S. Peng Oh

Second-Generation Objects in the Universe: Radiative Cooling and Collapse of Halos with Virial Temperatures above 104 K

The first generation of stars is thought to have formed in low-mass halos with Tvir 104 K, i.e., those that can cool in the absence of H2 via neutral atomic lines. The evolution of these halos differs from their less massive counterparts. Efficient atomic line radiation allows rapid cooling to ~8000 K; subsequently the gas can contract nearly isothermally at this temperature. In the absence of H2 molecules, the gas would likely settle into a locally stable disk, and only disks with unusually low spin would be unstable. However, we find that the initial atomic line cooling leaves a large, out-of-equilibrium residual free electron fraction. This allows the molecular fraction to build up to a universal value of x ≈ 10-3, almost independently of initial density and temperature. We show that this is a nonequilibrium freeze-out value that can be understood in terms of timescale arguments. Unlike in less massive halos, H2 formation and cooling is largely impervious to feedback from external UV fields, due to the high initial densities achieved by atomic cooling. The newly formed molecules cool the gas further to ~100 K and allow the gas to fragment on scales of a few times 100 M☉. We investigate the importance of various feedback effects such as H2 photodissociation from internal UV fields and radiation pressure due to Lyα photon trapping, which are likely to regulate the efficiency of star formation.

Quasar Feedback: The Missing Link in Structure Formation

We consider the impact of quasar outflows on structure formation. Such outflows are potentially more important than galactic winds, which appear insufficient to produce the level of preheating inferred from X-ray observations of galaxy clusters. At late times, energetic material from the densest objects in the centers of galaxies makes its way into the intergalactic medium (IGM), impacting structures on many scales, much as supernovae impact structures on many scales within the interstellar medium. Using a simple analytical model for the distribution of quasars with redshift, coupled with a one-dimensional Sedov-Taylor model for outflows, we are able to make robust statements about these interactions. As large regions of the IGM are heated above a critical entropy of Scrit ≈ 100 keV cm2, cooling becomes impossible within them, regardless of changes in density. On quasar scales, this has the effect of inhibiting further formation, resulting in their observed falloff in number densities below z ≈ 2. On galaxy scales, quasar feedback fixes the turnover scale in the galaxy luminosity function (L*) as the nonlinear scale at the redshift of strong feedback. The galaxy luminosity function then remains largely fixed after this epoch, consistent with recent observations and in contrast to the strong evolution predicted in more standard galaxy-formation models. Finally, strong quasar feedback explains why the intracluster medium is observed to have been preheated to entropy levels just above Scrit, the minimum excess that would not have been erased by cooling. The presence of such outflows is completely consistent with the observed properties of the Lyα forest at z ~ 2 but is expected to have a substantial and detectable impact on Compton distortions observed in the microwave background and the multiphase properties of the warm-hot (z = 0) circumgalactic medium.

Lyman α radiative transfer in a multiphase medium

Hydrogen Lyman a (Lya) is our primary emission-line window into high-redshift galaxies. Despite an extensive literature, Lya radiative transfer in the most realistic case of a dusty, multiphase medium has received surprisingly little detailed theoretical attention. We investigate Lya resonant scattering through an ensemble of dusty, moving, optically thick gas clumps. We treat each clump as a scattering particle and use Monte Carlo simulations of surface scattering to quantify continuum and Lya surface scattering angles, absorption probabilities, and frequency redistribution, as a function of the gas dust content. This atomistic approach speeds up the simulations by many orders of magnitude, making possible calculations which are otherwise intractable. Our fitting formulae can be readily adapted for fast radiative transfer in numerical simulations. With these surface scattering results, we develop an analytic framework for estimating escape fractions and line widths as a function of gas geometry, motion, and dust content. Our simple analytic model shows good agreement with full Monte Carlo simulations. We show that the key geometric parameter is the average number of surface scatters for escape in the absence of absorption, N 0 , and we provide fitting formulae for several geometries of astrophysical interest. We consider the following two interesting applications. (i) Equivalent widths (EWs). Lya can preferentially escape from a dusty multiphase interstellar medium if most of the dust lies in cold neutral clouds, which Lya photons cannot penetrate. This might explain the anomalously high EWs sometimes seen in high-redshift/submillimetre sources. (ii) Multiphase galactic outflows. We show the characteristic profile is asymmetric with a broad red tail, and relate the profile features to the outflow speed and gas geometry. Many future applications are envisaged.

