80 research outputs found
Verzweigung in einem Finite-Elemente Modell für das hydrostatische Skelett
Beyn W-J, Wadepuhl M. Verzweigung in einem Finite-Elemente Modell für das hydrostatische Skelett. Zeitschrift für angewandte Mathematik und Mechanik. 1990;70(4):T272-T274
Local Group Dwarf Galaxies: Nature And Nurture
We investigate the formation and evolution of dwarf galaxies in a high
resolution, hydrodynamical cosmological simulation of a Milky Way sized halo
and its environment. Our simulation includes gas cooling, star formation,
supernova feedback, metal enrichment and UV heating. In total, 90 satellites
and more than 400 isolated dwarf galaxies are formed in the simulation,
allowing a systematic study of the internal and environmental processes that
determine their evolution. We find that 95% of satellite galaxies are gas-free
at z=0, and identify three mechanisms for gas loss: supernova feedback, tidal
stripping, and photo-evaporation due to re-ionization. Gas-rich satellite
galaxies are only found with total masses above ~ 5x10^9 solar masses. In
contrast, for isolated dwarf galaxies, a total mass of ~ 10^9 solar masses
constitutes a sharp transition; less massive galaxies are predominantly
gas-free at z=0, more massive, isolated dwarf galaxies are often able to retain
their gas. In general, we find that the total mass of a dwarf galaxy is the
main factor which determines its star formation, metal enrichment, and its gas
content, but that stripping may explain the observed difference in gas content
between field dwarf galaxies and satellites with total masses close to 10^9
solar masses. We also find that a morphological transformation via tidal
stripping of infalling, luminous dwarf galaxies whose dark matter is less
concentrated than their stars, cannot explain the high total mass-light ratios
of the faint dwarf spheroidal galaxies.Comment: 18 pages, 13 figures, submitted to MNRA
Applying scale-free mass estimators to the Local Group in Constrained Local Universe Simulations
We use the recently proposed scale-free mass estimators to determine the
masses of the Milky Way (MW) and Andromeda (M31) galaxy in a dark matter only
Constrained Local UniversE Simulation (CLUES). While these mass estimators work
rather well for isolated spherical host systems, we examine here their
applicability to a simulated binary system with a unique satellite population
similar to the observed satellites of MW and M31. We confirm that the
scale-free estimators work also very well in our simulated Local Group galaxies
with the right number of satellites which follow the observed radial
distribution. In the isotropic case and under the assumption that the
satellites are tracking the total gravitating mass, the power-law index of the
radial satellite distribution is directly related
to the host's mass profile as .
The use of this relation for any given leads to highly accurate mass
estimations which is a crucial point for observer, since they do not know a
priori the mass profile of the MW and M31 haloes. We discuss possible bias in
the mass estimators and conclude that the scale-free mass estimators can be
satisfactorily applied to the real MW and M31 system.Comment: 14 pages, 6 figures, 6 tables. Accepted in MNRAS 2012 March 29.
Received 2012 March 29; in original form 2011 September 2
Stealth Galaxies in the Halo of the Milky Way
We predict that there is a population of low-luminosity dwarf galaxies
orbiting within the halo of the Milky Way that have surface brightnesses low
enough to have escaped detection in star-count surveys. The overall count of
stealth galaxies is sensitive to the presence (or lack) of a low-mass threshold
in galaxy formation. These systems have luminosities and stellar velocity
dispersions that are similar to those of known ultrafaint dwarf galaxies but
they have more extended stellar distributions (half light radii greater than
about 100 pc) because they inhabit dark subhalos that are slightly less massive
than their higher surface brightness counterparts. As a result, the typical
peak surface brightness is fainter than 30 mag per square arcsec. One
implication is that the inferred common mass scale for Milky Way dwarfs may be
an artifact of selection bias. If there is no sharp threshold in galaxy
formation at low halo mass, then ultrafaint galaxies like Segue 1 represent the
high-mass, early forming tail of a much larger population of objects that could
number in the hundreds and have typical peak circular velocities of about 8
km/s and masses within 300 pc of about 5 million solar masses. Alternatively,
if we impose a low-mass threshold in galaxy formation in order to explain the
unexpectedly high densities of the ultrafaint dwarfs, then we expect only a
handful of stealth galaxies in the halo of the Milky Way. A complete census of
these objects will require deeper sky surveys, 30m-class follow-up telescopes,
and more refined methods to identify extended, self-bound groupings of stars in
