A CO survey of the southern Milky Way - The mean radial distribution of molecular clouds within the solar circle

Bronfman, L.; Cohen, R. S.; Álvarez, H.; May, J.; Thaddeus, P.

doi:10.1086/165892

Cited by 233 publications

(289 citation statements)

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“…The same value was estimated for the southern telescope by Bronfman et al (1988). Gaussian Ðts to the azimuth and elevation scans plotted linearly (Figs.…”

Section: Local Molecular Gas Layersupporting

confidence: 71%

“…However, since these changes were accurately tracked with test sources and overlapping surveys, reported values (often called or T mb T R * for a source that just Ðlls the main beam ÏÏ in past "" T R papers) have only changed once, by D20%, owing to a reevaluation of the beam efficiencies by Bronfman et al (1988). Because the calibration of the northern telescope in its present conÐguration is so well understood, we compared its measured intensities to those of previous surveys with both telescopes in order to put all surveys on a common and self-consistent scale of T mb .…”

Section: Observationsmentioning

confidence: 99%

See 1 more Smart Citation

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

Dame

Hartmann

Thaddeus

2001

ApJ

Self Cite

2,120

178

2,713

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New large-scale CO surveys of the Ðrst 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 (1 8 ¡) strip 4¡È10¡ wide in latitude, and beamwidth or sample nearly all large local clouds at higher lati-1 4 ¡ tudes. 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 signiÐcant 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 o b o \ 32¡. The close agreement of the observed and predicted maps implies that only D2% of the total CO emission at o b o \ 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 D3 ] 1020 cm~2 at o b o \ 5¡ to D0.1 ] 1020 cm~2 at o b o \ 30¡ ; this drop is D6 times steeper than would be expected from a planeparallel 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 mass conversion factor Out of the Galactic plane, 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.

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“…The same value was estimated for the southern telescope by Bronfman et al (1988). Gaussian Ðts to the azimuth and elevation scans plotted linearly (Figs.…”

Section: Local Molecular Gas Layersupporting

confidence: 71%

Section: Observationsmentioning

confidence: 99%

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

Dame

Hartmann

Thaddeus

2001

ApJ

Self Cite

2,120

178

2,713

View full text Add to dashboard Cite

show abstract

“…the conversion of total gas mass to stars) to decrease within this radius, but it is not clear how this might relate to the rate of production of stars in dense clumps measured by the SFF. Koda et al (2016) show that the molecular gas fraction increases steadily with decreasing RGC, but we see in Figure 2 that the surface density of mass in dense clumps falls rapidly within 4 kpc (Bronfman et al 1988;Urquhart et al 2014a). The production of molecular clouds from neutral gas therefore is more efficient at small RGC where the H2/HI ratio is nearly 100% (Koda et al 2016).…”

Section: What Drives the Gradient In Sff With Rgc?mentioning

confidence: 78%

The prevalence of star formation as a function of Galactocentric radius

Ragan

Moore

Eden

et al. 2016

Mon. Not. R. Astron. Soc.

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We present large-scale trends in the distribution of star-forming objects revealed by the Hi-GAL survey. As a simple metric probing the prevalence of star formation in Hi-GAL sources, we define the fraction of the total number of Hi-GAL sources with a 70 µm counterpart as the "star-forming fraction" or SFF. The mean SFF in the inner galactic disc (3.1 kpc < R GC < 8.6 kpc) is 25%. Despite an apparent pile-up of source numbers at radii associated with spiral arms, the SFF shows no significant deviations at these radii, indicating that the arms do not affect the star-forming productivity of dense clumps either via physical triggering processes or through the statistical effects of larger source samples associated with the arms. Within this range of Galactocentric radii, we find that the SFF declines with R GC at a rate of −0.026±0.002 per kiloparsec, despite the dense gas mass fraction having been observed to be constant in the inner Galaxy. This suggests that the SFF may be weakly dependent on one or more largescale physical properties of the Galaxy, such as metallicity, radiation field, pressure or shear, such that the dense sub-structures of molecular clouds acquire some internal properties inherited from their environment.

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“…A similar problem exists in the LMC for the Quadrant and Sextant arcs, which also have circular projected shapes but larger radii than the thickness of the LMC gas disk (Efremov 2001a). The scale height of the Milky Way thin stellar disk is only about 100 pc (Gilmore 1989), and the FWHM thickness of the molecular gas disk is B140 pc (e.g., Bronfman et al 1988). A thick- FIG.…”

Section: T He Intrinsic Shape Of the Complexmentioning

confidence: 99%

Hubble Space Telescope Imaging of a Peculiar Stellar Complex in NGC 6946*

Larsen¹,

Efremov²,

Elmegreen³

et al. 2002

ApJ

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The stellar populations in a stellar complex in NGC 6946 are analyzed on images taken with the Wide Field Planetary Camera 2 on board the Hubble Space T elescope. The complex is peculiar because of its very high density of stars and clusters and semicircular shape. Its physical dimensions are about the same as for the local Gould Belt, but the stellar density is 1È2 orders of magnitude higher. In addition to an extremely luminous, D15 Myr old cluster discussed in an earlier paper, accounting for about 17% of the integrated V -band light, we identify 18 stellar clusters within the complex with luminosities similar to the brightest open clusters in the Milky Way. The color-magnitude diagram of individual stars in the complex shows a paucity of red supergiants compared to model predictions in the 10È20 Myr age range for a uniform star formation rate. We thus Ðnd tentative evidence for a gap in the dispersed star formation history, with a concentration of star formation into a young globular cluster during this gap. ConÐrmation of this result must, however, await a better understanding of the late evolution of stars in the corresponding mass range A reddening map based on individual reddenings for 373 early-(Z12 M _ ). type stars is presented, showing signiÐcant variations in the absorption across the complex. These may be responsible for some of the arclike structures previously identiÐed on ground-based images. We Ðnally discuss various formation scenarios for the complex and the star clusters within it.

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A CO survey of the southern Milky Way - The mean radial distribution of molecular clouds within the solar circle

Cited by 233 publications

References 0 publications

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

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

The prevalence of star formation as a function of Galactocentric radius

Hubble Space Telescope Imaging of a Peculiar Stellar Complex in NGC 6946*

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