The Milky Way in Molecular Clouds: A New Complete CO Survey

By T. Dame, D. Hartmann, P. Thaddeus. 2000

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New large-scale CO surveys of the first and second Galactic quadrants and the nearby molecular cloud complexes in Orion and Taurus, obtained with the CfA 1.2 m telescope, have been combined with 31 other surveys obtained over the past two decades with that instrument and a similar telescope on Cerro Tololo in Chile, to produce a new composite CO survey of the entire Milky Way. The survey consists of 488,000 spectra that Nyquist or beamwidth ( °) sample the entire Galactic plane over a strip 4°-10° wide in latitude, and beamwidth or ° sample nearly all large local clouds at higher latitudes. Compared with the previous composite CO survey of Dame et al. (1987), the new survey has 16 times more spectra, up to 3.4 times higher angular resolution, and up to 10 times higher sensitivity per unit solid angle. Each of the component surveys was integrated individually using clipping or moment masking to produce composite spatial and longitude-velocity maps of the Galaxy that display nearly all of the statistically significant emission in each survey but little noise. The composite maps provide detailed information on individual molecular clouds, suggest relationships between clouds and regions widely separated on the sky, and clearly display the main structural features of the molecular Galaxy. In addition, since the gas, dust, and Population I objects associated with molecular clouds contribute to the Galactic emission in every major wavelength band, the precise kinematic information provided by the present survey will form the foundation for many large-scale Galactic studies. A map of molecular column density predicted from complete and unbiased far-infrared and 21 cm surveys of the Galaxy was used both to determine the completeness of the present survey and to extrapolate it to the entire sky at |b| < 32°. The close agreement of the observed and predicted maps implies that only ~ 2% of the total CO emission at |b| < 32° lies outside our current sampling, mainly in the regions of Chamaeleon and the Gum Nebula. Taking into account this small amount of unobserved emission, the mean molecular column density decreases from ~ 3 × 1020 cm-2 at |b| = 5° to ~ 0.1 × 1020 cm-2 at |b| = 30°; this drop is ~ 6 times steeper than would be expected from a plane-parallel layer, but is consistent with recent measurements of the mean molecular column density at higher latitudes. The ratio of the predicted molecular column density map to the observed CO intensity map provides a calibration of the CO-to-H2 mass conversion factor X ≡ N/WCO. Out of the Galactic plane (|b| > 5°), X shows little systematic variation with latitude from a mean value of (1.8 ± 0.3) × 1020 cm-2 K-1 km-1 s. Given the large sky area and large quantity of CO data analyzed, we conclude that this is the most reliable measurement to date of the mean X value in the solar neighborhood.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

The Bar and Spiral Structure Legacy (BeSSeL) Survey: Mapping the Milky Way with VLBI Astrometry† (2011, methodology): “The Milky Way is a barred spiral galaxy, as seen from observations of CO and HI gas (e.g. Burton 1988; Dame et al. 2001), and star counts (e.g. Benjamin et al. 2005).”
Ammonia from cold high-mass clumps discovered in the inner Galactic disk by the ATLASGAL survey (2012, background): “It is well known that high-mass star formation starts in giant molecular clouds (Dame et al. 2001).”
GIANT GAMMA-RAY BUBBLES FROM FERMI-LAT: ACTIVE GALACTIC NUCLEUS ACTIVITY OR BIPOLAR GALACTIC WIND? (2010, methodology): “In this model, the π0 emission is modeled with maps of interstellar gas: H I from the Leiden/Argentine/Bonn (LAB) Galactic Survey (Kalberla et al. 2005) and CO from the CfA composite CO survey (Dame et al. 2001).”
VALIDATION OF THE EQUILIBRIUM MODEL FOR GALAXY EVOLUTION TO z ∼ 3 THROUGH MOLECULAR GAS AND DUST OBSERVATIONS OF LENSED STAR-FORMING GALAXIES (2013, background): “…conditions, both observations and models suggest that the value of αCO increases, as the CO molecule becomes a poor proxy for H2 (e.g. Israel 1997; Dame et al. 2001; Rosolowsky et al. 2003; Blitz et al. 2007; Leroy et al. 2011; Glover & Mac Low 2011; Shetty et al. 2011; Feldmann et al. 2012;…”
The Relationship between Gas Content and Star Formation in Molecule-rich Spiral Galaxies (2001, methodology): “We have scaled the CO fluxes derived by Young et al. (1995) to H2 masses using a conversion factor appropriate for the Galaxy: NH2 = 2× 1020 ( ICO K km s−1 ) cm−2 , (3) (Strong & Mattox 1996; Dame et al. 2001).”
Magnetic Fields in Dark Cloud Cores: Arecibo OH Zeeman Observations (2008, methodology): “To identify suitable cores, we first examined CO survey data (Dame et al. 2001) to identify nearby molecular cloud complexes observable from Arecibo.”
Structural Analysis of Molecular Clouds: Dendrograms (2008, methodology): “We have adopted a fiducial value of the CO-to-H2 conversion factor ofXCO = 2× 1020 cm−2(K km s−1)−1 based on12CO(1→ 0) observations in the Milky Way (Strong & Mattox 1996; Dame et al. 2001) and express changes relative to this value in terms of the parameterX2.”
The 6-GHz multibeam maser survey-I. Techniques (2008, methodology): “The centre velocities and range for observations were determined from the Galactic CO emission of Dame et al. (2001) (Fig.”
A LARGE CATALOG OF ACCURATE DISTANCES TO MOLECULAR CLOUDS FROM PS1 PHOTOMETRY (2014, methodology): “This dust cloud has been mapped in CO by Dame et al. (2001), but we are unaware of a previous estimate of its distance.”
The James Clerk Maxwell Telescope Legacy Survey of Nearby Star‐forming Regions in the Gould Belt (2007, background): “…(for an H2 number density of 10 5 cm 3) reveals the kinematics over the critical transition to gravitational dominance (Ossenkopf 2002), whereas existing low-J CO maps (e.g., Dame et al. 2001; Wilson et al. 2005) trace only the lower density large-scale molecular cloud structure and kinematics.”
Planck intermediate results: XXXV. Probing the role of the magnetic field in the formation of structure in molecular clouds (2015, neutral): “We obtain σ v (cid:107) from the most complete CO emission-line survey of the Milky Way currently available, that of Dame et al. (2001).”
Planck2018 results (2018, methodology): “To probe this possibility, we have selected a set of six 12◦×12◦ molecular regions in the Gould Belt (Dame et al. 2001).”
Mapping the interstellar dust with near-infrared observations: An optimized multi-band technique (2001, background): “Census of entire molecular cloud complexes are rare (CO line emission: Dame et al. 2001; Fukui et al. 1999; IRAS opacity maps: Wood & Myers 1995; optical star counts: Cambrésy 1999), have typical dynamical ranges of 1021 protons cm−2 to below or about 1022 protons cm−2, but suffer from the caveats…”
The DiskMass Survey VII. The distribution of luminous and dark matter in spiral galaxies (2013, methodology): “Distribution of luminous and dark matter in spiral galaxies Galactic measurement of XCO from Dame et al. (2001) with the measurements for M 31 and M 33 from Bolatto et al. (2008).”
Molecular line emission in NGC 1068 imaged with ALMA: I. An AGN-driven outflow in the dense molecular gas (2014, background): “Radiative and mechanical feedback is often invoked as a mechanism of self-regulation in galaxy evolution (Di Matteo et al. 2005, 2008).”