8 pages, for publication in Rev Mex Astron Astrof Conf Series proceedings of Magnetic Fields in the Universe II, eds. A. Esquivel, J. Franco, G. Garcia-Segura, A. Lazarian & A. Raga
I review the effects of magnetic fields on star formation in galaxies. This includes the effects of the magnetorotational instability (MRI) at galactic scales, magneto-Jeans and swing instabilities, Parker instabilities, and the effects of magnetic fields on the evolution of supernova-driven turbulence. I argue that currently turbulent support by the MRI appears likely to be the most important of these processes to regulating star formation.
The global star formation rate (SFR) density is estimated from the star formation histories (SFHs) of Local Group galaxies. This is found to be broadly consistent with estimates of the global SFH from existing redshift surveys for two favoured cosmologies. It also provides additional evidence for a relatively constant global SFR density at high redshift (z>1).
Motivated by the problem of the formation of active regions from a deep-seated solar magnetic field, we consider the nonlinear three-dimensional evolution of magnetic buoyancy instabilities resulting from a smoothly stratified horizontal magnetic field. By exploring the case for which the instability is continuously driven we have identified a new mechanism for the formation of concentrations of magnetic flux.
In this note we respond to Mouschovias and Kassis criticisms that our paper "Testing Magnetic Star Formation Theory" is flawed. We stand fully behind our paper.
Starting with the assertion that the problem of isolated, single star formation is essentially solved, this paper examines some of the missing steps needed to go from there to understanding the star formation history of the Universe. Along the way, some results on the formation of star clusters in the Milky Way and the properties of GMCs in nearby galaxies are briefly examined.
The effect of the global rotation of the universe on the formation of galaxies is investigated. It is found that the global rotation provides a natural origin for the rotation of galaxies, and the morphology of the objects formed from gravitational instability in a rotating and expanding universe depends on the amplitude of the density fluctuation, different values of the amplitude of the fluctuation lead to the formation of elliptical galaxies, spiral galaxies, and walls. The global rotation gives a natural explanation of the empirical relation between the angular momentum and mass of galaxie…
The star formation history of the universe shows strong evolution with cosmological epoch. Although we know mergers between galaxies can cause luminous bursts of star formation, the relative importance of such mergers to the global star formation rate (SFR) is unknown. We present a simple analytic formula for the rate at which halos merge to form higher-mass systems, derived from Press-Schechter theory and confirmed by numerical simulations (for high halo masses). A comparison of the evolution in halo formation rate with the observed evolution in the global SFR indicates that the latter is lar…
We outline a method to infer the global history of star formation in galaxies with input only from absorption-line observations of quasars. The application of the method to existing data leads to the conclusion that most stars formed at relatively low redshifts (z <~ 2). We combine the global rate of star formation with stellar population synthesis models to compute the mean comoving emissivity and mean intensity of background radiation from far-UV to far-IR wavelengths. These predictions are consistent with all the available measurements and observational limits, including recent results from…
We study numerically the ambipolar diffusion-driven evolution of non-rotating, magnetically subcritical, disk-like molecular clouds, assuming axisymmetry. Previous similar studies have concentrated on the formation of single magnetically supercritical cores at the cloud center, which collapse to form isolated stars. We show that, for a cloud with many Jeans masses and a relatively flat mass distribution near the center, a magnetically supercritical ring is produced instead. The supercritical ring contains a mass well above the Jeans limit. It is expected to break up, through both gravitational…
