Research

RESEARCH PROGRAMS

* not a member of the University

non-MSSSO member of the University

# formerly a faculty member

SOLAR SYSTEM

Infrared Observations of Jupiter

The 2.3m telescope was used with the CASPIR near-infrared camera in May and June 1997 to obtain images of Jupiter in support of the Galileo spacecraft mission. The images were used to assist in targeting Galileo instruments by monitor changing cloud patterns on Jupiter. MSSSO were contracted by NASA through Dr G Orton * to obtain images of Jupiter at a wavelength of 5 microns for up to 3 hours per night during a period when the NASA Infrared Telescope Facility in Hawaii was not available. The 2.3m observations (Figure 4) were performed by SSO technical staff during morning twilights.

Fig.4: Images of Jupiter at 4.8 microns obtained on 30 May 1997 (upper left), 31 May 1997 (upper right), 1 June 1997 (lower left), and 5 June 1997 (lower right). Celestial north is up and east is to the left. At 4.8 microns we see structures below the methane cloud layer that scatters sunlight strongly at visible wavelengths. This sequence shows how rapidly the cloud patterns change due to Jupiter's rapid rotation with a period of 10 hours.

MSSSO Annual Report 1997

STARS

Pre-main Sequence Stars in Galactic Clusters

Fig.5. Ha Emission in pre-main sequence stars

The initial mass-function of young cluster stars is of great interest. Many attempts have been made in the past, but difficulties of assigning cluster membership and in identifying pre-main sequence stars has meant the mass functions are very uncertain. Hwankyung Sung, Bessell and Lee* have shown that a good way of finding pre-main sequence stars is to select stars which show Ha emission (Figure 5). This method has been successfully applied to the two clusters, NGC 2264 and NGC 6231.

Alpha Cen A and B

Bessell and O'Mara* have obtained high resolution spectra of Alpha Cen A and B with the 120 inch camera of the 74 inch telescope. Preliminary analysis of Alpha Cen A by Nimish Hathi* (MSc Thesis U of Q) indicates that the strong-lines in Alpha Cen A are identical with those in the Sun.

Research

The Dependence of Kinematics on Abundance on the Giant Branch of Omega Centauri

Norris, Freeman, Mayor*, and Seitzer* have analysed accurate kinematic and chemical abundance data for a sample of some 400 red giants in the chemically inhomogeneous globular cluster Omega Centauri (Figure 6), and found correlations between the radial velocities of Mayor et al. and the calcium abundances of Norris et al. This is the first time that such correlations have been reported in a globular cluster, and contains important clues as to the manner in which the system may have formed. They found that the 20% metal-rich tail of the calcium abundance distribution, as well as being more centrally-concentrated, is kinematically cooler than the 80% metal-poorer component. At first glance one might interpret this as evidence consistent with a dissipative enrichment scenario of cluster formation. It obtains, however, that while the metal-poorer component exhibits well-defined systemic rotation, the metal-richer one shows no evidence of it, in contradistinction to the simple dissipative picture. A more natural explanation of the observed rotation and other chemical abundance data is provided by the merging of protocluster fragments as first postulated by Searle.

Fig. 6: The globular cluster Omega Centauri (copyright AAO)

MSSSO Annual Report 1997

The Stars near the Galactic Centre

The region near the center of our Galaxy is known to contain stars of several types, including very old red giants with masses near that of the sun and young stars with masses up to 100 times the sun's mass. This situation suggests that star formation may occur in an ongoing fashion near the Galactic Centre, in which case there should be intermediate age stars there also. One way of detecting intermediate age stars is to look for the very luminous, red, long-period variables that are usually associated with such a population. These stars, which are near the ends of their lives, are throwing off their outer layers into the interstellar medium via very strong stellar winds (the lost matter can then feed the black hole at the Galactic Centre). The OH molecules in the winds are excited by infrared radiation, leading to strong 1612 MHz maser emission. Such objects, the OH/IR stars, have already been detected near the Galactic Centre. In order to get estimates of masses and ages for the OH/IR stars, it is necessary to have measurements of the pulsation period (typically 400-2000 days) and the luminosity (energy output).

