MWMC - Milky Way Molecular Clouds from CO Measurements

Overview

This study presents a catalog of 8107 molecular clouds that covers the entire Galactic plane and includes 98% of the ¹²CO emission observed within b +/- 5 deg. The catalog was produced using a hierarchical cluster identification method applied to the result of a Gaussian decomposition of the Dame+ (2001ApJ...547..792D) data. The total H₂ mass in the catalog is 1.2 x 10⁹ M_sun, in agreement with previous estimates. The authors find that 30% of the sight lines intersect only a single cloud, with another 25% intersecting only two clouds. The most probable cloud size is R~30pc. In contrast with the general idea, the authors find a rather large range of values of surface densities, Sigma = 2 to 300 M_sun/pc², and a systematic decrease with increasing Galactic radius, R_gal. The cloud velocity dispersion and the normalization sigma₀ = sigma_v / R^1/2 both decrease systematically with R_gal. When studied over the whole Galactic disk, there is a large dispersion in the line width-size relation and a significantly better correlation between sigma_v and SigmaR. The normalization of this correlation is constant to better than a factor of two for R_gal < 20kpc. This relation is used to disentangle the ambiguity between near and far kinematic distances. The authors report a strong variation of the turbulent energy injection rate. In the outer Galaxy it may be maintained by accretion through the disk and/or onto the clouds, but neither source can drive the 100 times higher cloud-averaged injection rate in the inner Galaxy.

The data set used in this catalog come from that of Dame+ (2001ApJ...547..792D). Those authors combined observations obtained over a period of 20 yr with two telescopes, one in the north (first located in New York City and then moved to Cambridge, Massachusetts) and one in the south (Cerro Tololo, Chile). These 1.2m telescopes have an angular resolution of ~8.5' at 115GHz, the frequency of the ¹²CO 1-0 line. For the current study the authors used the data set covering the whole Galactic plane with +/- 5 deg in Galactic latitude.

Catalog Bibcode

2017ApJ...834...57M

References

Physical properties of molecular clouds for the entire Milky Way disk.
    Miville-Deschenes M.-A., Murray N., Lee E.J.
   <Astrophys. J., 834, 57-57 (2017)>
   =2017ApJ...834...57M    (SIMBAD/NED BibCode)

Provenance

This table was created by the HEASARC in March 2019 based upon the CDS Catalog J/ApJ/834/57 file table1.dat.

Parameters

Source_Number
The sequential identification number for each molecular cloud source in the catalog.

Name
The source designation based on the source_number.

Num_Components
The number of Gaussian components that describe the fit to the CO spectrum. For each sky position of the data cube, the CO spectrum is described by the sum of these Gaussian components. Each spectrum was fit with a maximum of N = 12 Gaussian components.

Num_Pixels
The number of pixels on the sky that describes the extent of the cloud.

Area
The angular area of the cloud, determined by A = num_pixels * 56.25 arcmin², where 56.25 arcmin² is the solid angle subtended by a single pixel.

RA
The Right Ascension of the cloud source in the selected equinox. The position was derived from the intensity-weighted mean coordinate.

Dec
The Declination of the cloud source in the selected equinox. The position was derived from the intensity-weighted mean coordinate.

LII
The Galactic longitude of the cloud source in the selected equinox. The position was derived from the intensity-weighted mean coordinate.

LII_Error
The uncertainty in the Galactic longitude. This also provides an estimate of the angular extent of the cloud along the longitude direction.

BII
The Galactic latitude of the cloud source in the selected equinox. The position was derived from the intensity-weighted mean coordinate.

BII_Error
The uncertainty in the Galactic latitude. This also provides an estimate of the angular extent of the cloud along the latitude direction.

Galactic_Plane_Angle
This is the angle (theta) with respect to the Galactic plane (bii = 0).

Int_Intensity
The integrated CO emission, summed over all the Gaussian spectral components, num_components, and the full extent of the cloud, num_pixels.

NH2
The average H₂ column density for the cloud is measured as: NH₂ = int_intensity * X_CO / num_pixels, where X_CO is the conversion factor from CO to H₂. The authors use X_CO = 2E20.

Surface_Density
The surface density for the cloud is measured as: surface_density = NH₂ * 2 * 1.36 * m_H pc² / M_sun, where the factor 1.36 takes into account the contribution of helium and metals, m_H is the mass of the hydrogen atom, and M_sun is the mass of the Sun.

Radial_Velocity
The emission-weighted mean velocity for the cloud is measured from the total CO spectrum, which is a sum of all the Gaussian components.

Velocity_Dispersion
The velocity dispersion for the cloud is measured from the total CO spectrum, which is a sum of all the Gaussian components. The contribution of instrumental broadening was removed quadratically. The velocity_dispersion is the quadratic sum of the turbulent and thermal broadening.

Extent_Upper
The maximum eigenvalue of the inertia matrix, which is a brightness-weighted half-axis of the projected structure and provides an estimate of the maximum extent of the cloud without assuming a spherical shape.

Extent_Lower
The minimum eigenvalue of the inertia matrix, which is a brightness-weighted half-axis of the projected structure and provides an estimate of the minimum extent of the cloud without assuming a spherical shape.

Extent
The angular size given as the cubed root of the maximum and minimum extents: (extent_upper * extent_lower²)^1/3. The authors assume that it is statistically more likely that the depth in the third dimension, along the line of sight, is closer to the smallest axis seen in projection on the sky, extent_lower.

Radial_Distance
The radial distance from the center of the Milky Way to the cloud, measured as: 8.5 kpc * sin(lii) * V(r) / (radial_velocity / cos(bii) + 220 km/s * sin(lii)), where the authors assumed 8.5 kpc for the galactocentric radius and 220 km/s for the orbital velocity of the Sun and V(r) is the rotation curve defined in Brand & Blitz (1993).

Distance_Flag
The flag to denote which distance indicator is more likely: 0=distance_near or 1=distance_far. The distance is measured as 8.5 * cos(lii) +/- {sqrt(radial_distance² - 8.5² sin²(lii))}. In the inner Galaxy, where radial_distance is less than 8.5 kpc, or the Sun's distance from the center of the Milky Way, there are two solutions for the distance. The authors provide this flag as an estimate of which distance is more likely based on the sigma_v - SigmaR relation:

     velocity_dispersion = 0.23(surface_density * size)^0.43+/-0.14; Eq. 26 in the paper).

where size is dependent on distance, e.g., size_near and size_far.

Distance_Near
The kinematic distance, based on near estimate (less than 8.5 kpc).

Distance_Far
The kinematic distance, based on far estimate (greater than 8.5 kpc).

Z_Distance_Near
The distance from the Galactic mid-plane, based on distance_near measurement.

Z_Distance_Far
The distance from the Galactic mid-plane, based on distance_far measurement.

Surface_Area_Near
The surface of the cloud in pc², based on num_pixels * area * distance_near².

Surface_Area_Far
The surface of the cloud in pc², based on num_pixels * area * distance_far².

Size_Near
The physical size of the cloud, in pc, measured as distance_near * tan(extent).

Size_Far
The physical size of the cloud, in pc, measured as distance_far * tan(extent).

Mass_Near
The mass of the cloud given by the surface_density * surface_area_near.

Mass_Far
The mass of the cloud given by the surface_density * surface_area_far.

Contact Person

Questions regarding the MWMC database table can be addressed to the HEASARC Help Desk.

Page Author: Browse Software Development Team
Last Modified: Monday, 16-Sep-2024 17:31:20 EDT