The Evolution of the Intergalactic Medium

The bulk of cosmic matter resides in a dilute reservoir that fills the space between galaxies, the intergalactic medium (IGM). The history of this reservoir is intimately tied to the cosmic histories of structure formation, star formation, and supermassive black hole accretion. Our models for the IGM at intermediate redshifts (2<z<5) are a tremendous success, quantitatively explaining the statistics of Lyman-alpha absorption of intergalactic hydrogen. However, at both lower and higher redshifts (and around galaxies) much is still unknown about the IGM. We review the theoretical models and measurements that form the basis for the modern understanding of the IGM, and we discuss unsolved puzzles (ranging from the largely unconstrained process of reionization at high-z to the missing baryon problem at low-z), highlighting the efforts that have the potential to solve them.Comment: 55 pages, 13 figures; published in the Annual Review of Astronomy and Astrophysic

Promising Observational Methods for Detecting the Epoch of Reionization

It has been several years since the first detection of Gunn-Peterson troughs in the z > 6 Ly-alpha forest and since the first measurement of the Thomson scattering optical depth through reionization from the cosmic microwave background (CMB). Present day CMB measurements provide a significant constraint on the mean redshift of reionization, and the Ly-alpha forest provides a lower bound on the redshift at which reionization ended. However, no observation has provided definitive information on the duration and morphology of this process. This article is intended as a short review on the most promising observational methods that aim to detect and study this cosmic phase transition, focusing on CMB anisotropies, gamma ray burst afterglows, Ly-alpha emitting galaxies, and redshifted 21cm emission.Comment: 15 pages; Invited Review, ASP conference proceedings of the "Frank N. Bash Symposium 2009: New Horizons in Astronomy

Locating the "missing" baryons with extragalactic dispersion measure estimates

Recently, Thornton and coworkers (2013) confirmed a class of millisecond radio bursts likely of extragalactic origin that is well-suited for estimating dispersion measures (DMs). We calculate the probability distribution of DM(z) in different models for how the cosmic baryons are distributed (both analytically and with cosmological simulations). We show that the distribution of DM is quite sensitive to whether the "missing" baryons lie around the virial radius of 10^11-10^13 Msun halos or further out, which is not easily constrained with other observational techniques. The intrinsic contribution to DM from each source could complicate studies of the extragalactic contribution. This difficulty is avoided by stacking based on the impact parameter to foreground galaxies. We show that a stacking analysis using a sample of ~100 DM measurements from arcminute-localized, z >~ 0.5 sources would place interesting constraints at 0.2-2 halo virial radii on the baryonic mass profile surrounding different galaxy types. Conveniently for intergalactic studies, sightlines that intersect intervening galactic disks should be easily identified owing to scattering. A detectable level of scattering may also result from turbulence in the circumgalactic medium.Comment: 6 pages, 3 figures; published in ApJ

Exploring the astrophysics of dark atoms

A component of the dark matter could consist of two darkly charged particles with a large mass ratio and a massless force carrier. This `atomic' dark sector could behave much like the baryonic sector, cooling and fragmenting down to stellar-mass or smaller scales. Past studies have shown that cosmic microwave background and large-scale structure constraints rule out $\gtrsim 5\%$ of the dark matter to behave in this manner. However, we show that, even with percent level mass fractions, a dark atomic sector could affect some extragalactic and galactic observables. We track the cooling and merger history of an atomic dark component for much of the interesting parameter space. Unlike the baryons, where stellar feedback (driven by nuclear physics) delays the formation and growth of galaxies, cooling dark atomic gas typically results in disks forming earlier, leaving more time for their destruction via mergers. Rather than disks in Milky Way sized halos, we find the end product is typically spheroidal structures on galactic scales or dark atom fragments distributed on halo scales. This result contrasts with previous studies, which had assumed that the dark atoms would result in dark disks. Furthermore the dark atoms condense into dense clumps, analogous to how the baryons fragment on solar-mass scales. We estimate the size of these dark clumps, and use these estimates to show that viable atomic dark matter parameter space is ruled out by stellar microlensing, by the half-light radii of ultra-faint dwarf galaxies, and by Milky Way mass-to-light inferences.Comment: 20 pages and 7 figures, added references and fixed minor typo

