456 research outputs found
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
Potential formation sites of super star clusters in ultra-luminous infrared galaxies
Recent observational results on high spatial resolution images of
ultra-luminous infrared galaxies (ULIGs) have revealed very luminous, young,
compact, and heavily obscured super star clusters in their central regions,
suggested to be formed by gas-rich major mergers. By using stellar and gaseous
numerical simulations of galaxy mergers, we firstly demonstrate that the
central regions of ULIGs are the most promising formation sites of super star
clusters owing to the rather high gaseous pressure of the interstellar medium.
Based on simple analytical arguments, we secondly discuss the possibility that
super star clusters in an ULIG can be efficiently transferred into the nuclear
region owing to dynamical friction and consequently merge with one another to
form a single compact stellar nucleus with a seed massive black hole. We thus
suggest that multiple merging between super star clusters formed by nuclear
starbursts in the central regions of ULIGs can result in the formation of
massive black holes.Comment: 12 pages 4 figures, 2001, accepted by ApJ
Formation of the Galactic globular clusters with He-rich stars in low-mass halos virialized at high redshift
Recent observations have reported that the Galactic globular clusters (GCs)
with unusually extended horizontal-branch (EHB) morphologies show a
significantly lower velocity dispersion compared with that of the entire
Galactic GC system. We consider that the observed distinctive kinematics of GCs
with EHB has valuable information on the formation epochs of GCs and
accordingly discuss this observational result based on cosmological N-body
simulations with a model of GC formation. We assume that GCs in galaxies were
initially formed in low-mass halos at high redshifts and we investigate final
kinematics of GCs in their host halos at . We find that GCs formed in
halos virialized at z>10 show lower velocity dispersions on average than those
formed at z>6 for halos with GCs at z=0. We thus suggest that the origin of the
observed lower velocity dispersion for the Galactic GCs with EHBs is closely
associated with earlier formation epochs (z>10) of halos initially hosting the
GCs in the course of the Galaxy formation. Considering that the origin of EHBs
can be due to the presence of helium-enhanced second-generation stars in GCs,
we discuss the longstanding second parameter problem of GCs in the context of
different degrees of chemical pollution in GC-forming gas clouds within
low-mass halos virialized at different redshifts.Comment: 5 pages, 3 figures, accepted by MNRAS Letter
Three-dimensional non-kinematic simulation of post-emergence evolution of bipolar magnetic regions and Babcock-Leighton dynamo of the Sun
The Babcock-Leighton (BL) flux-transport model is a widely-accepted dynamo
model of the Sun. This dynamo model has been extensively studied in a
two-dimensional (2D) mean-field framework in both kinematic and non-kinematic
regimes. Recent three-dimensional (3D) models have been restricted to the
kinematic regime. In these models, the surface poloidal flux is produced by the
emergence of bipolar magnetic regions (BMRs) that are tilted according to Joy's
law. We investigate the prescription for emergence of a BMR in 3D non-kinematic
simulations. We also report initial results of cyclic BL dynamo simulation. We
extend a conventional 2D mean-field model of the BL flux-transport dynamo into
3D non-kinematic regime. The large-scale mean flows are driven by the
parameterized -effect in this model. For the induction equation, we
use a BL source term by which the surface BMRs are produced in response to the
dynamo-generated toroidal field inside the convection zone. We find that, in
the 3D non-kinematic regime, the tilt angle of a newly-emerged BMR is very
sensitive to the prescription for the subsurface structure of the BMR. Anti-Joy
tilt angles are found unless the BMR is deeply embedded in the convection zone.
We also find that the leading spot tends to become stronger than the following
spot. The anti-Joy's law trend and the morphological asymmetry of the BMRs can
be explained by the Coriolis force acting on the Lorentz-force-driven flows.
