Excitation and charge transfer in H-H+ collisions at 5-80 keV and application to astrophysical shocks

In astrophysical regimes where the collisional excitation of hydrogen atoms is relevant, the cross-sections for the interactions of hydrogen atoms with electrons and protons are necessary for calculating line profiles and intensities. In particular, at relative velocities exceeding ∼1000 km s−1, collisional excitation by protons dominates over that by electrons. Surprisingly, the H-H+ cross-sections at these velocities do not exist for atomic levels of n≥ 4, forcing researchers to utilize extrapolation via inaccurate scaling laws. In this study, we present a faster and improved algorithm for computing cross-sections for the H-H+ collisional system, including excitation and charge transfer to the n≥ 2 levels of the hydrogen atom. We develop a code named bdscx which directly solves the Schrödinger equation with variable (but non-adaptive) resolution and utilizes a hybrid spatial-Fourier grid. Our novel hybrid grid reduces the number of grid points needed from ∼4000n6 (for a ‘brute force', Cartesian grid) to ∼2000n4 and speeds up the computation by a factor of ∼50 for calculations going up to n= 4. We present (l, m)-resolved results for charge transfer and excitation final states for n= 2-4 and for projectile energies of 5-80 keV, as well as fitting functions for the cross-sections. The ability to accurately compute H-H+ cross-sections to n= 4 allows us to calculate the Balmer decrement, the ratio of Hα to Hβ line intensities. We find that the Balmer decrement starts to increase beyond its largely constant value of 2-3 below 10 keV, reaching values of 4-5 at 5 keV, thus complicating its use as a diagnostic of dust extinction when fast (∼1000 km s−1) shocks are impinging upon the ambient interstellar mediu

Non-Gaussianity and large-scale structure in a two-field inflationary model

Single field inflationary models predict nearly Gaussian initial conditions and hence a detection of non-Gaussianity would be a signature of the more complex inflationary scenarios. In this paper we study the effect on the cosmic microwave background and on large scale structure from primordial non-Gaussianity in a two-field inflationary model in which both the inflaton and curvaton contribute to the density perturbations. We show that in addition to the previously described enhancement of the galaxy bias on large scales, this setup results in large-scale stochasticity. We provide joint constraints on the local non-Gaussianity parameter $\tilde f_{\rm NL}$ and the ratio $\xi$ of the amplitude of primordial perturbations due to the inflaton and curvaton using WMAP and SDSS data

The 21-cm signature of the first stars during the Lyman–Werner feedback era

The formation of the first stars is an exciting frontier area in astronomy. Early redshifts (z ∼ 20) have become observationally promising as a result of a recently recognized effect of a supersonic relative velocity between the dark matter and gas. This effect produces prominent structure on 100 comoving Mpc scales, which makes it much more feasible to detect 21-cm fluctuations from the epoch of first heating. We use semi-numerical hybrid methods to follow for the first time the joint evolution of the X-ray and Lyman–Werner radiative backgrounds, including the effect of the supersonic streaming velocity on the cosmic distribution of stars. We incorporate self-consistently the negative feedback on star formation induced by the Lyman–Werner radiation, which dissociates molecular hydrogen and thus suppresses gas cooling. We find that the feedback delays the X-ray heating transition by Δz ∼ 2, but leaves a promisingly large fluctuation signal over a broad redshift range. The large-scale power spectrum is predicted to reach a maximal signal-to-noise ratio of S/N ∼ 3–4 at z ∼ 18 (for a projected first-generation instrument), with S/N >1 out to z ∼ 22–23. We hope to stimulate additional numerical simulations as well as observational efforts focused on the epoch prior to cosmic reionization

Relative velocity of dark matter and baryonic fluids and the formation of the first structures

At the time of recombination, baryons and photons decoupled and the sound speed in the baryonic fluid dropped from relativistic to the thermal velocities of the hydrogen atoms. This is less than the relative velocities of baryons and dark matter computed via linear perturbation theory, so we infer that there are supersonic coherent flows of the baryons relative to the underlying potential wells created by the dark matter. As a result, the advection of small-scale perturbations (near the baryonic Jeans scale) by large-scale velocity flows is important for the formation of the first baryonic structures. This effect involves a quadratic term in the cosmological perturbation theory equations and hence has not been included in studies based on linear perturbation theory. We show that the relative motion suppresses the abundance of the first bound objects, even if one only investigates dark matter haloes, and leads to qualitative changes in their spatial distribution, such as introducing scale-dependent bias and stochasticity. We discuss the possible observable implications for high-redshift galaxy clustering and reionization

