273 research outputs found
The influence of Galactic wind upon the star formation histories of Local Group galaxies
We examine the possibility that ram pressure exerted by the galactic wind
from the Galaxy could have stripped gas from the Local Group dwarf galaxies,
thereby affecting their star formation histories. Whether gas stripping occurs
or not depends on the relative magnitudes of two counteracting forces acting on
gas in a dwarf galaxy: ram pressure force by the wind and the gravitational
binding force by the dwarf galaxy itself. We suggest that the galactic wind
could have stripped gas in a dwarf galaxy located within the distance of
kpc
(where is the surface radius and is the total binding
energy of the dwarf galaxy, respectively) from the Galaxy within a timescale of
Gyr, thereby preventing star formation there. Our result based on this Galactic
wind model explains the recent observation that dwarfs located close to the
Galaxy experienced star formation only in the early phase of their lifetimes,
whereas distant dwarfs are still undergoing star formation. The present star
formation in the Large Magellanic Cloud can also be explained through our
Galactic wind model.Comment: 7 pages LaTeX, no figures, to appear in MNRA
Star Formation Efficiency in the Central 1 kpc Region of Early-Type Spiral Galaxies
It has been reported recently that there are some early-type spiral (Sa--Sab)
galaxies having evident star-forming regions which concentrate in their own
central 1-kpc. In such central region, is the mechanism of the star formation
distinct from that in disks of spiral galaxies? To reveal this, we estimate the
star formation efficiency (SFE) in this central 1-kpc star-forming region of
some early-type spiral galaxies, taking account of the condition for this 1-kpc
region to be self-gravitating. Using two indicators of present star formation
rate (H and infrared luminosity), we estimate the SFE to be a few
percents. This is equivalent to the observational SFE in the disks of late-type
spiral (Sb--) galaxies. This coincidence may support the universality of the
mean SFE of spiral galaxies reported in the recent studies. That is, we find no
evidence of distinct mechanism of the star formation in the central 1-kpc
region of early-type galaxies. Also, we examine the structure of the central
star-forming region, and discuss a method for estimating the mass of
star-forming regions.Comment: accepted by A
Application of the Limit Cycle Model to Star Formation Histories in Spiral Galaxies: Variation among Morphological Types
We propose a limit-cycle scenario of star formation history for any
morphological type of spiral galaxies. It is known observationally that the
early-type spiral sample has a wider range of the present star formation rate
(SFR) than the late-type sample. This tendency is understood in the framework
of the limit-cycle model of the interstellar medium (ISM), in which the SFR
cyclically changes in accordance with the temporal variation of the mass
fraction of the three ISM components. When the limit-cycle model of the ISM is
applied, the amplitude of variation of the SFR is expected to change with the
supernova (SN) rate. Observational evidence indicates that the early-type
spiral galaxies show smaller rates of present SN than late-type ones. Combining
this evidence with the limit-cycle model of the ISM, we predict that the
early-type spiral galaxies show larger amplitudes in their SFR variation than
the late-types. Indeed, this prediction is consistent with the observed wider
range of the SFR in the early-type sample than in the late-type sample. Thus,
in the framework of the limit-cycle model of the ISM, we are able to interpret
the difference in the amplitude of SFR variation among the morphological
classes of spiral galaxies.Comment: 12 pages LaTeX, to appear in A
Emission from Dust in Galaxies: Metallicity Dependence
Infrared (IR) dust emission from galaxies is frequently used as an indicator
of star formation rate (SFR). However, the effect of the dust-to-gas ratio
(i.e., amount of the dust) on the conversion law from IR luminosity to SFR has
not so far been considered. Then, in this paper, we present a convenient
analytical formula including this effect. In order to obtain the dependence on
the dust-to-gas ratio, we extend the formula derived in our previous paper, in
which a theoretical formula converting IR luminosity to SFR was derived. That
formula was expressed as , where f is
the fraction of ionizing photons absorbed by hydrogen, is the
efficiency of dust absorption for nonionizing photons, is the cirrus
fraction of observed dust luminosity, and is the observed
luminosity of dust emission in the 8-1000-m range. Our formula explains
the IR excess of the Galaxy and the Large Magellanic Cloud. In the current
paper, especially, we present the metallicity dependence of our conversion law
between SFR and . This is possible since both f and can
be estimated via the dust-to-gas ratio, which is related to metallicity. We
have confirmed that the relation between the metallicity and the dust-to-gas
ratio is applied to both giant and dwarf galaxies. Finally, we apply the result
to the cosmic star formation history. We find that the comoving SFR at z=3
calculated from previous empirical formulae is underestimated by a factor of
4-5.Comment: 8 pages LaTeX, to appear in A&
On the Decelerating Shock Instability of Plane-Parallel Slab with Finite Thickness
Dynamical stability of the shock compressed layer with finite thickness is
investigated. It is characterized by self-gravity, structure, and shock
condition at the surfaces of the compressed layer. At one side of the shocked
layer, its surface condition is determined via the ram pressure, while at the
other side the thermal pressure supports its structure. When the ram pressure
dominates the thermal pressure, we expect deceleration of the shocked layer.
