Accepted for publication in the Astrophysical Journal Letters

Mid-infrared and optical spectroscopy of ultraluminous infrared galaxies: A comparison 1

Accepted for publication in the Astrophysical Journal Letters

Mid-infrared and optical spectroscopy of ultraluminous infrared galaxies: A comparison 1 D. Lutz2, S. Veilleux3, R. Genzel2

ABSTRACT New tools from Infrared Space Observatory (ISO) mid-infrared spectroscopy have recently become available to determine the power sources of dust-obscured ultraluminous infrared galaxies (ULIRGs). We compare ISO classi cations{ starburst or active galactic nucleus (AGN){ with classi cations from optical spectroscopy, and with optical/near-infrared searches for hidden broad-line regions. The agreement between mid-infrared and optical classi cation is excellent if optical LINER spectra are assigned to the starburst group. The starburst nature of ULIRG LINERs strongly supports the suggestion that LINER spectra in infrared-selected galaxies, rather than being an expression of the AGN phenomenon, are due to shocks that are probably related to galactic superwinds. Di erences between ISO and optical classi cation provide clues on the evolution of ULIRGs and on the con guration of obscuring dust. We nd few ISO AGN with optical H II or LINER identi cation, suggesting that highly obscured AGN exist but are not typical for the ULIRG phenomenon in general. Rather, our results indicate that strong AGN activity, once triggered, quickly breaks the obscuring screen at least in certain directions, thus becoming detectable over a wide wavelength range. Subject headings: infrared: galaxies, galaxies: starburst, galaxies: active, galaxies: Seyfert

1. Introduction Dust obscuration is a signi cant concern in studies of ultraluminous infrared galaxies (ULIRGs, L> 1012L, see Sanders& Mirabel 1996 for a review). Evidence for both starburst IR

on observations with ISO, an ESA project with instruments funded by ESA member states, with the participation of ISAS and NASA 2 MPE, Postfach 1603, 85740 Garching, Germany; lutz@mpe.mpg.de 3 University of Maryland, College Park, MD 20742, USA

1 Based