Impact of the relative motion between the dark matter and baryons on the first stars: semi-analytical modelling

Recently the initial supersonic relative velocity between the dark matter and baryons was shown to have an important effect on galaxy formation at high redshift. We study the impact of this relative motion on the distribution of the star-forming haloes and on the formation redshift of the very first star. We include a new aspect of the relative velocity effect found in recent simulations by fitting their results to obtain the spatially varying minimum halo mass needed for molecular cooling. Thus, the relative velocities have three separate effects: suppression of the halo abundance, suppression of the gas content within each halo and boosting of the minimum cooling mass. We show that the two suppressions (of gas content and of halo abundance) are the primary effects on the small minihaloes that cannot form stars, while the cooling mass boost combines with the abundance suppression to produce order unity fluctuations in stellar density. We quantify the large-scale inhomogeneity of galaxies, finding that 68 per cent of the star formation (averaged on a 3 Mpc scale) is confined to 35 per cent of the volume at z= 20 (and just 18 per cent at z= 40). In addition, we estimate the first observable star to be formed at redshift z= 65 (t∼ 33 Myr) which includes a delay of Δz∼ 5 (Δt∼ 3.6 Myr) due to the relative velocity

The signature of the first stars in atomic hydrogen at redshift 20

Dark and baryonic matter moved at different velocities in the early Universe, which strongly suppressed star formation in some regions. This was estimated to imprint a large-scale fluctuation signal of about 2 mK in the 21-cm spectral line of atomic hydrogen associated with stars at a redshift of 20, although this estimate ignored the critical contribution of gas heating due to X-rays and major enhancements of the suppression. A large velocity difference reduces the abundance of halos and requires the first stars to form in halos of about a million solar masses, substantially greater than previously expected. Here we report a simulation of the distribution of the first stars at z=20 (cosmic age of ~180 Myr), incorporating all these ingredients within a 400 Mpc box. We find that the 21-cm signature of these stars is an enhanced (10 mK) fluctuation signal on the 100-Mpc scale, characterized by a flat power spectrum with prominent baryon acoustic oscillations. The required sensitivity to see this signal is achievable with an integration time of a thousand hours with an instrument like the Murchison Wide-field Array or the Low Frequency Array but designed to operate in the range of 50-100 MHz.Comment: 27 pages, 5 figures, close (but not exact) match to accepted version. Basic results unchanged from first submitted version, but justification strengthened, title and abstract modified, and substantial Supplementary Material added. Originally first submitted for publication on Oct. 12, 201

Suppression and Spatial Variation of Early Galaxies and Minihalos

We study the effect of the relative velocity of dark matter and baryonic fluids after the epoch of recombination on the evolution of the first bound objects in the early universe. Recent work has shown that, although relative motion of the two fluids is formally a second order effect in density, it has a dramatic impact on the formation and distribution of the first cosmic structures. Focusing on the gas content, we analyze the effect of relative velocity on the properties of halos over a wide range of halo masses and redshifts. We calculate accurately the linear evolution of the baryon and dark matter fluctuations, and quantify the resulting effect on halos based on an analytical formalism that has been carefully checked with simulations in the case with no relative velocity. We estimate the effect on the abundance of and gas fraction in early halos. We find that the relative velocity effect causes several changes: (i) the characteristic mass that divides gas-rich and gas-poor halos is increased by roughly an order of magnitude, from 2 10^4 Msun to about 2 10^5 Msun; (ii) this characteristic mass has a large scatter (full width at half maximum is ~ 1.5 10^5 Msun at z=20); (iii) the fraction of baryons in star-less gas minihalos is suppressed by a factor of 4 at z=20; (iv) the fraction of baryons in halos that can cool and form stars is suppressed by a factor of 1.5 at z=20; and (v) there are enhanced spatial variations of these various fractions.Comment: 10 pages, 10 figure

