Alejandro C. Raga

Photoevaporation of cosmological minihaloes during reionization

Energy released by a small fraction of the baryons in the Universe, which condensed out while the intergalactic medium (IGM) was cold, dark and neutral, reheated and reionized it by redshift 6, exposing other baryons already condensed into dwarf-galaxy minihaloes to the glare of ionizing radiation. We present the first gas dynamical simulations of the photoevaporation of cosmological minihaloes overtaken by the ionization fronts which swept through the IGM during the reionization epoch in the currently favoured A cold dark matter (ACDM) universe, including the effects of radiative transfer. These simulations demonstrate the phenomenon of I-front trapping inside minihaloes, in which the weak, R-type fronts which travelled supersonically across the IGM decelerated when they encountered the dense, neutral gas inside minihaloes, and were thereby transformed into D-type I-fronts, preceded by shock waves. For a minihalo with virial temperature below 10 4 K, the I-front gradually burned its way through the minihalo which trapped it, removing all of its baryonic gas by causing a supersonic, evaporative wind to blow backwards into the IGM, away from the exposed layers of minihalo gas just behind the advancing I-front. We describe this process in detail, along with some of its observable consequences, for the illustrative case of a minihalo of total mass 10 7 M ○. , exposed to a distant source of ionizing radiation with either a stellar or quasar-like spectrum, after it was overtaken at redshift z = 9 by the weak, R-type I-front which ionized the IGM surrounding the source. For a source at z = 9 which emits 10 56 ionizing photons per second at 1 Mpc (or, equivalently, 10 52 ionizing photons per second at 10 kpc), the photoevaporation of this minihalo takes about 100-150 Myr, depending on the source spectrum, ending at about z = 7.5. Such hitherto neglected feedback effects were widespread during the reionization epoch. N-body simulations and analytical estimates of halo formation in the ACDM model suggest that sub-kpc minihaloes such as these, with virial temperatures below 10 4 K, were so common as to cover the sky around larger-mass source haloes and possibly dominate the absorption of ionizing photons during reionization. This means that previous estimates of the number of ionizing photons per hydrogen atom required to complete reionization which neglected this effect may be too low. Regardless of their effect on the progress of reionization, however, the minihaloes were so abundant that random lines of sight through the high-z Universe should encounter many of them, which suggests that it may be possible to observe the processes described here in the absorption spectra of distant sources.

Temporal evolution of the shock wave and hot core air in laser induced plasma

The temporal evolution of electric breakdown in air at atmospheric pressure by Nd:yttrium–aluminum–garnet Q-switched nanosecond laser pulses was studied from the nanosecond to the millisecond time scale by shadowgraphy and interferometry techniques. The results were modeled with a gasdynamic code with good agreement. It was possible to simultaneously model the whole evolution of the plasma, the shock wave, and the hot core air. The shock wave velocity was determined to be ⩾60 km s−1 at 20 ns. The plasma temperature was found to reach about 1.7×104 K at 1 μs and the hot core air temperature was determined to be <103 K at 100 μs. This letter presents an experimental work that extends the study of laser induced plasmas to millisecond time scales.

Stellar jets with intrinsically variable sources

Models for high Mach number, isothermal, pressure-matched jets ejected from sources with variable velocities are presented. It is found that the relaxation of the assumption of a time-independent source allows the complex position-velocity diagrams and multiple bow shock structures observed in some stellar jets to be explained in a straightforward way. Analytic and numerical models are presented for jets with variable velocity sources. A comparison with previously published observations of the HH 46/47 jet is performed which shows how the velocity variations observed along a jet can be used to reconstruct the past time variability of the source. This technique is promising for future studies of the mechanism by which stellar jets are produced. 41 refs.

Minihalo photoevaporation during cosmic reionization: Evaporation times and photon consumption rates

