Spatially Resolved PAH Emission Features in Nearby, Low Metallicity, Star-Forming Galaxies

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)~8.0, 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 micron)/(11.3 micron), (7.7 micron)/(11.3 micron), (8.6 micron)/(11.3 micron), (7.7 micron)/(6.2 micron), and (8.6 micron)/(6.2 micron) 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 micron) 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 micron)/(6.2 micron) and (8.6 micron)/(6.2 micron) to the mixed (CC/CH) mode PAH ratio (7.7 micron)/(11.3 micron). 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.

Astrophysical Journal, in press

References in corpus (10)

• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• Metallicity Effects on Dust Properties in Starbursting Galaxies
• Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
• The Behavior of the Aromatic Features in M101 HII Regions: Evidence for Dust Processing
• Spectral Mapping Reconstruction of Extended Sources
• The Spitzer View of Low-Metallicity Star Formation: III. Fine Structure Lines, Aromatic Features, and Molecules
• Aromatic emission from the ionised mane of the Horsehead nebula
• Hot Dust and PAH Emission at Low Metallicity: A Spitzer Survey of Local Group and Other Nearby Dwarf Galaxies
• Anomalously low PAH emission from low-luminosity galaxies
• The Nature of Infrared Emission in the Local Group Dwarf Galaxy NGC 6822 As Revealed by Spitzer

Cited by in corpus (5)

• Spitzer Local Volume Legacy (LVL) SEDs and Physical Properties
• Early-stage young stellar objects in the Small Magellanic Cloud
• Polycyclic aromatic hydrocarbons in spatially resolved extragalactic star forming complexes
• Pre-existing dwarfs, tidal knots and a tidal dwarf galaxy: an unbiased HI study of the gas-rich interacting galaxy group NGC 3166/9
• 3.3 μm PAH observations of the central kiloparsecs of Centaurus A