The Cosmic Stories: Beginning, Evolution and Present Days of the Universe

This work presents three studies of independent astrophysical phenomena which cover a full timeline of the universe from the epoch of inflation to the present day. Along with our results we provide concise overviews of the considered phenomena and outline major open questions. The first part of this work is focused on the epoch of inflation. We analyze the evolution of early density fluctuations which originate during inflation and connect physical fields driving inflation with observable parameters. We study several inflationary scenarios, specifically one field inflation, in which the only field present during that epoch is the inflaton field and two field inflation, in which along with the inflaton field the epoch of inflation is effected by the second scalar field - curvaton field. Single field inflationary models predict nearly Gaussian initial conditions and hence a detection of non-Gaussianity would be a signature of more complex inflationary scenarios. In this work we study the effect of primordial non-Gaussianity on the cosmic microwave background (CMB) and on large-scale structure in a two-field inflationary model in which both the inflaton and curvaton fields contribute to the primordial density fluctuations. We show that in addition to the previously described enhancement of the galaxy bias on large scales, this setup results in large-scale stochasticity. We provide joint constraints on the local non-Gaussianity parameter f nl and the ratio of the amplitude of primordial perturbations due to the inflaton and curvaton using WMAP and Sloan Digital Sky Survey (SDSS) data. The second and largest part of this study is focused on the formation of the first cosmic structures and the effect of relative velocity between dark matter and baryonic fluids. In that part we discuss a very important and previously unnoticed effect which significantly changes the process of structure formation in the early universe. At the time of recombination, baryons and photons decoupled and the sound speed in the baryonic fluid dropped from relativistic, to the thermal velocity of the hydrogen atoms. This is less than the relative velocity of baryons and dark matter computed via linear perturbation theory, so we infer that there are supersonic coherent flows of the baryons relative to the underlying potential wells created by the dark matter. As a result, the advection of small-scale perturbations (near the baryonic Jeans scale) by large-scale velocity flows is important for the formation of the first structures. This effect involves a quadratic term in the cosmological perturbation theory equations and hence has not been included in studies based on linear perturbation theory. We show that the relative motion suppresses the abundance of the first bound objects, even if one only investigates dark matter halos, and leads to qualitative changes in their spatial distribution, such as introducing scale-dependent bias and stochasticity. We further discuss possible observable implications of this effect for high-redshift galaxy clustering and reionization. Specifically we discuss in detail the effect of the relative velocity on the gas content in the early galaxies, minihaloes and the first stars. This part of the thesis also includes a concise overview of the recent studies that investigated various aspects of the relative velocity effect and showed its importance for topics ranging from star formation to precision cosmology. The third and final part of the thesis covers interaction between expanding shocks of the supernovae explosions with the interstellar medium. The shocks of supernovae remnants represent a unique cosmic environment which allows detailed studies of plasma physics and high-energy astrophysics phenomena in conditions unreachable in the Earth-based laboratories. Specifically, shocks of supernovae remnants are associated with production of cosmic rays - the most energetic particles that we can observe. In our study we are specifically focused on the science of Balmer-dominated shocks (BDS) - a subset of collisionless, fast shocks dominated by hydrogen line emission with both broad and narrow components. The unique feature of BDS is that they are directly observable and their observations provide an opportunity for direct testing of the phase space structure and ion velocity distribution inside of shocks. Understanding of physical phenomena occurring inside of astrophysical shocks requires precise knowledge of cross sections for high-nl proton-hydrogen collisions. Until now scientists have been using approximations for these cross sections which fall short of the precision needed for robust analysis of the observed data and can no longer satisfy needs of the astrophysical community. Guided by the demand in high-precision calculations of the cross sections we developed and implemented a robust method for direct solution of the Schroedinger partial differential equation on a grid. In this work we provide a detailed description of our computational algorithm for calculating cross sections in high-nl proton-hydrogen collisions and show results for n &#8804; 4. We describe the code we developed, show the results of consistency tests and describe possible extensions. Finally, we show how our results are applied to the studies of Balmer-dominated shocks and specifically how the precise cross sections for n &#8804; 4 can be used in computing Balmer decrement - the ratio of Halpha and Hbeta line intensities. </p