721 research outputs found
On DDO154 and Cold Dark Matter halo profiles
We investigate the claim by Burkert and Silk (1997) that the observed
rotation curve of the dwarf irregular galaxy DDO154 cannot be reconciled with
the universal CDM halo profile of Navarro, Frenk & White (1996,1997) even when
allowance is made for the effect of violent gas outflow events on the structure
of the galaxy. By means of N-body simulations we show that under certain
conditions it is possible to obtain a reasonable fit to the observed rotation
curve without invoking Burkert & Silk's proposed spheroidal MACHO component. We
are able to best reproduce the observed decline in the rotation curve by
postulating additional hidden disc mass, in an amount that is compatible with
disc stability requirements. In the process we improve upon the results of
Navarro, Eke & Frenk (1996) on the formation of halo cores by mass loss by
using actual haloes from Cold Dark Matter simulations instead of Hernquist
(1990) distributions.Comment: LaTeX (mn.sty), 8 pages, 6 figures included; updated to match final
version to appear in MNRA
Feedback from galactic stellar bulges and hot gaseous haloes of galaxies
We demonstrate that the feedback from stellar bulges can play an essential
role in shaping the halo gas of galaxies with substantial bulge components by
conducting 1-D hydrodynamical simulations. The feedback model we consider
consists of two distinct phases: 1) an early starburst during the bulge
formation and 2) a subsequent long-lasting mass and energy injection from
stellar winds of low-mass stars and Type Ia SNe. An energetic outward blastwave
is initiated by the starburst and is maintained and enhanced by the
long-lasting stellar feedback. For a MW-like galactic bulge, this blastwave
sweeps up the halo gas in the proto-galaxy and heats up the surrounding medium
to a scale much beyond the virial radius of the halo, thus the accretion of the
halo hot gas can be completely stopped. In addition, the long-lasting feedback
in the later phase powers a galactic bulge wind that is reverse-shocked at a
large radius in the presence of surrounding intergalactic medium and hence
maintains a hot gaseous halo. As the mass and energy injection decreases with
time, the feedback evolves to a subsonic and quasi-stable outflow, which is
enough to prevent halo gas from cooling. The two phases of the feedback thus
re-enforce each-other's impact on the gas dynamics. The simulation results
demonstrate that the stellar bulge feedback may provide a plausible solution to
the long-standing problems in understanding the MW type galaxies, such as the
"missing stellar feedback" problem and the "over-cooling" problem. The
simulations also show that the properties of the hot gas in the subsonic
outflow state depend sensitively on the environment and the formation history
of the bulge. This dependence and variance may explain the large dispersion in
the X-ray to B-band luminosity ratio of the low Es.Comment: v2, discussions added, accepted for publication in MNRA
Preheating by Previrialization and its Impact on Galaxy Formation
We use recent observations of the HI-mass function to constrain galaxy
formation. The data conflicts with the standard model where most of the gas in
a low-mass dark matter halo is assumed to settle into a disk of cold gas that
is depleted by star formation and supernova-driven outflows until the disk
becomes gravitationally stable. A consistent model can be found if low-mass
haloes are embedded in a preheated medium, with a specific gas entropy ~ 10Kev
cm^2. Such a model simultaneously matches the faint-end slope of the galaxy
luminosity function. We propose a preheating model where the medium around
low-mass haloes is preheated by gravitational pancaking. Since gravitational
tidal fields suppress the formation of low-mass haloes while promoting that of
pancakes, the formation of massive pancakes precedes that of the low-mass
haloes within them. We demonstrate that the progenitors of present-day dark
matter haloes with M<10^{12}h^{-1}\msun were embedded in pancakes of masses
~5x10^{12}h^{-1}\msun at z~2. The formation of such pancakes heats the gas to
a temperature of 5x10^5K and compresses it to an overdensity of ~10. Such gas
has a cooling time that exceeds the age of the Universe at z~2, and has a
specific entropy of ~15Kev cm^2, almost exactly the amount required to explain
the stellar and HI mass functions. (Abridged)Comment: 13 pages, 3 figures. Accepted for publication in MNRA
Galaxy Formation in Preheated Intergalactic Media
We outline a scenario of galaxy formation in which the gas in galaxy-forming
regions was preheated to high entropy by vigorous energy feedback associated
with the formation of stars in old ellipticals and bulges and with AGN
activity. Such preheating likely occurred at redshifts z ~ 2-3, and can produce
the entropy excess observed today in low-mass clusters of galaxies without
destroying the bulk of the Lyman alpha forest. Subsequent galaxy formation is
affected by the preheating, because the gas no longer follows the dark matter
on galaxy scales. The hot gas around galaxy haloes has very shallow profiles
and emits only weakly in the X-ray. Cooling in a preheated halo is not
inside-out, because the cooling efficiency does not change significantly with
radius. Only part of the gas in a protogalaxy region can cool and be accreted
into the final galaxy halo. The accreted gas is likely in diffuse clouds and so
does not lose angular momentum to the dark matter. Cluster ellipticals are
produced by mergers of stellar systems formed prior to the preheating, while
large galaxy disks form in low-density environments where gas accretion can
continue to the present time.Comment: 11 pages, 7 figures, MNRAS submitte
On the relation between the Schmidt and Kennicutt-Schmidt star formation laws and its implications for numerical simulations
When averaged over large scales, star formation in galaxies is observed to
follow the empirical Kennicutt-Schmidt (KS) law for surface densities above a
constant threshold. While the observed law involves surface densities,
theoretical models and simulations generally work with volume density laws
(i.e. Schmidt laws). We derive analytic relations between star formation laws
expressed in terms of surface densities, volume densities, and pressures and we
show how these relations depend on parameters such as the effective equation of
