Dynamics of Planetary Rings and Galaxies
(Dr. Heikki Salo, Division of Astronomy , Department of Physical Sciences , University of Oulu , Finland)

• Planetary ring dynamics
• Satellite resonances
• Dynamics and observations of interacting galaxies
• Dynamics of barred galaxies
• Development of numerical N-body simulation methods

At the moment the group consists of following members:

• Dos. Heikki Salo (planetary rings, galaxy dynamics and observations)
• Dos. Eija Laurikainen (galaxy dynamics and observations)
• Dr. Pertti Rautiainen (galaxy dynamics and observations)
• M. Sc. Tom Speltincx (galaxy dynamics and observations)
• + 3 undergraduate students (Eero Hatunen, Niina Stranius, Sami Airaksinen)

Former members of the group:

• Dr. Jyrki Hänninen (planetary rings)
• Dr. Ryuji Morishima (planetary rings)
• Dr. Raine Karjalainen (planetary rings)

The most important current collaborations are with the University of Alabama (Ron Buta), the Wellesley College (Richard French, Colleen McGhee), the Cornell university (Phil Nicholson, Matt Hedman), the University of Potsdam (Jürgen Schmidt, Frank Spahn), and the University of Colorado at Boulder (Miodrag Sremcevic, Nicole Albers).

Our recent planetary ring studies have concentrated on dense rings, especially the role of self-gravity. The most important result has been the numerical prediction of small-scale wake structure in Saturns's rings (Nature 359, 619). We are investigating the observational implications of such wakes with photometric MC-simulation, and comparing the results to Voyager data, and to HST observations by Dick French. Other current interest is the apparantly irregular density variations in Saturn's B-ring: we are studying the properties of such extremely dense rings both with N-body simulations and with hydrodynamical stability analysis, in collaboration with Frank Spahns's group in Potsdam.

Our previous satellite resonance studies have dealt with the angular momentum exchange between rings and satellites, confirming numerically the validity of Goldreich-Tremaine predictions, and to some degree extending them to non-linear regime. Also, we have demonstrated the possibility of one-sided confinement of narrow ringlet in the 2:1 ILR of external satellite. Our recent studies have dealt with the azimuthally incomplete ring arcs of Neptune. We have shown (Science 282, 1102) that self-gravity of massive arc particles can stabilize them agains mutual impacts, and help to confine dust to the arc region in general. We are currently seaking for possible explanations for the recently observed shift between the locations of the arcs and the predictions of the Porco-Goldreich-Tremaine-model.

Our studies of interacting galaxies concentrate on the observed properties and N-body modelling of M51-type closely interacting galaxies. Observations include both visual and IR broad-band imaging, and Fabry-Perot velocity fields. Fairly detailed tidal models have been constructed for two systems, Arp 86 and the M51 itself. Our current interests concentrate on the innermost spiral structures, and the interplay between intrinsic spiral modes and externally induced tidal waves.

The other main application of our N-body modelling of galaxies have been the barred galaxies , especially the ring structures related to the resonances of the rotating bar. For example, a detailed N-body model has been developed for IC 4214, a prototypical weakly barred galaxy, using ring morphology and kinematics as diagnostics for the bar strength and pattern speed. Recent studies concentrate on the outer spiral modes, and their relation to the dominant bar mode: for example we address the non-linear mode coupling.

We are also studying the observed properties of bars in collaboration with Ron Buta, and their correlations with the gravitational perturbations associated with bars, derived from near IR-images.

Starting almost 20 years ago, various N-body methods have been developed in our group, first for planetary ring simulations and later also for galaxy studies. The main current methods include

• Local simulations of planetary rings, including dissipative collisions and self-gravity.
• Numerical codes for long-term integration of particle-satellite interactions, including radiative pressure.
• 3D multi-grid code for simulation of interacting galaxies, composed of self-consistent disk+bulge+halo systems with both stars and gas.

Recent Publications:

