310 research outputs found
The Magellanic Squall: Gas Replenishment from the Small to Large Magellanic Cloud
We first show that a large amount of metal-poor gas is stripped from the
Small Magellanic Cloud (SMC) and fallen into the Large Magellanic Cloud (LMC)
during the tidal interaction between the SMC, the LMC, and the Galaxy over the
last 2 Gyrs. We propose that this metal-poor gas can closely be associated with
the origin of LMC's young and intermediate-age stars and star clusters with
distinctively low-metallicities with [Fe/H] < -0.6. We numerically investigate
whether gas initially in the outer part of the SMC's gas disk can be stripped
during the LMC-SMC-Galaxy interaction and consequently can pass through the
central region (R<7.5 kpc) of the LMC. We find that about 0.7 % and 18 % of the
SMC's gas can pass through the central region of the LMC about 1.3 Gyr ago and
0.2 Gyr ago, respectively. The possible mean metallicity of the replenished gas
from the SMC to LMC is about [Fe/H] = -0.9 to -1.0 for the two interacting
phases. These results imply that the LMC can temporarily replenish gas supplies
through the sporadic accretion and infall of metal-poor gas from the SMC. These
furthermore imply that if these gas from the SMC can collide with gas in the
LMC to form new stars in the LMC, the metallicities of the stars can be
significantly lower than those of stars formed from gas initially within the
LMC.Comment: 5 pages, 5 figures, accepted in MNRAS Letter
Impact of dark matter subhalos on extended HI disks of galaxies: Possible formation of HI fine structures and stars
Recent observations have discovered star formation activities in the extreme
outer regions of disk galaxies. However it remains unclear what physical
mechanisms are responsible for triggering star formation in such low-density
gaseous environments of galaxies. In order to understand the origin of these
outer star-forming regions, we numerically investigate how the impact of dark
matter subhalos orbiting a gas-rich disk galaxy embedded in a massive dark
matter halo influences the dynamical evolution of outer HI gas disk of the
galaxy. We find that if the masses of the subhalos () in a galaxy
with an extended HI gas disk are as large as , where
is the total mass of the galaxy's dark halo, local fine structures
can be formed in the extended HI disk. We also find that the gas densities of
some apparently filamentary structures can exceed a threshold gas density for
star formation and thus be likely to be converted into new stars in the outer
part of the HI disk in some models with larger . These results thus
imply that the impact of dark matter subhalos (``dark impact'') can be
important for better understanding the origin of recent star formation
discovered in the extreme outer regions of disk galaxies. We also suggest that
characteristic morphologies of local gaseous structures formed by the dark
impact can indirectly prove the existence of dark matter subhalos in galaxies.
We discuss the origin of giant HI holes observed in some gas-rich galaxies
(e.g., NGC 6822) in the context of the dark impact.Comment: 8 pages, 4 figures, accepted by ApJ
Massive stars and globular cluster formation
We first present chemodynamical simulations to investigate how stellar winds
of massive stars influence early dynamical and chemical evolution of forming
globular clusters (GCs). In our numerical models, GCs form in
turbulent,high-density giant molecular clouds (GMCs), which are embedded in a
massive dark matter halo at high redshifts. We show how high-density, compact
stellar systems are formed from GMCs influenced both by physical processes
associated with star formation and by tidal fields of their host halos. We also
show that chemical pollution of GC-forming GMCs by stellar winds from massive
stars can result in star-to-star abundance inhomogeneities among light elements
(e.g., C, N, and O) of stars in GCs. The present model with a canonical initial
mass function (IMF) also shows a C-N anticorrelation that stars with smaller
[C/Fe] have larger [N/Fe] in a GC. Although these results imply that
``self-pollution'' of GC-forming GMCs by stellar winds from massive stars can
cause abundance inhomogeneities of GCs, the present models with different
parameters and canonical IMFs can not show N-rich stars with [N/Fe] ~ 0.8
observed in some GCs (e.g., NGC 6752). We discuss this apparent failure in the
context of massive star formation preceding low-mass one within GC-forming GMCs
(``bimodal star formation scenario''). We also show that although almost all
stars (~97%) show normal He abundances (Y) of ~0.24 some stars later formed in
GMCs can have Y as high as ~0.3 in some models. The number fraction of He-rich
stars with Y >0.26 is however found to be small (~10^-3) for most models.Comment: 10 pages, 8 figures, accepted by Ap
Study of Field-Induced Magnetic Order in Singlet-Ground-State Magnet CsFeCl
The field-induced magnetic order in the singlet-ground-state system
CsFeCl has been studied by measuring magnetization and neutron diffraction.
