Planck early results: XIX. All-sky temperature and dust optical depth from Planck and IRAS. Constraints on the "dark gas" in our Galaxy

Jussi Tuovinen, Jean-Philippe Bernard (Corresponding Author), Planck Collaboration

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Abstract

An all sky map of the apparent temperature and optical depth of thermal dust emission is constructed using the Planck-HFI (350μm to 2 mm) andIRAS(100μm) data. The optical depth maps are correlated with tracers of the atomic (Hi) 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 NH, the correlation is consistent with that of the lowest NH, for a given choice of the CO-to-H2 conversion factor. In the intermediate NH 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 Hi and CO surveys. The 2D spatial distribution of the dark gas in the solar neighbourhood (|bII| > 10°) is shown to extend around known molecular regions traced by CO. The average dust emissivity in the Hi phase in the solar neighbourhood is found to be τD/NHtot = 5.2×10-26 cm2 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 fromFIRAS measurements at high latitudes within 10%. The threshold for the existence of the dark gas is found at NHtot = (8.0±0.58)×1020 H cm−2 (AV = 0.4mag). Assuming the same high frequency emissivity for the dust in the atomic and the molecular phases leads to an average XCO = (2.54 ± 0.13) × 1020 H2 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 H2 survives photodissociation but CO does not. The observed transition for the onsetof this phase in the solar neighbourhood (AV = 0.4mag) 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 Hi measurements.
Original languageEnglish
Article numberA19
Number of pages16
JournalAstronomy and Astrophysics
Volume536
DOIs
Publication statusPublished - 2011
MoE publication typeA1 Journal article-refereed

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Infrared Astronomy Satellite
optical thickness
optical depth
sky
dust
galaxies
solar neighborhood
gases
gas
emissivity
temperature
monatomic gases
thermal emission
molecular gases
photodissociation
polar regions
linearity
tracers
power law distribution
spatial distribution

Keywords

  • Clouds
  • dust
  • evolution
  • extinction
  • Galaxy
  • general
  • ISM
  • Solar neighborhood
  • submillimeter

Cite this

@article{0f071f607a2840639e870766433b72db,
title = "Planck early results: XIX. All-sky temperature and dust optical depth from Planck and IRAS. Constraints on the {"}dark gas{"} in our Galaxy",
abstract = "An all sky map of the apparent temperature and optical depth of thermal dust emission is constructed using the Planck-HFI (350μm to 2 mm) andIRAS(100μm) data. The optical depth maps are correlated with tracers of the atomic (Hi) 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 NH, the correlation is consistent with that of the lowest NH, for a given choice of the CO-to-H2 conversion factor. In the intermediate NH 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 Hi and CO surveys. The 2D spatial distribution of the dark gas in the solar neighbourhood (|bII| > 10°) is shown to extend around known molecular regions traced by CO. The average dust emissivity in the Hi phase in the solar neighbourhood is found to be τD/NHtot = 5.2×10-26 cm2 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 fromFIRAS measurements at high latitudes within 10{\%}. The threshold for the existence of the dark gas is found at NHtot = (8.0±0.58)×1020 H cm−2 (AV = 0.4mag). Assuming the same high frequency emissivity for the dust in the atomic and the molecular phases leads to an average XCO = (2.54 ± 0.13) × 1020 H2 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 H2 survives photodissociation but CO does not. The observed transition for the onsetof this phase in the solar neighbourhood (AV = 0.4mag) 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 Hi measurements.",
keywords = "Clouds, dust, evolution, extinction, Galaxy, general, ISM, Solar neighborhood, submillimeter",
author = "P.A.R. Ade and N. Aghanim and M. Arnaud and Jussi Tuovinen and Jean-Philippe Bernard and {Planck Collaboration}",
note = "Project code: 18080",
year = "2011",
doi = "10.1051/0004-6361/201116479",
language = "English",
volume = "536",
journal = "Astronomy and Astrophysics",
issn = "0004-6361",
publisher = "EDP Sciences",

}

Planck early results : XIX. All-sky temperature and dust optical depth from Planck and IRAS. Constraints on the "dark gas" in our Galaxy. / Tuovinen, Jussi; Bernard, Jean-Philippe (Corresponding Author); Planck Collaboration.

In: Astronomy and Astrophysics, Vol. 536, A19, 2011.

Research output: Contribution to journalArticleScientificpeer-review

TY - JOUR

T1 - Planck early results

T2 - XIX. All-sky temperature and dust optical depth from Planck and IRAS. Constraints on the "dark gas" in our Galaxy

AU - Ade, P.A.R.

AU - Aghanim, N.

AU - Arnaud, M.

AU - Tuovinen, Jussi

AU - Bernard, Jean-Philippe

AU - Planck Collaboration

N1 - Project code: 18080

PY - 2011

Y1 - 2011

N2 - An all sky map of the apparent temperature and optical depth of thermal dust emission is constructed using the Planck-HFI (350μm to 2 mm) andIRAS(100μm) data. The optical depth maps are correlated with tracers of the atomic (Hi) 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 NH, the correlation is consistent with that of the lowest NH, for a given choice of the CO-to-H2 conversion factor. In the intermediate NH 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 Hi and CO surveys. The 2D spatial distribution of the dark gas in the solar neighbourhood (|bII| > 10°) is shown to extend around known molecular regions traced by CO. The average dust emissivity in the Hi phase in the solar neighbourhood is found to be τD/NHtot = 5.2×10-26 cm2 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 fromFIRAS measurements at high latitudes within 10%. The threshold for the existence of the dark gas is found at NHtot = (8.0±0.58)×1020 H cm−2 (AV = 0.4mag). Assuming the same high frequency emissivity for the dust in the atomic and the molecular phases leads to an average XCO = (2.54 ± 0.13) × 1020 H2 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 H2 survives photodissociation but CO does not. The observed transition for the onsetof this phase in the solar neighbourhood (AV = 0.4mag) 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 Hi measurements.

AB - An all sky map of the apparent temperature and optical depth of thermal dust emission is constructed using the Planck-HFI (350μm to 2 mm) andIRAS(100μm) data. The optical depth maps are correlated with tracers of the atomic (Hi) 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 NH, the correlation is consistent with that of the lowest NH, for a given choice of the CO-to-H2 conversion factor. In the intermediate NH 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 Hi and CO surveys. The 2D spatial distribution of the dark gas in the solar neighbourhood (|bII| > 10°) is shown to extend around known molecular regions traced by CO. The average dust emissivity in the Hi phase in the solar neighbourhood is found to be τD/NHtot = 5.2×10-26 cm2 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 fromFIRAS measurements at high latitudes within 10%. The threshold for the existence of the dark gas is found at NHtot = (8.0±0.58)×1020 H cm−2 (AV = 0.4mag). Assuming the same high frequency emissivity for the dust in the atomic and the molecular phases leads to an average XCO = (2.54 ± 0.13) × 1020 H2 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 H2 survives photodissociation but CO does not. The observed transition for the onsetof this phase in the solar neighbourhood (AV = 0.4mag) 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 Hi measurements.

KW - Clouds

KW - dust

KW - evolution

KW - extinction

KW - Galaxy

KW - general

KW - ISM

KW - Solar neighborhood

KW - submillimeter

U2 - 10.1051/0004-6361/201116479

DO - 10.1051/0004-6361/201116479

M3 - Article

VL - 536

JO - Astronomy and Astrophysics

JF - Astronomy and Astrophysics

SN - 0004-6361

M1 - A19

ER -