96 research outputs found
Nuclear Spirals as Signatures of Supermassive Black Holes
Recent high resolution images of spiral galaxies show wide varieties of
features including nuclear spirals in the central parts. Some of the galaxies
show grand-design nuclear spirals. The morphology of grand-design spirals can
be further divided by the openness of the arms: tightly wound ones with winding
angle of around 3 radian and open spirals with winding angle of around
radian. Based on hydrodynamical simulations, we have investigated the
mechanism responsible for the openness of nuclear spirals. Since the gas flow
in the nuclear region is mainly governed by the central mass concentration near
the nuclei and the sound speed of the gas, we have examined various models with
different mass concentration represented by the mass of the central black hole
and different sound speeds. We found that the tightly wound spirals can be
formed when the mass of the black hole is large enough to remove the
inner-inner Lindblad resonances and sound speeds lie between 15 - 20 km/sec.
Thus, the presence of the tightly wound nuclear spiral could imply the presence
of relatively massive black hole in the center.Comment: 6 pages, 4 figure
Star Formation Histories of Dwarf Lenticular Galaxies
Star formation histories (SFHs) are essential for understanding galaxy
formation and evolution. We present the mean SFHs of 148 dwarf lenticular
galaxies (dS0s) derived from the Sloan Digital Sky Survey (SDSS) spectra. These
SFHs reveal two distinct periods of active star formation. The first period,
concluding approximately 6 Gyr ago, witnessed the formation of predominantly
old, metal-poor (Z=0.0004) stars, accounting for approximately 60\% of the
stellar mass and 30\% of the luminosity. The scarcity of extremely metal-poor
(Z=0.0001) stars suggests pre-enrichment during the re-ionization era. Star
formation gradually waned during this initial phase. In contrast, the second
period, ending around 1 Gyr ago, exhibited a peak in the middle of the period,
contributing to the formation of moderately old stellar populations with
intermediate metallicity. The SFHs of dS0 galaxies show a clear dependence on
stellar mass, with more massive dS0s forming stars earlier. But, we find no
significant correlation with morphological properties such as outer spiral arms
and nucleation. The SFHs of dS0 galaxies share many similarities with those of
dE galaxies, suggesting a common origin, mostly not primordial.Comment: 11 pages, 13+1 figures, submitted to MNRA
Formation of Warped Disks by Galactic Fly-by Encounters. I. Stellar Disks
Warped disks are almost ubiquitous among spiral galaxies. Here we revisit and
test the `fly-by scenario' of warp formation, in which impulsive encounters
between galaxies are responsible for warped disks. Based on N-body simulations,
we investigate the morphological and kinematical evolution of the stellar
component of disks when galaxies undergo fly-by interactions with adjacent dark
matter halos. We find that the so-called `S'-shaped warps can be excited by
fly-bys and sustained for even up to a few billion years, and that this
scenario provides a cohesive explanation for several key observations. We show
that disk warp properties are governed primarily by the following three
parameters; (1) the impact parameter, i.e., the minimum distance between two
halos, (2) the mass ratio between two halos, and (3) the incident angle of the
fly-by perturber. The warp angle is tied up with all three parameters, yet the
warp lifetime is particularly sensitive to the incident angle of the perturber.
Interestingly, the modeled S-shaped warps are often non-symmetric depending on
the incident angle. We speculate that the puzzling U- and L-shaped warps are
geometrically superimposed S-types produced by successive fly-bys with
different incident angles, including multiple interactions with a satellite on
a highly elongated orbit.Comment: 16 pages, 13 figures, 3 tables. Accepted for publication in Ap
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