the halo.Comment: 12 pages, 7 figures, accepted by ApJ. Several crucial references
added and the discussion has been expanded. Conclusions are unchanged
Satellite galaxies in hydrodynamical simulations of Milky Way sized galaxies
Collisionless simulations of the CDM cosmology predict a plethora of dark
matter substructures in the halos of Milky Way sized galaxies, yet the number
of known luminous satellites galaxies is very much smaller, a discrepancy that
has become known as the `missing satellite problem'. The most massive
substructures have been shown to be plausibly the hosts of the brightest
satellites, but it remains unclear which processes prevent star formation in
the many other, purely dark substructures. We use high-resolution hydrodynamic
simulations of the formation of Milky Way sized galaxies in order to test how
well such self-consistent models of structure formation match the observed
properties of the Galaxy's satellite population. For the first time, we include
in such calculations feedback from cosmic rays injected into the star forming
gas by supernovae as well as the energy input from supermassive black holes
growing at the Milky Way's centre and its progenitor systems. We find that
non-thermal particle populations quite strongly suppress the star formation
efficiency of the smallest galaxies. In fact, our cosmic ray model is able to
reproduce the observed faint-end of the satellite luminosity function, while
models that include only the effects of cosmic reionization, or galactic winds,
do significantly worse. Our simulated satellite population approximately
matches available kinematic data on the satellites and their observed spatial
distribution. We conclude that a proper resolution of the missing satellite
problem likely requires the inclusion of non-standard physics for regulating
star formation in the smallest halos, and that cosmic reionization alone may
not be sufficient.Comment: 20 pages, 17 figure
Galactic winds driven by cosmic-ray streaming
Galactic winds are observed in many spiral galaxies with sizes from dwarfs up
to the Milky Way, and they sometimes carry a mass in excess of that of newly
formed stars by up to a factor of ten. Multiple driving processes of such winds
have been proposed, including thermal pressure due to supernova-heating, UV
radiation pressure on dust grains, or cosmic ray (CR) pressure. We here study
wind formation due to CR physics using a numerical model that accounts for CR
acceleration by supernovae, CR thermalization, and advective CR transport. In
addition, we introduce a novel implementation of CR streaming relative to the
rest frame of the gas. We find that CR streaming drives powerful and sustained
winds in galaxies with virial masses M_200 < 10^{11} Msun. In dwarf galaxies
(M_200 ~ 10^9 Msun) the winds reach a mass loading factor of ~5, expel ~60 per
cent of the initial baryonic mass contained inside the halo's virial radius and
suppress the star formation rate by a factor of ~5. In dwarfs, the winds are
spherically symmetric while in larger galaxies the outflows transition to
bi-conical morphologies that are aligned with the disc's angular momentum axis.
We show that damping of Alfven waves excited by streaming CRs provides a means
of heating the outflows to temperatures that scale with the square of the
escape speed. In larger haloes (M_200 > 10^{11} Msun), CR streaming is able to
drive fountain flows that excite turbulence. For halo masses M_200 > 10^{10}
Msun, we predict an observable level of H-alpha and X-ray emission from the
heated halo gas. We conclude that CR-driven winds should be crucial in
suppressing and regulating the first epoch of galaxy formation, expelling a
large fraction of baryons, and - by extension - aid in shaping the faint end of
the galaxy luminosity function. They should then also be responsible for much
of the metal enrichment of the intergalactic medium.Comment: 25 pages, 14 figures, accepted by MNRA
Satellite Survival in Highly Resolved Milky Way Class Halos
Surprisingly little is known about the origin and evolution of the Milky
Way's satellite galaxy companions. UV photoionisation, supernova feedback and
interactions with the larger host halo are all thought to play a role in
shaping the population of satellites that we observe today, but there is still
no consensus as to which of these effects, if any, dominates. In this paper, we
revisit the issue by re-simulating a Milky Way class dark matter (DM) halo with
unprecedented resolution. Our set of cosmological hydrodynamic Adaptive Mesh
Refinement (AMR) simulations, called the Nut suite, allows us to investigate
the effect of supernova feedback and UV photoionisation at high redshift with
sub-parsec resolution. We subsequently follow the effect of interactions with
the Milky Way-like halo using a lower spatial resolution (50pc) version of the
simulation down to z=0. This latter produces a population of simulated
satellites that we compare to the observed satellites of the Milky Way and M31.
