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

Abstract

We present visible and infrared images, 1.5 to 2.3 micrometer infraredspectra and (Fe II) 1.64 micrometer, H2 (1 - 0) S(1) 2.121 micrometer,and Br_{gamma} 2.165 micrometer infrared line contour mapsobtained to investigate the nature of the activity and the origin of theinfrared line emission in two relatively nearby infrared luminousgalaxies. NGC 3256 is a merging system exhibiting starburst activityover a region several kpc across and a high infrared luminosity ofapproximately 3 x 10^{11} solar luminosity. A specific searchfor (Si VI) 1.96 micrometer coronal line emission from a possiblyvisually obscured Seyfert nucleus proved negative. Extremely highstar-formation and SN rates of approximately 20 solar mass/year andapproximately 1.5/year, respectively, however, are estimated using theobserved integrated Br_{gamma} line luminosites and He I 2.06micrometer/Br_{gamma} ratios together with an existing starburstmodel. At this rate, the injection energy associated with SN explosionsalone could account for the observed (Fe II) 1.64 micrometer and H2(1 -0)S(1) line luminosities. NGC 4945 is an almost edge-on spiral with anorder of magnitude lower infrared luminosity which is powered by both aapproximately 400 pc size starburst and a visually obscured Seyfertnucleus which is a variable source of hard X-ray emission and couldsubstantially contribute to the total luminosity. Its star formationefficiency is higher than in NGC 3256 whereas its luminosity and starformation and SN rates of (approximately 0.4 solar mass/year andapproximately 0.05/year are much lower although still adequate toaccount for the excitation of the observed (Fe II) 1.64 micrometer andH2(1 - 0)S(1) lines. Based on previous survey work, however, the largeintegrated H2(1 - 0)S(1)/Br_{gamma} ratio observed in thisgalaxy is also indicative of the presence of a Seyfert nucleus. Maps ofthe Br_{gamma}, (Fe II), and H2 (1 - 0)S(1) line emission revealsignificantly different spatial distributions in the two galaxies. InNGC 3256 the emission extends over a region approximately 4 kpc acrossand shows similar (Fe II) and Br_{gamma} morphologies, whereasthe H2 (1 - 0)S(1) emission does not correlate in detail with either. Inparticular, it is less peaked on the nucleus and traces a spiral arm andembedded source to the south of the nucleus. This source is alsoprominent in our K^{'} (2.1 micrometer) image but totallyobscured by dust in the visible and is probably the nucleus of themerging companion. Additional evidence that the merger is not yetcomplete is also provided by the fact that the light distribution in theK^{'} image does not correspond to that expected of a relaxedsystem. In NGC 4945 the Br_{gamma} emission is largely confinedto a approximately 380 pc diameter and less than or equal to 190 pcthick disk in the plane of the galaxy whereas both the (Fe II) 1.64micrometer and H2 (1 - 0)S(1) line emission extends over the full extentof the molecular complex which is a factor of 2 larger perpendicular tothe plane. This morphology is consistent with additional excitation byan embedded active galactic nucleus and/or a starburst driven superwind.The similarity of the observed (Fe II) and H2 intensity profilesperpendicular to the plane suggests a common excitation mechanism forthe 'excess' emission which we propose can be attributed to partialionization and heating of molecular gas respectively by X-rays from thenuclear source. The overall conclusion of this study is that NGC 4945 isprobably at an advanced stage of its evolution from a starburst to aSeyfert galaxy whereas there is no evidence that such an evolution isoccurring yet in the considerably more luminous, merging system, NGC3256.