An HST surface photometric study of ultraluminous infrared galaxies

Abstract

We study the surface photometry for 13 single-nucleus ultraluminous IRAS galaxies (ULIRGs), selected from a parent sample of 58 galaxies obtained in a Hubble Space Telescope I-band snapshot survey. These galaxies can be classified into two classes using a quantitative chi (2) criterion based on whether their surface brightness profiles are well fitted by the R(1) / 4 law. The surface brightness profiles of all four galaxies in the first class are well fitted by the R(1) / 4 law. Their isophotes are all disky at R <~ 1 h(-1) kpc, consistent with the molecular disks/rings found in nearby ULIRGs from CO observations. For the nine class II objects, their surface brightness profiles are less well fitted by the R(1) / 4 law. Qualitatively, the class II objects can be further divided into two sub-classes (IIa and IIb) based on their photometric behaviors. The four galaxies in class IIa have a bright nucleus, an inner R(1) / 4 component and an outer extension. Remarkably all these four galaxies are Seyfert 1 galaxies with luminosities in the quasar regime and with relatively narrow permitted and strong FeII emission lines. The remaining five galaxies in Class IIb have surface brightness profiles deviating to various degrees from the R(1) / 4 law, including one galaxy that is well fitted by an exponential law. In contrast to class IIa, spectral classifications of galaxies in class IIb are mostly HII regions or LINERs. We also present new spectroscopic observations for some of these galaxies. We establish the redshift of IR 09427+1929 to be 0.284, instead of 0.149 as adopted in the literature. Our analyzes support the idea that mergings of disk galaxies produce elliptical or S0 galaxies. These observations also suggest that the formation of QSOs may be an integral part of elliptical galaxy formation (at low redshifts). This research was based on observations obtained with the NASA/ESA Hubble Space Telescope through the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.