886 research outputs found
Origin of optically passive spiral galaxies with dusty star-forming regions: Outside-in truncation of star formation?
Recent observations have revealed that red, optically--passive spiral
galaxies with little or no optical emission lines, harbour significant amounts
of dust-obscured star formation. We propose that these observational results
can be explained if the spatial distributions of the cold gas and star-forming
regions in these spiral galaxies are significantly more compact than those in
blue star-forming spirals. Our numerical simulations show that if the sizes of
star-forming regions in spiral galaxies with disk sizes of R_d are ~ 0.3R_d,
such galaxies appear to have lower star formation rates as well as higher
degrees of dust extinction. This is mainly because star formation in these
spirals occurs only in the inner regions where both the gas densities and
metallicities are higher, and hence the dust extinction is also significantly
higher. We discuss whether star formation occurring preferentially in the inner
regions of spirals is closely associated with the stripping of halo and disk
gas via some sort of environmental effect. We suggest that the "outside-in
truncation of star formation" is the key to a better understanding of
apparently optically--passive spirals with dusty star-forming regions.Comment: 5 pages, 4 figures, accepted in MNRAS Letter
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
Passive spiral formation from halo gas starvation: Gradual transformation into S0s
Recent spectroscopic and high resolution -imaging observations have
revealed significant numbers of ``passive'' spiral galaxies in distant
clusters, with all the morphological hallmarks of a spiral galaxy (in
particular, spiral arm structure), but with weak or absent star formation.
Exactly how such spiral galaxies formed and whether they are the progenitors of
present-day S0 galaxies is unclear. Based on analytic arguments and numerical
simulations of the hydrodynamical evolution of a spiral galaxy's halo gas
(which is a likely candidate for the source of gas replenishment for star
formation in spirals), we show that the origin of passive spirals may well be
associated with halo gas stripping. Such stripping results mainly from the
hydrodynamical interaction between the halo gas and the hot intracluster gas.
Our numerical simulations demonstrate that even if a spiral orbits a cluster
with a pericenter distance 3 times larger than the cluster core radius,
80 % of the halo gas is stripped within a few Gyr and, accordingly,
cannot be accreted by the spiral. Furthermore, our study demonstrates that this
dramatic decline in the gaseous infall rate leads to a steady increase in the
parameter for the disk, with the spiral arm structure, although persisting,
becoming less pronounced as the star formation rate gradually decreases. These
results suggest that passive spirals formed in this way, gradually evolve into
red cluster S0s.Comment: 13 pages 4 figures (fig.1 = jpg format), accepted by Ap
Transformation from spirals into S0s with bulge growth in groups of galaxies
Recent observations have revealed that the time evolution of the S0 number
fraction at intermediate and high redshifts (0.2<z<0.8) is more dramatic in
groups of galaxies than in clusters. In order to understand the origin of S0s
in groups, we investigate numerically the morphological transformation of
spirals into S0s through group-related physical processes. Our chemodynamical
simulations show that spirals in group environments can be strongly influenced
by repetitive slow encounters with group member galaxies so that those with
thin disks and prominent spiral arm structures can be transformed into S0s with
thick disks and without prominent spiral arm structure. Such tidal interactions
can also trigger repetitive starbursts within the bulges of spirals and
consequently increase significantly the masses of their bulges. Owing to rapid
consumption of gas initially in spirals during the bulge growth, the S0s can
become gas-poor. The S0s transformed from spirals in this way have young and
metal-rich stellar populations in the inner regions of their bulges. The
simulated S0s have lower maximum rotational velocities and flatter radial
line-of-sight velocity dispersion profiles in comparison to their progenitor
spirals. The formation processes of S0s due to tidal interactions depend not
only on the masses and orbits of the progenitor spirals, but also on group
mass. A significant fraction (10-30%) of stars and gas can be stripped during
this spiral to S0 morphological transformation so that intragroup stars and gas
can be formed. Based on these results, we discuss structures, kinematics,
