279 research outputs found
Evolution of Multiphase Hot Interstellar Medium in Elliptical Galaxies
We present the results of a variety of simulations concerning the evolution
of multiphase (inhomogeneous) hot interstellar medium (ISM) in elliptical
galaxies. We assume the gases ejected from stars do not mix globally with the
circumferential gas. The ejected gas components evolve separately according to
their birth time, position, and origin. We consider cases where supernova
remnants (SNRs) mix with local ISM. The components with high metal abundance
and/or high density cool and drop out of the hot ISM gas faster than the other
components because of their high metal abundance and/or density. This makes the
average metal abundance of the hot ISM low. Furthermore, since the metal
abundance of mass-loss gas decreases with radius, gas inflow from outer region
makes the average metal abundance of the hot ISM smaller than that of mass-loss
gas in the inner region. As gas ejection rate of stellar system decreases, mass
fraction of mass-loss gas ejected at outer region increases in a galaxy. If the
mixing of SNRs is ineffective, our model predicts that observed [Si/Fe] and
[Mg/Fe] should decrease towards the galactic center because of strong iron
emission by SNRs. In the outer region, where the cooling of time of the ISM is
long, the selective cooling is ineffective and most of gas components remain
hot. Thus, the metal abundance of the ISM in this region directly reflects that
of the gas ejected from stars. Our model shows that supernovae are not
effective heating sources in the inner region of elliptical galaxies, because
most of the energy released by them radiates. Therefore, cooling flow is
established even if the supernova rate is high. Mixing of SNRs with ambient ISM
makes the energy transfer between supernova explosion and ambient ISM more
effective.Comment: 21 pages (AASTeX), 14 figures, accepted for publication in The
Astrophysical Journa
Formation of Sub-galactic Clouds under UV Background Radiation
The effects of the UV background radiation on the formation of sub-galactic
clouds are studied by means of one-dimensional hydrodynamical simulations. The
radiative transfer of the ionizing photons due to the absorption by HI, HeI and
HeII, neglecting the emission, is explicitly taken into account. We find that
the complete suppression of collapse occurs for the clouds with circular
velocities typically in the range V_c \sim 15-40 km/s and the 50% reduction in
the cooled gas mass with V_c \sim 20-55 km/s. These values depend most
sensitively on the collapse epoch of the cloud, the shape of the UV spectrum,
and the evolution of the UV intensity. Compared to the optically thin case,
previously investigated by Thoul & Weinberg (1996), the absorption of the
external UV photon by the intervening medium systematically lowers the above
threshold values by \Delta V_c \sim 5 km/s. Whether the gas can contract or
keeps expanding is roughly determined by the balance between the gravitational
force and the thermal pressure gradient when it is maximally exposed to the
external UV flux. Based on our simulation results, we discuss a number of
implications on galaxy formation, cosmic star formation history, and the
observations of quasar absorption lines. In Appendix, we derive analytical
formulae for the photoionization coefficients and heating rates, which
incorporate the frequency/direction-dependent transfer of external photons.Comment: 38 pages, 16 figures, accepted for publication in Ap
A model for the metallicity evolution of damped Lyman-alpha systems
We apply a physically motivated stellar feedback model to analyse the
statistical properties of damped Lyman-alpha systems (DLAs) expected in the
concordance cold dark matter (CDM) model. Our feedback model produces extended
low-metallicity cold gaseous discs around small galaxies. Since the space
density of galaxies with low circular speeds is high, these discs dominate the
cross-section for the identification of DLAs at all redshifts. The combined
effects of star formation, outflows and infall in our models result in mild
evolution of the N_{HI}-weighted metallicity content in DLAs with redshift,
consistent with observations. According to our model, DLAs contribute only a
small fraction of the volume averaged star formation rate at redshifts z \simlt
5. Our model predicts weak evolution in Omega_{HI} over the redshift range
z=0-5. Furthermore, we show that the cosmological evolution of Omega_{HI} and
the cosmic star formation rate are largely disconnected and conclude that the
evolution of Omega_{HI} as a function of redshift is more likely to tell us
about feedback processes and the evolution of the outer gaseous components of
small galaxies than about the cosmic history of star formation.Comment: 18 pages, 12 figures, accepted for publication in MNRAS (minor
