105 research outputs found
Detection of Molecular Clouds in the Interarm of the Flocculent Galaxy NGC 5055
We present high-resolution (~ 4") 12CO (J = 1 - 0) mapping observations with
high - velocity resolution (~ 2.6 km s^{-1}) toward the disk of flocculent
galaxy NGC 5055, using the Nobeyama Millimeter Array in order to study the
physical properties of the molecular clouds in the arm and the interarm. The
obtained map shows clumpy structures. Although these are mainly distributed
along a spiral arm seen in near-infrared observations, some clouds are located
far from the arm, namely in the interarm. These clouds in both the arm and the
interarm have a typical size and mass of a few 100 pc and a few 10^6 Mo,
respectively. These correspond to the largest Giant Molecular Cloud (GMC) in
our Galaxy, and are slightly smaller than Giant Molecular Associations (GMAs)
in the grand design spiral M 51. Their CO flux-based masses show good agreement
with their virial masses. A size - velocity dispersion relation is also plotted
on an extension of the relation for the Galactic GMCs. These facts suggest that
the properties of these clouds are similar to that of the Galactic GMCs. We
also found no clear systematic offset between the molecular gas and HII regions
unlike M 51. This fact and no existense of GMAs suggest the view that, in NGC
5055, cloud formation and following star formation in both the arm and the
interarm are due to enhancement of gas by local fluctuation. On the other hand,
in grand design spiral galaxies, such as M 51, GMA formations may occur only in
the arm due to a strong density wave also enhanced star formation in GMA
formation may also occur. These may control the optical morphology of spiral
arms in spiral galaxies.Comment: 16 pages, 8 figure
Formation of intermediate-mass black holes in circumnuclear regions of galaxies
Recent high-resolution X-ray imaging studies have discovered possible
candidates of intermediate-mass black holes with masses of M_\bullet \sim
10^{2-4} \MO in circumnuclear regions of many (disk) galaxies. It is known
that a large number of massive stars are formed in a circumnuclear giant H {\sc
ii} region. Therefore, we propose that continual merger of compact remnants
left from these massive stars is responsible for the formation of such an
intermediate-mass black hole within a timescale of years. A
necessary condition is that several hundreds of massive stars are formed in a
compact region with a radius of a few pc.Comment: 11 pages, PASJ in pres
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
Lyman Emitters beyond Redshift 5:The Dawn of Galaxy Formation
The 8m class telescopes in the ground-based optical astronomy together with
help from the ultra-sharp eye of the Hubble Space Telescope have enabled us to
observe forming galaxies beyond redshift . In particular, more than twenty
Ly-emitting galaxies have already been found at . These findings
provide us with useful hints to investigate how galaxies formed and then
evolved in the early universe. Further, detailed analysis of Ly
emission line profiles are useful in exploring the nature of the intergalactic
medium because the trailing edge of cosmic reionization could be close to -- 7, at which forming galaxies have been found recently. We also
discuss the importance of superwinds from forming galaxies at high redshift,
which has an intimate relationship between galaxies and the intergalactic
medium. We then give a review of early cosmic star formation history based on
recent progress in searching for Ly-emitting young galaxies beyond
redshift 5.Comment: 23 pages, 12 figures, jkas35.sty. To appear in the proceedings of the
APCTP WoFormation and Interaction of Galaxies, edited by Hyung Mok Leerkshop
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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
On the Origin of Lyman Blobs at High Redshift: Submillimetric Evidence for a Hyperwind Galaxy at z=3.1
The most remarkable class of high-redshift objects observed so far is
extended Ly emission-line blobs found in an over-density region at
redshift 3.1. They may be either a dust-enshrouded, extreme starburst galaxy
with a large-scale galactic outflow (superwind) or cooling radiation from dark
matter halos. Recently one of these Ly blobs has been detected at
submillimeter wavelengths (450 and 850 m). Here we show that its
rest-frame spectral energy distribution between optical and far-infrared is
quite similar to that of Arp 220, which is a typical ultraluminous
starburst/superwind galaxy in the local universe. This suggests strongly that
the superwind model proposed by Taniguchi & Shioya is applicable to this
Ly blob. Since the blob is more luminous in the infrared by a factor of
30 than Arp 220, it comprises a new population of hyperwind galaxies at high
redshift.Comment: 4 pages, 1 figure. ApJ (Letters), in pres
On the Hidden Nuclear Starburst in Arp 220
We construct a starburst model for the hidden starbursts in Arp 220 based on
the new Starburst99 models of Leitherer et al. Comparing these stellar
population synthesis models with observations, we show that the hidden power
source must be due to star formation (as opposed to an AGN) at the 50% level or
more in order to avoid an ionizing photon excess problem, and this starburst
must be young ( yr). We derive a current star formation rate
of yr, and an extinction mag for our line of
sight to this hidden starburst.Comment: 4 pages, 1 figure, Accepted for publication in ApJ
Formation of quasar nuclei in the hearts of ultraluminous infrared galaxies
We investigate whether or not supermassive black holes (SMBHs) with mass ≳ 108 M⊙ can be made in the hearts of ultraluminous infrared galaxies (ULIGs) during the course of mergers between/among gas-rich galaxies. (1) If one progenitor galaxy had a seed SMBH with mass of ∼107 M⊙, this seed SMBH can grow up to ≳108 M⊙ due to efficient Bondi-type gas accretion during the course of merger, given a gas density in the circumnuclear region of nH ∼ 103 cm-3. (2) Even if there was no progenitor galaxy with a seed SMBH, star clusters with compact remnants (neutron stars and/or black holes) produced in the circumnuclear starbursts can merge into the merger center within a dynamical timescale of ∼109 yr to form an SMBH with ≳108 M⊙. Note, however, that the contribution of compact remnants supplied from hidden star clusters is necessary to lead to the formation of an SMBH. In conclusion, the ULIGs observed in the local universe can make SMBHs in their centers during the course of merging either by gas accretion onto a seed SMBH or by dynamical relaxation of compact remnants made in the violent circumnuclear starbursts. Therefore, it is quite likely that the ULIGs will finally evolve to optically luminous quasars, as suggested by Sanders et al
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