1,523 research outputs found
Two Body Relaxation in Simulated Cosmological Haloes
This paper aims at quantifying discreetness effects, born of finite particle
number, on the dynamics of dark matter haloes forming in the context of
cosmological simulations. By generalising the standard calculation of two body
relaxation to the case when the size and mass distribution are variable, and
parametrising the time evolution using established empirical relations, we find
that the dynamics of a million particle halo is noise-dominated within the
inner percent of the final virial radius. Far larger particle numbers (~ 10^8)
are required for the RMS perturbations to the velocity to drop to the 10 %
level there. The radial scaling of the relaxation time is simple and strong:
t_relax ~ r^2, implying that numbers >> 10^8 are required to faithfully model
the very inner regions; artificial relaxation may thus constitute an important
factor, contributing to the contradictory claims concerning the persistence of
a power law density cusp to the very centre. The cores of substructure haloes
can be many relaxation times old. Since relaxation first causes their expansion
before recontraction occurs, it may render them either more difficult or easier
to disrupt, depending on their orbital parameters. It may thus modify the
characteristics of the subhalo distribution and effects of interactions with
the parent. We derive simple closed form formulas for the characteristic
relaxation times, as well as for the weak N-scaling reported by Diemand et al.
when the main contribution comes from relaxing subhaloes (abridged).Comment: 11 Pages, 7 figs, Monthly Notices styl
The coupling between the core/cusp and missing satellite problems
We calculate the energy that baryons must inject in cold dark matter (CDM)
haloes in order to remove centrally-divergent DM cusps on scales relevant to
observations of dwarf spheroidal galaxies (dSphs). We estimate that the CDM
haloes often associated with the Milky Way's dSphs (M_vir/M_\odot \sim
10^{9-10}) require \Delta E/erg \sim 10^{53-55} in order to form cores on
scales comparable to the luminous size of these galaxies. While supernova type
II (SNeII) explosions can in principle generate this energy, the actual
contribution is limited by the low star formation efficiency implied by the
abundance of luminous satellites. Considering that CDM's well-known `core/cusp'
and `missing satellite' problems place opposing demands on star formation
efficiencies, existing observational evidences for large cores in the most
luminous dSphs require that CDM models invoke some combination of the
following: (i) efficient (of order unity) coupling of SNeII energy into dark
matter particles, (ii) star formation histories peaking at unexpectedly high
redshifts (z>6), (iii) a top-heavy stellar IMF, and/or (iv) substantial
satellite disruption or other stochastic effects to ease the substructure
abundance constraints. Our models show that the tension between CDM problems on
small scales would increase if cored DM profiles were to be found in fainter
dwarves.Comment: Accepted for publication in ApJ Letters. Minor changes to match
published versio
Parkinson's disease medication alters small intestinal motility and microbiota composition in healthy rats
Parkinson’s disease (PD) is known to be associated with altered gastrointestinal function and microbiota composition. To date, the effect of PD medication on the gastrointestinal function and microbiota, at the site of drug absorption, the small intestine, has not been studied, although it may represent an important confounder in reported microbiota alterations observed in PD patients. To this end, healthy (non-PD) wild-type Groningen rats were employed and treated with dopamine, pramipexole (in combination with levodopa-carbidopa), or ropinirole (in combination with levodopa-carbidopa) for 14 sequential days. Rats treated with dopamine agonists showed a significant reduction in small intestinal motility and an increase in bacterial overgrowth in the distal small intestine. Notably, significant alterations in microbial taxa were observed between the treated and vehicle groups; analogous to the changes previously reported in human PD versus healthy control microbiota studies. These microbial changes included an increase in Lactobacillus and Bifidobacterium and a decrease in Lachnospiraceae and Prevotellaceae. Markedly, certain Lactobacillus species correlated negatively with levodopa levels in the systemic circulation, potentially affecting the bioavailability of levodopa. Overall, the study highlights a significant effect of PD medication intrinsically on disease-associated comorbidities, including gastrointestinal dysfunction and small intestinal bacterial overgrowth, as well as the gut microbiota composition. The results urge future studies to take into account the influence of PD medication per se when seeking to identify microbiota-related biomarkers for PD. IMPORTANCE Parkinson’s disease (PD) is the second most common neurodegenerative disorder and is known to be associated with altered gastrointestinal function and microbiota composition. We previously showed that the gut bacteria harboring tyrosine decarboxylase enzymes interfere with levodopa, the main treatment for PD (S. P. van Kessel, A. K. Frye, A. O. El-Gendy, M. Castejon, A. Keshavarzian, G. van Dijk, and S. El Aidy, Nat Commun 10:310, 2019). Although PD medication could be an important confounder in the reported alterations, its effect, apart from