Cosmological perturbation theory in 1+1 dimensions

Many recent studies have highlighted certain failures of the standard Eulerian-space cosmological perturbation theory (SPT). Its problems include (1) not capturing large-scale bulk flows [leading to an O(1) error in the 1-loop SPT prediction for the baryon acoustic peak in the correlation function], (2) assuming that the Universe behaves as a pressureless, inviscid fluid, and (3) treating fluctuations on scales that are non-perturbative as if they were. Recent studies have highlighted the successes of perturbation theory in Lagrangian space or theories that solve equations for the effective dynamics of smoothed fields. Both approaches mitigate some or all of the aforementioned issues with SPT. We discuss these physical developments by specializing to the simplified 1D case of gravitationally interacting sheets, which allows us to substantially reduces the analytic overhead and still (as we show) maintain many of the same behaviors as in 3D. In 1D, linear-order Lagrangian perturbation theory ("the Zeldovich approximation") is exact up to shell crossing, and we prove that n^{th}-order Eulerian perturbation theory converges to the Zeldovich approximation as n goes to infinity. In no 1D cosmology that we consider (including a CDM-like case and power-law models) do these theories describe accurately the matter power spectrum on any mildly nonlinear scale. We find that theories based on effective equations are much more successful at describing the dynamics. Finally, we discuss many topics that have recently appeared in the perturbation theory literature such as beat coupling, the shift and smearing of the baryon acoustic oscillation feature, and the advantages of Fourier versus configuration space. Our simplified 1D case serves as an intuitive review of these perturbation theory results.Comment: 28 pages + appendices; 10 figures; matches version accepted to JCA

A physical understanding of how reionization suppresses accretion onto dwarf halos

We develop and test with cosmological simulations a physically motivated theory for how the interplay between gravity, pressure, cooling, and self-shielding set the redshift--dependent mass scale at which halos can accrete intergalactic gas. This theory provides a physical explanation for the halo mass scale that can accrete unshocked intergalactic gas, which has been explained with ad hoc criteria tuned to reproduce the results of a few simulations. Furthermore, it provides an intuitive explanation for how this mass scale depends on the reionization redshift, the amplitude of the ionizing background, and the redshift. We show that accretion is inhibited onto more massive halos than had been thought because previous studies had focused on the gas fraction of halos rather than the instantaneous mass that can accrete gas. A halo as massive as 10^11 Msun cannot accrete intergalactic gas at z=0, even though typically its progenitors were able to accrete gas at higher redshifts. We describe a simple algorithm that can be implemented in semi-analytic models, and we compare the predictions of this algorithm to numerical simulations.Comment: 13 pages, 8 figures; submitted to MNRA

On using angular cross-correlations to determine source redshift distributions

We investigate how well the redshift distribution of a population of extragalactic objects can be reconstructed using angular cross-correlations with a sample whose redshifts are known. We derive the minimum variance quadratic estimator, which has simple analytic representations in very applicable limits and is significantly more sensitive than earlier proposed estimation procedures. This estimator is straightforward to apply to observations, it robustly finds the likelihood maximum, and it conveniently selects angular scales at which fluctuations are well approximated as independent between redshift bins and at which linear theory applies. We find that the linear bias times number of objects in a redshift bin generally can be constrained with cross-correlations to fractional error (10^2 n/N)^1/2, where N is the total number of spectra per dz and n is the number of redshift bins spanned by the bulk of the unknown population. The error is often independent of the sky area and sampling fraction. Furthermore, we find that sub-percent measurements of the angular source density per unit redshift, dN/dz, are in principle possible, although cosmic magnification needs to be accounted for at fractional errors of <~ 10 per cent. We discuss how the sensitivity to dN/dz changes as a function of photometric and spectroscopic depth and how to optimize the survey strategy to constrain dN/dz. We also quantify how well cross-correlations of photometric redshift bins can be used to self-calibrate a photometric redshift sample. Simple formulae that can be quickly applied to gauge the utility of cross correlating different samples are given.Comment: 23 pages plus 6 pages of appendix; 15 figures; eqn. 31 corrected (after publication

