Observational tasks that require photometry of large numbers of objects, or over large fields, are still best approached using photographic photometry. Nevertheless, there are well-known problems in using photographic emulsions as two-dimensional detectors; these include their non-linearity and unknown photometric zero-points. The non-linearity can be calibrated using standard sensitometer techniques but the provision of photometric zero-points (and the verification of the validity of the intensity calibration) can be done properly only by providing a sequence of standards down to the plate limit. One approach to this latter problem, that has recently been revived by Racine (1969), is to use a sub-beam prism as originally suggested by Pickering (1891).

We present the proceedings from a two-day workshop held at Swinburne University on 2005 May 24–25. The workshop participants highlighted current Australian research on both theoretical and observational aspects of galaxy groups. These proceedings include short one-page summaries of a number of the talks presented at the workshop. The talks presented ranged from reconciling N-body simulations with observations, to the Hı content of galaxies in groups and the existence of ‘dark galaxies’. The formation and existence of ultra-compact dwarfs in groups, and a new supergroup in Eridanus were also discussed.

We review the evidence that the ultra-compact dwarf (UCD) galaxies we recently discovered in the Fornax Cluster form a new, previously unknown class of galaxies and we discuss possible scenarios for their formation. We then present recent results that UCDs are also present in the Virgo Cluster, and that there is a much larger than expected population of fainter UCDs in the Fornax Cluster. The size and properties of this population may lead us to revise our original ‘galaxy threshing’ hypothesis for the formation of UCDs.

Some unsolved cosmological questions remain in relation to the formation of structure in the universe. One way of addressing such questions is to use rich galaxy clusters as tracers of the growth of large-scale structure. To date, studies of rich clusters of galaxies have concentrated on systems generally at either high redshift or in the local universe. The properties of clusters and their constituent galaxies at these extrema are becoming well understood. In particular, it is becoming clear that rich clusters have undergone considerable evolution both dynamically and in their galaxy populations over the last 5–8 Gyr. We are undertaking a detailed study of rich clusters of galaxies in the range 0·05 ≲ z ≲ 0·15. Our results will be directly comparable to those of previous studies both at high and low redshift and, for the first time, provide continuous coverage across this important and unexplored transitory epoch in terms of galaxy evolution and structure growth.

We present a statistical comparison between spectral line strength variations in Coma galaxies and galaxies in three rich clusters at z ~ 0.3. Using a principal component analysis, we have transformed the observable quantities, the line strengths, into new mutually orthogonal axes and found two specific results. First, more independent parameters are required to account for the line strength variations in the distant cluster data than in the Coma data, and second, line strengths which are not correlated in the distant cluster data are strongly coupled in the Coma data. These results suggest that galaxies in clusters have been homogenised such that most of the information from the conditions present at their formation has been destroyed. Hence, it may be possible that the present-day homogeneity of early-type galaxy properties, e.g. the fundamental plane relation, does not require a single formation scenario, but that a variety of formation scenarios for different galaxies could still yield the observed behaviour of nearby galaxies.

We have used the Low Resolution Imaging Spectrograph on the W. M. Keck I telescope to obtain spatially resolved spectroscopy of a small sample of six ‘post-starburst’ and three ‘dusty-starburst’ galaxies in the rich cluster CL 0016+16 at z=0.55. We use this to measure radial profiles of the Hδ and [OII]λ3727 lines which are diagnostic probes of the mechanisms that give rise to the abrupt changes in star formation rates in these galaxies. In the post-starburst sample we are unable to detect any radial gradients in the Hδ line equivalent width — although one galaxy exhibits a gradient from one side of the galaxy to the other. The absence of Hδ gradients in these galaxies is consistent with their production via interaction with the intracluster medium; however, our limited spatial sampling prevents us from drawing robust conclusions. All members of the sample have early-type morphologies, typical of post-starburst galaxies in general, but lack the high incidence of tidal tails and disturbances seen in local field samples. This argues against a merger origin and adds weight to a scenario where truncation by the intra-cluster medium is at work. The post-starburst spectral signature is consistent over the radial extent probed with no evidence of [OII]λ3727 emission and strong Hδ absorption at all radii, i.e. the post-starburst classification is not an aperture effect. In contrast the ‘dusty-starburst’ sample shows a tendency for a central concentration of [OII]λ3727 emission. This is most straightforwardly interpreted as the consequence of a central starburst. However, other possibilities exist such as a non-uniform dust distribution (which is expected in such galaxies) and/or a non-uniform starburst age distribution. The members of the sample exhibit late-type and irregular morphologies.

We investigate, via numerical simulations, the tidal stripping and accretion of globular clusters (GCs). In particular, we focus on creating models that simulate the situation for the GC systems of NGC 1404 and NGC 1399 in the Fornax cluster, which have poor (specific frequency SN ~ 2) and rich (SN ~ 10) GC systems respectively. We initially assign NGC 1404 in our simulation a typical SN (~ 5) for cluster ellipticals, and find that its GC system can only be reduced through stripping to the presently observed value, if its orbit is highly eccentric (with orbital eccentricity of > 0.5) and if the initial scale length of the GCs system is about twice as large as the effective radius of NGC 1404 itself. These stripped GCs can be said to have formed a ‘tidal stream’ of intra-cluster globular clusters (ICGCs) orbiting the center of Fornax cluster (many of which would be assigned to NGC 1399 in an imaging study). The physical properties of these GCs (e.g., number, radial distribution, and kinematics) depend on the orbit and initial distribution of GCs in NGC 1404. Our simulations also predict a trend for 5N to rise with increasing cluster-centric distance – a trend for which there is some observational support in the Fornax cluster. We demonstrate that since the kinematical properties of ICGCs formed by tidal stripping in the cluster tidal field depend strongly on the orbits of their previous host galaxies, observations of ICGC kinematics provides a new method for probing galaxy dynamics in a cluster.

