  We analyze a sample of galaxies chosen to have F24>0.5 mJy and satisfy a
certain IRAC color criterion. IRS spectra yield redshifts, spectral types, and
PAH luminosities, to which we add multi-wavelength broadband photometry.
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-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
24mu-luminous z~1.9 sample dominated by AGN. L(PAH)/L(IR) for the new z~1.9
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 0.9-2.6x 10^7
Lsol/Mpc^3.
