28 research outputs found
Rest-frame UV line emission from the intergalactic medium at 2<z<5
Rest-frame UV emission lines offer the possibility to directly image the gas
around high-redshift galaxies with upcoming optical instruments. We use a suite
of large, hydrodynamical simulations to predict the nature and detectability of
emission lines from the intergalactic medium at 2<z<5. The brightest emission
comes from HI Ly-alpha and the strongest metal line, CIII, is about an order of
magnitude fainter, although HI Ly-alpha may be fainter if the gas is
self-shielded to the UV background or if dust is important. The highest surface
brightness regions for CIV, SiIII, SiIV and OVI are fainter than CIII by
factors of a few. The NV and NeVIII lines, as well as HeII H-alpha, are
substantially weaker but their maximum surface brightnesses still exceed 100
photon/cm^2/s/sr at z=2 (for 2" pixels). Lower ionisation lines arise in denser
and colder gas that produces clumpier emission. The brightest HI Ly-alpha
emission arises in highly overdense gas, but the highest surface brightness
emission from high-ionisation metal lines traces a wider range of
overdensities. Bright metal-line emission traces gas with temperatures close to
the peak of the corresponding emissivity curve. While HI Ly-alpha, HeII
H-alpha, CIII, SiIII, and SiIV are excellent probes of cold accretion flows and
the colder parts of outflows, CIV, NV, OVI, and NeVIII are powerful tracers of
the diffuse WHIM and galactic winds. A comparison of results from simulations
with varying physical prescriptions demonstrates that the predictions for the
brighter metal-line emission are robust to within factors of a few. Several
emission lines from the high-redshift IGM will become detectable in the near
future, possibly starting with the Cosmic Web Imager on Palomar. MUSE and the
Keck Cosmic Web Imager have the potential to revolutionise studies of the
interactions between high-redshift galaxies and their environment. (Abridged)Comment: 21 pages, 17 figures. Accepted for publication by MNRA
A Comparison of Galaxy Merger History Observations and Predictions from Semi-Analytic Models
We present a detailed analysis of predicted galaxy-galaxy merger fractions
and rates in the Millennium simulation and compare these with the most up to
date observations of the same quantities up to z~3. We carry out our analysis
by considering the predicted merger history in the Millennium simulation within
a given time interval, as a function of stellar mass. This method, as opposed
to pair fraction counts, considers mergers that have already taken place, and
allows a more direct comparison with the observed rates and fractions measured
with the concentration-asymmetry-clumpiness (CAS) method. We examine the
evolution of the predicted merger fraction and rate in the Millennium
simulation for galaxies with stellar masses M_* ~ 10^9 - 10^12 M_sun. We find
that the predicted merger rates and fractions match the observations well for
galaxies with M_* > 10^11 M_sun at z<2, while significant discrepancies occur
at lower stellar masses, and at z>2 for M_* > 10^11 M_sun systems. At z>2 the
simulations underpredict the observed merger fractions by a factor of 4-10. The
shape of the predicted merger fraction and rate evolutions are similar to the
observations up to z~2, and peak at 1<z<2 in almost all mass bins. The
exception is the merger rate of galaxies with M_* > 10^11 M_sun. We discuss
possible reasons for these discrepancies, and compare different realisations of
the Millennium simulation to understand the effect of varying the physical
implementation of feedback. We conclude that the comparison is potentially
affected by a number of issues, including uncertainties in interpreting the
observations and simulations in terms of the assumed merger mass ratios and
merger time-scales. (abridged)Comment: 15 pages, 9 figures. References update
The effect of dwarf galaxies disruption in semi-analytic models
We present results for a galaxy formation model that includes a simple
treatment for the disruption of dwarf galaxies by gravitational forces and
galaxy encounters within galaxy clusters. This is implemented a posteriori in a
semi-analytic model by considering the stability of cluster dark matter
sub-haloes at z=0. We assume that a galaxy whose dark matter substructure has
been disrupted will itself disperse, while its stars become part of the
population of intracluster stars responsible for the observed intracluster
light. Despite the simplicity of this assumption, our results show a
substantial improvement over previous models and indicate that the inclusion of
galaxy disruption is indeed a necessary ingredient of galaxy formation models.
