A Test of the Collisional Dark Matter Hypothesis from Cluster Lensing

Spergel & Steinhardt proposed the possibility that the dark matter particles are self-interacting, as a solution to two discrepancies between the predictions of cold dark matter models and the observations: first, the observed dark matter distribution in some dwarf galaxies has large, constant-density cores, as opposed to the predicted central cusps; and second, small satellites of normal galaxies are much less abundant than predicted. The dark matter self-interaction would produce isothermal cores in halos, and would also expel the dark matter particles from dwarfs orbiting within large halos. However, another inevitable consequence of the model is that halos should become spherical once most particles have interacted. Here, I rule out this model by the fact that the innermost regions of dark matter halos in massive clusters of galaxies are elliptical, as shown by gravitational lensing and other observations. The absence of collisions in the lensing cores of massive clusters implies that any dark matter self-interaction is too weak to have affected the observed density profiles in the dark-matter dominated dwarf galaxies, or to have eased the destruction of dwarf satellites in galactic halos. If $s_x$ is the cross section and $m_x$ the mass of the dark matter particle, then s_x/m_x < 10^{-25.5} \cm^2/\gev.Comment: to appear in ApJ, January 1 200

Evolution of Structure in the Intergalactic Medium and the Nature of the Ly-alpha Forest

We have performed a detailed statistical study of the evolution of structure in a photoionized intergalactic medium (IGM) using analytical simulations to extend the calculation into the mildly non-linear density regime found to prevail at z = 3. Our work is based on a simple fundamental conjecture: that the probability distribution function of the density of baryonic diffuse matter in the universe is described by a lognormal (LN) random field. The LN field has several attractive features and follows plausibly from the assumption of initial linear Gaussian density and velocity fluctuations at arbitrarily early times. Starting with a suitably normalized power spectrum of primordial fluc- tuations in a universe dominated by cold dark matter (CDM), we compute the behavior of the baryonic matter, which moves slowly toward minima in the dark matter potential on scales larger than the Jeans length. We have computed two models that succeed in matching observations. One is a non-standard CDM model with Omega=1, h=0.5 and \Gamma=0.3, and the other is a low density flat model with a cosmological constant(LCDM), with Omega=0.4, Omega_Lambda=0.6 and h=.65. In both models, the variance of the density distribution function grows with time, reaching unity at about z=4, where the simulation yields spectra that closely resemble the Ly-alpha forest absorption seen in the spectra of high z quasars. The calculations also successfully predict the observed properties of the Ly-alpha forest clouds and their evolution from z=4 down to at least z=2, assuming a constant intensity for the metagalactic UV background over this redshift range. However, in our model the forest is not due to discrete clouds, but rather to fluctuations in a continuous intergalactic medium. (This is an abreviated abstract; the complete abstract is included with the manuscript.)Comment: Wrong Fig. 10 is corrected. Our custom made postscript is available at ftp://hut4.pha.jhu.edu/incoming/igm, or contact Arthur Davidsen ([email protected]) for nice hardcopies; accepted for publication in Ap

Probing the Slope of Cluster Mass Profile with Gravitational Einstein Rings: Application to Abell 1689

The strong lensing modelling of gravitational ``rings'' formed around massive galaxies is sensitive to the amplitude of the external shear and convergence produced by nearby mass condensations. In current wide field surveys, it is now possible to find out a large number of rings, typically 10 gravitational rings per square degree. We propose here, to systematically study gravitational rings around galaxy clusters to probe the cluster mass profile beyond the cluster strong lensing regions. For cluster of galaxies with multiple arc systems, we show that rings found at various distances from the cluster centre can improve the modelling by constraining the slope of the cluster mass profile. We outline the principle of the method with simple numerical simulations and we apply it to 3 rings discovered recently in Abell~1689. In particular, the lens modelling of the 3 rings confirms that the cluster is bimodal, and favours a slope of the mass profile steeper than isothermal at a cluster radius \sim 300 \kpc. These results are compared with previous lens modelling of Abell~1689 including weak lensing analysis. Because of the difficulty arising from the complex mass distribution in Abell~1689, we argue that the ring method will be better implemented on simpler and relaxed clusters.Comment: Accepted for publication in MNRAS. Substantial modification after referee's repor

How neutral is the intergalactic medium surrounding the redshift z=7.085 quasar ULAS J1120+0641?

