Secular interactions between inclined planets and a gaseous disk

In a planetary system, a secular particle resonance occurs at a location where the precession rate of a test particle (e.g. an asteroid) matches the frequency of one of the precessional modes of the planetary system. We investigate the secular interactions of a system of mutually inclined planets with a gaseous protostellar disk that may contain a secular nodal particle resonance. We determine the normal modes of some mutually inclined planet-disk systems. The planets and disk interact gravitationally, and the disk is internally subject to the effects of gas pressure, self-gravity, and turbulent viscosity. The behavior of the disk at a secular resonance is radically different from that of a particle, owing mainly to the effects of gas pressure. The resonance is typically broadened by gas pressure to the extent that global effects, including large-scale warps, dominate. The standard resonant torque formula is invalid in this regime. Secular interactions cause a decay of the inclination at a rate that depends on the disk properties, including its mass, turbulent viscosity, and sound speed. For a Jupiter-mass planet embedded within a minimum-mass solar nebula having typical parameters, dissipation within the disk is sufficient to stabilize the system against tilt growth caused by mean-motion resonances.Comment: 30 pages, 6 figures, to be published in The Astrophysical Journa

Reply to "Comment on 'Scalar-tensor gravity coupled to a global monopole and flat rotation curves' "

In Brans-Dicke theory of gravity we explain how the extra constant value in the formula for rotation velocities of stars in a galactic halo can be obtained due to the global monopole field. We argue on a few points of the preceding Comment and discuss improvement of our model.Comment: 4 pages, RevTeX4 fil

The Shape and Orientation of NGC 3379: Implications for Nuclear Decoupling

The intrinsic shape and orientation of the elliptical galaxy NGC 3379 are estimated by dynamical modeling. The maximal ignorance shape estimate, an average over the parameter space, is axisymmetric and oblate in the inner parts, with an outward triaxiality gradient. The 1 sigma limits on total-mass triaxiality T are T < 0.13 at 0.33 kpc and T = 0.08 +/- 0.07 at 3.5 kpc from the center. The luminous short-to-long axis ratio c_L = 0.79 +0.05-0.1 inside 0.82 kpc, flattening to c_L = 0.66 +0.07-0.08 at 1.9 kpc. The results are similar if the galaxy is assumed to rotate about its short axis. Estimates for c_L are robust, but those for T are dependent on whether the internal rotation field is disklike or spheroid-like. Short-axis inclinations between 30 and 50 degrees are preferred for nearly axisymmetric models; but triaxial models in high inclination are also allowed, which can affect central black hole mass estimates. The available constraints on orientation rule out the possibility that the nuclear dust ring at R = 1.5" is in a stable equilibrium in one of the galaxy's principal planes. The ring is thus a decoupled nuclear component not linked to the main body of the galaxy. It may be connected with ionized gas that extends to larger radii, since the projected gas rotation axis is near the minor axis of the ring. The gas and dust may both be part of a strongly warped disk; however, if caused by differential precession, the warp will wind up on itself in a few 10^7 years. The decoupling with the stellar component suggests that the gas has an external origin, but no obvious source is present.Comment: Astronomical Journal, accepted. 15 pages, incl. 5 figs, 1 table. AASTeX 4.0. Paper with better quality figures in PDF format at http://www.phy.ohiou.edu/~tss/Shape3379.pd

A Neutral Hydrogen Survey of Polar-Ring Galaxies: I. Green Bank Observations of the Northern Sample

We present the results of a neutral hydrogen survey conducted with the Green Bank 140-foot radio telescope of 47 northern objects in the polar-ring galaxy atlas of Whitmore \etal\ (1990). We detected 39 of these above our detection limit of 1.7 \hbox{Jy\CDOT\KMS}; the average measured flux of 21 Jy\CDOT\KMS\ corresponds to an average neutral hydrogen mass of $\rm 5.3 \times 10^9$ \MSUN for a Hubble constant of $\rm H_0 = 75$ \KMS \ Mpc$^{-1}$. For the polar-ring galaxies in our sample that have also been observed with radio arrays, we find that the 21\AMIN\ (FWHM) Green Bank beam often includes much more flux than found by the synthesis instruments for the polar rings alone; some of these galaxies are known to have gas-rich companions. We compare the neutral hydrogen content of the sample to the blue luminosity and IRAS fluxes. The \HI-to-blue- light ratios of the confirmed and probable polar rings are around unity in solar units, indicating that polar ring galaxies (or their environments) are as gas-rich as typical irregular galaxies. For their blue luminosity, the confirmed polar rings are underluminous in the far-infrared, as compared with the rest of the sample. They are also FIR-underluminous for their \HI\ masses, which suggests that most of the gas in the ring may be in stable orbits, rather than flowing inward to trigger star formation in the central galaxy. The more disordered class of `related objects,' which includes a number of obvious mergers, is highly luminous in the far-infrared.Comment: 34 pages, LaTeX file, Institute for Advanced Study number AST 93/4

