Multi-density model of the ionised gas in NGC 253 using radio recombination lines^*

¹ Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France
² Raman Research Institute, C.V. Raman Avenue, Sadashivanagar Post Office, Bangalore 560080, India
³ Joint Astronomy Program, Department of Physics, Indian Institute of Science, Bangalore 560012, India
⁴ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA e-mail: mgoss@aoc.nrao.edu

Received: 17 June 2004
Accepted: 26 October 2004

Abstract

We have imaged the H92α  (8.3 GHz), H75α  (15 GHz), and H166α (1.4 GHz) Radio Recombination Lines (RRLs) from NGC 253 at resolutions of 4.5 pc (0.4″) , 2.5 pc (0.2″) and 53 pc (4.5″) respectively. The H92α  line arises from individual compact sources, most of which possess radio continuum counterparts. The line widths range from ~200 km s^-1  for the sources near the radio nucleus to 70–100 km s^-1  for the extranuclear ones. These lines are emitted by gas at a density ~10⁴ cm^-3. The remainder of the cm-wave RRLs arise in lower density gas (~500 cm^-3) with a higher area filling factor and with ten times higher mass. A third component of higher density gas (>10⁴ cm^-3) is required to explain the mm-wave RRLs.