Keck-I MOSFIRE spectroscopy of the z~12 candidate galaxy UDFj-39546284
  We report the results of deep (4.6h) H band spectroscopy of the well studied
z~12 H-band dropout galaxy candidate UDFj-39546284 with MOSFIRE on Keck-I.
These data reach a sensitivity of 5-10 x 10^-19 erg s^-1 cm^-2 per 4.4 Angstrom
resolution element between sky lines. Previous papers have argued this source
could either be a large equivalent width line emitting galaxy at 2<z<3.5 or a
luminous galaxy at z~12. We find a 2.2 sigma peak associated with a line
candidate in deep Hubble-Space-Telescope Wide-Field-Camera 3 Infrared grism
observations, but at a lower flux than what was expected. After considering
several possibilities we conclude these data can not conclusively confirm or
reject the previous line detection, and significantly deeper spectroscopic
observations are required. We also search for low-redshift emission lines in
ten other 7<z<10 z, Y, and J-dropout candidates in our mask and find no
significant detections.
