Aravind Natarajan

MEASUREMENT OF 21 cm BRIGHTNESS FLUCTUATIONS AT z ∼ 0.8 IN CROSS-CORRELATION

In this Letter, 21 cm intensity maps acquired at the Green Bank Telescope are cross-correlated with large-scale structure traced by galaxies in the WiggleZ Dark Energy Survey. The data span the redshift range 0.6 < z < 1 over two fields totaling {approx}41 deg. sq. and 190 hr of radio integration time. The cross-correlation constrains {Omega}{sub HI} b{sub HI} r = [0.43 {+-} 0.07(stat.) {+-} 0.04(sys.)] Multiplication-Sign 10{sup -3}, where {Omega}{sub HI} is the neutral hydrogen (H I) fraction, r is the galaxy-hydrogen correlation coefficient, and b{sub HI} is the H I bias parameter. This is the most precise constraint on neutral hydrogen density fluctuations in a challenging redshift range. Our measurement improves the previous 21 cm cross-correlation at z {approx} 0.8 both in its precision and in the range of scales probed.

DARK MATTER ANNIHILATION AND PRIMORDIAL STAR FORMATION

We investigate the effects of weakly interacting massive particle (WIMP) dark matter annihilation on the formation of Population III.1 (Pop III.1) stars. We consider the relative importance of cooling due to baryonic radiative processes and heating due to WIMP annihilation. We analyze the dark matter and gas profiles of several halos formed in cosmological-scale numerical simulations. The heating rate depends sensitively on the dark matter density profile, which we approximate with a power law , in the numerically unresolved inner regions of the halo. If we assume a self-similar structure so that αχ 1.5 as measured on the resolved scales ~1 pc, then for a fiducial WIMP mass of 100 GeV, the heating rate is typically much smaller (<10–3) than the cooling rate for densities up to n H = 1017 cm–3. In one case, where αχ = 1.65, the heating rate becomes similar to the cooling rate by a density of n H = 1015 cm–3. The dark matter density profile is expected to steepen in the central baryon-dominated region 1 pc due to adiabatic contraction, and we observe this effect, though with low resolution, in our numerical models. From these we estimate αχ 2.0. Heating now dominates cooling above n H 1014 cm–3, in agreement with the study of Spolyar, Freese, & Gondolo. We expect that this leads to the formation of an equilibrium structure with baryonic and dark matter density distributions exhibiting a flattened central core. Examining such equilibria, we find that the total luminosities due to WIMP annihilation are relatively constant and ~103 L ☉, set by the radiative luminosity of the baryonic core. We discuss the implications for Pop III.1 star formation, particularly the subsequent growth of the protostar. Even if the initial protostar fails to accumulate any additional dark matter, its contraction to the main sequence could be significantly delayed by WIMP annihilation heating, potentially raising the mass scale of Pop III.1 stars to masses 100 M ☉.

Dark matter and Higgs bosons in the MSSM

A bstractWe investigate dark matter (DM) in the context of the minimal supersymmetric extension of the standard model (MSSM). We scan through the MSSM parameter space and search for solutions that (a) are consistent with the Higgs discovery and other collider searches; (b) satisfy the flavor constraints from B physics; (c) give a DM candidate with the correct thermal relic density; and (d) are allowed by the DM direct detection experiments. For the surviving models with our parameter scan, we find the following features: (1) The DM candidate is largely a Bino-like neutralino with non-zero but less than 20% Wino and Higgsino fractions; (2) The relic density requirement clearly pins down the solutions from the Z and Higgs resonances (Z, h, H, A funnels) and co-annihilations; (3) Future direct search experiments will likely fully cover the Z, h funnel regions, and H, A funnel regions as well except for the “blind spots”; (4) Future indirect search experiments will be more sensitive to the CP-odd Higgs exchange due to its s-wave nature; (5) The branching fraction for the SM-like Higgs decay to DM can be as high as 10%, while those from heavier Higgs decays to neutralinos and charginos can be as high as 20%. We show that collider searches provide valuable information complementary to what may be obtained from direct detections and astroparticle observations. In particular, the Z- and h-funnels with a predicted low LSP mass should be accessible at future colliders. Overall, the Higgs bosons may play an essential role as the portal to the dark sector.

