96 research outputs found

    Nuclear Spirals as Signatures of Supermassive Black Holes

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    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 3π\pi radian and open spirals with winding angle of around π\pi 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

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    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

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    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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