We find that supernova feedback reduces star formation in the least massive
satellites but enhances it in the more massive ones. Photoionisation appears to
play a very minor role in suppressing star and galaxy formation in all
progenitors of satellite halos. By far the largest effect on the satellite
population is found to be the mass of the host and whether gas cooling is
included in the simulation or not. Indeed, inclusion of gas cooling
dramatically reduces the number of satellites captured at high redshift which
survive down to z=0.Comment: 22 pages, 16 figures, accepted for publication in MNRA
The Milky Way's bright satellites as an apparent failure of LCDM
We use the Aquarius simulations to show that the most massive subhalos in
galaxy-mass dark matter halos in LCDM are grossly inconsistent with the
dynamics of the brightest Milky Way dwarf spheroidal galaxies. While the
best-fitting hosts of the dwarf spheroidals all have 12 < Vmax < 25 km/s, LCDM
simulations predict at least ten subhalos with Vmax > 25 km/s. These subhalos
are also among the most massive at earlier times, and significantly exceed the
UV suppression mass back to z ~ 10. No LCDM-based model of the satellite
population of the Milky Way explains this result. The problem lies in the
satellites' densities: it is straightforward to match the observed Milky Way
luminosity function, but doing so requires the dwarf spheroidals to have dark
matter halos that are a factor of ~5 more massive than is observed. Independent
of the difficulty in explaining the absence of these dense, massive subhalos,
there is a basic tension between the derived properties of the bright Milky Way
dwarf spheroidals and LCDM expectations. The inferred infall masses of these
galaxies are all approximately equal and are much lower than standard LCDM
predictions for systems with their luminosities. Consequently, their implied
star formation efficiencies span over two orders of magnitude, from 0.2% to 20%
of baryons converted into stars, in stark contrast with expectations gleaned
from more massive galaxies. We explore possible solutions to these problems
within the context of LCDM and find them to be unconvincing. In particular, we
use controlled simulations to demonstrate that the small stellar masses of the
bright dwarf spheroidals make supernova feedback an unlikely explanation for
their low inferred densities.Comment: 18 pages, 10 figures; matches version published in MNRA
Galactic star formation and accretion histories from matching galaxies to dark matter haloes
We present a new statistical method to determine the relationship between the
stellar masses of galaxies and the masses of their host dark matter haloes over
the entire cosmic history from z~4 to the present. This multi-epoch abundance
matching (MEAM) model self-consistently takes into account that satellite
galaxies first become satellites at times earlier than they are observed. We
employ a redshift-dependent parameterization of the stellar-to-halo mass
relation to populate haloes and subhaloes in the Millennium simulations with
galaxies, requiring that the observed stellar mass functions at several
redshifts be reproduced simultaneously. Using merger trees extracted from the
dark matter simulations in combination with MEAM, we predict the average
assembly histories of galaxies, separating into star formation within the
galaxies (in-situ) and accretion of stars (ex-situ). The peak star formation
efficiency decreases with redshift from 23% at z=0 to 9% at z=4 while the
corresponding halo mass increases from 10^11.8M\odot to 10^12.5M\odot. The star
formation rate of central galaxies peaks at a redshift which depends on halo
mass; for massive haloes this peak is at early cosmic times while for low-mass
galaxies the peak has not been reached yet. In haloes similar to that of the
Milky-Way about half of the central stellar mass is assembled after z=0.7. In
low-mass haloes, the accretion of satellites contributes little to the assembly
of their central galaxies, while in massive haloes more than half of the
central stellar mass is formed ex-situ with significant accretion of satellites
at z<2. We find that our method implies a cosmic star formation history and an
evolution of specific star formation rates which are consistent with those
inferred directly. We present convenient fitting functions for stellar masses,
star formation rates, and accretion rates as functions of halo mass and
redshift.Comment: 20 pages, 12 figures, 1 table, submitted to MNRA
The Aquila comparison project: the effects of feedback and numerical methods on simulations of galaxy formation
We compare the results of various cosmological gas-dynamical codes used to simulate the formation of a galaxy in the Λ cold dark matter structure formation paradigm. The various runs (13 in total) differ in their numerical hydrodynamical treatment [smoothed particle hydrodynamics (SPH), moving mesh and adaptive mesh refinement] but share the same initial conditions and adopt in each case their latest published model of gas cooling, star formation and feedback. Despite the common halo assembly history, we find large code-to-code variations in the stellar mass, size, morphology and gas content of the galaxy at z= 0, due mainly to the different implementations of star formation and feedback. Compared with observation, most codes tend to produce an overly massive galaxy, smaller and less gas rich than typical spirals, with a massive bulge and a declining rotation curve. A stellar disc is discernible in most simulations, although its prominence varies widely from code to code. There is a well-defined trend between the effects of feedback and the severity of the disagreement with observed spirals. In general, models that are more effective at limiting the baryonic mass of the galaxy come closer to matching observed galaxy scaling laws, but often to the detriment of the disc component. Although numerical convergence is not particularly good for any of the codes, our conclusions hold at two different numerical resolutions. Some differences can also be traced to the different numerical techniques; for example, more gas seems able to cool and become available for star formation in grid-based codes than in SPH. However, this effect is small compared to the variations induced by different feedback prescriptions. We conclude that state-of-the-art simulations cannot yet uniquely predict the properties of the baryonic component of a galaxy, even when the assembly history of its host halo is fully specified. Developing feedback algorithms that can effectively regulate the mass of a galaxy without hindering the formation of high angular momentum stellar discs remains a challeng
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