Evidence for Environmental Changes in the Submillimeter Dust Opacity

Abstract

The submillimeter opacity of dust in the diffuse interstellar medium (ISM) in the Galactic plane has been quantified using a pixel-by-pixel correlation of images of continuum emission with a proxy for column density. We used multi-wavelength continuum data: three Balloon-borne Large Aperture Submillimeter Telescope bands at 250, 350, and 500 μm and one IRAS band at 100 μm. The proxy is the near-infrared color excess, E(J − K s ), obtained from the Two Micron All Sky Survey. Based on observations of stars, we show how well this color excess is correlated with the total hydrogen column density for regions of moderate extinction. The ratio of emission to column density, the emissivity, is then known from the correlations, as a function of frequency. The spectral distribution of this emissivity can be fit by a modified blackbody, whence the characteristic dust temperature T and the desired opacity σ e (1200) at 1200 GHz or 250 μm can be obtained. We have analyzed 14 regions near the Galactic plane toward the Vela molecular cloud, mostly selected to avoid regions of high column density (N H > 10 22 cm −2 ) and small enough to ensure a uniform dust temperature. We find σ e (1200) is typically (2-4) × 10 −25 cm 2 H −1 and thus about 2-4 times larger than the average value in the local high Galactic latitude diffuse atomic ISM. This is strong evidence for grain evolution. There is a range in total power per H nucleon absorbed (and re-radiated) by the dust, reflecting changes in the strength of the interstellar radiation field and/or the dust absorption opacity. These changes in emission opacity and power affect the equilibrium T, which is typically 15 K, colder than at high latitudes. Our analysis extends, to higher opacity and lower temperature, the trend of increasing σ e (1200) with decreasing T that was found at high latitudes. The recognition of changes in the emission opacity raises a cautionary flag because all column densities deduced from dust emission maps, and the masses of compact structures within them, depend inversely on the value adopted.

Cosmic rays, gas, and dust in local clouds

“…The small amplitude of the E(B−V)/N H variations with increasing N H confirms that the large rise in opacity τ 353 /N H seen toward dense CO clouds is primarily due to changes in dust emissivity. The environmental changes are qualitatively compatible with model predictions based on mantle accretion on the grains and the formation of grain aggregates (Martin et al 2012;Köhler et al 2015). The confirmation of large opacity variations across clouds directly 85 impacts the gas mass estimates inferred from dust emission at sub-mm and mm wavelengths to derive star-forming efficiencies in the Galaxy and in external galaxies.…”

“…Those assumptions still need to be tested in a variety of clouds and across complex phase changes, especially on consideration that dust grains and their radiation properties vary from the diffuse ISM to molecular clouds (Stepnik et al 2003;Flagey et al 2009;Martin et al 2012;Roy et al 2013;Ysard et al 2013;Planck Collaboration 2014a,b;20 Planck and Fermi Collaborations 2015). One can also test the uniformity of the Local γ-ray emissivity spectrum of the gas (Casandjian 2015) in well-resolved nearby clouds to verify the smooth penetration of CRs with energies above ∼1 GeV (Skilling & Strong 1976;Cesarsky & Volk 1978;Morlino & Gabici 2015).…”

Submillimetre Studies of Prestellar and Starless Cores in the Ophiuchus, Taurus and Cepheus Molecular Clouds

“…Our low value of β may be due to there being multiple dust temperature components along the lines of sight toward our sources, which would broaden the SEDs of the sources, and hence lower their apparent β values. This effect was discussed in detail by Martin et al (2012).…”

On the possible correlation of Galactic very-high energy source locations and enhancements of the surface density in the Galactic plane

“…An environment more similar to the mass enhancements studied here is the Orion molecular cloud, where the opacities can be larger by a factor of ∼2−4 (Herschel data from Roy et al 2013). As they note, much higher opacities than what is obtained for the ISM are not unprecedented; another example is σ e (1200 GHz) = 3.8×10 −25 cm 2 H −1 for a dense molecular region in the Vela molecular ridge where the column density ranged between [10 ... 40] × 10 21 cm −2 (Martin et al 2012). A study of the impact of dust grain composition is given in Coupeaud et al (2011), where values as high as κ 850 μm 0.8 m 2 /kg are shown.…”

“…In the case of PWN, the VHE emission needs to also consider the inverse Compton scattering (ICS) of the accelerated electrons off the thermal radiation associated to the dust (e.g., Martin et al 2012). The VHE emission has also been predicted to be produced in regions of massive-star formation.…”