Impact of Dark Matter Annihilation on the High-Redshift Intergalactic Medium

We reexamine the impact of dark matter (DM) annihilation on the intergalactic medium, taking into account the clumping of DM particles. We find that energy injection from the annihilation of the thermal relic DM particles may significantly raise the gas temperature at high redshifts and leave a strong imprint on the cosmological 21-cm signal, provided the particle mass is below ~1 TeV. Further, we find that while the energy injection from DM annihilation could not alone complete the reionization of the Universe, it could make a significant contribution to the electron optical depth.Comment: 4 pages, 5 figures, ApJL, in prin

Radiative Transfer Effect on Ultraviolet Pumping of the 21cm Line in the High Redshift Universe

During the epoch of reionization the 21cm signal is sensitive to the scattering rate of the ultraviolet photons, redshifting across the Lyman_alpha resonance. Here we calculate the photon scattering rate profile for a single ultraviolet source. After taking into account previously neglected natural broadening of the resonance line, we find that photons approach the resonance frequency and experience most scatterings at a significantly smaller distance from the source than naively expected r=(dnu/nu_0)(c/H), where dnu=nu-nu_0 is the initial frequency offset, and the discrepancy increases as the initial frequency offset decreases. As a consequence, the scattering rate P(r) drops much faster with increasing distance than the previously assumed 1/r^2 profile. Near the source (r<1Mpc comoving), the scattering rate of photons that redshift into the Ly_alpha resonance converges to P(r) \propto r^{-7/3}. The scattering rate of Ly_alpha photons produced by splitting of photons that redshift into a higher resonance (Ly_gamma, Ly_delta, etc.) is only weakly affected by the radiative transfer, while the sum of scattering rates of Ly_alpha photons produced from all higher resonances also converges to P(r) \propto r^{-7/3} near the source. At 15<z<35, on scales of ~0.01-20Mpc/h (comoving), the total scattering rate of Ly_alpha photons from all Lyman resonances is found to be higher by a factor of ~1+0.3[(1+z)/20]^{2/3} than obtained without full radiative transfer. Consequently, during the early stage of reionization, the differential brightness of 21cm signal against the cosmic microwave background is also boosted by a similar factor.Comment: 7 pages, 4 figures, submitted to Ap

Recognizing the First Radiation Sources Through Their 21-cm Signature

At the beginning of the reionization epoch, radiation sources produce fluctuations in the redshifted 21-cm background. We show that different types of sources (such as miniquasars, Pop II and III stars, supernovae, etc.) produce distinct signatures in the 21-cm signal radial profiles and statistical fluctuations, through which they can be identified. Further, we show that the 21-cm signal from X-ray emitting sources is much easier to observe than was expected, due to a previously neglected pumping mechanism.Comment: 4 pages, 4 figures, accepted by ApJ

Heating and cooling of the intergalactic medium by resonance photons

During the epoch of reionization a large number of photons were produced with frequencies below the hydrogen Lyman limit. After redshifting into the closest resonance, these photons underwent multiple scatterings with atoms. We examine the effect of these scatterings on the temperature of the neutral intergalactic medium (IGM). Continuum photons, emitted between the Ly_alpha and Ly_gamma frequencies, heat the gas after being redshifted into the H Ly_alpha or D Ly_beta resonance. By contrast, photons emitted between the Ly_gamma and Ly-limit frequencies, produce effective cooling of the gas. Prior to reionization, the equilibrium temperature of ~100 K for hydrogen and helium atoms is set by these two competing processes. At the same time, Ly_beta resonance photons thermally decouple deuterium from other species, raising its temperature as high as 10^4 K. Our results have important consequences for the cosmic 21-cm background and the entropy floor of the early IGM which can affect star formation and reionization.Comment: 4 pages, 4 figures, ApJ, in pres

Consequences of short range interactions between dark matter and protons in galaxy clusters

Protons gain energy in short range collisions with heavier dark matter particles (DMPs) of comparable velocity dispersion. We examine the conditions under which the heating of baryons by scattering off DMPs can offset radiative cooling in the cores of galaxy clusters. Collisions with a constant cross section independent of the relative velocity of the colliding particles, cannot produce stable thermal balance. In this case, avoiding an unrealistic increase of the central temperatures yields the upper bound on the cross-section, \sigma_xp<10^-25 cm^2 (m_x/m_p), where m_x and m_p are the DMP and proton mass, respectively. A stable balance, however, can be achieved for a power law dependence on the relative velocity, V, of the form \sigma_xp \propto V^a with a<-3. An advantage of this heating mechanism is that it preserves the metal gradients observed in clusters.Comment: 7 pages, new calculations include

Element segregation in giant galaxies and X-ray clusters

We examine the process of element segregation by gravity in giant elliptical galaxies and X-ray clusters. We solve the full set of flow equations developed by Burgers for a multi-component fluid, under the assumption that magnetic fields slow the diffusion by a constant factor F_B. Compared to the previous calculations that neglected the residual heat flow terms, we find that diffusion is faster by ~20%. In clusters we find that the diffusion changes the local abundance typically by factors of 1+0.3(T/10^8 K)^1.5/F_B and 1+0.15(T/10^8 K)^1.5/F_B for helium and heavy elements, respectively, where T is the gas temperature. In elliptical galaxies, the corresponding factors are 1+0.2(T/10^7 K)^1.5/F_B and 1+0.1(T/10^7 K)^1.5/F_B, respectively. If the suppression factor F_B is modest, diffusion could significantly affect observational properties of hot X-ray clusters and cD galaxies. In particular, diffusion steepens the baryon distribution, increases the total X-ray luminosity, and changes the spectrum and evolution of stars that form out of the helium-rich gas. Detection of these diffusion signatures would allow to gauge the significance of the magnetic fields, which also inhibit thermal heat conduction as a mechanism for eliminating cooling flows in the same environments.Comment: Accepted for publication in MNRAS. Minor changes. (5 pages, 6 figures