Furthermore, we demonstrate that the solar-like magnetic cycles can be
successfully obtained if the Joy's law is explicitly given in the BL
-effect. In these cyclic dynamo simulation, a strong Lorentz force
feedback leads to cycle modulations in the differential rotation and meridional
circulation. The non-axisymmetric components of the flows are found to exist as
inertial modes such as the equatorial Rossby modes.Comment: 13 pages, 13 figures, submitted to A&
The Galactic globular cluster system as a fossil record of reionization
We propose that structural, kinematical, and chemical properties of the
Galactic globular clusters (GCs) can contain fossil information of the cosmic
reionization history. We first summarize possible observational evidences for
the influence of reionization on the Galactic GC formation. We then show how
structural properties of the GC system (GCS) in the Galaxy can be influenced by
suppression of GC formation due to reionization during the Galaxy formation
through hierarchical merging of subgalactic clumps, by using numerical
simulations with and without suppression of GC formation by reionization. In
particular, we show that if GC formation in dwarf galaxies that are building
blocks of the Galaxy and virialized after reionization era () are
completely suppressed, the present-day radial distribution of the Galactic GCs
depends strongly on . Our numerical results imply that if GC
formation after 15 is strongly suppressed, the origin of the
observed structural properties of the Galactic GCS can be more naturally
explained in the framework of the hierarchical clustering scenario.Comment: 8 pages 3 figures (1 color jpg) ApJL in press (626, L93
Gas fueling and nuclear disk formation in merging between a central black hole and a gas clump
We numerically investigate dynamical evolution of a merger between a central
massive black hole (MBH) and a gas clump with the mass of
in the central tens pc of a galactic bulge. We found that strong
tidal gravitational field of the MBH transforms the initial spherical clump
into a moderately thick gaseous disk (or torus) around the MBH. The developed
disk is also found to show rotation, essentially because the tidal field
changes efficiently the orbital angular momentum of the clump into intrinsic
angular momentum of the disk. Furthermore about a few percent of gas mass
(corresponding to a few ) in the clump is found to be
transferred to the central sub-parsec region around the MBH within an order of
yr. We thus suggest that successive merging of gas clumps onto a MBH can
not only be associated closely with the formation of nuclear disk around the
MBH but also can provide gas fuel for the MBH.Comment: 9 pages 4 figures,2000,ApJ,545 in press. See:
http://newt.phys.unsw.edu.au/~bekki/res.dir/paper.dir/apjdir11/paper.tar.g
On the Origin of Mass--Metallicity Relations, Blue Tilts, and Scaling Relations for Metal-poor Globular Cluster Systems
We investigate formation processes and physical properties of globular
cluster systems (GCSs) in galaxies based on high-resolution cosmological
simulations with globular clusters. We focus on metal-poor clusters (MPCs) and
correlations with their host galaxies by assuming that MPC formation is
truncated at a high redshift (z_trun > 6). We find that the correlation between
mean metallicities (Z_gc) of MPCs and their host galaxy luminosities (L)
flattens from z=z_trun to z=0. We also find that the observed relation (Z_gc ~
L^0.15) in MPCs can be reproduced well in the models with Z_gc ~ L^0.5 at
z=z_trun when z_trun ~ 10, if mass-to-light-ratios are assumed to be constant
at z=z_trun. However, better agreement with the observed relation is found for
models with different mass-to-light-ratios between z=z_trun and z=0. It is also
found that the observed color-magnitude relation of luminous MPCs (i.e., ``blue
tilts'') may only have a small contribution from the stripped stellar nuclei of
dwarf galaxies, which have nuclei masses that correlate with their total mass
at z=z_trun. The simulated blue tilts are found to be seen more clearly in more
massive galaxies, which reflects the fact that more massive galaxies at z=0 are
formed from a larger number of dwarfs with stellar nuclei formed at z>z_trun.
The half-number radii (R_e) of GCSs, velocity dispersions of GCSs (sigma), and
their host galaxy masses (M_h) are found to be correlated with one another such
that R_e ~ M_h^{0.57} and sigma ~ M_h^{0.32}.Comment: 15 pages, 20 figures, accepted by MNRA
Disordered Regimes of the one-dimensional complex Ginzburg-Landau equation
I review recent work on the ``phase diagram'' of the one-dimensional complex
Ginzburg-Landau equation for system sizes at which chaos is extensive.
Particular attention is paid to a detailed description of the spatiotemporally
disordered regimes encountered. The nature of the transition lines separating
these phases is discussed, and preliminary results are presented which aim at
evaluating the phase diagram in the infinite-size, infinite-time, thermodynamic
limit.Comment: 14 pages, LaTeX, 9 figures available by anonymous ftp to
amoco.saclay.cea.fr in directory pub/chate, or by requesting them to
[email protected]
Spatially Resolved Spectroscopy of the E+A Galaxies in the z=0.32 Cluster AC114
We present spatially resolved intermediate resolution spectroscopy of a
sample of twelve E+A galaxies in the z=0.32 rich galaxy cluster AC 114,
obtained with the FLAMES multi-integral field unit system on the European
Southern Observatory's VLT. Previous integrated spectroscopy of all these
galaxies by Couch & Sharples (1987) had shown them to have strong Balmer line
absorption and an absence of [OII 3727] emission -- the defining
characteristics of the``E+A'' spectral signature, indicative of an abrupt halt
to a recent episode of quite vigorous star formation. We have used our spectral
data to determine the radial variation in the strength of Hdelta absorption in
these galaxies and hence map out the distribution of this recently formed
stellar population. Such information provides important clues as to what
physical event might have been responsible for this quite dramatic change in
star formation activity in these galaxies' recent past. We find a diversity of
behaviour amongst these galaxies in terms of the radial variation in Hdelta
absorption: Four galaxies show little Hdelta absorption across their entire
extent; it would appear they were misidentified as E+A galaxies in the earlier
integrated spectroscopic studies. The remainder show strong Hdelta absorption,
with a gradient that is either negative (Hdelta equivalent width decreasing
with radius), flat, or positive. By comparing with numerical simulations we
suggest that the first of these different types of radial behaviour provides
evidence for a merger/interaction origin, whereas the latter two types of
behaviour are more consistent with the truncation of star formation in normal
disk galaxies. It would seem therefore that more than one physical mechanism is
responsible for E+A formation in the same environment.Comment: 15 pages, 10 figures, accepted MNRA
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