First year of Australia hunting for Black Holes and Dark Energy

Sep 2, 2015

Type Ia supernovae provided some of the first evidence for the accelerating Universe and remain among the most import tools for cosmology. The Dark Energy Survey (DES) Supernova Search is a new generation experiment that will discover more than 3000 Type Ia supernovae. This will be the largest coherent sample to date and used to make the best ever measurement of the Universe’s  expansion history. Traditionally, spectroscopy of a live supernova within a few weeks of maximum light serves the dual purpose of “typing” the transient (i.e. determining whether it is a Type Ia supernova) and measuring its redshift, both of which parameters are critical for putting the supernova onto a Hubble diagram. However, this method requires an unrealistically large amount of resources for a large number of supernovae such as DES will collect. Alternatively, redshifts can be measured by observing supernova host galaxies at any time after the supernova discoveries. With the help of these redshifts, light curves can be used to reliably identify Type Ia supernovae.

The Australian Dark Energy Survey (OzDES) is a five-year, 100-night program that will use the 400 fibre 2dF system and AAOmega spectrograph on the Anglo-Australian Telescope to measure redshifts of 2,500 Type Ia supernovae host galaxies from DES. Bringing together the power of multi-fibre spectrograph and time series observations (roughly monthly during the DES season), OzDES will also monitor more than 500 active galactic nuclei and quasars to measure their black hole masses, classify live DES transients and provide calibration data for other DES programs probing the Universe’s structure and evolution.

In this paper, we present an overview of the OzDES program and results from its first year of operation. In the first year, OzDES observed over 10,000 objects and measured more than 6,000 redshifts. We are achieving goals in terms of efficiency, redshift reliability and precision, while we continue to improve data quality. The large number of spectra taken by OzDES also guarantees discoveries of rare events or objects. We highlight a few cases that stand out, including some galaxy-galaxy lens candidates.

Publication details:

Fang Yuan, C. Lidman, T. M. Davis, M. Childress et al. in MNRAS (2015) “OzDES multi-fibre spectroscopy for the Dark Energy Survey: first-year operation and results