Mid-infrared spectroscopy of Spitzer-selected ultra-luminous starbursts
  at z~2
  We want to study the mid-infrared properties and the starburst and AGN
contributions, of 24um sources at z~2, through analysis of mid-infrared spectra
combined with millimeter, radio, and infrared photometry. Mid-infrared
spectroscopy allows us to recover accurate redshifts. A complete sample of 16
Spitzer-selected sources (ULIRGs) believed to be starbursts at z~2
("5.8um-peakers") was selected in the (0.5 sq.deg.) J1064+56 SWIRE Lockman Hole
field. These sources have S(24um)>0.5mJy, a stellar emission peak redshifted to
5.8um, and r'(Vega)>23. The entire sample was observed with the low resolution
units of the Spitzer/IRS infrared spectrograph. These sources have 1.2mm
observations with IRAM 30m/MAMBO and very deep 20cm observations from the VLA.
Nine of our sources also benefit from 350um observation and detection from
CSO/SHARC-II. The entire sample shows good quality IRS spectra dominated by
strong PAH features. The main PAH features at 6.2, 7.7, 8.6, and 11.3um have
high S/N average luminosities of 2.90, 10.38, 3.62, and 2.29x10^{10}Lsun,
respectively. We derived accurate redshifts spanning from 1.75 to 2.28. The
average of these redshifts is 2.017. This result confirms that the selection
criteria of "5.8um-peakers" associated with a strong detection at 24um are
reliable to select sources at z~2. We have analyzed the different correlations
between PAH emission and infrared, millimeter, and radio emission. Practically
all our sources are strongly dominated by starburst emission. We have also
defined two subsamples based on the equivalent width at 7.7um to investigate
AGN contributions. Our sample contains strong starbursts and represents a
particularly 24um-bright class of SMGs. The very good correlation between PAH
and far-IR luminosities is now confirmed in high-z starburst ULIRGs. These
sources show a small AGN contribution to the mid-IR, around ~20% in most cases.
