Stephen D. Murray

RADIATIVE SHOCK-INDUCED COLLAPSE OF INTERGALACTIC CLOUDS

Accumulating observational evidence for a number of radio galaxies suggests an association between their jets and regions of active star formation. The standard picture is that shocks generated by the jet propagate through an inhomogeneous medium and trigger the collapse of overdense clouds, which then become active star-forming regions. In this contribution, we report on recent hydrodynamic simulations of radiative shock-cloud interactions using two different cooling models: an equilibrium cooling-curve model assuming solar metallicities and a nonequilibrium chemistry model appropriate for primordial gas clouds. We consider a range of initial cloud densities and shock speeds in order to quantify the role of cooling in the evolution. Our results indicate that for moderate cloud densities (1 cm-3) and shock Mach numbers (20), cooling processes can be highly efficient and result in more than 50% of the initial cloud mass cooling to below 100 K. We also use our results to estimate the final H2 mass fraction for the simulations that use the nonequilibrium chemistry package. This is an important measurement, since H2 is the dominant coolant for a primordial gas cloud. We find peak H2 mass fractions of 10-2 and total H2 mass fractions of 10-5 for the cloud gas, consistent with cosmological simulations of first star formation. Finally, we compare our results with the observations of jet-induced star formation in Minkowskis Object, a small irregular starburst system associated with a radio jet in the nearby cluster of galaxies Abell 194. We conclude that its morphology, star formation rate (~0.3 M☉ yr-1) and stellar mass (~1.2 × 107 M☉) can be explained by the interaction of a ~9 × 104 km s-1 jet with an ensemble of moderately dense (~10 cm-3), warm (104 K) intergalactic clouds in the vicinity of its associated radio galaxy at the center of the galaxy cluster.

THREE-DIMENSIONAL MOVING-MESH SIMULATIONS OF GALACTIC CENTER CLOUD G2

Using three-dimensional, moving-mesh simulations, we investigate the future evolution of the recently discovered gas cloud G2 traveling through the galactic center. We consider the case of a spherical cloud initially in pressure equilibrium with the background. Our suite of simulations explores the following parameters: the equation of state, radial profiles of the background gas, and start times for the evolution. Our primary focus is on how the fate of this cloud will affect the future activity of Sgr A*. From our simulations we expect an average feeding rate in the range of (5-19) × 10–8 M ☉ yr–1 beginning in 2013 and lasting for at least 7 years (our simulations stop in year 2020). The accretion varies by less than a factor of three on timescales ≤1 month, and shows no more than a factor of 10 difference between the maximum and minimum observed rates within any given model. These rates are comparable to the current estimated accretion rate in the immediate vicinity of Sgr A*, although they represent only a small ( 5%) increase over the current expected feeding rate at the effective inner boundary of our simulations (r = 750, RS ≈ 1015 cm), where RS is the Schwarzschild radius of the black hole. Therefore, the breakup of cloud G2 may have only a minimal effect on the brightness and variability of Sgr A* over the next decade. This is because current models of the galactic center predict that most of the gas will be caught up in outflows. However, if the accreted G2 material can remain cold, it may not mix well with the hot, diffuse background gas, and instead accrete efficiently onto Sgr A*. Further observations of G2 will give us an unprecedented opportunity to test this idea. The breakup of the cloud itself may also be observable. By tracking the amount of cloud energy that is dissipated during our simulations, we are able to get a rough estimate of the luminosity associated with its tidal disruption; we find values of a few 1036 erg s–1.

Magnetohydrodynamic simulations of shock interactions with radiative clouds

We present results from two-dimensional numerical simulations of the interactions between magnetized shocks and radiative clouds. Our primary goal is to characterize the dynamical evolution of the shocked clouds. We perform runs in both the strong and weak magnetic field limits and consider three different field orientations. For the geometries considered, we generally find that magnetic fields external to, but concentrated near, the surface of the cloud suppress the growth of destructive hydrodynamic instabilities. External fields also increase the compression of the cloud by effectively acting as a confinement mechanism driven by the interstellar flow and local field stretching. This can have a dramatic effect on both the efficiency of radiative cooling, which tends to increase with increasing magnetic field strength, and on the size and distribution of condensed cooled fragments. In contrast, fields acting predominately internally to the cloud tend to resist compression, thereby inhibiting cooling. We observe that, even at modest strengths (β0 100), internal fields can completely suppress low-temperature (T < 100 K) cooling in two-dimensional clouds.

