What Powers Ultra-Luminous IRAS Galaxies ?

R. Genzel, D. Lutz, E. Sturm, E. Egami, D. Kunze, A.F.M. Moorwood, D. Rigopoulou, H.W.W. Spoon, A. Sternberg, L.E. Tacconi-Garman, L. Tacconi and N. Thatte

Abstract:

We present an ISOSWS and ISOPHOT-S, mid-infrared spectroscopic survey of fifteen ultra-luminous IRAS galaxies (LIR >10^12 Lo). We combine the survey results with a detailed case study, based on arcsecond resolution, near-IR and mm imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these ultra-luminous galaxies to ISO and literature data of thirty starburst and active galactic nuclei (AGN), template galaxies.

We find that:

  1. 70 to 80% of the ultra-luminous IRAS galaxies in our sample are predominantly powered by recently formed massive stars. 20 to 30% are powered by a central AGN. These conclusions are based on a new infrared 'diagnostic diagram' involving the ratio of high to low excitation mid-IR emission lines on the one hand, and on the strength of the 7.7(m PAH feature on the other hand.
  2. at least half of the sources probably have simultaneously an active nucleus and starburst activity in a 1-2 kpc diameter circum-nuclear disk/ring.
  3. the mid-infrared emitting regions are highly obscured (AV(screen)~5 to 50 or AV(mixed)~50 to 1000). In a model where star forming regions and dense molecular clouds are fully mixed the ISO derived, V-band dust extinctions approach the dust column densities inferred from CO mm measurements. After correction for these extinctions, we estimate that the star forming regions in ULIRGs have ages between 107 and 108 years, similar to but somewhat larger than those found in lower luminosity starburst galaxies.
  4. in the sample we have studied there is no obvious trend for the AGN component to dominate in the most compact, and thus most advanced mergers. Instead, at any given time during the merger evolution, the time dependent compression of the circum-nuclear interstellar gas, the accretion rate onto the central black hole and the associated radiation efficiency may determine whether star formation or AGN activity dominates the luminosity of the system.

Keywords: galaxies: stellar content - galaxies: starburst - galaxies: nuclei - infrared: sources - infrared: galaxies - stars: formation

Status: to appear in Astrophysical Journal (accepted)

Address: mailto: genzel@mpe-garching.mpg.de

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Figure: ISO SWS/PHOT-S Diagnostic Diagram

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