The MUSE Hubble Ultra Deep Field Survey: II. Spectroscopic redshifts and
  comparisons to color selections of high-redshift galaxies
  We have conducted a two-layered spectroscopic survey (1'x1' ultra deep and
3'x3' deep regions) in the Hubble Ultra Deep Field (HUDF) with the Multi Unit
Spectroscopic Explorer (MUSE). The combination of a large field of view, high
sensitivity, and wide wavelength coverage provides an order of magnitude
improvement in spectroscopically confirmed redshifts in the HUDF; i.e., 1206
secure spectroscopic redshifts for HST continuum selected objects, which
corresponds to 15% of the total (7904). The redshift distribution extends well
beyond z>3 and to HST/F775W magnitudes as faint as ~30 mag (AB, 1-sigma). In
addition, 132 secure redshifts were obtained for sources with no HST
counterparts that were discovered in the MUSE data cubes by a blind search for
emission-line features. In total, we present 1338 high quality redshifts, which
is a factor of eight increase compared with the previously known spectroscopic
redshifts in the same field. We assessed redshifts mainly with the spectral
features [OII] at z<1.5 (473 objects) and Lya at 2.9<z<6.7 (692 objects). With
respect to F775W magnitude, a 50% completeness is reached at 26.5 mag for ultra
deep and 25.5 mag for deep fields, and the completeness remains >~20% up to
28-29 mag and ~27 mag, respectively. We used the determined redshifts to test
continuum color selection (dropout) diagrams of high-z galaxies. The selection
condition for F336W dropouts successfully captures ~80% of the targeted z~2.7
galaxies. However, for higher redshift selections (F435W, F606W, and F775W
dropouts), the success rates decrease to ~20-40%. We empirically redefine the
selection boundaries to make an attempt to improve them to ~60%. The revised
boundaries allow bluer colors that capture Lya emitters with high Lya
equivalent widths falling in the broadbands used for the color-color selection.
Along with this paper, we release the redshift and line flux catalog.
