Active galactic nuclei as powerful cosmic probes and the role of OzDES


Venue: Parnell Building (07) Rm 222, St Lucia Campus
Ms Anthea King, UQ (CAASTRO)

This colloquium will be held at 3 pm on Friday 14th August.

Active galactic nuclei (AGN; aka accreting supermassive black holes) have been used as powerful probes of black hole growth for several decades, allowing us to investigate the co-evolution of the central black-hole with their host galaxies. Recently, our team showed that AGN can also potentially be used to probe the early time expansion history of the universe as a distance indicator (Watson et al. 2011). The technique of reverberation mapping allows one to infer the intrinsic luminosity of the AGN by measuring the time delay between continuum variations from the nuclear region and spectral line strength variations in the surrounding gas clouds.  The more highly luminous, the larger the ionised region, and the longer the time delay.  AGN are extremely luminous (up to 100x greater than galaxies starlight), and as a consequence they can be observed to much earlier epochs of the expansion history of the Universe than the currently used probes.  Information from these high-redshift regions can help constrain the properties of dark energy,  in particular whether it varies over time.

OzDES is a 5-year spectroscopic survey currently underway at the AAT working in conjunction with the Dark Energy Survey (DES). As part of this survey, OzDES is performing reverberation mapping on the order of 500 AGN. This is a 10-fold increase in number over the current sample of AGN, and will extend the redshift range from its current limit of z<~0.3 to z~4. As a consequence, OzDES will provide a useful sample for testing the reliability of this method for cosmological measurements and will provide a baseline calibration sample for future work. In this talk, I will describe how AGN are expected to be reliable cosmic probes and discuss the predicted capabilities of OzDES.