Early Cosmological H II/He III Regions and Their Impact on Second-Generation Star Formation

We present the results of three-dimensional radiation hydrodynamics simulations of the formation and evolution of early H II/He III regions around the first stars. Cooling and recollapse of the gas in the relic H II region is also followed in a full cosmological context, until second-generation stars are formed. We first carry out ray-tracing simulations of ionizing radiation transfer from the first star. Hydrodynamics is directly coupled with photoionization heating as well as radiative and chemical cooling. The photoionized hot gas is evacuated out of the host halo at a velocity of ~30 km s-1. This radiative feedback effect quenches further star formation within the halo for over tens to a hundred million years. We show that the thermal and chemical evolution of the photoionized gas in the relic H II region is remarkably different from that of a neutral primordial gas. Efficient molecular hydrogen production in the recombining gas enables it to cool to ~100 K, where fractionation of HD/H2 occurs. The gas further cools by HD line cooling down to a few tens of kelvins. Interestingly, at high redshifts (z > 10), the minimum gas temperature is limited by that of the cosmic microwave background with TCMB = 2.728(1 + z). The gas cloud experiences runaway collapse when its mass is ~40 M☉, which is significantly smaller than a typical clump mass of ~200-300 M☉ for early primordial gas clouds. We argue that massive, rather than very massive, primordial stars may form in the relic H II region. Such stars might be responsible for early metal enrichment of the interstellar medium from which recently discovered hyper-metal-poor stars were born.

Reionization by Hard Photons. I. X-Rays from the First Star Clusters

Observations of the Lyα forest at z ~ 3 reveal an average metallicity Z ~ 10-2 Z☉. The high-redshift supernovae that polluted the intergalactic medium also accelerated relativistic electrons. Since the energy density of the cosmic microwave background is proportional to (1 + z)4, at high redshift these electrons cool via inverse Compton scattering. Thus, the first star clusters emit X-rays. Unlike stellar UV ionizing photons, these X-rays can escape easily from their host galaxies. This has a number of important physical consequences:

Chaotic cold accretion on to black holes

Using 3D AMR simulations, linking the 50 kpc to the sub-pc scales over the course of 40 Myr, we systematically relax the classic Bondi assumptions in a typical galaxy hosting a SMBH. In the realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the nonlinear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when t_cool/t_ff 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (t_turb/t_cool < 1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions and tidal forces between clouds, filaments and the central clumpy torus promote angular momentum cancellation, hence boosting accretion. On sub-pc scales the clouds are channelled to the very centre via a funnel. A good approximation to the accretion rate is the cooling rate, which can be used as subgrid model, physically reproducing the boost factor of 100 required by cosmological simulations, while accounting for fluctuations. Chaotic cold accretion may be common in many systems, such as hot galactic halos, groups, and clusters, generating high-velocity clouds and strong variations of the AGN luminosity and jet orientation. In this mode, the black hole can quickly react to the state of the entire host galaxy, leading to efficient self-regulated AGN feedback and the symbiotic Magorrian relation. During phases of overheating, the hot mode becomes the single channel of accretion (with a different cuspy temperature profile), though strongly suppressed by turbulence.

ON LYMAN-LIMIT SYSTEMS AND THE EVOLUTION OF THE INTERGALACTIC IONIZING BACKGROUND

We study the properties of self-shielding intergalactic absorption systems and their implications for the ionizing background. We find that cosmological simulations post-processed with detailed radiative transfer calculations generally are able to reproduce the observed abundance of Lyman-limit systems, and we highlight possible discrepancies between the observations and simulations. This comparison tests cosmological simulations at overdensities of ~100. Furthermore, we show that the properties of Lyman-limit systems in these simulations, in simple semianalytic arguments, and as suggested by recent observations indicate that a small change in the ionizing emissivity of the sources would have resulted in a much larger change in the amplitude of the intergalactic H I-ionizing background (with this scaling strengthening with increasing redshift). This strong scaling could explain the rapid evolution in the Lyα forest transmission observed at z ≈ 6. Our calculations agree with the suggestion of simpler models that the comoving ionizing emissivity was constant or even increasing from z = 3 to 6. Our calculations also provide a more rigorous estimate than in previous studies for the clumping factor of intergalactic gas after reionization, which we estimate was ≈2-3 at z = 6.

