169 research outputs found
Re-examining the Too-Big-To-Fail Problem for Dark Matter Haloes with Central Density Cores
Recent studies found the densities of dark matter (DM) subhaloes which
surround nearby dwarf spheroidal galaxies (dSphs) to be significantly lower
than those of the most massive subhaloes expected around Milky Way sized
galaxies in cosmological simulations, the so called "too-big-to-fail" (TBTF)
problem. A caveat of previous work has been that dark substructures were
assumed to contain steep density cusps in the center of DM haloes even though
the central density structure of DM haloes is still under debate. In this
study, we re-examine the TBTF problem for models of DM density structure with
cores or shallowed cusps. Our analysis demonstrates that the TBTF problem is
alleviated as the logarithmic slope of the central cusp becomes shallower. We
find that the TBTF problem is avoided if the central cusps of DM haloes
surrounding dSphs are shallower than .Comment: 8 pages, 5 figures, accepted for publication in MNRA
The Core-Cusp Problem in Cold Dark Matter Halos and Supernova Feedback: Effects of Oscillation
This study investigates the dynamical response of dark matter (DM) halos to
recurrent starbursts in forming less-massive galaxies to solve the core-cusp
problem. The gas, which is heated by supernova feedback after a starburst,
expands and the star formation then terminates. This expanding gas loses energy
by radiative cooling and then falls back toward the galactic center.
Subsequently, the starburst is enhanced again. This cycle of expansion and
contraction of the interstellar gas leads to a repetitive change in the
gravitational potential of the gas. The resonance between DM particles and the
density wave excited by the oscillating potential plays a key role in
understanding the physical mechanism of the cusp-core transition of DM halos.
DM halos effectively gain kinetic energy from the baryon potential through the
energy transfer driven by the resonance between the particles and density
waves. We determine that the critical condition for the cusp-core transition is
such that the oscillation period of the gas potential is approximately the same
as the local dynamical time of DM halos. We present the resultant core radius
of a DM halo after the cusp-core transition induced by the resonance by using
the conventional mass density profile predicted by the cold dark matter models.
Moreover, we verify the analytical model by using -body simulations, and the
results validate the resonance model.Comment: 12 pages, 12 figures, 3 table
Formation of dense filaments induced by runaway supermassive black holes
A narrow linear object extending kpc from the centre of a galaxy at
redshift has been recently discovered and interpreted as a shocked
gas filament forming stars. The host galaxy presents an irregular morphology,
implying recent merger events. Supposing that each of the progenitor galaxies
has a central supermassive black hole (SMBH) and the SMBHs are accumulated at
the centre of the merger remnant, a fraction of them can be ejected from the
galaxy with a high velocity due to interactions between SMBHs. When such a
runaway SMBH (RSMBH) passes through the circumgalactic medium (CGM), converging
flows are induced along the RSMBH path, and star formation could be ignited
eventually. We find that the CGM temperature prior to the RSMBH perturbation
should be below the peak temperature in the cooling function to trigger
filament formation. While the gas is heated up temporarily due to compression,
the cooling efficiency increases, and gas accumulation becomes allowed along
the path. When the CGM density is sufficiently high, the gas can cool down and
develop a dense filament by . The mass and velocity of the RSMBH
determine the scale of the filament formation. Hydrodynamical simulations
validate the analytical expectations. Therefore, we conclude that the
perturbation by RSMBHs is a viable channel for forming the observed linear
object. We also expect the CGM around the linear object to be warm ( K) and dense ().Comment: 10 pages, 10 figures, 1 table, submitted to MNRA
Perception of depth and motion from ambiguous binocular information
AbstractThe visual system can determine motion and depth from ambiguous information contained in images projected onto both retinas over space and time. The key to the way the system overcomes such ambiguity lies in dependency among multiple cues—such as spatial displacement over time, binocular disparity, and interocular time delay—which might be established based on prior knowledge or experience, and stored in spatiotemporal response characteristics of neurons at an early cortical stage. We conducted a psychophysical investigation of whether a single ambiguous cue (specifically, interocular time delay) permits depth discrimination and motion perception. Data from this investigation are consistent with the predictions derived from the response profiles of V1 neurons, which show interdependency in their responses to each cue, indicating that spatial and temporal information is jointly encoded in early vision
Universal dark halo scaling relation for the dwarf spheroidal satellites
Motivated by a recently found interesting property of the dark halo surface
density within a radius, , giving the maximum circular velocity,
, we investigate it for dark halos of the Milky Way's and
Andromeda's dwarf satellites based on cosmological simulations. We select and
analyze the simulated subhalos associated with Milky Way-sized dark halos and
find that the values of their surface densities, , are in
good agreement with those for the observed dwarf spheroidal satellites even
without employing any fitting procedures. This implies that this surface
density would not be largely affected by any baryonic feedbacks and thus
universal. Moreover, all subhalos on the small scales of dwarf satellites are
expected to obey the relation ,
irrespective of differences in their orbital evolutions, host halo properties,
and observed redshifts. Therefore, we find that the universal scaling relation
for dark halos on dwarf galaxy mass scales surely exists and provides us
important clues to understanding fundamental properties of dark halos. We also
investigate orbital and dynamical evolutions of subhalos to understand the
origin of this universal dark halo relation and find that most of subhalos
evolve generally along the sequence, even
though these subhalos have undergone different histories of mass assembly and
tidal stripping. This sequence, therefore, should be the key feature to
understand the nature of the universality of .Comment: 12 pages, 5 figures and 3 tables, submitted to Ap
Shedding light on low-mass subhalo survival and annihilation luminosity with numerical simulations
In this work, we carry out a suite of specially designed numerical simulations to shed light on dark matter (DM) subhalo
survival at mass scales relevant for gamma-ray DM searches, a topic subject to intense debate nowadays. We have employed
an improved version of DASH, a GPU N-body code, to study the evolution of low-mass subhaloes inside a Milky-Way-like
halo with unprecedented accuracy, reaching solar-mass and sub-parsec resolution. We simulate subhaloes with varying mass,
concentration, and orbital properties, and consider the effect of baryons in the host. We analyse the evolution of the bound
mass fraction and annihilation luminosity, finding that most subhaloes survive until present, yet losing in some cases more than
99 per cent of their initial mass. Baryons induce a much greater mass-loss, especially when the subhalo orbit is more parallel
to the Galactic disc. Many of these subhaloes cross the solar Galactocentric radius, making it easier to detect their annihilation
fluxes from Earth. We find subhaloes orbiting a DM-only halo with a pericentre in the solar vicinity to lose 70–90 per cent
of their initial annihilation luminosity at present, which increases up to 99 per cent when including baryons. We find a strong
relation between subhalo’s mass-loss and the effective tidal field at pericentre. Indeed, much of the dependence on all considered
parameters can be explained through this single parameter. In addition to shedding light on the survival of low-mass Galactic
subhaloes, our results can provide detailed predictions that will aid current and future quests for the nature of D
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