Planck early results. XIX. All-sky temperature and dust optical depth from Planck and IRAS. Constraints on the (original) (raw)
Related papers
Astronomy & Astrophysics, 2011
An all sky map of the apparent temperature and optical depth of thermal dust emission is constructed using the Planck-HFI (350 µm to 2 mm) and IRAS (100 µm) data. The optical depth maps are correlated with tracers of the atomic (H i) and molecular gas traced by CO. The correlation with the column density of observed gas is linear in the lowest column density regions at high Galactic latitudes. At high N H , the correlation is consistent with that of the lowest N H , for a given choice of the CO-to-H 2 conversion factor. In the intermediate N H range, a departure from linearity is observed, with the dust optical depth in excess of the correlation. This excess emission is attributed to thermal emission by dust associated with a dark gas phase, undetected in the available H i and CO surveys. The 2D spatial distribution of the dark gas in the solar neighbourhood (|b II | > 10 • ) is shown to extend around known molecular regions traced by CO. The average dust emissivity in the H i phase in the solar neighbourhood is found to be τ D /N tot H = 5.2 × 10 −26 cm 2 at 857 GHz. It follows roughly a power law distribution with a spectral index β = 1.8 all the way down to 3 mm, although the SED flattens slightly in the millimetre. Taking into account the spectral shape of the dust optical depth, the emissivity is consistent with previous values derived from FIRAS measurements at high latitudes within 10%. The threshold for the existence of the dark gas is found at N tot H = (8.0±0.58)×10 20 Hcm −2 (A V = 0.4 mag). Assuming the same high frequency emissivity for the dust in the atomic and the molecular phases leads to an average X CO = (2.54 ± 0.13) × 10 20 H 2 cm −2 /(K km s −1 ). The mass of dark gas is found to be 28% of the atomic gas and 118% of the CO emitting gas in the solar neighbourhood. The Galactic latitude distribution shows that its mass fraction is relatively constant down to a few degrees from the Galactic plane. A possible explanation for the dark gas lies in a dark molecular phase, where H 2 survives photodissociation but CO does not. The observed transition for the onset of this phase in the solar neighbourhood (A V = 0.4 mag) appears consistent with recent theoretical predictions. It is also possible that up to half of the dark gas could be in atomic form, due to optical depth effects in the H i measurements.
2011
An all sky map of the apparent temperature and optical depth of thermal dust emission is constructed using the Planck-HFI (350 µm to 2 mm) and IRAS (100 µm) data. The optical depth maps are correlated with tracers of the atomic (H i) and molecular gas traced by CO. The correlation with the column density of observed gas is linear in the lowest column density regions at high Galactic latitudes. At high N H , the correlation is consistent with that of the lowest N H , for a given choice of the CO-to-H 2 conversion factor. In the intermediate N H range, a departure from linearity is observed, with the dust optical depth in excess of the correlation. This excess emission is attributed to thermal emission by dust associated with a dark gas phase, undetected in the available H i and CO surveys. The 2D spatial distribution of the dark gas in the solar neighbourhood (|b II | > 10 • ) is shown to extend around known molecular regions traced by CO. The average dust emissivity in the H i phase in the solar neighbourhood is found to be τ D /N tot H = 5.2 × 10 −26 cm 2 at 857 GHz. It follows roughly a power law distribution with a spectral index β = 1.8 all the way down to 3 mm, although the SED flattens slightly in the millimetre. Taking into account the spectral shape of the dust optical depth, the emissivity is consistent with previous values derived from FIRAS measurements at high latitudes within 10%. The threshold for the existence of the dark gas is found at N tot H = (8.0±0.58)×10 20 Hcm −2 (A V = 0.4 mag). Assuming the same high frequency emissivity for the dust in the atomic and the molecular phases leads to an average X CO = (2.54 ± 0.13) × 10 20 H 2 cm −2 /(K km s −1 ). The mass of dark gas is found to be 28% of the atomic gas and 118% of the CO emitting gas in the solar neighbourhood. The Galactic latitude distribution shows that its mass fraction is relatively constant down to a few degrees from the Galactic plane. A possible explanation for the dark gas lies in a dark molecular phase, where H 2 survives photodissociation but CO does not. The observed transition for the onset of this phase in the solar neighbourhood (A V = 0.4 mag) appears consistent with recent theoretical predictions. It is also possible that up to half of the dark gas could be in atomic form, due to optical depth effects in the H i measurements.
