240 research outputs found
On the evolution of the density pdf in strongly self-gravitating systems
The time evolution of the probability density function (PDF) of the mass
density is formulated and solved for systems in free-fall using a simple
appoximate function for the collapse of a sphere. We demonstrate that a
pressure-free collapse results in a power-law tail on the high-density side of
the PDF. The slope quickly asymptotes to the functional form
for the (volume-weighted) PDF and
for the corresponding mass-weighted
distribution. From the simple approximation of the PDF we derive analytic
descriptions for mass accretion, finding that dynamically quiet systems with
narrow density PDFs lead to retarded star formation and low star formation
rates. Conversely, strong turbulent motions that broaden the PDF accelerate the
collapse causing a bursting mode of star formation. Finally, we compare our
theoretical work with observations. The measured star formation rates are
consistent with our model during the early phases of the collapse. Comparison
of observed column density PDFs with those derived from our model suggests that
observed star-forming cores are roughly in free-fall.Comment: accepted for publication, 13 page
An IFU investigation of possible Lyman continuum escape from Mrk 71/NGC 2366
Mrk 71/NGC 2366 is the closest Green Pea (GP) analog and candidate Lyman
Continuum (LyC) emitter. Recently, 11 LyC-leaking GPs have been detected
through direct observations of the ionizing continuum, making this the most
abundant class of confirmed LyC-emitters at any redshift. High resolution,
multi-wavelength studies of GPs can lead to an understanding of the method(s),
through which LyC escapes from these galaxies. The proximity of Mrk 71/NCG 2366
offers unprecedented detail on the inner workings of a GP analog, and enables
us to identify the mechanisms of LyC escape. We use 5825-7650{\AA} integral
field unit PMAS observations to study the kinematics and physical conditions in
Mrk 71. An electron density map is obtained from the [S II] ratio. A fortuitous
second order contamination by the [O II]3727 doublet enables the construction
of an electron temperature map. Resolved maps of sound speed, thermal
broadening, "true" velocity dispersion, and Mach number are obtained and
compared to the high resolution magneto-hydrodynamic SILCC simulations. Two
regions of increased velocity dispersion indicative of outflows are detected to
the north and south of the super star cluster, knot B, with redshifted and
blueshifted velocities, respectively. We confirm the presence of a faint broad
kinematical component, which is seemingly decoupled from the outflow regions,
and is fainter and narrower than previously reported in the literature. Within
uncertainties, the low- and high-ionization gas move together. Outside of the
core of Mrk 71, an increase in Mach numbers is detected, implying a decrease in
gas density. Simulations suggest this drop in density can be as high as ~4 dex,
down to almost optically thin levels, which would imply a non-zero LyC escape
fraction along the outflows... [abridged]Comment: Accepted for publication in A&A. 17 pages, 16 figures, 4 table
Simulations of cosmic ray propagation
We review numerical methods for simulations of cosmic ray (CR) propagation on
galactic and larger scales. We present the development of algorithms designed
for phenomenological and self-consistent models of CR propagation in kinetic
description based on numerical solutions of the Fokker-Planck equation. The
phenomenological models assume a stationary structure of the galactic
interstellar medium and incorporate diffusion of particles in physical and
momentum space together with advection, spallation, production of secondaries
and various radiation mechanisms. The self-consistent propagation models of CRs
include the dynamical coupling of the CR population to the thermal plasma. The
CR transport equation is discretized and solved numerically together with the
set of magneto-hydrodynamic (MHD) equations in various approaches treating the
CR population as a separate relativistic fluid within the two-fluid approach or
as a spectrally resolved population of particles evolving in physical and
momentum space. The relevant processes incorporated in self-consistent models
include advection, diffusion and streaming well as adiabatic compression and
several radiative loss mechanisms.
We discuss applications of the numerical models for the interpretation of CR
data collected by various instruments. We present example models of
astrophysical processes influencing galactic evolution such as galactic winds,
the amplification of large-scale magnetic fields and instabilities of the
interstellar medium.Comment: 99 pages, 13 figures, to be published in the Living Reviews of
Computational Astrophysic
Gamma-ray emission from spectrally resolved cosmic rays in galaxies
Cosmic rays (CRs) are ubiquitous in the interstellar medium (ISM) of nearby
galaxies, but many of their properties are not well-constrained. Gamma-ray
observations provide a powerful tool in this respect, allowing us to constrain
both the interaction of CR protons with the ISM and their transport properties.
