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The use of roman numerals for stellar populations represents a classification approach to galaxy formation which is now well behind us. Nevertheless, the concept of a pristine generation of stars, followed by a protogalactic era, and finally the mainstream stellar population is a plausible starting point for testing our physical understanding of early star formation. This will be observationally driven as never before in the coming decade. In this paper, we search out observational tests of an idealised coeval and homogeneous distribution of population II stars. We examine the spatial distribution of quasars, globular clusters, and the integrated free electron density of the intergalactic medium, in order to test the assumption of homogeneity. Any real inhomogeneity implies a population II that is not coeval.
The high rate of gas consumption and evidence for massive stars in NGC 253, implies a prodigious star formation and supernovae rate. However there is no optical confirmation of these supernovae, presumably due to the enormous dust obscuration present. Radio observations (Antonucci and Ulvestad 1988) reveal compact radio sources i.e. young supernova remnants (SNRs).
In this overview of the key properties of globular cluster (GC) systems I show that the GCs in elliptical and spiral host galaxies have more in common than previously thought. After contrasting these properties I briefly comment on GC formation.
NGC 1052 is a flat-spectrum radio elliptical exhibiting a characteristic low excitation LINER spectrum. Independent evidence, such as extensive and infalling HI gas, and misalignment of the stellar and ionized gas components, suggest a merger has occurred. Optical CCD imaging reveals the presence of dust and a spiral morphology for the Hα emission (see Forbes, Sparks and Macchetto 1990). The X-ray luminosity is consistent with a hot gaseous halo in which a cooling flow is claimed to be operating (Thomas et al. 1986).
Integral field unit spectrographs allow the 2D exploration of the kinematics and stellar populations of galaxies, although they are generally restricted to small fields-of-view. Using the large field-of-view of the DEIMOS multislit spectrograph on Keck and our Stellar Kinematics using Multiple Slits technique, we are able to extract sky-subtracted stellar light spectra to large galactocentric radii. Here, we present a new DEIMOS mask design named SuperSKiMS that explores large spatial scales without sacrificing high spatial sampling. We simulate a set of observations with such a mask design on the nearby galaxy NGC 1023, measuring stellar kinematics and metallicities out to where the galaxy surface brightness is orders of magnitude fainter than the sky. With this technique we also reproduce the results from literature integral field spectroscopy in the innermost galaxy regions. In particular, we use the simulated NGC 1023 kinematics to model its total mass distribution to large radii, obtaining comparable results with those from published integral field unit observation. Finally, from new spectra of NGC 1023, we obtain stellar 2D kinematics and metallicity distributions that show good agreement with integral field spectroscopy results in the overlapping regions. In particular, we do not find a significant offset between our Stellar Kinematics using Multiple Slits and the ATLAS3D stellar velocity dispersion at the same spatial locations.
We summarise the first year of operation of the Medium Deep Survey - a key project of the HST. Two fields in the LMC are discussed and some preliminary scientific results presented. We also comment on image deconvolution for the extragalactic fields observed as part of the Medium Deep Survey.
We discuss new observations of the starburst galaxy NGC 7552. From optical and near–infrared colour maps we find a red, dusty circumnuclear ring. High-resolution radio mapping from the ATCA reveals the same ring, and a number of bright blobs (probably SNRs). The ring is probably associated with gas and dust which have lost angular momenta due to torques in the bar potential and settled at the inner Lindblad resonance. These circumnuclear starburst rings may be relatively common (when mapped without the obscuring affects of dust) and may play a role in collimating material of a nuclear outflow.
There exists a relationship between globular cluster mean metallicity and parent galaxy luminosity (e.g. Brodie & Huchra 1991; Forbes et al. 1996), which appears to be similar to that between stellar metallicity and galaxy luminosity. The globular cluster relation has a similar slope but is offset by about 0.5 dex to lower metallicity. The similarity of these relations suggests that both the globular cluster system and their parent galaxy have shared a common chemical enrichment history. If we can understand the formation and evolution of the globulars, we will also learn something about galaxy formation. With this aim in mind we have created the SAGES (Study of the Astrophysics of Globular clusters in Extragalactic Systems) project. Project members include Brodie, Elson, Forbes, Freeman, Grillmair, Huchra, Kissler–Patig and Schroder. We are using HST Imaging and Keck spectroscopy to study extragalactic globular cluster systems. Further details are given at http://www.ucolick.org/~mkissler/Sages/sages.html.
