Optical versus Mid‐infrared Spectroscopic Classification of Ultraluminous Infrared Galaxies

Abstract

The origin of huge infrared luminosities of ultraluminous infrared galaxies (ULIGs) is still in question. Recently, Genzel et al. made mid-infrared (MIR) spectroscopy of a large number of ULIGs and found that the major energy source in them is massive stars formed in the recent starburst activity; i.e., ∼ 70%-80% of the sample are predominantly powered by the starburst. However, it is known that previous optical spectroscopic observations showed that the majority of ULIGs are classified as Seyferts or LINERs (low-ionization nuclear emission-line regions). In order to reconcile this difference, we compare types of emission-line activity for a sample of ULIGs which have been observed in both optical and MIR. We confirm the results of previous studies that the majority of ULIGs classified as LINERs based on the optical emission-line diagnostics turn to be starburst-dominated galaxies based on the MIR ones. Since the MIR spectroscopy can probe more heavily-reddened, inner parts of the ULIGs, it is quite unlikely that the inner parts are powered by the starburst while the outer parts are powered by non-stellar ionization sources. The most probable resolution of this dilemma is that the optical emission-line nebulae with the LINER properties are powered predominantly by shock heating driven by the superwind activity; i.e., a blast wave driven by a collective effect of a large number of supernovae in the central region of galaxy mergers.

Outflows in Infrared‐Luminous Starbursts at z < 0.5. II. Analysis and Discussion

“…See x 3.2 for further discussion. [See the electronic edition of the Supplement for a color version of this figure. ] Some authors have suggested that the shock-like line ratios found in the nuclei of galaxies classified as LINERs are due to the presence of shocks in outflowing gas ( Veilleux et al 1995;Lutz et al 1999;Taniguchi et al 1999). We can test this hypothesis by comparing the velocities in LINERs and H ii galaxies.…”

Optical Classification of Southern Warm Infrared Galaxies

“…The ionizing source of LINERs is currently a hot topic of debate (see Filipenko 1989 for a review) and has been attributed to either collisional ionization by fast shocks (Taniguchi et al 1999 ;Lutz et al 1999 ;Dopita & Sutherland 1995 ;Veilleux et al 1995) or photoionization by hot stars (Terashima et al 2000). In many of these studies, LINERs have been classiÐed using either the Heckman (1980) Veilleux et al (1995) selection criterion includes many of the objects which follow the curve of a possible starburst-AGN mixing sequence.…”

“…The LINERs classed using our extreme mixing line fall in the region of the diagnostic diagram corresponding to either photoionization by a hard radiation Ðeld with a very low ionization parameter (Veilleux & Osterbrock 1987) or emission by fast shocks in a gas-poor environment without an ionizing precursor. The fast shocks may be due to one or a combination of the following ; superwind activity (Taniguchi et al 1999 ;Kim, Veilleux, & Sanders 1998), cooling Ñows (Dopita et al 1997), or a shocked accretion disk around an AGN (Dopita et al 1997). …”

ISOCAM‐CVF 5–12 Micron Spectroscopy of Ultraluminous Infrared Galaxies

“…The polycyclic aromatic hydrocarbon (PAH) method has been extended to a larger sample (Lutz et al 1998 ;Rigopoulou et al 1999), allowing us to probe for evolutionary e †ects expected in a scenario in which starburst activity gives way to a quasar-like AGN buried inside the ULIRG (Sanders et al 1988a). Comparison of optical and MIR spectroscopic diagnostics (Lutz, Veilleux, & Genzel 1999 ;Taniguchi et al 1999) demonstrated surprisingly good agreement if optical LINERs are interpreted as starbursts. This Ðnding also suggests that AGNs in ULIRGs usually make their presence known optically at least in certain directions, instead of being fully embedded by large obscuring columns of dust.…”