A Unified Model for the Evolution of Galaxies and Quasars

We incorporate a simple scheme for the growth of supermassive black holes into semi-analytic models that follow the formation and evolution of galaxies in a cold dark matter dominated Universe. We assume that supermassive black holes are formed and fuelled during major mergers. If two galaxies of comparable mass merge, their central black holes coalesce and a few percent of the gas in the merger remnant is accreted by the new black hole over a timescale of a few times 10^7 years. With these simple assumptions, our model not only fits many aspects of the observed evolution of galaxies, but also reproduces quantitatively the observed relation between bulge luminosity and black hole mass in nearby galaxies, the strong evolution of the quasar population with redshift and the relation between the luminosities of nearby quasars and those of their host galaxies. The strong decline in the number density of quasars from z=2 to z=0 is due to the combination of three effects: i) a decrease in the merging rate, ii) a decrease in the amount of cold gas available to fuel black holes, and iii) an increase in the timescale for gas accretion. In a LCDM cosmology the predicted decline in the total content of cold gas in galaxies is consistent with that inferred from observations of damped Lyman-alpha systems. Our results strongly suggest that the evolution of supermassive black holes, quasars and starbursts is inextricably linked to the hierarchical build-up of galaxies.Comment: 30 pages, Latex, 18 figures included, submitted to MNRA

Ionized Gas in Damped Lyman Alpha Protogalaxies: II. Comparison Between Models and the Kinematic Data

We test semi-analytic models for galaxy formation with accurate kinematic data of damped Lyman alpha protogalaxies (DLAs) presented in the companion paper I. The models envisage centrifugally supported exponential disks at the centers of dark matter halos which are filled with ionized gas undergoing radial infall to the disks. The halo masses are drawn from cross-section weighted mass distributions predicted by CDM cosmogonies, or by the null hypothesis (TF model) that the dark matter mass distribution has not evolved since z ~ 3. In our models, C IV absorption lines detected in DLAs arise in infalling ionized clouds while the low-ion absorption lines arise from neutral gas in the disks. Using Monte Carlo methods we find: (a) The CDM models are incompatible with the low-ion statistics at more than 99% confidence whereas some TF models cannot be excluded at more than 88% confidence. (b) Both CDM and TF models agree with the observed distribution of C IV velocity widths. (c) The CDM models generate differences between the mean velocities of C IV and low ion profiles in agreement with the data, while the TF model produces differences in the means that are too large. (d) Both CDM and TF models produce ratios of C IV to low-ion velocity widths that are too large. (e) Both CDM and TF models generate C IV versus low-ion cross-correlation functions incompatible with the data. While it is possible to select model parameters resulting in consistency with the data, the disk-halo configuration assumed in both cosmogonies still does not produce significant overlap in velocity space between C IV low-ion velocity profiles. We conjecture that including angular momentum of the infalling clouds will increase the overlap between C IV and low-ion profiles.Comment: 18 pages, 12 Figures, Accepted for publication in the Dec. 20 issue of the Astrophysical Journa

Importance of an Astrophysical Perspective for Textbook Relativity

The importance of a teaching a clear definition of the ``observer'' in special relativity is highlighted using a simple astrophysical example from the exciting current research area of ``Gamma-Ray Burst'' astrophysics. The example shows that a source moving relativistically toward a single observer at rest exhibits a time ``contraction'' rather than a ``dilation'' because the light travel time between the source and observer decreases with time. Astrophysical applications of special relativity complement idealized examples with real applications and very effectively exemplify the role of a finite light travel time.Comment: 5 pages TeX, European Journal of Physics, in pres

Compton dragged gamma-ray bursts: the spectrum

We calculate the spectrum resulting from the interaction of a fireball with ambient soft photons. These photons are assumed to be produced by the walls of a funnel in a massive star. By parameterizing the radial dependence of the funnel temperature we calculate the deceleration of the fireball self-consistently, taking into account the absorption of high energy gamma-rays due to interaction with the softer ambient photons. The resulting spectrum is peaked at energies in agreement with observations, has a nu^2 slope in the X-ray band and a steep power-law high energy tail.Comment: 5 pages, 3 figures, accepted for publication in MNRAS, pink page

Immunotherapeutic potential of DISC-HSV and OX40L in cancer

Several vectors, viral and bacterial, have been developed over the past few years for means of generating an effective anti-tumor immune response. We have developed and studied a “model for immunotherapy” using a viral vector DISC-HSV, which efficiently transduces various tumor cell lines and offers a useful vehicle for the further development of cell based vaccines. The immunotherapeutic potential of DISC-HSV encoding GMCSF was demonstrated in a number of murine carcinoma models, leading to complete regression of well established tumors in up to 70% of the mice. Moreover, the therapeutic potential of DISC-HSV-GMCSF was significantly enhanced when used in combination therapy with either OX40L or dendritic cells (DC), even in poorly immunogenic tumor model. The ability of this vector to accept large gene inserts, its good safety profile, its ability to undergo only a single round of infection, the inherent viral immunostimulatory properties and its ability to infect various tumor cell lines efficiently, make DISC-HSV an ideal candidate vector for immunotherapy. The DISC- CT-26 tumor model has been used to investigate these mechanisms associated with immunotherapy – induced tumor rejection. Although CTL induction, was positively correlated with regression, MHC class I down regulation and accumulation of immature Gr1+ myeloid cells were shown to be the main immuno-suppressor mechanisms operating against regression and associated with progressive tumor growth

