Heating Of Intergalactic Medium Research Articles

Primordial black holes as near infrared background sources

Abstract The near infrared background (NIRB) is the collective light from unresolved sources observed in the band 1–10 $\rm{\mu m}$. The measured NIRB angular power spectrum on angular scales θ ≳ 1 arcmin exceeds by roughly two order of magnitudes predictions from known galaxy populations. The nature of the sources producing these fluctuations is still unknown. Here we test primordial black holes (PBHs) as sources of the NIRB excess. Considering PBHs as a cold dark matter (DM) component, we model the emission of gas accreting onto PBHs in a cosmological framework. We account for both accretion in the intergalactic medium (IGM) and in DM haloes. We self consistently derive the IGM temperature evolution, considering ionization and heating due to X-ray emission from PBHs. Besides ΛCDM, we consider a model that accounts for the modification of the linear matter power spectrum due to the presence of PBHs; we also explore two PBH mass distributions, i.e. a δ-function and a lognormal distribution. For each model, we compute the mean intensity and the angular power spectrum of the NIRB produced by PBHs with mass 1–$10^3~\rm M_{\odot}$. In the limiting case in which the entirety of DM is made of PBHs, the PBH emission contributes <1 per cent to the observed NIRB fluctuations. This value decreases to <0.1 per cent if current constraints on the abundance of PBHs are taken into account. We conclude that PBHs are ruled out as substantial contributors to the NIRB.

Signatures of cosmic ray heating in 21-cm observables

ABSTRACT Cosmic rays generated by supernovae carry away a significant portion of the lifetime energy emission of their parent star, making them a plausible mechanism for heating the early universe intergalactic medium (IGM). Following a review of the existing literature on cosmic ray heating, we develop a flexible model of this heating mechanism for use in 3D seminumerical 21-cm signal simulations and conduct the first investigations of the signatures it imprints on the 21-cm power spectrum and tomographic maps. We find that cosmic ray heating of the IGM is short-ranged, leading to heating clustered around star-forming sites, and a sharp contrast between heated regions of 21-cm emission and unheated regions of absorption. This contrast results in greater small-scale power for cosmic ray heated scenarios compared to what is found for X-ray heating, thus suggesting a way to test the nature of IGM heating with future 21-cm observations. Finally, we find an unexpectedly rich thermal history in models where cosmic rays can only escape efficiently from low-mass haloes, such as in scenarios where these energetic particles originate from population III star supernovae remnants. The interplay of heating and the Lyman–Werner feedback in these models can produce a local peak in the IGM kinetic temperature and, for a limited parameter range, a flattened absorption trough in the global 21-cm signal.

Elevated Hot Gas and High-mass X-Ray Binary Emission in Low-metallicity Galaxies: Implications for Nebular Ionization and Intergalactic Medium Heating in the Early Universe

High-energy emission associated with star formation has been proposed as a significant source of interstellar medium (ISM) ionization in low-metallicity starbursts and an important contributor to the heating of the intergalactic medium (IGM) in the high-redshift (z ≳ 8) universe. Using Chandra observations of a sample of 30 galaxies at D ≈ 200–450 Mpc that have high specific star formation rates of 3–9 Gyr−1 and metallicities near Z ≈ 0.3Z ⊙, we provide new measurements of the average 0.5–8 keV spectral shape and normalization per unit star formation rate (SFR). We model the sample-combined X-ray spectrum as a combination of hot gas and high-mass X-ray binary (HMXB) populations and constrain their relative contributions. We derive scaling relations of /SFR = 40.19 ± 0.06 and /SFR significantly elevated compared to local relations. The HMXB scaling is also somewhat higher than –SFR-Z relations presented in the literature, potentially due to our galaxies having relatively low HMXB obscuration and young and X-ray luminous stellar populations. The elevation of the hot gas scaling relation is at the level expected for diminished attenuation due to a reduction of metals; however, we cannot conclude that an –SFR-Z relation is driven solely by changes in ISM metal content. Finally, we present SFR-scaled spectral models (both emergent and intrinsic) that span the X-ray-to-IR band, providing new benchmarks for studies of the impact of ISM ionization and IGM heating in the early universe.

