Multidimensional Simulations of Rotating Pair Instability Supernovae

We study the effects of rotation on the dynamics, energetics and Ni-56 production of Pair Instability Supernova explosions by performing rotating two-dimensional ("2.5-D") hydrodynamics simulations. We calculate the evolution of eight low metallicity (Z = 10^-3, 10^-4 Zsun) massive (135-245 Msun) PISN progenitors with initial surface rotational velocities 50% that of the critical Keplerian value using the stellar evolution code MESA. We allow for both the inclusion and the omission of the effects of magnetic fields in the angular momentum transport and in chemical mixing, resulting in slowly-rotating and rapidly-rotating final carbon-oxygen cores, respectively. Increased rotation for carbon-oxygen cores of the same mass and chemical stratification leads to less energetic PISN explosions that produce smaller amounts of Ni-56 due to the effect of the angular momentum barrier that develops and slows the dynamical collapse. We find a non-monotonic dependence of Ni-56 production on rotational velocity in situations when smoother composition gradients form at the outer edge of the rotating cores. In these cases, the PISN energetics are determined by the competition of two factors: the extent of chemical mixing in the outer layers of the core due to the effects of rotation in the progenitor evolution and the development of angular momentum support against collapse. Our 2.5-D PISN simulations with rotation are the first presented in the literature. They reveal hydrodynamic instabilities in several regions of the exploding star and increased explosion asymmetries with higher core rotational velocity.Comment: 31 pages, 23 figures, accepted for publication in the Ap

HST Observations of Gravitationally Lensed Features in the Rich Cluster Ac114

Deep Hubble Space Telescope images of superlative resolution obtained for the distant rich cluster AC114 (z=0.31) reveal a variety of gravitational lensing phenomena for which ground-based spectroscopy is available. We present a luminous arc which is clearly resolved by HST and appears to be a lensed z=0.64 sub-L star spiral galaxy with a detected rotation curve. Of greatest interest is a remarkably symmetrical pair of compact blue images separated by 10 arcsec and lying close to the cluster cD. We propose that these images arise from a single very faint background source gravitationally lensed by the cluster core. Deep ground-based spectroscopy confirms the lensing hypothesis and suggests the source is a compact star forming system at a redshift z=1.86. Taking advantage of the resolved structure around each image and their very blue colours, we have identified a candidate third image of the same source roughly 50 arcsec away. The angular separation of the three images is much larger than previous multiply-imaged systems and indicates a deep gravitational potential in the cluster centre. Resolved multiply-imaged systems, readily recognised with HST, promise to provide unique constraints on the mass distribution in the cores of intermediate redshift clusters.Comment: submitted to ApJ, 6 pages (no figures), uuencoded Postscript, compressed TAR of Postscript figures available via anonymous ftp in users/irs/figs/ac114_figs.tar.gz on astro.caltech.edu. PAL-IRS-

Potential formation sites of super star clusters in ultra-luminous infrared galaxies

Recent observational results on high spatial resolution images of ultra-luminous infrared galaxies (ULIGs) have revealed very luminous, young, compact, and heavily obscured super star clusters in their central regions, suggested to be formed by gas-rich major mergers. By using stellar and gaseous numerical simulations of galaxy mergers, we firstly demonstrate that the central regions of ULIGs are the most promising formation sites of super star clusters owing to the rather high gaseous pressure of the interstellar medium. Based on simple analytical arguments, we secondly discuss the possibility that super star clusters in an ULIG can be efficiently transferred into the nuclear region owing to dynamical friction and consequently merge with one another to form a single compact stellar nucleus with a seed massive black hole. We thus suggest that multiple merging between super star clusters formed by nuclear starbursts in the central regions of ULIGs can result in the formation of massive black holes.Comment: 12 pages 4 figures, 2001, accepted by ApJ

Ultra-compact dwarf galaxies: a new class of compact stellar system discovered in the Fornax Cluster

We have used the 2dF spectrograph on the Anglo-Australian Telescope to obtain a complete spectroscopic sample of all objects in the magnitude range, Bj= 16.5 to 19.8, regardless of morphology, in an area centred on the Fornax Cluster of galaxies. Among the unresolved targets are five objects which are members of the Fornax Cluster. They are extremely compact stellar systems with scale lengths less than 40 parsecs. These ultra-compact dwarfs are unlike any known type of stellar system, being more compact and significantly less luminous than other compact dwarf galaxies, yet much brighter than any globular cluster.Comment: To appear in IAU Symposium 207: Extragalactic Star Cluster

HST Observations of Giant Arcs: High Resolution Imaging Of Distant Field Galaxies.

We present HST imaging of eight spectroscopically-confirmed giant arcs, pairs and arclets. These objects have all been extensively studied from the ground and we demonstrate the unique advantages of HST imaging in the study of such features by a critical comparison of our data with the previous observations. In particular we present new estimates of the core radii of two clusters (Cl0024+16, A370) determined from lensed features which are identifiable in our HST images. 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. A number of the objects in our sample are multiply-imaged and we illustrate the ease of identification of such features when working at high resolution. We discuss the morphological and scale information on these distant field galaxies in the light of HST studies of lower redshift samples. We conclude that the dominant population of star-forming galaxies at z=1 is a factor of 1.5-2 times smaller than the similar group in the local field. This implies either a considerable evolution in the sizes of star-forming galaxies within the last $\sim$10 Gyrs or a shift in the relative space densities of massive and dwarf star-forming systems over the same timescale.Comment: 9 pages (no figures), uuencoded, compressed Postscript. Postscript text, tables and figures (803 Kb) available via anonymous ftp in at ftp://ociw.edu//pub/irs/pub/hstarcs.tar.

Formation and evolution of E+A galaxies in dusty starburst galaxies

The formation and evolution of the ``E+A'' (also named ``k+a'' and ``a+k'' types by Dressler et al. 1999) galaxies found in significant numbers in the cores of intermediate redshift clusters has been extensively discussed by many authors. In this paper, we model the spectral, dynamical and morphological evolution of a prime candidate for producing this spectral signature: a dusty starburst associated with a major galaxy merger. We show that as this system evolves dynamically, its spectral type changes from and ``e(a)'' type (exhibiting strong H$\delta$ absorption and modest [OII] emission -- the identifying features of local dusty starburst galaxies) to a k+a type and then finally to a passive ``k'' type. This result shows that galaxies with an e(a) spectral type can be precursors to the k+a systems and that dynamical evolution greatly controls the spectral evolution in these merger cases. Our simulations also show that a merger with very high infrared luminosity ($L_{\rm IR}$ $>$ $10^{11}$ $L_{\odot}$) is more likely to show an e(a) spectrum, which implies that spectral types can be correlated with infrared fluxes in dusty starburst galaxies. Based on these results, we discuss the origin of the evolution of k+a/a+k galaxies in distant clusters and the role merging is likely to have.Comment: 14 pages 4 figures,2001,ApJL,in pres