Combining the results from Keck spectral and HST imaging data (Forbes et al. 1994), we have derived various quantitative parameters for 17 faint (I ∼ 21), distant (z ∼ 0.5) galaxies. Such redshifts correspond to a look–back time that is about half the age of the Universe and for which some scenarios predict significant galaxy evolution. We have measured disk scale lengths (with sizes ranging from 1–5 kpc) from fits to the surface brightness profiles and internal velocities with a rest frame resolution of σ = 55 to 80 km s–1 by fitting to the emission lines. The luminosity–disk size and luminosity–internal velocity relations for our moderate redshift galaxies are similar to the scaling relations seen for local galaxies, albeit with modest ΔMB ∼ 1m brightening. We do not see evidence for a dominant population of starbursting dwarf galaxies, that have disappeared by the present epoch. Further details of this study can be found in Forbes et al. (1995). When large samples of kinematic data on distant galaxies are available, we will be able to trace galaxy evolution by mass as distinct from light.
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.
We present recent results from our long-term Gemini/GMOS study of globular clusters (GCs) in early-type galaxies. To date, we have obtained photometry and spectroscopy for GCs in NGCs 3379, 4649, 524, 7332, and IC 1459. We find a clear bimodality in the NGC 4649 GC color distribution, with the fraction of blue/red clusters increasing with galacto-centric radius. We derive ages and metallicities for 22 GCs in NGC 3379, finding that most of the clusters appear old (10–15 Gyr); however, there is a group of 4 metal-rich, younger clusters with ages of 2–6 Gyr. The NGC 3379 GC velocity dispersion decreases with radius, as does the inferred (local) mass-to-light ratio: there is no evidence for a dark matter halo in NGC 3379 based on our GC data.
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.
Cold Dark Matter simulations predict 10-100 x more dwarf satellite galaxies than are observed. Some of these ‘missing satellites’ may have been accreted, along with their globular clusters (GCs), by giant galaxies (Cote et al. 1998). But examples of dwarfs in the early stages of disruption have remained elusive.
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.
Galaxy groups have been under-studied relative to their richer counterparts — clusters. The Group Evolution Multiwavelength Study (GEMS) aims to redress some of the balance. Here we describe the GEMS sample selection and resulting sample of 60 nearby (< 130 Mpc) galaxy groups and our multiwavelength dataset of X-ray, optical, and Hı imaging. ROSAT X-ray images of each group are presented. GEMS also utilizes near-infrared imaging from the 2MASS survey and optical spectra from the 6dFGS. These observational data are complemented by mock group catalogues generated from the latest ΓCDM simulations with gas physics included. Existing GEMS publications are briefly highlighted as are future publication plans.
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.
Although originally classified as galaxies, Ultra-Compact Dwarfs (UCDs) have many properties in common with globular star clusters. The debate on the origin and nature of UCDs, and the recently discovered ultra-faint dwarf spheroidal (dSph) galaxies which contain very few stars, has motivated us to as the question ‘What is a galaxy?’ Our aim here is to promote further discussion of how to define a galaxy and, in particular, what separates it from a star cluster. Like most previous definitions, we adopt the requirement of gravitationally bound stellar system as a minimum. In order to distinguish a dwarf galaxy from a globular cluster, we discuss other possible requirements, such as a minimum size, a long two-body relaxation time a satellite system, the presence of complex stellar populations and non-baryonic dark matter. We briefly mention the implications of the adoption of each of these definitions. Some special cases of objects with a ambiguous nature are also discussed. Finally, we give our favoured criteria, and in the spirit of ‘collective wisdom’, invite readers to vote on their prefered definition of a galaxy via a dedicated website.
We detail an innovative new technique for measuring the 2-D velocity moments (rotation velocity, velocity dispersion and Gauss-Hermite coefficients h3 and h4) using spectra from Keck DEIMOS multi-object spectroscopic observations. The data are used to reconstruct 2-D rotation velocity maps.