21 cm forest constraints on primordial black holes

Abstract Primordial black holes (PBHs) as part of the dark matter (DM) would modify the evolution of large-scale structures and the thermal history of the universe. Future 21 cm forest observations, sensitive to small scales and the thermal state of the intergalactic medium (IGM), could probe the existence of such PBHs. In this article, we show that the shot noise isocurvature mode on small scales induced by the presence of PBHs can enhance the amount of low-mass halos, or minihalos, and thus, the number of 21 cm absorption lines. However, if the mass of PBHs is as large as MPBH ≳ 10 $M_\odot$, with an abundant enough fraction of PBHs as DM, fPBH, the IGM heating due to accretion on to the PBHs counteracts the enhancement due to the isocurvature mode, reducing the number of absorption lines instead. The concurrence of both effects imprints distinctive signatures on the number of absorbers, allowing the abundance of PBHs to be bound. We compute the prospects for constraining PBHs with future 21 cm forest observations, finding achievable competitive upper limits on the abundance as low as fPBH ∼ 10−3 at MPBH = 100 $M_\odot$, or even lower at larger masses, in regions of the parameter space unexplored by current probes. The impact of astrophysical X-ray sources on the IGM temperature is also studied, which could potentially weaken the bounds.

Impacts of new small-scale N -body simulations on dark matter annihilations constrained from cosmological 21-cm line observations

We revisit constraints on annihilating dark matter based on the global 21cm signature observed by EDGES. For this purpose, we used the numerical data of the latest N-body simulation performed by state-of-the-art standard in order to estimate the boost factor at high redshifts ($z$ = 10 - 100), which enhances the annihilation of dark matter in course of structure formation. By taking into account to what fraction injected energy from dark matter annihilation contributes to ionization, excitation and heating of intergalactic medium during dark ages, we estimated how large the global 21cm absorption can be. By assuming the thermal freezeout scenario, we find that $m_{\rm DM} < 15$ GeV and $m_{\rm DM} < 3$ GeV have been excluded at 95$\%$ C.L. for the annihilation modes into $b\bar{b}$ and $e^+ e^-$, respectively.

Jet feedback and the photon underproduction crisis in simba

ABSTRACT We examine the impact of black hole jet feedback on the properties of the low-redshift intergalactic medium (IGM) in the simba simulation, with a focus on the Lyα forest mean flux decrement DA. Without jet feedback, we confirm the photon underproduction crisis (PUC) in which ΓH i at $z$ = 0 must be increased by 6 times over the Haardt &amp; Madau value in order to match the observed DA. Turning on jet feedback lowers this discrepancy to ∼2.5 times, and additionally using the recent Faucher–Giguère background mostly resolves the PUC, along with producing a flux probability distribution function in accord with observations. The PUC becomes apparent at late epochs ($z \lesssim 1$) where the jet and no-jet simulations diverge; at higher redshifts simba reproduces the observed DA with no adjustment, with or without jets. The main impact of jet feedback is to lower the cosmic baryon fraction in the diffuse IGM from 39 per cent to 16 per cent at $z$ = 0, while increasing the warm-hot intergalactic medium (WHIM) baryon fraction from 30 per cent to 70 per cent; the lowering of the diffuse IGM content directly translates into a lowering of DA by a similar factor. Comparing to the older mufasa simulation that employs different quenching feedback but is otherwise similar to simba, mufasa matches DA less well than simba, suggesting that low-redshift measurements of DA and ΓH i could provide constraints on feedback mechanisms. Our results suggest that widespread IGM heating at late times is a plausible solution to the PUC, and that simba’s jet active galactic nucleus feedback model, included to quench massive galaxies, approximately yields this required heating.

