Neutral hydrogen in isolated galaxies. II - The large angular diameter galaxies

Abstract

Fifty-two of the large angular diameter (a> or =2.'5) galaxies included in the 21-cm survey of isolated galaxies being undertaken at Arecibo by Haynes and Giovanelli have been mapped along their optical major axes using the 305-m telescope and the low sidelobe flat feed system. The observed fluxes have then been compared with a numerical convolution of an assumed H I distribution sigma/sub H/(r) and the beam pattern G(theta) to produce an estimate of the characteristic size. The derived H I scale length R/sub 70/ is that radius interior to which 70% of the model's H I mass is contained. With still a significant scatter, the H I diameter D/sub 70/ = 2R/sub 70/ is on the average about 1.2 times the UGC blue diameter a, and shows no dependence on morphological type for types later than Sa. Application of the observed relationship between H I size and optical diameter and of an assumed double Gaussian (centrally depressed) H I surface density distribution produces proper corrections for source to beam size. Corrected fluxes derived from Arecibo data do not differ systematically from those measured at the 91-m telescope, proving the applicability of the calibration and correction procedure. The H Imore » mass M/sub H/ is shown to be nearly proportional to the square of the linear H I diameter, implying that the H I surface density averaged over the disk is almost constant.« less