Galaxies and Dark Matter
The Large-Scale Structure of the Cosmos

posted Apr 20, 2010, jeb
revised Apr 25, 2010, jeb

Outline Dark Matter in the Universe Galaxy Collisions Galaxy Formation and Evolution Black Holes in Galaxies The Universe on Large Scales Summary

Dark Matter in the Universe

The Milky Way Galaxy appears to be surrounded by a dark matter halo
• The orbital velocities of stars and gas in the Milky Way reveal the presence of this invisible matter.

Mass Measurement (Galaxies and Galaxy Clusters)
• Rotation curves of galaxies are measured by observing the Doppler shift of light versus distance from the galactic center
• As with the the Milky Way, these curves show an anomaly from what is expected based on the distribution of visible mass
• Distant galaxies can be measured using the Tully-Fisher relation
  • Broadening of spectral lines reveals overall rotation speed
  • Estimate of the galaxies size leads to an estimate of the mass
  • This technique works to measure mass within about 50 kpc of galaxy's center
  
  
• Mass beyond 50 kpc of the galaxy's center can be determined for Binary galaxy systems
  • Estimates of period and size of galactic orbits translate into approximate measures of mass
  
  
• Total mass within a galactic cluster can be determined from observing the motions using Doppler shifts
  • Estimate the mass required to bind the galaxies, given the values of their relative speeds


Dark Matter, Dark Halos, and Dark Galaxies
• The rotation curves of spiral galaxies suggest an invisible dark halo
• Velocities within galaxy clusters suggest a lot more mass than we can see is needed to bind the galaxies together
• Upwards of 90 percent of the matter in the universe appears to be composed of dark matter


• Dark Galaxies
  • Could some galaxies be composed almost entirely of dark matter?
    • Perhaps UGC 10214 is the result of a Dark Galaxy companion ripping matter from the dwarf elliptical galaxy
    • VIRGOHI 21 is a controversial candidate, a mysterious cloud of hydrogen (revealed by 21 cm radiation) in the Virgo cluster, without visible stars
      • report


• What is the dark matter?
  • Stellar remnants?
  • Exotic subatomic particles?
  • We do not yet know
    • My expectation - mostly exotic subatomic particles, some stellar remnants
  
  
Intracluster Gas
• Intergalactic Matter (between galaxies)
  • Interstellar medium (between stars within a galaxy)
    • about 1 atom per cubic cm
  • Intergalactic medium (between galaxies within the cluster - intracluster)
    • about 10^-6 atoms/cm³, or about 1 atom per cubic meter
  • Intercluster medium (between galactic clusters)
    • very little matter
• Hot gas is observed from X-ray emission
  • Abell 85
  • A distant galaxy cluster
  • gas extends into the space between galaxies, but it does not add up sufficiently to solve the missing matter problem above (need dark matter)
• Further evidence of gas external to galaxies from radio lobes of radio galaxies
  • head-tail radio galaxy shows what appears to result from galaxy traveling through intergalactic medium
• The amount of mass required to bind this gas to the clusters ( see the Virgo Cluster) greatly exceeds the total visible mass of the galaxies (more evidence of dark matter)
• There is no evidence for such gas beyond the clusters, in the "extracluster" spaces

Galaxy Collisions

Galaxies in clusters often collide
• Cartwheel galaxy at a distance of 150 Mpc from Earth
  • Smaller galaxy has collided with larger one
  • Density wave generated in larger galaxy resulted in ring of star formation
• Recent encounter
  • Two spiral galaxies (NGC 2207 and IC 2163) have swung past each other
  • Close approach 40 million years ago
    • Resulting in burst of star formation due to shock to interstellar medium
  • Likely will merge into a single galaxy in about a billion years
• The antennae
  • Long tidal tails mark final plunge of colliding galaxies
  • High resolution image from Hubble Space Telescope clearly shows star formation resulting from violent shock waves produced by collision
  • computer simulation

Encounters have substantial effects on galaxies

Andromeda is approaching the Milky Way at 120 km/s, and will collide in a few billion years!

