Variation in the dust emissivity index across M 33 withHerschelandSpitzer(HerM 33es)

Abstract

We study the wavelength dependence of the dust emission as a function of position and environment across the disk of M 33 using Spitzer and Herschel photometric data. M 33 is a Local Group spiral with slightly subsolar metallicity, which makes it an ideal stepping-stone to less regular and lower-metallicity objects such as dwarf galaxies and, probably, young-universe objects. Expressing the emissivity of the dust as a power law, the power-law exponent (β) was estimated from two independent approaches designed to properly treat the degeneracy between β and the dust temperature (T ). Both β and T are higher in the inner than in the outer disk, contrary to reported β − T anti-correlations found in other sources. In the cold + warm dust model, the warm component and the ionized gas (Hα) have a very similar distribution across the galaxy, demonstrating that the model separates the components in an appropriate way. Both cold-and warm-dust column densities are high in star-forming regions and reach their maxima toward the giant star-forming complexes NGC 604 and NGC 595. β declines from close to 2 in the center to about 1.3 in the outer disk. β is positively correlated with star formation and with the molecular gas column, as traced by the Hα and CO emission. The lower dust-emissivity index in the outer parts of M 33 is most likely related to the reduced metallicity (different grain composition) and possibly to a different size distribution. It is not due to the decrease in stellar radiation field or temperature in a simple way because the far-infrared-bright regions in the outer disk also have a low β. Like most spirals, M 33 has a (decreasing) radial gradient in star formation and molecular-to-atomic gas ratio such that the regions bright in Hα or CO tend to trace the inner disk, which makes it difficult to distinguish between their effects on the dust. The assumption of a constant emissivity index β is obviously not appropriate.

Dusty star-forming galaxies at high redshift

“…The spectral emissivity index, β, is often assumed to be 1.5 (and is found to usually range between 1-2 in starburst galaxies Hildebrand, 1983;Dunne & Eales, 2001;Chapin et al, 2011). Note that several works on nearby molecular clouds and dusty regions in nearby galaxies debate whether or not β also has temperature dependence, with laboratory experiments suggesting an anti-correlation (Lisenfeld et al, 2000;Dupac et al, 2003;Paradis et al, 2009;Shetty et al, 2009a,b;Veneziani et al, 2010;Bracco et al, 2011;Tabatabaei et al, 2013), although that has little impact on the implied SED fit for unresolved distant DSFGs where the effective temperature is only representative of the aggrigate dust temperatures contained within. Note that some works assume that DSFGs galaxies can be approximated as optically thin modified blackbodies, such that the (1−e −τ(ν) ) term reduces to ν β .…”

Radial distribution of dust, stars, gas, and star-formation rate in DustPedia face-on galaxies

“…A decreasing spectral index for the absorption cross section with radius, and thus possible variations in the dust properties along the disk, has been reported for MBB fits to M 31 (Smith et al 2012a) and M 33 (Tabatabaei et al 2014). Though this change in properties might affect the dust gradients, we did not try fits with varying β, as the dust mass determination would also need a knowledge of the unknown variations of the absorption cross section normalization with the environment.…”

High-resolution, 3D radiative transfer modeling

“…We, therefore, argue that the assumed relative dust-to-stellar scale height h z,d /h z, = 0.5 provides a good representation of the true vertical distribution of stars and dust in M 51. We do need to be cautious regarding uncertainties on the assumed dust model parameters, i.e., deviations from the assumed dust opacity κ, and its variation with wavelength (e.g., Tabatabaei et al 2014;Gordon et al 2014), which will reflect in a similar uncertainty on the dust mass determination, FUV attenuation, and dust emission in IR/submm wavebands.…”