• Salo, H. & Hänninen, J. (1998): Neptune's Partial Rings: Action of Galatea on Self-Gravitating Particles. Science 282, 1102-1104.
• Moss, D., Korpi, M., Rautiainen, P. & H. Salo (1998): The evolution of dynamo maintained magnetic fields in a barred spiral galaxy. Astron. Astrophys. 329, 895-905.
• Laurikainen, E. & Salo, H. & Aparacio, A. (1998): BVRI Imaging of M51-type Interacting Galaxy Pairs. Astron. Astrophys. Suppl. 129, 517-540.
• Moss, D., Rautiainen, P. & H. Salo (1999): A dynamo model for the magnetic field of the weakly barred galaxy IC 4214. MNRAS, 303, 125-132.
• Buta, R., Purcell, G.B., Cobb, M., Crocker, D.A., Rautiainen, P., & Salo, H. (1999): The Structure and Dynamics of the Early-Type Resonance Ring Galaxy IC 4214. I. Observations. Astron. J. 117, 778-791.
• Salo, H., Rautiainen, P., Buta, R., Purcell, G.B., Cobb, M., Crocker, D.A., & Laurikainen, E. (1999): The Structure and Dynamics of the Early-Type Resonance Ring Galaxy IC 4214. II. Models. Astron. J. 117, 792-810.
• Salo, H. & Laurikainen, E. (1999): Inner Structure of M51. Astrophys. Space Sci. 269-270, 589-592.
• Salo, H. & Laurikainen, E. (1999): A multiple encounter model of M51. Astrophysics and Space Science 269-270: 663-664.
• Schmidt, J., Salo, H., Petzshman, O. & Spahn, F. (1999): Vertical distribution of temperature and density in a planetary ring. Astron. Astrophys. 345, 646-652.
• Rautiainen, P. & Salo, H. (1999): Multiple pattern speeds in barred galaxies. Astron. Astrophys. 348, 737-754.
• Laurikainen, E. & Salo, H. (2000): BVRI Imaging of M51-type Interacting Galaxy Pairs II. Bulge and disk parameters. Astronomy Astrophys. Suppl. 141, 103-111 (+19 pages in electronic form)
• Spahn, F., Schmidt, J., Petzschmann, O. & Salo, H. (2000): Stability analysis of a Keplerian disk of granular grains: influence of thermal diffusion. Icarus 145, 657-660.
• Rautiainen, P. & Salo, H. (2000) N-body simulations of resonance rings in galactic disks. Astron. Astrophys. 362, 465-485.
• Salo, H. & Laurikainen, E. (2000): N-body model for M51: I. Multiple encounter vs. single passage?. MNRAS 319, 377-392 .
• Salo, H. & Laurikainen, E. (2000): N-body model for M51: II. Inner structure. MNRAS 319, 393-413.
• Laurikainen, E. & Salo, H. (2001) : BVRI Imaging of M51-type Interacting Galaxy Pairs III. Analysis of the photometric parameters. MNRAS 324, 685-698.
• Salo, H., Schmidt. J. & Spahn, F. (2001) : Viscous overstability in Saturn's B-ring: I. Direct simulations and measurement of transport coefficients. Icarus 153, 295-315.
• Schmidt. J., Salo, H., Spahn, F. & Petzschmann, O. (2001) : Viscous overstability in Saturn's B-ring: II. Hydrodynamic theory and comparison to simulations. Icarus 153, 316-331.
• Salo, H. & Laurikainen, E. (2001): N-body models of M51: From extended tail to central spiral structure. Astron. Astrophys Trans. 20, 93-101.
• Laurikainen, E, Salo, H. & Rautiainen, P.(2002): Comparison of bar strengths in active and non-axtive galaxies. MNRAS , 331, 880-893
• Rautiainen, P., Salo, H & Laurikainen, (2002) : Simulations of central structure in barred galaxies. MNRAS 337, 1233-1244.
• Laurikainen, E, Salo, H. (2002): Bar strengths in spiral galaxies estimated from 2MASS images. MNRAS 337, 1118-1138.
• Schmidt, J, Salo, H. (2003) : A weakly nonlinear model for oscillatory instability in Saturn's dense rings. Phys. Rev. Lett 90, 061102-1 - 061102-4
• Bergvall N., Laurikainen E., Aalto S. (2003): Galaxy interactions - poor starburst triggers: A study of a complete sample of interacting galaxies. III. Astron. Astrophys. 405, 31-52.
• Salo, H, Karjalainen, R (2003): Photometric modeling of Saturn's rings: I. Monte Carlo method and the effect of non-zero volume filling factor. Icarus 164, 428-460.
• Buta, R, Laurikainen, E., Salo, H (2004): The distribution of maximum relative gravitational torque in disk galaxies. Astron. J 127, 279-294.
• Fuentes-Carrera, I, Rosado, M, Amram, P., Dultzin-Hacyan, D. Cruz-Conzales, I, Salo, H, Laurikainen, E, Bernal, A, Ambrocio-Cruz, P, Le Coarer, E (2004): The isolated interacting galaxy pair NGC 5426/27 (Arp 271). Astron. Astrophys. 415, 451-469
• Rautiainen, P., Salo, H., Buta, R. (2004): Dynamical modelling of a remarkable four-armed barred spiral galaxy ESO 566-24. MNRAS 349, 933-944.
• Morishima, R, Salo., H (2004): Spin rates of small moonlets embedded in planetary rings: I. Three body calculations. Icarus 167, 330-346
• Morishima, R, Watanabe, S. (2004): Co-accretion of the Earth-Moon system after the giant impact: reduction of the total angular momentum by lunar impact ejecta. Icarus 168, 60-79