Wood, Habing^* and McGregor have used the ANU 2.3 m telescope to monitor 102 fields near the Galactic Centre for 1200 days in the infrared in order to measure periods and luminosities of a sample of long-period variables near the Galactic Centre. Because visible light coming from the Galactic Center to us is attenuated by a factor of about 10¹⁰ by intervening dust, it is necessary to use infrared light to see to the Galactic Centre. Pulsation periods have been determined for 79 known OH/IR stars and a further 26 previously unknown long-period variables. The complete period distribution is dominated by variables with periods less than 600 days, although the very bright and easily detected OH/IR stars mostly have periods longer than this. The long-period variables near the Galactic Centre have lower luminosities and higher wind velocities at a given pulsation period than similar stars in the solar vicinity, indicating that stars in the Galactic Centre region are 2-5 times more metal-rich than the sun. The OH/IR stars with higher wind velocities also have longer periods at a given luminosity, a result that indicates that these stars are more metal-rich and younger than their counterparts with lower wind velocity. The overall results suggest that continuous star formation and metal enrichment have occurred near the Galactic Centre.

Research

THE GALAXY

Dark Matter in the Galaxy

The Massive Astronomical Compact Halo Object (MACHO) Project is searching for galactic dark matter by looking for its gravitational lensing effects on the light of background stars. The changing geometry of the lensing system as the lens moves relative to the background star causes the brightness of the background star to change with time. These 'microlensing' events can be detected by monitoring the brightnesses of large numbers of stars. MACHO began gathering data in 1992 using the 50 inch telescope at Mt Stromlo Observatory, and now monitors roughly 40 million stars in the Large and Small Magellanic Clouds, and in the Galactic Bulge. A large number of microlensing events have now been detected, with well over 100 along lines of sight to the Bulge, about 16 toward the LMC and, very recently, the first toward the SMC.

During 1997 our major emphasis was on completing the analysis of the first four years of LMC data. The analysis computer runs are now completed, and a preliminary look at the results indicates that the inferred halo dark matter mass and individual lens masses will be consistent with the analysis of the first two years of data, which was published last year. The team expect to complete the analysis and submit results for publication early in 1998. We have also performed a preliminary analysis of the first two years of SMC observations, and in the process detected the first ever microlensing event toward the SMC. This result was recently published, and is especially significant in the light of recent suggestions that the LMC lenses may not in fact be in the halo, but rather are in an intervening strand of material tidally stripped from the LMC. The presence of SMC microlensing, especially if further events are detected as the analysis proceeds, substantially strengthens the likelihood that the detected lenses are in fact a galactic halo population.

MSSSO Annual Report 1997

We have also utilised the large MACHO variable star database to address the possibility that the LMC lenses belong to a non-halo population. In particular, the RR Lyrae distribution has been utilised to rule out the possibility that the lenses are located in a hitherto undetected intervening dwarf galaxy, similar to the Sagittarius dwarf. Similar work is underway to investigate the LMC tidal tail hypothesis for the lens origin. The MACHO database of clump giants will be used to generate co-ordinates for a proposed 2dF run at the Anglo-Australian Telescope, which should definitively resolve the issue by measuring the radial velocities of the stars which belong to the possible tidal tail.

Bulge Image Subtraction

Axelrod has been supervising the PhD thesis work of Andrew Drake, who is performing a pixel lensing analysis of MACHO bulge data through image subtraction. This work has been going well, and promises to substantially increase the number of detected bulge microlensing events.

