A velocity separation of stars and gas in Carina

In the Carina spiral feature near galactic longitude of 290 deg, a significant difference of 10-20 km/sec is observed between the motions of the young stars and neutral hydrogen gas. The region of these anomalous motions may extend from 3 to 7 kpc from the sun and is only 2 deg wide in longitude. This region may best be described as a narrow sheet, particularly since the extent in latitude is uncertain. Two possible interpretations of the optical and radio evidence for spiral structure would place this peculiar region either on the inner side of the spiral feature or along the central axis of the optical spiral arm. The observed velocity anomaly has the right sign and a reasonable amplitude for a shock-front effect, and the applicability of Roberts's shock-wave model of spiral structure to this anomalous region is discussed.