Active Galaxies
What are Active Galaxies?
Since their discovery in the 1960s, quasars have been a hot topic of research. It was soon realised that the most likely power source for these distant ultra-luminous sources was accretion of material onto a super-massive black hole. These black holes typically have masses a million to a billion times that of the Sun, and when accreting efficiently can outshine their entire host galaxy (which may contain 10^12 stars or more). Quasars are now known to be part of a family of objects generally refered to as active galaxies, or active galactic nuclei (AGN). These objects are all though to be powered by accretion onto a super-massive black hole. The detailed physics, orientation and evolutionary stages that such objects go through are characterized by different observational signatures. They are then often classified by astronomers into a zoo of different AGN, including quasars, quasi-stellar objects (QSOs) Seyfert galaxies (type I and II), BL Lacs, FRI/FRII radio galaxies...etc. Underlying all these different sources in the same physical mechanism - accretion onto a black hole.
Disk at the heart of the elliptical
galaxy NGC 4261
The ultra-high luminosity of many types of AGN allow them to be used as very effective probes of the distant Universe, were it is much harder to observe more normal (fainter) galaxies. They have been used to study the growth and evolution of structure in the Universe, as well as probe the inter-galactic medium (via absorption in the spectra of quasars). The revelation in the 1990s that most (if not all) galaxies (including our own Milky Way) contain super-massive black holes has made AGN research an even more crucial aspect of galaxy formation. It is expected that the super-massive black holes have a profound impact on the evolutionary history of their host galaxies, for example, regulating star formation and injecting vaste amounts of energy in the inter-stellar medium.
Current research areas
Large Spectroscopic Quasar Surveys
The new generation of large spectroscopic surveys for quasars, utilizing powerful multi-object spectrographs (such as 2dF on the Anglo-Australian Telescope - AAT) have revolutionized quasar astronomy. The 2dF QSO Redshift Survey (2QZ) was a ground breaking survey of 25000 quasars (2.5 times the number of objects discovered in the previous thirty years and 25 times the previous largest single sample) at redshifts z=0.3-3.0. The survey was completed in 2002, and resulted in major advances in the understanding of the space distribution, clustering, evolution and spectral properties of quasars. Various analyses are still ongoing. The 2df-SDSS Luminous-red-galaxy and Quasar Survey (2SLAQ) follows on from 2QZ, probing the distribution of fainter quasars and bright red galaxies in tandem. This sample contains ~8000 new quasars and is being analyses to measure the clustering of quasars as a function of luminosity and determine how the black hole mass distribution varies as a function of luminosity. The latest quasar survey makes use of the new AAOmega spectrograph at the AAT and deep optical and infrared imaging from the SDSS and UKIDSS Survey to select faint high redshift quasars (the AUS Survey). This is ongoing and aims to measure the clustering and space distribution of faint quasars at high redshifts, z=3-6. It will use the infrared data from UKIDSS to select quasars free from bias due to absorption by dust.
For more information contact Scott Croom
The Hosts and Environments of AGN
As well as large statistical studies, there is also significant effort being put into understanding the impact the AGN have on their hosts (and visa versa). Scott Croom is currently collaborating with Rob Sharp (at the Anglo-Australian Observatory) on an integral field spectroscopic study of low redshift (z~0.1) AGN host galaxies. This project will obtain spatially resolved spectroscopy of the hosts to address a wide range of issues, including: are AGN triggered by galaxy mergers? What are the ages of the stellar populations in AGN host galaxies? Do AGN have the same black-hole vs. host relations as quiescent galaxies? For this we are using data taken by the AAT using the Spiral integral field unit (IFU) and the VLT using the VIMOS IFU.
For more information contact Scott Croom