Not the way one might have thought. In hydrodynamic simulations of galaxy formation, some gas follows the traditionally envisioned route, shock heating to the halo virial temperature before cooling to the much lower temperature of the neutral ISM. But most gas enters galaxies without ever heating close to the virial temperature, gaining thermal energy from weak shocks and adiabatic compression, and radiating it just as quickly. This “cold mode” accretion is channeled along filaments, while the conventional, “hot mode” accretion is quasi-spherical. Cold mode accretion dominates high redshift growth by a substantial factor, while at z < 1 the overall accretion rate declines and hot mode accretion has greater relative importance. The decline of the cosmic star formation rate at low z is driven largely by geometry, as the typical cross section of filaments begins to exceed that of the galaxies at their intersections.

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How do galaxies get their gas

The James Webb Space Telescope (JWST) is a large (6.6 m), cold (<50 K), infrared (IR)-optimized space observatory that will be launched early in the next decade into orbit around the second Earth–Sun Lagrange point. The observatory will have four instruments: a near-IR camera, a near-IR multiobject spectrograph, and a tunable filter imager will cover the wavelength range, 0.6 < ; < 5.0 μ m, while the mid-IR instrument will do both imaging and spectroscopy from 5.0 < ; < 29 μ m. The JWST science goals are divided into four themes. The key objective of The End of the Dark Ages: First Light and Reionization theme is to identify the first luminous sources to form and to determine the ionization history of the early universe. The key objective of The Assembly of Galaxies theme is to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and active nuclei within them evolved from the epoch of reionization to the present day. The key objective of The Birth of Stars and Protoplanetary Systems theme is to unravel the birth and early evolution of stars, from infall on to dust-enshrouded protostars to the genesis of planetary systems. The key objective of the Planetary Systems and the Origins of Life theme is to determine the physical and chemical properties of planetary systems including our own, and investigate the potential for the origins of life in those systems. Within these themes and objectives, we have derived representative astronomical observations. To enable these observations, JWST consists of a telescope, an instrument package, a spacecraft, and a sunshield. The telescope consists of 18 beryllium segments, some of which are deployed. The segments will be brought into optical alignment on-orbit through a process of periodic wavefront sensing and control. The instrument package contains the four science instruments and a fine guidance sensor. The spacecraft provides pointing, orbit maintenance, and communications. The sunshield provides passive thermal control. The JWST operations plan is based on that used for previous space observatories, and the majority of JWST observing time will be allocated to the international astronomical community through annual peer-reviewed proposal opportunities.

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The James Webb Space Telescope

The history of the transition from a neutral intergalactic medium (IGM) to one that is almost fully ionized can reveal the character of cosmological ionizing sources. We study the evolution of the volume filling factors of H II and He III regions in a clumpy IGM and discuss the implications for rival reionization scenarios of the rapid decline observed at z3 in the space density of optical and radio-loud quasars and of the large population of star-forming galaxies recently discovered at the same epoch. The hydrogen component in a highly inhomogeneous universe is completely reionized when the number of photons emitted above 1 ryd in one recombination time equals the mean number of hydrogen atoms. If stellar sources are responsible for keeping the IGM ionized at z=5, the rate of star formation at this epoch must be comparable or greater than the one inferred from optical observations of galaxies at z≈3 and the mean metallicity per baryon in the universe 0.002 solar. An early generation of stars in dark matter halos with circular velocities, vcirc≈50 km s-1, possibly one of the main sources of UV photons at high z, could be detectable with the Next Generation Space Telescope. Models in which the quasar emissivity declines rapidly at z3 predict a late He II reionization epoch, a feature that could explain the recent detection of patchy He II Lyα at z=2.9 by Reimers et al. and the abrupt change observed by Songaila at about the same epoch of the Si IV /C IV ratio, but appear unable to provide the required number of hydrogen-ionizing photons at z≈5.

http://iopscience.iop.org/article/10.1086/306975/pdf

Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Sources

We investigate the 21 cm signature that may arise from the intergalactic medium (IGM) prior to the epoch of full reionization (z > 5) In scenarios in which the IGM is reionized by discrete sources of photoionizing radiation, the neutral gas that has not yet been engulfed by an H II region may easily be preheated to temperatures well above that of the cosmic background radiation (CBR), rendering the IGM invisible in absorption against the CBR We identify three possible preheating mechanisms: (1) photoelectric heating by soft X-rays from QSOs, (2) photoelectric heating by soft X-rays from early galactic halos, and (3) resonant scattering of the continuum UV radiation from an early generation of stars We find that bright quasars with only a small fraction of the observed comoving density at z ~ 4 will suffice to preheat the entire universe at z 6 We also show that, in a cold dark matter dominated cosmology, the thermal bremsstrahlung radiation associated with collapsing galactic mass halos (1010-1011 M?) may warm the IGM to ~100 K by z ~ 7 Alternatively, the equivalent of ~10% of the star formation rate density in the local universe, whether in isolated pregalactic stars, dwarf, or normal galaxies, would be capable of heating the entire IGM to a temperature above that of the CBR by Ly? scattering in a small fraction of the Hubble time at z ~ 6 In the presence of a sufficiently strong ambient flux of Ly? photons, the hyperfine transition in the warmed H I will be excited A beam differencing experiment would detect a patchwork of emission, both in frequency and in angle across the sky This patchwork could serve as a valuable tool for understanding the epoch, nature, and sources of the reionization of the universe, and their implications for cosmology We demonstrate that isolated QSOs will produce detectable signals at meter wavelengths within their spheres of influence over which they warm the IGM As a result of the redshifted 21 cm radiation emitted by warm H I bubbles, the spectrum of the radio extragalactic background will display frequency structure with velocity widths up to 10,000 km s-1 Broad beam observations would reveal corresponding angular fluctuations in the sky intensity with ?T/T 10-3 on scales ? ~ 1? This scale is set either by the thermalization distance from a QSO within which Ly? pumping determines the spin temperature of the IGM or by the quasar lifetime Radio measurements near 235 and 150 MHz, as will be possible in the near future using the Giant Metrewave Radio Telescope, may provide the first detection of a neutral IGM at 5 z 10 A next generation facility like the Square Kilometer Array Interferometer could effectively open much of the universe to a direct study of the reheating epoch and possibly probe the transition from a neutral universe to one that is fully ionized