Cosmic ray heating of intergalactic medium: patchy or uniform?

We study the heating of the intergalactic medium (IGM) surrounding high redshift star forming galaxies due to cosmic rays (CR). We take into account the diffusion of low energy cosmic rays and study the patchiness of the resulting heating. We discuss the case of IGM heating around a high redshift minihalo ($z\sim 10\hbox{--}20$, M$\sim 10^5\hbox{--}10^7$ M$_\odot$),and put an upper limit on the diffusion coefficient $D\le 1\times 10^{26}$ cm$^2$ s$^{-1}$ for the heating to be inhomogeneous at $z\sim 10$ and $D\le 5\hbox{--}6 \times 10^{26}$ cm$^2$ s$^{-1}$ at $z\sim 20$. For typical values of $D$, our results suggest uniform heating by CR at high redshift, although there are uncertainties in magnetic field and other CR parameters. We also discuss two cases with continuous star formation, one in which the star formation rate (SFR) of a galaxy is high enough to make the IGM in the vicinity photoionized, and another in which the SFR is low enough to keep it neutral but high enough to cause significant heating by cosmic ray protons. In the neutral case (low SFR), we find that the resulting heating can make the gas hotter than the cosmic microwave background (CMB) radiation for $D < 10^{30}$ cm$^2$ s$^{-1}$, within a few kpc of the galaxy, and unlikely to be probed by near future radio observations. In the case of photoionized IGM (high SFR), the resulting heating of the gas in the vicinity of high redshift ($z\sim 4)$ galaxies of mass $\ge 10^{12}$ M$_\odot$ can suppress gas infall into the galaxy. At lower redshifts ($z\sim 0$), an SFR of $\sim 1$ M$_\odot$ yr$^{-1}$ can suppress the infall into galaxies of mass $\le 10^{10}$ M$_\odot$.

Radiation Backgrounds at Cosmic Dawn: X-Rays from Compact Binaries

We compute the expected X-ray diffuse background and radiative feedback on the intergalactic medium (IGM) from X-ray binaries prior and during the epoch of reionization. The cosmic evolution of compact binaries is followed using a population synthesis technique that treats separately neutron stars and black hole binaries in different spectral states and is calibrated to reproduce the observed X-ray properties of galaxies at z<4. Together with an updated empirical determination of the cosmic history of star formation, recent modeling of the stellar mass-metallicity relation, and a scheme for absorption by the IGM that accounts for the presence of ionized HII bubbles during the epoch of reionization, our detailed calculations provide refined predictions of the X-ray volume emissivity and filtered radiation background from "normal" galaxies at z>6. Radiative transfer effects modulate the background spectrum, which shows a characteristic peak between 1 and 2 keV. While the filtering of X-ray radiation through the IGM slightly increases the mean excess energy per photoionization, it also weakens the radiation intensity below 1 keV, lowering the mean photoionization and heating rates. Numerical integration of the rate and energy equations shows that the contribution of X-ray binaries to the ionization of the bulk IGM is negligible, with the electron fraction never exceeding 1%. Direct HeI photoionizations are the main source of IGM heating, and the temperature of the largely neutral medium in between HII cavities increases above the temperature of the cosmic microwave background (CMB) only at z<10, when the volume filling factor of HII bubbles is already >0.1. Therefore, in this scenario, it is only at relatively late epochs that the bulk of neutral intergalactic hydrogen may be observable in 21-cm emission against the CMB.