Feedback heating by cosmic rays in clusters of galaxies

Recent observations show that the cooling flows in the central regions of galaxy clusters are highly suppressed. Observed active galactic nuclei (AGN)-induced cavities/bubbles are a leading candidate for suppressing cooling, usually via some form of mechanical heating. At the same time, observed X-ray cavities and synchrotron emission point towards a significant nonthermal particle population. Previous studies have focused on the dynamical effects of cosmic ray pressure support, but none has built successful models in which cosmic ray heating is significant. Here, we investigate a new model of AGN heating, in which the intracluster medium is efficiently heated by cosmic rays, which are injected into the intra-cluster medium (ICM) through diffusion or the shredding of the bubbles by Rayleigh‐Taylor or Kelvin‐Helmholtz instabilities. We include thermal conduction as well. Using numerical simulations, we show that the cooling catastrophe is efficiently suppressed. The cluster quickly relaxes to a quasiequilibrium state with a highly reduced accretion rate and temperature and density profiles which match observations. Unlike the conduction-only case, no fine-tuning of the Spitzer conduction suppression factor f is needed. The cosmic ray pressure, Pc/Pg � 0.1 and ∇Pc � 0.1ρg, is well within observational bounds. Cosmic ray heating is a very attractive alternative to mechanical heating, and may become particularly compelling if Gamma-ray Large Array Space Telescope (GLAST) detects the γ -ray signature of cosmic rays in clusters.

SHAKEN AND STIRRED: CONDUCTION AND TURBULENCE IN CLUSTERS OF GALAXIES

Uninhibited radiative cooling in clusters of galaxies would lead to excessive mass accretion rates contrary to observations. One of the key proposals to offset radiative energy losses is thermal conduction from outer, hotter layers of cool core (CC) clusters to their centers. However, thermal conduction is sensitive to magnetic field topology. In CC clusters where temperature decreases inwards, the heat buoyancy instability (HBI) leads to magnetic fields ordered preferentially in the direction perpendicular to that of gravity, which significantly reduces the level of conduction below the classical Spitzer-Braginskii value. However, the CC clusters are rarely in perfect hydrostatic equilibrium. Sloshing motions due to minor mergers and stirring motions induced by cluster galaxies or active galactic nuclei can significantly perturb the gas. The turbulent cascade can then affect the topology of the magnetic field and the effective level of thermal conduction. We perform three-dimensional adaptive mesh refinement magnetohydrodynamical simulations of the effect of turbulence on the properties of the anisotropic thermal conduction in CC clusters. We show that very weak subsonic motions, well within observational constraints, can randomize the magnetic field and significantly boost effective thermal conduction beyond the saturated values expected in the pure unperturbed HBI case. We find that the turbulent motions can essentially restore the conductive heat flow to the CC to level comparable to the theoretical maximum of ~1/3 Spitzer for a highly tangled field. Runs with radiative cooling show that the cooling catastrophe can be averted and the cluster core stabilized; however, this conclusion may depend on the central gas density. Above a critical Froude number, these same turbulent motions also eliminate the tangential bias in the velocity and magnetic field that is otherwise induced by the trapped g-modes, and possibly allow significant turbulent heat diffusion. Our results can be tested with future radio polarization measurements and have implications for efficient metal dispersal in clusters.

Foregrounds for 21-cm observations of neutral gas at high redshift

We investigate a number of potential foregrounds for an ambitious goal of future radio telescopes such as the Square Kilometer Array (SKA) and the Low Frequency Array (LOFAR): spatial tomography of neutral gas at high redshift in 21-cm emission. While the expected temperature fluctuations due to unresolved radio point sources is highly uncertain, we point out that free–free emission from the ionizing haloes that reionized the Universe should define a minimal bound. This emission is likely to swamp the expected brightness temperature fluctuations, making proposed detections of the angular patchwork of 21-cm emission across the sky unlikely to be viable. Hα observations with JWST could place an upper bound on the contribution of high-redshift sources to the free–free background. An alternative approach is to discern the topology of reionization from spectral features due to 21-cm emission along a pencil-beam slice. This requires tight control of the frequency-dependence of the beam in order to prevent foreground sources from contributing excessive variance. We also investigate potential contamination by galactic and extragalactic radio recombination lines (RRLs). These are unlikely to be show-stoppers, although little is known about the distribution of RRLs away from the Galactic plane. The mini-halo emission signal is always less than that of the intergalactic medium (IGM), making mini-haloes unlikely to be detectable. If they are seen, it will be only in the very earliest stages of structure formation at high redshift, when the spin temperature of the IGM has not yet decoupled from the cosmic microwave background.