Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

Large reservoirs of cold (~10⁴ K) gas exist out to and beyond the virial radius in the circumgalactic medium (CGM) of all types of galaxies. Photoionization modeling suggests that cold CGM gas has significantly lower densities than expected by theoretical predictions based on thermal pressure equilibrium with hot CGM gas. In this work, we investigate the impact of cosmic-ray physics on the formation of cold gas via thermal instability. We use idealized three-dimensional magnetohydrodynamic simulations to follow the evolution of thermally unstable gas in a gravitationally stratified medium. We find that cosmic-ray pressure lowers the density and increases the size of cold gas clouds formed through thermal instability. We develop a simple model for how the cold cloud sizes and the relative densities of cold and hot gas depend on cosmic-ray pressure. Cosmic-ray pressure can help counteract gravity to keep cold gas in the CGM for longer, thereby increasing the predicted cold mass fraction and decreasing the predicted cold gas inflow rates. Efficient cosmic-ray transport, by streaming or diffusion, redistributes cosmic-ray pressure from the cold gas to the background medium, resulting in cold gas properties that are in between those predicted by simulations with inefficient transport and simulations without cosmic rays. We show that cosmic rays can significantly reduce galactic accretion rates and resolve the tension between theoretical models and observational constraints on the properties of cold CGM gas

Column Density, Kinematics, and Thermal State of Metal-bearing Gas within the Virial Radius of z ∼ 2 Star-forming Galaxies in the Keck Baryonic Structure Survey

We present results from the Keck Baryonic Structure Survey (KBSS) including the first detailed measurements of the column densities, kinematics, and internal energy of metal-bearing gas within the virial radius (35–100 physical kpc) of eight ~L* galaxies at z ~ 2. From our full sample of 130 metal-bearing absorbers, we infer that halo gas is kinematically complex when viewed in singly, doubly, and triply ionized species. Broad O vi and C iv absorbers are detected at velocities similar to the lower-ionization gas but with a very different kinematic structure, indicating that the circumgalactic medium (CGM) is multiphase. There is a high covering fraction of metal-bearing gas within 100 kpc, including highly ionized gas such as O vi; however, observations of a single galaxy probed by a lensed background QSO suggest the size of metal-bearing clouds is small (<400 pc for all but the O vi-bearing gas). The mass in metals found within the halo is substantial, equivalent to ≳25% of the metal mass within the interstellar medium. The gas kinematics unambiguously show that 70% of galaxies with detected metal absorption have some unbound metal-enriched gas, suggesting galactic winds may commonly eject gas from halos at z ~ 2. When modeled assuming that ions with different ionization potentials can originate within a single gaseous structure, significant thermal broadening is detected in CGM absorbers that dominates the internal energy of the gas. Some 40% of the detected gas has temperatures in the range 10^(4.5-5.5) K where cooling times are short, suggesting the CGM is dynamic, with constant heating or cooling to produce this short-lived thermal phase

Column Density, Kinematics, and Thermal State of Metal-bearing Gas within the Virial Radius of z ∼ 2 Star-forming Galaxies in the Keck Baryonic Structure Survey

We present results from the Keck Baryonic Structure Survey (KBSS) including the first detailed measurements of the column densities, kinematics, and internal energy of metal-bearing gas within the virial radius (35–100 physical kpc) of eight ~L* galaxies at z ~ 2. From our full sample of 130 metal-bearing absorbers, we infer that halo gas is kinematically complex when viewed in singly, doubly, and triply ionized species. Broad O vi and C iv absorbers are detected at velocities similar to the lower-ionization gas but with a very different kinematic structure, indicating that the circumgalactic medium (CGM) is multiphase. There is a high covering fraction of metal-bearing gas within 100 kpc, including highly ionized gas such as O vi; however, observations of a single galaxy probed by a lensed background QSO suggest the size of metal-bearing clouds is small (<400 pc for all but the O vi-bearing gas). The mass in metals found within the halo is substantial, equivalent to ≳25% of the metal mass within the interstellar medium. The gas kinematics unambiguously show that 70% of galaxies with detected metal absorption have some unbound metal-enriched gas, suggesting galactic winds may commonly eject gas from halos at z ~ 2. When modeled assuming that ions with different ionization potentials can originate within a single gaseous structure, significant thermal broadening is detected in CGM absorbers that dominates the internal energy of the gas. Some 40% of the detected gas has temperatures in the range 10^(4.5-5.5) K where cooling times are short, suggesting the CGM is dynamic, with constant heating or cooling to produce this short-lived thermal phase

Exploring Halo Substructure with Giant Stars III: First Results from the Grid Giant Star Survey and Discovery of a Possible Nearby Sagittarius Tidal Structure in Virgo

