OzDES aimed to maximise the cosmological constraining power of the DES photometric survey by providing a critical spectroscopic survey. Spectra of DES targets will facilitate the science goals of numerous DES Science Working Groups, as outlined below.
Observations of a few dozen high-redshift Type Ia supernovae led to the first discovery of the accelerating expansion of the universe. The last decade saw supernova searches discovering thousands of supernovae, with many hundreds being spectroscopically classified as SNe Ia and placed on the Hubble Diagram. Next generation surveys such as DES and LSST will discover far more SNe than can be spectroscopically classified in real time. Redshifts from the SN host galaxies will be critical for reliable photometric classification of the SNe and accurate measurement of the SN Ia light curve properties, which can then be used to constrain the properties of dark energy. OzDES obtained host redshifts for the DES SNe using the AAT, which is ideally suited for the efficient collection of host galaxy redshifts. OzDES also targeted active transients in order to obtain a sub-sample of spectroscopically classified SNe, useful for both the training of photometric classifiers and the study of SN spectroscopic properties.
Active Galactic Nuclei
We regularly monitored almost 800 AGN as part of OzDES in order to measure the masses of their central, supermassive black holes. The 25 spectroscopic observation epochs from AAOmega, combined with over a hundred photometric epochs from DES, are well suited to accomplish this with the technique of reverberation mapping. This method measures black hole masses through the application of the virial theorem, and the key measurements are the velocity dispersion of the broad-line region (BLR) and the distance of the BLR from the central black hole. The intensity of the broad emission lines are determined by the intensity of the central continuum source, and as AGN vary in luminosity, the intensity of the broad lines vary in response. Reverberation mapping measures the time lag between variations in continuum and broad line intensity, and thus the light travel time from the central source to the BLR. This time lag ranges from a few light days up to a light year or more for the most luminous AGN, and thus the 6-year DES+OzDES program is well suited to measure even quite luminous AGN.
Before OzDES began, the reverberation mapping technique had been used to measure black hole masses in approximately 50 AGN, nearly all of which are of low luminosity and at low redshift (z<0.3). With OzDES, our goal is to measure black hole masses in up to an order of magnitude more sources, and specifically target the luminous AGN at z>1 that represent the bulk of black hole mass assembly in the universe. Reverberation mapping is the only technique that can determine black hole masses outside of the nearby universe, as it relies on the time domain, rather than spatial resolution. With these measurements we plan to better calibrate black hole mass scaling relations that will be used to track the evolution of supermassive black hole growth throughout cosmic history. We will also use these scaling relationships to determine distances to individual AGN, and thus use them as a completely independent form of standard candle to probe cosmological parameters.
Rich galaxy clusters are the most massive bound structures in the universe. Containing thousands of galaxies and reaching masses as high as a few times 1015 solar masses. The number density and clustering of galaxy clusters are sensitive probes of dark energy and gravity. The Dark Energy Survey will discover 100,000 galaxy clusters and will be the largest, most complete catalogue of its kind until the advent of the Legacy Survey of Space and Time (LSST). In addition to constraining the properties of dark energy, the catalogue will be used to study how galaxies evolve in the dense environment of a galaxy cluster. OzDES has gathered redshifts of galaxy clusters in the DES supernova fields, which will hep calibrate photometric redshifts for the remainder of the DES cluster sample. For many clusters have also obtained spectra of multiple cluster members, which can allow us to measure their masses dynamically.
Strong gravitational lenses provide unique information on both the lensing galaxy and the lensed object. For the lens we obtain measurements of total projected mass densities of individual galaxies, enabling cosmological tests and measurements of mass-to-light ratios, dark matter fractions, IMF slope, internal structures and central source properties not accessible by any other means. In addition, the magnified lensed galaxy allows important measurements of galaxy evolution at high redshift. The increased spatial resolution enables morphological probes of structure down to sub-kpc scales (e.g. stellar clumps) and the light amplification results in higher signal-to-noise for detailed spectroscopy. OzDES followed-up gravitational lenses, providing important redshift confirmation of the lensing systems, enabling a range of studies. The AAT will also enable lenses to be discovered spectroscopically (via background emission lines), as our LRG targets have large lensing cross-sections. This will allow detection of lenses with small Einstein radii not accessible to ground-based imaging.
The Australian Telescope Large Area Survey (ATLAS) is a project to image six square degrees of the sky to an RMS of around 15 uJy. ATLAS is the widest radio survey of this depth and covers the ECDFS and ELAIS S1 regions. The fields were chosen to cover the areas of the southern sky well studied at many other wavelengths, e.g. with Spitzer, Herschel etc. These deep radio observations cut through the obscuring dust and will allow us to trace the cosmic star formation history of the Universe as well as the history of jet-powered AGN. To study these phenomena we require luminosities, and hence redshifts, for all our sources. Radio sources tend to have unique optical photometry, which does not lend itself to traditional photometric techniques, since they are powered by diverse astrophysical processes. ATLAS is a precursor to the Evolutionary Map of the Universe (EMU) project with the Australian SKA Pathfinder which will have a similar depth and resolution, but will cover the entire southern sky visible from Western Australia. The OzDES ATLAS redshifts will provide a sample of radio sources with extremely high spectroscopic completeness providing a vital training set for statistical redshifts to be provided for the entire EMU dataset enabling many science goals covering cosmology and galaxy evolution.