Self-consistent Modeling of Reionization in Cosmological Hydrodynamical Simulations

Abstract The ultraviolet background (UVB) emitted by quasars and galaxies governs the ionization and thermal state of the intergalactic medium (IGM), regulates the formation of high-redshift galaxies, and is thus a key quantity for modeling cosmic reionization. The vast majority of cosmological hydrodynamical simulations implement the UVB via a set of spatially uniform photoionization and photoheating rates derived from UVB synthesis models. We show that simulations using canonical UVB rates reionize and, perhaps more importantly, spuriously heat the IGM, much earlier ( ) than they should. This problem arises because at , where observational constraints are nonexistent, the UVB amplitude is far too high. We introduce a new methodology to remedy this issue, and we generate self-consistent photoionization and photoheating rates to model any chosen reionization history. Following this approach, we run a suite of hydrodynamical simulations of different reionization scenarios and explore the impact of the timing of reionization and its concomitant heat injection on the thermal state of the IGM. We present a comprehensive study of the pressure smoothing scale of IGM gas, illustrating its dependence on the details of both hydrogen and helium reionization, and argue that it plays a fundamental role in interpreting Lyα forest statistics and the thermal evolution of the IGM. The premature IGM heating we have uncovered implies that previous work has likely dramatically overestimated the impact of photoionization feedback on galaxy formation, which sets the minimum halo mass able to form stars at high redshifts. We make our new UVB photoionization and photoheating rates publicly available for use in future simulations.

Fossil Systems; a Multi-wavelength Approach towards Understanding Galaxy Formation

Fossil systems are understood to be the end product of galaxy mergers within groups and clusters. Their halo morphology points to their relaxed/virialised nature, thus allowing them to be employed as observational probes for the evolution of cosmic structures, their thermodynamics and dark matter distribution. Cosmological simulations, and their underlying models, are broadly consistent with the early formation epoch for fossils. In a series of studies we have looked into galaxy properties and intergalactic medium (IGM) in fossils, across a wide range of wavelengths, from X-ray through optical to the radio, to have a better understanding of their nature, the attributed halo age, IGM heating and their AGNs and use them as laboratories to constrain galaxy formation models. Adhering to one of less attended properties of fossils, using the the Millennium Simulation, we combine luminosity gap with luminosity segregation (the brightest galaxy offset from the group luminosity centroid) to identify the most dynamically relaxed galaxy groups which allows us to reveal brand new observational connections between galaxies and their environments.

Another look at distortions of the Cosmic Microwave Background spectrum

We review aspects of Cosmic Microwave Background (CMB) spectral distortions which do not appear to have been fully explored in the literature. In particular, implications of recent evidences of heating of the intergalactic medium (IGM) by feedback from active galactic nuclei are investigated. Taking also into account the IGM heating associated to structure formation, we argue that values of the y parameter of several × 10−6, i.e. a factor of a few below the COBE/FIRAS upper limit, are to be expected. The Compton scattering by the re-ionized plasma also re-processes primordial distortions, adding a y-type contribution. Hence no pure Bose-Einstein-like distortions are to be expected. An assessment of Galactic and extragalactic foregrounds, taking into account the latest results from the Planck satellite as well as the contributions from the strong CII and CO lines from star-forming galaxies, demonstrates that a foreground subtraction accurate enough to fully exploit the PIXIE sensitivity will be extremely challenging. Motivated by this fact we also discuss methods to detect spectral distortions not requiring absolute measurements and show that accurate determinations of the frequency spectrum of the CMB dipole amplitude may substantially improve over COBE/FIRAS limits on distortion parameters. Such improvements may be at reach of next generation CMB anisotropy experiments. The estimated amplitude of the Cosmic Infrared Background (CIB) dipole might be detectable by careful analyses of Planck maps at the highest frequencies. Thus Planck might provide interesting constraints on the CIB intensity, currently known with a ≃ 30% uncertainty.

