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

Abstract

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.

The orbital motion of the Arches cluster — clues on cluster formation near the galactic center

“…Clouds on stable orbits in the central molecular zone (r GC ≤ 200 pc) are suggested to follow spheroidal x2 orbits in the Milky Way bar potential (e.g., Binney et al 1991, Englmaier andGerhard 1999). Molecular clouds with high densities capable of massive cluster formation display line-of-sight velocities of v los < 120 km/s in radio CS surveys (Dame et al 2001). A velocity of 120 km/s is in agreement with the terminal velocity of x2 orbits in the Milky Way bar potential.…”

“…Candidates for such a collision are clouds on x1 orbits following the bar's major axis outside the x2 (minor axis) orbital zone, as already suggested by Binney et al 1991, or clouds falling in from the outer parts of the Galaxy (Crawford et al 2002). A collision between an x1 and an x2 orbit cloud is particularly intriguing, as x1 clouds display line-of-sight velocities of up to 270 km/s (Dame et al 2001), which could account for the orbital velocity of 232 km/s observed for the Arches today. One particular problem with this scenario is that the cluster would need to inherit the high velocity of the infalling cloud, which requires a massive, high density cloud capable to transfer its momentum to the local cloud on the x2 orbit.…”

ISM gas studies towards the TeV PWN HESS J1825−137 and northern region

“…HESS excess counts image (> 100 GeV) towards HESS J1825−137 overlaid by the CO(1-0) integrated intensity contour (40,60,80, 100 K km/s) between v lsr = 40 − 60 km/s from Dame, Hartmann & Thaddeus (2001) as revealed by Lemiere, Terrier & DjannatiAtaï (2006). The white circles represent the different SNRs detected (Brogan et al 2006).…”

Maser astrometry with VERA and Galactic structure

“…The list consists of 44 star-forming regions with precise astrometry, from which we determine the basic structure of the Galaxy (Honma et al 2012). In figure 2, we show distributions of 44 star-forming regions in the l-v diagram of the Galaxy, overlaid on the CO l-v diagram (Dame et al 2001). As seen in figure 2, the sources are basically distributed in the northern part of the Galaxy, with a hole around the region between l = 240 • to 350 • due to the bias in array location.…”