An H alpha Survey of 8 Abell Clusters: the dependence of tidally-induced star formation on cluster density

We have undertaken a survey of H alpha emission in a substantially complete sample of CGCG galaxies of types Sa and later within 1.5 Abell radii of the centres of 8 low-redshift Abell clusters. Some 320 galaxies were surveyed, of which 116 were detected in emission (39% of spirals, 75% of peculiars). Detected emission was classified as `compact' or `diffuse'. From an analysis of the full survey sample, we reconfirm our previous identification of compact and diffuse emission with circumnuclear starburst and disk emission respectively. The circumnuclear emission is associated either with the presence of a bar, or with a disturbed galaxy morphology indicative of on-going tidal interactions. The frequency of such tidally-induced (circumnuclear) starburst emission in spirals increases from regions of lower to higher local galaxy surface density, and from clusters with lower to higher central galaxy space density. We conclude that tidal interactions are likely to be the main mechanism for the transformation of spirals to S0s in clusters. Finally, for regions of comparable local density, the frequency of tidally-induced starburst emission is greater in clusters with higher central galaxy density. This implies that, for a given local density, morphological transformation of disk galaxies proceeds more rapidly in clusters of higher central galaxy density. This effect is considered to be due to subcluster merging and could account for the previously considered anomalous absence of a significant type - local surface density relation for irregular clusters at intermediate redshift.Comment: 22 pages including 4 figures. Accepted for publication in MNRA

The luminous X-ray hotspot in 4C 74.26: synchrotron or inverse-Compton emission?

We report the discovery of an X-ray counterpart to the southern radio hotspot of the largest-known radio quasar 4C 74.26 (whose redshift is z=0.104). Both XMM-Newton and Chandra images reveal the same significant (10arcsec, i.e. 19kpc) offset between the X-ray hotspot and the radio hotspot imaged with MERLIN. The peak of the X-ray emission may be due to synchrotron or inverse-Compton emission. If synchrotron emission, the hotspot represents the site of particle acceleration and the offset arises from either the jet exhibiting Scheuer's `dentist's drill' effect or a fast spine having less momentum than the sheath surrounding it, which creates the radio hotspot. If the emission arises from the inverse-Compton process, it must be inverse-Compton scattering of the CMB in a decelerating relativistic flow, implying that the jet is relativistic (Gamma >= 2) out to a distance of at least 800kpc. Our analysis, including optical data from the Liverpool Telescope, rules out a background AGN for the X-ray emission and confirms its nature as a hotspot, making it the most X-ray luminous hotspot yet detected.Comment: 9 pages, 9 figures, definitive version published by MNRA

Dilepton Production at Fermilab and RHIC

Some recent results from several fixed-target dimuon production experiments at Fermilab are presented. In particular, we discuss the use of Drell-Yan data to determine the flavor structure of the nucleon sea, as well as to deduce the energy-loss of partons traversing nuclear medium. Future dilepton experiments at RHIC could shed more light on the flavor asymmetry and possible charge-symmetry-violation of the nucleon sea. Clear evidence for scaling violation in the Drell-Yan process could also be revealed at RHIC.Comment: 5 pages, talk presented at the RIKEN-BNL Workshop on 'Hard Parton Physics in Nucleus-Nucleus collisions, March 199

Star-forming galaxies in low-redshift clusters: Data and integrated galaxy properties

This paper is a continuation of an ongoing study of the evolutionary processes affecting cluster galaxies. Both CCD R band and H alpha narrow-band imaging was used to determine photometric parameters (m_(r), r_(24), H alpha flux and equivalent width) and derive star formation rates for 227 CGCG galaxies in 8 low-redshift clusters. The galaxy sample is a subset of CGCG galaxies in an objective prism survey of cluster galaxies for H alpha emission. It is found that detection of emission-line galaxies in the OPS is 85%, 70%, and 50% complete at the mean surface brightness values of 1.25 x 10^(-19), 5.19 x 10^(-20), and 1.76 x 10^(-20) W m^(-2) arcsec^(-2), respectively, measured within the R band isophote of 24 mag arcsec^(-2) for the galaxy. The CCD data, together with matched data from a recent H alpha galaxy survey of UGC galaxies within 3000 km s^(-1), will be used for a comparative study of R band and H alpha surface photometry between cluster and field spirals.Comment: Accepted for publication in A&A. 11 pages, including 6 figure

Cosmological Constraints on Dissipative Models of Inflation

(Abridged) We study dissipative inflation in the regime where the dissipative term takes a specific form, \Gamma=\Gamma(\phi), analyzing two models in the weak and strong dissipative regimes with a SUSY breaking potential. After developing intuition about the predictions from these models through analytic approximations, we compute the predicted cosmological observables through full numerical evolution of the equations of motion, relating the mass scale and scale of dissipation to the characteristic amplitude and shape of the primordial power spectrum. We then use Markov Chain Monte Carlo techniques to constrain a subset of the models with cosmological data from the cosmic microwave background (WMAP three-year data) and large scale structure (SDSS Luminous Red Galaxy power spectrum). We find that the posterior distributions of the dissipative parameters are highly non-Gaussian and their allowed ranges agree well with the expectations obtained using analytic approximations. In the weak regime, only the mass scale is tightly constrained; conversely, in the strong regime, only the dissipative coefficient is tightly constrained. A lower limit is seen on the inflation scale: a sub-Planckian inflaton is disfavoured by the data. In both weak and strong regimes, we reconstruct the limits on the primordial power spectrum and show that these models prefer a {\it red} spectrum, with no significant running of the index. We calculate the reheat temperature and show that the gravitino problem can be overcome with large dissipation, which in turn leads to large levels of non-Gaussianity: if dissipative inflation is to evade the gravitino problem, the predicted level of non-Gaussianity might be seen by the Planck satellite.Comment: 14 pages, 9 figures, Accepted by JCAP without text changes, References adde