We analyze a sample of galaxies chosen to have F _ { 24 \micron } > 0.5 mJy and satisfy a certain IRAC color criterion .
IRS spectra yield redshifts , spectral types , and PAH luminosities , to which we add broadband photometry from optical through IRAC wavelengths , MIPS from 24-160 µm , 1.1 millimeter , 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 \sim 1.9 .
This is the first starburst-dominated 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 SEDs for starburst galaxies at extremely high luminosity and at all wavelengths .
Similar broadband data are assembled for three other galaxy samples – local starburst galaxies , local AGN/ULIRGS , and a second 24µm-luminous z \sim 2 sample dominated by AGN . L _ { PAH } / L _ { IR } for the new z \sim 2 starburst sample is the highest ever seen , some three times higher than in local starbursts , whereas in AGNs this ratio is depressed below the starburst trend , often severely .
Several pieces of evidence imply that AGNs exist in this starburst dominated sample , except two of which even host very strong AGN , while they still have very strong PAH emission .
The ACS images show most objects have very extended morphologies in the rest-frame UV 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 starburst ; and the total infrared luminosity density contributed by this type of objects is 0.9 - 2.6 \times 10 ^ { 7 } L _ { \odot } / Mpc ^ { 3 } .