the disease itself, on the microbiota composition or the gastrointestinal function at the site of drug absorption, the small intestine, has not been studied. The findings presented here show a significant impact of commonly prescribed PD medication on the small intestinal motility, small intestinal bacterial overgrowth, and microbiota composition, irrespective of the PD. Remarkably, we observed negative associations between bacterial species harboring tyrosine decarboxylase activity and levodopa levels in the systemic circulation, potentially affecting the bioavailability of levodopa. Overall, this study shows that PD medication is an important factor in determining gastrointestinal motility and, in turn, microbiota composition and may, partly, explain the differential abundant taxa previously reported in the cross-sectional PD microbiota human studies. The results urge future studies to take into account the influence of PD medication on gut motility and microbiota composition when seeking to identify microbiota-related biomarkers for PD
Galactic halo cusp-core: tidal compression in mergers
We explain in simple terms how the buildup of dark haloes by merging compact
satellites, as in the CDM cosmology, inevitably leads to an inner cusp of
density profile with \alpha \gsim 1, as seen in
cosmological N-body simulations. A flatter halo core with exerts on
the satellites tidal compression in all directions, which prevents deposit of
stripped satellite material in the core region. This makes the satellite orbits
decay from the radius where to the halo centre with no local
tidal mass transfer and thus causes a rapid steepening of the inner profile to
. These tidal effects, the resultant steepening of the profile to a
cusp, and the stability of this cusp to tandem mergers with compact satellites,
are demonstrated using N-body simulations. The transition at is
then addressed using toy models in the limiting cases of impulse and adiabatic
approximations and using tidal radii for satellites on radial and circular
orbits. In an associated paper we address the subsequent slow convergence from
either side to an asymptotic stable cusp with \alpha \gsim 1. Our analysis
thus implies that an inner cusp is enforced when small haloes are typically
more compact than larger haloes, as in the CDM scenario, such that enough
satellite material makes it intact into the inner halo and is deposited there.
We conclude that a necessary condition for maintaining a flat core, as
indicated by observations, is that the inner regions of the CDM satellite
haloes be puffed up by about 50% such that when they merge into a bigger halo
they would be disrupted outside the halo core. This puffing up could be due to
baryonic feedback processes in small haloes, which may be stimulated by the
tidal compression in the halo cores.Comment: 19 pages, Latex, mn2e.cls, some revisions, MNRAS in pres
A profile in FIRE: resolving the radial distributions of satellite galaxies in the Local Group with simulations
While many tensions between Local Group (LG) satellite galaxies and LCDM
cosmology have been alleviated through recent cosmological simulations, the
spatial distribution of satellites remains an important test of physical models
and physical versus numerical disruption in simulations. Using the FIRE-2
cosmological zoom-in baryonic simulations, we examine the radial distributions
of satellites with Mstar > 10^5 Msun around 8 isolated Milky Way- (MW) mass
host galaxies and 4 hosts in LG-like pairs. We demonstrate that these
simulations resolve the survival and physical destruction of satellites with
Mstar >~ 10^5 Msun. The simulations broadly agree with LG observations,
spanning the radial profiles around the MW and M31. This agreement does not
depend strongly on satellite mass, even at distances <~ 100 kpc. Host-to-host
variation dominates the scatter in satellite counts within 300 kpc of the
hosts, while time variation dominates scatter within 50 kpc. More massive host
galaxies within our sample have fewer satellites at small distances, likely
because of enhanced tidal destruction of satellites via the baryonic disks of
host galaxies. Furthermore, we quantify and provide fits to the tidal depletion
of subhalos in baryonic relative to dark matter-only simulations as a function
of distance. Our simulated profiles imply observational incompleteness in the
LG even at Mstar >~ 10^5 Msun: we predict 2-10 such satellites to be discovered
around the MW and possibly 6-9 around M31. To provide cosmological context, we
compare our results with the radial profiles of satellites around MW analogs in
the SAGA survey, finding that our simulations are broadly consistent with most
SAGA systems.Comment: 18 pages, 10 figures, plus appendices. Main results in figures 2, 3,
and 4. Accepted versio
Dynamical friction for accelerated motion in a gaseous medium
Dynamical friction arises from the interaction of a perturber and the
gravitational wake it excites in the ambient medium. This interaction is
usually derived assuming that the perturber has a constant velocity. In
realistic situations, motion is accelerated as for instance by dynamical
friction itself. Here, we study the effect of acceleration on the dynamical
friction force. We characterize the density enhancement associated with a
constantly accelerating perturber with rectilinear motion in an infinite
homogeneous gaseous medium and show that dynamical friction is not a local
force and that its amplitude may depend on the perturber's initial velocity.
