What Must the First Generation of Stars Produce to Reionize the Gas in the Early Universe?

three. THE REIONIZATION PROCESS

The inflationary process that occurred very early in the Universe has created the initial tiny fluctuations in matter density field. The loftier density peaks in these fluctuations field are the seeds around which galaxies course. The formation process is initially driven by gravitational instability alone only later gas physics, cooling, heating, radiations processes and feedback furnishings play an important role also [132, 156]. The first galaxies class when primordial gas (H Iand He I) condenses within dark matter potential wells which leads to radiative cooling driven mostly by the Lyman line transition [53, 75, 107, 106, 105, 153] and, probably, by H₂ cooling. To appointment Lyman emission has been observed in many loftier redshift galaxies [98, 140, 149, 150]. This gas condenses farther to form the first stars and black holes which in plow produce radiations that starts ionizing the Universe. The efficiency with which these objects produce ionizing radiation is subject field to many different concrete processes and assumptions (meet east.g., [44]). Since this book's topic is the get-go galaxies, the reader is referred to the other chapters in this book for detailed discussion of how the beginning radiation emitting objects form and how efficient are they in producing ionizing radiation.

An important unknown in these galaxies is the so chosen escape fraction, namely the fraction of ionizing radiation that escapes the galaxy into the IGM. It is these ionizing photons that are relevant to the Universe's reionization. Determining the escape fraction of ionizing radiations observationally is very difficult peculiarly at loftier redshifts where the available data is very express. Nevertheless, such observations take been carried out by a number of authors [70, 91, 92, 181, 189] where the measured fraction is found to be between 0.1-0.5. Theoretical prediction of the escape fraction is besides hard. Early studies have causeless idealized smooth galaxies [55, 56, 167, 214] merely later studies have false more realistic galaxies (see e.g., [43]). Each of these studies accept considered different set up and different sources merely all conclude that the escape fraction of radiation is roughly in the range of 0.ane-0.5.

The most accustomed picture of how reionization unfolds is simple. The commencement radiations-emitting objects ionize their immediate environs, forming bubbling that expand until their ionizing photons are consume by the neutral IGM. As the number of radiating sources increases, so do the number and size of the ionization bubbles, which somewhen spread to make full the whole Universe. However, nigh of the details of this scenario are yet to be clarified. For instance: what controls the formation of the beginning objects and how much ionizing radiation practise they produce? How do the bubbling aggrandize into the intergalactic medium and what do they ionize first, high-density or low density regions? The answer to these important questions and many others touch upon many cardinal questions in cosmology, milky way germination, quasars activeness and the physical properties of very metal poor stars [nine, 30, 44, 41, 67, 135].

To ionize hydrogen 1 needs photons with energy of xiii.6 eV or higher meaning the reionization of the Universe requires ultraviolet photons. A crucial question is which sources in the Universe provide the UV photons needed to ionize the Universe and maintain information technology in that state. Obvious candidates are the starting time stars (so called Population III stars), 2d generation stars (Population II stars) and (mini)quasars which are objects powered by intermediate mass blackness holes (10^{three-half dozen} M ). There are other candidate sources of reionization, like decomposable or cocky-annihilating night matter particles or decaying catholic strings. However, the constraints on such objects make it unlikely that they could reionize the Universe past themselves [40, 100, 121, 122, 142, 151, 168, 231].

Massive black holes powering quasars convert mass to radiation extremely efficiently. They produce a large amount of UV and X-ray radiations higher up the ionization threshold. In fact, ane of the main discoveries of the last decade is that quasars, powered by very large black holes with masses in excess of 10⁹ M , already existed at redshift above 7 (e.yard., QSO ULAS J1120+0641 [137] from the UKIDSS survey [108]). How these black holes managed to accumulate so much mass in such a short time is a puzzle in its own right [137, twenty]. Nonetheless, the mass distribution of the massive black holes in the early Universe is unknown, rendering the role played by quasars during reionization very uncertain.

Population III stars formed from the primordial mix of the elements and thus only contain hydrogen and helium. This composition makes them very different from present-day stars. In club for a star to class, the initial proto-star has to radiate some of the free energy gained past gravitational contraction, or the plummet will apace halt equally the cloud reaches hydrostatic equilibrium. Population 3 stars are poor radiators until the cloud from which they form reaches high temperatures. This causes them to exist very massive, and hence, they are very efficient and abundant sources of UV photons, nonetheless are very short lived. Theoretically, these objects could have reionized the Universe only our knowledge of them, including the question of whether they existed in sufficient numbers, is very uncertain.

The kickoff stars' early on metallic enrichment was likely the dominant consequence that brought well-nigh the transition from Population III to Population 2 star formation. Recent numerical simulations of collapsing primordial objects with masses of ≈ 10^half-dozen M , have shown that the gas has to be enriched with heavy elements to a minimum level of Z _crit ≈ 10^-4 Z , in order to have whatever upshot on the dynamics and fragmentation backdrop of the system. Normal, low-mass (Population 2) stars are hypothesized to class merely out of gas with metallicity Z ≥ Z _crit. Thus, the characteristic mass scale for star formation is expected to be a role of metallicity, with a discontinuity at Z _crit where the mass scale changes by about 2 orders of magnitude. The redshift where this transition occurs has important implications for the early growth of catholic structure, and the resulting observational signature includes the extended nature of reionization (see the review by Ciardi and Ferrara [44]).