The quasar ULAS J1120+0641 at redshift z=7.085 has a highly ionised near zone which is smaller than those around quasars of similar luminosity at z~6. The spectrum also exhibits evidence for a damping wing extending redward of the systemic Lya redshift. We use radiative transfer simulations in a cosmological context to investigate the implications for the ionisation state of the inhomogeneous IGM surrounding this quasar. Our simulations show that the transmission profile is consistent with an IGM in the vicinity of the quasar with a volume averaged HI fraction of f_HI>0.1 and that ULAS J1120+0641 has been bright for 10^6--10^7 yr. The observed spectrum is also consistent with smaller IGM neutral fractions, f_HI ~ 10^-3--10-4, if a damped Lya system in an otherwise highly ionised IGM lies within 5 proper Mpc of the quasar. This is, however, predicted to occur in only ~5 per cent of our simulated sight-lines for a bright phase of 10^6--10^7 yr. Unless ULAS J1120+0641 grows during a previous optically obscured phase, the low age inferred for the quasar adds to the theoretical challenge of forming a 2x10^9 M_sol black hole at this high redshift.Comment: 5 pages, 4 figures, accepted to MNRAS letter

Monte Carlo Simulation of Lyman Alpha Scattering and Application to Damped Lyman Alpha Systems

A Monte Carlo code to solve the transfer of Lyman alpha (Lya) photons is developed, which can predict the Lya image and two-dimensional Lya spectra of a hydrogen cloud with any given geometry, Lya emissivity, neutral hydrogen density distribution, and bulk velocity field. We apply the code to several simple cases of a uniform cloud to show how the Lya image and emitted line spectrum are affected by the column density, internal velocity gradients, and emissivity distribution. We then apply the code to two models for damped Lya absorption systems: a spherical, static, isothermal cloud, and a flattened, axially symmetric, rotating cloud. If the emission is due to fluorescence of the external background radiation, the Lya image should have a core corresponding to the region where hydrogen is self-shielded. The emission line profile has the characteristic double peak with a deep central trough. We show how rotation of the cloud causes the two peaks to shift in wavelength as the slit is perpendicular to the rotation axis, and how the relative amplitude of the two peaks is changed. In reality, damped Lya systems are likely to have a clumpy gas distribution with turbulent velocity fields, which should smooth the line emission profile, but should still leave the rotation signature of the wavelength shift across the system.Comment: 19 pages, 17 eps figures. One panel is added in Fig.1 to show the recoil effect. Revisions are made in response to the referee's comments. Accepted for publication in Ap

The Observed Probability Distribution Function, Power Spectrum, and Correlation Function of the Transmitted Flux in the Lyman-alpha Forest

A sample of eight quasars observed at high resolution and signal-to-noise is used to determine the probability distribution function (PDF), the power spectrum, and the correlation function of the transmitted flux in the \lya forest, in three redshift bins centered at z=2.41, 3.00, and 3.89. All the results are presented in tabular form, with full error covariance matrices to allow for comparisons with any numerical simulations and with other data sets. The observations are compared with a numerical simulation of the \lya forest of a Lambda-CDM model with Omega=0.4, known to agree with other large-scale structure observational constraints. There is excellent agreement for the PDF, if the mean transmitted flux is adjusted to match the observations. A small difference between the observed and predicted PDF is found at high fluxes and low redshift, which may be due to the uncertain effects of fitting the spectral continuum. Using the numerical simulation, we show how the flux power spectrum can be used to recover the initial power spectrum of density fluctuations. From our sample of eight quasars, we measure the amplitude of the mass power spectrum to correspond to a linear variance per unit ln(k) of $\Delta^2_\rho(k)=0.72\pm0.09$ at k=0.04(km/s)^{-1} and z=3, and the slope of the power spectrum near the same k to be $n_p=-2.55\pm0.10$ (statistical error bars). The results are statistically consistent with Croft et. al. (1999), although our value for the rms fluctuation is lower by a factor 0.75. For the Lambda-CDM model we use, the implied primordial slope is $n=0.93\pm0.10$, and the normalization is $\sigma_8=0.68+1.16(0.95-n)\pm0.04$.Comment: submitted to Ap

The Top Ten List of Gravitational Lens Candidates from the HST Medium Deep Survey

A total of 10 good candidates for gravitational lensing have been discovered in the WFPC2 images from the HST Medium Deep Survey (MDS) and archival primary observations. These candidate lenses are unique HST discoveries, i.e. they are faint systems with sub-arcsecond separations between the lensing objects and the lensed source images. Most of them are difficult objects for ground-based spectroscopic confirmation or for measurement of the lens and source redshifts. Seven are ``strong lens'' candidates which appear to have multiple images of the source. Three are cases where the single image of the source galaxy has been significantly distorted into an arc. The first two quadruply lensed candidates were reported in Ratnatunga et al 1995 (ApJL, 453, L5) We report on the subsequent eight candidates and describe them with simple models based on the assumption of singular isothermal potentials. Residuals from the simple models for some of the candidates indicate that a more complex model for the potential will probably be required to explain the full structural detail of the observations once they are confirmed to be lenses. We also discuss the effective survey area which was searched for these candidate lens objects.Comment: 26 pages including 12 figures and 10 tables. AJ Vol. 117, No.