Stellar Bar Evolution in Cuspy and Flat-Cored Triaxial CDM Halos

We analyze the evolution of stellar bars in galactic disks in mildly triaxial flat-core and cuspy CDM halos. We use tailored simulations of rigid and live halos which include the feedback from disk/bar onto the halo in order to test the work by El-Zant & Shlosman (2002). The latter used the Liapunov exponents to analyze the fate of bars in analytical asymmetric halos. We find: (1) The bar growth is similar in all rigid axisymmetric and triaxial halos. (2) Bars in live models vertically buckle and form a pseudobulge with a boxy/peanut shape. (3) In live axisymmetric halos, the bar strength varies little during the secular evolution. The bar pattern speed anticorrelates with the halo core size. The bar strength is larger for smaller disk-to-halo mass ratios within disk radii, the bar size correlates with the halo core sizes, and the bar pattern speeds -- with the halo central mass concentration. Bars embedded in live triaxial halos have a starkly different fate: they dissolve on ~1.5-5 Gyr due to the onset of chaos over continuous zones, leaving behind a weak oval distortion. The onset of chaos is related to the halo triaxiality, the fast rotating bar and the halo cuspiness. Before the bar dissolves, the region outside it develops strong spiral structures, especially in the live triaxial halos. (4) More angular momentum is absorbed by the triaxial halos as compared to the axisymmetric models and its exchange is mediated by resonances. (5) Cuspy halos are more susceptible than flat-core halos to having their prolateness washed out by the bar. We analyze these results in terms of the stability of trajectories and development of chaos. We set constraints on the triaxiality of DM halos by comparing our predictions to recent observations of bars out to z~1.Comment: 17 pages, 14 figures, Astrophysical Journal, in press, Vol. 637. Updated version (text, references

A Magellanic Origin for the Warp of the Galaxy

We show that a Magellanic Cloud origin for the warp of the Milky Way can explain most quantitative features of the outer HI layer recently identified by Levine, Blitz & Heiles (2005). We construct a model similar to that of Weinberg (1998) that produces distortions in the dark matter halo, and we calculate the combined effect of these dark-halo distortions and the direct tidal forcing by the Magellanic Clouds on the disk warp in the linear regime. The interaction of the dark matter halo with the disk and resonances between the orbit of the Clouds and the disk account for the large amplitudes observed for the vertical m=0,1,2 harmonics. The observations lead to six constraints on warp forcing mechanisms and our model reasonably approximates all six. The disk is shown to be very dynamic, constantly changing its shape as the Clouds proceed along their orbit. We discuss the challenges to MOND placed by the observations.Comment: 4 pages, 3 figures, submitted to ApJ Letters. Additional graphics, 3d visualizations and movies available at http://www.astro.umass.edu/~weinberg/lm

Low frequency m=1 normal mode oscillations of a self-gravitating disc

A continuous system such as a galactic disc is shown to be well approximated by an N-ring differentially rotating self-gravitating system. Lowest order (m=1) non-axisymmetric features such as lopsidedness and warps are global in nature and quite common in the discs of spiral galaxies. Apparently these two features of the galactic discs have been treated like two completely disjoint phenomena. The present analysis based on an eigenvalue approach brings out clearly that these two features are fundamentally similar in nature and they are shown to be very Low frequency Normal Mode (LNM) oscillations manifested in different symmetry planes of the galactic disc. Our analysis also show that these features are actually long-lived oscillating pattern of the N-ring self-gravitating system.Comment: 5 figures. Accepted for publication in MNRAS Letter

Evidence for coupling between the Sagittarius dwarf galaxy and the Milky Way warp

Using recent determinations of the mass and orbit of Sagittarius, I calculate its orbital angular momentum. From the latest observational data, I also calculate the angular momentum of the Milky Way's warp. I find that both angular momenta are directed toward l=270, b=0, and have magnitude 2-8x10^12 M_Sun kpc km s^-1, where the range in both cases reflects uncertainty in the mass. The coincidence of the angular momenta is suggestive of a coupling between these systems. Direct gravitational torque of Sgr on the disk is ruled out as the coupling mechanism. Gravitational torque due to a wake in the halo and the impulsive deposition of momentum by a passage of Sgr through the disk are still both viable mechanisms pending better simulations to test their predictions on the observed Sgr-MW system.Comment: 11 pages, to appear in the February 1 issue of ApJ

First Interferometric Observations of Molecular Gas in a Polar Ring: The Helix Galaxy NGC 2685

We have detected four Giant Molecular cloud Associations (GMAs) (sizes < 6.6'' ~ 430 pc) in the western and eastern region of the polar ring in NGC2685 (the Helix galaxy) using the Owens Valley Radio Observatory (OVRO) millimeter interferometer. Emission from molecular gas is found close to the brightest Halpha and HI peaks in the polar ring and is confirmed by new IRAM 30m single dish observations. The CO and HI line velocities are very similar, providing additional kinematic confirmation that the CO emission emerges from the polar ring. For the first time, the total molecular mass within a polar ring is determined (M_H2~(8-11)x10^6 M_sol, using the standard Galactic conversion factor). We detect about M_H2~4.4x10^6 M_sol in the nuclear region with the single dish. Our upper limit derived from the interferometric data is lower (M_H2<0.7x10^6 M_sol) suggesting that the molecular gas is distributed in an extended (< 1.3 kpc) diffuse disk. These new values are an order of magnitude lower than in previous reports. The total amount of molecular gas and the atomic gas content of the polar ring are consistent with formation due to accretion of a small gas-rich object, such as a dwarf irregular. The properties of the NGC2685 system suggest that the polar ring and the host galaxy have been in a stable configuration for a considerable time (few Gyr). The second (outer) HI ring within the disk of NGC2685 is very likely at the outer Lindblad resonance (OLR) of the ~ 11 kpc long stellar bar.Comment: 8 pages, 4 figures, accepted by ApJ Letter