We carried out 2.5-dimensional resistive magnetohydrodynamic simulations to study the effects of magnetic diffusivity on magnetically driven mass accretion and jet formation. We found that (1) when the normalized magnetic diffusivity, is small, mass accretion and jet formation take place intermittently; (2) when diffusivity is middle, the system evolves toward a quasi-steady state; and the system evolves toward a quasi-steady state; and (3) when diffusivity is large, the accretion/mass outflow rate decreases with diffusivity and approaches 0. The results of these simulations indicate magnetic …
The efficiency of star formation, defined as the ratio of the stellar to total (gas and stellar) mass, is observed to vary from a few percent in regions of dispersed star formation to about a third in cluster-forming cores. This difference may reflect the relative importance of magnetic fields and turbulence in controlling star formation. We investigate the interplay between supersonic turbulence and magnetic fields using numerical simulations, in a sheet-like geometry. We demonstrate that star formation with an efficiency of a few percent can occur over several gravitational collapse times in…
We report high-angular-resolution measurements of polarized dust emission toward the low-mass protostellar system NGC 1333 IRAS 4A. We show that in this system the observed magnetic field morphology is in agreement with the standard theoretical models of the formation of Sun-like stars in magnetized molecular clouds at scales of a few hundred astronomical units; gravity has overcome magnetic support, and the magnetic field traces a clear hourglass shape. The magnetic field is substantially more important than turbulence in the evolution of the system, and the initial misalignment of the magnet…
We discuss gravitational effects of global scalar fields and, especially, of global topological defects. We first give an introduction to the dynamics of global fields and the formation of defects. Next we investigate the induced gravitational fields, first in a flat background and then in the expanding universe. In flat space, we explicitly calculate the gravitational fields of exact global monopole and global texture solutions and discuss the motion of photons and massive particles in these geometries. We also show that slowly moving particles and the energy of photons are not affected in st…
A multi-wavelength set of photometric data including UV (GALEX), optical, near-IR, infrared (Spitzer) and radio (VLA 20cm) images and spectroscopic observations are used to map the dust-obscured and unobscured star formation in the galaxy pair ARP 32. The system consists of an actively star-forming galaxy and another one with depressed star formation. The most active galaxy has disrupted morphology and different sites of star formation. Spectroscopic data show hints of nuclear activity in its core, intense star formation in limited regions of the galaxy as well as an underlying population of s…
We examine the structure of cool magnetic spots in the photospheres of evolved stars, specifically asymptotic giant branch (AGB) stars and R Coronae Borealis stars. We find that the photosphere of a cool magnetic spot will be above the surrounding photosphere of AGB stars, opposite to the situation in the sun. This results from the behavior of the opacity, which increases with decreasing temperature, opposite to the behavior of the opacity near the effective temperature of the sun. We analyze the formation of dust above the cool magnetic spots, and suggest that the dust formation is facilitate…
We present first results of our simulations of magnetic fields in the formation of single and binary stars using a recently developed method for incorporating Magnetohydrodynamics (MHD) into the Smoothed Particle Hydrodynamics (SPH) method. An overview of the method is presented before discussing the effect of magnetic fields on the formation of circumstellar discs around young stars. We find that the presence of magnetic fields during the disc formation process can lead to significantly smaller and less massive discs which are much less prone to gravitational instability. Similarly in the cas…
We present the first large sample of accurate iron abundances and temperatures for clusters at redshifts greater than 0.14. We find that the Fe abundance shows little or no evolution out to redshift 0.3. This and the early formation epoch of elliptical galaxies in clusters indicate that most of the enrichment of the intracluster medium occurred at redshifts greater than 1. If clusters represent fair samples of the universe, then global metal production is 2-5 times greater than is inferred from recent studies of galaxies at low and high redshifts.