The imprint of the cosmic supermassive black hole growth history on the 21 cm background radiation

The redshifted 21 cm transition line of hydrogen tracks the thermal evolution of the neutral intergalactic medium (IGM) at "cosmic dawn," during the emergence of the first luminous astrophysical objects (~100 Myr after the Big Bang) but before these objects ionized the IGM (~400-800 Myr after the Big Bang). Because X-rays, in particular, are likely to be the chief energy courier for heating the IGM, measurements of the 21 cm signature can be used to infer knowledge about the first astrophysical X-ray sources. Using analytic arguments and a numerical population synthesis algorithm, we argue that the progenitors of supermassive black holes (SMBHs) should be the dominant source of hard astrophysical X-rays---and thus the primary driver of IGM heating and the 21 cm signature---at redshifts $z < 20$, if (i) they grow readily from the remnants of Population III stars and (ii) produce X-rays in quantities comparable to what is observed from active galactic nuclei and high-mass X-ray binaries. We show that models satisfying these assumptions dominate over contributions to IGM heating from stellar populations, and cause the 21 cm brightness temperature to rise at $z > 20$. An absence of such a signature in the forthcoming observational data would imply that SMBH formation occurred later (e.g. via so-called direct collapse scenarios), that it was not a common occurrence in early galaxies and protogalaxies, or that it produced far fewer X-rays than empirical trends at lower redshifts, either due to intrinsic dimness (radiative inefficiency) or Compton-thick obscuration close to the source.

Preheating of the Universe by cosmic rays from primordial supernovae at the beginning of cosmic reionization: Figure 1.

The 21-cm signal from the cosmic reionization epoch can shed light on the history of heating of the primordial intergalactic medium (IGM) at z~30-10. It has been suggested that X-rays from the first accreting black holes could significantly heat the Universe at these early epochs. Here we propose another IGM heating mechanism associated with the first stars. As known from previous work, the remnants of powerful supernovae (SNe) ending the lives of massive Population III stars could readily expand out of their host dark matter minihalos into the surrounding IGM, aided by the preceeding photoevaporation of the halo's gas by the UV radiation from the progenitor star. We argue that during the evolution of such a remnant a significant fraction of the SN kinetic energy can be put into low-energy (E<30 MeV) cosmic rays that will eventually escape into the IGM. These subrelativistic cosmic rays could propagate through the Universe and heat the IGM by ~10-100 K by z~15, before more powerful reionization/heating mechanisms associated with the first galaxies and quasars came into play. Future 21-cm observations could thus constrain the energetics of the first supernovae and provide information on the magnetic fields in the primordial IGM.

Unveiling the nature of dark matter with high redshift 21 cm line experiments

Observations of the redshifted 21 cm line from neutral hydrogen will open a new window on the early Universe. By influencing the thermal and ionization history of the intergalactic medium (IGM), annihilating dark matter (DM) can leave a detectable imprint in the 21 cm signal. Building on the publicly available 21cmFAST code, we compute the 21 cm signal for a 10 GeV WIMP DM candidate. The most pronounced role of DM annihilations is in heating the IGM earlier and more uniformly than astrophysical sources of X-rays. This leaves several unambiguous, qualitative signatures in the redshift evolution of the large-scale (k ≃ 0.1 Mpc-1) 21 cm power amplitude: (i) the local maximum (peak) associated with IGM heating can be lower than the other maxima; (ii) the heating peak can occur while the IGM is in emission against the cosmic microwave background (CMB); (iii) there can be a dramatic drop in power (a global minimum) corresponding to the epoch when the IGM temperature is comparable to the CMB temperature. These signatures are robust to astrophysical uncertainties, and will be easily detectable with second generation interferometers. We also briefly show that decaying warm dark matter has a negligible role in heating the IGM.

IGM heating in fossil galaxy groups

We study intergalactic medium (IGM) heating in a sample of five fossil galaxy groups by using their radio properties at 610 MHz and 1.4 GHz. The power by radio jets introducing mechanical heating for the sampled objects is not sufficient enough to suppress the cooling flow. Therefore, we discussed shock-, vortex heating, and conduction as alternative heating processes. Further, the 1.4 GHz and 610 MHz radio luminosities of fossil groups are compared to a sample of normal galaxy groups of the same radio brightest (BGGs), stellar mass, and total group stellar mass, quantified using the $K$-band luminosity. It appears that the fossil BGGs are under luminous at 1.4 GHz and 610 MHz for a given BGG stellar mass and luminosity, in comparison to a general population of the groups. In addition, we explore how the bolometric radio luminosity of fossil sample depends on clusters and groups characteristics. Using the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS) as a control sample we found that the large-scale behaviours of fossil galaxy groups are consistent with their relaxed and virialised nature.