changes
Phase shift keyed systems based on a gain switched laser transmitter
Return-to-Zero (RZ) and Non-Return-to-Zero (NRZ) Differential Phase Shift Keyed (DPSK) systems require cheap and optimal transmitters for widespread implementation. The authors report on a gain switched Discrete Mode (DM) laser that can be employed as a cost efficient transmitter in a 10.7 Gb/s RZ DPSK system and compare its performance to that of a gain switched Distributed Feed-Back (DFB) laser. Experimental results show that the gain switched DM laser readily provides error free performance and a receiver sensitivity of -33.1 dBm in the 10.7 Gbit/s RZ DPSK system. The standard DFB laser on the other hand displays an error floor at 10(-1) in the same RZ DPSK system. The difference in performance, between the two types of gain switched transmitters, is analysed by investigating their linewidths. We also demonstrate, for the first time, the generation of a highly coherent gain switched pulse train which displays a spectral comb of approximately 13 sidebands spaced by the 10.7 GHz modulation frequency. The filtered side-bands are then employed as narrow linewidth Continuous Wave (CW) sources in a 10.7 Gb/s NRZ DPSK system
First-line treatment and outcome of elderly patients with primary central nervous system lymphoma (PCNSL)—a systematic review and individual patient data meta-analysis
Evidence for prognosis and treatment of elderly patient with primary central nervous system is limited. High-dose methotrexate should be applied whenever possible, especially combination with oral alkylating agents is a promising approach. Further combinations with other intravenous drugs do not seem to improve outcome. More prospective trials designed for elderly PCNSL patients are warrante
CHAMPION: Chalmers Hierarchical Atomic, Molecular, Polymeric & Ionic Analysis Toolkit
We present CHAMPION: a software developed to automatically detect
time-dependent bonds between atoms based on their dynamics, classify the local
graph topology around them, and analyze the physicochemical properties of these
topologies by statistical physics. In stark contrast to methodologies where
bonds are detected based on static conditions such as cut-off distances,
CHAMPION considers pairs of atoms to be bound only if they move together and
act as a bound pair over time. Furthermore, the time-dependent global bond
graph is possible to split into dynamically shifting connected components or
subgraphs around a certain chemical motif and thereby allow the physicochemical
properties of each such topology to be analyzed by statistical physics.
Applicable to condensed matter and liquids in general, and electrolytes in
particular, this allows both quantitative and qualitative descriptions of local
structure, as well as dynamical processes such as speciation and diffusion. We
present here a detailed overview of CHAMPION, including its underlying
methodology, implementation and capabilities.Comment: 11 pages, 8 figure
Atomic Hydrogen Gas in Dark-Matter Minihalos and the Compact High Velocity Clouds
We calculate the coupled hydrostatic and ionization structures of
pressure-supported gas clouds that are confined by gravitationally dominant
dark-matter (DM) mini-halos and by an external bounding pressure provided by a
hot medium. We focus on clouds that are photoionized and heated by the
present-day background metagalactic field and determine the conditions for the
formation of warm (WNM), and multi-phased (CNM/WNM) neutral atomic hydrogen
(HI) cores in the DM-dominated clouds. We consider LCDM dark-matter halos, and
we compute models for a wide range of halo masses, total cloud gas masses, and
external bounding pressures. We present models for the pressure-supported HI
structures observed in the Local Group dwarf galaxies Leo A and Sag DIG. We
then construct minihalo models for the multi-phased (and low-metallicity)
compact high-velocity HI clouds (CHVCs). If the CHVCs are drawn from the same
family of halos that successfully reproduce the dwarf galaxy observations, then
the CHVCs must be "circumgalactic objects" with characteristic distances of 150
kpc. For such systems we find that multi-phased behavior occurs for peak WNM HI
column densities between 2e19 and 1e20 cm^-2, consistent with observations. If
the large population of CHVCs represent "missing low-mass satellites" of the
Galaxy, then these clouds must be pressure-confined to keep the gas neutral.
For an implied CHVC minihalo scale velocity of v_s=12 km s^-1, the confining
pressure must exceed ~50 cm^-3 K. A hot (2e6 K) Galactic corona could provide
the required pressure at 150 kpc.Comment: 82 pages, 17 figures. To appear in Astrophysical Journal Supplement.
(Abridged abstract
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