Using a complete sample of ∼300 star-forming galaxies within 11 Mpc of the Milky Way, we evaluate the consistency between star formation rates (SFRs) inferred from the far ultraviolet (FUV) non-ionizing continuum and Hα nebular emission, assuming standard conversion recipes in which the SFR scales linearly with luminosity at a given wavelength. Our analysis probes SFRs over 5 orders of magnitude, down to ultra-low activities on the order of ∼10 −4 M yr −1 . The data are drawn from the 11 Mpc Hα and Ultraviolet Galaxy Survey (11HUGS), which has obtained Hα fluxes from ground-based narrowband imaging, and UV fluxes from imaging with GALEX. For normal spiral galaxies (SFR ∼ 1 M yr −1 ), our results are consistent with previous work which has shown that FUV SFRs tend to be lower than Hα SFRs before accounting for internal dust attenuation, but that there is relative consistency between the two tracers after proper corrections are applied. However, a puzzle is encountered at the faint end of the luminosity function. As lower luminosity dwarf galaxies, roughly less active than the Small Magellanic Cloud, are examined, Hα tends to increasingly underpredict the total SFR relative to the FUV. The trend is evident prior to corrections for dust attenuation, which affects the FUV more than the nebular Hα emission, so this general conclusion is robust to the effects of dust. Although past studies have suggested similar trends, this is the first time this effect is probed with a statistical sample for galaxies with SFR 0.1 M yr −1 . By SFR ∼ 0.003 M yr −1 , the average Hα-to-FUV flux ratio is lower than expected by a factor of two, and at the lowest SFRs probed, the ratio exhibits an order of magnitude discrepancy for the handful of galaxies that remain in the sample. A range of standard explanations does not appear to be able to fully account for the magnitude of the systematic. Some recent work has argued for a stellar initial mass function which is deficient in high-mass stars in dwarf and low surface brightness galaxies, and we also consider this scenario. Under the assumption that the FUV traces the SFR in dwarf galaxies more robustly, the prescription relating Hα luminosity to SFR is re-calibrated for use in the low SFR regime when FUV data are not available.

The survey description and the near-, mid-, and far-infrared flux properties are presented for the 258 galaxies in the Local Volume Legacy (LVL). LVL is a Spitzer Space Telescope legacy program that surveys the local universe out to 11 Mpc, built upon a foundation of ultraviolet, Hα, and Hubble Space Telescope imaging from 11HUGS (11 Mpc Hα and Ultraviolet Galaxy Survey) and ANGST (ACS Nearby Galaxy Survey Treasury). LVL covers an unbiased, representative, and statistically robust sample of nearby star-forming galaxies, exploiting the highest extragalactic spatial resolution achievable with Spitzer. As a result of its approximately volume-limited nature, LVL augments previous Spitzer observations of present-day galaxies with improved sampling of the lowluminosity galaxy population. The collection of LVL galaxies shows a large spread in mid-infrared colors, likely due to the conspicuous deficiency of 8 μm polycyclic aromatic hydrocarbon emission from low-metallicity, lowluminosity galaxies. Conversely, the far-infrared emission tightly tracks the total infrared emission, with a dispersion in their flux ratio of only 0.1 dex. In terms of the relation between the infrared-to-ultraviolet ratio and the ultraviolet spectral slope, the LVL sample shows redder colors and/or lower infrared-to-ultraviolet ratios than starburst galaxies, suggesting that reprocessing by dust is less important in the lower mass systems that dominate the LVL sample. Comparisons with theoretical models suggest that the amplitude of deviations from the relation found for starburst galaxies correlates with the age of the stellar populations that dominate the ultraviolet/optical luminosities.

We present MMT spectroscopic observations of H II regions in 42 low luminosity galaxies in the Spitzer Local Volume Legacy (LVL) survey. For 31 of the 42 galaxies in our sample, we were able to measure the temperature sensitive [O III] λ4363 line at a strength of 4σ or greater, and thus determine oxygen abundances using the "direct" method. Our results provide the first "direct" estimates of oxygen abundance for 19 of these galaxies. "Direct" oxygen abundances were compared to B -band luminosities, 4.5 µm luminosities, and stellar masses in order to characterize the luminosity-metallicity and mass-metallicity relationships at low-luminosity.We present and analyze a "Combined Select" sample composed of 38 objects (drawn from a sub-set of our parent sample and the literature) with "direct" oxygen abundances and reliable distance determinations (based on the tip of the red giant branch or Cepheid variables). Consistent with previous studies, the B-band and 4.5 µm luminosity-metallicity relationships for the 38 objects were found to be 12 + log (O/H) = (6.27±0.21)+(−0.11±0.01)M B and 12 + log(O/H) = (6.10±0.21)+(−0.10±0.01)M [4.5] † Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. arXiv:1205.6782v1 [astro-ph.CO] 30 May 2012 Low-Luminosity LVL SampleWe selected a sample of 42 low luminosity galaxies in the LVL survey in order to obtain new MMT high-resolution spectra. These low luminosity spirals and dwarf irregulars span a range in distance of 2.5 ≤ D ≤ 14.0 Mpc 2 . The luminosities for this sample range in the near-IR (determined from IRAC (Fazio et al. 2004) photometry) from M [4.5] = −13.1 to −21.7, with B -band magnitudes of −10.8 ≥ M B ≥ −18.8. Most of the objects were chosen because they lack "direct" oxygen abundances in the literature, their abundance estimates are dated, or were studied with instruments which were known to have problems.Although not LVL objects, two additional galaxies were added to the sample (increasing the sample total to 44 objects) because they played a role in motivating this project. Both UGC 4393 and UGC 10818 were identified by Engelbracht et al. (2008) as low metallicity outliers from the global trend of 70/160 µm color temperature as a function of metallicity. These two galaxies affect the interpretation of the trend for aromatic emission to weaken below 12 + log(O/H) = 7.9 in the mid-IR (see e.g., Engelbracht et al. 2008) and the far-IR (see e.g., Draine et al. 2007;Engelbracht et al. 2008).