We find that galaxy disruption suppresses the number density of dwarf galaxies
by about a factor of two. This makes the slope of the faint end of the galaxy
luminosity function shallower, in agreement with observations. In particular,
the abundance of faint, red galaxies is strongly suppressed. As a result, the
luminosity function of red galaxies and the distinction between the red and the
blue galaxy populations in colour-magnitude relationships are correctly
predicted. Finally, we estimate a fraction of intracluster light comparable to
that found in clusters of galaxies.Comment: 7 pages, 6 figures, accepted for publication in MNRAS, 2 figures
changed and references adde
Semi-analytic Simulations of Galactic Winds: Volume Filling Factor, Ejection of Metals and Parameter Study
We present a semi-analytic treatment of galactic winds within high
resolution, large scale cosmological N-body simulations of a LCDM Universe. The
evolution of winds is investigated by following the expansion of supernova
driven superbubbles around the several hundred thousand galaxies that form in
an approximately spherical region of space with diameter 52 Mpc/h and mean
density close to the mean density of the Universe. We focus our attention on
the impact of winds on the diffuse intergalactic medium. Initial conditions for
mass loss at the base of winds are taken from Shu, Mo and Mao (2003). Results
are presented for the volume filling factor and the mass fraction of the IGM
affected by winds and their dependence on the model parameters is carefully
investigated. The mass loading efficiency of bubbles is a key factor to
determine the evolution of winds and their global impact on the IGM: the higher
the mass loading, the later the IGM is enriched with metals. Galaxies with 10^9
< M_* < 10^10 M_sun are responsible for most of the metals ejected into the IGM
at z=3, while galaxies with M_* < 10^9 M_sun give a non negligible contribution
only at higher redshifts, when larger galaxies have not yet assembled. We find
a higher mean IGM metallicity than Lyalpha forest observations suggest and we
argue that the discrepancy may be explained by the high temperatures of a large
fraction of the metals in winds, which may not leave detectable imprints in
absorption in the Lyalpha forest.Comment: 18 pages, 15 figures. Major changes in the model. Manuscript with
high resolution figures available upon request. MNRAS in pres
Freak Waves in Random Oceanic Sea States
Freak waves are very large, rare events in a random ocean wave train. Here we
study the numerical generation of freak waves in a random sea state
characterized by the JONSWAP power spectrum. We assume, to cubic order in
nonlinearity, that the wave dynamics are governed by the nonlinear Schroedinger
(NLS) equation. We identify two parameters in the power spectrum that control
the nonlinear dynamics: the Phillips parameter and the enhancement
coefficient . We discuss how freak waves in a random sea state are more
likely to occur for large values of and . Our results are
supported by extensive numerical simulations of the NLS equation with random
initial conditions. Comparison with linear simulations are also reported.Comment: 7 pages, 6 figures, to be published in Phys. Rev. Let
Numerical simulations of the Warm-Hot Intergalactic Medium
In this paper we review the current predictions of numerical simulations for
the origin and observability of the warm hot intergalactic medium (WHIM), the
diffuse gas that contains up to 50 per cent of the baryons at z~0. During
structure formation, gravitational accretion shocks emerging from collapsing
regions gradually heat the intergalactic medium (IGM) to temperatures in the
range T~10^5-10^7 K. The WHIM is predicted to radiate most of its energy in the
ultraviolet (UV) and X-ray bands and to contribute a significant fraction of
the soft X-ray background emission. While O VI and C IV absorption systems
arising in the cooler fraction of the WHIM with T~10^5-10^5.5 K are seen in
FUSE and HST observations, models agree that current X-ray telescopes such as
Chandra and XMM-Newton do not have enough sensitivity to detect the hotter
WHIM. However, future missions such as Constellation-X and XEUS might be able
to detect both emission lines and absorption systems from highly ionised atoms
such as O VII, O VIII and Fe XVII.Comment: 18 pages, 5 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 14; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Introduction to Surface Avatar: the First Heterogeneous Robotic Team to be Commanded with Scalable Autonomy from the ISS
Robotics is vital to the continued development toward Lunar and Martian exploration, in-situ resource utilization, and surface infrastructure construction. Large-scale extra-terrestrial missions will require teams of robots with different, complementary capabilities, together with a powerful, intuitive user interface for effective commanding. We introduce Surface Avatar, the newest ISS-to-Earth telerobotic experiment series, to be conducted in 2022-2024. Spearheaded by DLR, together with ESA, Surface Avatar builds on expertise on commanding robots with different levels of autonomy from our past telerobotic experiments: Kontur-2, Haptics, Interact, SUPVIS Justin, and Analog-1. A team of four heterogeneous robots in a multi-site analog environment at DLR are at the command of a crew member on the ISS. The team has a humanoid robot for dexterous object handling, construction and maintenance; a rover for long traverses and sample acquisition; a quadrupedal robot for scouting and exploring difficult terrains; and a lander with robotic arm for component delivery and sample stowage. The crew's command terminal is multimodal, with an intuitive graphical user interface, 3-DOF joystick, and 7-DOF input device with force-feedback. The autonomy of any robot can be scaled up and down depending on the task and the astronaut's preference: acting as an avatar of the crew in haptically-coupled telepresence, or receiving task-level commands like an intelligent co-worker. Through crew performing collaborative tasks in exploration and construction scenarios, we hope to gain insight into how to optimally command robots in a future space mission. This paper presents findings from the first preliminary session in June 2022, and discusses the way forward in the planned experiment sessions