IGM Heating and AGN activity in Fossil Galaxy Groups

AbstractFossil galaxy groups are energetically and morphologically ideal environments to study the intergalactic medium (IGM) heating, because their inter-galactic gas is undisturbed due to the lack of recent group scale mergers. We study the role of active galactic nuclei (AGN) in heating the IGM in a sample of five fossil galaxy groups by employing properties at 610 MHz and 1.4 GHz. We find that two of the dominant galaxies in fossil groups, ESO 3060170 and RX J1416.4+2315, are associated with the radio lobes. We evaluate the PdV work of the radio lobes and their corresponding heating power and compare to the X-ray emission loss within cooling radius. Our results show that the power due to mechanical heating is not sufficiently high to suppress the cooling.

Signatures of X-rays in the early Universe

[abridged] With their long mean free paths and efficient heating of the intergalactic medium (IGM), X-rays could have a dramatic impact on the thermal and ionization history of the Universe. We explore this in various signals: (i) Reionization history: including X-rays results in an earlier, more extended reionization. Efficient thermal feedback from X-ray heating could yield an extended, ~10% ionized epoch. (ii) Reionization morphology: a sizable (~10%) contribution of X-rays to reionization results in a more uniform morphology, though the impact is modest when compared at the same global neutral fraction, xH. However, changes in morphology cannot be countered by increasing the bias of the ionizing sources, making them a robust signature. (iii) The kinetic Sunyaev-Zel'dovich (kSZ) effect: at a fixed reionization history, X-rays decrease the kSZ power at l=3000 by ~0.5 microK^2. Our extreme model in which X-rays dominate reionization is the only one that is marginally consistent with upper limits from the South Pole Telescope, assuming no thermal Sunyaev-Zel'dovich (tSZ) - dusty galaxy correlation. Since this extreme model is unlikely, we conclude that there should be a sizable tSZ-dusty galaxy signal. (iv) The cosmic 21cm signal: the impact of X-rays on the 21cm power spectrum during the advanced stages of reionization (xH<0.7) is modest, except in extreme, X-ray dominated models. The largest impact of X-rays is to govern IGM heating. In fact, unless thermal feedback is efficient, the epoch of X-ray heating likely overlaps with the beginning of reionization (xH>0.9). This results in a 21cm power spectrum which is ~ 10-100 times higher than obtained from naive estimates ignoring this overlap. However, if thermal feedback is efficient, the resulting extended epoch between X-ray heating and reionization could provide a clean probe of the matter power spectrum in emission.

The nature of dark matter from the global high-redshift H i 21 cm signal

We study the imprint of dark matter (DM) annihilation on the global 21 cm signal from the Dark Ages to Cosmic Reionization. Motivated by recent observations, we focus on three DM candidates: (i) a 10 GeV Bino-like neutralino (ii) a 200 GeV Wino and (iii) a 1 TeV heavier particle annihilating into leptons. For each DM candidate we assume two values for the thermally averaged annihilation cross section \sigma v, the standard thermal value \sigma v_th = 3 x 10^-26 cm^3 s^-1 and the maximum value allowed by WMAP7 data, \sigma v_max. We include the enhancement of DM annihilations due to collapsed structures, detailed estimates of energy deposition into the intergalactic medium (IGM), as well realistic prescriptions for astrophysical sources of UV and X-ray radiation. In these models, the additional heat input from DM annihilation suppresses the mean 21cm brightness temperature offset by \delta Tb ~ few-100 mK. In particular, the very deep \delta Tb ~ -150 mK absorption feature at ~20 < z < 25 predicted by popular models of the first galaxies is considerably reduced or totally erased by some of the considered DM candidates. Such an enhancement in IGM heating could come from either DM annihilations or a stronger-than-expected astrophysical component (i.e. abundant early X-ray sources). However, we find that the two signatures are not degenerate, since the DM heating is dominated by halos several orders of magnitude smaller than those hosting galaxies, whose fractional abundance evolves more slowly resulting in a smaller gradient: d \delta Tb/d \nu < 4 mK/MHz in the range \nu ~60-80 MHz. The detection of such signals by future radio telescopes would be clear evidence of DM energy injection at high-redshifts.

