Extended infrared line emission excited by starburst and Seyfert activity in NGC 3256 and NGC 4945

Abstract

view Abstract Citations (70) References (70) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Extended Infrared Line Emission Excited by Starburst and Seyfert Activity in NGC 3256 and NGC 4945 Moorwood, A. F. M. ; Oliva, E. Abstract We present visible and infrared images, 1.5-2.3 micron infrared spectra and [Fe II] 1.64 micron, H_2_(1-0)S(1)2.121 microns, and Brγ 2.165 micron infrared line contour maps obtained to investigate the nature of the activity and the origin of the infrared line emission in two relatively nearby infrared luminous galaxies. NGC 3256 is a merging system exhibiting starburst activity over a region several kpc across and a high infrared luminosity of ~3 x 10^11^ L_sun_. A specific search for [Si VI] 1.96 micron coronal line emission from a possibly visually obscured Seyfert nucleus proved negative. Extremely high star- formation and SN rates of ~20 M_sun_ yr^-1^ and ~1.5 yr^-1^, respectively, however, are estimated using the observed integrated Brγ line luminosities and He I 2.06 micron/Brγ ratios together with an existing starburst model. At this rate, the injection energy associated with SN explosions alone could account for the observed [Fe II] 1.64 micron and H_2_ (1-0)S(1) line luminosities. NGC 4945 is an almost edge-on spiral with an order of magnitude lower infrared luminosity which is powered by both a 400 Pc size starburst and a visually obscured Seyfert nucleus which is a variable source of hard X- ray emission and could substantially contribute to the total luminosity. Its star formation efficiency is higher than in NGC 3256 whereas its luminosity and star formation and SN rates of ~0.4 M_sun_ yr^-1^ and ~0.05 yr^-1^ are much lower although still adequate to account for the excitation of the observed [Fe II] 1.64 micron and H_2_ (1-0)S(1) lines. Based on previous survey work, however, the large integrated H_2_ (1-0)S(I)/Brγ ratio observed in this galaxy is also indicative of the presence of a Seyfert nucleus. Maps of the Brγ, [Fe II], and H_2_ (1-0)S(1) line emission reveal significantly different spatial distributions in the two galaxies. In NGC 3256 the emission extends over a region ~4 kpc across and shows similar [Fe II] and Brγ morphologies, whereas the H_2_(1-0)S(1) emission does not correlate in detail with either. In particular, it is less peaked on the nucleus and traces a spiral arm and embedded source to the south of the nucleus. This source is also prominent in our K' (2.1 micron) image but totally obscured by dust in the visible and is probably the nucleus of the merging companion. Additional evidence that the merger is not yet complete is also provided by the fact that the light distribution in the K' image does not correspond to that expected of a relaxed system. In NGC 4945 the Brγ emission is largely confined to a ~380 Pc diameter and <= 190 Pc thick disk in the plane of the galaxy whereas both the [Fe II] 1.64 micron and H_2_ (1-0)S(1) line emission extends over the full extent of the molecular complex which is a factor of 2 larger perpendicular to the plane. This morphology is consistent with additional excitation by an embedded active galactic nucleus and/or a starburst driven superwind. The similarity of the observed [Fe II] and H_2_ intensity profiles perpendicular to the plane suggests a common excitation mechanism for the "excess" emission which we propose can be attributed to partial ionization and heating of molecular gas respectively by X-rays from the nuclear source. The overall conclusion of this study is that NGC 4945 is probably at an advanced stage of its evolution from a starburst to a Seyfert galaxy whereas there is no evidence that such an evolution is occurring yet in the considerably more luminous, merging system, NGC 3256. Publication: The Astrophysical Journal Pub Date: July 1994 DOI: 10.1086/174347 Bibcode: 1994ApJ...429..602M Keywords: Infrared Imagery; Infrared Spectra; Light (Visible Radiation); Luminosity; Mapping; Seyfert Galaxies; Star Formation; Starburst Galaxies; Arrays; Calibrating; Cameras; Contours; Detectors; Infrared Spectrometers; Infrared Telescopes; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 3256; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 4945; GALAXIES: INTERACTIONS; GALAXIES: STARBURST; GALAXIES: SEYFERT; INFRARED: GALAXIES; GALAXIES: ISM full text sources ADS | data products SIMBAD (2) NED (2)