The field dependence of intensity for the neutron magnetic reflection has
clearly demonstrated that the field-induced ordered phase is described by the
order parameter . A condensate growth of magnons is investigated through
the temperature dependence of and , and this ordering is
discussed in the context of a magnon Bose-Einstein condensation. Development of
the coherent state and the static correlation length has been observed in the
incommensurate phase in the field region of , a satellite peak was found in coexistence with the commensurate
peak at the phase boundary around 10 T, which indicates that the tilt of the
c-axis would be less than in the whole experiments.Comment: 5 pages, 5 figure
Origin of structural and kinematical properties of the Small Magellanic Cloud
We investigate structural, kinematical, and chemical properties of stars and
gas in the Small Magellanic Cloud (SMC) interacting with the Large Magellanic
Cloud (LMC) and the Galaxy based on a series of self-consistent chemodynamical
simulations. We adopt a new "dwarf spheroidal model" in which the SMC initially
has both old stars with a spherical spatial distribution and an extended HI gas
disk. We mainly investigate SMC's evolution for the last 3 Gyr within which the
Magellanic stream (MS) and the Magellanic bridge (MB) can be formed as a result
of the LMC-SMC-Galaxy interaction. Our principal results, which can be tested
against observations, are as follows. The final spatial distribution of the old
stars projected onto the sky is spherical even after the strong LMC-SMC-Galaxy
interaction, whereas that of the new ones is significantly flattened and
appears to form a bar structure. Old stars have the line-of-sight velocity
dispersion (sigma) of ~ 30 km/s and slow rotation with the maximum rotational
velocity (V) of less than slow rotation with the maximum rotational velocity
(V) of less than 20 km/s and show asymmetry in the radial profiles. New stars
have a smaller sigma than old ones and a significant amount of rotation
(V/sigma >1). HI gas shows velocity dispersions of sigma = 10-40 km/s a high
maximum rotational velocity (V ~ 50 km/s), and the spatial distribution similar
to that of new stars. The new stars with ages younger than 3 Gyr show a
negative metallicity gradient in the sense that more metal-rich stars are
located in the inner regions of the SMC.Comment: 21 pages, 21 figures (5 color), accepted by PAS
Therapeutic Effects of the Sphingosine 1-Phosphate Receptor Modulator, Fingolimod (FTY720), on Experimental Autoimmune Encephalomyelitis
Fossil records of cosmic reionization in galactic stellar halos
Galactic stellar halos have long been considered to contain fossil
information on early dynamical and chemical evolution of galaxies. We propose
that the surface brightness distributions of old stellar halos contain the
influence of reionization on early formation histories of galaxies. By assuming
that reionization significantly suppresses star formation in small subgalactic
clumps virialized after reionization redshift (), we first
numerically investigate how structural and kinematical properties of stellar
halos formed from merging of these subgalactic clumps depend on . We then discuss what observable properties of galactic stellar halos
offer us the fossil records of reionization influence on hierarchical formation
of halos based on the current results of numerical simulations. We particularly
suggest that both the half-light radius of stellar halos and the slope of their
surface brightness profile contain useful information on when star formation in
subgalactic clumps were significantly influenced by reionization. By using the
simulated surface brightness distributions of galactic stellar halos for models
with different , we also discuss how wide-field imaging studies
of extragalactic halos will help us to elucidate the epoch of cosmic
reionization.Comment: 4 pages, 3 figures, ApJL in pres
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