124 research outputs found
Effects of bariatric surgery on adipokine-induced inflammation and insulin resistance
Over a third of the US population is obese and at high risk for developing type 2 diabetes, insulin resistance, and other metabolic disorders. Obesity is considered a chronic lowgrade inflammatory condition that is primarily attributed to expansion and inflammation of adipose tissues. Indeed, adipocytes produce and secrete numerous proinflammatory and anti-inflammatory cytokines known as adipokines.When the balance of these adipokines is shifted toward higher production of proinflammatory factors, local inflammation within adipose tissues and subsequently systemic inflammation occur. These adipokines including leptin, visfatin, resistin, apelin, vaspin, and retinol binding protein-4 can regulate inflammatory responses and contribute to the pathogenesis of diabetes.These effects are mediated by key inflammatory signaling molecules including activated serine kinases such as c-Jun N-terminal kinase and serine kinases inhibitor κB kinase and insulin signaling molecules including insulin receptor substrates, protein kinase B (PKB, also known as Akt), and nuclear factor kappa B. Bariatric surgery can decrease body weight and improve insulin resistance in morbidly obese subjects. However, despite reports suggesting reduced inflammation and weight-independent effects of bariatric surgery on glucose metabolism, mechanisms behind such improvements are not yet well understood. This review article focuses on some of these novel adipokines and discusses their changes after bariatric surgery and their relationship to insulin resistance, fat mass, inflammation, and glucose homeostasis
Dwarfs Gobbling Dwarfs: A Stellar Tidal Stream Around NGC 4449 and Hierarchical Galaxy Formation on Small Scales
A candidate diffuse stellar substructure was previously reported in the halo
of the nearby dwarf starburst galaxy NGC 4449 by Karachentsev et al. We map and
analyze this feature using a unique combination of deep integrated-light images
from the Black Bird 0.5-meter telescope, and high-resolution wide-field images
from the 8-meter Subaru telescope, which resolve the nebulosity into a stream
of red giant branch stars, and confirm its physical association with NGC 4449.
The properties of the stream imply a massive dwarf spheroidal progenitor, which
after complete disruption will deposit an amount of stellar mass that is
comparable to the existing stellar halo of the main galaxy. The ratio between
luminosity or stellar-mass between the two galaxies is ~1:50, while the
indirectly measured dynamical mass-ratio, when including dark matter, may be
~1:10-1:5. This system may thus represent a "stealth" merger, where an
infalling satellite galaxy is nearly undetectable by conventional means, yet
has a substantial dynamical influence on its host galaxy. This singular
discovery also suggests that satellite accretion can play a significant role in
building up the stellar halos of low-mass galaxies, and possibly in triggering
their starbursts.Comment: ApJ Letters, in press. Minor changes. 6 pages, 3 figures. A high
resolution version of the paper and Subaru image are available at
http://www.cosmotography.com/images/small_ngc4449.htm
An analytical model for the accretion of dark matter subhalos
An analytical model is developed for the mass function of cold dark matter
subhalos at the time of accretion and for the distribution of their accretion
times. Our model is based on the model of Zhao et al. (2009) for the median
assembly histories of dark matter halos, combined with a simple log-normal
distribution to describe the scatter in the main-branch mass at a given time
for halos of the same final mass. Our model is simple, and can be used to
predict the un-evolved subhalo mass function, the mass function of subhalos
accreted at a given time, the accretion-time distribution of subhalos of a
given initial mass, and the frequency of major mergers as a function of time.
We test our model using high-resolution cosmological -body simulations, and
find that our model predictions match the simulation results remarkably well.