We describe first results of a spectroscopic probe of selected fields from the Grid Giant Star Survey. Multifiber spectroscopy of several hundred stars in a strip of eleven fields along delta approximately -17^{circ}, in the range 12 <~ alpha <~ 17 hours, reveals a group of 8 giants that have kinematical characteristics differing from the main field population, but that as a group maintain coherent, smoothly varying distances and radial velocities with position across the fields. Moreover, these stars have roughly the same abundance, according to their MgH+Mgb absorption line strengths. Photometric parallaxes place these stars in a semi-loop structure, arcing in a contiguous distribution between 5.7 and 7.9 kpc from the Galactic center. The spatial, kinematical, and abundance coherence of these stars suggests that they are part of a diffuse stream of tidal debris, and one roughly consistent with a wrapped, leading tidal arm of the Sagittarius dwarf spheroidal galaxy.Comment: 8 pages including 4 figures. Accepted for publication in ApJ

Organization of Multinational Activities and Ownership Structure

We develop a model in which multinational investors decide about the modes of organization, the locations of production, and the markets to be served. Foreign investments are driven by market-seeking and cost-reducing motives. We further assume that investors face costs of control that vary among sectors and increase in distance. The results show that (i) production intensive sectors are more likely to operate a foreign business independent of the investment motive, (ii) that distance may have a non-monotonous effect on the likelihood of horizontal investments, and (iii) that globalization, if understood as reducing distance, leads to more integration

A Two Micron All-Sky Survey View of the Sagittarius Dwarf Galaxy: II. Swope Telescope Spectroscopy of M Giant Stars in the Dynamically Cold Sagittarius Tidal Stream

We present moderate resolution (~6 km/s) spectroscopy of 284 M giant candidates selected from the Two Micron All Sky Survey photometry. Radial velocities (RVs) are presented for stars mainly in the south, with a number having positions consistent with association to the trailing tidal tail of the Sagittarius (Sgr) dwarf galaxy. The latter show a clear RV trend with orbital longitude, as expected from models of the orbit and destruction of Sgr. A minimum 8 kpc width of the trailing stream about the Sgr orbital midplane is implied by verified RV members. The coldness of this stream (dispersion ~10 km/s) provides upper limits on the combined contributions of stream heating by a lumpy Galactic halo and the intrinsic dispersion of released stars, which is a function of the Sgr core mass. The Sgr trailing arm is consistent with a Galactic halo containing one dominant, LMC-like lump, however some lumpier halos are not ruled out. An upper limit to the total M/L of the Sgr core is 21 in solar units. A second structure that roughly mimics expectations for wrapped, leading Sgr arm debris crosses the trailing arm in the Southern Hemisphere; however, this may also be an unrelated tidal feature. Among the <13 kpc M giants toward the South Galactic Pole are some with large RVs that identify them as halo stars, perhaps part of the Sgr leading arm near the Sun. The positions and RVs of Southern Hemisphere M giants are compared with those of southern globular clusters potentially stripped from the Sgr system and support for association of Pal 2 and Pal 12 with Sgr debris is found. Our discussion includes description of a masked-filtered cross-correlation methodology that achieves better than 1/20 of a resolution element RVs in moderate resolution spectra.Comment: 41 pages, 6 figures, Astronomical Journal, in press (submitted Nov. 24, 2003; tentatively scheduled for July 2004 issue

The GALEX Arecibo SDSS Survey. VI. Second Data Release and Updated Gas Fraction Scaling Relations

We present the second data release from the GALEX Arecibo SDSS Survey (GASS), an ongoing large Arecibo program to measure the HI properties for an unbiased sample of ~1000 galaxies with stellar masses greater than 10^10 Msun and redshifts 0.025<z<0.05. GASS targets are selected from the Sloan Digital Sky Survey (SDSS) spectroscopic and Galaxy Evolution Explorer (GALEX) imaging surveys, and are observed until detected or until a gas mass fraction limit of a few per cent is reached. This second data installment includes new Arecibo observations of 240 galaxies, and marks the 50% of the complete survey. We present catalogs of the HI, optical and ultraviolet parameters for these galaxies, and their HI-line profiles. Having more than doubled the size of the sample since the first data release, we also revisit the main scaling relations of the HI mass fraction with galaxy stellar mass, stellar mass surface density, concentration index, and NUV-r color, as well as the gas fraction plane introduced in our earlier work.Comment: 30 pages, 12 figures. Accepted for publication in A&A. Version with complete Appendix A available at http://www.mpa-garching.mpg.de/GASS/pubs.php . GASS released data can be found at http://www.mpa-garching.mpg.de/GASS/data.ph

GRACKLE: a chemistry and cooling library for astrophysics

We present the Grackle chemistry and cooling library for astrophysical simulations and models. Grackle provides a treatment of non-equilibrium primordial chemistry and cooling for H, D, and He species, including H2 formation on dust grains; tabulated primordial and metal cooling; multiple UV background models; and support for radiation transfer and arbitrary heat sources. The library has an easily implementable interface for simulation codes written in C, C++, and Fortran as well as a Python interface with added convenience functions for semi-analytical models. As an open-source project, Grackle provides a community resource for accessing and disseminating astrochemical data and numerical methods. We present the full details of the core functionality, the simulation and Python interfaces, testing infrastructure, performance, and range of applicability. Grackle is a fully open-source project and new contributions are welcome.Comment: 20 pages, 8 figures, accepted for publication in MNRAS. For more info, visit grackle.readthedocs.i