The Three Kiloparsec Arm

The so called '3-kpc arm' is a prominent large-scale feature seen in the distribution of the Galactic hydrogen. The 21-cm emission can be followed over a range of 30 degrees in Galactic longitude, which turns into absorption when it passes in front of the Galactic Centre. The absorption profile at -53 km/sec is also the most convincing evidence for non-circular motions in the Galactic Disk. There have been many proposed explanations for the origin of the arm, ranging from an explosive event in the Galactic Centre to non-circular motions forced by a rotating bar-shaped mass distribution. The explanation favoured by Kalnajs is that the '3-kpc arm' is the gas swept into a single filled elliptical orbit by a rotating bar. Such filled orbits, also known as 'inner rings', are common in barred galaxies.

The recent survey of Galactic OH/IR stars by Sevenster has provided direct evidence that favours the elliptical orbit explanation. Figure 7 below shows the distribution of the radial velocities for the hydrogen and the OH/IR stars in the Galactic Plane. Most of the emission at the low velocities comes from nearby gas. The '3-kpc arm' is the long narrow ridge at negative velocities extending from about +7 to -20 degrees and

Research

crossing the Galactic Centre at -53 km/sec. It is nicely outlined by 10 or 11 OH/IR stars. According to Sevenster the ages of these OH/IR stars lie between 100 and 350 million years.

Fig. 7: The distribution of 21-cm emission from Galactic hydrogen and OH/IR stars within 1 degree of the Galactic Plane.

The simplest explanation for the close association of the OH/IR stars and the hydrogen which has survived for 1 to 3.5 galactic revolutions is that both are moving along the same orbit. Had the '3-kpc arm' been a density wave, the stars and the gas would have separated during that time. The ages of the stars and the 53 km/sec expansion rate also rules out the explosive origin of the '3-kpc arm'. The debris would have reached the present position in only 40 million years. The current position of the '3-kpc arm' is about half way between the Sun and the Galactic Centre.

MSSSO Annual Report 1997

The '3-kpc arm' is the nearest piece of an elliptical orbit. One would expect to see the far side of the orbit in the direction of the Galactic Center to be moving away from us at +53 km/sec. There does not appear to be anything recognisable at this velocity, but there is a faint feature at +73 km/sec. If this turns out to be the far side, it will mean that the solar neighbourhood is moving away from the Galactic Center at a rate of 10 km/sec.

Wide Field Imaging

The 16 inch telescope at SSO has been used as a mount for a wide-field CCD camera. A Nikon Nikkor lens and a Hasselblad lens provide fields of 7 and 30 degrees respectively, and are used with a Site 2Kx2K thinned CCD detector. By placing narrow band filters between the lens and the CCD, Bessell has been able to restrict the field angles through the filters and thus obtain good monochromatic response across the 30 degree fields. Dome flat fields provide good vignetting corrections.

Bessell, Sutherland, Watson* and Buxton have surveyed the Galactic Plane, Orion and the Magellanic Clouds in Ha, [OIII] and the red continuum. Selected regions have also been imaged in broad-band blue, visual and infrared colours.

Ha minus red continuum images have proved excellent for isolating weak hydrogen emission features, while combining Ha and [OIII] highlights excitation and ionisation variations in the interstellar medium. Combining Ha, [OIII] and B images of galactic HII regions and external galaxies into red-green-blue (RGB) images have produced many strikingly beautiful astronomical pictures suitable for research and educational purposes. Some of the processed images are accessible from the MSSSO homepage. The image of Eta Carina (Figure 8) has been published in most of the popular astronomy magazines around the world.

Research

Fig. 8: Eta Carina, taken with the 16 inch telescope at Siding Spring Observatory

Bessell and Green* will compare the optical Galactic Plane Survey with the Molonglo Radio Survey, obtained at radio frequencies.

Microlensing of Globular Cluster Stars

Axelrod has been exploring the possibility that stars in globular clusters can be used as source stars for detecting microlensing. This appears feasible using pixel lensing techniques and moderate sized telescopes. It is under active investigation as a possible extension of the MACHO project, which formally ends in 2000.

