Galactic-Center Hyper-Shell Model for the North Polar Spurs

The bipolar-hyper shell (BHS) model for the North Polar Spurs (NPS-E, -W, and Loop I) and counter southern spurs (SPS-E and -W) is revisited based on numerical hydrodynamical simulations. Propagations of shock waves produced by energetic explosive events in the Galactic Center are examined. Distributions of soft X-ray brightness on the sky at 0.25, 0.7, and 1.5 keV in a +/-50 deg x +/-50 deg region around the Galactic Center are modeled by thermal emission from high-temperature plasma in the shock-compressed shell considering shadowing by the interstellar HI and H2 gases. The result is compared with the ROSAT wide field X-ray images in R2, 4 and 6 bands. The NPS and southern spurs are well reproduced by the simulation as shadowed dumbbell-shaped shock waves. We discuss the origin and energetics of the event in relation to the starburst and/or AGN activities in the Galactic Center. [ High resolution pdf is available at http://www.ioa.s.u-tokyo.ac.jp/~sofue/htdocs/2016bhs/ ]Comment: 13 pages, 20 figures; To appear in MNRA

Effects of a Supermassive Black Hole Binary on a Nuclear Gas Disk

We study influence of a galactic central supermassive black hole (SMBH) binary on gas dynamics and star formation activity in a nuclear gas disk by making three-dimensional Tree+SPH simulations. Due to orbital motions of SMBHs, there are various resonances between gas motion and the SMBH binary motion. We have shown that these resonances create some characteristic structures of gas in the nuclear gas disk, for examples, gas elongated or filament structures, formation of gaseous spiral arms, and small gas disks around SMBHs. In these gaseous dense regions, active star formations are induced. As the result, many star burst regions are formed in the nuclear region.Comment: 19 pages, 11 figures, accepted for publication in Ap

Application of the Limit Cycle Model to Star Formation Histories in Spiral Galaxies: Variation among Morphological Types

We propose a limit-cycle scenario of star formation history for any morphological type of spiral galaxies. It is known observationally that the early-type spiral sample has a wider range of the present star formation rate (SFR) than the late-type sample. This tendency is understood in the framework of the limit-cycle model of the interstellar medium (ISM), in which the SFR cyclically changes in accordance with the temporal variation of the mass fraction of the three ISM components. When the limit-cycle model of the ISM is applied, the amplitude of variation of the SFR is expected to change with the supernova (SN) rate. Observational evidence indicates that the early-type spiral galaxies show smaller rates of present SN than late-type ones. Combining this evidence with the limit-cycle model of the ISM, we predict that the early-type spiral galaxies show larger amplitudes in their SFR variation than the late-types. Indeed, this prediction is consistent with the observed wider range of the SFR in the early-type sample than in the late-type sample. Thus, in the framework of the limit-cycle model of the ISM, we are able to interpret the difference in the amplitude of SFR variation among the morphological classes of spiral galaxies.Comment: 12 pages LaTeX, to appear in A

Isolating signatures of major cloud-cloud collisions - II. The lifetimes of broad bridge features

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.We investigate the longevity of broad bridge features in position–velocity diagrams that appear as a result of cloud–cloud collisions. Broad bridges will have a finite lifetime due to the action of feedback, conversion of gas into stars and the time-scale of the collision. We make a series of analytic arguments with which to estimate these lifetimes. Our simple analytic arguments suggest that for collisions between clouds larger than R ∼ 10 pc the lifetime of the broad bridge is more likely to be determined by the lifetime of the collision rather than the radiative or wind feedback disruption time-scale. However, for smaller clouds feedback becomes much more effective. This is because the radiative feedback time-scale scales with the ionizing flux Nly as R7/4N−1/4ly R7/4Nly−1/4 so a reduction in cloud size requires a relatively large decrease in ionizing photons to maintain a given time-scale. We find that our analytic arguments are consistent with new synthetic observations of numerical simulations of cloud–cloud collisions (including star formation and radiative feedback). We also argue that if the number of observable broad bridges remains ∼ constant, then the disruption time-scale must be roughly equivalent to the collision rate. If this is the case, our analytic arguments also provide collision rate estimates, which we find are readily consistent with previous theoretical models at the scales they consider (clouds larger than about 10 pc) but are much higher for smaller clouds.Peer reviewe

Tidal disruption of dark matter halos around proto-globular clusters

Tidal disruption of dark matter halos around proto-globular clusters in a halo of a small galaxy is studied in the context of the hierarchical clustering scenario by using semi-cosmological N-body/SPH simulations assuming the standard cold dark matter model ($\Omega_0 = 1$). Our analysis on formation and evolution of the galaxy and its substructures archives until $z = 2.0$. In such a high-redshift universe, the Einstein-de Sitter universe is still a good approximation for a recently favored $\Lambda$-dominated universe, and then our results does not depend on the choice of cosmology. In order to resolve small gravitationally-bound clumps around galaxies and consider radiative cooling below $T = 10^4 K$, we adopt a fine mass resolution (m_{\rm SPH} = 1.12 \times 10^3 \Msun). Because of the cooling, each clump immediately forms a `core-halo' structure which consists of a baryonic core and a dark matter halo. The tidal force from the host galaxy mainly strips the dark matter halo from clumps and, as a result, theses clumps get dominated by baryons. Once a clump is captured by the host halo, its mass drastically decreases each pericenter passage. At $z = 2$, more than half of the clumps become baryon dominated systems (baryon mass/total mass $> 0.5$). Our results support the tidal evolution scenario of the formation of globular clusters and baryon dominated dwarf galaxies in the context of the cold dark matter universe.Comment: 9page, 13 figures. Accepted for publication in ApJ. A high-resolution PDF of the paper can be obtained from http://th.nao.ac.jp/~takayuki/ApJ05

Magnetohydrodynamics of Cloud Collisions in a Multi-phase Interstellar Medium

We extend previous studies of the physics of interstellar cloud collisions by beginning investigation of the role of magnetic fields through 2D magnetohydrodynamic (MHD) numerical simulations. We study head-on collisions between equal mass, mildly supersonic diffuse clouds. We include a moderate magnetic field and two limiting field geometries, with the field lines parallel (aligned) and perpendicular (transverse) to the colliding cloud motion. We explore both adiabatic and radiative cases, as well as symmetric and asymmetric ones. We also compute collisions between clouds evolved through prior motion in the intercloud medium and compare with unevolved cases. We find that: In the (i) aligned case, adiabatic collisions, like their HD counterparts, are very disruptive, independent of the cloud symmetry. However, when radiative processes are taken into account, partial coalescence takes place even in the asymmetric case, unlike the HD calculations. In the (ii) transverse case, collisions between initially adjacent unevolved clouds are almost unaffected by magnetic fields. However, the interaction with the magnetized intercloud gas during the pre-collision evolution produces a region of very high magnetic energy in front of the cloud. In collisions between evolved clouds with transverse field geometry, this region acts like a ``bumper'', preventing direct contact between the clouds, and eventually reverses their motion. The ``elasticity'', defined as the ratio of the final to the initial kinetic energy of each cloud, is about 0.5-0.6 in the cases we considered. This behavior is found both in adiabatic and radiative cases.Comment: 40 pages in AAS LaTeX v4.0, 13 figures (in degraded jpeg format). Full resolution images as well as mpeg animations are available at http://www.msi.umn.edu:80/Projects/twj/mhd-cc/ . Accepted for publication in The Astrophysical Journa