15 research outputs found
From Large to Small Scales: Global Models of the ISM
We review large scale modelling of the ISM with emphasis on the importance to
include the disk-halo-disk duty cycle and to use a dynamical refinement of the
grid (in regions where steep variations of density and pressure occur) for a
realistic modelling of the ISM. We also discuss the necessity of convergence of
the simulation results by comparing 0.625, 1.25 and 2.5 pc resolution
simulations and show that a minimum grid resolution of 1.25 pc is required for
quantitatively reliable results, as there is a rapid convergence for Delta x
\leq 1.1 pc.Comment: 10 pages, 8 figures that include 2 simulation images (in jpeg format)
and 6 plots (4 in ps and 2 in jpeg formats), to appear in the proceedings of
``From Observations to Self-Consistent Modelling of the ISM in Galaxies'',
Kluwe
Turbulence and galactic structure
Interstellar turbulence is driven over a wide range of scales by processes
including spiral arm instabilities and supernovae, and it affects the rate and
morphology of star formation, energy dissipation, and angular momentum transfer
in galaxy disks. Star formation is initiated on large scales by gravitational
instabilities which control the overall rate through the long dynamical time
corresponding to the average ISM density. Stars form at much higher densities
than average, however, and at much faster rates locally, so the slow average
rate arises because the fraction of the gas mass that forms stars at any one
time is low, ~10^{-4}. This low fraction is determined by turbulence
compression, and is apparently independent of specific cloud formation
processes which all operate at lower densities. Turbulence compression also
accounts for the formation of most stars in clusters, along with the cluster
mass spectrum, and it gives a hierarchical distribution to the positions of
these clusters and to star-forming regions in general. Turbulent motions appear
to be very fast in irregular galaxies at high redshift, possibly having speeds
equal to several tenths of the rotation speed in view of the morphology of
chain galaxies and their face-on counterparts. The origin of this turbulence is
not evident, but some of it could come from accretion onto the disk. Such high
turbulence could help drive an early epoch of gas inflow through viscous
torques in galaxies where spiral arms and bars are weak. Such evolution may
lead to bulge or bar formation, or to bar re-formation if a previous bar
dissolved. We show evidence that the bar fraction is about constant with
redshift out to z~1, and model the formation and destruction rates of bars
required to achieve this constancy.Comment: in: Penetrating Bars through Masks of Cosmic Dust: The Hubble Tuning
Fork strikes a New Note, Eds., K. Freeman, D. Block, I. Puerari, R. Groess,
Dordrecht: Kluwer, in press (presented at a conference in South Africa, June
7-12, 2004). 19 pgs, 5 figure
The History of the Baryon Budget: Cosmic Logistics in a Hierarchical Universe
Using a series of high-resolution N-body hydrodynamical numerical
simulations, we investigate several scenarios for the evolution of the baryon
budget in galactic halos. We derive individual halo star formation history
(SFH), as well as the global star formation rate in the universe. We develop a
simple analytical model that allows us to compute surprisingly accurate
predictions, when compared to our simulations, but also to other simulations
presented in Springel & Hernquist (2003). The model depends on two main
parameters: the star formation time scale t* and the wind efficiency eta_w. We
also compute, for halos of a given mass, the baryon fraction in each of the
following phases: cold disc gas, hot halo gas and stars. Here again, our
analytical model predictions are in good agreement with simulation results, if
one correctly takes into account finite resolution effect.
We compare predictions of our analytical model to several observational
constraints, and conclude that a very narrow range of the model parameters is
allowed. The important role played by galactic winds is outlined, as well as a
possible `superwind' scenario in groups and clusters. The `anti-hierarchical'
behavior of observed SFH is well reproduced by our best model with t*=3Gyr and
eta_w=1.5. We obtain in this case a present-day cosmic baryon budget of Omega*=
0.004, Omega_cold=0.0004, Omega_hot=0.01 and Omega_back=0.02 (diffuse
background).Comment: 33 pages, 21 figures, accepted for publication in A&
Early galaxy formation and its large-scale effects
Galaxy formation is at the heart of our understanding of cosmic evolution. Although there is a consensus that galaxies emerged from the expanding matter background by gravitational instability of primordial fluctuations, a number of additional physical processes must be understood and implemented in theoretical models before these can be reliably used to interpret observations. In parallel, the astonishing recent progresses made in detecting galaxies that formed only a few hundreds of million years after the Big Bang is pushing the quest for more sophisticated and detailed studies of early structures. In this review, we combine the information gleaned from different theoretical models/studies to build a coherent picture of the Universe in its early stages which includes the physics of galaxy formation along with the impact that early structures had on large-scale processes as cosmic reionization and metal enrichment of the intergalactic medium
Consequences of Starbursts for the Interstellar and Intergalactic Medium
Star formation in general, and starbursts in particular, drive the evolution of galaxies. To understand the process of galactic matter cycle quantitatively, it is absolutely necessary to follow the evolution of the components of the interstellar medium, such as gas, magnetic fields, cosmic rays in detail over sufficiently long time scales. Due to the non-linearity of the interactions between the various components, and the turbulent nature of the plasma, high resolution numerical simulations offer the best strategy to further our understanding. The results of our numerical studies can be summarized as follows: (i) Supernova explosions are the most important energy input sources in the ISM and lead to a high level of turbulence in the plasma, coupling structures on all scales, (ii) more than half of the disk mass resides in classically thermally unstable temperature regimes, (iii) turbulent mixing is the dominant energy transport process over a wide range of scales, (iv) proportionality between magnetic field and density is generally weak, except for the densest regions, (v) magnetic fields, even if they are parallel to the galactic disk, cannot prevent outflow into the halo, (vi) the ionization structure of the plasma depends on its thermal history, and is in general not in collisional ionization equilibrium, (vii) the cooling function varies in space and time, (viii) X-rays can be emitted even at plasma temperatures of the order of 104K due to delayed recombination, both in the disk and the halo, (ix) cosmic rays can help driving a galactic wind, (x) cosmic rays can be accelerated to high energies beyond 1015eV (the âkneeâ) in long lived shocks propagating into the galactic halo, because of time-dependent star formation
PRISM (Polarized Radiation Imaging and Spectroscopy Mission): an extended white paper
Contains fulltext :
126057.pdf (preprint version ) (Open Access