Formation of the Black Holes in the Highest Redshift Quasars

The recent discovery of luminous quasars up to a redshift z=6.43 has renewed interest in the formation of black holes massive enough to power quasars. If black holes grow by Eddington-limited gas accretion with a radiative efficiency of at least 10%, the time required to grow from a stellar black hole to ~10^9 msun is ~10^9 years, close to the age of the universe at z=6. Black hole mergers may accelerate the rate of mass growth, but can also completely eject black holes from halo centers owing to the gravitational wave recoil effect. Recently, Haiman concluded that black hole ejections likely do not allow black holes to grow to ~10^9 msun by z=6.43. We reexamine this problem and show that, by using a different halo escape velocity, accounting for the dependence of the recoil velocity on the black hole binary mass ratio and spins, and allowing seed black holes to form in all halos down to virial temperatures of 2000 K, black hole masses may reach ~10^9 msun as early as z=9 starting from stellar seeds, without super-Eddington accretion. In this particular case, we find that these massive black holes form from the merger of ~10^4 stellar black holes formed in low-mass halos at z~20, which must all grow close to the maximum Eddington rate over most of the time available from their birth to z~6. The alternative is that black holes can grow more rapidly by super-Eddington accretion.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter

The Impact of Temperature Fluctuations on the Lyman-alpha Forest Power Spectrum

We explore the impact of spatial fluctuations in the intergalactic medium temperature on the Lyman-alpha forest flux power spectrum near z ~ 3. We develop a semianalytic model to examine temperature fluctuations resulting from inhomogeneous HI and incomplete HeII reionizations. Detection of these fluctuations might provide insight into the reionization histories of hydrogen and helium. Furthermore, these fluctuations, neglected in previous analyses, could bias constraints on cosmological parameters from the Lyman-alpha forest. We find that the temperature fluctuations resulting from inhomogeneous HI reionization are likely to be very small, with an rms amplitude of < 5%, $\sigma_{T_0}/ < 0.05$. More important are the temperature fluctuations that arise from incomplete HeII reionization, which might plausibly be as large as 50%, $\sigma_{T_0}/ ~ 0.5$. In practice, however, these temperature fluctuations have only a small effect on flux power spectrum predictions. The smallness of the effect is possibly due to density fluctuations dominating over temperature fluctuations on the scales probed by current measurements. On the largest scales currently probed, k ~ 0.001 s/km (~0.1 h/Mpc), the effect on the flux power spectrum may be as large as ~10% in extreme models. The effect is larger on small scales, up to ~20% at k = 0.1 s/km, due to thermal broadening. Our results suggest that the omission of temperature fluctuations effects from previous analyses does not significantly bias constraints on cosmological parameters.Comment: 11 pages, 5 figures, ApJ accepte

The Spin-Kinetic Temperature Coupling and the Heating Rate due to Lyman Alpha Scattering before Reionization: Predictions for 21cm Emission and Absorption

We investigate the interaction of Lyman alpha photons produced by the first stars in the universe with the intergalactic medium (IGM) prior to reionization. The background Lyman alpha spectral profile is obtained by solving a Fokker-Planck equation. Accurate values of the heating and scattering rates, and the spin-kinetic temperature coupling coefficient, are presented. We show that the heating rate induced by the Lyman alpha scatterings is much lower than found previously, and is basically negligible. The dominant heating source is most likely the X-rays from the first ionizing sources, which are able to penetrate into the atomic medium. The scattering of Lyman alpha photons couples the hydrogen spin temperature to the kinetic temperature. If the first ionizing sources in the universe did not emit significant X-rays, the spin temperature would be rapidly brought down to the very low gas kinetic temperature, and a 21cm absorption signal against the CMB larger than 100 mK would be predicted. However, we argue that sufficient X-rays are likely to have been emitted by the first stellar population, implying that the gas kinetic temperature should rapidly increase, turning a reduced and brief absorption signal into emission, with a smaller amplitude of about 10 mK. The detection of the 21cm absorption and emission feature would be a hallmark in unravelling the history of the ``dark age'' before reionization.Comment: AASTEX 5.02, 23 pages with 8 figures, submitted to ApJ. Our code for solving the Lyman alpha line profile and calculating the scattering and heating rates can be downloaded from http://theory.itp.ucsb.edu/~xuelei/LAST/index.html replaced with revised version, added more discussion on the physical process of heating and coolin