1	Astr 5460 Wed., Oct. 6, 2004 This week: Finish up galaxy interactions (Ch. 7, Combes et al., parts) Unless noted, all figs and eqs from Combes et al or Longair.
2	Homework old and new Old: HW#2 returned. Some small issues, but overall I am pretty happy with the effort. Discuss assumptions in first problem, different data, inclination For the Coma problems, more thought into discussions, about all the assumptions, the dark matter, stars, gas, etc. Pretty nice overall. Discuss the pros/cons of the galaxy spectrum modeling as currently implemented. Also, what of the redshift business? New: for several problems you will need to run a galaxy simulator described in the hand-out from Caroll & Ostlie’s appendix H. There is a Windows based implementation that can be downloaded from http://tnewton.solarbotics.net/galaxy.html which you should get and verify ASAP. I will demonstrate in class. Also, five parts to a scientific article.
3	Three-Body Simualtions Toomre & Toomre (1972) is pretty simple, as is GALAXY based on their work. Most of the simulations we watched on Monday are n-body codes. These simpler ones are three-body codes. Please look at the GALAXY code to see how it works. All the stars are really just test particles, no self gravity, moving in the average potential of the two galaxies. This illustrates tidal effects very well (1/D³) but not effects like dynamic friction (later today).
4	Tidal Filaments Already saw filaments, tidal “tails” as in the Antennae system and the Mice. Tidal forces strong distance dependence draws these structures out. Duration is 1-2 Gyrs, and are relics of an interaction that persist. Star-forming knots can and do form in the filaments, and can become dwarf galaxies in their own right.
5	Ring Galaxies Cartwheel is an example we’ve already seen. Appear to result from small impact parameters. The core of the impacted galaxy can be pulled out entirely! The rings are kinematic waves. Companion temporarily draws stars into the center. Oscillations are set up and driven by disk gravity, and a density wave slowly propogates toward the outer part of the disk. Density waves cause star formation! Will examine some aspects of this in the homework. Watch carefully!
6	Ring Galaxies Toomre (1977) on Ring Galaxies from Arp’s 1996 Atlas of Peculiar Galaxies.
7	Ring Galaxies Lynds and Toomre (1976) in the system II Hz 4. A Double Ring Galaxy.
8	Vertical Oscillations and Warps Talked about companions driving spiral structure, rings, tidal tails. All can be relatively confined to disk plane. Distortions out of plane as well, e.g., “warped disks.”
9	3D Halos and Polar Ring Galaxies Merger product probes TWO dimensions.
10	Dynamic Friction This is the effect of braking of a massive body P by the stars of a galaxy when P penetrates or passes nearby.
11	Dynamic Friction Dissipates energy and causes mergers to be relatively fast as seen in n-body simulations. See Chandrasekhar (1943). Highlights here. Estimating the “frictional force” associated with an interaction. Extend to a multi-body interaction.
12	Dynamic Friction Start with a two-body interaction. Top consider as a reduced mass undergoing a deflection. Bottom, lab reference. Collision is elastic, treated in center of mass frame. Velocity is perturbed, break into parallel and perpendicular components.
13	Dynamic Friction Chandrasekhar’s (1960) formula: dv/dt = -(v/v³)16π²(ln Λ)G²m(m+M)∫f(v)v²dv where Λ is a factor of order 1 involving the masses, impact parameter and initial velocity, f is the stellar density, M is the intruder, m is the stellar mass. If the intruder is mass, M >> m, then dv/dt ~ -v 16π²(ln Λ)G²mM f(0) So the force proportional to velocity in this case, a viscous friction. Concerned with the mass density of stars, not individual masses. Force dependent on M, intruder mass. The FORCE goes as mass squared.
14	Chandrasekhar and Simulations
15	Merger Conditions
16	Merger Products Messes, yes that’s what I call them. Tidal tails. Loops, arcs, shells, lumps. Messes. Cool! Things fade and may result in triaxial systems.
17	“Magic” Shells
18	“Magic” Shells Schweizer (1986) Top: Fornax Middle: Centaurus Bottom: NGC 7252 Left is direct images. Right is filtering low-order frequencies to increase contrast.
19	“Magic” Shells Giant elliptical consuming a small spiral (1/100 the mass). Giant elliptical hardly perturbed. Spiral shredded. Peter Quinn did simulations in 1984 and succeeded in reproducing effect. Shells form from “quasi-radial” orbits that are eccentric, so spend most of their time at large radii, depending on energy. Different distribution than the elliptical galaxy stars.
20	“Magic” Shells Quinn (1984): stars arrive at right and oscillate in the potential of the major galaxy.
21	Shells and 3D Galaxy shape Top: Simulation of large prolate elliptical eating small galaxy. Shells aligned with major axis. Bottom: Oblate. Shells spiral randomly.
22	Formation of Ellipticals Many have shells (20%). Not every merger event will produce shells (average consumed then is 4-5). Are all ellipticals merger products? Globular clusters and issue (common around ellipticals, less so around spirals). Remember that starbursts accompany mergers – ULIRGS – and make new GCs? Issue of heirarchical galaxy formation coming…
23	For Next Time Chapter 8 in Combes et al. on radio sources. Continue on Data Reduction and Analysis WIRO open house, anyone?