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ABSTRACT: We present a semianalytic model for the thermal and ionization history of the universe at 1000 z 3. This model incorporates much of the essential physics included in full-scale hydrodynamical simulations, such as (1) gravitational collapse and virialization; (2) star/quasar formation and subsequent ionizing radiation; (3) heating and cooling; (4) atomic and molecular physics of hydrogen; and (5) the feedback relationships between these processes. In addition, we model the process of reheating and reionization using two separate phases, self-consistently calculating the filling factor of each phase. Thus, radiative transfer is treated more accurately than simulations published to date have done: we allow to lowest order for the inhomogeneity of the sources and the sinks of radiation. After calibrating and checking the results of this model against a hydrodynamical simulation, we apply our model to a variety of Gaussian (adiabatic power spectra) and non-Gaussian (texture and isocurvature) cold dark matter (CDM)-dominated cosmologies normalized to cluster abundances. Our model is also normalized to observations of the ionizing UV intensity J21 ≈ 1 at redshift z = 4. Our major conclusions include: (1) the epoch of reheating (starting late at z ~ 30 or early at z ~ 80) depends most strongly on the power spectrum (late: adiabatic; early: texture or isocurvature); (2) because of the effects of gas clumping, full reionization occurs at z ~ 10 in all models; (3) the cosmic microwave background radiation (CMBR) polarization anisotropy will be a strong discriminant between late and early reheating models; (4) the fraction of baryons sequestered in stars and quasars in early reheating models appears to be greater than the observational limit, while the fraction in late reheating models is well below it; (5) the average degree of nonlinearity for collapsing baryons remains roughly constant during reheating, a possible explanation of which is feedback, which regulates the pace of reheating through the Jeans criterion; and (6) the evolution of the bias of luminous objects can potentially discriminate strongly between Gaussian and non-Gaussian probability density functions.
The Astrophysical Journal 12/2008; 534(2):507. · 6.02 Impact Factor
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ABSTRACT: We investigate the effect of non-evaporating primordial black holes (PBHs) on the ionization and thermal history of the universe. X-rays emitted by gas accretion onto PBHs modify the cosmic recombination history, producing measurable effects on the spectrum and anisotropies of the Cosmic Microwave Background (CMB). Using the third-year WMAP data and FIRAS data we improve existing upper limits on the abundance of PBHs with masses >0.1 Msun by several orders of magnitude. Fitting WMAP3 data with cosmological models that do not allow for non-standard recombination histories, as produced by PBHs or other early energy sources, may lead to an underestimate of the best-fit values of the amplitude of linear density fluctuations (sigma_8) and the scalar spectral index (n_s). Cosmological parameter estimates are affected because models with PBHs allow for larger values of the Thomson scattering optical depth, whose correlation with other parameters may not be correctly taken into account when PBHs are ignored. Values of tau_e=0.2, n_s=1 and sigma_8=0.9 are allowed at 95% CF. This result that may relieve recent tension between WMAP3 data and clusters data on the value of sigma_8. PBHs may increase the primordial molecular hydrogen abundance by up to two orders of magnitude, this promoting cooling and star formation. The suppression of galaxy formation due to X-ray heating is negligible for models consistent with the CMB data. Thus, the formation rate of the first galaxies and stars would be enhanced by a population of PBHs. Comment: 17 pages (Apj style), 9 figures, submitted to ApJ
09/2007;
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ABSTRACT: We discuss the possibilities for primordial black holes (PBHs) to grow via
the accretion of dark matter. In agreement with previous works, we find that
accretion during the radiation-dominated era does not lead to a significant
mass increase. However, during matter-domination, PBHs may grow by up to two
orders of magnitude in mass through the acquisition of large dark matter halos.
