| This week: Finish up galaxy interactions | |
| (Ch. 7, Combes et al., parts) | |
| Unless noted, all figs and eqs from Combes et al or Longair. | |
| 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. | ||
| 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). |
| 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. | |
| 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! |
| Toomre (1977) on Ring Galaxies from Arp’s 1996 Atlas of Peculiar Galaxies. |
| Lynds and Toomre (1976) in the system II Hz 4. | |
| A Double Ring Galaxy. |
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.” |
3D Halos and Polar Ring Galaxies
| Merger product probes TWO dimensions. |
| This is the effect of braking of a massive body P by the stars of a galaxy when P penetrates or passes nearby. |
| 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. |
| 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. |
| 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. |
| Messes, yes that’s what I call them. Tidal tails. Loops, arcs, shells, lumps. Messes. Cool! Things fade and may result in triaxial systems. |
| Schweizer (1986) | |
| Top: Fornax | |
| Middle: Centaurus | |
| Bottom: NGC 7252 | |
| Left is direct images. | |
| Right is filtering low-order frequencies to increase contrast. | |
| 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. |
| Quinn (1984): stars arrive at right and oscillate in the potential of the major galaxy. |
| Top: Simulation of large prolate elliptical eating small galaxy. Shells aligned with major axis. | |
| Bottom: Oblate. Shells spiral randomly. |
| 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… |
| Chapter 8 in Combes et al. on radio sources. | |
| Continue on Data Reduction and Analysis | |
| WIRO open house, anyone? |