Recent spectroscopic observations of galaxies in distant clusters have revealed that the rate of star formation in star-forming galaxies is significantly suppressed with respect to their counterparts in the field at a similar redshift. It is, however, highly uncertain which physical processes are responsible for this suppression. We present the results of a numerical investigation of how the global tidal field of a cluster dynamically influences the reservoir of halo gas surrounding a disk galaxy as it falls into the cluster from the surrounding field. We find that the tidal field of the clust…
We investigate numerically the combined effects of supersonic turbulence, strong magnetic fields and ambipolar diffusion on cloud evolution leading to star formation. We find that, in clouds that are initially magnetically subcritical, supersonic turbulence can speed up star formation, through enhanced ambipolar diffusion in shocks. The speedup overcomes a major objection to the standard scenario of low-mass star formation involving ambipolar diffusion, since the diffusion time scale at the average density of a molecular cloud is typically longer than the cloud life time. At the same time, the…
Magnetic fields are important at every scale in the star formation process: from the dynamics of the ISM in galaxies, to the collapse of turbulent molecular clouds to form stars and in the fragmentation of individual star forming cores. The recent development of a robust algorithm for MHD in the Smoothed Particle Hydrodynamics method has enabled us to perform simulations of star formation including magnetic fields at each of these scales. This paper focusses on three questions in particular: What is the effect of magnetic fields on fragmentation in star forming cores? How do magnetic fields af…
We study the implications of primordial magnetic fields for the thermal and ionization history of the post-recombination era. In particular we compute the effects of dissipation of primordial magnetic fields owing to ambipolar diffusion and decaying turbulence in the intergalactic medium (IGM) and the collapsing halos and compute the effects of the altered thermal and ionization history on the formation of molecular hydrogen. We show that, for magnetic field strengths in the range $2 \times 10^{-10} {\rm G} \la B_0 \la 2 \times 10^{-9} {\rm G}$, the molecular hydrogen fraction in IGM and colla…
We present cosmological magnetohydrodynamic simulations of the formation of a galaxy cluster with magnetic energy feedback from an active galactic nuclei (AGN). We demonstrate that X-ray cavities can be produced by the magnetically dominated jet-lobe system that is supported by a central axial current. The cavities are magnetically dominated and their morphology is determined jointedly by the magnetic fields and the background cluster pressure profile. The expansion and motion of the cavities are driven initially by the Lorentz force of the magnetic fields, and the cavities only become buoyant…
Magnetic fluctuations generated by a tangling of the mean magnetic field by velocity fluctuations are studied in a developed turbulent convection with large magnetic Reynolds numbers. We show that the energy of magnetic fluctuations depends on magnetic Reynolds number only when the mean magnetic field is smaller than $B_{eq} / 4 Rm^{1/4}$, where $B_{eq}$ is the equipartition mean magnetic field determined by the turbulent kinetic energy and Rm is magnetic Reynolds number. Generation of magnetic fluctuations in a turbulent convection with a nonzero mean magnetic field results in a decrease of t…
Active star formation (SF) is tightly related to the dense molecular gas in the giant molecular clouds' dense cores. Our HCN (measure of the dense molecular gas) survey in 65 galaxies (including 10 ultraluminous galaxies) reveals a tight linear correlation between HCN and IR (SF rate) luminosities, whereas the correlation between IR and CO (measure of the total molecular gas) luminosities is nonlinear. This suggests that the global SF rate depends more intimately upon the amount of dense molecular gas than the total molecular gas content. This linear relationship extends to both the dense core…
The decimetric luminosity of many galaxies appears to be dominated by synchrotron emission excited by supernova explosions. Simple models suggest that the luminosity is directly proportional to the rate of supernova explosions of massive stars averaged over the past 30 Myr. The proportionality may be used together with models of the evolving 1.4 GHz luminosity function to estimate the global star formation rate density in the era z < 1. The local value is estimated to be 0.026 solar masses per year per cubic megaparsec, some 50% larger than the value inferred from the Halpha luminosity density…
We examine the effect of magnetic fields on star cluster formation by performing simulations following the self-gravitating collapse of a turbulent molecular cloud to form stars in ideal MHD. The collapse of the cloud is computed for global mass-to-flux ratios of infinity, 20, 10, 5 and 3, that is using both weak and strong magnetic fields. Whilst even at very low strengths the magnetic field is able to significantly influence the star formation process, for magnetic fields with plasma beta < 1 the results are substantially different to the hydrodynamic case. In these cases we find large-scale…
It is established that the formation of rotationally supported disks during the main accretion phase of star formation is suppressed by a moderately strong magnetic field in the ideal MHD limit. Non-ideal MHD effects are expected to weaken the magnetic braking, perhaps allowing the disk to reappear. We concentrate on one such effect, ambipolar diffusion, which enables the field lines to slip relative to the bulk neutral matter. We find that the slippage does not sufficiently weaken the braking to allow rotationally supported disks to form for realistic levels of cloud magnetization and cosmic …
In two previous papers we formulated and solved, for a fiducial set of free parameters, the problem of the formation and evolution of a magnetically supercritical core inside a magnetically subcritical parent cloud. In this paper we present a parameter study to assess the sensitivity of the results (1) to the density at which the equation of state becomes adiabatic; (2) to the initial mass-to-flux ratio of the parent cloud; and (3) to ionization by radioactive decay of different nuclei (40K and 26Al) at high densities (number density > 10^12 particles per cubic cm). We find that (1) the result…
We argue the behavior of Poynting flux-dominated outflows from AGN in the galactic cluster systems by performing three-dimensional MHD simulations within the framework of the "magnetic tower" model. Of particular interests are the structure of MHD waves, the cylindrical radial force balance, the (de)collimation, and the stability properties of magnetic tower jets. Transition between the jet/lobe and the formation of wiggling jet by growing current-driven instability are discussed.
