We report 4.5 µm luminosities for 27 nearby (D 5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 µm luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 µm 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 µm. 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 five 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 Space Telescope 4.5 and 8.0 µm imaging of 15 Local Group and nearby dwarf galaxies. We find that the diffuse 8 micron emission is spatially correlated with regions of active star formation in these spatially resolved 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 µm emission, originating from hot dust and PAHs, on these parameters. We detect prominent diffuse 8 µm 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. These data are the first spatially resolved images of diffuse 8 µm emission -2from such low-metallicity objects (12+log(O/H)∼7.5). We observe general correlations of the diffuse 8 µm 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 µm emission cannot be attributed to young galaxy ages. Also, we find that winds are unlikely to explain the paucity of diffuse 8 µm 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 µm 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 µm emission is at least partly due to a general absence of dust (including PAHs), in agreement with their low metallicities.a The systematic uncertainty in the current star formation rates derived from Hα luminosities is estimated by Kennicutt (1983) to be roughly 50% or less. b 4.5 µm fluxes are from Lee et al. (2006) except Antlia, DDO 210, Leo A, Phoenix, and UGCA 438. Fluxes for these targets were calculated as described in Lee et al. (2006). c The diffuse 8 µm flux listed for Sextans A is only for the large H II region in the southeast. See §3.6.

We present Spitzer/IRAC observations at 3.6 and 4.5 µm 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 asymptotic giant branch (AGB) stars. We find 39% of the infrared-detected AGB stars are not detected in the optical data, even though our 50% completeness limit is three magnitudes 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 narrow-band 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 Ṁ = 0.7-2.4×10 −3 M ⊙ yr −1 . The distribution 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 present Australia Telescope Compact Array mosaic H I imaging of the Local Group dwarf irregular galaxy WLM. We find an integrated flux of 149 Jy km s −1 and a total H I mass of (3.2±0.3)×10 7 M ⊙ . The major axis of the H I is aligned with the stellar component of the galaxy. The overall H I distribution is relatively smooth at our resolution and has a double peaked central core. One of these peaks is aligned with a region found to have extinction that is internal to WLM and we take this as possible evidence of a large molecular gas complex in the southern half of the galaxy. The other H I peak is in close proximity to the brightest H II regions. WLM's overall velocity field is consistent with rigid body rotation. A rotation curve is derived, and we find a total dynamical mass of (3.00±0.80)×10 8 M ⊙ . We also performed a wide field search, 38 ′ in radius, for H I companions or evidence of recent interactions (e.g., tidal tails), and found no detections to an H I mass limit of M HI > 8.4×10 5 M ⊙ .

No abstract

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

For a sample of 25 dwarf irregular galaxies with distances D ∼ < 5 Mpc and measured oxygen abundances, we present results derived from galaxy luminosities at 4.5 µm and stellar masses from near-infrared imaging with IRAC on the Spitzer Space Telescope. We have constructed the appropriate luminosity-metallicity (L-Z) and mass-metallicity (M -Z) relations, and compared these relations with their corresponding relations from the Sloan Digital Sky Survey (SDSS). We obtain the following results. 1. The dispersion in the near-infrared L-Z relation is reduced with respect to the dispersion in the optical L-Z relation, which agrees with expectations for reduced variations of stellar mass-to-light ratios at longer wavelengths compared to optical wavelengths. 2. The dispersion in the optical L-Z relation is similar over approximately 11 mag in optical luminosity. 3. With our constructed M -Z relation, we have extended the SDSS M -Z relation to lower masses by about 2.5 dex in stellar mass. 4. The dispersion in the M -Z relation appears to be comparable over a range of 5.5 dex in stellar mass.

No abstract