Dwarf Sphs/First-galaxies connection

I analyze the properties of the first galaxies in cosmological simulations with radiative feedback. Preliminary results indicate similarities with the observed properties of the bulk of dwarf spheroidal galaxies (dSphs) in the Local Group and Andromeda. I briefly discuss observational tests that could help in understanding the impact of a population of small primordial objects on the cosmic evolution.Comment: 6 pages, 3 figures. To appear in the proceedings of "The IGM/Galaxy Connection - The Distribution of Baryons at z=0", ed. M. Putman & J. Rosenber

Massive Milky Way Satellites in Cold and Warm Dark Matter: Dependence on Cosmology

We investigate the claim that the largest subhaloes in high resolution dissipationless cold dark matter (CDM) simulations of the Milky Way are dynamically inconsistent with observations of its most luminous satellites. We find that the inconsistency is largely attributable to the large values of \sigma_8 and n_s adopted in the discrepant simulations producing satellites that form too early and therefore are too dense. We find the tension between observations and simulations adopting parameters consistent with WMAP9 is greatly diminished making the satellites a sensitive test of CDM. We find the Via Lactea II halo to be atypical for haloes in a WMAP3 cosmology, a discrepancy that we attribute to its earlier formation epoch than the mean for its mass. We also explore warm dark matter (WDM) cosmologies for 1--4 keV thermal relics. In 1 keV cosmologies subhaloes have circular velocities at kpc scales ~ 60% lower than their CDM counterparts, but are reduced by only 10% in 4 keV cosmologies. Since relic masses < 2-3 keV are ruled out by constraints from the number of Milky Way satellites and Lyman-\alpha\ forest, WDM has a minor effect in reducing the densities of massive satellites. Given the uncertainties on the mass and formation epoch of the Milky Way, the need for reducing the satellite densities with baryonic effects or WDM is alleviated.Comment: 11 pages, 7 figures, submitted to MNRA

Constraints on the Dark Matter Particle Mass from the Number of Milky Way Satellites

We have conducted N-body simulations of the growth of Milky Way-sized halos in cold and warm dark matter cosmologies. The number of dark matter satellites in our simulated Milky Ways decreases with decreasing mass of the dark matter particle. Assuming that the number of dark matter satellites exceeds or equals the number of observed satellites of the Milky Way we derive lower limits on the dark matter particle mass. We find with 95% confidence m_s > 13.3 keV for a sterile neutrino produced by the Dodelson and Widrow mechanism, m_s > 8.9 keV for the Shi and Fuller mechanism, m_s > 3.0 keV for the Higgs decay mechanism, and m_{WDM} > 2.3 keV for a thermal dark matter particle. The recent discovery of many new dark matter dominated satellites of the Milky Way in the Sloan Digital Sky Survey allows us to set lower limits comparable to constraints from the complementary methods of Lyman-alpha forest modeling and X-ray observations of the unresolved cosmic X-ray background and of dark matter halos from dwarf galaxy to cluster scales. Future surveys like LSST, DES, PanSTARRS, and SkyMapper have the potential to discover many more satellites and further improve constraints on the dark matter particle mass.Comment: 17 pages, 13 figures, replaced with final version published in Physical Review

Constraints on First-Light Ionizing Sources from Optical Depth of the Cosmic Microwave Background

We examine the constraints on high-redshift star formation, ultraviolet and X-ray pre-ionization, and the epoch of reionization at redshift z_r, inferred from the recent WMAP-5 measurement, tau_e = 0.084 +/- 0.016, of the electron scattering optical depth of the cosmic microwave background (CMB). Half of this scattering can be accounted for by the optical depth, tau_e = 0.04-0.05, of a fully ionized intergalactic medium (IGM) at z < z_GP = 6-7, consistent with Gunn-Peterson absorption in neutral hydrogen. The required additional optical depth, Delta-tau_e = 0.03 +/- 0.02 at z > z_GP, constrains the ionizing contributions of first light sources. WMAP-5 also measured a significant increase in small-scale power, which lowers the required efficiency of star formation and ionization from mini-halos. Early massive stars (UV radiation) and black holes (X-rays) can produce a partially ionized IGM, adding to the residual electrons left from incomplete recombination. Inaccuracies in computing the ionization history, x_e(z), and degeneracies in cosmological parameters (Omega_m, Omega_b, sigma_8, n_s) add systematic uncertainty to the measurement and modeling of $\tau_e$. From the additional optical depth from sources at z > z_GP, we limit the star-formation efficiency, the rate of ionizing photon production for Pop III and Pop II stars, and the photon escape fraction, using standard histories of baryon collapse, minihalo star formation, and black-hole X-ray preionization.Comment: Greatly revised version, based on WMAP-5 results and new models. Accepted for ApJ (2008

On Nonshearing Magnetic Configurations in Differentially Rotating Disks

A new class of disk MHD equilibrium solutions is described, which is valid within the standard local (``shearing sheet'') approximation scheme. These solutions have the following remarkable property: velocity streamlines and magnetic lines of force rotate rigidly, even in the presence of differential rotation. This situation comes about because the Lorentz forces acting upon modified epicycles compel fluid elements to follow magnetic lines of force. Field line (and streamline) configurations may be elliptical or hyperbolic, prograde or retrograde. These structures have previously known hydrodynamical analogs: the ``planet'' solutions described by Goodman, Narayan, & Goldreich. The primary focus of this investigation is configurations in the disk plane. A related family of solutions lying in a vertical plane is briefly discussed; other families of solutions may exist. Whether these MHD structures are stable is not yet known, but could readily be determined by three-dimensional simulations. If stable or quasi-stable, these simple structures may find important applications in both accretion and galactic disks

The Fate of the First Galaxies. III. Properties of Primordial Dwarf Galaxies and their Impact on the Intergalactic Medium

In two previous papers, we presented simulations of the first galaxies in a representative volume of the Universe. The simulations are unique because we model feedback-regulated galaxy formation, using time-dependent, spatially-inhomogeneous radiative transfer coupled to hydrodynamics. Here, we study the properties of simulated primordial dwarf galaxies with masses <2x10^8 Msolar and investigate their impact on the intergalactic medium. While many primordial galaxies are dark, about 100--500 per comoving Mpc^3 are luminous but relatively faint. They form preferentially in chain structures, and have low surface brightness stellar spheroids extending to 20% of the virial radius. Their interstellar medium has mean density n_H~10--100 cm^-3, metallicity Z~ 0.01--0.1 Zsolar and can sustain a multi-phase structure. With large scatter, the mean efficiency of star formation scales with halo mass, \propto M_dm^2, independent of redshift. Because of feedback, halos smaller than a critical mass, M_crit(z), are devoid of most of their baryons. More interestingly, we find that dark halos have always a smaller M_crit(z) than luminous ones. Metal enrichment of the intergalactic medium is inhomogeneous, with only a 1%--10% volume filling factor of enriched gas with [Z/H]>-3.0 and 10%--50% with [Z/H]>-5.0. At z=10, the fraction of stars with metallicity Z<10^-3 Zsolar is 10^-6 of the total stellar mass. Although detections of high-redshift dwarf galaxies with the James Webb Space Telescope will be a challenge, studies of their fossil records in the local Universe are promising because of their large spatial density.Comment: 19 pages, 13 figures, 2 tables (emulateapj). Submitted to Ap