Helene Roussel

We present observations of the dust and atomic gas phase in seven dwarf irregular galaxies of the M81 group from the Spitzer SINGS and VLA THINGS surveys. The Spitzer observations provide a first glimpse of the nature of the nonatomic ISM in these metal-poor (Z ~ 0.1 Z_☉), quiescent (SFR ~ 0.001-0.1 M_☉ yr^-1) dwarf galaxies. Most detected dust emission is restricted to H I column densities >1 × 10²¹ cm^-2, and almost all regions of high H I column density (>2.5 × 10²¹ cm^-2) have associated dust emission. Spitzer spectroscopy of two regions in the brightest galaxies (IC 2574 and Holmberg II) show distinctly different spectral shapes and aromatic features, although the galaxies have comparable gas-phase metallicities. This result emphasizes that the strength of the aromatic features is not a simple linear function of metallicity. We estimate dust masses of ~10⁴-10⁶ M_☉ for the M81 dwarf galaxies, resulting in an average dust-to-gas ratio (M_dust/M_{H I}) of ~3 × 10^-4 (1.5 × 10^-3 if only the H I that is associated with dust emission is considered); this is an order of magnitude lower than the typical value derived for the SINGS spirals. The dwarf galaxies are underluminous per unit star formation rate at 70 μm as compared to the more massive galaxies in SINGS by a factor of ~2. However, the average 70/160 μm ratio in the sample dwarf galaxies is higher than what is found in the other galaxies of the SINGS sample. This can be explained by a combination of a lower dust content in conjunction with a higher dust temperature in the dwarfs.

We present Spitzer imaging of the metal-deficient (Z 30% Z_☉) Local Group dwarf galaxy NGC 6822. On spatial scales of ~130 pc, we study the nature of IR, Hα, H I, and radio continuum emission. Nebular emission strength correlates with IR surface brightness; however, roughly half of the IR emission is associated with diffuse regions not luminous at Hα (as found in previous studies). The global ratio of dust to H I gas in the ISM, while uncertain at the factor of ~2 level, is ~25 times lower than the global values derived for spiral galaxies using similar modeling techniques; localized ratios of dust to H I gas are about a factor of 5 higher than the global value in NGC 6822. There are strong variations (factors of ~10) in the relative ratios of Hα and IR flux throughout the central disk; the low dust content of NGC 6822 is likely responsible for the different Hα/IR ratios compared to those found in more metal-rich environments. The Hα and IR emission is associated with high column density (10²¹ cm^-2) neutral gas. Increases in IR surface brightness appear to be affected by both increased radiation field strength and increased local gas density. Individual regions and the galaxy as a whole fall within the observed scatter of recent high-resolution studies of the radio-far-IR correlation in nearby spiral galaxies; this is likely the result of depleted radio and far-IR emission strengths in the ISM of this dwarf galaxy.

We present Spitzer imaging of the metal-deficient (Z ~30% Z_sun) Local Group dwarf galaxy NGC 6822. On spatial scales of ~130 pc, we study the nature of IR, H alpha, HI, and radio continuum emission. Nebular emission strength correlates with IR surface brightness; however, roughly half of the IR emission is associated with diffuse regions not luminous at H alpha (as found in previous studies). The global ratio of dust to HI gas in the ISM, while uncertain at the factor of ~2 level, is ~25 times lower than the global values derived for spiral galaxies using similar modeling techniques; localized ratios of dust to HI gas are about a factor of five higher than the global value in NGC 6822. There are strong variations (factors of ~10) in the relative ratios of H alpha and IR flux throughout the central disk; the low dust content of NGC 6822 is likely responsible for the different H alpha/IR ratios compared to those found in more metal-rich environments. The H alpha and IR emission is associated with high-column density (> ~1E21 cm^-2) neutral gas. Increases in IR surface brightness appear to be affected by both increased radiation field strength and increased local gas density. Individual regions and the galaxy as a whole fall within the observed scatter of recent high-resolution studies of the radio-far IR correlation in nearby spiral galaxies; this is likely the result of depleted radio and far-IR emission strengths in the ISM of this dwarf galaxy.

We present Spitzer observations of the dwarf starburst galaxy NGC 1705 obtained as part of SINGS. The galaxy morphology is very different shortward and longward of ~5 microns: short-wavelength imaging shows an underlying red stellar population, with the central super star cluster (SSC) dominating the luminosity; longer-wavelength data reveals warm dust emission arising from two off-nuclear regions offset by ~250 pc from the SSC. These regions show little extinction at optical wavelengths. The galaxy has a relatively low global dust mass (~2E5 solar masses, implying a global dust-to-gas mass ratio ~2--4 times lower than the Milky Way average). The off-nuclear dust emission appears to be powered by photons from the same stellar population responsible for the excitation of the observed H Alpha emission; these photons are unassociated with the SSC (though a contribution from embedded sources to the IR luminosity of the off-nuclear regions cannot be ruled out). Low-resolution IRS spectroscopy shows moderate-strength PAH emission in the 11.3 micron band in the eastern peak; no PAH emission is detected in the SSC or the western dust emission complex. There is significant diffuse 8 micron emission after scaling and subtracting shorter wavelength data; the spatially variable PAH emission strengths revealed by the IRS data suggest caution in the interpretation of diffuse 8 micron emission as arising from PAH carriers alone. The metallicity of NGC 1705 falls at the transition level of 35% solar found by Engelbracht and collaborators; the fact that a system at this metallicity shows spatially variable PAH emission demonstrates the complexity of interpreting diffuse 8 micron emission. A radio continuum non-detection, NGC 1705 deviates significantly from the canonical far-IR vs. radio correlation. (Abridged)