We discuss the possibility of PBHs being an important component in dark matter
halos of galaxies as well as their potential to explain the ultra-luminous
x-ray sources (ULXs) observed in nearby galactic disks. We point out that
although PBHs are ruled out as the dominant component of dark matter, there is
still a great deal of parameter space open to them playing a role in the
modern-day universe. For example, a primordial halo population of PBHs each at
$10^{2.5} M_\odot$ making up 0.1% of the dark matter grow to $10^{4.5} M_\odot$
via the accumulation of dark matter halos to account for $\sim 10%$ of the dark
matter mass by a redshift of $z \approx 30$. These intermediate mass black
holes may then ``light up'' when passing through molecular clouds, becoming
visible as ULXs at the present day, or they may form the seeds for supermassive
black holes at the centers of galaxies.
08/2006;
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ABSTRACT: We use numerical simulations of a cosmological volume to study the X-ray ionization and heating of the intergalactic medium (IGM) by an early population of accreting black holes (BHs). By considering theoretical limits on the accretion rate and observational constraints from the X-ray background and faint X-ray source counts, we find that the maximum value of the optical depth to Thompson scattering which can be produced using these models is τe≃ 0.17, in agreement with previous semi-analytic results. The redshifted soft X-ray background produced by these early sources is important in producing a fully ionized atomic hydrogen in the low-density intergalactic medium before stellar reionization at redshift z∼ 6–7. As a result, stellar re-ionization is characterized by an almost instantaneous ‘overlap phase’ of H ii regions. The background also produces a second He ii re-ionization at about redshift 3 and maintains the temperature of the IGM at about 10 000 K even at low redshifts.If the spectral energy distribution of these sources has a non-negligible high-energy power-law component, the luminosity in the soft X-ray band of the ‘typical’ galaxies hosting intermediate-mass accreting BHs is maximum at z∼ 15 and is about one or two orders of magnitude below the sensitivity limit of the Chandra Deep Field. We find that about a thousand of these sources may be present per square arcmin of the sky, producing potentially detectable fluctuations. We also estimate that a few rare objects, not present in our small simulated volume, could be luminous enough to be visible in the Chandra Deep Field. The XEUS and Constellation-X satellites will be able to detect more of these sources that, if radio loud, could be used to study the 21-cm forest in absorption.A signature of an early X-ray pre-ionization is the production of secondary cosmic microwave background (CMB) anisotropies on small angular scales (<1 arcmin). We find that in these models the power spectrum of temperature fluctuations increases with decreasing angular scale (ΔT∼ 16 μK at ∼1-arcsec scales), while for stellar re-ionization scenarios the power decreases on smaller scales. We also show that the redshifted 21-cm radiation from neutral hydrogen can be marginally detected in emission at redshifts 7 < z < 12. At a redshift of about z∼ 30 a stronger and narrower (in redshift space) signal in absorption against the CMB, that is peculiar to these models, could be detectable.
Monthly Notices of the Royal Astronomical Society 01/2005; 357(1):207 - 219. · 4.90 Impact Factor
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ABSTRACT: In this paper we investigate the possibility that there is a first phase of partial ionization due to X-rays produced by black hole (BH) accretion in small-mass galaxies at redshifts 7 < z < 20. This is followed by complete reionization by stellar sources at z≃ 7. This scenario is motivated by the large optical depth to Thomson scattering, τe≃ 0.17 ± 0.04, recently measured by the Wilkinson Microwave Anisotropy Probe (WMAP). However, it is also consistent with the observed Gunn–Peterson trough in the spectra of quasars at z∼ 5–6. We use a semi-analytic code to explore models with different BH accretion histories and cosmological parameters. We find that ‘pre-ionization’ by X-rays can increase the intergalactic medium (IGM) optical depth from τe≈ 0.06 given by stellar sources only, to 0.1 ≲τe≲ 0.2, if a fraction of baryons 10−5≲ωac≲ 10−4 is accreted on to seed BHs produced in the collapse of low-metallicity, high-mass stars before z≃ 15. To be effective, pre-ionization requires a non-negligible star formation in the first small-mass galaxies in which seed BHs are formed. By z∼ 20–25 the IGM is reheated to 10 000 K and the ionization fraction is about 20 per cent. The increase of the IGM Jeans mass is effective in reducing star formation in the smaller mass haloes. Large values of τe are obtained in models with top-heavy stellar initial mass function only if pair-instability supernovae (SNe) are not important. Seed BHs are assumed to accrete at near the Eddington limit with a duty cycle that decreases slowly with increasing time. Alternatively, a moderate fraction of the black holes must be ejected from the host galaxy or exist without merging into the supermassive BHs in galactic centres. The model predicts that dwarf spheroidal galaxies, if they are preserved fossils of the first galaxies, may host a mass in BHs that is 5–40 per cent of their stellar mass. The redshifted X-ray background produced by this early epoch of BH accretion constitutes about 5–10 per cent of the hard X-ray background in the 2–50 keV bands and roughly half of the currently estimated BH mass density was formed at early times. Moreover, in most models, the photons from the redshifted background are sufficient to fully reionize He ii at redshift z∼ 3 without any additional contribution from quasars at lower redshifts and the temperature of the mean density IGM remains close to 104 K down to redshift z∼ 1.
