trends are not consistent with those expected from temperature uncertainty effects. The N/O and He/H ratios are relatively insensitive to temperature errors and should serve as good indicators of abundance inhomogeneities wherever they may exist.

We present a long-slit optical spectrophotometric survey covering 0.05 kpc2 in the nearby irregular "post-starburst" galaxy NGC 1569 to search for chemical gradients and inhomogeneities in the interstellar medium. Despite the presence of two massive evolved star clusters and numerous H II 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 at all locations are consistent with the results from the highest signal-to-noise ratio spectra: 12 + log (O/H) = 8.19 +- 0.02, log (N/O) = -1.39 +- 0.05, He/H = 0.080 +- 0.003. 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, even though the expected yield of heavy elements produced by massive stars in young starbursts is substantial. Based on a typical IMF, a dynamical mass of ~3 x 105 M, and an age of 20 Myr, roughly 450 stars in excess of 20 M should have already exploded as supernovae within the star cluster, A, in NGC 1569, releasing ~1000 M of oxygen and ~8 M of nitrogen. 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 <107 yr and are mixed instantaneously and uniformly; or (3) freshly synthesized elements remain unmixed with the surrounding interstellar medium and reside in a hard-to-observe hot 106 K phase or a cold, dusty, molecular phase. We advance the third scenario as the most plausible, and we suggest ways to locate the chemical products of massive star formation in starburst galaxies. Any successful model for chemical enrichment in these systems must be able to reproduce the appearance of chemical homogeneity on spatial scales of ~20-1000 pc and on temporal scales that are longer than the lifetimes of prominent H II regions (~107 yr). Such long timescales imply that the instantaneous recycling approximation sometimes used in galactic chemical evolution modeling is not generally applicable.

Optical and ultraviolet spectroscopy obtained with the HubbleSpaceTelescopeFaint Object Spectrograph at three locations in the central H IIcomplex of the amorphous starburst galaxy NGC 5253 confirms an apparent area of enhanced nitrogen abundance seen in ground-based studies. At two positions, N appears enhanced by a factor of 3 [log (N/O) = -0.85] compared to a third location 50 pc away where the measured N abundance is typical of metal-poor galaxies [log (N/O) = -1.30]. No other elemental species shows spatial abundance fluctuations, including C as measured from the C III] ??1907, 1909 Å emission lines. Extinction to the emission-line regions is quite low (AV= 0.25), and we rule out reddening uncertainties as the cause of the apparent N enhancement. Comparison of the observed ionic fractions to predictions of photoionization models shows that ionization uncertainties cannot be responsible for the anomalously high N/O ratio. These findings require the existence of a N production mechanism that is decoupled from C and O production. Although the high N/He enrichment ratio is consistent with that expected from non-Type I planetary nebulae (PNs), the localized nature of the enrichment, the large number of PNs (150-500), and the long timescales (109yr) required make this explanation highly unlikely. Given the proximity of the N overabundance to a very young, heavily obscured star cluster, we attribute the N excess to recent "pollution" from massive stars. Plausible N enrichment scenarios involve O star winds, He-deficient W-R star winds, and/or ejection events from luminous blue variables. If the N enrichment is due to localized pollution from the winds of W-R stars observed in the burst, then an accompanying He enrichment of 40% ought to be observed, but we are able to rule out He enhancements at the 2 ? level at bothlocations showing N enhancement. We propose that if the N-rich wind or ejecta is incorporated into self-gravitating clumps of molecular gas, and if the 106yr old clusters of massive stars in starburst galaxies are precursors of globular clusters, then N-overabundant halo and globular cluster stars in the Galaxy may owe their chemical peculiarity to similar N-enrichment episodes early in the history of the Milky Way.

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. These observations, in conjunction with photoionization models, indicate that the C/O ratios estimated from C III] l1909 and [O III] l5007 lines are consistent with those measured from C II] l2326 and [O II] l3727 lines. Although inherently more uncertain than C++/O++ due to poorly known ionization correction factors, we develop the use of the C+/O+ ratio as a reasonable substitute diagnostic of the carbon abundance in H II regions. The C II] l2326 multiplet is typically quite weak, but its proximity to the [O II] ll3726, 3729 lines makes these transitions a potential tool for measuring the carbon abundance in high-redshift emission-line objects. The derived chemical properties are consistent with a statistically significant correlation between N and C abundances in metal-poor extragalactic H II regions, in the sense that systems with the largest N/O ratios also have the highest C/O ratios. 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, instead, 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. The C/N ratio is, then, fixed by the initial mass function. Since the occurrence 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. As such, 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.

We include new data in an updated analysis of helium in low-metallicity extragalactic H II regions with the goal of deriving the primordial abundance of 4He (YP). We show that the new observations of Izotov et al. are consistent with previous data. However, they should not be taken in isolation to determine YP, owing to the lack of sufficiently low-metallicity points. We use the extant data in a semi-empirical approach to bounding the size of possible systematic uncertainties in the determination of YP. Our best estimate for the primordial abundance of 4He assuming a linear relation between 4He and O/H is YP = 0.230 +- 0.003(stat) based on the subset of H II regions with the lowest metallicity; for our full data set we find YP = 0.234 +- 0.002(stat). Both values are entirely consistent with our previous results. We discuss the implications of these values for standard big bang nucleosynthesis (SBBN), particularly in the context of recent measurements of deuterium in high-redshift, low-metallicity QSO absorption-line systems.

H II regions which exhibit surprisingly strong nebular He II ?4686 emission, with I(?4686)/I (H?) as high as 0.3 in some cases. The He II emission in-these objects is associated with a rather diverse set of stars, with WO stars, very early WN stars, and luminous X-ray binaries involved, and in one case what appears to be a normal O-type star. Extension of a thermal X-ray spectrum to lower energies appears to satisfactorily explain the He II emission associated with the X-ray binaries. For the remaining objects, stellar effective temperatures of at least 60,000 K are required to account for the observed He II emission, if photoionization by a hot stellar continuum is responsible. The existence of stars hot enough to produce such emission presents an interesting problem. for stellar atmospheric models and stellar evolution and may provide an explanation for the He II ? 4686 emission commonly observed in metal-poor extragalactic H II regions.