We study the effects of bulge elongation on the star formation activity in the centers of spiral galaxies using the data from the Sloan Digital Sky Survey Data Release 7. We construct a volume-limited sample of face-on spiral galaxies with $M_r < -$1
9.5 mag at 0.02 $leq z <$ 0.055 by excluding barred galaxies, where the aperture of the SDSS spectroscopic fibre covers the bulges of the galaxies. We adopt the ellipticity of bulges measured by Simard et al. (2011) who performed two-dimensional bulge+disc decompositions using the SDSS images of galaxies, and identify nuclear starbursts using the fibre specific star formation rates derived from the SDSS spectra. We find a statistically significant correlation between bulge elongation and nuclear starbursts in the sense that the fraction of nuclear starbursts increases with bulge elongation. This correlation is more prominent for fainter and redder galaxies, which exhibit higher ratios of elongated bulges. We find no significant environmental dependence of the correlation between bulge elongation and nuclear starbursts. These results suggest that non-axisymmetric bulges can efficiently feed the gas into the centre of galaxies to trigger nuclear starburst activity.
We present hydrodynamic simulations of gas clouds inflowing from the disk to a few hundred parsec region of the Milky Way. A gravitational potential is generated to include realistic Galactic structures by using thousands of multipole expansions that
describe 6.4 million stellar particles of a self-consistent Galaxy simulation. We find that a hybrid multipole expansion model, with two different basis sets and a thick disk correction, accurately reproduces the overall structures of the Milky Way. Through non-axisymmetric Galactic structures of an elongated bar and spiral arms, gas clouds in the disk inflow to the nuclear region and form a central molecular zone (CMZ)-like nuclear ring. We find that the size of the nuclear ring evolves into ~240 pc at T~1500 Myr, regardless of the initial size. For most simulation runs, the rate of gas inflow to the nuclear region is equilibrated to ~0.02 M_sun/yr. The nuclear ring is off-centered, relative to the Galactic center, by the lopsided central mass distribution of the Galaxy model, and thus an asymmetric mass distribution of the nuclear ring arises accordingly. The vertical asymmetry of the the Galaxy model also causes the nuclear ring to be tilted along the Galactic plane. During the first ~100 Myr, the vertical frequency of the gas motion is twice that of the orbital frequency, thus the projected nuclear ring shows a twisted, infinity-like shape.
We study the behaviors of galactic disks in triaxial halos both numerically and analytically to see if warps can be excited and sustained in triaxial potentials. We consider the following two scenarios: 1) galactic disks that are initially tilted rel
ative to the equatorial plane of the halo (for a pedagogical purpose), and 2) tilted infall of dark matter relative to the equatorial plane of the disk and the halo. With numerical simulations of 100,000 disk particles in a fixed halo potential, we find that in triaxial halos, warps can be excited and sustained just as in spherical or axisymmetric halos but they show some oscillatory behaviors and even can be transformed to a polar-ring system if the halo has a prolate-like triaxiality. The non-axisymmetric component of the halo causes the disk to nutate, and the differential nutation between the inner and outer parts of the disk generally makes the magnitude of the warp slightly diminish and fluctuate. We also find that warps are relatively weaker in oblate and oblate-like triaxial halos, and since these halos are the halo configurations of disk galaxies inferred by cosmological simulations, our results are consistent with the fact that most of the observed warps are quite weak. We derive approximate formulae for the torques exerted on the disk by the triaxial halo and the dark matter torus, and with these formulae we successfully describe the behaviors of the disks in our simulations. The techniques used in deriving these formulae could be applied for realistic halos with more complex structures.
With the Infrared Space Observatory, we conducted 3x3-pixel imaging photometry of twelve luminosity class III stars, which were previously presumed to have dust particles around them, at far infrared wavelengths (60 and 90 um). Eleven out of twelve t
argets show a peak of excess (above photosphere) far infrared emission at the location of the star, implying that the dust particles are truly associated with stars. To estimate the size of the excess emission source, the flux ratio of center to boundary pixels of the 3x3 array was examined. The radius of the dust emission is found to be ~3000 to ~10000 AU for a thin shell distribution, and ~5000 to ~25000 AU for a uniform distribution. We consider three models for the origin of the dust: disintegration of comets, sporadic dust ejection from the star, and emission from nearby interstellar cirrus. The data seem to rule out the first model (as far as the Kuiper--belt like particles are assumed to be large blackbody grains), but do not enable us to choose between the other two models.
Tidal captures can produce objects that are observationally and dynamically important in dense stellar systems. Recent discoveries of compact young clusters in and out of the Galaxy have prompted the studies of dynamics of star clusters with a large
range in stellar masses. The tidal interactions between high and low mass stars are found to be rather frequent in such clusters. In this Research Note, we present fitting formulae for the cross sections of tidal capture binary formation between two stars with a large mass ratio. We present the cases between two main-sequence stars, and between a degenerate star and a main-sequence star.
We examine the corecollapse times of isolated, two-mass-component star clusters using Fokker-Planck models. With initial condition of Plummer models, we find that the corecollapse times of clusters with M_1/M_2 >> 1 are well correlated with (N_1/N_2)
^{1/2}(m_1/m_2)^{2} t_{rh}, where M_1/M_2 and m_1/m_2 are the light to heavy component total and individual mass ratios, respectively, N_1/N_2 is the number ratio, and t_{rh} is the initial half-mass relaxation time scale. We also find two-component cluster parameters that best match multi-component (thus more realistic) clusters with power-law mass functions.
Two-component (normal and degenerate stars) models are the simplest realization of clusters with a mass spectrum because high mass stars evolve quickly into degenerates, while low mass stars remain on the main-sequence for the age of the universe. He
re we examine the evolution of isolated globular clusters using two-component Fokker-Planck (FP) models that include heating by binaries formed in tidal capture and in three-body encounters. Three-body binary heating dominates and the postcollapse expansion is self-similar, at least in models with total mass M <= 3 x 10^5 M_odot, initial half-mass radius r_{h,i} >= 5 pc, component mass ratio m_2/m_1 <= 2, and number ratio N_1/N_2 <= 300 when m_2=1.4 M_odot. We derive scaling laws for rho_c, v_c, r_c, and r_h as functions of m_1/m_2, N, M, and time t from simple energy-balance arguments, and these agree well with the FP simulations. We have studied the conditions under which gravothermal oscillations (GTOs) occur. If E_{tot} and E_c are the energies of the cluster and of the core, respectively, and t_{rh} and t_c are their relaxation times, then epsilon equiv (E_{tot}/t_{rh})/(E_c/t_{rc}) is a good predictor of GTOs: all models with epsilon>0.01 are stable, and all but one with epsilon < 0.01 oscillate. We derive a scaling law for epsilon against N and m_1/m_2 and compared with our numerical results. Clusters with larger m_2/m_1 or smaller N are stabler.
