2,452 research outputs found
Dark satellites and the morphology of dwarf galaxies
One of the strongest predictions of the LambdaCDM cosmological model is the
presence of dark satellites orbiting all types of galaxies. We focus here on
the dynamical effects of such satellites on disky dwarf galaxies, and
demonstrate that these encounters can be dramatic. Although mergers with M_sat
> M_d are not very common, because of the lower baryonic content they occur
much more frequently on the dwarf scale than for L_*-galaxies. As an example,
we present a numerical simulation of a 20% (virial) mass ratio merger between a
dark satellite and a disky dwarf (akin to the Fornax dwarf galaxy in
luminosity) that shows that the merger remnant has a spheroidal morphology. We
conclude that perturbations by dark satellites provide a plausible path for the
formation of dSph systems and also could trigger starbursts in gas rich dwarf
galaxies. Therefore the transition from disky to the often amorphous,
irregular, or spheroidal morphologies of dwarfs could be a natural consequence
of the dynamical heating of hitherto unobservable dark satellites.Comment: Submitted to ApJ Letters. 5 pages, 4 figures. A movie showing an
encounter between a disky dwarf galaxy and a dark satellite can be found at
http://www.astro.rug.nl/~ahelmi/dwarfs-morphologies.htm
Properties of Galaxy Groups in the SDSS: II.- AGN Feedback and Star Formation Truncation
Successfully reproducing the galaxy luminosity function and the bimodality in
the galaxy distribution requires a mechanism that can truncate star formation
in massive haloes. Current models of galaxy formation consider two such
truncation mechanisms: strangulation, which acts on satellite galaxies, and AGN
feedback, which predominantly affects central galaxies. The efficiencies of
these processes set the blue fraction of galaxies as function of galaxy
luminosity and halo mass. In this paper we use a galaxy group catalogue
extracted from the Sloan Digital Sky Survey (SDSS) to determine these
fractions. To demonstrate the potential power of this data as a benchmark for
galaxy formation models, we compare the results to the semi-analytical model
for galaxy formation of Croton et al. (2006). Although this model accurately
fits the global statistics of the galaxy population, as well as the shape of
the conditional luminosity function, there are significant discrepancies when
the blue fraction of galaxies as a function of mass and luminosity is compared
between the observations and the model. In particular, the model predicts (i)
too many faint satellite galaxies in massive haloes, (ii) a blue fraction of
satellites that is much too low, and (iii) a blue fraction of centrals that is
too high and with an inverted luminosity dependence. In the same order, we
argue that these discrepancies owe to (i) the neglect of tidal stripping in the
semi-analytical model, (ii) the oversimplified treatment of strangulation, and
(iii) improper modeling of dust extinction and/or AGN feedback. The data
presented here will prove useful to test and calibrate future models of galaxy
formation and in particular to discriminate between various models for AGN
feedback and other star formation truncation mechanisms.Comment: 16 pages, 5 figures, submitted to MNRA
On the Hierarchy of Block Deterministic Languages
A regular language is -lookahead deterministic (resp. -block
deterministic) if it is specified by a -lookahead deterministic (resp.
-block deterministic) regular expression. These two subclasses of regular
languages have been respectively introduced by Han and Wood (-lookahead
determinism) and by Giammarresi et al. (-block determinism) as a possible
extension of one-unambiguous languages defined and characterized by
Br\"uggemann-Klein and Wood. In this paper, we study the hierarchy and the
inclusion links of these families. We first show that each -block
deterministic language is the alphabetic image of some one-unambiguous
language. Moreover, we show that the conversion from a minimal DFA of a
-block deterministic regular language to a -block deterministic automaton
not only requires state elimination, and that the proof given by Han and Wood
of a proper hierarchy in -block deterministic languages based on this result
is erroneous. Despite these results, we show by giving a parameterized family
that there is a proper hierarchy in -block deterministic regular languages.
We also prove that there is a proper hierarchy in -lookahead deterministic
regular languages by studying particular properties of unary regular
expressions. Finally, using our valid results, we confirm that the family of
-block deterministic regular languages is strictly included into the one of
-lookahead deterministic regular languages by showing that any -block
deterministic unary language is one-unambiguous
How research-based theatre is a solution for community engagement and advocacy at regional medical campuses: The Health and Equity through Advocacy, Research, and Theatre (HEART) program
Background: Regional medical campuses are often located in geographic regions that have different populations than the main campus, and are well-positioned to advocate for the health needs of their local community to promote social accountability within the medical school.Methods: At the Niagara Regional Campus of McMaster University, medical students developed a framework which combined research, advocacy, and theatre to advocate for the needs of the local population of the regional campus to which they were assigned. This involved a qualitative study using semi-structured interviews with homeless individuals to explore their experience accessing the healthcare system and using a transformative framework to identify barriers to receiving quality healthcare services. Findings from the qualitative study informed a play script that presented the experiences of homeless individuals in the local health system, which was presented to health sciences learners and practicing health professionals. Participants completed two instruments to examine the utility of this framework.Results: Research-based theatre was a useful intervention to educate current and future health professionals about the challenges faced by homeless individuals in the region. Participants from both shows felt the framework of research-based theatre was an effective strategy to promote change and advocate for marginalized populations.Conclusion: Research-based theatre is an innovative approach which can be utilized to promote social accountability at regional medical campuses, advocating for the health needs of the communities in which they are located, with the added bonus of educating current and future health professionals
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
The environmental dependence of clustering in hierarchical models
In hierarchical models, density fluctuations on different scales are
correlated. This induces correlations between dark halo masses, their formation
histories, and their larger-scale environments. In turn, this produces a
correlation between galaxy properties and environment. This correlation is
entirely statistical in nature. We show how the observed clustering of galaxies
can be used to quantify the importance of this statistical correlation relative
to other physical effects which may also give rise to correlations between the
properties of galaxies and their surroundings. We also develop a halo model
description of this environmental dependence of clustering.Comment: 11 pages, 6 figures, MNRAS in pres
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
Internal properties and environments of dark matter halos
We use seven high-resolution -body simulations to study the correlations
among different halo properties (assembly time, spin, shape and substructure),
and how these halo properties are correlated with the large-scale environment
in which halos reside. The large-scale tidal field estimated from halos above a
mass threshold is used as our primary quantity to characterize the large-scale
environment, while other parameters, such as the local overdensity and the
morphology of large-scale structure, are used for comparison. For halos at a
fixed mass, all the halo properties depend significantly on environment,
particularly the tidal field. The environmental dependence of halo assembly
time is primarily driven by local tidal field. The mass of the unbound fraction
in substructure is boosted in strong tidal force region, while the bound
fraction is suppressed. Halos have a tendency to spin faster in stronger tidal
field and the trend is stronger for more massive halos. The spin vectors show
significant alignment with the intermediate axis of the tidal field, as
expected from the tidal torque theory. Both the major and minor axes of halos
are strongly aligned with the corresponding principal axes of the tidal field.
In general, a halo that can accrete more material after the formation of its
main halo on average is younger, is more elongated, spins faster, and contains
a larger amount of substructure. Higher density environments not only provide
more material for halo to accrete, but also are places of stronger tidal field
that tends to suppress halo accretion. The environmental dependencies are the
results of these two competing effects. The tidal field based on halos can be
estimated from observation, and we discuss the implications of our results for
the environmental dependence of galaxy properties.Comment: Accepted for publication in MNRA
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