103 research outputs found
The Formation and Destruction of Molecular Clouds and Galactic Star Formation
We describe an overall picture of galactic-scale star formation. Recent
high-resolution magneto-hydrodynamical simulations of two-fluid dynamics with
cooling/heating and thermal conduction have shown that the formation of
molecular clouds requires multiple episodes of supersonic compression. This
finding enables us to create a scenario in which molecular clouds form in
interacting shells or bubbles on a galactic scale. First we estimate the
ensemble-averaged growth rate of molecular clouds over a timescale larger than
a million years. Next we perform radiation hydrodynamics simulations to
evaluate the destruction rate of magnetized molecular clouds by the stellar FUV
radiation. We also investigate the resultant star formation efficiency within a
cloud which amounts to a low value (a few percent) if we adopt the power-law
exponent -2.5 for the mass distribution of stars in the cloud. We finally
describe the time evolution of the mass function of molecular clouds over a
long timescale (>1Myr) and discuss the steady state exponent of the power-law
slope in various environments.Comment: 7 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysic
Can we use Weak Lensing to Measure Total Mass Profiles of Galaxies on 20 kiloparsec Scales?
Current constraints on dark matter density profiles from weak lensing are
typically limited to radial scales greater than 50-100 kpc. In this paper, we
explore the possibility of probing the very inner regions of galaxy/halo
density profiles by measuring stacked weak lensing on scales of only a few tens
of kpc. Our forecasts focus on scales smaller than the equality radius (Req)
where the stellar component and the dark matter component contribute equally to
the lensing signal. We compute the evolution of Req as a function of lens
stellar mass and redshift and show that Req=7-34 kpc for galaxies with the
stellar mass of 10^{9.5}-10^{11.5} solar masses. Unbiased shear measurements
will be challenging on these scales. We introduce a simple metric to quantify
how many source galaxies overlap with their neighbours and for which shear
measurements will be challenging. Rejecting source galaxies with close-by
companions results in about a 20 per cent decrease in the overall source
density. Despite this decrease, we show that Euclid and WFIRST will be able to
constrain galaxy/halo density profiles at Req with signal-to-noise ratio >20
for the stellar mass of >10^{10} solar masses. Weak lensing measurements at
Req, in combination with stellar kinematics on smaller scales, will be a
powerful means by which to constrain both the inner slope of the dark matter
density profile as well as the mass and redshift dependence of the stellar
initial mass function.Comment: 19 pages, 14 figures, 3 tables, submitted to MNRAS, included the
referee comment
Metallicity Dependence of Molecular Cloud Hierarchical Structure at Early Evolutionary Stages
The formation of molecular clouds out of HI gas is the first step toward star
formation. Its metallicity dependence plays a key role to determine star
formation through the cosmic history. Previous theoretical studies with
detailed chemical networks calculate thermal equilibrium states and/or thermal
evolution under one-zone collapsing background. The molecular cloud formation
in reality, however, involves supersonic flows, and thus resolving the cloud
internal turbulence/density structure in three dimension is still essential. We
here perform magnetohydrodynamics simulations of 20 km s^-1 converging flows of
Warm Neutral Medium (WNM) with 1 uG mean magnetic field in the metallicity
range from the Solar (1.0 Zsun) to 0.2 Zsun environment. The Cold Neutral
Medium (CNM) clumps form faster with higher metallicity due to more efficient
cooling. Meanwhile, their mass functions commonly follow dn/dm proportional to
m^-1.7 at three cooling times regardless of the metallicity. Their total
turbulence power also commonly shows the Kolmogorov spectrum with its 80
percent in the solenoidal mode, while the CNM volume alone indicates the
transition towards the Larson's law. These similarities measured at the same
time in the unit of the cooling time suggest that the molecular cloud formation
directly from the WNM alone requires a longer physical time in a lower
metallicity environment in the 1.0-0.2 Zsun range. To explain the rapid
formation of molecular clouds and subsequent massive star formation possibly
within 10 Myr as observed in the Large/Small Magellanic Clouds (LMC/SMC), the
HI gas already contains CNM volume instead of pure WNM.Comment: 23 pages, 11 figures. Accepted for publication in Ap
Multi log-normal density structure in Cygnus-X molecular clouds: A fitting for N-PDF without power-law
We studied the H column density probability distribution function (N-PDF)
based on molecular emission lines using the Nobeyama 45-m Cygnus X CO survey
data. Using the DENDROGRAM and SCIMES algorithms, we identified 124 molecular
clouds in the CO data. From these identified molecular clouds, an N-PDF
was constructed for 11 molecular clouds with an extent of more than 0.4
deg. From the fitting of the N-PDF, we found that the N-PDF could be
well-fitted with one or two log-normal distributions. These fitting results
provided an alternative density structure for molecular clouds from a
conventional picture. We investigated the column density, dense molecular cloud
cores, and radio continuum source distributions in each cloud and found that
the N-PDF shape was less correlated with the star-forming activity over a whole
cloud. Furthermore, we found that the log-normal N-PDF parameters obtained from
the fitting showed two impressive features. First, the log-normal distribution
at the low-density part had the same mean column density ( 10
cm) for almost all the molecular clouds. Second, the width of the
log-normal distribution tended to decrease with an increasing mean density of
the structures. These correlations suggest that the shape of the N-PDF reflects
the relationship between the density and turbulent structure of the whole
molecular cloud but is less affected by star-forming activities.Comment: 14 pages, 7 Figures, Accepted in MNRA
Insights on the Sun birth environment in the context of star-cluster formation in hub-filament systems
Cylindrical molecular filaments are observed to be the main sites of Sun-like
star formation, while massive stars form in dense hubs, at the junction of
multiple filaments. The role of hub-filament configurations has not been
discussed yet in relation to the birth environment of the solar system and to
infer the origin of isotopic ratios of Short-Lived Radionuclides (SLR, such as
Al) of Calcium-Aluminum-rich Inclusions (CAIs) observed in meteorites.