https://arxiv.org/pdf/astro-ph/9608010.pdf

21 centimeter tomography of the intergalactic medium at high redshift

I use simulations of cosmological reionization to quantify the effect of photoionization on the gas fraction in low-mass objects, in particular the characteristic mass scale below which the gas fraction is reduced compared to the universal value. I show that this characteristic scale can be up to an order of magnitude lower than the linear-theory Jeans mass, and that even if one defines the Jeans mass at a higher overdensity, it does not track the evolution of this characteristic suppression mass. Instead, the filtering mass, which corresponds directly to the scale over which baryonic perturbations are smoothed in linear perturbation theory, provides a remarkably good fit to the characteristic mass scale. Thus, it appears that the effect of reionization on structure formation in both the linear and nonlinear regimes is described by a single characteristic scale, the filtering scale of baryonic perturbations. In contrast to the Jeans mass, the filtering mass depends on the full thermal history of the gas instead of the instantaneous value of the sound speed, so it accounts for the finite time required for pressure to influence the gas distribution in the expanding universe. In addition to the characteristic suppression mass, I study the full shape of the probability distribution to find an object with a given gas mass among all the objects with the same total mass, and I show that the numerical results can be described by a simple fitting formula that again depends only on the filtering mass. This simple description of the probability distribution may be useful for semianalytical modeling of structure formation in the early universe.

http://iopscience.iop.org/article/10.1086/317042/pdf

Effect of Reionization on Structure Formation in the Universe

In most models of the reionization of the intergalactic medium, sources of UV radiation create ionized regions around them which expand until they overlap. The gas is impulsively heated when it enters one of these ionized regions. We discuss the various physical effects that determine the temperature of the gas after the passage of the ionization front. Sources with different spectra may heat the surrounding gas to different temperatures, creating large-scale inhomogeneities of the gas temperature in the Universe. The presence of helium is particularly important, since it allows the post-reionization temperature to reach values as high as ∼ 5×10 4 K. Subsequent adiabatic heating as gas collapses into haloes can raise the temperature even more, explaining large b-parameters in Lyα clouds in the minihalo model

Reionization and thermal evolution of a photoionized intergalactic medium.

When a source of constant luminosity moves behind a gravitational lens composed of compact objects, its flux is observed to change due to the varying amplification of the images produced by different components of the lens. A new method is presented to calculate such microlensed light curves for any given field of point masses. The method is sufficiently efficient and fast to allow one to calculate any desired statistical property of the light curves for a large number of models in a reasonable time, with present computers. We compare the light curves generated by this method with those produced by the ray-shooting method, and find good agreement

Microlensing light curves: a new and efficient numerical method

According to the minihalo model for the Ly α forest, the absorption lines arise from photoionized gas gravitationally confined in dark matter haloes of low velocitydispersion. Possible tests for this hypothesis are analysed. The predicted column density distribution is mostly dependent on the density profile of the haloes, and is in basic agreement with the observations. At low column densities, the lines should be probing the infalling, non-virialized regions of the clouds, and they should deviate from Voigt profiles. The Gunn-Peterson absorption should also fluctuate with wavelength, and it may thus be difficult to distinguish from a superposition of many weak absorption lines

Tests for the minihalo model of the Lyman alpha forest

The flux of a quasar below a rest-frame wavelength of 304 A (the Ly α line of He II) should be reduced owing to the He II Gunn-Peterson effect and the He II absorption lines of the Ly α forest. This region of the spectrum may soon be observed for high-redshift quasars with the HST, HUT and other satellites. The predicted decrement depends mainly on the spectrum of the ionizing background at high redshift. If this background is produced by quasars with a power-law spectrum joining their observed UV and X-ray luminosities, the He II decrement should be about four times larger than the decrement of the H I Ly α forest, being a factor of ∼ 3 at z = 3, and should be easily seen on any quasar detected down to the He II Ly a wavelength

On the He II Gunn–Peterson effect and the He II forest

We propose that a large number of gravitationally lensed rings should be observable in the optical. We argue that the probability of forming a ring is likely to be similar for optical and radio sources. The number of optical rings in the sky can then be estimated from the rate of occurrence of rings in radio sources, and the observed number of optical sources. This gives ∼100 optical rings per square degree, or a few million over the whole sky, to a magnitude limit of B=26. Most of the rings should be substantially bluer than the foreground lenses, allowing them to be detected with multicolour imaging

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Jordi Miralda-Escudé

Papers

Reionization and thermal evolution of a photoionized intergalactic medium.

Microlensing light curves: a new and efficient numerical method

Tests for the minihalo model of the Lyman alpha forest

On the He II Gunn–Peterson effect and the He II forest

Optical rings: a large number of gravitational lenses?