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17 - Galaxy Evolution. (and interactions). Hints. Galaxies separated by ~10-100 times their sizes Rich dense “ regular ” clusters - E ’ s & SO ’ s at centers compared to outer regions
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17 - Galaxy Evolution (and interactions)
Hints • Galaxies separated by ~10-100 times their sizes • Rich dense “regular” clusters - E’s & SO’s at centers compared to outer regions • Rich dense “regular” clusters (i.e. Coma Cluster) also have a greater fraction of E’s & S0’s than less-dense “irregular” clusters (such as the Hercules Cluster)
“Models” • Forms are primordial - E’s form more easily at the bottom of the cluster potential well • Forms are evolutionary - higher collision rate near cluster cores may both strip star-forming gas from the galaxies and increase the rate of mergers to make giant E’s • S0’s? Disk galaxies that were stripped by collisions, mergers, and star-formation related superwinds?
v Dynamical Friction M ρ v An object (globular cluster, small galaxy) of mass M moving through a medium of “responsive” gravitational bodies (i.e. stars, dark matter) with density ρ with a velocity vM will experience a drag force fd: If strong enough, the moving object can be slowed sufficiently to merge with the medium.
Consequences • If strong enough, the moving object can be slowed sufficiently to merge with the medium. It will be “eaten” by the surrounding medium. This is true even if the “medium” is another galaxy of comparable mass. (CMa Galaxy) • An object (satellite galaxy) will have its orbital velocity reduced, causing it to spiral into the host galaxy (orbital decay) - This is happening to the Magellanic Clouds.
The Virial Theorem The total energy E of a system is the sum of its kinetic energy T and its potential energy Ω: E=T+ Ω It is also possible to show that: 2T+ Ω=0 or 2T=- Ω or T=- Ω/2 For example, this is true of the gas of a protostar contracting under its own gravitational potential energy. As it collapses, Ω becomes more negative, and the star responds by heating up (increasing T). Q: where does the other Ω/2 go? Now, let us consider galaxies as being composed of “gas particles” that we call “stars”.
Galaxy Collisions • Two colliding galaxies will “heat up” by increasing the velocity dispersions of their stars. • The “heat” may go into expanding the “gas” - puffing up the galaxies. The system may even cool by “evaporating” the highest-energy stars - ejecting them.
Examples Ring Galaxies A Polar Ring Galaxy
Simulations “The Antennae”
Galaxy Formation “Top-down and bottom-up”
Top-down: Eggen, Lynden-Bell, and Sandage (1962) The kinematics and metallicities of stars in the MWG can be explained if the MWG formed from the collapse of a galaxy-mass cloud. Stars formed early in the near-spherical cloud are metal-poor and on highly elliptical orbits - high velocities w.r.t. LSR. Remaining gas forms flattened disk, from which younger more metal-rich stars form - even today. Metal enrichment occurs throughout the entire process.
Bottom-up Formation • Many globular clusters are on retrograde orbits w.r.t. rotation of the MWG. • Age spread of GCs (few x 109 yrs) is much greater than that of the ELS model (few x 108 yrs). • GCs near center are more metal-rich and older, while those in the halo exhibit a wide range in metallicity and tend to be younger. Also there is that “bifurcation” at [Fe/H]=-0.8 with the more metal rich GCs being associated with the disk. • Many of these problems are avoided if the MWG was built from the bottom-up through the merger of smaller systems with time.
Evidence for Bottom-Up Formation • We see galaxy collisions today. It happens! • We see stripping of gas and stars from nearby systems (such as the Magellanic Clouds). • Cen - multiple epochs of star-formation? • 1994 - Sag dE galaxy currently being consumed by the MWG! M 54 shares motion of Sag DEG - a member!?
Black Holes & Galaxy Bulges There seems to be a correlation between galaxy bulge mass and the mass of the central BH. One seems to govern the other, but which one came first?
