High‐Resolution Mid‐Infrared Spectroscopy of Ultraluminous Infrared Galaxies

Abstract

We present R � 600, 10Y37 � m spectra of 53 ultraluminous infrared galaxies (ULIRGs), taken using the Infrared SpectrographonboardSpitzer.Thespectra showfine-structureemissionlines of neon,oxygen,sulfur,silicon, argon, chlorine, iron, and phosphorous; molecular hydrogen lines, and C2H2, HCN, and OHabsorption features. We em- ploy diagnostics based on the fine-structure lines, the polycyclic aromatic hydrocarbon (PAH) features and the 9.7 � m silicate absorption feature, to show that the infrared emission from most ULIRGs is powered mostly by star formation, with only � 20% of ULIRGs hosting an AGN with a greater IR luminosity than the starburst. The detection of (Ne v) k14.32injustunderhalfthesample,however,impliesthatanAGNcontributessignificantlytothemid-IRfluxin � 42% of ULIRGs.ThestarburstsandAGNsinULIRGsappearmoreextincted,andforthestarburstsmorecompactthanthose in lower luminosity systems. The excitations and electron densities in the narrow-line regions of ULIRGs appear comparable to those of starbursts with LP10 11.5 L� , although the NLR gas in ULIRGs may be more dense. We show thatthe(Neii)k12.81+(Neiii)k15.56 luminositycorrelateswithbothinfraredluminosityandtheluminosityof the 6.2 and 11.2 � m PAH features, and derive a calibration between PAH luminosity and star formation rate. Finally, we show thatULIRGswithsilicateabsorptionstrengthsSsilof 0:8PSsilP2:4 arelikelytobepoweredmainlybystarformation, but that ULIRGs with Ssil P0:8, and possibly those with Ssil k2:4, contain an IR-luminous AGN. Subject headingg galaxies: active — galaxies: evolution — galaxies: starburst — infrared: galaxies Online material: color figures