Abstract
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). As a result of 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 10 <sup>9</sup> ≤ L <sub>8-1000μm</sub> ≤ 10 <sup>13.57</sup> L <sub>⊙</sub> . Our sample includes three hyperluminous infrared galaxies (L <sub>8-1000μm</sub> > 10 <sup>13</sup> L <sub>⊙</sub> ) and the large number of 178 ultraluminous infrared galaxies (10 <sup>12</sup> < L <sub>8-1000μm</sub> ≤ 10 <sup>13</sup> L <sub>⊙</sub> ). In addition, it is important to have a statistical number of lower-luminosity infrared galaxies (L <sub>8-1000μm</sub> ∼ 10 <sup>10</sup> L <sub>⊙</sub> ) in order to link the well-studied ultraluminous infrared galaxies to normal star-forming galaxies. Our findings are as follows. (i) We found that more infrared luminous galaxies tend to have a 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 active galactic nuclei (AGNs) increase as a function of L <sub>ir</sub> . (iii) There is a good correlation between L <sub>ir</sub> and L <sub>[O III]</sub> for AGNs, suggesting that both of the parameters can be a good estimator of the total power of AGNs. (iv) A good correlation is found between L <sub>ir</sub> and the optically estimated star formation rate (SFR) for star-forming galaxies, suggesting L <sub>ir</sub> is a good indicator of galaxy SFR. However, caution is needed when SFR is estimated using L <sub>ir</sub> , i.e. high SFR galaxies selected by L <sub>ir</sub> are frequently as a result of merger/interaction, whereas high SFR galaxies selected by optical emission are often normal spiral or Magellanic-cloud-like irregular galaxies. (v) More IR luminous galaxies have a slightly larger Hα/Hβ ratio, (vi) More IR luminous galaxies have a more centrally concentrated morphology, being consistent with the morphological appearance of galaxy-galaxy merger remnants. The optical images of ultra-/very luminous infrared galaxies also show frequent signs of merger/interaction, (vii) Comparison with the spectral energy distribution synthesis models indicates that the majority of luminous infrared galaxies (L <sub>8-1000μm</sub> > 10 <sup>11</sup> L <sub>⊙</sub> ) may be in a post-starburst phase, sharing a similar (but not the same) merger/interaction origin with E+A galaxies. © 2005 RAS.