X-ray 'ridge' in galactic center.

The more ways astronomers look at the center of our galaxy the more curious things they find. This time, an-ray survey of the center of the galaxy using the satellite Exosat has found a mysterious "ridge" of X-ray emissions stretching for 40[deg.' on each side of the galactic center, two ninths of the way around the sky. According to the observers, R.S. Warwick, M.J.L. Turner, M.G. Watson and R. Willingale of the University of Leicester in England, this is the first survey of the X-ray emissions of the galactic center region. Previous X-ray observations with the HEAO-1 satellite were unable to get data on this area.

HEAO-1 did find similar ridgelike emissions along the galactic plane at galactic longitudes greater than 50[deg.]. (Galactic longitude is counted from the center of the galaxy, which is longitude 0[deg.].) One of the people involved with the HEAO-1 work, Diana Worrall of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., says therefore that she is not surprised that the Leicester group has found what they did. She suggests that the ridgelike features at large galactic longitudes may be extensions of this central piece.

The central ridge is quite narrow. At its ends it extends over only about 2[deg.] of galactic latitude, but in the center it bulges to cover about 10[deg.] of galactic latitude. (Zero galactic latitude is the plane in which most of the galaxy lies.) Thus the X-ray ridge bulges in just the region where the galaxy bulges out from its central plane.

Worral says the Leicester group's work is "a nice measurement" by which astrophysicists can test their models of what is there. At the moment the origin of the ridgelike emissions is very much a mystery. In their paper in the Sept. 19 NATURE reporting the discovery, the Leicester observers consider a few possibilities. One way such a ridgelike appearance could arise is from the overlapping of emissions by a large number of compact discrete sources that the observing equipment is unable to resolve separately. If this is the case, however, the individual sources have to be quite faint, emitting less than 10.sup.33.5 ergs per second each. Another possibility is that the source itself is diffuse, some kind of hot, possibly ionized, gas, but such a gas would be hard to confine in the galactic plane by either gravity or pressure, the Leicester group calculates.