Most studies of reionization have focused on stars as being the main source [126, 1, 2, 29, 221]. Due to the deficiency of hard photons in the spectral energy distributions (SEDs) of these "first stars", heating due to these objects is limited in extent [204]. On the other hand, miniquasars (miniqsos), characterized by fundamental black pigsty masses < x⁶ M , take also been considered every bit an of import contributor to reionization [120, 165, 166, 144, 66, 69, 217, 204]. Ionization aspects of the miniquasar radiation have been explored by several authors [120, 165, 166, 204, 206, 229]. Thomas & Zaroubi [204] accept shown that although the ionization blueprint around miniqsos is similar to that of stellar-blazon sources, the heating due to the presence of hard photons in miniqsos is very different. The reason existence is that stars produce thermal radiation that is mostly in the UV range, which is very efficient in ionization, merely in one case it is captivated by H I, the energy left will be too small to be converted to heat finer. On the other hand black hole powered sources have hard x-ray photons as their spectral energy distribution (SED) follows a ability police force (typically assumed to exist -1). Such x-ray photons have lower leap-free cross section relative to UV photons but once they are absorbed, their leftover energy is very large and tin easily exist converted to heat. Too, x-ray photons penetrate much deeper into the IGM and can oestrus information technology up much further from the source than UV radiation.

Miniqsos heat the surrounding IGM well across their ionization front [204, 42]. Several authors (e.one thousand., [120, 144, 230] have shown the importance of heating the IGM with respect to the observability of the redshifted 21 cm radiation in either emission or assimilation. Figure 13 shows the ionization and heating patterns around a number of stars (upper panels) and miniqsos (lower panel). The mass of the stars and black-holes are indicated side by side to the lines, and their SEDs are assumed to exist thermal or to have a ability law dependence on the photon energy, ∝ E ^-i, respectively. The calculation here is spherically symmetric and assumes a single object forming in the IGM [230, 204]. The ionization design effectually stars and blackness holes are very similar, they both evidence a very sharp increase in H I with a clear ionization front (meet east.g. [102, 204, 230]). Of form the radius at which such front is seen depends on the mass of the star or the black hole but the design is the aforementioned (see the left mitt side panels of Figure 13). The heating profile effectually the ii types of sources, on the other manus, is dissimilar since in power police force sources (miniqsos) the radiations tin can penetrate the neutral gas and reach big distances from the sources (encounter right hand panels of Figure thirteen). This high energy radiations is efficient in heating the IGM gas through secondary electrons [183] (see word after) whereas UV radiation is efficient in ionizing the gas but has little energy left to heat as well much and can not penetrate the neutral gas as far as 10-ray radiation does.

Figure thirteen. This figure shows the ionization and heating contour effectually a unmarried star and black hole forming in the IGM assuming spherical symmetry. The upper panels show the stars case whereas the lower panels show the blackness holes case. The left paw panels testify the neutral fraction of H I every bit a function of distance from the star and the right paw panels prove the gas temperature equally a function of distance from the source [204].

We accept seen that unlike stars, x-ray source a (e.g., miniquasars) take an additional property of heating the IGM to a large extent and through secondary Lyman radiation making the neutral IGM visible to a 21-cm experiment. However, some authors (e.g., [52, 175]) contend that miniquasars lonely tin can not reionize the Universe as they volition produce far more than soft X-ray background radiation than currently observed [136, 186] while simultaneously satisfying the WMAP3 polarisation results [152, 187]. It should be noted, withal, that these calculations take been carried out assuming specific models for the development of blackness pigsty mass density and spectral energy distributions of UV/X-ray radiations of the miniquasars, whereas i can easily construct other models in which the discrepancy is not then severe [230, 169].

Some authors [99] take claimed a detection of excess IR background radiations and argued that it provides evidence for stars existence the primary source of reionization. This too has been discipline to controversy because of the sensitivity of the result to the subtraction of the contaminants, e.g., Zodiacal light, within the same waveband [48].

Although dubiety looms about the sources that resided during the night ages, it is believable from observations of our Universe upwardly to redshifts of vi.five, that sources of reionization could have been a mixture of both stellar and miniquasar sources. Implementing radiative transfer that includes both ionizing and difficult X-ray photons has been difficult and, every bit a result, well-nigh 3-D radiative transfer schemes restrict themselves to ionization due to stars [12, 45, 73, 126, 127, 141, 154, 170, 163, 196, 211, 222]. In [165], a "semi" hybrid model of stars and quasars like the one hinted higher up used, albeit in sequential club instead of a simultaneous implementation. That is, pre-ionization due to quasars has been invoked between 7 ≤ z ≤ 20, after which stars reionize the Universe at redshift 7.

Given the numerical cost of the full 3D radiative transfer schemes, exploring a big parameter space for models of reionization, is non feasible. Such an exploration is needed in club to understand the various physical effects introduced past each such parameter. It is also needed to assist interpret the available data. A number of authors have been pursuing "quick-and-dingy" methods to simulate the reionization process. These schemes can include very rough methods that utilize the initial density field to produce a reionization cube without the demand for cosmological Northward-body and hydro simulations, such as 21cmFAST ([129, 222, 223]) and SimFast21 [177]. They also include more accurate (yet still fast) methods like BEARS [204, 206, 205] that utilise N-body and hydro simulations but reduces the numerical cost by restricting the ionization bubbling around the radiations sources to exist spherical.

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Source: https://ned.ipac.caltech.edu/level5/March14/Zaroubi/Zaroubi3.html

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