MSSSO Annual Report 1997

THE MAGELLANIC CLOUDS

HI and Ha Surveys of the LMC

A high resolution HI survey of the Large Magellanic Cloud has been completed with the Australia Telescope Compact Array by Sungeun Kim and collaborators at ATNF. This is a 'mosaiced' image made from 1344 fields, with a resolution of 1 arcmin (15 pc, using a distance to the LMC of 50 kpc) - a significant improvement on the 14.9 arcmin (220 pc) resolution achieved in previous surveys. A new Ha survey taken with the 16 inch telescope at SSO covers the same area, with a spatial resolution of 20 arcsec. Together the two surveys offer a unique probe of the detailed relationship between the ionised phase and atomic phase in the gas of the LMC.

Cepheids in the Large Magellanic Cloud

Cepheids are relatively massive stars (3-5 times the mass of the Sun) that regularly pulsate with periods of a few days typically. They are young (by astronomical standards) and, as such, are tracers of the recent star formation history. By combining models that describe the evolution of these stars with models describing their pulsational properties, one can reproduce the distribution of periods of Cepheids in a galaxy. This has been done by Bersier for the Large Magellanic Cloud (LMC), using data accumulated by the MACHO project. This international collaboration, of which Axelrod, Freeman, Rodgers and Peterson are members, is observing the LMC on a nightly basis in order to determine the nature of the dark matter in our Galaxy. As a by-product, this provides an enormous body of data on variable stars. As an example, there are about 1500 Cepheids in the LMC, two-thirds of which are newly discovered.

The period distribution of Cepheids in the LMC has revealed a strong increase in the star formation rate in this galaxy 100 million years ago, lasting about 25 million years. Another result is the discovery of about 300 Cepheids that represent a challenge to stellar evolution theory. A plausible solution put forward is that these stars are the result of the merging of two stars, that were previously orbiting around their common centre-of-mass. Conventional stellar evolution in certain binary systems can lead to the merging of the two companion stars.

Research

Spectroscopy of Red Giant Stars in SMC Star Clusters

The Small Magellanic Cloud (SMC) is one of the nearest galaxies to our own. For this reason it offers opportunities for detailed studies, for example involving individual stars, that are impossible for more distant galaxies. Da Costa, in collaboration with Hatzidimitriou*, has undertaken a study of the chemical enrichment of the SMC, i.e. the change in abundance of SMC stars with time, by obtaining spectra of red giant stars in a number of SMC star clusters. The star clusters range in age from approximately 4 billion years to 12 billion years and the individual stellar spectra reveal the strengths of calcium absorption lines and thus yield the abundance of the elements in each star cluster. When combined with existing data for other objects, particularly those with younger ages, the resulting age-abundance relation for the SMC is generally consistent with a simple model of chemical enrichment, as expected for a galaxy of the SMC's type. However, two of the clusters studied have abundances that are approximately three times lower than expected for their ages from the mean age-abundance relation. In galaxies such as the SMC the chemical enrichment is thought to be uniform across the system, so the existence of these anomalous clusters indicates either they were formed in "special" events involving the infall of less enriched gas, or the chemical history of the SMC was more complex than the simple models predict.

Planetary Nebulae - Deep Imaging

The difference between the theoretical masses of planetary nebula (PN) shells and those measured in practice has long been known. The measured mass of the whole system (central star + nebula) always reveals a "mass deficit", rarely exceeding 1 solar mass, whilst theoretical calculations predict that this should be several solar masses for the more massive precursor stars. In addition, the PN morphology is closely connected with the evolution of the ensemble PN - central star. One possibility is that the "missing" mass is there, but is now at so low a density that it is optically very faint.

MSSSO Annual Report 1997

Fig. 9: An Ha image of planetary nebula NGC 6302

In order to test this idea, Dopita and Hua* have made deep monochromatic images of 21 galactic planetary nebulae which on the basis of their morphology and chemical composition, should be derived from the more massive end of the possible precursor stellar masses. In many cases, these monochromatic images, obtained through narrow-band interference filters using the 2.3m telescope, reveal conspicuous secondary structures far outside the previously known nebula, and which account for a significant fraction of the total mass of the system. These structures are probably relics of early mass ejections associated with episodes of helium-burning during the post-AGB phase of the progenitors.

