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Search Results: 1 - 10 of 262493 matches for " B. K. Gibson "
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Metallicity Gradients in Simulated Disk Galaxies
K. Pilkington,B. K. Gibson
Physics , 2012,
Abstract: The stellar metallicity and abundance ratio gradients from the fiducial late-type galaxy simulation of Stinson et al. (2010) are presented. Over 1-3 scalelengths, gradients are shown to flatten with time, consistent with empirical evidence at high- and low-redshifts. Kinematic effects, including radial migration, though, flatten these intrinsicly steep gradients such that by redshift z=0, the measured gradients of these (now) old stars are flatter than their young counterparts, in contradiction to what is observed locally. Conversely, the stellar [O/Fe] gradient is (to first order) robust against migration, remaining fairly flat for both young and old populations today.
Chemical Evolution of the Carina Dwarf Spheroidal
K. Pilkington,B. K. Gibson
Physics , 2012,
Abstract: We explore a range of chemical evolution models for the Local Group dwarf spheroidal (dSph) galaxy, Carina. A novel aspect of our work is the removal of the star formation history (SFH) as a `free parameter' in the modeling, making use, instead, of its colour-magnitude diagram (CMD)-constrained SFH. By varying the relative roles of galactic winds, re-accretion, and ram-pressure stripping within the modeling, we converge on a favoured scenario which emphasises the respective roles of winds and re-accretion. While our model is successful in recovering most elemental abundance patterns, comparable success is not found for all the neutron capture elements. Neglecting the effects of stripping results in predicted gas fractions approximately two orders of magnitude too high, relative to that observed.
Galaxy evolution: the effect of dark matter on the chemical evolution of ellipticals and galaxy clusters
F. Matteucci,B. K. Gibson
Physics , 1996,
Abstract: In this paper we discuss the chemical evolution of elliptical galaxies and its consequences on the evolution of the intracluster medium (ICM). We use chemical evolution models taking into account dark matter halos and compare the results with previous models where dark matter was not considered. In particular, we examine the evolution of the abundances of some relevant heavy elements such as oxygen, magnesium and iron and conclude that models including dark matter halos and an initial mass function (IMF) containing more massive stars than the Salpeter (1955) IMF, better reproduce the observed abundances of Mg and Fe both in the stellar populations and in the ICM (ASCA results). We also discuss the origin of gas in galaxy clusters and conclude that most of it should have a primordial origin.
On the Trend of [Mg/Fe] among Giant Elliptical Galaxies
F. Matteucci,R. Ponzone,B. K. Gibson
Physics , 1998,
Abstract: We revisit the problem of the flat slope of the Mg2 versus relationship found for nuclei of elliptical galaxies (Faber et al. 1992; Worthey et al. 1992; Carollo et al. 1993; Davies et al. 1993), indicating that the Mg/Fe ratio should increase with galactic luminosity and mass. We transform the abundance of Fe, as predicted by classic wind models and alternative models for the chemical evolution of elliptical galaxies, into the metallicity indices Mg2 and , by means of the more recent index calibrations and show that none of the current models for the chemical evolution of elliptical galaxies is able to reproduce exactly the observed slope of the versus Mg2 relation, although the existing spread in the data makes this comparison quite difficult. In other words, we can not clearly discriminate between models predicting a decrease (classic wind model) or an increase of such a ratio with galactic mass. The reason for this resides in the fact that the available observations show a large spread due mostly to the errors in the derivation of the index. In our opinion this fact prevents us from drawing any firm conclusion on the behaviour of Mg and Fe in these galaxies.
Type Ia Supernovae and the Value of the Hubble Constant
Brad K. Gibson,Chris B. Brook
Physics , 2000,
Abstract: The methodology involved in deriving the Hubble Constant via the calibration of the corrected peak luminosities of Type Ia supernovae (SNe) is reviewed. We first present a re-analysis of the Calan-Tololo (C-T) and Center for Astrophysics (CfA) Type Ia SN surveys. Bivariate linear least squares and quadratic boot-strapped fits in peak apparent magnitude and light curve shape are employed to correct this heterogeneous sample of peak apparent magnitudes, resulting in an homogeneous (and excellent) secondary distance indicator: the so-called corrected peak luminosity. We next provide an empirical calibration for this corrected luminosity, using Cepheid-based distances for seven nearby spiral galaxies host to Type Ia SNe. Included in this sample is the spectroscopically peculiar SN 1991T (in NGC 4527), whose corrected peak luminosity is shown to be indistinguishable from that of so-called ``normal'' SNe. A robust value of the Hubble Constant is derived and shown to be H0=73+/-2(r)+/-7(s) km/s/Mpc.
