303 research outputs found
A unique X-ray unabsorbed Seyfert 2 galaxy IRAS F01475-0740
X-ray unabsorbed Seyfert 2 galaxies appear to have X-ray absorption column
densities that are too low (NH < 10^22 cm-2) to explain the absence of broad
emission lines in their optical spectra, challenging the standard AGN
unification model. In this paper we report Suzaku exposure on the X-ray
unabsorbed Seyfert 2 galaxy IRAS F01475-0740, in which a hidden broad line
region was detected through spectropolarimetric observation. The X-ray data
show rapid and significant variations on time scales down to 5 ks, indicating
that we are viewing its central engine directly. A newly obtained optical
spectrum and previous optical/X-ray data suggest that state transition is
unlikely in this source. These make IRAS F01475-0740 a very peculiar X-ray
unabsorbed Seyfert 2 galaxy which can only be explained by absorption from
materials with abnormally high dust-to-gas ratio (by a factor of > 4 larger
than Galactic). This is in contrast to most AGNs, which typically show
dust-to-gas ratios 3 - 100 times lower than the Galactic.Comment: 13 pages, 3 figures, 1 table, accepted to the Astrophysical Journal
Letter
Evaporation and Condensation of HI clouds in thermally bistable interstellar media: semi-analytic description of isobaric dynamics of curved interfaces
We analyze the evaporation and condensation of spherical and cylindrical HI
clouds of the cold neutral medium surrounded by the warm neutral medium.
Because the interstellar medium including those two phases is well described as
a thermally bistable fluid, it is useful to apply pattern formation theories to
the dynamics of the interface between the two phases. Assuming isobaric
evolution of fluids and a simple cubic form of the heat-loss function, we show
the curvature effects of the interface. We find that approximate solutions for
spherical clouds are in good agreement with numerically obtained solutions. We
extend our analysis to general curved fronts taking into account the curvature
effects explicitly. We find that the curvature effects always stabilise curved
interfaces under assumptions such as isobaric evolution we adopt in this
Letter.Comment: 5 pages, 4 figures, to appear in MNRAS Letter
Two-Fluid MHD Simulations of Converging Hi Flows in the Interstellar Medium. II: Are Molecular Clouds Generated Directly from Warm Neutral Medium?
Formation of interstellar clouds as a consequence of thermal instability is
studied using two-dimensional two-fluid magnetohydrodynamic simulations. We
consider the situation of converging, supersonic flows of warm neutral medium
in the interstellar medium that generate a shocked slab of thermally unstable
gas in which clouds form. We found, as speculated in paper I, that in the
shocked slab magnetic pressure dominates thermal pressure and the thermal
instability grows in the isochorically cooling, thermally unstable slab that
leads formation of HI clouds whose number density is typically n < 100 cm^-3,
even if the angle between magnetic field and converging flows is small. We also
found that even if there is a large dispersion of magnetic field, evolution of
the shocked slab is essentially determined by the angle between the mean
magnetic field and converging flows. Thus, the direct formation of molecular
clouds by piling up warm neutral medium does not seem a typical molecular cloud
formation process, unless the direction of supersonic converging flows is
biased to the orientation of mean magnetic field by some mechanism. However,
when the angle is small, the HI shell generated as a result of converging flows
is massive and possibly evolves into molecular clouds, provided gas in the
massive HI shell is piled up again along the magnetic field line. We expect
that another subsequent shock wave can pile up again the gas of the massive
shell and produce a larger cloud. We thus emphasize the importance of multiple
episodes of converging flows, as a typical formation process of molecular
clouds.Comment: 9 pages, 8 figures, accepted by Ap
An Origin of Supersonic Motions in Interstellar Clouds
The propagation of a shock wave into an interstellar medium is investigated
by two-dimensional numerical hydrodynamic calculation with cooling, heating and
thermal conduction. We present results of the high-resolution two-dimensional
calculations to follow the fragmentation due to the thermal instability in a
shock-compressed layer. We find that geometrically thin cooling layer behind
the shock front fragments into small cloudlets. The cloudlets have supersonic
velocity dispersion in the warm neutral medium in which the fragments are
embedded as cold condensations. The fragments tend to coalesce and become
larger clouds.Comment: 4 pages, 2 figures. ApJL accepte
Self-similar solutions for the dynamical condensation of a radiative gas layer
A new self-similar solution describing the dynamical condensation of a
radiative gas is investigated under a plane-parallel geometry. The dynamical
condensation is caused by thermal instability. The solution is applicable to
generic flow with a net cooling rate per unit volume and time , where , and are density, temperature and a free
parameter, respectively. Given , a family of self-similar solutions
with one parameter is found in which the central density and pressure
evolve as follows: and
, where
is an epoch when the central density becomes infinite. For , the
solution describes the isochoric mode, whereas for , the solution
describes the isobaric mode. The self-similar solutions exist in the range
between the two limits; that is, for . No self-similar solution is
found for . We compare the obtained self-similar solutions with the
results of one-dimensional hydrodynamical simulations. In a converging flow,
the results of the numerical simulations agree well with the self-similar
solutions in the high-density limit. Our self-similar solutions are applicable
to the formation of interstellar clouds (HI cloud and molecular cloud) by
thermal instability.Comment: Accepted for Monthly Notices of the Royal Astronomical Society: 9
pages, 7 figure
Toward Understanding the Origin of Turbulence in Molecular Clouds: Small Scale Structures as Units of Dynamical Multi-Phase Interstellar Medium
In order to investigate the origin of the interstellar turbulence, detailed
observations in the CO J=1--0 and 3--2 lines have been carried out in an
interacting region of a molecular cloud with an HII region. As a result,
several 1,000 to 10,000 AU scale cloudlets with small velocity dispersion are
detected, whose systemic velocities have a relatively large scatter of a few
km/s. It is suggested that the cloud is composed of small-scale dense and cold
structures and their overlapping effect makes it appear to be a turbulent
entity as a whole. This picture strongly supports the two-phase model of
turbulent medium driven by thermal instability proposed previously. On the
surface of the present cloud, the turbulence is likely to be driven by thermal
instability following ionization shock compression and UV irradiation. Those
small scale structures with line width of ~ 0.6 km/s have a relatively high CO
line ratio of J=3--2 to 1--0, 1 < R(3-2/1-0) < 2. The large velocity gradient
analysis implies that the 0.6 km/s width component cloudlets have an average
density of 10^{3-4} cm^{-3}, which is relatively high at cloud edges, but their
masses are only < 0.05 M_{sun}.Comment: 12 pages, 9 figures. To be published in the Astrophysical Journa
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