Planckearly results. XXV. Thermal dust in nearby molecular clouds

Abstract

Planck allows unbiased mapping of Galactic sub-millimetre and millimetre emission from the most diffuse regions to the densest parts of molecular clouds. We present an early analysis of the Taurus molecular complex, on line-of-sight-averaged data and without component separation. The emission spectrum measured by Planck and IRAS can be fitted pixel by pixel using a single modified blackbody. Some systematic residuals are detected at 353 GHz and 143 GHz, with amplitudes around −7% and +13%, respectively, indicating that the measured spectra are likely more complex than a simple modified blackbody. Significant positive residuals are also detected in the molecular regions and in the 217 GHz and 100 GHz bands, mainly caused by the contribution of the J = 2 → 1 and J = 1 → 0 12 CO and 13 CO emission lines. We derive maps of the dust temperature T , the dust spectral emissivity index β, and the dust optical depth at 250 μm τ 250 . The temperature map illustrates the cooling of the dust particles in thermal equilibrium with the incident radiation field, from 16−17 K in the diffuse regions to 13−14 K in the dense parts. The distribution of spectral indices is centred at 1.78, with a standard deviation of 0.08 and a systematic error of 0.07. We detect a significant T − β anti-correlation. The dust optical depth map reveals the spatial distribution of the column density of the molecular complex from the densest molecular regions to the faint diffuse regions. We use near-infrared extinction and H i data at 21-cm to perform a quantitative analysis of the spatial variations of the measured dust optical depth at 250 μm per hydrogen atom τ 250 /N H . We report an increase of τ 250 /N H by a factor of about 2 between the atomic phase and the molecular phase, which has a strong impact on the equilibrium temperature of the dust particles.

The Atacama Large Millimeter/submillimeter Array Spectroscopic Survey in the Hubble Ultra Deep Field: CO Emission Lines and 3 mm Continuum Sources

“…In Figure 13 we present the 3 mm number counts scaled from 1.1 mm using a range of parameters motivated by previous studies (blue-shaded region). For the dust emissivity index we use a range of β=1.5-2.0 (Dunne & Eales 2001;Chapin et al 2009;Clements et al 2010;Draine 2011;Planck Collaboration et al 2011a, 2011b, for the dust temperature we take a range of 25-40 K (Magdis et al 2012;Magnelli et al 2014;Schreiber et al 2018), and for the redshift we take the median redshift of z=2.5 found for sources detected in this work ( Table 7). We also show in Figure 13 the 3 mm number counts scaled from 1.1 mm when assuming a dust temperature of 35 K, which is the average dust temperature expected for star-forming galaxies at z=2.5 as presented by Schreiber et al (2018).…”