state of the multiphase interstellar medium. Our analytic relations enable us
to implement observed surface density laws into simulations. Because the
parameters of our prescription for star formation are observables, we are not
free to tune them to match the observations. We test our theoretical framework
using high-resolution simulations of isolated disc galaxies that assume an
effective equation of state for the multiphase interstellar medium. We are able
to reproduce the star formation threshold and both the slope and the
normalisation of arbitrary input KS laws without tuning any parameters and with
very little scatter, even for unstable galaxies and even if we use poor
numerical resolution. Moreover, we can do so for arbitrary effective equations
of state. Our prescription therefore enables simulations of galaxies to bypass
our current inability to simulate the formation of stars. On the other hand,
the fact that we can reproduce arbitrary input thresholds and KS laws, rather
than just the particular ones picked out by nature, indicates that simulations
that lack the physics and/or resolution to simulate the multiphase interstellar
medium can only provide limited insight into the origin of the observed star
formation laws.Comment: Accepted for publication in MNRAS, 14 pages and 9 figures. Minor
change
Tidal disruption of satellite galaxies in a semi-analytic model of galaxy formation
We introduce a new physical recipe into the De Lucia and Blaizot version of the Munich semi-analytic model built upon the Millennium dark matter simulation: the tidal stripping of stellar material from satellite galaxies during mergers. To test the significance of the new physical process we apply a Monte Carlo Markov Chain parameter estimation technique constraining the model with the -band luminosity function, colours and the black hole-bulge mass relation. The differences in parameter correlations, and in the allowed regions in likelihood space, reveal the impact of the new physics on the basic ingredients of the model, such as the star-formation laws, feedback recipes and the black hole growth model. With satellite disruption in place, we get a model likelihood four times higher than in the original model, indicating that the new process seems to be favoured by observations. This is achieved mainly due to a reduction in black hole growth that produces a better agreement between the properties of central black holes and host galaxies. Compared to the best-fit model without disruption, the new model removes the excess of dwarf galaxies in the original recipe with a more modest supernova heating. The new model is now consistent with the three observational data sets used to constrain it, while significantly improving the agreement with observations for the distribution of metals in stars. Moreover, the model now follows the build up of intra-cluster light
Intentional and unintentional contributions to nonspecific preparation: Electrophysiological evidence.
The authors hypothesized that there are distinct intentional and unintentional influences on nonspecific preparation for a future event. In 2 experiments, participants responded to an imperative stimulus (
Surface photometry and structure of high redshift disk galaxies in the HDF-S NICMOS field
A photometric study of 22 disk galaxies at redshifs z=0.5-2.6 is conducted,
using deep NICMOS J and H band and STIS open mode observations of the HDF-S
NICMOS parallel field. Rest-frame B-profiles and (U-V) color profiles are
constructed. A number of disks show steeper decrease of luminosity than
exponential, referring to disk truncation. Shape of the luminosity profiles
does not vary with redshift, but galactic sizes decrease significantly. (U-V)
colors and color gradients suggest more intense and centrally concentrated star
formation at earlier epochs. On the basis of (U-V) color and chemical evolution
models, the disks at z~2.5 have formed between z=3.5-7. The studied parameters
are idependent of absolute B luminosity within the sample.Comment: 13 pages, 8 figures, Astron. Astrophys. accepte
Anaerobic Metazoans: No longer an oxymoron
The sediments of a deep-sea hypersaline and sulfidic Mediterranean basin have yielded an unexpected discovery, the first multicellular animals living entirely without oxygen. Reported by Danovaro et al. in BMC Biology, these three new species of Loricifera add a new and remarkable dimension to anoxic ecosystems previously thought to support only unicellular life
On the Origin of Exponential Galaxy Disks
We use a disk galaxy evolution model to investigate whether galaxies with
exponential surface brightness profiles can be produced in a cosmologically
motivated framework for disk galaxy formation. Our model follows the accretion,
cooling, and ejection of baryonic mass, as a function of radius, inside growing
dark matter haloes. The surface density profile of the disk is determined by
detailed angular momentum conservation, starting from the distribution of
specific angular momentum as found in cosmological simulations. Exponential and
quasi-exponential disks can be produced by our model through a combination of
supernova driven galactic outflows (which preferentially remove low angular
momentum material), intrinsic variation in the angular momentum distribution of
the halo gas, and the inefficiency of star formation at large radii. We use
observations from the SDSS NYU-VAGC to show that the median Sersic index of
blue galaxies is a strong function of stellar mass. For blue galaxies, low mass
galaxies have a median n=1.3, while high mass galaxies have a median n=4. Our
model with energy driven outflows correctly reproduces this trend, whereas our
models with momentum driven outflows and no outflows over predict the Sersic
indices in low mass galaxies. We show that the observed fraction of
"bulge-less" exponential galaxies is a strong function of stellar mass. For
Milky-Way mass galaxies less than 0.1% of blue galaxies are bulge-less, whereas
for M33 mass galaxies bulge-less and quasi-bulgeless galaxies are typical.
These results suggest that the difficulty of hierarchical formation models to
produce bulge-less Milky-Way mass galaxies is in fact not a problem. However,
the problem of producing M33 like galaxies remains, and will provide a key test
for hierarchical galaxy formation models. [Abridged]Comment: 23 pages, 13 figures, accepted to MNRAS, two new figure
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