Cosmic ray feedback in the FIRE simulations: constraining cosmic ray propagation with GeV gamma ray emission

We present the implementation and the first results of cosmic ray (CR) feedback in the Feedback In Realistic Environments (FIRE) simulations. We investigate CR feedback in non-cosmological simulations of dwarf, sub-$L\star$ starburst, and $L\star$ galaxies with different propagation models, including advection, isotropic and anisotropic diffusion, and streaming along field lines with different transport coefficients. We simulate CR diffusion and streaming simultaneously in galaxies with high resolution, using a two moment method. We forward-model and compare to observations of $\gamma$-ray emission from nearby and starburst galaxies. We reproduce the $\gamma$-ray observations of dwarf and $L\star$ galaxies with constant isotropic diffusion coefficient $\kappa \sim 3\times 10^{29}\,{\rm cm^{2}\,s^{-1}}$. Advection-only and streaming-only models produce order-of-magnitude too large $\gamma$-ray luminosities in dwarf and $L\star$ galaxies. We show that in models that match the $\gamma$-ray observations, most CRs escape low-gas-density galaxies (e.g.\ dwarfs) before significant collisional losses, while starburst galaxies are CR proton calorimeters. While adiabatic losses can be significant, they occur only after CRs escape galaxies, so they are only of secondary importance for $\gamma$-ray emissivities. Models where CRs are ``trapped'' in the star-forming disk have lower star formation efficiency, but these models are ruled out by $\gamma$-ray observations. For models with constant $\kappa$ that match the $\gamma$-ray observations, CRs form extended halos with scale heights of several kpc to several tens of kpc.Comment: 31 pages, 26 figures, accepted for publication in MNRA

Strongly Time-Variable Ultra-Violet Metal Line Emission from the Circum-Galactic Medium of High-Redshift Galaxies

We use cosmological simulations from the Feedback In Realistic Environments (FIRE) project, which implement a comprehensive set of stellar feedback processes, to study ultra-violet (UV) metal line emission from the circum-galactic medium of high-redshift (z=2-4) galaxies. Our simulations cover the halo mass range Mh ~ 2x10^11 - 8.5x10^12 Msun at z=2, representative of Lyman break galaxies. Of the transitions we analyze, the low-ionization C III (977 A) and Si III (1207 A) emission lines are the most luminous, with C IV (1548 A) and Si IV (1394 A) also showing interesting spatially-extended structures. The more massive halos are on average more UV-luminous. The UV metal line emission from galactic halos in our simulations arises primarily from collisionally ionized gas and is strongly time variable, with peak-to-trough variations of up to ~2 dex. The peaks of UV metal line luminosity correspond closely to massive and energetic mass outflow events, which follow bursts of star formation and inject sufficient energy into galactic halos to power the metal line emission. The strong time variability implies that even some relatively low-mass halos may be detectable. Conversely, flux-limited samples will be biased toward halos whose central galaxy has recently experienced a strong burst of star formation. Spatially-extended UV metal line emission around high-redshift galaxies should be detectable by current and upcoming integral field spectrographs such as the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope and Keck Cosmic Web Imager (KCWI).Comment: 16 pages, 8 figures, accepted for publication in MNRA

Transitions between Inherent Structures in Water

The energy landscape approach has been useful to help understand the dynamic properties of supercooled liquids and the connection between these properties and thermodynamics. The analysis in numerical models of the inherent structure (IS) trajectories -- the set of local minima visited by the liquid -- offers the possibility of filtering out the vibrational component of the motion of the system on the potential energy surface and thereby resolving the slow structural component more efficiently. Here we report an analysis of an IS trajectory for a widely-studied water model, focusing on the changes in hydrogen bond connectivity that give rise to many IS separated by relatively small energy barriers. We find that while the system \emph{travels} through these IS, the structure of the bond network continuously modifies, exchanging linear bonds for bifurcated bonds and usually reversing the exchange to return to nearly the same initial configuration. For the 216 molecule system we investigate, the time scale of these transitions is as small as the simulation time scale ($\approx 1$ fs). Hence for water, the transitions between each of these IS is relatively small and eventual relaxation of the system occurs only by many of these transitions. We find that during IS changes, the molecules with the greatest displacements move in small ``clusters'' of 1-10 molecules with displacements of $\approx 0.02-0.2$ nm, not unlike simpler liquids. However, for water these clusters appear to be somewhat more branched than the linear ``string-like'' clusters formed in a supercooled Lennar d-Jones system found by Glotzer and her collaborators.Comment: accepted in PR

Magnetic fileds of coalescing neutron stars and the luminosity function of short gamma-ray burst

