Spectroscopic identification of a redshift 1.55 supernova host galaxy   from the Subaru Deep Field Supernova Survey
  Context: The Subaru Deep Field (SDF) Supernova Survey discovered 10 Type Ia supernovae (SNe Ia) in the redshift range 1.5<z<2.0, as determined solely from photometric redshifts of the host galaxies. However, photometric redshifts might be biased, and the SN sample could be contaminated by active galactic nuclei (AGNs).   Aims: We aim to obtain the first robust redshift measurement and classification of a z > 1.5 SDF SN Ia host galaxy candidate   Methods: We use the X-shooter (U-to-K-band) spectrograph on the Very Large Telescope to allow the detection of different emission lines in a wide spectral range.   Results: We measure a spectroscopic redshift of 1.54563 +/- 0.00027 of hSDF0705.25, consistent with its photometric redshift of 1.552 +/- 0.018. From the strong emission-line spectrum we rule out AGN activity, thereby confirming the optical transient as a SN. The host galaxy follows the fundamental metallicity relation defined in Mannucci et al. (2010, 2011) showing that the properties of this high-redshift SN Ia host galaxy is similar to other field galaxies.   Conclusions: Spectroscopic confirmation of additional SDF SN hosts would be required to confirm the cosmic SN rate evolution measured in the SDF. 
Teddy F. Frederiksen, Or Graur, Jens Hjorth, Dan Maoz, Dovi Poznanski
astro-ph.CO