Monthly Notices of the Royal Astronomical Society 07/2004; 352(2):547 - 562. · 4.90 Impact Factor
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ABSTRACT: We present constraints on cosmological and star formation parameters based on combining observations of the Wilkinson Microwave Anisotropy Probe (WMAP) and high-redshift quasars from the Sloan Digital Sky Survey (SDSS). We use a semi-analytic model for reionization (Chiu and Ostriker 2000) that takes into account a number of important physical processes both within collapsing halos and in the intergalactic medium. Assuming that the efficiency of producing UV photons per baryon is constant, we derive a constraint of the form sigma_8 Omega_0^0.5~0.33 in a flat, Lambda-dominated universe with h=0.72, n=0.99, and Omega_b h^2=0.024. However, the calculated optical depth to electron scattering of tau_es~0.06 is well below the value found by WMAP of 0.17+/-(0.04~0.07) (Spergel et al 2003). Since the WMAP constraints on tau_es are somewhat degenerate with the value of the spectral index n, we then permit the primordial spectral index n to float and fix Omega_0 h^2=0.14, while normalizing the power spectrum using WMAP. In addition, we allow the UV-efficiency to have time-dependence. Combining the WMAP constraints with the quasar transmission data, our analysis then favors a model with tau_es=0.11^{+0.02}_{-0.03}, n=0.96^{+0.02}_{-0.03}$, implying sigma_8=0.83^{+0.03}_{-0.05} (95% confidence), and an effective UV-efficiency that was at least ~10x greater at z >> 6. These results indicate that the quasar and WMAP observations are consistent. If future observations confirm an optical depth to electron scattering tau_es~0.1, then it would appear that no more "exotic" sources of UV-photons, such as mini-quasars or AGNs, are necessary; but our analysis indicates that a determination of tau_es>~0.17 would require a more radical solution. Comment: 31 pages, 7 figures, 5 tables
04/2003;
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ABSTRACT: It is well known that the mass function for_halos_ in CDM cosmology is a relatively steep power law for low masses, possibly too steep to be consistent with observations. But how steep is the_galaxy_ mass function? We have analyzed the stellar and gas mass functions of the first massive luminous objects formed in a \Lambda CDM universe, as calculated in the numerical simulation described in Gnedin (2000ab). We found that while the dark matter mass function is steep, the stellar and gas mass functions are flatter for low mass objects. The stellar mass function is consistently flat at the low mass end. Moreover, while the gas mass function follows the dark matter mass function until reionization at z~7, between z=7 and z=4, the gas mass function also flattens considerably at the low mass end. At z=4, the gas and stellar mass functions are fit by a Schechter function with \alpha ~ -1.2 +/- 0.1, significantly shallower than the dark matter halo mass function and consistent with some recent observations. The baryonic mass functions are shallower because (a) the dark matter halo mass function is consistent with the Press-Schechter formulation at low masses n(M) M^-2 and (b) heating/cooling and ionization processes appear to cause baryons to collect in halos with the relationship M_b M_d^4 at low masses. Combining (a) and (b) gives n(M_b) M_b^-5/4, comparable to the simulation results. Thus, the well known observational fact that low mass galaxies are underabundant as compared to expectations from numerical dark matter simulations or Press-Schechter modeling of CDM universes emerges naturally from these results, implying that perhaps no ``new physics'' beyond the standard model is needed. Comment: Submitted to ApJ, 17 pages including 6 figures
03/2001;