ABSTRACT The weak, R-type ionization fronts (I-fronts) which swept across the intergalacticmedium (IGM) during the reionization of the universe often found their paths blockedby cosmological minihaloes (haloes with virial temperatures T vir 6 10 4 K). When thishappened, the neutral gas which filled each minihalo was photoevaporated; as theI-front burned its way through the halo, decelerating from R-type to D-type, all thehalo gas was eventually blown back into the IGM as an ionized, supersonic wind. Ina previous paper (Shapiro, Iliev & Raga 2004, hereafter Paper I), we described thisprocess and presented our results of the first simulations of it by numerical gas dy-namics with radiation transport in detail. For illustration we focused on the particularcase of a 10 7 M ⊙ minihalo which is overrun at z = 9 by an intergalactic I-front causedby a distant source of ionizing radiation, for different types of source spectra (eitherstellar from massive Pop. II or III stars, or QSO-like) and a flux level typical of that ex-pected during reionization. In a Cold Dark Matter (CDM) universe, minihaloes formedin abundance before and during reionization and, thus, their photoevaporation is animportant, possibly dominant, feature of reionization, which slowed it down and cost itmany ionizing photons. In view of the importance of minihalo photoevaporation, bothas a feedback mechanism on the minihaloes and as an effect on cosmic reionization,we have now performed a larger set of high-resolution simulations to determine andquantify the dependence of minihalo photoevaporation times and photon consumptionrates on halo mass, redshift, ionizing flux level and spectrum. We use these results toderive simple expressions for the dependence of the evaporation time and photon con-sumption rate on these halo and external flux parameters which can be convenientlyapplied to estimate the effects of minihaloes on the global reionization process in bothsemi-analytical calculations and larger-scale, lower-resolution numerical simulationswhich cannot adequately resolve the minihaloes and their photoevaporation. We findthat the average number of ionizing photons each minihalo atom absorbs during itsphotoevaporation is typically in the range 2-10. For the collapsed fraction in mini-haloes expected during reionization, this can add ≈1 photon per total atom to therequirements for completing reionization, potentially doubling the minimum numberof photons required to reionize the universe.Key words: hydrodynamics—radiative transfer—galaxies: halos—galaxies: high-redshift—intergalactic medium—cosmology: theory

A model for the post‐collapse equilibrium of cosmological structure: truncated isothermal spheres from top‐hat density perturbations

ABSTRA C T The post-collapse structure of objects that form by gravitational condensation out of the expanding cosmological background universe is a key element in the theory of galaxy formation. Towards this end, we have reconsidered the outcome of the non-linear growth of a uniform, spherical density perturbation in an unperturbed background universe ‐ the cosmological ‘top-hat’ problem. We adopt the usual assumption that the collapse to infinite density at a finite time predicted by the top-hat solution is interrupted by a rapid virialization caused by the growth of small-scale inhomogeneities in the initial perturbation. We replace the standard description of the post-collapse object as a uniform sphere in virial equilibrium by a more self-consistent one as a truncated, non-singular, isothermal sphere in virial and hydrostatic equilibrium, including for the first time a proper treatment of the finite-pressure boundary condition on the sphere. The results differ significantly from both the uniform sphere and the singular isothermal sphere approximations for the post-collapse objects. The virial temperature that results is more than twice the previously used ‘standard value’ of the post-collapse uniform sphere approximation, but 1.4 times smaller than that of the singular, truncated isothermal sphere approximation. The truncation radius is 0.554 times the radius of the top-hat at maximum expansion, and the ratio of the truncation radius to the core radius is 29.4, yielding a central density that is 514 times greater than at the surface and 1:8 10 4 times greater than that of the unperturbed background density at the epoch of infinite collapse predicted by the top-hat solution. For the top-hat fractional overdensity d L predicted by extrapolating the linear solution into the non-linear regime, the standard top-hat model assumes that virialization is instantaneous at dLa dca 1:686 i.e. the epoch at which the non-linear top-hat reaches infinite density. The surface of the collapsing sphere meets that of the post-collapse equilibrium sphere slightly earlier, however, when dLa 1:52. These results will have a significant effect on a wide range of applications of the Press‐Schechter and other semi-analytical models to cosmology. We discuss the density profiles obtained here in relation to the density profiles for a range of cosmic structures, from dwarf galaxies to galaxy clusters, indicated by observation and by N-body simulation of cosmological structure formation, including the recent suggestion of a universal density profile for haloes in the cold dark matter (CDM) model. The non-singular isothermal sphere solution presented here predicts the virial temperature and integrated mass distribution of the X-ray clusters formed in the CDM model as found by detailed, 3D, numerical gas and N-body dynamical simulations remarkably well. This solution allows us to derive analytically the numerically calibrated mass‐temperature and radius‐temperature scaling laws for X-ray clusters, which were derived empirically by Evrard, Metzler & Navarro from simulation results for the CDM model.