• Laurikainen, E., Salo, H., Buta, R. (2004): Comparison of bar strengths and fraction of bars in active and non-active galaxies. Astrophys. J 607, 103-124.
• Salo, H., Karjalainen, R., French, R.G (2004): Photometric modeling of Saturn's rings: II. Azimuthal asymmetry in reflected and transmitted ligth. Icarus 170, 70-90.
• Karjalainen, R., Salo, H (2004): Gravitational accretion of particles in Saturn's rings. Icarus 172, 328-348.
• Laurikainen, E., Salo, H., Buta, R., Vasylyev, S. (2004): Bar-induced perturbations strengths of the galaxies in the Ohio State University Bright Galaxy Survey (OSUBGS). MNRAS 355, 1251-1271.
• McGhee, C., French,R.C., Dones, L., Cuzzi, J.N., Salo., H., Danos, R (2005) : HST observations of spokes in Saturn's B ring. Icarus 173, 508-521.
• Buta, R., Vasylyev, S., Salo, H., Laurikainen, E. (2005) : The distribution of bar and spiral arm strengths in disk galaxies. Astronomical Journal 130, 506-523.
• Seiss, M., Spahn, F., Sremcevic, M., Salo, H. (2005) : Structures induced by small moonlets in Saturn's rings: Implications for the Cassini Mission. Geophysical Research Letters 32, L11205(1-4).
• Nicholson, P.D., French, R.G., Campbell, D.B., Margot, J.-L., Nolan, M.C., Black, G.J., Salo, H. (2005): Radar imaging of Saturn's rings. Icarus 177, 32-62.
• Laurikainen, E., Salo, H., Buta, R. (2005) : Multicomponent decomposition for a sample of S0 galaxies. MNRAS . 362, 1319-1347.
• Rautiainen, P, Salo, H., Laurikainen, E. (2005) : The pattern speeds of 38 galaxies. Astrophys. Journal 631, L129-L132. .
• Villata, M. ....Laurikainen, E... (78 authors) (2006): The unprecedented optical outburst of the quasar 3C 454.3. The WEBT campaign of 2004-2005 Astrophys. Journal 817-822
• Morishima, R., Salo, H. (2006): Simulations of dense planetary rings: IV Rotating self-gravitating particles with size distribution. Icarus 181, 272-291.
• Buta, R., Laurikainen, E., Salo, H., Block, D. L., Knapen, J. H. (2006) : The Near-Infrared S0 Survey. II. Fourier Dissection of Early-Type Galaxy Bars. Astron. Journal 132, 1859-1876.
• Laurikainen, E., Salo, H., Buta, R, Knapen, J. H., Speltincx, T., Block, D. L. (2006) : The Near-Infrared S0 Survey. III. Morphology of 15 Southern Early-type Disk Galaxies. Astron. Journal 132, 2634-2652.
• Fuentes-Carrera, I. Rosado, M. Amram, P. Salo, H., Laurikainen, E. (2007) : Kinematics and dynamics of the M51-type galaxy pair NGC 3893/96 (KPG 302). Astron. Astrophys. 466, 847-854.
• Hedman, M., Nicholson, P., Salo, H., Wallis, B., Buratti, B, Baines, K., Brown, R., Clark, R. (2007): Self-Gravity wake structures in Saturn's A ring revealed by Cassini-VIMS. Astron. J. 133:2624-2629.
• French, R. G., Salo, H. , McGhee, C \& Dones, L. (2007) : Hubble Space Telescope observations of azimuthal asymmetry in Saturn's Rings. Icarus 189, 493-522.
• Karjalainen, R (2007) : Aggregate impacts in Saturn's rings Icarus 189, 523-537.
• French, R. G., Verbischer, A., Salo, H., McGhee, C., Dones, L. (2007) . Saturn's rings at true opposition. PASP 119: 623-642
• Treuthardt, P, Buta, R, Salo, H \& Laurikainen, E. (2007) . The Kinematically Measured Pattern Speeds of NGC 2523 and NGC 4245. Astron. J. 134: 1195 - 2005.
• Laurikainen, E., Salo, H., Buta, R.\& Knapen, J. : Properties of bars and bulges in the Hubble sequence manuscript, Accepted to MNRAS
• Sremcevic, M., Schmidt, J., Salo, H., Seiss, M., Spahn, F., Albers, N.: A Moonlet Belt in Saturn's A ring. Accepted to Nature
• Speltincx, T., Laurikainen, E. Salo, H.: Comparison of bar strengths in optical and near-infrared for the OSUBSGS sample . Accepted to MNRAS

Invited talks:

• Salo, H. (1999): ``Numerical exploration of planetary ring dynamics'', 215th WE-Hereaus Conference on Granular Gases, Bad Honnef, Germany, 8-13.3.1999
• Salo, H. and Laurikainen, E. (2000): ``Simulation modelling of M51: from extended tail to inner spiral structure'', European Astronomical Society, JENAM-2000, Moscow, 29.5-4.6.2000
• Salo, H., Scmidt, J., Spahn, F (2002): ``Viscous stability properties of dense planetary rings'', Granular Gases: Theory-Experiments-Simulations Conference, CECAM, Lyon, France, 2-5.9.2002
• Salo, H. Laurikainen, E. Buta, R (2004): "Estimation of bar strengths form near-IR images", ``Penetrating Bars through Masks of Cosmic Dust'' Pilanesberg, South Africa, 7-12.2004.
• Salo, H , Schmidt J(2007): ``Dynamical and photometric modeling of dense planetary rings'', European Planetary Science Conference, Potsdam, Germany 20-24.8.2007.

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