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ABSTRACT: We use cosmological hydrodynamic simulations to investigate formation of galactic bulges within the framework of hierarchical clustering in a representative CDM cosmological model. We show that largest objects forming at cosmological redshifts z~4 resemble observed bulges of spiral galaxies or moderate size ellipticals in their general properties like sizes, shapes, and density profiles. This is consistent with observational data indicating the existence of ``old'' bulges and ellipticals at more moderate redshifts. These bulges are gas dominated at redshift z=3, with high rates of star formation and would appear to be good candidates for small blue galaxies seen in the Hubble Deep Field. Comment: Revised version accepted for publication in ApJ
12/1999;
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ABSTRACT: We simulate a plausible cosmological model in considerable physical and numerical detail through the successive phases of reheating (at 10<z<20), formation of Pop III stars at z=15 (due to molecular hydrogen cooling), with subsequent reionization at z=7. We assume an efficiency of high mass star formation appropriate to leave the universe, after it becomes transparent, with an ionizing background J_21=0.4 (at z=4), near (and perhaps slightly below) the observed value. Since the same stars produce the ionizing radiation and the first generation of heavy elements, a mean metallicity of Z/Z_\sun=1/200 is produced in this early phase, but there is a large variation about this mean, with the high density regions having Z/Z_\sun=1/30 and low density regions essentially no metals. Reionization, when it occurs, is very rapid, which will leave a signature which may be detectable by very large area meter-wavelength radio instruments. Also, the background UV radiation field will show a sharp drop from 1Ryd to 4Ryd due to absorption edges. The simulated volume is too small to form L_* galaxies, but the smaller objects which are found in the simulation obey the Faber-Jackson relation. In order to explore theoretically this domain of "the end of the dark ages" quantitatively, numerical simulations must have a mass resolution of the order of 10^{4.5} M_\sun} in baryons, high spatial resolution (1 kpc) to resolve strong clumping, and allow for detailed and accurate treatment of both the radiation field and atomic/molecular physics. Comment: submitted to ApJ
12/1996;
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ABSTRACT: We note that current observational evidence strongly favors a conventional recombination of ionized matter subsequent to redshift z=1200, followed by reionization prior to redshift z=5 and compute how this would have occurred in a standard scenario for the growth of structure. Extending prior semi-analytic work, we show by direct, high-resolution numerical simulations (of a COBE normalized CDM+Lambda model) that reheating, will occur in the interval 15>z>7, followed by reionization and accompanied by a significant increase in the Jeans mass. However, the evolution of the Jeans mass does not significantly affect star formation in dense, self-shielded clumps of gas, which are detached from the thermal evolution of the rest of the universe. On average, the growth of the Jeans mass tracks the growth of the nonlinear mass scale, a result we suspect is due to nonlinear feedback effects. Cooling on molecular hydrogen leads to a burst of star formation prior to reheating which produces Population III stars with Omega_* reaching 10^{-5.5} and Z/Z_sun reaching 10^{-3.7} by z=14. Star formation subsequently slows down as molecular hydrogen is depleted by photo-destruction and the rise of the temperature. At later times, z<10, when the characteristic virial temperature of gas clumps reach 10,000 degrees, star formation increases again as hydrogen line cooling become efficient. Objects containing Pop III stars accrete mass with time and, as soon as they reach 10,000 K virial temperature, they engage in renewed star formation and turn into normal Pop II objects having an old Pop III metal poor component. Comment: six postscript figures included, submitted to ApJL
08/1996;