CRITICAL STAR FORMATION RATES FOR REIONIZATION: FULL REIONIZATION OCCURS AT REDSHIFTz≈ 7

We assess the probable redshift (z{sub rei} Almost-Equal-To 7) for full reionization of the intergalactic medium (IGM) using a prescription for the comoving star formation rate (SFR) density ({rho}-dot{sub SFR}) required to maintain photoionization against recombination. Our newly developed online reionization simulator allows users to assess the required SFR and ionization histories, using a variety of assumptions for galactic and stellar populations, IGM clumping factor and temperature, and Lyman continuum (LyC) escape fraction. The decline in high-redshift galaxy candidates and Ly{alpha} emitters at z = 6-8 suggests a rising neutral fraction, with reionization at z {approx}> 7 increasingly difficult owing to increased recombination rates and constraints from the ionizing background and LyC mean free path. The required rate is {rho}-dot{sub SFR}{approx}(.018 M{sub sun}yr{sup -1}Mpc{sup -3})[(1+z)/8]{sup 3}(C{sub H}/3)(0.2/f{sub esc})T{sub 4}{sup -0.845} scaled to fiducial values of clumping factor C{sub H} = 3, escape fraction f{sub esc} = 0.2, electron temperature T{sub e} = 10{sup 4} K, and low-metallicity initial mass functions (IMFs) and stellar atmospheres. Our hydrodynamical + N-body simulations find a mean clumping factor C{sub H} Almost-Equal-To (2.9)[(1 + z)/6]{sup -1.1} in the photoionized, photoheated filaments at z = 5-9. The critical SFR could be reduced by increasing the minimummore » stellar mass, invoking a top-heavy IMF, or systematically increasing f{sub esc} at high z. The cosmic microwave background optical depth, {tau}{sub e} = 0.088 {+-} 0.015, could be explained by full reionization, producing {tau}{sub e} = 0.050 back to z{sub rei} Almost-Equal-To 7, augmented by {Delta}{tau}{sub e} Almost-Equal-To 0.01-0.04 in a partially ionized IGM at z > 7. In this scenario, the strongest 21 cm signal should occur at redshifted frequencies 124-167 MHz owing to IGM heating over an interval {Delta}z Almost-Equal-To 3 in the range z Almost-Equal-To 7.5-10.5.« less

Leptonic dark matter annihilation in the evolving universe:constraints and implications

The cosmic electron and positron excesses have been explained aspossible dark matter (DM) annihilation products. In this work weinvestigate the possible effects of such a DM annihilation scenarioduring the evolution history of the Universe. We first calculate theextragalactic γ-ray background (EGRB), which is producedthrough the final state radiation of DM annihilation to chargedleptons and the inverse Compton scattering betweenelectrons/positrons and the cosmic microwave background. The DM haloprofile and the minimal halo mass, which are not yet well determinedfrom the current N-body simulations, are constrained by the EGRBdata from EGRET and Fermi telescopes. Then we discuss the impact ofsuch leptonic DM models on cosmic evolution, such as thereionization and heating of intergalactic medium, neutral Hydrogen21 cm signal and suppression of structure formation. We show thatthe impact on the Hydrogen 21 cm signal might show interestingsignatures of DM annihilation, but the influence on star formation isnot remarkable. Future observations of the 21 cm signals could beused to place new constraints on the properties of DM.