The neutron star formation during the collapse with the strong magnetic field may lead to a mirror symmetry violation and formation of an asymmetric magnetic field. Dependence of the week interaction cross-section on the magnetic field strength lead to the asymmetric neutrino flux and formation of rapidly mooving pulsars due to the recoil action as well as rapidly moving black holes.
An efficient technique for computing perturbation power spectra in field ordering theories of cosmic structure formation is introduced, enabling computations to be carried out with unprecedented precision. Large scale simulations are used to measure unequal time correlators of the source stress energy, taking advantage of scaling during matter and radiation domination, and causality, to make optimal use of the available dynamic range. The correlators are then re-expressed in terms of a sum of eigenvector products, a representation which we argue is optimal, enabling the computation of the fina…
We investigate cosmological structure formation seeded by topological defects which may form during a phase transition in the early universe. First we derive a partially new, local and gauge invariant system of perturbation equations to treat microwave background and dark matter fluctuations induced by topological defects or any other type of seeds. We then show that this system is well suited for numerical analysis of structure formation by applying it to seeds induced by fluctuations of a global scalar field. Our numerical results are complementary to previous investigations since we use sub…
We investigate the dissipation of magnetic flux in primordial star-forming clouds throughout their collapse including the run-away collapse phase as well as the accretion phase. We solve the energy equation and the non-equilibrium chemical reactions in the collapsing gas, in order to obtain the radial distribution of the ionized fraction during the collapse. As a result, we find the ionized fraction is high enough for the magnetic field to couple with the gas throughout the evolution of the cloud. This result suggests that the jet formation from protostars as well as the activation of magneto-…
(ABRIDGED) We use hydrodynamical simulations of disk galaxies to study relations between star formation and properties of the molecular interstellar medium (ISM). We implement a model for the ISM that includes low-temperature (T<10^4K) cooling, directly ties the star formation rate to the molecular gas density, and accounts for the destruction of H2 by an interstellar radiation field from young stars. We demonstrate that the ISM and star formation model simultaneously produces a spatially-resolved molecular-gas surface density Schmidt-Kennicutt relation of the form Sigma_SFR \propto Sigma_Hmol…
Gamma-rays from the decay of $^{26}$Al offer a stringent constraint on the Galaxy's global star formation rate over the past million years, supplementing other methods for quantifying the recent Galactic star formation rate, such as equivalent widths of H$α$ emission. Advantages and disadvantages of using $^{26}$Al gamma-ray measurements as a tracer of the massive star formation rate are analyzed. Estimates of the Galactic $^{26}$Al mass derived from COMPTEL measurements are coupled with a simple, analytical model of the $^{26}$Al injection rate from massive stars and restrict the Galaxy's rec…
We simulate the formation and the evolution of global strings taking into account the expansion of the universe and the concomitant change of the effective potential, that is, the change from the restoration stage of the global {\it U}(1)-symmetry to the broken stage. Starting from the thermal equilibrium state, we run simulations for 100 sets of different initial conditions in 2+1 dimensions. As a result, we find deviation from scale invariance. This is because the energy loss mechanism is pair annihilation rather than intercommutation, so that a string and an anti-string attract each other u…
Using a Newtonian approximation, we developed a quantitative criterion for the collapse of a spherical distribution of matter under an isolated texture field. In particular, we found that the evolution of an overdense region is strongly determined by two parameters: the energy scale of symmetry breaking and the initial radius of the system.
