1308.3232
Using cumulative number densities to compare galaxies across cosmic time
Behroozi, Marchesini, Wechsler, Muzzin, Papovich, Stefanon
To estimate the evolution of specific galaxy populations, compare galaxies across redshifts at fixed cumulative number density [I guess the brightest remain the brightest, so they must be the same population? Can only track the brightest galaxy evolution this way?]. This method ignores scatter in mass accretion histories and galaxy-galaxy mergers, which can lead to errors when comparing galaxies over large redshift ranges (Delta z>1). Use abundance matching in the LCDM paradigm to estimate the median change in number density with redshift and provide a simple fit (0.16 dex per unit Delta z) for progenitors of z=0 galaxies. Find that galaxy descendants do not evolve in the same way as galaxy progenitors, largely due to scatter in mass accretion histories. Also provide estimates for the 1-sigma range of number densities corresponding to galaxy progenitors and descendants. Finally, discuss some limits on number density comparisons, which arise due to difficulties measuring physical quantities (e.g., stellar mass) consistently across redshifts. A public tool to calculate number density evolution for galaxies, as well as approximate halo masses, is available online.
1308.3236
Jet luminosity from neutrino-dominated accretion flows in GRBs
Kawanaka
A hyperaccretion disk around a stellar-mass BH is a plausible model for the central engine that powers GRBs. Estimate the luminosity of a jet driven by magnetohydrodynamic processes such as the Blandford-Znajek (BZ) mechanism as a function of mass accretion rate, the BH mass, and the other accretion parameters. Show that the jet is most efficient when the accretion flow is cooled via optically-thin neutrino emission, and that its luminosity is much larger than the energy deposition rate through neutrino annihilation provided that the BH is spinning rapidly enough. Find a significant jump in the jet luminosity at the transition mass accretion rate between the advection dominated accretion flow (ADAF) regime and the neutrino-dominated accretion flow (NDAF) regime. This may cause the large variability observed in the prompt emission of GRBs.
1308.3240
Precision measures of the primordial abundance of deuterium
Cooke, Pettini, Jorgenson, Murphy, Steidel
D absorption found in the very metal-poor ([Fe/H]=-2.87) damped Ly-a system at z_abs=3.067 towards QSO SDSS J1358+6522. On the basis of 13 resolved D I absorption lines and the damping wings of the H I Ly-a transition, obtain a new, precise measure of the primordial abundance of D. Reanalyze all of the known D absorption-ine systems that satisfy a set of strict criteria, to bolster the present statistics of Precision D/H measures. Adopted a blind analysis strategy, and developed a SW package that is specifically designed for precision D/H abundance measurements. For this reanalyzed sample of systems, obtain a weighted mean of (D/H)_p = 2.53e-5, corresponding to a Universal baryon density 100 Omega_b h^2=2.2 for the standard model of BBN [about right, I guess]. By combining measure of (D/H)_p with observations of the CMB, derive the effective number of light fermion species, N_eff=3.28. Rule out the existence of an additional (sterile) neutrino (i.e., N_eff=4.046) at 99.3 confidence (2.7 sigma), provided that N_eff and the baryon-to-photon ratio (eta_10) did not change between BBN and recombination. Also place a strong bound on the lepton asymmetry, xi_D=0.05pm0.13 based only on the CMB+(D/H)_p observations. Combining xi_D with the current best literature measure of Y_p, find |xi|<=0.066. In future, improved measurements of several key reaction rates, in particular d(p,gamma)3He, and further measures of (D/H)_p with a precision comparable to those considered here, should allow even more stringent limits to be placed on new physics beyond the standard model.
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