Near Infrared Observations of IRAS 09104+4109

Abstract

Near infrared imaging and grism spectroscopy of the high luminosity infrared bright galaxy IRAS 09104+4109 have been obtained with the W. M. Keck Telescope. The imaging shows 6 “knots” of emission projected against the extended stellar envelope of the cD galaxy thought to be the source of the large far infrared luminosity. The luminosities of the knots are consistent with the bulges of galaxies accreting onto the central galaxy. In addition, there are 11 companion galaxies seen at radii of 40-150 kpc from the cD nucleus. These objects have colors in the range R—K~ 3.5±0.5 mag, J-H~0.9±0.2 mag H-K ~0.7±0.2 mag, which are consistent with galaxies at a redshift of 0.4. The companion galaxies have luminosities comparable to or less than the characteristic luminosity (L^*) of field galaxies. While the central cD galaxy is identified with the luminous infrared source, it appears to be a quiescent, radio-quiet galaxy, showing no evidence from its near infrared colors for a highly reddened nucleus as seen in other infrared luminous galaxies. The grism spectroscopy shows forbidden lines of low ionization stages of sulfer, iron, and oxygen, as well as hydrogen recombination lines and a strong line of neutral helium. A visual extinction of Av—2 mag is derived to the narrow line region surrounding the galaxy nucleus, based on the line ratios [S II]1.03 µm/0.407 µm and Pδ/Hβ. The near infrared spectrum is consistent with the optical classification of this system being a Seyfert 2 nucleus.