17 Collsion & Formation of Galaxies

1. Galaxy Collisions

  • The “Cartwheel” galaxy (Left image - upper left) appears to be the result of a collision that has led to an expanding ring of star formation moving outward through the galactic disk.
  • Four bands: IR (red) + Optical (green) + UV (blue) + X-ray (purple)
  • The offending galaxy might be one of the smaller objects at right.

Galaxy Encounter

  • This encounter between two spirals, NGC 2207 (left) and IC 2163, has already led to bursts of star formation in each.
  • Eventually the two will merge, but not for a billion years or so.
  • (a) The “Antennae” galaxies collided a few tens of millions of years ago.
  • The long tidal “tails” (black and white image at the left) mark their final plunge.
  • Strings of young, bright “super star clusters” (color image at center) are the result of violent shock waves produced in the gas disks of the two colliding galaxies.
  • (b) A computer simulation of the encounter shows many of the same features as the real thing, strengthening the case that we really are seeing a collision in progress.

2. Galaxy Formation and Evolution

  • (a) The present view of galaxy formation holds that large systems were built up from smaller ones through collisions and mergers, as shown schematically in this drawing.
  • (b) This photograph provides “fossil evidence” for hundreds of galaxy shards and fragments, up to 5000 Mpc distant.
  • (c) Enlargements of selected portions of (b) reveal rich (billion-star) “star clusters,” all lying within a relatively small volume of space (about 1 Mpc across).
  • Their proximity to one another suggests that we may be seeing a group of pregalactic fragments about to merge to form a galaxy.
  • The events pictured took place about 10 billion years ago.

Hubble Deep Field

  • Numerous small, irregularly shaped young galaxies can be seen in this very deep (i.e catching fainter, therefore distant objects) optical image (made with an exposure of approximately 100 hours).
  • Redshift measurements (numbers next to objects) indicate that some of these galaxies lie well over 1000 Mpc from Earth.
  • The field of view is about 2 arc minutes across, or less than 1 percent of the area subtended by the full Moon.
  • The most distant appear irregular, supporting the theory of galaxy formation by merger.

Starburst Galaxies

  • (a) Collision. This interacting galaxy pair (IC 694, at the left, and NGC 3690) shows starbursts now under way in both galaxies — hence the bluish tint.
  • Such intense, short-lived bursts probably last for no more than a few tens of millions of years—a small fraction of a typical galaxy’s lifetime.
  • (b) Black Hole Interaction. This infrared image of a pair of starburst galaxies (called Arp 107) shows numerous young star clusters arrayed as though along a pearl necklace.
  • (c) Collision. The peculiar (Irr II) galaxy NGC 1275 contains a system of long filaments that seem to be exploding outward into space.
  • Its blue blobs are probably young globular clusters formed by the collision of two galaxies.

Galactic Cannibalism

  • Here we have a dramatic glimpse of a large and massive galaxy under assembly by the merging of smaller, lighter galaxies.
  • This is the way most galaxies probably developed in the earlier universe—by means of a “bottom up” scenario that hierarchically built really big objects by merging star-rich building blocks.
  • This image captures a formative process that occurred about 10 billion years ago, only a few billion years after the Big Bang.
  • The bigger image highlights a region at upper left (in the white box) and nicknamed the “Spiderweb” Galaxy.
  • The inset shows more clearly dozens of small galaxies about to merge into a single huge object, in fact altogether one of the most massive galaxies known.

Tidal Streams in the Milky Way

  • This illustration depicts the breakup and dispersion of the contents of an incoming star-rich galaxy companion captured by our Milky Way.
  • Such streams of stars, all in similar orbits and having much the same composition, are found in the halo of our Galaxy today.
  • Eventually, the smaller galaxy dissolves within the larger one much as other dwarf companion galaxies were probably “consumed” by our Galaxy long ago.
  • This simulation shows how interaction with a smaller galaxy could turn a larger one into a spiral (e.g over several 100 million years).

Galaxy Mergers

  • (Top) When comparably-sized galaxies come together the result is probably an elliptically shaped galaxy, as their original arms and disks do not likely survive the encounter.
  • (Bogttom) By contrast, if a large spiral absorbs a smaller companion, the probable result is merely a larger spiral, with much of its original geometry unchanged.

3. Black Holes in Galaxies

  • (Left) This galaxy is viewed in the radio spectrum, mostly from 21-cm radiation.
  • Doppler shifts of emissions from the core show that the gas clouds around the center obey Kepler’s third law perfectly and have revealed a slightly warped disk.
  • These enormous speeds imply that the cental object is very close to a massive object - a black hole.
  • (Right) The mass of the central black hole is well correlated with the mass of the galactic bulge, for those galaxies where both have been measured.

Binary Black Hole

  • Starburst galaxy NGC6240: (a) optical, (b) X-ray images
  • They show two supermassive black holes (the blue-white objects near the center of the X-ray image) orbiting about 1 kpc apart.
  • According to theoretical estimates, they will merge in about 400 million years, releasing an intense burst of gravitational radiation in the process.

Quasar Host Galaxies

  • Images of distant quasars.
  • They clearly show the young host galaxies in which the quasars reside
  • They imply that quasars represent an early phase of galactic evolution (but highly luminious).
  • Note that several of the quasars appear to be associated with interacting galaxies, consistent with current theories of galaxy evolution.

Quasar Era

  • Quasar Epoch ended 10 billion years ago.
  • All quasars we have seen are older than this.
  • Therefore most probably the black holes (at the centers of the galaxies) powering the quasars.

Galaxy Evolution

  • Some possible evolutionary sequences for galaxies:
    • It begins with galaxy mergers;
    • leading to the highly luminous quasars;
    • decreasing in violence through the radio and Seyfert galaxies;
    • ending with normal ellipticals and spirals.
  • The central black holes that powered the early activity are still there at later times; they simply run out of fuel as time goes on.