Lyα absorption at a high velocity in NGC1275

Abstract

The peculiar galaxy NGC1275 = Per A = 3C84 remains a problem in the classification of extragalactic sources despite intensive study in all available wavebands. Included in Seyfert's original list¹, it was described as a Seyfert 2 by Khachikian and Weedman² on the basis of observed emission line profiles. Later Weedman³, however, declined to classify NGC1275 and most recently Véron⁴ has suggested that on the basis of its emission line spectrum, variability and radio and polarimetric properties we should regard NGC1275 as a (rather anomalous) BL Lac object. We present here results of IUE spectrophotometry of NGC1275 which show an absorption feature identified with Lyα at a velocity of 8,200 km s⁻¹. We attribute the origin of this feature 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 N_H ∼ 1.8 × 10²⁰ atoms cm⁻². Combining this result with available 21-cm absorption data yields a spin temperature T_s ∼ 52 K for the absorbing gas, typical of that in interstellar H I clouds seen in our Galaxy. The implications of these results are discussed.