Mid-infrared and optical spectroscopy of ultraluminous infrared
galaxies: A comparison
D. Lutz, S. Veilleux and R. Genzel
Abstract:
New tools from Infrared Space Observatory (ISO) mid-infrared
spectroscopy
have recently become available to determine the
power sources of dust-obscured ultraluminous infrared galaxies (ULIRGs).
We compare ISO classifications - starburst or active galactic
nucleus (AGN) - with classifications from optical spectroscopy, and
with optical/near-infrared searches for hidden broad-line regions.
The agreement between
mid-infrared and optical classification is excellent if optical
LINER spectra are assigned to the starburst group. The starburst nature
of
ULIRG LINERs strongly supports the
suggestion that LINER spectra in infrared-selected
galaxies, rather than being an expression of the AGN phenomenon,
are due to shocks that are probably related to galactic superwinds.
Differences between ISO and optical classification provide clues on the
evolutionof ULIRGs and on the configuration of obscuring dust.
We find few ISO AGN with optical HII or LINER identification,
suggesting that highly obscured AGN exist
but are not typical for the ULIRG phenomenon in general. Rather, our
results
indicate that strong AGN activity, once triggered, quickly breaks
the obscuring screen at least in certain directions, thus becoming
detectable
over a wide wavelength range.
Keywords: infrared: galaxies, galaxies: starburst,
galaxies: active, galaxies: Seyfert
Status: ApJL accepted
Address: mailto:
lutz@mpe.mpg.de
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