Astron. Astrophys. Suppl. Ser. 132, 181-193
H. Wu1,4 - Z.L. Zou1,4 - X.Y. Xia1,2,4 - Z.G. Deng1,3,4
Send offprint request: H. Wu
1 - Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, China
2 - Department of Physics, Tianjin Normal University, Tianjin 300074, China
3 - Department of Physics, Chinese Academy of Sciences Graduate School, Beijing 100039, China
4 - Beijing Astrophysics Center (BAC), Beijing 100871, China
Received November 5, 1997; accepted April 1, 1998
Spectroscopic observations of a sample of 73 very luminous IRAS galaxies
( for
,
) from the 2 Jy redshift survey catalogue were carried out using
the 2.16 m telescope at the
Beijing Astronomical Observatory. The observational data, including
the optical images (extracted
from Digital Sky Survey) and spectra for these galaxies, are presented
in Paper I (Wu et al. 1998).
In this paper, we give the spectral and morphological classifications
for these very luminous IRAS galaxies (VLIRGs). We show that about
60% of VLIRGs exhibit AGN-like spectra (Seyfert 1s, Seyfert 2s, LINER-like galaxies).
This fraction goes
up to 82% for the ultraluminous IRAS galaxies (ULIRGs) subsample
(
). 56% of the VLIRGs show strong interaction
or merging signatures; this fraction rises to 91% for the ULIRGs.
These statistical results strongly suggest that
interaction triggers nuclear activities and enhances the infrared luminosity. We find that
LINER and a mixture type which have optical properties of both HII galaxies and LINERs
could be at the transition stage from infrared luminous HII galaxies to AGNs;
their main energy production is from starbursts as well as AGNs.
Both infrared luminosities
and
equivalent widths increase dramatically as nuclear
separations between VLIRGs and their nearest neighbors decrease.
There is little doubt that strong starbursts happen in the nuclei of VLIRGs.
Assuming class 0 as advanced merger, we construct a simple merger sequence,
from morphological classes 1 to 4 (with near or far companions),
to class 5 and 6 (interacting pairs and mergers) and then to class 0 (isolated galaxies).
Along this sequence, VLIRGs evolve from HII galaxies to AGNs.
Key words: infrared: galaxies -- galaxies: active; starburst; interaction
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