Ultraluminous Star-Forming Galaxies and Extremely Luminous Warm
  Molecular Hydrogen Emission at z=2.16 in the PKS 1138-26 Radio Galaxy
  Protocluster
  A deep Spitzer Infrared Spectrograph map of the PKS 1138-26 galaxy
protocluster reveals ultraluminous PAH emission from obscured star formation in
three protocluster galaxies, including Halpha-emitter (HAE) 229, HAE 131, and
the central Spiderweb Galaxy. Star formation rates of 500-1100 Msun/yr are
estimated from the 7.7 micron PAH feature. At such prodigious formation rates,
the galaxy stellar masses will double in 0.6-1.1 Gyr. We are viewing the peak
epoch of star formation for these protocluster galaxies. However, it appears
that extinction of Halpha is much greater (factor of 40) in the two ULIRG HAEs
compared to the Spiderweb. This may be attributed to different spatial
distributions of star formation--nuclear star formation in the HAEs versus
extended star formation in accreting satellite galaxies in the Spiderweb. We
find extremely luminous mid-IR rotational line emission from warm molecular
hydrogen in the Spiderweb Galaxy, with L(H2 0-0 S(3))= 1.4E44 erg/s (3.7E10
Lsun), 20 times more luminous than any previously known H2 emission galaxy
(MOHEG). Depending on temperature, this corresponds to a very large mass of
>9E6-2E9 Msun of T>300 K molecular gas, heated by the PKS 1138-26 radio jet,
acting to quench nuclear star formation. There is >8 times more warm H2 at
these temperatures in the Spiderweb than what has been seen in low-redshift
(z<0.2) radio galaxies, indicating that the Spiderweb may have a larger
reservoir of molecular gas than more evolved radio galaxies. This is the
highest redshift galaxy yet in which warm molecular hydrogen has been directly
detected.
