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.

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

The Faint Irregular Galaxies GMRT Survey (FIGGS) is a Giant Metrewave Radio Telescope (GMRT) based H I imaging survey of a systematically selected sample of extremely faint nearby dwarf irregular galaxies. The primary goal of FIGGS is to provide a comprehensive and statistically robust characterization of the neutral interstellar medium properties of faint, gas-rich dwarf galaxies. The FIGGS galaxies represent the extremely low mass end of the dwarf irregular galaxies population, with a median M B ∼ −13.0 and median H I mass of ∼3 × 10 7 M , extending the baseline in mass and luminosity space for a comparative study of galaxy properties. The H I data are supplemented with observations at other wavelengths. In addition, distances accurate to ∼10 per cent are available for most of the sample galaxies. This paper gives an introduction to FIGGS, describes the GMRT observations and presents the first results from the H I observations. From the FIGGS data, we confirm the trend of increasing H I to optical diameter ratio with decreasing optical luminosity; the median ratio of D H I /D Ho for the FIGGS sample is 2.4. Further, on comparing our data with aperture synthesis surveys of bright spirals, we find at best marginal evidence for a decrease in average surface density with decreasing H I mass. To a good approximation, the discs of gas-rich galaxies, ranging over three orders of magnitude in H I mass, can be described as being drawn from a family with fixed H I average surface density.

In this Letter, we announce the discovery of a new dwarf galaxy, Leo T, in the Local Group. It was found as a stellar overdensity in the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram of Leo T shows two well-defined features, which we interpret as a red giant branch and a sequence of young, massive stars. As judged from fits to the color-magnitude diagram, it lies at a distance of about 420 kpc and has an intermediate-age stellar population with a metallicity of [Fe/H]= -1.6, together with a young population of blue stars of age of 200 Myr. There is a compact cloud of neutral hydrogen with mass roughly 10^5 solar masses and radial velocity 35 km/s coincident with the object visible in the HIPASS channel maps. Leo T is the smallest, lowest luminosity galaxy found to date with recent star-formation. It appears to be a transition object similar to, but much lower luminosity than, the Phoenix dwarf.Comment: Ap J (Letters) in press, the subject of an SDSS press release toda

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To investigate the fundamental principles of H 2 formation in a giant molecular cloud, we derive the Hi and H 2 surface density (Σ Hi and Σ H2 ) images of the Perseus molecular cloud on sub-pc scales (∼0.4 pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V -band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the Hi data from the Galactic Arecibo L-band Feed Array Hi Survey and an estimate of the local dust-to-gas ratio, we then derive the Σ H2 distribution across Perseus. We find a relatively uniform Σ Hi ∼ 6-8 M pc −2 for both dark and star-forming regions, suggesting a minimum Hi surface density required to shield H 2 against photodissociation. As a result, a remarkably tight and consistent relation is found between Σ H2 /Σ Hi and Σ Hi + Σ H2 . The transition between the Hi-and H 2 -dominated regions occurs at N (Hi) + 2N (H 2 ) ∼ (8-14) × 10 20 cm −2 . Our findings are consistent with predictions for H 2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H 2 formation. However, the importance of warm neutral medium for H 2 shielding, internal radiation field, and the timescale of H 2 formation still remain as open questions. We also compare H 2 and CO distributions and estimate the fraction of "CO-dark" gas, f DG ∼ 0.3. While significant spatial variations of f DG are found, we do not find a clear correlation with the mean V -band extinction.

We present Giant Meterwave Radio Telescope (GMRT) and Westerbork Synthesis Radio Telescope (WSRT) observations of the recently discovered Local Group dwarf galaxy, Leo T. The peak H i column density is measured to be 7 × 1020 cm−2, and the total H i mass is 2.8 × 105 M⊙, based on a distance of 420 kpc. Leo T has both cold (∼500 K) and warm (∼6000 K) H i at its core, with a global velocity dispersion of 6.9 km s−1, from which we derive a dynamical mass within the H i radius of 3.3 × 106 M⊙, and a mass‐to‐light ratio of >50. We calculate the Jeans mass from the radial profiles of the H i column density and velocity dispersion, and predict that the gas should be globally stable against star formation. This finding is inconsistent with the half light radius of Leo T, which extends to 170 pc, and indicates that local conditions must determine where star formation takes place. Leo T is not only the lowest luminosity galaxy with on‐going star formation discovered to date, but it is also the most dark matter‐dominated, gas‐rich dwarf in the Local Group.

We compare the gas distribution, kinematics and the current star formation in a sample of 10 very faint (-13.37 < M_B < -9.55) dwarf galaxies. For 5 of these galaxies we present fresh, high sensitivity, GMRT HI 21cm observations. For all our galaxies we construct maps of the HI column density at a constant linear resolution of ~300 pc; this forms an excellent data set to check for the presence of a threshold column density for star formation. We find that while current star formation (as traced by Halpha emission) is confined to regions with relatively large (N_HI > (0.4 -1.7) X 10^{21} atoms cm^{-2}) HI column density, the morphology of the Halpha emission is in general not correlated with that of the high HI column density gas. Thus, while high column density gas may be necessary for star formation, in this sample at least, it is not sufficient to ensure that star formation does in fact occur. We examine the line profiles of the HI emission, but do not find a simple relation between regions with complex line profiles and those with on-going star formation. Finally, we examine the very fine scale (~20-100 pc) distribution of the HI gas, and find that at these scales the emission exhibits a variety of shell like, clumpy and filamentary features. The Halpha emission is sometimes associated with high density HI clumps, sometimes the Halpha emission lies inside a high density shell, and sometimes there is no correspondence between the Halpha emission and the HI clumps. In summary, the interplay between star formation and gas density in these galaxy does not seem to show the simple large scale patterns observed in brighter galaxies (abridged).Comment: 15 pages, 6 tables, 13 figures. Accepted for publication in MNRA