Dark matter annihilation and its effect on CMB and hydrogen 21 cm observations

If dark matter is made up of weakly interacting massive particles, the annihilation of these particles in halos results in energy being released, some of which is absorbed by gas, causing partial ionization and heating. Dark matter annihilation may result in partial ionization and gas heating at high redshifts, even before the formation of the first stars. It is shown that early ionization results in a transfer of power to higher multipoles in the large angle CMB polarization power spectra. Future CMB experiments may be able to place constraints on certain light dark matter models. We also investigate the effect of gas heating on the expected H21 cm power spectrum. Heating by particle annihilation results in a decrease in the amplitude of the H21 cm power spectrum as the gas temperature T becomes comparable to the CMB temperature T-gamma, and then an increase as T > T-gamma. The result is a minimum in the power spectrum at the redshift for which T approximate to T-gamma. Only certain models (low particle masses similar to 10 GeV, or favorable halo parameters) show this effect. Within these models, observations of the H21 cm power spectrum at multiple redshifts can help us obtain constraints on dark matter particle and halo properties.

A closer look at CMB constraints on WIMP dark matter

We use Cosmic Microwave Background data from the WMAP, SPT, BICEP, and QUaD experiments to obtain constraints on the dark matter particle mass m , and show that the combined data requires m > 7:6 GeV at the 95% condence level for the ! b b channel assuming s wave annihilation and a thermal cross sectionh avi = 3 10 26 cm 3 =s. We examine whether the bound on m is sensitive to 8 measurements made by galaxy cluster observations. The large uncertainty in 8 and the degeneracy with m allow only small improvements in the dark matter mass bound. Increasing the number of eective neutrino-like degrees of freedom to Ne = 3:85 improves the mass bound to m > 8:6 GeV at 95% condence, for the ! b b channel. We also study models in which dark matter halos at z < 60 reionize the Universe. We compute the Ostriker-Vishniac power resulting from partial reionization at intermediate redshifts 10 < z < 60, but nd the eect to be small. We discuss the importance of the large angle polarization as a complementary probe of dark matter annihilation. By performing Monte Carlo simulations, we show that future experiments that measure the EE power spectrum from 20 < l < 50 can exclude m 10 GeV at the 2 (3) level provided the error bars are smaller than 4 (3) cosmic variance. We show that the Planck experiment will signicantly improve our knowledge of dark matter properties.

Distinguishing standard reionization from dark matter models

The Wilkinson Microwave Anisotropy Probe (WMAP) experiment has detected reionization at the 5:5 level and has reported a mean optical depth of 0:088 0:015. A powerful probe of reionization is the large-angle EE polarization power spectrum, which is now (since the rst ve years of data from WMAP) cosmic variance limited for 2 l 6. Here we consider partial reionization caused by WIMP dark matter annihilation, and calculate the expected polarization power spectrum. We compare the dark matter models with a standard 2-step reionization theory, and examine whether the models may be distinguished using current, and future CMB observations. We consider dark matter annihilation at intermediate redshifts (z < 60) due to halos, as well as annihilation at higher redshifts due to free particles. In order to study the eect of high redshift dark

Effect of early dark matter halos on reionization

The annihilation of dark matter particles releases energy, ionizing some of the gas in the Universe. We investigate the effect of dark matter halos on reionization. We show that the effect depends on the assumed density profile, the particle mass, and the assumed minimum halo mass. For Navarro-Frenk-White halos and typical WIMPs, we find the effect to be quite small. However, light dark matter candidates in the MeV range can contribute significantly to reionization and can make an important contribution to the measured optical depth. This effect may be used to constrain light dark matter models. We also study the effect of varying the halo density profile on reionization.

Probing dark matter streams with CoGeNT

We examine the future sensitivity of CoGeNT to the presence of dark matter streams and find that consideration of streams in the data may lead to differences in the interpretation of the results. W ...

Does the second caustic ring of dark matter cause the Monoceros Ring of stars

Caustic rings of dark matter were predicted to exist in the plane of the Galaxy at radii a n ≃ 40 kpc /n for n = 1, 2, 3,.... The recently discovered Monoceros Ring of stars is located near the n = 2 caustic, prompting us to consider a possible connection between these two objects. We identify two processes through which the Monoceros Ring of stars may have formed. One process is the migration of gas to an angular velocity minimum at the caustic leading to enhanced star formation there. The other is the adiabatic deformation of star orbits as the caustic slowly grows in mass and radius. The second process predicts an order 100% enhancement of the density of disk stars at the location of the caustic ring.

Further look at particle annihilation in dark matter caustics

Dark matter caustics are small scale, high density structures believed to exist in galaxies like ours. If the dark matter consists of weakly interacting massive particles, these caustics may be detected by means of the gamma rays produced by dark matter particle annihilation. We discuss particle annihilation in outer and inner caustics and provide sky maps of the expected gamma ray distribution.