In this work, we present simple analytical estimates of the impact of stellar
feedback on the young solar system forming along a filament of a hub-filament
system. We find that the host filament can shield the young solar system from
the stellar feedback, both during the formation and evolution of stars (stellar
outflow, wind, and radiation) and at the end of their life (supernovae). We
show that the young solar system formed along a dense filament can be enriched
with supernova ejecta (e.g., Al) during the formation timescale of CAIs.
We also propose that the streamers recently observed around protostars may be
channeling the SLR-rich material onto the young solar system. We conclude that
considering hub-filament configurations as the birth environment of the Sun is
important when deriving theoretical models explaining the observed properties
of the solar system.Comment: Accepted for publication in The Astrophysical Journal Letter
CO Multi-line Imaging of Nearby Galaxies (COMING). IX. CO(=2-1)/CO(=1-0) line ratio on kiloparsec scales
While molecular gas mass is usually derived from CO(=1-0) - the
most fundamental line to explore molecular gas - it is often derived from
CO(=2-1) assuming a constant CO(=2-1)/CO(=1-0)
line ratio (). We present variations of and effects of the
assumption that is a constant in 24 nearby galaxies using CO
data obtained with the Nobeyama 45-m radio telescope and IRAM 30-m telescope.
The median of for all galaxies is 0.61, and the weighted mean of
by CO(=1-0) integrated-intensity is 0.66 with a standard
deviation of 0.19. The radial variation of shows that it is high
(~0.8) in the inner ~1 kpc while its median in disks is nearly constant at 0.60
when all galaxies are compiled. In the case that the constant of 0.7
is adopted, we found that the total molecular gas mass derived from
CO(=2-1) is underestimated/overestimated by ~20%, and at most by 35%.
The scatter of a molecular gas surface density within each galaxy becomes
larger by ~30%, and at most by 120%. Indices of the spatially resolved
Kennicutt-Schmidt relation by CO(=2-1) are underestimated by 10-20%,
at most 39% in 17 out of 24 galaxies. has good positive correlations
with star-formation rate and infrared color, and a negative correlation with
molecular gas depletion time. There is a clear tendency of increasing
with increasing kinetic temperature (). Further, we found that not
only but also pressure of molecular gas is important to
understand variations of . Special considerations should be made when
discussing molecular gas mass and molecular gas properties inferred from
CO(=2-1) instead of CO(=1-0).Comment: 29 pages, 18 figures, 5 tables. Accepted for publication in PASJ. The
original resolution version is available here
(https://astro3.sci.hokudai.ac.jp/~radio/coming/publications/COMING_IX_org_res.pdf
Scalar cosmological perturbations in the Gauss-Bonnet braneworld
We study scalar cosmological perturbations in a braneworld model with a bulk
Gauss-Bonnet term. For an anti-de Sitter bulk, the five-dimensional
perturbation equations share the same form as in the Randall-Sundrum model,
which allows us to obtain metric perturbations in terms of a master variable.
We derive the boundary conditions for the master variable from the generalized
junction conditions on the brane. We then investigate several limiting cases in
which the junction equations are reduced to a feasible level. In the low energy
limit, we confirm that the standard result of four-dimensional Einstein gravity
is reproduced on large scales, whereas on small scales we find that the
perturbation dynamics is described by the four-dimensional Brans-Dicke theory.
In the high energy limit, all the non-local contributions drop off from the
junction equations, leaving a closed system of equations on the brane. We show
that, for inflation models driven by a scalar field on the brane, the
Sasaki-Mukhanov equation holds on the high energy brane in its original
four-dimensional form.Comment: 18 pages, v2: minor changes, reference added, v3: comments and
references added, accepted for publication in JCA
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