A New Molecular Arm in the Far Outer Galaxy

Details A New Molecular Arm in the Far Outer Galaxy A New Molecular Arm in the Far Outer Galaxy

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21821703d&link_type=abstract

American Astronomical Society, AAS Meeting #218, #217.03; Bulletin of the American Astronomical Society, Vol. 43, 2011

Astronomy and Astrophysics

Astronomy

Scientific paper

We have identified in existing 21 cm surveys and subsequently traced with new CO observations a Galactic spiral arm lying well beyond the Outer Arm in the first quadrant. This arm was largely overlooked in the past, probably because it lies mainly out of the Galactic plane, its Galactic latitude increasing steadily with longitude as it follows the Galactic warp. The arm appears roughly linear in Galactic longitude, latitude, and LSR velocity and extrapolates very close to zero velocity at zero longitude. Such an l-v locus is the unmistakable signature of an arm lying well beyond the solar circle on the far side of the Galaxy. This is in marked contrast to the Outer Arm, which is clearly inside the solar circle (at positive velocities) at l < 20 deg.
We have carried out a search for molecular gas in the new arm using the CfA 1.2 meter telescope and detected CO at 10 of 200 positions. The detections are well distributed along the arm from l = 13 deg, v = -21 km/s to l = 55 deg, v = -84 km/s and coincide with most of the arm's main H I concentrations. We also fully mapped one object, finding a large molecular cloud with a radius of 50 pc and molecular mass of 50,000 Mo. At a mean distance of 21 kpc, the molecular gas in this arm is the most distant yet detected in the Milky Way.
The new arm appears to be the continuation of the Scutum-Centaurus Arm in the outer Galaxy and a symmetric counterpart of the nearby Perseus Arm; these two may be the main density wave arms of the Milky Way, extending symmetrically from opposite ends of the central Galactic bar (Churchwell et al. 2009).

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