Planetary Nebulae - The Nature of FLIERs

In young planetary nebulae, we sometimes see small, low ionisation features appearing as either a pair of "ears" jets or pairs of symmetric knots on either side of the main body of the nebula. Recent observations with HST show that these knots are very small, dense and display highly supersonic motions. The nature of these microstructures,

Research

dubbed "FLIERs" or "Fast Low Ionisation Emission Regions" was somewhat of a mystery. Dopita has shown that the excitation, temperature, size and electron density of FLIERs can all be understood if these are shocks in an outflowing gas. Since these shocks are embedded in the nebula they are also illuminated by the ultraviolet light from the central star. In fact, the energetic flux of ionising photons across the shock is about two orders of magnitude higher than the mechanical energy flux through the shock itself. In this case, the shock serves to compress the gas. This weakens the relative strength of the ionizing radiation in the post-shock photoionised region, and enhances the emission lines of low excitation.

GALAXIES

Gas in the Outer Regions of Spiral Galaxies

Using the TAURUS-2 Fabry-Perot Interferometer at the AAT, Bland-Hawthorn*, Freeman and Quinn# detected very faint Ha and [NII] emission in the outer regions of NGC 253, beyond the edge of the neutral hydrogen distribution. This emission was expected from the ionization of the diffuse outer HI by the metagalactic UV radiation field. However the observed emission was much stronger than expected, and suggests that the ionization comes from UV photons emitted by the inner disk of the galaxy itself. These observations open up the possibility of extending the measured rotation curves of spiral galaxies beyond the edge of their HI distributions.

MSSSO Annual Report 1997

Fig. 10: The beautiful spiral galaxy NGC 253 (courtesy: NASA)

Extragalactic Planetary Nebulae

Arnaboldi# and Freeman, with Gerhard*, Kudritzki*, Mendez*, Ford*, and Capaccioli*, continued their work on planetary nebulae in the outer regions of giant elliptical galaxies. Following their spectroscopic discovery of a few intracluster planetary nebulae in the Virgo cluster, they used the William Herschel Telescope (WHT) to survey a region in the Virgo cluster, well away from the major galaxies. Another 11 likely intracluster planetary nebulae were discovered. The goal now is to understand the origin of this population: does it come from debris torn from galaxies as they interact, or is it perhaps an early pre-galactic population of stars?

Research

Separating Shocks and Photoionization in Active Galaxies

Active galaxies have bright cores that cannot be explained by starlight and are thought to be powered by a central black hole. The gas in the nuclear environment glows with a rich spectrum of emission lines, but the detailed processes responsible for the glowing gas are not understood. Candidates for the excitation mechanism include photoionization by energetic photons from the nucleus, and shock excitation of the gas. On one hand photoionization is favoured because of the presence of a bright nucleus, but on the other, we see large scale outflows of gas and turbulence which must include shocks.

In order to discriminate between the different mechanisms, Allen, Dopita and Tsvetanov* have developed a set of diagnostic diagrams which employ ultraviolet emission lines. Ultraviolet emission lines are more sensitive to the excitation mechanism than standard optical lines. Model grids of shock and photoionized gas can be separated on these diagrams making this set of diagrams a useful observational tool for discriminating between shocks and photoionization.

These diagnostics have been applied to HST Faint Object Spectrograph observations of nearby Seyfert and LINER galaxies, and also to a sample of high redshift radio galaxy spectra. It has been shown that M87 has a shock excited nuclear disk, but that Seyfert galaxies indicate both shock and photoionization characteristics.

Bar Diagnostics in Edge-On Spiral Galaxies: The Closed-Orbits Approach

Bureau, with assistance from Kalnajs and Sparke*, has been developing computer codes to find and compute closed orbits in a variety of rotating potentials found in barred spiral galaxies. Closed orbits represent a first order approximation to the gas dynamics of galaxies. The position-velocity diagrams obtained from the computed models can then be compared directly with spectroscopic observations and used to infer the presence and to a lesser extent the structure of a bar in these galaxies.