chemical properties, and the Tully-Fisher relation of S0s in groups.Comment: 15 pages, 16 figures, accepted in MNRA
Galaxy threshing and the formation of ultra-compact dwarf galaxies
Recent spectroscopic and morphological observational studies of galaxies
around NGC 1399 in the Fornax Cluster (Drinkwater et al. 2000b) have discovered
several `ultra-compact dwarf' galaxies with intrinsic sizes of 100 pc
and absolute band magnitudes ranging from -13 to -11 mag. In order to
elucidate the origin of these enigmatic objects, we perform numerical
simulations on the dynamical evolution of nucleated dwarf galaxies orbiting NGC
1399 and suffering from its strong tidal gravitational field. Adopting a
plausible scaling relation for dwarf galaxies, we find that the outer stellar
components of a nucleated dwarf are totally removed. This is due to them being
tidally stripped over the course of several passages past the central region of
NGC 1399. The nucleus, however, manages to survive. We also find that the size
and luminosity of the remnant are similar to those observed for ultra-compact
dwarf galaxies, if the simulated precursor nucleated dwarf has a mass of
. These results suggest that ultra-compact dwarf galaxies
could have previously been more luminous dwarf spheroidal or elliptical
galaxies with rather compact nuclei.Comment: 9 pages 4 figures,2001, ApJL, 552, 10
Galaxy threshing and the origin of intracluster stellar objects
We numerically investigate dynamical evolution of non-nucleated dwarf
elliptical/spiral galaxies (dE) and nucleated ones (dE,Ns) in clusters of
galaxies in order to understand the origin of intracluster stellar objects,
such as intracluster stars (ICSs), GCs (ICGCs), and ``ultra-compact dwarf''
(UCDs) recently discovered by all-object spectroscopic survey centred on the
Fornax cluster of galaxies. We find that the outer stellar components of a
nucleated dwarf are removed by the strong tidal field of the cluster, whereas
the nucleus manages to survive as a result of its initially compact nature. The
developed naked nucleus is found to have physical properties (e.g., size and
mass) similar to those observed for UCDs. We also find that the UCD formation
processes does depend on the radial density profile of the dark halo in the
sense that UCDs are less likely to be formed from dwarfs embedded in dark
matter halos with central `cuspy' density profiles. Our simulations also
suggest that very massive and compact stellar systems can be rapidly and
efficiently formed in the central regions of dwarfs through the merging of
smaller GCs. GCs initially in the outer part of dE and dE,Ns are found to be
stripped to form ICGCs.Comment: 6 pages and 3 figures (JPG file for Fig. 1), in the proceedings of
IAU 217 ``Recycling intergalactic and interstellar matter'
Dissipative transformation of non-nucleated dwarf galaxies into nucleated systems
Recent photometric observations by the {\it Hubble Space Telescope (HST)}
have revealed the physical properties of stellar galactic nuclei in nucleated
dwarf galaxies in the Virgo cluster of galaxies. In order to elucidate the
formation processes of nucleated dwarfs, we numerically investigate gas
dynamics, star formation, and chemical evolution within the central 1 kpc of
gas disks embedded within the galactic stellar components of non-nucleated
dwarfs. We find that high density, compact stellar systems can be formed in the
central regions of dwarfs as a result of dissipative, repeated merging of
massive stellar and gaseous clumps developed from nuclear gaseous spiral arms
as a result of local gravitational instability. The central stellar components
are found to have stellar masses which are typically 5% of their host dwarfs
and show very flattened shapes, rotational kinematics, and central velocity
dispersions significantly smaller than those of their host dwarfs. We also find
that more massive dwarfs can develop more massive, more metal-rich, and higher
density stellar systems in their central regions, because star formation and
chemical enrichment proceed more efficiently owing to the less dramatic
suppression of star formation by supernovae feedback effects in more massive
dwarfs. Based on these results, we suggest that gas-rich, non-nucleated dwarfs
can be transformed into nucleated ones as a result of dissipative gas dynamics
in their central regions. We discuss the origin of the observed correlations
between physical properties of stellar galactic nuclei and those of their host
galaxies.Comment: 13 pages, 4 figures (1 color), ApJL in pres
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
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