A LARGE-SCALE SURVEY OF OH IN THE GALACTIC-CENTER

Results are presented from an absorption line survey of the galactic centre region in the OH main lines (frequencies 1667.359 MHz and 1665.402 MHz). The survey was made using the 250ft Lovell Telescope at Jodrell Bank which has a beamwidth of 10 arcmin at these frequencies. Spectra were taken on a grid with 0 degrees.2 spacing. The entire region between 354 degrees less than or equal to l less than or equal to 8 degrees.6 and -1 degrees.0 less than or equal to b less than or equal to+1 degrees.0 was surveyed with extensions out to b = -2 degrees.0 and b = +1 degrees.6 at some longitudes. The spectra were taken using an autocorrelation spectrometer with spectral bandwidth 5 MHz, a velocity range of 900 km s(-1) and a resolution of 3.1 km s(-1). The rms noise level achieved was typically 80-90 mK. The data are presented as a series of longitude-velocity (l-V), latitude-velocity (b-V) and longitude-latitude (l-b) contour maps of line temperature and apparent opacity. The survey reveals that the molecular gas in the galactic centre is much more extensive than previously thought. The molecular nuclear disk can be traced over the entire region 354 degrees<1 <6 degrees, a projected size of similar to 1.8 kpc. The disk appears to be warped, lying preferentially to positive latitudes at negative longitudes and negative latitudes at positive longitudes. The gas reaches latitudes of +1 degrees 6, corresponding to z-distances of +/-240 pc out of the galactic plane. The new features are described in detail. There are 7 broadline ''clumps''; concentrations of molecular gas with angular sizes typically 0 degrees.6 and large velocity extents of typically 200 kms(-1). The clumps have positive gradients of velocity with galactic longitude. Some of the clumps appear to be connected by narrow filaments which have velocity widths of typically 30 kms(-1). There are also a number of molecular spur features which extend outwards from the nucleus to latitudes of +/-1 degrees.0. These appear to be associated with radio continuum features, including the galactic centre lobe.