Browsing by Subject "cosmology : theory"

Now showing 1 - 8 of 8

Cosmic Microwave Background-Weak Lensing Correlation: Analytical and Numerical Study of Nonlinearity and Implications for Dark Energy
(2008-04) Nishizawa, Atushi J.; Komatsu, Eiichiro; Yoshida, Naoki; Takahashi, Ryuichi; Sugiyama, Naoshi; Komatsu, Eiichiro
Evolution of density fluctuations yields secondary anisotropies in the cosmic microwave background ( CMB), which are correlated with the same density fluctuations that can be measured by weak lensing (WL) surveys. We study the CMB-WL correlation induced by the integrated Sachs-Wolfe (ISW) effect and its nonlinear extension, the Rees-Sciama (RS) effect, using analytical models as well as N-body simulations. We show that an analytical model based on the time derivative of matter power spectrum agrees with simulations. All-sky cosmic-variance-limited CMB and WL surveys allow us to measure the correlation from the nonlinear RS effect with high significance (50 sigma) for l(max) = 10(4) whereas forthcoming missions such as Planck and LSST are expected to yield 4 l p 10 1.5 sigma detections, on the assumption of that the point-source contributions are negligible. We find that the CMB-WL correlation has a characteristic scale which is sensitive to the nature of dark energy.
The Cosmic Near Infrared Background: Remnant Light From Early Stars
(2008-03) Fernandez, E. R.; Komatsu, E.; Fernandez, Elizabeth R.; Komatsu, Eiichiro
The redshifted ultraviolet light from early stars at z similar to 10 contributes to the cosmic near infrared background (NIRB). We present detailed calculations of its spectrum with various assumptions about metallicity and mass spectrum of the early stars. We show that if the NIRB has a stellar origin, metal-free stars are not the only explanation of the excess NIRB; stars with significant metals (e.g., Z=1/50 Z(circle dot)) can produce the same amount of background intensity as metal-free stars. This is because the average intensity at 1-2 microns is determined by the efficiency of nuclear burning in stars, which is not very sensitive to metallicity. We predict v*Iv/(rho) over dot*similar to 4 - 8nW m(-2) sr(-1), where (rho) over dot* is the mean star formation rate at z=7-15 (solar masses per year per cubic megaparsec) for stars more massive than 5 solar masses. On the other hand, since we have very little knowledge about the form of the mass spectrum of early stars, the uncertainty in the average intensity due to the mass spectrum could be large. An accurate determination of the near-infrared background allows us to probe the formation history of early stars, which is difficult to constrain by other means. While the star formation rate at z=7-15 inferred from the current data is significantly higher than the local rate at z<5, it does not rule out the stellar origin of the cosmic near-infrared background. In addition, we show that a reasonable initial mass function, coupled with this star formation rate, does not overproduce metals in the universe in most cases and may produce as little as less than 1% of the metals observed in the universe today.
The First Supernova Explosions: Energetics, Feedback, And Chemical Enrichment
(2007-11) Greif, Thomas H.; Johnson, Jarrett L.; Bromm, Volker; Klessen, Ralf S.; Greif, Thomas H.; Johnson, Jarrett L.; Bromm, Volker
We perform three-dimensional smoothed particle hydrodynamics simulations in a realistic cosmological setting to investigate the expansion, feedback, and chemical enrichment properties of a 200M(circle dot) pair-instability supernova in the high-redshift universe. We find that the SN remnant propagates for a Hubble time at z similar or equal to 20 to a final mass-weighted mean shock radius of 2.5 kpc (proper), roughly half the size of the H II region, and in this process sweeps up a total gas mass of 2: 5; 10(5) M-circle dot. The morphology of the shock becomes highly anisotropic once it leaves the host halo and encounters filaments and neighboring minihalos, while the bulk of the shock propagates into the voids of the intergalactic medium. The SN entirely disrupts the host halo and terminates further star formation for at least 200 Myr, while in our specific case it exerts positive mechanical feedback on neighboring minihalos by shock-compressing their cores. In contrast, we do not observe secondary star formation in the dense shell via gravitational fragmentation, due to the previous photoheating by the progenitor star. We find that cooling by metal lines is unimportant for the entire evolution of the SN remnant, while the metal-enriched, interior bubble expands adiabatically into the cavities created by the shock, and ultimately into the voids with a maximum extent similar to the final mass-weighted mean shock radius. Finally, we conclude that dark matter halos of at least M-vir greater than or similar to 10(8) M-circle dot must be assembled to recollect all components of the swept-up gas.
GRB Cosmology: Probing The Early Universe
(2007-10) Bromm, V.; Loeb, A.; Bromm, V.
Current observations are about to open up a direct observational window into the final frontier of cosmology: the crucial first billion years in cosmic history when the first stars and galaxies formed. Even before the launch of the James Webb Space Telescope, it would be possible to utilize Gamma-Ray Bursts (GRBs) as unique probes of cosmic star formation and the state of the intergalactic medium up to redshifts of the first stars. The ongoing Swift mission might be the first observatory to detect individual Population III stars, provided that massive metal-free stars were able to trigger GRBs. Swift will empirically constrain the redshift at which Population III star formation was terminated, thus providing crucial input to models of cosmic reionization and metal enrichment.