Implications of the large OVI columns around low-redshift L∗L_* galaxies

Observations reveal massive amounts of OVI around star-forming $L_*$ galaxies, with covering fractions of near unity extending to the host halo's virial radius. This OVI absorption is typically kinematically centered upon photoionized gas, with line widths that are suprathermal and kinematically offset from the galaxy. We discuss various scenarios and whether they could result in the observed phenomenology (cooling gas flows, boundary layers, shocks, virialized gas, photoionized clouds in thermal equilibrium). If predominantly collisionally ionized, as we argue is most probable, the OVI observations require that the circumgalactic medium (CGM) of $L_*$ galaxies holds nearly all the associated baryons within a virial radius ($\sim 10^{11}M_\odot$) and hosts massive flows of cooling gas with $\approx30[nT/30{\rm~cm^{-3}K}]~M_\odot~$yr$^{-1}$, which must be largely prevented from accreting onto the host galaxy. Cooling and feedback energetics considerations require $10 <\langle nT\rangle<100{\rm~cm^{-3}K}$ for the warm and hot halo gases. We argue that virialized gas, boundary layers, hot winds, and shocks are unlikely to directly account for the bulk of the OVI. Furthermore, we show that there is a robust constraint on the number density of many of the photoionized $\sim10^4$K absorption systems that yields upper bounds in the range $n<(0.1-3)\times10^{-3}(Z/0.3)$cm$^{-3}$, where $Z$ is the metallicity, suggestive that the dominant pressure in some photoionized clouds is nonthermal. This constraint, which requires minimal ionization modeling, is in accord with the low densities inferred from more complex photoionization modeling. The large amount of cooling gas that is inferred could re-form these clouds in a fraction of the halo dynamical time, as some arguments require, and it requires much of the feedback energy available from supernovae and stellar winds to be dissipated in the CGM.Comment: 16 pages, matches version accepted to Ap

Inference of dispersion measure from incoherent time-steady sources

Several recent papers have proposed schemes by which a dispersion measure, and hence electron column, could be obtained from a time-steady, incoherent radio source at a cosmological distance (such as an active galactic nucleus). If correct, this would open a new window on the distribution of intergalactic baryons. These schemes are based on the statistical properties of the received radiation, such as the 2- or 4-point correlation function of the received electric field, and in one case on the quantum nature of the electromagnetic field. We show, on the basis of general principles, that these schemes are not sensitive to dispersion measure (or have an extremely small signal-to-noise ratio), because (i) the classical 2-point correlation function is unaffected by dispersion; (ii) for a source with a large number of incoherently emitting electrons, the central limit theorem obliterates additional information in higher-order functions; and (iii) such an emitter produces a radiation density matrix that is equivalent to a statistical distribution of coherent states, which contains no information that is not already in the statistics of the classical waveforms. Why the proposed observables do not depend on dispersion measure (or have extremely tiny dependences) is discussed in detail.Comment: 16 pages, 1 figur

On the intergalactic temperature-density relation

Cosmological simulations of the low-density intergalactic medium exhibit a strikingly tight power-law relation between temperature and density that holds over two decades in density. It is found that this relation should roughly apply Delta z ~ 1-2 after a reionization event, and this limiting behavior has motivated the power-law parameterizations used in most analyses of the Ly-alpha forest. This relation has been explained by using equations linearized in the baryonic overdensity (which does not address why a tight power-law relation holds over two decades in density) or by equating the photoheating rate with the cooling rate from cosmological expansion (which we show is incorrect). Previous explanations also did not address why recombination cooling and Compton cooling off of the cosmic microwave background, which are never negligible, do not alter the character of this relation. We provide an understanding for why a tight power-law relation arises for unshocked gas at all densities for which collisional cooling is unimportant. We also use our results to comment on (1) how quickly fluctuations in temperature redshift away after reionization processes, (2) how much shock heating occurs in the low-density intergalactic medium, and (3) how the temperatures of collapsing gas parcels evolve.Comment: 8 pages, 5 figures; published in MNRA