Finally, we discuss the implications of our model for the evolution of subhalos
in their hosts and for the construction of a self-consistent model to link
galaxies and dark matter halos at different cosmic times.Comment: 14 pages, 10 figures (caption for figure 10 fixed). Accepted for
publication in Ap
Large scale bias and the inaccuracy of the peak-background split
The peak-background split argument is commonly used to relate the abundance
of dark matter halos to their spatial clustering. Testing this argument
requires an accurate determination of the halo mass function. We present a
Maximum Likelihood method for fitting parametric functional forms to halo
abundances which differs from previous work because it does not require binned
counts. Our conclusions do not depend on whether we use our method or more
conventional ones. In addition, halo abundances depend on how halos are
defined. Our conclusions do not depend on the choice of link length associated
with the friends-of-friends halo-finder, nor do they change if we identify
halos using a spherical overdensity algorithm instead. The large scale halo
bias measured from the matter-halo cross spectrum b_x and the halo
autocorrelation function b_xi (on scales k~0.03h/Mpc and r ~50 Mpc/h) can
differ by as much as 5% for halos that are significantly more massive than the
characteristic mass M*. At these large masses, the peak background split
estimate of the linear bias factor b1 is 3-5% smaller than b_xi, which is 5%
smaller than b_x. We discuss the origin of these discrepancies: deterministic
nonlinear local bias, with parameters determined by the peak-background split
argument, is unable to account for the discrepancies we see. A simple linear
but nonlocal bias model, motivated by peaks theory, may also be difficult to
reconcile with our measurements. More work on such nonlocal bias models may be
needed to understand the nature of halo bias at this level of precision.Comment: MNRAS accepted. New section with Spherical Overdensity identified
halos included. Appendix enlarge
The impact of environment on the dynamical structure of satellite systems
We examine the effects of environment on the dynamical structure of satellite
systems based on the Millennium--II Simulation. Satellite halos are defined as
sub--halos within the virial radius of a host halo. The satellite sample is
restricted to those sub--halos which showed a maximum circular velocity above
30 km/s at the time of accretion. Host halo masses range from 10^11 to 10^14
Msol/h. We compute the satellites' average accretion redshift, z_acc, velocity
dispersion, sigma, and velocity anisotropy parameter, beta, utilising stacked
satellite samples of equal mass hosts at similar background densities. The main
results are: (1) On average satellites within hosts in high density
environments are accreted earlier (Delta z~ 0.1$) compared to their
counterparts at low densities. For host masses above 5 times10^13 Msol/h this
trend weakens and may reverse for higher host masses; (2) The velocity
dispersion of satellites in low density environments follows that of the host,
i.e. no velocity bias is observed for host halos at low densities independent
of host mass. However, for low mass hosts in high density environments the
velocity dispersion of the satellites can be up to ~30% larger than that of the
host halo, i.e. the satellites are dynamically hotter than their host halos.
(3) The anisotropy parameter depends on host mass and environment. Satellites
of massive hosts show more radially biased velocity distributions. Moreover in
low density environments satellites have more radially biased velocities (Delta
beta > 0.1) compared to their counterparts in high density environments. We
believe that our approach allows to predict a similar behaviour for observed
satellite galaxy systems.Comment: 7 pages, 4 figures, accepted for publication in MNRA
The nature of assembly bias - I. Clues from a LCDM cosmology
We present a new proxy for the overdensity peak height for which the
large-scale clustering of haloes of a given mass does not vary significantly
with the assembly history. The peak height, usually taken to be well
represented by the virial mass, can instead be approximated by the mass inside
spheres of different radii, which in some cases can be larger than the virial
radius and therefore include mass outside the individual host halo. The sphere
radii are defined as r = delta_t + log_10(M_vir/M_nl), where delta_t is
the age relative to the typical age of galaxies hosted by haloes with virial
mass M_vir, M_nl is the non-linear mass, and =0.2 and =-0.02 are the free
parameters adjusted to trace the assembly bias effect. Note that depends on
both halo mass and age. In this new approach, some of the objects which were
initially considered low-mass peaks belong to regions with higher
overdensities. At large scales, i.e. in the two-halo regime, this model
properly recovers the simple prescription where the bias responds to the height
of the mass peak alone, in contrast to the usual definition (virial mass) that
shows a strong dependence on additional halo properties such as formation time.
The dependence on the age in the one-halo term is also remarkably reduced. The
population of galaxies whose "peak height" changes with this new definition
consists mainly of old stellar populations and are preferentially hosted by
low-mass haloes located near more massive objects. The latter is in agreement
with recent results which indicate that old, low-mass haloes would suffer
truncation of mass accretion by nearby larger haloes or simply due to the high
density of their surroundings, thus showing an assembly bias effect. The change
in mass is small enough that the Sheth et al. (2001) mass function is still a
good fit to the resulting distribution of new masses.Comment: 13 pages, 10 figures, submitted to MNRAS, comments welcom
On the Orbits of Infalling Satellite Halos
The orbital properties of infalling satellite halos set the initial
conditions which control the subsequent evolution of subhalos and the galaxies
that they host, with implications for mass stripping, star formation quenching,
and merging. Using a high-resolution, cosmological N-body simulation, I examine
the orbital parameters of satellite halos as they merge with larger host halos,
focusing primarily on orbital circularity and pericenter. I explore in detail
how these orbital parameters depend on mass and redshift. Satellite orbits
become more radial and plunge deeper into their host halo at higher host halo
mass, but they do not significantly depend on satellite halo mass.
