Local Benchmarks for the Evolution of Major-Merger Galaxies -- Spitzer
  Observations of a K-Band Selected Sample
  We present Spitzer observations for a sample of close major-merger galaxy
pairs (KPAIR sample) selected from 2MASS/SDSS-DR3 cross-matches. The goals are
to study the star formation activity in these galaxies and to set a local bench
mark for the cosmic evolution of close major mergers. The Spitzer KPAIR sample
(27 pairs, 54 galaxies) includes all spectroscopically confirmed S+S and S+E
pairs in a parent sample that is complete for primaries brighter than K=12.5
mag, projected separations of 5< s < 20 kpc/h, and mass ratios<2.5. The Spitzer
data consist of images in 7 bands (3.6, 4.5, 5.8, 8, 24, 70, 160 um). Compared
to single spiral galaxies in a control sample, only spiral galaxies in S+S
pairs show significantly enhanced specific star formation rate (sSFR=SFR/M),
whereas spiral galaxies in S+E pairs do not. Furthermore, the SFR enhancement
of spiral galaxies in S+S pairs is highly mass-dependent. Only those with $\rm
M \gsim 10^{10.5} M_\sun$ show significant enhancement. Relatively low mass
($\rm M \sim 10^{10} M_\sun$) spirals in S+S pairs have about the same SFR/M
compared to their counterparts in the control sample. There is evidence for a
correlation between the global star formation activities (but not the nuclear
activities) of the component galaxies in massive S+S major-merger pairs (the
"Holmberg effect"). There is no significant difference in the SFR/M between the
primaries and the secondaries, nor between spirals of SEP<1 and those of SEP.1.
The contribution of KPAIR galaxies to the cosmic SFR density in the local
universe is only 1.7%.
