Effect of Streaming Motion of Baryons Relative to Dark Matter on the Formation of the First Stars

We evaluate the effect of a supersonic relative velocity between the baryons and dark matter on the thermal and density evolution of the first gas clouds at z < 50. Through a series of cosmological simulations, initialized at z=100 with a range of relative streaming velocities and minihalo formation redshifts, we find that the typical streaming velocities will have little effect on the gas evolution. Once the collapse begins, the subsequent evolution of the gas will be nearly indistinguishable from the case of no streaming, and star formation will still proceed in the same way, with no change in the characteristic Pop III stellar masses. Reionization is expected to be dominated by halo masses of > 10^8 M_sun, for which the expected effect of streaming is negligible.Comment: 5 pages, 4 figures, accepted to ApJ Letter

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 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

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

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

On the Equivalence of Barrier Crossing, Peak-Background Split, and Local Biasing

Several, apparently distinct, formalisms exist in the literature for predicting the clustering of dark matter halos. It has been noticed on a case-by-case basis that the predictions of these different methods agree in specific examples, but there is no general proof that they are equivalent. In this paper, we give a simple proof of the mathematical equivalence of barrier crossing, peak-background split, and local biasing.Comment: 20 pages, 1 figur

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 Opacity of the Intergalactic Medium During Reionization: Resolving Small-Scale Structure

Early in the reionization process, the intergalactic medium (IGM) would have been quite inhomogeneous on small scales, due to the low Jeans mass in the neutral IGM and the hierarchical growth of structure in a cold dark matter Universe. This small-scale structure acted as an important sink during the epoch of reionization, impeding the progress of the ionization fronts that swept out from the first sources of ionizing radiation. Here we present results of high-resolution cosmological hydrodynamics simulations that resolve the cosmological Jeans mass of the neutral IGM in representative volumes several Mpc across. The adiabatic hydrodynamics we follow are appropriate in an unheated IGM, before the gas has had a chance to respond to the photoionization heating. Our focus is determination of the resolution required in cosmological simulations in order to sufficiently sample and resolve small-scale structure regulating the opacity of an unheated IGM. We find that a dark matter particle mass of m_dm 1 Mpc are required. With our converged results we show how the mean free path of ionizing radiation and clumping factor of ionized hydrogen depends upon the ultraviolet background (UVB) flux and redshift. We find, for example at z = 10, clumping factors typically of 10 to 20 for an ionization rate of Gamma ~ 0.3 - 3 x 1e-12 s^-1, with corresponding mean free paths of ~ 3 - 15 Mpc, extending previous work on the evolving mean free path to considerably smaller scales and earlier times.Comment: Accepted for publication in the Astrophysical Journa