Dark gas: a new possible link between low and high-energy phenomena
Proceedings of the International Astronomical Union, 2011
Galactic-scale studies of γ-rays and sub-mm radiation suggest that a significant amount of neutral interstellar medium is not detectable either in CO or HI (Grenier et al. 2005; Ade et al. 2011). This component is called “dark gas”. Here we argue that cool and dense atomic gas without molecules is responsible for the dark gas. This interpretation is supported by a recent finding of cool HI gas corresponding to the TeV γ-ray shell in the SNR RX J1713.7-3946 (Fukui et al. 2011). Such HI gas is not recognized under a usual assumption of optically thin HI emission but is identified by a careful analysis considering optically thick HI. The typical column density of such HI gas is a few times 1021 cm−2 and is also identified as visual extinction.
The nearby Chamaeleon clouds have been observed in γ rays by the Fermi Large Area Telescope (LAT) and in thermal dust emission by Planck and IRAS. Cosmic rays and large dust grains, if smoothly mixed with gas, can jointly serve with the H i and 12 CO radio data to: (i) map the hydrogen column densities, N H , in the different gas phases, in particular at the dark neutral medium (DNM) transition between the H i-bright and CO-bright media; (ii) constrain the CO-to-H 2 conversion factor, X CO ; and (iii) probe the dust properties per gas nucleon in each phase and map their spatial variations across the clouds. We have separated clouds at local, intermediate, and Galactic velocities in H i and 12 CO line emission to model in parallel the γ-ray intensity recorded between 0.4 and 100 GeV, the dust optical depth at 353 GHz, τ 353 , the thermal radiance of the large grains, and an estimate of the dust extinction, A VQ , empirically corrected for the starlight intensity. The dust and γ-ray models have been coupled to account for the DNM gas. The consistent γ-ray emissivity spectra recorded in the different phases confirm that the GeV-TeV cosmic rays probed by the LAT uniformly permeate all gas phases up to the 12 CO cores. The dust and cosmic rays both reveal large amounts of DNM gas, with comparable spatial distributions and twice as much mass as in the CO-bright clouds. We give constraints on the H i-DNM-CO transitions for five separate clouds. CO-dark H 2 dominates the molecular columns up to A V 0.9 and its mass often exceeds the one-third of the molecular mass expected by theory. The corrected A VQ extinction largely provides the best fit to the total gas traced by the γ rays. Nevertheless, we find evidence for a marked rise in A VQ /N H with increasing N H and molecular fraction, and with decreasing dust temperature. The rise in τ 353 /N H is even steeper. We observe variations of lesser amplitude and orderliness for the specific power of the grains, except for a coherent decline by half in the CO cores. This combined information suggests grain evolution. We provide average values for the dust properties per gas nucleon in the different phases. The γ rays and dust radiance yield consistent X CO estimates near 0.7 × 10 20 cm −2 K −1 km −1 s. The A VQ and τ 353 tracers yield biased values because of the large rise in grain opacity in the CO clouds. These results clarify a recurrent disparity in the γ-ray versus dust calibration of X CO , but they confirm the factor of 2 difference found between the X CO estimates in nearby clouds and in the neighbouring spiral arms.
Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations
The Astrophysical Journal
Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be under-abundant and/or sub-thermally excited. We performed one of the most sensitive (down to τ CO rms ∼ 0.002 and τ HCO + rms ∼ 0.0008) sub-millimeter molecular absorption line observations towards 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to A v < 0.32 mag and HCO + was down to A v < 0.2 mag, where atomic gas and dark molecular gas (DMG) starts to dominate. Multiple transitions measured in absorption toward 3C454.3 allow for a direct determination of excitation temperatures T ex of 4.1 K and 2.7 K, for CO and for HCO + , respectively, which are close to the cosmic microwave background (CMB) and provide explanation for their being undercounted in emission surveys. A stronger linear correlation was found between N HCO + and N H2 (Pearson correlation coefficient P ∼ 0.93) than that of N CO and N H2 (P ∼ 0.33), suggesting HCO + being a better tracer of H 2 than CO in diffuse gas. The derived CO-to-H 2 conversion factor (the CO X-factor) of (14 ± 3) × 10 20 cm −2 (K km s −1) −1 is approximately 6 times larger than the average value found in the Milky Way.