The force on an accelerating perturber is maximal between Mach 1 and Mach 2,
where it is smaller than the corresponding uniform motion friction. In the
limit where the perturber's size is much smaller than the distance needed to
change the Mach number by unity through acceleration, a subsonic perturber
feels a force similar to uniform motion friction only if its past history does
not include supersonic episodes. Once an accelerating perturber reaches large
supersonic speeds, accelerated motion friction is marginally stronger than
uniform motion friction. The force on a decelerating supersonic perturber is
weaker than uniform motion friction as the velocity decreases to a few times
the sound speed. Dynamical friction on a decelerating subsonic perturber with
an initial Mach number larger than 2 is much larger than uniform motion
friction and tends to a finite value as the velocity vanishes in contrast to
uniform motion friction.Comment: Published in MNRAS. Revised version (minor typos corrected
Dark and luminous satellites of LMC-mass galaxies in the FIRE simulations
Within lambda cold dark matter (CDM), dwarf galaxies like the Large Magellanic Cloud (LMC) are expected to host numerous dark matter subhaloes, several of which should host faint dwarf companions. Recent Gaia proper motions confirm new members of the LMC system in addition to the previously known SMC, including two classical dwarf galaxies (M∗ > 105 M; Carina and Fornax) as well as several ultrafaint dwarfs (Car2, Car3, Hor1, and Hyd1). We use the Feedback In Realistic Environments (FIRE) simulations to study the dark and luminous (down to ultrafaint masses, M∗ ∼6×103 M) substructure population of isolated LMC-mass hosts (M200m = 1–3×1011 M) and place the Gaia + DES results in a cosmological context. By comparing number counts of subhaloes in simulations with and without baryons, we find that, within 0.2 r200m, LMC-mass hosts deplete ∼30 per cent of their substructure, significantly lower than the ∼70 per cent of substructure depleted by Milky Way (MW) mass hosts. For our highest resolution runs (mbary = 880 M), ∼ 5–10 subhaloes form galaxies with M∗ ≥ 104 M , in agreement with the seven observationally inferred pre-infall LMC companions. However, we find steeper simulated luminosity functions than observed, hinting at observation incompleteness at the faint end. The predicted DM content for classical satellites in FIRE agrees with observed estimates for Carina and Fornax, supporting the case for an LMC association. We predict that tidal stripping within the LMC potential lowers the inner dark matter density of ultrafaint companions of the LMC. Thus, in addition to their orbital consistency, the low densities of dwarfs Car2, Hyd1, and Hyd2 reinforce their likelihood of Magellanic association
Star formation histories of dwarf galaxies in the FIRE simulations: dependence on mass and Local Group environment
We study star formation histories (SFHs) of dwarf galaxies
(stellar mass ) from FIRE-2 cosmological zoom-in
simulations. We compare dwarfs around individual Milky Way (MW)-mass galaxies,
dwarfs in Local Group (LG)-like environments, and true field (i.e. isolated)
dwarf galaxies. We reproduce observed trends wherein higher-mass dwarfs quench
later (if at all), regardless of environment. We also identify differences
between the environments, both in terms of "satellite vs. central" and "LG vs.
individual MWvs. isolated dwarf central." Around the individual MW-mass hosts,
we recover the result expected from environmental quenching: central galaxies
in the "near field" have more extended SFHs than their satellite counterparts,
with the former more closely resemble isolated ("true field") dwarfs (though
near-field centrals are still somewhat earlier forming). However, this
difference is muted in the LG-like environments, where both near-field centrals
and satellites have similar SFHs, which resemble satellites of single MW-mass
hosts. This distinction is strongest for but
exists at other masses. Our results suggest that the paired halo nature of the
LG may regulate star formation in dwarf galaxies even beyond the virial radii
of the MW and Andromeda. Caution is needed when comparing zoom-in simulations
targeting isolated dwarf galaxies against observed dwarf galaxies in the LG.Comment: Main text: 11 pages, 8 figures; appendices: 4 pages, 4 figures.
Submitted to MNRAS; comments welcom
A detailed statistical analysis of the mass profiles of galaxy clusters
The distribution of mass in the halos of galaxies and galaxy clusters has
been probed observationally, theoretically, and in numerical simulations. Yet
there is still confusion about which of several suggested parameterized models
is the better representation, and whether these models are universal. We use
the temperature and density profiles of the intracluster medium as measured by
X-ray observations of 11 relaxed galaxy clusters to investigate mass models for
the halo using a thorough Bayesian statistical analysis. We make careful
comparisons between two- and three-parameter models, including the issue of a
universal third parameter. We find that, of the two-parameter models, the NFW
is the best representation, but we also find moderate statistical evidence that
a generalized three-parameter NFW model with a freely varying inner slope is
preferred, despite penalizing against the extra degree of freedom. There is a
strong indication that this inner slope needs to be determined for each cluster
individually, i.e. some clusters have central cores and others have steep
cusps. The mass-concentration relation of our sample is in reasonable agreement
with predictions based on numerical simulations.Comment: 10 pages, 5 figures, accepted by ApJ. Matches accepted versio
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