What Powers Ultraluminous IRAS Galaxies?
R.Genzel, D.Lutz, E.Sturm, E.Egami, D.Kunze, H.W.W.Spoon (MPE)
A.F.M.Moorwood (ESO), A.Sternberg (Tel Aviv)
We have obtained ISO-SWS and ISOPHOT-S spectroscopy of fifteen ultraluminous
IRAS galaxies (ULIRGs) and compare their characteristics to those of thirty
starburst and AGN template galaxies. Evidence from various wavelength regimes
has suggested the presence of both AGNs and strong star formation in many
ULIRGs, but their relative importance remained unclear.
ISO spectra provide two new diagnostics which can distinguish whether
starburst or AGN powering prevails in these highly dust-obscured galaxies:
>From a new diagnostic diagram involving this ratio
of high and low excitation fine structure lines and the PAH line-to-continuum
ratio, we find that 70 to 80% of the ultraluminous IRAS galaxies are
predominantly powered by recently formed massive stars. The diagram is displayed
below, with squares representing AGNs, triangles starbursts, and filled circles
ULIRGs. The mid-infrared
emitting regions are highly obscured. In a model where star forming regions
and molecular clouds are fully mixed, extinctions approach average values
inferred from CO mm measurements. After correcting for these extinctions, we
estimate ages for the star forming regions of ULIRGs of 107 to 108,
similar but somewhat larger than those found in lower luminosity starbursts.
- AGNs have strong fine structure emission lines from highly excited species
like [OIV]26 µm or [NeV]14.3 µm, as well as from lower excitation species
like [NeII]12.8 µm or [SIII]33.5 µm. The high excitation lines are
extremely faint or absent in starbursts.
- Strong emission in the 7.7 µm `PAH4 feature is observed in starbursts
but not in AGNs.
Lutz, D., Genzel, R., Sternberg, A., et al.: Astron. Astrophys. 315,
Genzel, R., Lutz, D., Sturm, E., et al., submitted to ApJ.
postscript version of figures
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