MSSSO Annual Report 1997

The Dusty Accretion Torus of Seyfert Galaxies

The so-called "unified models" of the active galactic nuclei (AGN) of Seyfert galaxies require that there exists a thick torus of gas and dust which obscures the nucleus from direct view at optical wavelengths when the AGN is viewed over a fairly wide range about the equator of this torus. When the line of sight is close to the polar direction of the torus, the nuclear region can be seen in all its glory. These are the Seyfert I galaxies which display broad emission lines due to gas in rapid rotation very close to the nuclear black hole. On the other hand, when line of sight to the AGN passes through the torus, the existence of the AGN has to be inferred indirectly from the excitation it produces in the interstellar gas located near the poles. This excitation may be the result of photoionisation by the nucleus, or as a result of shocks powered by an outflow. These AGN are called Seyfert II galaxies. In a few cases, the nuclear region may be seen directly in polarised light as a result of scattering off material located close to the nucleus which acts as some kind of a diffuse "mirror". These are the so-called hidden-broad-line regions (HBLRs). Such a model, while being capable of qualitatively describing the difference between the different classes of Seyferts, does not provide a physical explanation for either the existence or the maintenance of such a torus.

As reported last year, Heisler, Lumsden* and Bailey* have shown that the HBLR Seyferts display warmer far-infrared colours and lower dust extinctions than the "true" Seyfert II galaxies. Both these observations suggest that the HBLR Seyferts are observed closer to the polar direction than the Seyfert II class as a whole. This year, with Dopita, they have refined this observation to use it as a probe of the physical conditions in the torus itself. They have shown that the intrinsic IRAS colours of all Seyferts are likely to be similar, but that the observed colours are the result of reddening of the nuclear region seen through the torus, mixed in with varying amounts of warm dust emission resulting from circumnuclear star formation. They showed that the properties of the dust torus are consistent with it being an accretion flow toward the nucleus, with most of the far-IR emission arising in the dust sublimation region a few light years from the nucleus itself. In these regions, the effect of radiation outflow in the polar regions is consistent with observations. The density of the accretion disk in the mid-plane of the accretion flow is likely to be much higher, and it is here that the H₂O maser activity is likely to occur.

Research

The Pattern Speed of NGC 2915

Fig. 11: The distribution of hydrogen in NGC 2915

Bureau and Freeman, with Meurer*, have been studying the extraordinary galaxy NGC 2915 (Fig. 11). The stellar component represents only a small blue compact dwarf galaxy, but the neutral hydrogen (HI) distribution extends to vast distances and possesses a well-defined bar and spiral arms. However, because the galaxy is dominated by dark matter everywhere, it is hard to explain the presence of the pattern. The Tremaine-Weinberg method has been used to measure the pattern speed in the disk of the galaxy. NGC 2915 is only the second object on which this method has been successfully applied. The result is puzzling: the rotation rate of the pattern is much slower than one has come to expect in barred spiral galaxies. The group argues that the pattern must be created by a triaxial dark halo which is in slow rotation. If this interpretation is true, it represents the first time the figure rotation of a dark halo is measured.

