INFRARED SPECTROGRAPH SPECTROSCOPY AND MULTI-WAVELENGTH STUDY OF LUMINOUS STAR-FORMING GALAXIES AT z similar or equal to 1.9
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2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A. We analyze a sample of galaxies chosen to have F24 m > 0.5 mJy and satisfy a certain IRAC color criterion. Infrared Spectrograph (IRS) spectra yield redshifts, spectral types, and polycyclic aromatic hydrocarbons (PAH) luminosities, to which we add broadband photometry from optical through IRAC wavelengths, MIPS from 24- 160 m, 1.1 mm, and radio at 1.4 GHz. Stellar population modeling and IRS spectra together demonstrate that the double criteria used to select this sample have efficiently isolated massive star-forming galaxies at z 1.9. This is the first starburst (SB)-dominated ultraluminous infrared galaxies (ULIRG) sample at high redshift with total infrared luminosity measured directly from FIR and millimeter photometry, and as such gives us the first accurate view of broadband spectral energy distributions for SB galaxies at extremely high luminosity and at all wavelengths. Similar broadband data are assembled for three other galaxy samples-local SB galaxies, local active galactic nucleus (AGN)/ULIRGs, and a second 24 m-luminous z 2 sample dominated by AGN. LPAH/LIR for the new z 2 SB sample is the highest ever seen, some three times higher than in local SBs, whereas in AGNs this ratio is depressed below the SB trend, often severely. Several pieces of evidence imply that AGNs exist in this SBdominated sample, except two of which even host very strong AGN, while they still have very strong PAH emission. The Advanced Camera for Surveys images show that most objects have very extended morphologies in the restframe ultraviolet band, thus extended distribution of PAH molecules. Such an extended distribution prevents further destruction PAH molecules by central AGNs.We conclude that objects in this sample are ULIRGs powered mainly by SB; and the total infrared luminosity density contributed by this type of objects is 0.9-2.6 107 L Mpc-3.