Strongly Time-Variable Ultra-Violet Metal Line Emission from the Circum-Galactic Medium of High-Redshift Galaxies

We use cosmological simulations from the Feedback In Realistic Environments (FIRE) project, which implement a comprehensive set of stellar feedback processes, to study ultra-violet (UV) metal line emission from the circum-galactic medium of high-redshift (z=2-4) galaxies. Our simulations cover the halo mass range Mh ~ 2x10^11 - 8.5x10^12 Msun at z=2, representative of Lyman break galaxies. Of the transitions we analyze, the low-ionization C III (977 A) and Si III (1207 A) emission lines are the most luminous, with C IV (1548 A) and Si IV (1394 A) also showing interesting spatially-extended structures. The more massive halos are on average more UV-luminous. The UV metal line emission from galactic halos in our simulations arises primarily from collisionally ionized gas and is strongly time variable, with peak-to-trough variations of up to ~2 dex. The peaks of UV metal line luminosity correspond closely to massive and energetic mass outflow events, which follow bursts of star formation and inject sufficient energy into galactic halos to power the metal line emission. The strong time variability implies that even some relatively low-mass halos may be detectable. Conversely, flux-limited samples will be biased toward halos whose central galaxy has recently experienced a strong burst of star formation. Spatially-extended UV metal line emission around high-redshift galaxies should be detectable by current and upcoming integral field spectrographs such as the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope and Keck Cosmic Web Imager (KCWI).Comment: 16 pages, 8 figures, accepted for publication in MNRA

Dynamic Ly alpha jets

The solar chromosphere and transition region are highly structured and complex regimes. A recent breakthrough has been the identification of dynamic fibrils observed in H alpha as caused by field-aligned magnetoacoustic shocks. We seek to find whether such dynamic fibrils are also observed in Ly alpha. We used a brief sequence of four high-resolution Ly alpha images of the solar limb taken by the Very high Angular resolution ULtraviolet Telescope (VAULT), which displays many extending and retracting Ly alpha jets. We measured their top trajectories and fitted parabolas to the 30 best-defined ones. Most jet tops move supersonically. Half of them decelerate, sometimes superballistically, the others accelerate. This bifurcation may arise from incomplete sampling of recurrent jets. The similarities between dynamic Ly alpha jets and H alpha fibrils suggest that the magnetoacoustic shocks causing dynamic H alpha fibrils also affect dynamic Ly alpha jets.Comment: 5 pages, 7 figures; changed title and content; accepted in Astronomy and Astrophysics; eps figures in full resolution are available at http://www.astro.sk/~koza/publications/vault/figs

The effect of magnetic fields on properties of the circumgalactic medium

We study the effect of magnetic fields on a simulated galaxy and its surrounding gaseous halo, or circumgalactic medium (CGM), within cosmological 'zoom-in' simulations of a Milky Way-mass galaxy as part of the Simulating the Universe with Refined Galaxy Environments (SURGE) project. We use three different galaxy formation models, each with and without magnetic fields, and include additional spatial refinement in the CGM to improve its resolution. The central galaxy's star formation rate and stellar mass are not strongly affected by the presence of magnetic fields, but the galaxy is more disc dominated and its central black hole is more massive when B > 0. The physical properties of the CGM change significantly. With magnetic fields, the circumgalactic gas flows are slower, the atomic hydrogen-dominated extended discs around the galaxy are more massive and the densities in the inner CGM are therefore higher, the temperatures in the outer CGM are higher, and the pressure in the halo is higher and smoother. The total gas fraction and metal mass fraction in the halo are also higher when magnetic fields are included, because less gas escapes the halo. Additionally, we find that the CGM properties depend on azimuthal angle and that magnetic fields reduce the scatter in radial velocity, whilst enhancing the scatter in metallicity at fixed azimuthal angle. The metals are thus less well-mixed throughout the halo, resulting in more metal-poor halo gas. These results together show that magnetic fields in the CGM change the flow of gas in galaxy haloes, making it more difficult for metal-rich outflows to mix with the metal-poor CGM and to escape the halo, and therefore should be included in simulations of galaxy formation

Neutron star mergers and rare core-collapse supernovae as sources of r-process enrichment in simulated galaxies

We use cosmological, magnetohydrodynamical simulations of Milky Way-mass galaxies from the Auriga project to study their enrichment with rapid neutron capture (r-process) elements. We implement a variety of enrichment models from both binary neutron star mergers and rare core-collapse supernovae. We focus on the abundances of (extremely) metal-poor stars, most of which were formed during the first ∼Gyr of the Universe in external galaxies and later accreted on to the main galaxy. We find that the majority of metal-poor stars are r-process enriched in all our enrichment models. Neutron star merger models result in a median r-process abundance ratio, which increases with metallicity, whereas the median trend in rare core-collapse supernova models is approximately flat. The scatter in r-process abundance increases for models with longer delay times or lower rates of r-process-producing events. Our results are nearly perfectly converged, in part due to the mixing of gas between mesh cells in the simulations. Additionally, different Milky Way-mass galaxies show only small variation in their respective r-process abundance ratios. Current (sparse and potentially biased) observations of metal-poor stars in the Milky Way seem to prefer rare core-collapse supernovae over neutron star mergers as the dominant source of r-process elements at low metallicity, but we discuss possible caveats to our models. Dwarf galaxies that experience a single r-process event early in their history show highly enhanced r-process abundances at low metallicity, which is seen both in observations and in our simulations. We also find that the elements produced in a single event are mixed with ≈108 M≥ of gas relatively quickly, distributing the r-process elements over a large region

The impact of natal kicks on galactic r-process enrichment by neutron star mergers

We study galactic enrichment with rapid neutron capture (r-process) elements in cosmological, magnetohydrodynamical simulations of a Milky Way-mass galaxy. We include a variety of enrichment models, based on either neutron star mergers or a rare class of core-collapse supernova as sole r-process sources. For the first time in cosmological simulations, we implement neutron star natal kicks on-the-fly to study their impact. With kicks, neutron star mergers are more likely to occur outside the galaxy disc, but how far the binaries travel before merging also depends on the kick velocity distribution and shape of the delay time distribution for neutron star mergers. In our fiducial model, the median r-process abundance ratio is somewhat lower and the trend with metallicity is slightly steeper when kicks are included. In a model 'optimized' to better match observations, with a higher rate of early neutron star mergers, the median r-process abundances are fairly unaffected by kicks. In both models, the scatter in r-process abundances is much larger with natal kicks, especially at low metallicity, giving rise to more r-process enhanced stars. We experimented with a range of kick velocities and find that with lower velocities, the scatter is reduced, but is still larger than without natal kicks. We discuss the possibility that the observed scatter in r-process abundances is predominantly caused by natal kicks removing the r-process sources far from their birth sites, making enrichment more inhomogeneous, rather than the usual interpretation that the scatter is set by the rarity of its production source