Neutral hydrogen associated with the S0 galaxy NGC 1023

Abstract

NGC 1023, one of the few gas-rich lenticular galaxies known to date, has been observed with the Westerbork Synthesis Radio Telescope. The hydrogen, amounting to $$1.5 \times 10^9 M_\odot$$, has remarkable density distribution and kinematics, suggesting probably an intergalactic origin and recent arrival. The presence of an H I cloud in the region to the north-east of NGC 1023, reported by Hart, Davies & Johnson (1980), is confirmed but it seems to be part of the H I complex associated with NGC 1023, rather than an isolated intergalactic cloud. The structure of H I around NGC 1023 appears irregular and clumpy. One of the densest H I clumps is associated with the small optical companion visible near the eastern edge of NGC 1023. The gas kinematics are quite complex; overall rotation is in evidence, but the presence over a large area of double-peaked velocity profiles indicates that at least part of the gas surrounding NGC 1023 must have systematic non-circular motions of order 100 km s⁻¹. The observed H I density distribution and kinematics are reminiscent of the tails and bridges found in interacting multiple systems of galaxies, and may have originated, in a similar way, from an encounter of NGC 1023 with one or more late-type hydrogen-rich systems. In this picture the latter would be tidally disrupted and would eventually merge with NGC 1023. In addition to the Eastern companion, two other dwarf-like systems have been detected in H I in the neighbourhood of NGC 1023, one 19 arcmin to the north and the other 10 arcmin to the south.