Especially, in this paper, we examine how the stratification of the
decelerating layer has an effect on its dynamical stability. Performing the
linear perturbation analysis, a {\it more general} dispersion relation than the
previous one obtained by one of the authors is derived. It gives us an
interesting information about the stability of the decelerating layer.
Importantly, the DSI (Decelerating Shock Instability) and the gravitational
instability are always incompatible. We also consider the evolution effect of
the shocked layer. In the early stages of its evolution, only DSI occurs. On
the contrary, in the late stages, it is possible for the shocked layer to be
unstable for the DSI (in smaller scale) and the gravitational instability (in
larger scale). Furthermore, we find there is a stable range of wavenumbers
against both the DSI and the gravitational instability between respective
unstable wavenumber ranges. These stable modes suggest the ineffectiveness of
DSI for the fragmentation of the decelerating slab.Comment: 17 pages, 6 figures. The Astrophysical Journal Vol.532 in pres
Is Thermal Instability Significant in Turbulent Galactic Gas?
We investigate numerically the role of thermal instability (TI) as a
generator of density structures in the interstellar medium (ISM), both by
itself and in the context of a globally turbulent medium. Simulations of the
instability alone show that the condenstion process which forms a dense phase
(``clouds'') is highly dynamical, and that the boundaries of the clouds are
accretion shocks, rather than static density discontinuities. The density
histograms (PDFs) of these runs exhibit either bimodal shapes or a single peak
at low densities plus a slope change at high densities. Final static situations
may be established, but the equilibrium is very fragile: small density
fluctuations in the warm phase require large variations in the density of the
cold phase, probably inducing shocks into the clouds. This result suggests that
such configurations are highly unlikely. Simulations including turbulent
forcing show that large- scale forcing is incapable of erasing the signature of
the TI in the density PDFs, but small-scale, stellar-like forcing causes
erasure of the signature of the instability. However, these simulations do not
reach stationary regimes, TI driving an ever-increasing star formation rate.
Simulations including magnetic fields, self-gravity and the Coriolis force show
no significant difference between the PDFs of stable and unstable cases, and
reach stationary regimes, suggesting that the combination of the stellar
forcing and the extra effective pressure provided by the magnetic field and the
Coriolis force overwhelm TI as a density-structure generator in the ISM. We
emphasize that a multi-modal temperature PDF is not necessarily an indication
of a multi-phase medium, which must contain clearly distinct thermal
equilibrium phases.Comment: 18 pages, 11 figures. Submitted to Ap
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
Sonographic Evaluation for Endometrial Polyps
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135550/1/jum201635112381.pd
Dust-to-gas ratio and star formation history of blue compact dwarf galaxies
This paper investigates the origin of the observed large variety in
dust-to-gas ratio among blue compact dwarf galaxies (BCDs). By applying our
chemical evolution model, we find that the dust destruction can largely
suppress the dust-to-gas ratio when the metallicity of a BCD reaches
, i.e., a typical metallicity level of BCDs. We also
show that dust-to-gas ratio is largely varied owing to the change of dust
destruction efficiency that has two effects: (i) a significant contribution of
Type Ia supernovae to total supernova rate; (ii) variation of gas mass
contained in a star-forming region. While mass loss from BCDs was previously
thought to be the major cause for the variance of dust-to-gas ratio, we suggest
that the other two effects are also important. We finally discuss the
intermittent star formation history, which naturally explains the large
dispersion of dust-to-gas ratio among BCDs.Comment: 7 pages LaTeX, to appear in A&
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