Formaldehyde in the Galactic Centre - I. The observations

Formaldehyde 6-cm absorption in the direction of the Galactic Centre has been surveyed using the Jodrell Bank MK II radio telescope (beam-width 10 × 9 arcmin). The observations sample the region |$-2^\circ \leqslant l \leqslant 5^\circ$| and |$-0^\circ.5 \leqslant b \leqslant 0^\circ.5$|⁠, with a velocity range of 620 km s−1 a velocity resolution of 2.1 km s−1 and an rms noise level of ∼ 0.03 K. The data are presented as contour maps showing line temperature as a function of latitude and velocity (b–V maps) and as a function of longitude and velocity (l–V maps). Similar maps of the line-to-continuum ratio are also presented. The radial distribution of formaldehyde (H2CO) in the Galactic Centre region is derived using two different kinematic models which give similar results. Formaldehyde is strongly concentrated in the Galactic Centre in a layer of latitude extent ∼ 0°.5 and longitude extent ∼ 4° which contains one quarter of all the H2CO in the Galaxy. The distribution is centred on l ∼ 1°. The individual H2CO features are described in detail. Several have not previously been detected in H2CO, although they were known from H I and OH studies. The Galactic Centre features all have unusually large velocity widths. Some of the strongest features appear to be associated with H II regions.