To help better understand the link between observational signatures and CR
physics, we use a series of magneto-hydrodynamical (MHD) AREPO simulations of
isolated galaxies performed using spectrally-resolved CR transport in every
computational cell, with subsequent gamma-ray emission calculated using the
CRAYON+ (Cosmic RAY emissiON) code. In each of our simulated halos, modelling
the energy-dependent spatial diffusion of CRs leads to a more extended
distribution of high-energy (~100 GeV) gamma rays compared to that predicted by
a 'grey' steady-state model, which is especially visible in the corresponding
emission maps and radial profiles. Despite this, the total gamma-ray spectra
can often be well approximated by the steady-state model, although recovering
the same spectral index typically requires a minor variation of the energy
dependence of the diffusion coefficient. Our simulations reproduce the observed
spectral indices and gamma-ray spectra of nearby star-forming galaxies and also
match recent observations of the far infrared--gamma-ray relation. We find,
however, that the spectrally resolved model yields marginally smaller
luminosities for lower star formation rates compared to grey simulations of
CRs. Our work highlights the importance of modelling spectrally resolved CR
transport for an accurate prediction of spatially resolved high-energy
gamma-ray emission, as will be probed by the upcoming Cherenkov Telescope Array
observatory.Comment: 18 pages, 11 figures, submitted to MNRAS, comments are welcome
Understanding star formation in molecular clouds I. Effects of line-of-sight contamination on the column density structure
Column-density maps of molecular clouds are one of the most important
observables in the context of molecular cloud- and star-formation (SF) studies.
With the Herschel satellite it is now possible to determine the column density
from dust emission. We use observations and simulations to demonstrate how LOS
contamination affects the column density probability distribution function
(PDF). We apply a first-order approximation (removing a constant level) to the
molecular clouds of Auriga, Maddalena, Carina and NGC3603. In perfect agreement
with the simulations, we find that the PDFs become broader, the peak shifts to
lower column densities, and the power-law tail of the PDF flattens after
correction. All PDFs have a lognormal part for low column densities with a peak
at Av~2, a deviation point (DP) from the lognormal at Av(DP)~4-5, and a
power-law tail for higher column densities. Assuming a density distribution
rho~r^-alpha, the slopes of the power-law tails correspond to alpha(PDF)=1.8,
1.75, and 2.5 for Auriga, Carina, and NGC3603 (alpha~1.5-2 is consistent
gravitational collapse). We find that low-mass and high-mass SF clouds display
differences in the overall column density structure. Massive clouds assemble
more gas in smaller cloud volumes than low-mass SF ones. However, for both
cloud types, the transition of the PDF from lognormal shape into power-law tail
is found at the same column density (at Av~4-5 mag). Low-mass and high-mass SF
clouds then have the same low column density distribution, most likely
dominated by supersonic turbulence. At higher column densities, collapse and
external pressure can form the power-law tail. The relative importance of the
two processes can vary between clouds and thus lead to the observed differences
in PDF and column density structure.Comment: A&A accepted, 15.12. 201
Importance of the initial conditions for star formation
This thesis investigates the impact of the initial conditions on present-day star formation. Using numerical simulations, we follow the gravitational collapse of dense molecular clouds under different initial turbulent motions and initial density distributions. Our analysis focuses on the morphology of the cloud, the time and location of the formation of stars, the energetics during the collapse, the formation of clusters including their internal structure, their accretion behaviour as well as their mass distribution. The morphology of the cloud and the total number of stars are strongly influenced by the initial type of turbulence and the initial density profile. The results range from almost unperturbed cores with a single star to strongly filamentary cores with hundreds of stars in disconnected clusters. The internal structure of protostellar clusters is systematically but not significantly influenced by the initial conditions. Concerning the accretion rates as well as the dynamical interactions of stars within the clusters, we observe a fairly uniform behaviour, not reflecting the large variations in the initial conditions. The simulations presented in this thesis were performed using the grid-based code FLASH, developed mainly at the University of Chicago
- …