The distribution of pairwise peculiar velocities in the nonlinear regime

The distribution of pairwise, relative peculiar velocities, $f(u;r)$, on small nonlinear scales, $r$, is derived from the Press--Schechter approach. This derivation assumes that Press--Schechter clumps are virialized and isothermal. The virialized assumption requires that the circular velocity, $V_c \propto M^{1/3}$, where $M$ denotes the mass of the clump. The isothermal assumption means that the circular velocity is independent of radius. Further, it is assumed that the velocity distribution within a clump is Maxwellian, that the pairwise relative velocity distribution is isotropic, and that on nonlinear scales clump-clump motions are unimportant when calculating the distribution of velocity differences. Comparison with $N$-body simulations shows that, on small nonlinear scales, all these assumptions are accurate. For most power spectra of interest, the resulting line of sight, pairwise, relative velocity distribution, $f(u_{\rm r})$, is well approximated by an exponential, rather than a Gaussian distribution. This simple Press--Schechter model is also able to provide a natural explanation for the observed, non-Maxwellian shape of $f(v)$, the distribution of peculiar velocities.Comment: (MNRAS, in press) 16 pages, uuencode

A sharp estimate for cover times on binary trees

We compute the second order correction for the cover time of the binary tree of depth $n$ by (continuous-time) random walk, and show that with probability approaching 1 as $n$ increases, \sqrt{\tau_{\mathrm{cov}}}=\sqrt{|E|}[\sqrt{2\log 2}\cdot n - {\log n}/{\sqrt{2\log 2}} + O((\log\logn)^8], thus showing that the second order correction differs from the corresponding one for the maximum of the Gaussian free field on the tree.Comment: 14 pages, no figur

Deuterium Abundance in the Most Metal-Poor Damped Lyman alpha System: Converging on Omega_baryons

The most metal-poor DLA known to date, at z = 2.61843 in the spectrum of the QSO Q0913+072, with an oxygen abundance only about 1/250 of the solar value, shows six well resolved D I Lyman series transitions in high quality echelle spectra recently obtained with the ESO VLT. We deduce a value of the deuterium abundance log (D/H) = -4.56+/-0.04 which is in good agreement with four out of the six most reliable previous determinations of this ratio in QSO absorbers. We find plausible reasons why in the other two cases the 1 sigma errors may have been underestimated by about a factor of two. The addition of this latest data point does not change significantly the mean value of the primordial abundance of deuterium, suggesting that we are now converging to a reliable measure of this quantity. We conclude that = -4.55+/-0.03 and Omega_b h^2 (BBN) = 0.0213+/-0.0010 (68% confidence limits). Including the latter as a prior in the analysis of the five year data of WMAP leads to a revised best-fitting value of the power-law index of primordial fluctuations n_s = 0.956+/-0.013 (1 sigma) and n_s < 0.990 with 99% confidence. Considering together the constraints provided by WMAP 5, (D/H)_p, baryon oscillations in the galaxy distribution, and distances to Type Ia supernovae, we arrive at the current best estimates Omega_b h^2 = 0.0224+/-0.0005 and n_s = 0.959+/-0.013.Comment: 13 pages, 8 Figures. Revised version following referee's comments. Accepted for publication in Monthly Notices of the Royal Astronomical Society. A few typos correcte

A Morning Coffee in Melbourne: Discussing the Contentious Spaces of Media Practice Research

This is a conversation that took place between three practitioner-academics one morning in Melbourne. All three work and practice in the field of the moving image: from screen production to audiovisual installation to screenwriting. Our conversation is underpinned by previous research we have undertaken in this field, namely the launching of a moving image journal, Sightlines, and a companion journal article on the process of setting it up, which focussed on the issues presented when trying to establish peer review protocols and guidelines for moving image works

The host galaxies of strong CaII QSO absorption systems at z<0.5

We present new imaging and spectroscopic observations of the fields of five QSOs with very strong intervening CaII absorption systems at redshifts z<0.5 selected from the Sloan Digital Sky Survey. Recent studies of these very rare absorbers indicate that they may be related to damped Lyman alpha systems (DLAs). In all five cases we identify a galaxy at the redshift of the CaII system with impact parameters up to ~24 kpc. In four out of five cases the galaxies are luminous (L ~L*), metal-rich (Z ~Zsun), massive (velocity dispersion, sigma ~100 km/s) spirals. Their star formation rates, deduced from Halpha emission, are high, in the range SFR = 0.3 - 30 Msun/yr. In our analysis, we paid particular attention to correcting the observed emission line fluxes for stellar absorption and dust extinction. We show that these effects are important for a correct SFR estimate; their neglect in previous low-z studies of DLA-selected galaxies has probably led to an underestimate of the star formation activity in at least some DLA hosts. We discuss possible links between CaII-selected galaxies and DLAs and outline future observations which will help clarify the relationship between these different classes of QSO absorbers.Comment: Accepted for publication in MNRAS, 14 pages, 9 figures. Version with full resolution images available at http://www.ast.cam.ac.uk/~bjz/papers/Zych_etal_2007a.pd

Inclusion of Evaporation Physics in the Modeling of Water Availability in the Savannah River Basin

2012 S.C. Water Resources Conference - Exploring Opportunities for Collaborative Water Research, Policy and Managemen

C, N, O Abundances in the Most Metal-Poor Damped Lyman alpha Systems

This study focuses on some of the most metal-poor damped Lyman alpha absorbers known in the spectra of high redshift QSOs, using new and archival observations obtained with UV-sensitive echelle spectrographs on the Keck and VLT telescopes. The weakness and simple velocity structure of the absorption lines in these systems allows us to measure the abundances of several elements, and in particular those of C, N, and O, a group that is difficult to study in DLAs of more typical metallicities. We find that when the oxygen abundance is less than about 1/100 of solar, the C/O ratio in high redshift DLAs and sub-DLAs matches that of halo stars of similar metallicity and shows higher values than expected from galactic chemical evolution models based on conventional stellar yields. Furthermore, there are indications that at these low metallicities the N/O ratio may also be above simple expectations and may exhibit a minimum value, as proposed by Centurion and her collaborators in 2003. Both results can be interpreted as evidence for enhanced production of C and N by massive stars in the first few episodes of star formation, in our Galaxy and in the distant proto-galaxies seen as QSO absorbers. The higher stellar yields implied may have an origin in stellar rotation which promotes mixing in the stars' interiors, as considered in some recent model calculations. We briefly discuss the relevance of these results to current ideas on the origin of metals in the intergalactic medium and the universality of the stellar initial mass function.Comment: 17 pages, 9 Figures, Accepted for publication in Monthly Notices of the Royal Astronomical Societ