Additionally, satellite orbits become more radial and plunge deeper into their
host halos at higher redshift. I also examine satellite velocities, finding
that most satellites infall with less specific angular momentum than the host
halo virial value, but that satellites are `hotter' than the host virial
velocity. I discuss the implications of these results to the processes of
galaxy formation and evolution, and I provide fitting formulas to the mass and
redshift dependence of satellite orbital circularity and pericenter.Comment: 11 pages, 12 figures, accepted for publication in MNRA
Removal and mixing of the coronal gas from satellites in galaxy groups: cooling the intragoup gas
The existence of an extended hot gaseous corona surrounding clusters, groups
and massive galaxies is well established by observational evidence and
predicted by current theories of galaxy formation. When a small galaxy collides
with a larger one, their coronae are the first to interact, producing
disturbances that remove gas from the smaller system and settle it into the
corona of the larger one. For a Milky-Way-size galaxy merging into a low-mass
group, ram pressure stripping and the Kelvin-Helmholtz instability are the most
relevant of these disturbances. We argue that the turbulence generated by the
latter mixes the material of both coronae in the wake of the orbiting satellite
creating a "warm phase" mixture with a cooling time a factor of several shorter
than that of the ambient intragroup gas. We reach this conclusion using
analytic estimates, as well as adiabatic and dissipative high resolution
numerical simulations of a spherical corona subject to the ablation process of
a constant velocity wind with uniform density and temperature. Although this is
a preliminary analysis, our results are promising and we speculate that the
mixture could potentially trigger in situ star formation and/or be accreted
into the central galaxy as a cold gas flow resulting in a new mode of star
formation in galaxy groups and clusters.Comment: 15 pages, 5 figures, accepted for publication in MNRA
The specific star formation rate of high redshift galaxies: the case for two modes of star formation
We study the specific star formation rate (SSFR) and its evolution at
z\gtsim 4, in models of galaxy formation, where the star formation is driven
by cold accretion flows. We show that constant star formation and feedback
efficiencies cannot reproduce the observed trend of SSFR with stellar mass and
its observed lack of evolution at . Model galaxies with \log(M_*) \ltsim
9.5 M show systematically lower specific star formation rates by
orders of magnitudes, while massive galaxies with M_* \gtsim 5 \times 10^{10}
M have up to an order of magnitude larger SSFRs, compared to recent
observations by Stark et al.. To recover these observations we apply an
empirical star formation efficiency in galaxies that scales with the host halo
velocity dispersion as during galaxy mergers. We find that
this modification needs to be of stochastic nature to reproduce the
observations, i.e. only applied during mergers and not during accretion driven
star formation phases. Our choice of star formation efficiency during mergers
allows us to capture both, the boost in star formation at low masses and the
quenching at high masses, and at the same time produce a constant SSFR-stellar
mass relation at z\gtsim 4 under the assumption that most of the observed
galaxies are in a merger triggered star formation phase. Our results suggest
that observed high-z low mass galaxies with high SSFRs are likely to be
frequently interacting systems, which experienced bursts in their star
formation rate and efficiency (mode 1), in contrast to low redshift z \ltsim
3 galaxies which are cold accretion-regulated star forming systems with lower
star formation efficiencies (mode 2).Comment: 5 pages, accepted to MNRAS, replaced by version with including
referees comment
From dwarf spheroidals to cDs: Simulating the galaxy population in a LCDM cosmology
We apply updated semi-analytic galaxy formation models simultaneously to the
stored halo/subhalo merger trees of the Millennium and Millennium-II
simulations. These differ by a factor of 125 in mass resolution, allowing
explicit testing of resolution effects on predicted galaxy properties. We have
revised the treatments of the transition between the rapid infall and cooling
flow regimes of gas accretion, of the sizes of bulges and of gaseous and
stellar disks, of supernova feedback, of the transition between central and
satellite status as galaxies fall into larger systems, and of gas and star
stripping once they become satellites. Plausible values of efficiency and
scaling parameters yield an excellent fit not only to the observed abundance of
low-redshift galaxies over 5 orders of magnitude in stellar mass and 9
magnitudes in luminosity, but also to the observed abundance of Milky Way
satellites. This suggests that reionisation effects may not be needed to solve
the "missing satellite" problem except, perhaps, for the faintest objects. The
same model matches the observed large-scale clustering of galaxies as a
function of stellar mass and colour. The fit remains excellent down to ~30kpc
for massive galaxies. For M* < 6 x 10^10Msun, however, the model overpredicts
clustering at scales below 1 Mpc, suggesting that the sigma_8 adopted in the
simulations (0.9) is too high. Galaxy distributions within rich clusters agree
between the simulations and match those observed, but only if galaxies without
dark matter subhalos (so-called orphans) are included. Our model predicts a
larger passive fraction among low-mass galaxies than is observed, as well as an
overabundance of ~10^10Msun galaxies beyond z~0.6, reflecting deficiencies in
the way star-formation rates are modelled.Comment: Accepted for publication in MNRAS. SQL databases containing the full
galaxy data at all redshifts and for both the Millennium and Millennium-II
simulations are publicly released at
http://www.mpa-garching.mpg.de/millenniu
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