MSSSO Annual Report 1997

HST/WFPC2 Observations of Dwarf Elliptical Galaxies in the M81 Group

The dwarf elliptical (dE) galaxies of the Local Group show a surprising variety of star formation histories in that some consist of only old stars, while others have gone on forming stars until as recently as one or two billion years ago. At the present time, however, there is no clear explanation for this diversity. We can expect though that by studying dwarf ellipticals outside the Local Group, we can learn whether this is purely a local phenomenon or whether it is a characteristic of dE galaxies in general. However, given the large distance to even the nearest dwarf elliptical beyond the Local Group, such a study requires the use of the Hubble Space Telescope. Caldwell, Armandroff*, Da Costa and Seitzer* have used the WFPC2 camera aboard HST to image two dEs in the M81 group of galaxies. This nearby galaxy group is much more compact than the Local Group and thus has provided a different environment for its member galaxies. The HST observations have been used to generate colour-magnitude diagrams for each dE and these have revealed the presence of many bright red stars in both dEs. The number and luminosity of these stars indicates that, like many Local Group dEs, the M81 group dEs have had star formation over a substantial interval: in one system stars as young as 3 to 4 billion years are probably present, while the youngest stars in the second dE are somewhat older, with ages of perhaps 8 billion years. Clearly then, whatever the mechanism is for producing the extended star formation histories seen in these galaxies, it is not restricted solely to the Local Group.

Improvements to Galaxy Cluster Distance Estimates using Elliptical Galaxies

A fundamental problem in astronomy is the accurate estimation of distance to extra-galactic objects. Understanding the relationship between the structure, dynamics and luminosity of elliptical galaxies is a key aspect in the use of these objects for measuring distances to clusters of galaxies.

Graham continued his investigation of the popular assumption about the homologous structure of elliptical galaxies (i.e. that the light distribution of all elliptical galaxies declines with the one-quarter power of galaxy radius). He showed this to be a poor approximation to reality. By taking into account the variations in the light profile

Research

from galaxy to galaxy, he obtained an improved distance ratio between the Virgo and Fornax clusters. The new result places Fornax 12 ± 6% further away than Virgo. This is to be compared with the previous value of 23 ± 8%, obtained by treating all the elliptical galaxies as scaled replicas of each other.

Compact Radio Cores in 60 Micron Peakers

Heisler and her collaborators Norris*, Jauncey*, Reynolds* and King* found that the IRAS-selected sample of 60 micron Peaker galaxies have radio core luminosities which are intermediate between "normal" Seyferts (i.e. far-infrared selected Seyferts chosen without regard to far-infrared spectral energy distribution) and radio-loud ellipticals. The 60 micron Peakers follow the same relationship of core-to-total flux as radio elliptical galaxies. Thus the correlation between core and total power for radio ellipticals extends to 60 micron Peakers with radio luminosities as low as 10²⁰ W/Hz. The radio emission in 60 micron Peakers is not like that of classical radio galaxies - it is unresolved on scales of ~1 kpc, and high resolution radio mapping is crucial for investigating the source of the total radio flux.

IC 5063 is a reasonably close 60 micron Peaker and has been mapped in the radio as part of a survey of radio bright Seyfert galaxies. This project was carried out by Heisler in collaboration with Norris *, Appleton*, Bransford * and Marston*. They discovered a linear triple of radio emission sources at 3cm and a possible large-scale bow-shock at 6cm. The components of the triple are separated by approximately 3 arcseconds, corresponding to a linear scale of 500 pc. They interpret the linear triple source as the ejection of radio plasma from the nucleus of the galaxy. This is an excellent candidate for a "baby" radio elliptical galaxy. VLBI mapping of the more distant 60 micron Peakers will be required in order to address the question of whether 60 micron Peakers are in a transitional stage of evolution which is intermediate between radio-quiet Seyferts and radio ellipticals.

Intermediate Radio-Loud IRAS Galaxies

Many galaxies show signs of activity in their central regions which range from intense bursts of recent star formation in some objects to broad line-emitting gas regions and narrow jets of radio-emitting relativistic plasma in extreme objects. The latter are

MSSSO Annual Report 1997

most likely powered ultimately by accretion of gas onto a central massive black hole. The evolutionary connection, if one exists, between starburst galaxies and more active galaxies is poorly understood. However, the existence of extremely luminous, dusty, and often interacting galaxies, emitting most of their energy at far-infrared wavelengths, suggests one way in which extreme activity in galaxies may arise: Gas funnelled into the central regions of galaxies during interactions first undergoes an intense starburst and later settles into the central few parsecs of the galaxy where it forms a massive black hole.

