808 research outputs found
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
Photometric evolution of dusty starburst mergers:On the nature of ultra-luminous infrared galaxies
By performing N-body simulations of chemodynamical evolution of galaxies with
dusty starbursts, we investigate photometric evolution of gas-rich major
mergers in order to explore the nature of ultraluminous infrared galaxies
(ULIRGs) with the total infrared luminosity ( for
m) of . Main results are the following three.
(1) Global colors and absolute magnitudes the during dusty starburst of a major
merger do not change with time significantly, because interstellar dust heavily
obscures young starburst populations that could cause rapid evolution of
photometric properties of the merger. (2) Dust extinction of stellar
populations in a galaxy merger with large infrared luminosity (
) is selective in the sense that younger stellar
populations are preferentially obscured by dust than old ones. This is because
younger populations are located in the central region where a larger amount of
dusty interstellar gas can be transferred from the outer gas-rich regions of
the merger. (3) Both and the ratio of to band
luminosity ) increases as the star formation rate increase during
the starburst of the present merger model, resulting in the positive
correlation between and .Comment: 32 pages 25 figures,2001,ApJ,in press. For all 25 PS figures
(including fig25.ps), see
http://newt.phys.unsw.edu.au/~bekki/res.dir/paper.dir/apj06.dir/fig.tar.g
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
Formation of the Galactic disk globular clusters in early dissipative minor merging
The origin of metal-rich, highly flattened, and rapidly rotating disk
globular cluster system in the Galaxy is one of longstanding issues in the
context of the Galaxy formation. Our numerical simulations suggest a new
``two-fold'' scenario that the disk globular clusters are firstly formed in the
high-pressure, dense central region of a gas-rich dwarf galaxy, as induced
during the tidal interaction with the pre-existing, young thin disk of the
Galaxy, and then dispersed into the disk region owing to the final tidal
destruction of the merging dwarf. We also demonstrate that spatial
distribution, total number, and metallicity distribution of the clusters formed
in this minor merging depend on the mass ratio of the host to dwarf galaxy and
the orbital configuration of merging. Based on these results, we discuss
whether a minor merging event about 10 Gyr ago can explain both the Galactic
thick disk and the disk globular clusters. Several other implications for the
possible relation between the properties of disk galaxies and their disk
globular clusters are also discussed.Comment: 18 pages 5 figures, 2002, ApJ 566, 24
How Does Feedback Affect Milky Way Satellite Formation?
We use sub-parsec resolution hydrodynamic resimulations of a Milky Way (MW)
like galaxy at high redshift to investigate the formation of the MW satellite
galaxies. More specifically, we assess the impact of supernova feedback on the
dwarf progenitors of these satellite, and the efficiency of a simple
instantaneous reionisation scenario in suppressing star formation at the
low-mass end of this dwarf distribution. Identifying galaxies in our high
redshift simulation and tracking them to z=0 using a dark matter halo merger
tree, we compare our results to present-day observations and determine the
epoch at which we deem satellite galaxy formation must be completed. We find
that only the low-mass end of the population of luminous subhalos of the
Milky-Way like galaxy is not complete before redshift 8, and that although
supernovae feedback reduces the stellar mass of the low-mass subhalos
(log(M/Msolar) < 9), the number of surviving satellites around the Milky-Way
like galaxy at z = 0 is the same in the run with or without supernova feedback.
If a luminous halo is able to avoid accretion by the Milky-Way progenitor
before redshift 3, then it is likely to survive as a MW satellite to redshift
0.Comment: Oral Presentation, Proceedings of "A Universe of Dwarf Galaxies"
Conference, Lyon 201
Ultra-compact dwarf galaxies: a new class of compact stellar system discovered in the Fornax Cluster
We have used the 2dF spectrograph on the Anglo-Australian Telescope to obtain
a complete spectroscopic sample of all objects in the magnitude range, Bj= 16.5
to 19.8, regardless of morphology, in an area centred on the Fornax Cluster of
galaxies. Among the unresolved targets are five objects which are members of
the Fornax Cluster. They are extremely compact stellar systems with scale
lengths less than 40 parsecs. These ultra-compact dwarfs are unlike any known
type of stellar system, being more compact and significantly less luminous than
other compact dwarf galaxies, yet much brighter than any globular cluster.Comment: To appear in IAU Symposium 207: Extragalactic Star Cluster
An HI study of the collisional ring galaxy NGC 922
We present new atomic hydrogen (HI) observations of the collisional ring
galaxy NGC 922 obtained using the Australia Telescope Compact Array. Our
observations reveal for the first time the vast extent of the HI disc of this
galaxy. The HI morphology and kinematics of NGC 922 show that this galaxy is
not the product of a simple drop-through interaction, but has a more complex
interaction history. The integrated HI flux density of NGC 922 from our
observations is 24.7 Jy km s, which is within the error of the flux
value obtained using the -m Parkes radio telescope. This flux density
translates to a total HI mass of M and corresponds to
an HI to total mass fraction (M/M) of approximately . The
gaseous structures of NGC 922 are more extended to the north and include an HI
tail that has a projected physical length of kpc. Gas warps are also
evident in the velocity field of NGC 922 and are more prominent on the
approaching and the western side of the disc. In comparison with a large sample
of star-forming galaxies in the local Universe, NGC 922 possesses a high gas
fraction relative to galaxies with a similar stellar mass of ~
M, and exhibits a high specific star formation rate.Comment: 11 pages, 8 figures, published in MNRA
On the efficiency of field star capture by star clusters
An exciting recent finding regarding scaling relations among globular
clusters is the so-called 'blue tilt': clusters of the blue sub-population
follow a trend of redder colour with increasing luminosity. In this paper we
evaluate to which extent field star capture over a Hubble time may explain the
'blue tilt'. We perform collisional N-body simulations to quantify the amount
of field star capture occuring over a Hubble time to star clusters with 10^3 to
10^6 stars. In the simulations we follow the orbits of field stars passing
through a star cluster and calculate the energy change that the field stars
experience due to gravitational interaction with cluster stars during one
passage through the cluster. The capture condition is that their total energy
after the passage is smaller than the gravitational potential at the cluster's
tidal radius. By folding this with the fly-by rates of field stars with an
assumed space density as in the solar neighbourhood and a range of velocity
dispersions, we derive estimates on the mass fraction of captured field stars
as a function of environment. We find that integrated over a Hubble time, the
ratio between captured field stars and total number of clusters stars is very
low (<~ 10^(-4)), even for the smallest considered field star velocity
dispersion sigma=15 km/s. This holds for star clusters in the mass range of
both open clusters and globular clusters. We furthermore show that tidal
friction has a negligible effect on the energy distribution of field stars
after interaction with the cluster. We conclude that field star capture is not
a probable mechanism for creating the colour-magnitude trend of metal-poor
globular clusters.Comment: 8 pages, 5 figures, accepted for publication in A&
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