Cold gas in massive early-type galaxies: the case of NGC 1167^⋆

¹ Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
e-mail: struve@astron.nl
² Kapteyn Institute, University of Groningen, Landleven 12, 9747 AD, Groningen, The Netherlands
³ INAF – Observatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Australia Telescope National Facility, CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW, 1710, Australia

Received: 30 June 2010
Accepted: 31 August 2010

Abstract

We present a study of the morphology and kinematics of the neutral hydrogen in the gas-rich (M_HI = 1.5 × 10¹⁰ M_⊙), massive early-type galaxy NGC 1167, which was observed with the Westerbork Synthesis Radio Telescope (WSRT). The H i is located in a 160 kpc disk (≈3 × D₂₅) and has low surface density (≤2 M_⊙ pc^-2). The disk shows regular rotation for r < 65 kpc but several signs of recent and ongoing interaction and merging with fairly massive companions are observed. No population of cold gas clouds is observed – in contrast to what is found in some spiral galaxies. This suggests that currently the main mechanism bringing in cold gas to the disk is the accretion of fairly massive satellite galaxies, rather than the accretion of a large number of small gas clumps. NGC 1167 is located in a (gas-) rich environment: we detect eight companions with a total H i mass of ~6 × 10⁹ M_⊙ within a projected distance of 350 kpc. Deep optical images show a disrupted satellite at the northern edge of the H i disk. The observed rotation curve shows a prominent bump of about 50 km s^-1 (in the plane of the disk) at r ≈ 1.3 × R₂₅. This feature in the rotation curve occurs at the radius where the H i surface density drops significantly and may be due to large-scale streaming motions in the disk. We suspect that both the streaming motions and the H i density distribution are the result of the interaction/accretion with the disrupted satellite. Like in other galaxies with wiggles and bumps in the rotation curve, H i scaling describes the observed rotation curve best. We suggest that interactions create streaming motions and features in the H i density distribution and that this is the reason for the success of H i scaling in fitting such rotation curves.