Observations of formaldehyde along the galactic plane

Extensive observations of the|${1}_{11}-{1}_{10}$| formaldehyde (HCHO) transition line (frequency 4.83 GHz) along the galactic plane between|${8}^{\circ}\,\lt\,l\,\lt\,{60}^{\circ }$| have been made using the Jodrell Bank Mark II telescope (beam l0arcmin). The data are presented as spectra and as longitude–velocity maps of both line temperature and apparent optical depth. The l–υ diagrams are similar to equivalent recombination line and carbon monoxide diagrams. The radial distribution of HCHO has been determined from the data. It has a peak at about |$R\,\simeq 5$| kpc with a monotonic decline at larger radii and a trough in the distribution at |$R\,\simeq 3.5$| kpc. Molecular hydrogen densities derived from this distribution are substantially in agreement with those obtained from CO work. Both the total mass of hydrogen gas and the ratio |${n}_{{H}_{2}}$|/nHI show exponential increases towards the Galactic Centre until |$R\,\simeq 5$| kpc. The column densities of formaldehyde towards three H II regions are incompatible with the HCHO radial distribution. This is simply explained by the large dense molecular clouds found near these regions and indicates that the clouds associated with regions of star formation are relatively uncommon. Evidence for the concentration of HCHO in spiral arms is weak, since the large-scale structure in the l– υ diagram can be interpreted as due to the velocity field alone. This could be due to a real confusion of the spiral structure in the region of the Galaxy that was sampled, but is more likely to be due to a masking of spiral arm structure by small random velocity components that the molecular clouds may have.