1 Based on observations obtained with the Spitzer Space Telescope, which is operated by JPL, CalTech, under NASA Contract 1407.

The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map $7000 deg 2 of the high Galactic latitude sky visible from Arecibo, providing a H i line spectral database covering the redshift range between À1600 and 18,000 km s À1 with $5 km s À1 resolution. Exploiting Arecibo's large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the H i mass function in the local universe and will provide a census of H i in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide-area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local Supercluster and nearby superclusters, allow measurement of the H i diameter function, and enable a first wide-area blind search for local H i tidal features, H i absorbers at z < 0:06, and OH megamasers in the redshift range 0:16 < z < 0:25. Although completion of the survey will require some 5 years, public access to the ALFALFA data and data products will be provided in a timely manner, thus allowing its application for studies beyond those targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum intrusion, drift scan observing technique that samples the same region of sky at two separate epochs to aid in the discrimination of cosmic signals from noise and terrestrial interference. Survey simulations, which take into account large-scale structure in the mass distribution and incorporate experience with the ALFA system gained from tests conducted during its commissioning phase, suggest that ALFALFA will detect on the order of 20,000 extragalactic H i line sources out to z $ 0:06, including several hundred with H i masses M H i < 10 7:5 M .

We present the results of low-dispersion optical spectroscopy of 186 H II regions spanning a range of radius in 13 spiral galaxies. Abundances for several elements (oxygen, nitrogen, neon, sulfur, and argon) were determined for 185 of the H II regions. As expected, low metallicities were found for the outlying H II regions of these spiral galaxies. Radial abundance gradients were derived for the 11 primary galaxies ; similar to results for other spiral galaxies, the derived abundance gradients are typically [0.04 to [0.07 dex kpc~1.

We present results from a GALEX ultraviolet (UV) survey of a complete sample of 390 galaxies within ∼11 Mpc of the Milky Way. The UV data are a key component of the composite Local Volume Legacy (LVL), an ultravioletto-infrared imaging program designed to provide an inventory of dust and star formation in nearby spiral and irregular galaxies. The ensemble dataset is an especially valuable resource for studying star formation in dwarf galaxies, which comprise over 80% of the sample. We describe the GALEX survey programs which obtained the data and provide a catalog of far-UV (∼ 1500Å) and near-UV (∼2200Å) integrated photometry. General UV properties of the sample are briefly discussed. We compute two measures of the global star formation efficiency, the SFR per unit HI gas mass and the SFR per unit stellar mass, to illustrate the significant differences that can arise in our understanding of dwarf galaxies when the FUV is used to measure the SFR instead of Hα. We find that dwarf galaxies may not be as drastically inefficient in coverting gas into stars as suggested by prior Hα studies. In this context, we also examine the UV properties of late-type dwarf galaxies that appear to be devoid of star formation because they were not detected in previous Hα narrowband observations. Nearly all such galaxies in our sample are detected in the FUV, and have FUV SFRs that fall below the limit where the Hα flux is robust to Poisson fluctuations in the formation of massive stars. Otherwise, the UV colors and star formation efficiencies of Hα-undetected, UV-bright dwarf irregulars appear to be relatively unremarkable with respect to those exhibited by the general population of star-forming galaxies.

We present the results of high spatial resolution HI observations of five intrinsically compact dwarf galaxies which are currently experiencing a strong burst of star formation. The HI maps indicate that these systems have a complex and clumpy interstellar medium. Unlike typical dwarf irregular galaxies, these Blue Compact Dwarf (BCD) galaxies exhibit strong central concentrations in their neutral gas distributions which may provide a clue to the origin of their strong star-burst activity. Furthermore, while all of the systems do appear to be rotating, based on observed velocity gradients, the kinematics are complex. All systems have non-ordered kinematic structure at some level; some of the extended gas is not necessarily kinematically connected to the main system.The observed gas distributions and kinematics place constraints on evolutionary scenarios for BCDs. Evolutionary links between BCDs, dwarf irregulars, and dwarf ellipticals have been postulated to explain their high star formation rates and low 1 Jansky Fellow 2 The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities Inc.3 NSF Presidential Faculty Fellow luminosity, low metallicity nature. The BCDs appear to have higher central mass concentrations in both gas and stellar content than the dwarf irregulars, indicating that evolutionary scenarios connecting these two classes will require mass redistribution. In addition, the fact that BCDs are rotationally supported systems indicates that BCDs are unlikely to evolve into dwarf ellipticals without substantial loss of angular momentum. Thus, while such evolutionary scenarios may still be possible with the aid of mergers or tidal interactions, the isolated nature of BCDs suggests that the majority of BCDs will not fade to become objects similar to the present day dwarf ellipticals.