Ly-alpha absorption at a high velocity in NGC1275

Abstract

The results of a study of three spectra of the peculiar galaxy NGC1275 obtained with the SWP camera aboard the IUE satellite are reported and discussed. The absorption feature identified with Ly-alpha at 8200 km/s was clearly seen in all three spectra. The origin of this feature is attributed to the intervening galaxy responsible for the high velocity emission filaments seen around NGC1275. Profile fitting yields a neutral hydrogen column density in the absorbing medium of about 1.8 x 10 to the 20th atoms/sq cm. This result was combined with available 21-cm absorption data to yield a spin temperature of about 52 K for the absorbing gas. The linear dimension of the cloud is estimated at 2 pc, consistent with the dimensions of interstellar clouds in the Galaxy. The mean thermal velocity in the cloud is of the order of 1 km/s, suggesting that the velocity width of 6 km/s seen in the 21 cm absorption feature arises as a result of turbulent motion within the cloud.