ALMA OBSERVATIONS OF THE HH 46/47 MOLECULAR OUTFLOW

The morphology, kinematics and entrainment mechanism of the HH 46/47 molecular outow were studied using new ALMA Cycle 0 observations. Results show that the blue and red lobes are strikingly dierent. We argue that these dierences are partly due to contrasting ambient densities that result in dierent wind components having a distinct eect on the entrained gas in each lobe. A 29-point mosaic, covering the two lobes at an angular resolution of about 3 00 , detected outow emission at much higher velocities than previous observations, resulting in signicantly higher estimates of the outow momentum and kinetic energy than previous studies of this source, using the CO(1{0) line. The morphology and the kinematics of the gas in the blue lobe are consistent with models of outow entrainment by a wide-angle wind, and a simple model describes the observed structures in the position-velocity diagram and the velocity-integrated intensity maps. The red lobe exhibits a more complex structure, and there is evidence that this lobe is entrained by a wide-angle wind and a collimated episodic wind. Three major clumps along the outow axis show velocity distribution consistent

VERY LARGE ARRAY OBSERVATIONS OF PROPER MOTIONS IN L1551 IRS 5

Using high angular resolution (~01) Very Large Array observations made at 2 cm during the period 1983 to 1998, we report the detection of proper motions in the components of the binary radio source in L1551 IRS 5. The absolute proper motions observed in these two protostars, of order 25 mas yr-1 or ~17 km s-1 at a distance of 140 pc, are very similar in magnitude and direction to those of T Tauri stars in the same region and are attributed to the large-scale motion of the parent molecular complex. The relative astrometry between the two components reveals orbital proper motions that suggest that the total mass and period of the binary system are ~1.2 M☉ and ~260 yr, respectively.

Radio Continuum Detection of the Exciting Sources of the DG Tauri B and L1551NE Outflows

The exciting sources of molecular outflows are characteristically associated with centimeter radio continuum emission, most probably originating in a partially ionized, collimated outflow. DG Tau B and L1551NE are two low-luminosity pre-main-sequence objects that have recently been found to be associated with molecular outflows. We have used 3.5 cm Very Large Array observations with angular resolution of 02 to detect compact radio continuum sources at both the positions of DG Tau B and L1551NE. The DG Tau B radio source has deconvolved dimensions (038 ± 002) × (022 ± 002), and its major axis is aligned along a position angle of 298° ± 5°, coincident within a few degrees with the position angle of the axis of the optical and molecular outflows (~294°). This result suggests that we are observing the base of the collimated jet that powers the region, on scales of tens of astronomical units. A comparison of the widths of the radio and optical jets in this source supports this interpretation.

A precessing jet model for the PN K 3 − 35: simulated radio-continuum emission

The bipolar morphology of the planetary nebula (PN) K 3 - 35 observed in radio-continuum images was modelled with 3D hydrodynamic simulations with the adaptive grid code YGUAZU-A. We find that the observed morphology of this PN can be reproduced considering a precessing jet evolving in a dense AGB circumstellar medium, given by a mass-loss rate M csm = 5 x 10 -5 M ⊙ yr -1 and a terminal velocity v w = 10 km s -1 . Synthetic thermal radio-continuum maps were generated from numerical results for several frequencies. Comparing the maps and the total fluxes obtained from the simulations with the observational results, we find that a model of precessing dense jets, where each jet injects material into the surrounding CSM at a rate M j = 2.8 x 10 -4 M ⊙ yr -1 (equivalent to a density of 8 x 10 4 cm -3 ), a velocity of 1500 km s -1 , a precession period of 100 yr and a semi-aperture precession angle of 20° agrees well with the observations.

Effects of Thermal Conduction on the X-Ray and Synchrotron Emission from Supernova Remnants

Several physical mechanisms have been studied in order to explain why composite supernova remnants (SNRs) have shell-like morphologies in the radio continuum while their X-ray emission is centrally peaked. One of the proposed mechanisms has been the presence of thermal conduction, which can raise the density at the center of SNRs, increasing the X-ray emission from these regions. In this work, we have carried out axisymmetric numerical simulations with the adaptive grid Yguaz?-a code, which includes advection of the magnetic field and thermal conduction. We have considered cases with anisotropic and isotropic thermal conduction, as well as with no conduction. We have simulated X-ray (as well as radio synchrotron) emission maps from our numerical simulations, which can be directly compared with observations.