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 spiral-spiral (S+S) and spiral-elliptical (S+E) pairs in a parent sample that is complete for primaries brighter than K=12.5 mag, projected separations of 5 ≤ s ≤ 20h −1 kpc, and mass ratios ≤ 2.5. The Spitzer data, consisting of images in 7 bands (3.6, 4.5, 5.8, 8, 24, 70, 160µm), show very diversified IR emission properties. 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 M > ∼ 10 10.5 M show significant enhancement. Relatively low mass (M ∼ 10 10 M ) spirals in S+S pairs have about the same SFR/M compared -2to their counterparts in the control sample, while those with 10 11 M have on average a ∼ 3 times higher SFR/M than single spirals. 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, SEP being the normalized separation parameter. The contribution of KPAIR galaxies to the cosmic SFR density in the local universe is only 1.7%, and amounts to ρ . KPAIR = 2.54 × 10 −4 (M yr −1 Mpc −3 ).

Using a sample of BzK-selected galaxies at z ∼ 2 identified from the CFHT/WIRCAM near-infrared survey of GOODS-North, we discuss the relation between star formation rate (SFR), specific star formation rate (SSFR), and stellar mass (M * ), and the clustering of galaxies as a function of these parameters. For star-forming galaxies (sBzKs), the UV-based SFR, corrected for extinction, scales with the stellar mass as SFR ∝ M * α with α = 0.74 ± 0.20 down to M * ∼ 10 9 M ⊙ , indicating a weak dependence on the stellar mass of the star formation rate efficiency, namely, SSFR. We also measure the angular correlation function and hence infer the correlation length for sBzK galaxies as a function of M * , SFR, and SSFR, as well as K-band apparent magnitude. We show that passive galaxies (pBzKs) are more strongly clustered than sBzK galaxies at a given stellar mass, mirroring the color−density relation seen at lower redshifts. We also find that the correlation length of sBzK galaxies ranges from 4 to 20 h −1 Mpc, being a strong function of M K , M * , and SFR. On the other hand, the clustering dependence on SSFR changes abruptly at 2 × 10 −9 yr −1 , which is the typical value for "main sequence" starforming galaxies at z ∼ 2. We show that the correlation length reaches a minimum at this characteristic value, and is larger for galaxies with both smaller and larger SSFRs; a dichotomy that is only marginally implied from the predictions of the semi-analytical models. Our results suggest that there are two types of environmental effects at work at z ∼ 2. Stronger clustering for relatively quiescent galaxies implies that the environment has started to play a role in quenching star formation. At the same time, stronger clustering for galaxies with elevated SSFRs ("starbursts") might be attributed to an increased efficiency for galaxy interactions and mergers in dense environments.