Supernova 1991T and the Value of the Hubble Constant
Brad K. Gibson,Peter B. Stetson
Physics , 2000, DOI: 10.1086/318905
Abstract: Based upon multi-epoch Hubble Space Telescope observations, we present the discovery of sixteen high-quality Cepheid candidates in NGC 4527. Corrected for metallicity effects in the Cepheid period-luminosity relation, we derive a distance, including both random (r) and systematic (s) uncertainties, of 13.0+/-0.5(r)+/-1.2(s) Mpc. Our result is then used to provide a calibration of the peak B-, V-, and I-band luminosities of the peculiar Type Ia supernova SN 1991T, a resident of NGC 4527. Despite its documented spectroscopic peculiarities, after correction for the decline rate-luminosity correlation the corrected peak luminosity is indistinguishable from those of so-called ``normal'' Type Ia SNe. With now nine local calibrators at our disposal, we determine a robust value for the Hubble Constant of H_0=73+/-2(r)+/-7(s) km/s/Mpc.
The First Results from the Parkes Multibeam High-Velocity Cloud Survey
M. E. Putman,B. K Gibson
Physics , 1999, DOI: 10.1071/AS99070
Abstract: With the onset of the HI Parkes All-Sky Survey, a new view of the high-velocity cloud (HVC) distribution in the southern sky is being revealed. The dense spatial sampling and unbiased coverage of HIPASS gives it multiple advantages over previous surveys of the southern sky. Detailed views of the clouds' structure and large mosaics of the HI sky allow us to link individual concentrations to larger structures (such as the Magellanic Clouds and the Galaxy), providing pictorial clues as to the origins of HVCs. It is clear that HVCs cannot be uniformly assigned a single origin scenario, and must be categorised appropriately.
The Distance to Supernova 1998aq in NGC3982
Peter B. Stetson,Brad K. Gibson
Physics , 2001, DOI: 10.1046/j.1365-8711.2001.05005.x
Abstract: The distance to NGC3982, host galaxy to the Type Ia supernova SN1998aq, is derived using 32 Cepheids discovered in archival multi-epoch Hubble Space Telescope observations. Employing recent Large Magellanic Cloud Cepheid period-luminosity relations and absolute zero point, we find a distance to NGC3982 of 20.5+/-0.8(r)+/-1.7(s) Mpc, including both random (r) and systematic (s) uncertainties, and ignoring any metallicity dependency in the Cepheid period-luminosity relation. Still unpublished light curve photometry promises to make SN1998aq one of the most important calibrators for the Type Ia supernova decline rate-peak luminosity relationship.
The Distribution of Young Stars and Metals in Simulated Cosmological Disk Galaxies
K. Pilkington,B. K. Gibson,D. H. Jones
Physics , 2012,
Abstract: We examine the distribution of young stars associated with the spiral arms of a simulated L* cosmological disk galaxy. We find age patterns orthogonal to the arms which are not inconsistent with the predictions of classical density wave theory, a view further supported by recent observations of face-on Grand Design spirals such as M51. The distribution of metals within a simulated ~0.1L* disk is presented, reinforcing the link between star formation, the age-metallicity relation, and the metallicity distribution function.
Heat Capacity of MgB_2: Evidence for Moderately Strong Coupling Behavior
R. K. Kremer,B. J. Gibson,K. Ahn
Physics , 2001,
Abstract: We characterize the superconducting state of phase pure polycrystalline samples of the new layered superconductor MgB$_2$ by specific heat measurements in magnetic fields up to 9 T. The characteristic jump at the superconducting transition is observed and compared with the predictions of weak coupling BCS-theory and the $\alpha$-model. Our analysis shows excellent agreement with the predictions for 2$\Delta/k_BT_{\rm C}$ = 4.2(2) with a Sommerfeld term $\gamma_{\rm exp}$ of 3.1(1) mJ/mol K$^2$ indicating that MgB$_2$ is a superconductor in the moderately strong electron-phonon coupling regime.
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