HST/NICMOS IMAGING OF BRIGHT HIGH-REDSHIFT 24μm SELECTED GALAXIES: MERGING PROPERTIES

We present new results on the physical nature of infrared-luminous sources at 0.5 < z < 2.8 as revealed by HST/NICMOS imaging and Infrared Spectrograph mid-infrared spectroscopy. Our sample consists of 134 galaxies selected at 24 mu m with a flux of S(24 mu m) > 0.9 mJy. We find many (similar to 60%) of our sources to possess an important bulge and/or central point source component, most of which reveal additional underlying structures after subtraction of a best-fit Sersic (or Sersic+PSF) profile. Based on visual inspection of the NIC2 images and their residuals, we estimate that similar to 80% of all our sources are mergers. We calculate lower and upper limits on the merger fraction to be 62% and 91%, respectively. At z < 1.5, we observe objects in early (pre-coalescence) merging stages to be mostly disk and star formation dominated, while we find mergers to be mainly bulge dominated and active galactic nucleus (AGN)-starburst composites during coalescence and then AGN dominated in late stages. This is analogous to what is observed in local ULIRGs. At z >= 1.5, we find a dramatic rise in the number of objects in pre-coalescence phases of merging, despite an increase in the preponderance of AGN signatures in their mid-IR spectra and luminosities above 10(12.5) L-circle dot. We further find the majority of mergers at those redshifts to retain a disk-dominated profile during coalescence. We conclude that, albeit still driven by mergers, these high-z ULIRGs are substantially different in nature from their local counterparts and speculate that this is likely due to their higher gas content. Finally, we observe obscured (tau(9.7 mu m) > 3.36) quasars to live in faint and compact hosts and show that these are likely high-redshift analogs of local dense-coremergers. We find late-stage mergers to possess predominantly unobscured AGN spectra, but do not observe other morphological classes to carry any specific combination of tau(9.7 mu m) and polycyclic aromatic hydrocarbon (PAH) equivalent width. This suggests a high degree of variation in the PAH emission and silicate absorption properties of these mergers, and possibly throughout the merging process itself.