THE SUPERNOVA TRIGGERED FORMATION AND ENRICHMENT OF OUR SOLAR SYSTEM

We investigate the enrichment of the pre-solar cloud core with short-lived radionuclides, especially 26Al. The homogeneity and the surprisingly small spread in the ratio 26Al/27Alxa0 observed in the overwhelming majority of calcium-aluminium-rich inclusions in a vast variety of primitive chondritic meteorites places strong constraints on the formation of the solar system. Freshly synthesized radioactive 26Alxa0 has to be included and well mixed within 20 kyr. After discussing various scenarios including X-winds, asymptotic giant branch stars, and Wolf-Rayet stars, we come to the conclusion that triggering the collapse of a cold cloud core by a nearby supernova (SN) is the most promising scenario. We then narrow down the vast parameter space by considering the pre-explosion survivability of such a clump as well as the cross-section necessary for sufficient enrichment. We employ numerical simulations to address the mixing of the radioactively enriched SN gas with the pre-existing gas and the forced collapse within 20 kyr. We show that a cold clump of 10 M ☉ at a distance of 5 pc can be sufficiently enriched in 26Alxa0and triggered into collapse fast enough—within 18 kyr after encountering the SN shock—for a range of different metallicities and progenitor masses, even if the enriched material is assumed to be distributed homogeneously in the entire SN bubble. In summary, we envision an environment for the birthplace of the solar system 4.567 Gyr ago similar to the situation of the pillars in M16 nowadays, where molecular cloud cores adjacent to an H II region will be hit by an SN explosion in the future. We show that the triggered collapse and formation of the solar system as well as the required enrichment with radioactive 26Alxa0 are possible in this scenario.

Ejection of Supernova-enriched Gas from Dwarf Disk Galaxies

We examine the efficiency with which supernova-enriched gas may be ejected from dwarf disk galaxies, using a methodology previously employed to study the self-enrichment efficiency of dwarf spheroidal systems. Unlike previous studies that focused on highly concentrated starbursts, in the current work we consider discrete supernova events spread throughout various fractions of the disk. We model disk systems having gas masses of 108 and 109 M☉ with supernova rates of 30, 300, and 3000 Myr-1. The supernova events are confined to the midplane of the disk but distributed over radii of 0%, 30%, and 80% of the disk radius, consistent with expectations for Type II supernovae. In agreement with earlier studies, we find that the enriched material from supernovae is largely lost when the supernovae are concentrated near the nucleus, as expected for a starburst event. In contrast, however, we find the loss of enriched material to be much less efficient when the supernovae occur over even a relatively small fraction of the disk. The difference is due to the ability of the system to relax following supernova events that occur over more extended regions. Larger physical separations also reduce the likelihood of supernovae going off within low-density chimneys swept out by previous supernovae. We also find that for the most distributed systems, significant metal loss is more likely to be accompanied by significant mass loss. A comparison with theoretical predictions indicates that when undergoing self-regulated star formation, galaxies in the mass range considered will efficiently retain the products of Type II supernovae.

STAR FORMATION AND FEEDBACK IN DWARF GALAXIES

We examine the star formation history and stellar feedback effects of dwarf galaxies under the influence of extragalactic ultraviolet radiation. Previous work has indicated that the background UV flux can easily ionize the gas within typical dwarf galaxies, delaying or even preventing cooling and star formation within them. Many dwarf galaxies within the Local Group are, however, observed to contain multiple generations of stars, the oldest of which formed in the early epochs of cosmic evolution, when the background UV flux was intense. In order to address this paradox, we consider the dynamical evolution of gas in dwarf galaxies using a one-dimensional, spherically symmetric, Lagrangian numerical scheme to compute the effects of radiative transfer and photoionization. We include a physically motivated star formation recipe and consider the effects of feedback. This scheme allows us to follow the history of the gas and of star formation within dwarf galaxies, as influenced by both external and internal UV radiation. Our results indicate that star formation in the severe environment of dwarf galaxies is a difficult and inefficient process. In potentials with total mass less than a few times 106 M☉ and velocity dispersion less than a few kilometers per second, residual gas is efficiently photoionized by cosmic background UV radiation. Since the density scale height of the gas within these galaxies is comparable to their size, gas may be tidally removed from them, leaving behind starless residual dark matter clumps. For intermediate-mass systems, such as the dSphs around the Galaxy, star formation can proceed within early cosmic epochs despite the intense background UV flux. Triggering processes such as merger events, collisions, and tidal disturbance can lead to density enhancements, reducing the recombination timescale, allowing gas to cool and star formation to proceed. However, the star formation and gas retention efficiency may vary widely in galaxies with similar dark matter potentials because they depend on many factors, such as the baryonic fraction, external perturbation, initial mass function, and background UV intensity. We suggest that the presence of very old stars in these dwarf galaxies indicates that their initial baryonic-to-dark matter content was comparable to the cosmic value. This constraint suggests that the initial density fluctuation of baryonic matter may be correlated with that of the dark matter. For the more massive dwarf elliptical galaxies, the star formation efficiency and gas retention rate are much higher. Their mass-to-light ratio is regulated by star formation feedback and is expected to be nearly independent of their absolute luminosity. The results of our theoretical models reproduce the observed (M/L)-Mv correlation.