McGregor, Kewley, and van Breugel* have identified a sample of unusual active galaxies by correlating far-infrared detections in the 60~µm Infrared Astronomical Satellite Faint Source Catalogue with radio detections in the 5GHz Parkes-MIT-NRAO catalogue. These dusty galaxies have radio luminosities intermediate between those of starburst galaxies and radio-quiet active galaxies on one hand and powerful radio galaxies and radio-loud quasars on the other. Their optical spectra, far-infrared luminosities, and optical emission-line luminosities are all similar to those of other well-studied classes of objects. The intermediate radio-loud IRAS galaxies may therefore be the active galaxies in which the radio activity has only recently commenced. Further observations are underway to clarify the nature of the radio emission in these galaxies.

Radiative 3D Hydrodynamics

Sutherland continued the development of plasma hydrodynamic simulations in 3D (Figure 12). Making extensive use of the supercomputing facilities available at the ANU, he has for the first time combined a realistic treatment of non-equilibrium atomic cooling processes into a robust magneto-hydrodynamic code, ZEUS. This has produced extremely high resolution simulations of thermal instabilities in shocks and cooling plasmas of astrophysical interest.

Research

Fig. 12: This plot shows the development of a quasi-steady state in a mach 20 double shock structure with radiative losses from a solar composition plasma. The initial formation of the double shock (a) is adiabatic, until cooling losses in the centre halt (b) the shock growth and eventually bring on the collapse of the shock (c). A second period of shock formation occurs, but is modified by internal secondary shocks and the growth of RT instabilities in the dense central layer (d). Cooling is no longer confined to the central regions and the stratified structure of the first pulse is no longer present. The collapse (e) is complicated by secondary shocks, and the dense central layer (f) is growing. In the third phase (g-j) the two shocks are no longer symmetrical and cooling occurs throughout the inhomogeneous shocked layers of gas. Fluctuations (h-i) on the 30-50 percent level occur with a period of roughly the cooling time of the main shock, but total collapse or rapid growth no longer occur.

MSSSO Annual Report 1997

THE UNIVERSE

The 2dF Galaxy Redshift Survey

Cosmography, or the mapping of the Universe, has a history extending back to the naming of the constellations and beyond. In the last hundred years it has been joined by cosmology, which seeks to explain in scientific terms how the Universe came to be

the way it is.

This year saw the start of the 2dF Galaxy Redshift Survey, an Anglo-Australian program to map the positions and learn the properties of 250,000 galaxies filling a statistically-representative volume of the Universe. This ambitious new attempt to extend cosmography and give a comprehensive inventory of the Universe is being undertaken by twenty researchers from Australia and the UK, headed by Colless and Ellis* and uses the new Two Degree Field multi-fibre spectrograph (2dF) at the Anglo-Australian Observatory. The survey, which is expected to take at least two more years to complete, will map the large-scale structure of the galaxy distribution, yielding cosmological information including an estimate of the mean mass density of the Universe (which determines its ultimate fate, either expanding forever or re-collapsing) and clues to the nature of the dark matter which makes up over 90% of the total mass. By examining how the properties of the galaxies vary according to the local density of matter and the type of large-scale structures they inhabit, the survey will also illuminate the links between structure formation and galaxy evolution.

Research

The Nearby Universe

Schmidt, Germany, Stubbs, and Reiss* are using the MSO 50 inch telescope to monitor rich clusters of galaxies, looking for the infrequent supernova explosions which suddenly appear in galaxies a couple of times per millennium.

Fig. 13: Distant supernovae

By looking at thousands of galaxies every few nights, this 18 month old program has uncovered more than 23 objects, making it the most effective current search for supernovae in the nearby Universe. These objects will form the benchmark for the comparison with extremely distant supernovae. The distances measured to these objects will allow Schmidt and collaborators to measure the gravitationally induced motions of galaxies in the local Universe, and thereby gauge the distribution of matter across areas where we can see it (galaxies) and where we cannot (voids) (Fig. 13).