Coalescing neutron star binaries are believed to be the most reliable sources for ground-based detectors of gravitational waves and likely progenitors of short gamma-ray bursts. In the process of coalescence, magnetic fields of neutron stars can induce interesting observational manifestations and affect the form of gravitational wave signal. In this papaer we use the population synthesis method to model the expected distribution of neutron star magnetic fields during the coalescence under different assumptions on the initial parameters of neutron stars and their magnetic field evolution. We discuss possible elecotrmagnetic phenomena preceding the coalescence of magnetized neutron star binaries and the effect of magnetic field on the gravitational wave signal. We find that a log-normal (Gaussian in logarithms) distribution of the initial magnetic fields of neutron stars, which agrees with observed properties of radio pulsars, produces the distribution of the magnetic field energy during the coalescence that adequately describes the observed luminosity function of short gamma-ray bursts under different assumptions on the field evolution and initial parameters of neutron stars. This agreement lends further support to the model of coalescing neutron star binaries as progenitors of gamma-ray bursts.Comment: v.2, LATEX, 25 pages, inc. 7 ps figures, Astron. Lett., in press. Typos corrected, reference adde

Diffuse Lyman Alpha Emitting Halos: A Generic Property of High Redshift Star Forming Galaxies

Using a sample of 92 UV continuum-selected, spectroscopically identified galaxies with = 2.65, all of which have been imaged in the Ly-a line with extremely deep narrow-band imaging, we examine galaxy Ly-a emission profiles to very faint surface brightness limits. The galaxies are representative of spectroscopic samples of LBGs at similar redshifts in terms of apparent magnitude, UV luminosity, inferred extinction, and star formation rate, and were selected without regard to Ly-a emission properties. We use extremely deep stacks of UV continuum and Ly-a emission line images to show that all sub-samples exhibit diffuse Ly-a emission to radii of at least 10" (80 physical kpc), including galaxies whose spectra exhibit Ly-a in net absorption. The intensity scaling, but not the surface brightness distribution, is strongly correlated with the emission observed in the central ~1". The characteristic scale length for Ly-a line emission exceeds that of the UV continuum light for the same galaxies by factors of at least 5-10, regardless of the spectral morphology of Ly-a. Including the extended Ly-a halos increases the total Ly-a flux [and rest equivalent width W_0(Lya)] by an average factor of 5. We argue that most, if not all, of the observed Ly-a emission in the diffuse halos originates in the galaxy H II regions and is scattered in our direction by H I gas in the galaxy's circum-galactic medium (CGM). We show that whether or not a galaxy is classified as a giant "Lyman-a Blob" (LAB) depends sensitively on the Ly-a surface brightness threshold reached by an observation. Accounting for diffuse Ly-a halos, all LBGs would be LABs if surveys were routinely sensitive to 10 times lower surface brightness thresholds; also, essentially all LBGs would qualify as LAEs (W_0(Lya) > 20 A).Comment: Updated to match final version to appear in ApJ; 20 pages, 14 figure

Birth and Evolution of Isolated Radio Pulsars

We investigate the birth and evolution of Galactic isolated radio pulsars. We begin by estimating their birth space velocity distribution from proper motion measurements of Brisken et al. (2002, 2003). We find no evidence for multimodality of the distribution and favor one in which the absolute one-dimensional velocity components are exponentially distributed and with a three-dimensional mean velocity of 380^{+40}_{-60} km s^-1. We then proceed with a Monte Carlo-based population synthesis, modelling the birth properties of the pulsars, their time evolution, and their detection in the Parkes and Swinburne Multibeam surveys. We present a population model that appears generally consistent with the observations. Our results suggest that pulsars are born in the spiral arms, with a Galactocentric radial distribution that is well described by the functional form proposed by Yusifov & Kucuk (2004), in which the pulsar surface density peaks at radius ~3 kpc. The birth spin period distribution extends to several hundred milliseconds, with no evidence of multimodality. Models which assume the radio luminosities of pulsars to be independent of the spin periods and period derivatives are inadequate, as they lead to the detection of too many old simulated pulsars in our simulations. Dithered radio luminosities proportional to the square root of the spin-down luminosity accommodate the observations well and provide a natural mechanism for the pulsars to dim uniformly as they approach the death line, avoiding an observed pile-up on the latter. There is no evidence for significant torque decay (due to magnetic field decay or otherwise) over the lifetime of the pulsars as radio sources (~100 Myr). Finally, we estimate the pulsar birthrate and total number of pulsars in the Galaxy.Comment: 27 pages, including 15 figures, accepted by Ap

Crucial Physical Dependencies of the Core-Collapse Supernova Mechanism

We explore with self-consistent 2D F{\sc{ornax}} simulations the dependence of the outcome of collapse on many-body corrections to neutrino-nucleon cross sections, the nucleon-nucleon bremsstrahlung rate, electron capture on heavy nuclei, pre-collapse seed perturbations, and inelastic neutrino-electron and neutrino-nucleon scattering. Importantly, proximity to criticality amplifies the role of even small changes in the neutrino-matter couplings, and such changes can together add to produce outsized effects. When close to the critical condition the cumulative result of a few small effects (including seeds) that individually have only modest consequence can convert an anemic into a robust explosion, or even a dud into a blast. Such sensitivity is not seen in one dimension and may explain the apparent heterogeneity in the outcomes of detailed simulations performed internationally. A natural conclusion is that the different groups collectively are closer to a realistic understanding of the mechanism of core-collapse supernovae than might have seemed apparent.Comment: 25 pages; 10 figure