Instability of toroidal magnetic field in jets and plerions

Jets and pulsar-fed supernova remnants (plerions) tend to develop highly organized toroidal magnetic field. Such a field structure could explain the polarization properties of some jets, and contribute to their lateral confinement. A toroidal field geometry is also central to models for the Crab Nebula - the archetypal plerion - and leads to the deduction that the Crab pulsar's wind must have a weak magnetic field. Yet this `Z-pinch' field configuration is well known to be locally unstable, even when the magnetic field is weak and/or boundary conditions slow or suppress global modes. Thus, the magnetic field structures imputed to the interiors of jets and plerions are unlikely to persist. To demonstrate this, I present a local analysis of Z-pinch instabilities for relativistic fluids in the ideal MHD limit. Kink instabilities dominate, destroying the concentric field structure and probably driving the system toward a more chaotic state in which the mean field strength is independent of radius (and in which resistive dissipation of the field may be enhanced). I estimate the timescales over which the field structure is likely to be rearranged and relate these to distances along relativistic jets and radii from the central pulsar in a plerion. I conclude that a concentric toroidal field is unlikely to exist well outside the Crab pulsar's wind termination shock. There is thus no dynamical reason to conclude that the magnetic energy flux carried by the pulsar wind is much weaker than the kinetic energy flux. Abandoning this inference would resolve a long-standing puzzle in pulsar wind theory.Comment: 28 pages, plain TeX. Accepted for publication in Ap

Excess noise characteristics of Al0.8Ga0.2As avalanche photodiodes

The avalanche noise characteristics of Al0.8Ga0.2 As have been measured in a range of p-i-n and n-i-p diodes with i-region widths ω varying from 1.02 to 0.02 μm. While thick bulk diodes exhibit low excess noise from electron initiated multiplication, owing to the large α/β ratio (1/k), the excess noise of diodes with ω < 0.31 μm were found to be greatly reduced by the effects of dead space. The thinnest diodes exhibit very low excess noise, corresponding to k = 0.08, up to a multiplication value of 90. In contrast to most III-V materials, it was found that both thick and thin Al0.8Ga0.2As multiplication layers can give very low excess noise and that electrons must initiate multiplication to minimize excess noise, even in thin structure

Formation and Disruption of Cosmological Low Mass Objects

We investigate the evolution of cosmological low mass (low virial temperature) objects and the formation of the first luminous objects. First, the `cooling diagram' for low mass objects is shown. We assess the cooling rate taking into account the contribution of H_2, which is not in chemical equilibrium generally, with a simple argument of time scales. The reaction rates and the cooling rate of H_2 are taken from the recent results by Galli & Palla (1998). Using this cooling diagram, we also estimate the formation condition of luminous objects taking into account the supernova (SN) disruption of virialized clouds. We find that the mass of the first luminous object is several times 10^7 solar mass, because smaller objects may be disrupted by the SNe before they become luminous. Metal pollution of low mass (Ly-alpha) clouds also discussed. The resultant metallicity of the clouds is about 1/1000 of the solar metallicity.Comment: 11 pages, 2 figures, To appear in ApJ

Border parasites: schistosomiasis control among Uganda's fisherfolk

Copyright @ 2012 Taylor & Francis. This article has been made publically available through the Brunel Open Access Publishing Fund.It is recognized that the control of schistosomisais in Uganda requires a focus on fisherfolk. Large numbers suffer from this water-borne parasitic disease; notably along the shores of lakes Albert and Victoria and along the River Nile. Since 2004, a policy has been adopted of providing drugs, free of charge, to all those at risk. The strategy has been reported to be successful, but closer investigation reveals serious problems. This paper draws upon long-term research undertaken at three locations in northwestern and southeastern Uganda. It highlights consequences of not engaging with the day to day realities of fisherfolk livelihoods; attributable, in part, to the fact that so many fisherfolk live and work in places located at the country’s international borders, and to a related tendency to treat them as "feckless" and "ungovernable". Endeavours to roll out treatment end up being haphazard, erratic and location-specific. In some places, concerted efforts have been made to treat fisherfolk; but there is no effective monitoring, and it is difficult to gauge what proportion have actually swallowed the tablets. In other places, fisherfolk are, in practice, largely ignored, or are actively harassed in ways that make treatment almost impossible. At all sites, the current reliance upon resident "community" drug distributors or staff based at static clinics and schools was found to be flawed.The Schistosomiasis Control Initiative, Imperial College, under the auspices of the Bill and Melinda Gates Foundation