Optical Properties of 4248 IRAS Galaxies

We have investigated optical properties of 4248 infrared galaxies (IRGs) by positionally matching data from the Infrared Astronomical Satellite (IRAS) and the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Due to the large sky area coverage of both surveys, we have obtained an opportunity to study an unprecedentedly large number of 4248 infrared galaxies spanning four orders of infrared luminosity range 10L⊙ 6 L8−1000μm 6 10L⊙. Our sample includes three hyper luminous infrared galaxies (L8−1000μm > 10L⊙) and the large number of 178 ultra luminous infrared galaxies (10L⊙ < L8−1000μm 6 10L⊙). In addition, it is important to have statistical number of lower luminosity infrared galaxies (L8−1000μm ∼ 10L⊙) in order to link the well-studied ultra luminous infrared galaxies to normal star-forming galaxies. Our findings are as follows: (i) we found that more IR luminous galaxies tend to have smaller local galaxy density, being consistent with the picture where luminous IRGs are created by the merger-interaction of galaxies that happens more often in lower density regions; (ii) the fractions of AGNs increase as a function of Lir; (iii) there is a good correlation between Lir and L[OIII] for AGNs, suggesting both of the parameters can be a good estimator of the total power of AGNs; (iv) a good correlation is found between Lir and optically estimated star formation rate (SFR) for star-forming galaxies, suggesting Lir is a good indicator of galaxy SFR. However, caution is needed when SFR is estimated using Lir, i.e., high SFR galaxies selected by Lir is frequently merger/interaction, whereas high SFR galaxies selected by optical emission is often normal spiral or Magellanic-cloud like irregular galaxies. (v) more IR luminous galaxies have slightly larger Hα/Hβ ratio; (vi) more IR luminous galaxies have more centrally-concentrated morphology, being consistent with the morphological appearance of galaxy-galaxy merger remnants. Optical image of ultra/very luminous infrared galaxies also show frequent signs of merger/interaction. (vii) comparison with the SED synthesis models indicate that majority of luminous infrared galaxies (L8−1000μm > 10L⊙) may be in a post-starburst phase, sharing a similar (but not the same) merger/interaction origin with E+A galaxies.