Our numerical simulations first demonstrate that the pressure of ISM in a major merger becomes so high (> 105 kB K cm-3) that GMCs in the merger can collapse to form globular clusters (GCs) within a few Myr. The star formation efficiency within a GMC in galaxy mergers can rise up from a few percent to ~ 80 percent, depending on the shapes and the temperature of the GMC. This implosive GC formation due to external high pressure of warm/hot ISM can be more efficient in the tidal tails or the central regions of mergers. The developed clusters have King-like profiles with an effective radius of a few pc. The structural, kinematical, and chemical properties of these GC systems can depend on the orbital and chemical properties of major mergers.

We demonstrate that single and binary star clusters can be formed during cloud-cloud collisions triggered by the tidal interaction between the Large and Small Magellanic clouds. We run two different sets of self-consistent numerical simulations which show that compact, bound star clusters can be formed within the centers of two colliding clouds due to strong gaseous shocks, compression, and dissipation, providing the clouds have moderately large relative velocities (10 — 60 km s-1). The impact parameter determines whether the two colliding clouds become a single or a binary cluster. The star formation efficiency in the colliding clouds is dependent upon the initial ratio of the relative velocity of the clouds to the sound speed of the gas. Based on these results, we discuss the observed larger fraction of binary clusters, and star clusters with high ellipticity, in the Magellanic clouds.

We present some preliminary results from an on-going study of the evolution of stellar populations in rich clusters of galaxies. Our baseline sample contains core line-strength measurements from 183 galaxies with $b_{\rm J} \le 19.5$ from four clusters with $\bar{z}\sim0.04$, against which observations of higher-redshift clusters can be compared. Using predictions from stellar population models to compare with our measured line strengths we can derive relative luminosity-weighted mean ages and metallicities for the stellar populations in each of our clusters. It must be stressed that these ages and metallicities are only accurate when used in a relative sense as the stellar population models, due to differing abundance ratios used in the models compared to those observed in elliptical galaxies, provide inaccurate absolute ages and metallicities. We also investigate the ${\rm Mg}b^\prime$-$\sigma_0$ and H$\beta_G^\prime$-$\sigma_0$ scaling relations. We find that ${\rm Mg}b^\prime$ is correlated with $\sigma_0$, the likely explanation being that larger galaxies are better at retaining their heavier elements due to their larger potentials. ${\rm H}\beta_G^\prime$, on the other hand, we find to be anti-correlated with $\sigma_0$. This result implies that the stellar populations in larger galaxies are older than in smaller galaxies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We propose that the giant HI ring recently discovered by HIPASS for S0 galaxy NGC 1533 is formed by unequal-mass merging between gas-rich LSB (low surface brightness: “ghost”) galaxies and HSB disks. The NGC 1533 progenitor HSB spiral is transformed into a barred S0 during merging and the outer HI gas disk of the LSB is transformed into the giant HI ring. We also discuss two different possibilities for the origin of isolated star-forming regions (“ELdot” objects) in the giant gas ring.

We numerically investigate the dynamical evolution of non-nucleated dwarf elliptical/spiral galaxies (dE) and nucleated ones (dE,Ns) in clusters of galaxies in order to understand the origin of intracluster stellar objects, such as intracluster stars (ICSs), GCs (ICGCs), and “ultra-compact dwarfs” (UCDs) recently discovered by all-object spectroscopic survey centred on the Fornax cluster of galaxies. We find that the outer stellar components of a nucleated dwarf are removed by the strong tidal field of the cluster, whereas the nucleus manages to survive as a result of its initially compact nature. The developed naked nucleus is found to have physical properties (e.g., size and mass) similar to those observed for UCDs. We also find that the UCD formation process does depend on the radial density profile of the dark halo in the sense that UCDs are less likely to be formed from dwarfs embedded in dark matter halos with central ‘cuspy’ density profiles. Our simulations also suggest that very massive and compact stellar systems can be rapidly and efficiently formed in the central regions of dwarfs through the merging of smaller GCs. GCs initially in the outer part of dE and dE,Ns are found to be stripped to form ICGCs.

We discuss HST imaging of eight spectroscopically-confirmed giant arcs, pairs and arclets. Although our HST observations include both pre- and post-refurbishment images, the depth of the exposures guarantees that the majority of the arcs are detected with diffraction-limited resolution. We present the size information on these distant field galaxies in the light of HST studies of lower redshift samples. We suggest that the dominant population of star-forming galaxies at z ∼ 1 is a factor of 1.5–2 times smaller in size than the equivalent population in the local field. This implies either a considerable evolution in the sizes of star-forming galaxies within the last ∼10 Gyrs or a shift in the relative space densities of massive and dwarf star-forming systems over the same time scale.