Mass-To-Light Ratio Of Ly Alpha Emitters: Implications Of Ly Alpha Surveys At Redshifts Z = 5.7, 6.5, 7, And 8.8
(2008-03) Fernandez, E. R.; Komatsu, E.; Fernandez, Elizabeth R.; Komatsu, Eiichiro
We present a simple, relatively model-independent method to interpret the galaxy number count data at z > 6. The only free parameter is a mass-to->observed light> ratio, M-h/L-band, where M-h refers to the total mass of the host halo, and L-band refers to the observed luminosity of the source. For narrow-band surveys, L-band is simply related to the intrinsic Ly alpha luminosity with a survival fraction of Ly alpha photons, alpha(esc). The mass-to->bolometric light>, M-h/L-bol, can also be found, once the metallicity and initial mass function of stellar populations are given. We find constraints on the mass-to-light ratio of Ly-alpha emitters from 5.7 < z < 8.8 of (M-h/L-bol) (alpha(esc)epsilon(1)/gamma)(-1) = 21 - 38, 14 - 26, and 9 - 17 for Z = 0, 1150, and 1 2 D, respectively, where E is the duty cycle and 7 - 2 is the local shape of the cumulative luminosity function. Therefore, Lya emitters are consistent with either starburst galaxies (M-h/L-bol similar to 0.1 - 1) for reasonable values of the Lya survival fraction, alpha(esc)epsilon(1)/gamma similar to 0.01 -0.05, or normal populations (M-h/L-bol similar to 10) if a good fraction of Ly alpha photons survived. We find no evidence for the end of reionization from current survey observations. The data are consistent with no evolution of intrinsic properties of Lya emitters or neutral fraction in the intergalactic medium. We also show that the lack of detections at z = 8.8 does not rule out the high-z galaxies being the origin of the excess near infrared background.
Radiative Transfer Effect On Ultraviolet Pumping Of The 21 cm Line In The High-Redshift Universe
(2007-12) Chuzhoy, Leonid; Zheng, Zheng; Chuzhoy, Leonid
During the epoch of reionization the 21 cm signal is sensitive to the scattering rate of the ultraviolet photons, redshifting across the Ly 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 = (Delta nu/nu(0))(c/H), where Delta nu = 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-alpha(r) drops much faster with increasing distance than the previously assumed 1/r(2) profile. Near the source, (r less than or similar to 1 comoving Mpc), the scattering rate of photons that redshift into the Ly alpha resonance converges to P-alpha(r) proportional to 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-alpha(r) proportional to r(-7/3) near the source. At 15 < z < 35, on scales of similar to 0.01-20 h(-1) Mpc (comoving), the total scattering rate of Ly alpha photons from all Lyman resonances is found to be higher by a factor of similar to 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 21 cm signal against the cosmic microwave background is also boosted by a similar factor.
The Theory And Simulation Of The 21-Cm Background From The Epoch Of Reionization
(2008-08) Shapiro, P. R.; Iliev, I. T.; Mellerna, G.; Pen, U. L.; Merz, H.; Shapiro, Paul R.
The redshifted 21-cm line of distant neutral H atoms provides a probe of the cosmic >dark ages> and the epoch of reionization (>EOR>) which ended them, within die first billion years of cosmic time. The radio continuum produced by this redshifted line can be seen in absorption or emission against die cosmic microwave background (>CMB>) at meterwaves, yielding information about tire thermal and ionization history of the universe and the primordial density perturbation spectrum that led to galaxy and large-scale structure formation. Observing this 21-cm background is a great challenge,as it is necessary to detect a diffuse signal at a brightness temperature that differs from that of die CM B at millikelvin levels and distinguish this from foreground continuum sources. A new generation of low-frequency radio arrays is currently under development to search for this background. Accurate theoretical predictions of tire spectrum and anisotropy of this back-ground, necessary to guide and interpret future observations, are also quite challenging. Toward this end, it is necessary to model the inhomogeneous reionization of the intergalactic medium and determine the spin temperature of the 21-cm transition and its variations in time and space as it decouples from the temperature of the CMB, In my talk, I summarized some of the theoretical progress in this area. Here, I will focus on just a few of the predictions for the 21-cm background from the EOR, based on our newest, large-scale simulations of patchy reionization. These simulations are the first with enough N-body particles (from 5 to 29 billion) and radiative transfer rays to resolve the formation of and trace the ionizing radiation front each of the millions of dwarf galaxies believed responsible for reionization, down to 10(8) M-circle dot, in a cubic volume large enough (90 and 163 comoving Mpc on a side) to make meaningful statistical predictions of the fluctuating 21-cm background.
Uncovering The Chemical Signature Of The First Stars In The Universe
(2008-05) Karlsson, Torgny; Johnson, Jarrett L.; Bromm, Volker; Johnson, Jarrett L.; Bromm, Volker
The chemical abundance patterns observed in metal-poor Galactic halo stars contain the signature of the first supernovae, and thus allow us to probe the first stars that formed in the universe. We construct a theoretical model for the early chemical enrichment history of the Milky Way, aiming in particular at the contribution from pair-instability supernovae (PISNe). These are a natural consequence of current theoretical models for primordial star formation at the highest masses. However, no metal-poor star displaying the distinct PISN signature has yet been observed. We here argue that this apparent absence of any PISN signature is due to an observational selection effect. Whereas most surveys traditionally focus on the most metal-poor stars, we predict that early PISN enrichment tends to "overshoot,'' reaching enrichment levels of [Ca/H] similar or equal to -2.5 that would be missed by current searches. We utilize existing observational data to place constraints on the primordial initial mass function (IMF). The number fraction of PISNe in the primordial stellar population is estimated to be < 0.07, or <= 40% by mass, assuming that metal-free stars have masses in excess of 10 M-circle dot. We further predict, based on theoretical estimates for the relative number of PISNe, that the expected fraction of second-generation stars below [Ca/H] = -2 with a dominant (i. e., > 90%) contribution from PISNe is merely similar to 10(-4) to 5 x 10(-4). The corresponding fraction of stars formed from gas exclusively enriched by PISNe is a factor of similar to 4 smaller. With the advent of next-generation telescopes and new, deeper surveys, we should be able to test these predictions.