Energy Dissipation in Multiphase Infalling Clouds in Galaxy Halos

During the epoch of large galaxy formation, thermal instability leads to the formation of a population of cool fragments that are embedded within a background of tenuous hot gas. The hot gas attains a quasi-hydrostatic equilibrium. Although the cool clouds are pressure confined by the hot gas, they fall into the galactic potential, and their motion is subject to drag from the hot gas. The release of gravitational energy due to the infall of the cool clouds is first converted into their kinetic energy and is subsequently dissipated as heat. The cool clouds therefore represent a potentially significant energy source for the background hot gas, depending on the ratio of thermal energy deposited within the clouds versus the hot gas. In this paper, we show that most of the dissipated energy is deposited in the tenuous hot halo gas, providing a source of internal energy to replenish losses in the hot gas through bremsstrahlung emission and conduction into the cool clouds. The heating from the motion of the cool clouds allows the multiphase structure of the interstellar medium to be maintained.

Supernova Enrichment of Dwarf Spheroidal Galaxies

Many dwarf galaxies exhibit subsolar metallicities, with some star-to-star variation, despite often containing multiple generations of stars. The total metal content in these systems is much less than expected from the heavy-element production of massive stars in each episode of star formation. Such a deficiency implies that a substantial fraction of the enriched material has been lost from these small galaxies. Mass ejection from dwarf galaxies may have important consequences for the evolution of the intergalactic medium and for the evolution of massive galaxies, which themselves may have formed via the merger of smaller systems. We report here the results of three-dimensional simulations of the evolution of supernova-enriched gas within dwarf spheroidal galaxies (dSphs), with the aim of determining the retention efficiency of supernova ejecta. We consider two galaxy models, selected to represent opposite ends of the dSph sequence. One contains 106 M☉ of gas, and the other 5.5 × 106 M☉. In both, the baryonic-to-dark matter ratio is assumed to be 0.1. The total binding energies of the gas in the two models are 9.8 × 1050 and 1.6 × 1052 ergs. For each model galaxy we investigate a number of scenarios. The simplest is a single supernova within a smooth gas distribution. We also investigate the evolution of 10 supernovae, within initially smooth gas distributions, occurring over time spans of either 10 or 100 Myr. Finally, we investigate the effects of 10 supernovae occurring over 10 Myr in a medium filled with hot bubbles, such as would be expected in the presence of an initial generation of hot stars. For models with only a single supernova, no enriched material is lost from the galaxies. When multiple supernovae occur within an initially smooth gas distribution, less than one-half the enriched gas is lost from the galaxy (fractional losses range from 0% to 47%). Most of the enriched gas is lost, however, from the cores of the galaxies. In the presence of an initially disturbed gas distribution, 6% or less of the enriched gas remains in the core, and much is lost from the galaxies as a whole (47% and 71% for the larger and smaller galaxy models, respectively). If subsequent star formation occurs predominantly within the core where most of the residual gas is concentrated, then these results could explain the poor self-enrichment efficiency observed in dwarf galaxies.

Cosmos: A Radiation-Chemo-Hydrodynamics Code for Astrophysical Problems

We have developed a new massively parallel radiation-hydrodynamics code (Cosmos) for Newtonian and relativistic astrophysical problems that also includes radiative cooling, self-gravity, and nonequilibrium, multispecies chemistry. Several numerical methods are implemented for the hydrodynamics, including options for both internal and total energy conserving schemes. Radiation is treated using flux-limited diffusion. The chemistry incorporates 27 reactions, including both collisional and radiative processes for atomic hydrogen and helium gases, and molecular hydrogen chains. In this paper we discuss the equations and present results from test problems carried out to verify the robustness and accuracy of our code in the Newtonian regime. An earlier paper presented tests of the relativistic capabilities of Cosmos.

Gas Accretion by Globular Clusters and Nucleated Dwarf Galaxies and the Formation of the Arches and Quintuplet Clusters

We consider here the collective accretion of gas by globular clusters and dwarf galaxies moving through the interstellar medium. In the limit of high velocity and/or sound speed of the ISM, the collective potential of the cluster is insufficient to accrete significant amounts of gas, and stars within the systems accrete gas individually. We show, however, that when the sound speed or the relative velocity of the ambient medium is less than the central velocity dispersion of the cluster, it is accreted into the collective potential of the cluster prior to being accreted onto the individual stars within the cluster. The collective rate is strongly enhanced relative to the individual rates. This effect may potentially modify the white dwarf cooling sequence in globular clusters with low-inclination and low-eccentricity Galactic orbits and lead to the rejuvenation of some marginally surviving cores of globular clusters and nucleated dwarf galaxies near the Galactic center. Such effects will only occur rarely, but may explain the existence of clusters of young, massive stars near the Galactic center.