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Radio galaxies and their evolution

At the centre of every large galaxy is a central supermassive black hole. Enormous amounts of energy can be released when this black hole swallows infalling material, with the energy emerging as radiation from an accretion disk or mechanical power in a radio-emitting jet of particles accelerated to relativistic speeds.

One of the most remarkable astronomical discoveries of the past decade is that the mass of a galaxy and the mass of its central black hole are closely correlated, implying that the growth of a galaxy over cosmic time is closely coupled to the growth of its central black hole.

Radio-loud AGN in 'normal' galaxies

Massive galaxies, which contain the most massive black holes, provide the most stringent tests of current galaxy formation models. It has been proposed that the energy injected by radio jets in massive galaxies heats the gas in and around the galaxy, preventing it from cooling and forming new stars. This proposed radio-mode feedback (Bower et al. 2006; Croton et al. 2006) provides a natural coupling between the central black hole and the large-scale stellar population of the galaxy.

Scott Croom and I were recently awarded an ARC DP grant, Relativistic jets and radio-mode feedback in massive galaxies, to investigate the ways in which radio galaxies are triggered and measure the energy input by radio jets into these galaxies at different epochs over cosmic time.


SUMSS contours of 843 MHz radio continuum emission (blue) overlaid on a greyscale image of the nearby elliptical galaxy NGC 3250. As with many low-power radio galaxies, the optical spectrum of NGC 3250 shows only stellar absorption lines, with no optical signature of an active nucleus. The total radio luminosity at 1.4 GHz is relatively low (about 1022 W/Hz), yet the radio emission shows an edge-brightened (FR-II) morphology which is generally seen only in powerful (> 1025 W/Hz) radio galaxies.


The high-frequency radio-source population

We recently completed the first high-frequency radio survey of the southern sky, the Australia Telescope 20 GHz (AT20G) survey, using a wide-band analogue correlator on the Australia Telescope Compact Array (ATCA). More details of the survey, as well as the public data catalogue released in 2010, are available here.


Radio colour-colour plot (from Murphy et al. 2010) for a set of 3763 AT20G sources with near-simultaneous observations at 20, 8 and 5 GHz (crosses). Open circles show a newly-identifed class of Ultra-Inverted Spectrum (UIS) sources with a spectral index of α(5, 20) > +0.7.


Neutral hydrogen in the universe

21cm HI absorption studies with ASKAP

I lead the ASKAP-FLASH survey (see http://www.physics.usyd.edu.au/sifa/Main/FLASH/), which will carry out the first blind survey for HI in individual galaxies at 0.5 < z < 1.0 by detecting the HI line in absorption against background continuum sources.

We are currently carrying out a series of demonstration science observations with the BETA test array which links six ASKAP antenas with Mk1 Phased Array Feeds (see here for more details).


BETA Band 1 (712- 1015 MHz) observation of the 1 Jy radio galaxy PKS2252-089. The (known) absorption line associated with HI gas intrinsic to PKS2252-089 can clearly be seen at 884 MHz. The inset shows a close-up of the detected absorption line, with the BETA spectrum in red and an earlier GBT observation in blue. [Figure courtesy of CSIRO]


HI in nearby E/S0 galaxies

I have an ongoing research program with Raffaella Morganti and Tom Oosterloo (ASTRON, Netherlands) to investigate the origin and kinematics of neutral hydrogen gas in elliptical and S0 galaxies in the local universe.


NGC5266 is an E4 elliptical galaxy (green in figure) with a prominent dust lane seen almost edge-on (not visible in the image). Observations with the Australia Telescope Compact Array ATCA showed that it has a disk of neutral hydrogen, perpendicular to the dust lane, extending to almost 10 R_e (red in figure). The estimated HI mass in NGC 5266 is M_HI ~ 2 x 1010 solar masses, a very large amount for an elliptical galaxy (for more details see our paper: Morganti et al. 1997, AJ, 113,937). [Figure courtesy of Raffaella Morganti and Tom Oosterloo, ASTRON]