Stars glide past one another (with essential no effect on individual stars)
• while tidal forces cause bursts of new star formation with major effects on the galaxies themselves

Galaxy Formation and Evolution

Galaxy formation started in the very early universe
• When small density fluctuations in the primordial matter began to grow

We know of no simple evolutionary path
• Rather, the development of any given galaxy is the consequence of its particular environment

Hierarchical merging
• Motion of galaxies in clusters often leads to close encounters or collisions
  • Computer simulations of the early universe show many mergers
• Galaxies at large redshifts (very distant, large look-back times) are smaller and more irregular than nearby galaxies
  • Hubble image showing mergers roughly 10 billion years ago
  • Images reveal galactic mergers were common ten billion years ago
• The Hubble Deep Field image shows evolution over billions of years
  • small, irregularly shaped galaxies at largest redshifts

A galaxy left alone will evolve slowly, in a predictable way
• Elliptical, lacking interstellar gas, will become fainter and redder as stars burn out
• Spiral or Irregular, with much gas, will remain bluish as long as new star formation continues

Within a cluster, close encounters and collisions lead to:
• Starburst Galaxies
  • Rapid star formation
  • NGC 1275 shows young cluster formation initiated by the collision of two galaxies
• Excitation of galactic nucleus, as fuel is diverted to central black hole
  • Active galaxies

Galactic Cannibalism
• Brightest galaxies devour weaker ones
• Galactic cannibalism close by
  • Sagittarus dwarf galaxy and Magellanic Clouds will be merged with the Milky Way
  • Halo stars of Milky Way are evidence of past Milky Way "consumption"
• Shape distortion
  • The Whirlpool Galaxy, M51
  • computer simulation

Galaxy Mergers
• Major mergers turn spirals into ellipticals
  • collisions between galaxies of comparable size
  • destroy galactic disks
• Minor mergers leave larger galaxy intact
  • collisions of small galaxies with large ones
  • smaller galaxy is absorbed

Black Holes in Galaxies

Quasars and Active Galaxies are powered by supermassive black holes
• Much more prevalent in distant past
  • How does this relate to galaxy evolution?
• Many bright normal galaxies contain supermassive black holes at their centers
  • Galactic black hole
    • Core probed by VLBA (Very Large Baseline Array)
      • VLBA is continent-wide array of 10 radio telescopes

Supermassive black holes are common
• Every bright galaxy - active or not - may contain a central supermassive black hole
• Black hole masses are correlated with the mass of the galaxy

Quasar Epoch
• The supermassive black holes needed to power quasars took time to build
  • black holes form in galaxies, and merge during galaxy merger
• Some quasar appear very early (redshift 6, about 13 billion year look-back times)
• But most appear after another 2 billion years (redshifts 2-3)
• Most central black holes are assumed to merge during galactic merger, but no always
  • Binary black hole

Long-exposure Hubble images of a distant quasars
• Clearly shows the young host galaxies in which the quasars reside
  • lending strong support to the view that quasars represent an early, very luminous phase of galaxy evolution
• Several quasars appear to be associated with interacting galaxies.
• The quasar at the top left has the catalog name PG0052+251 and resides roughly 700 Mpc (2.3 billion light-years) from Earth.

A possible evolutionary sequence for galaxies
• Beginning with the highly luminous quasars
• Decreasing in violence through the radio and Seyfert galaxies
• and ending with normal spirals and ellipticals
  • 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

The Universe on Large Scales

Superclusters
• Galaxy clusters are themselves clustered into superclusters
• Virgo Supercluster
  • Local Supercluster, includes Local Group
  • Virgo Supercluster in 3D
  • Tens of thousands of galaxies (about 10¹⁵ solar masses)
  • Includes Coma Cluster, more than 100 Mpc from Earth
  • Local Group lies on periphery, about 18 Mpc from the center

Red Shift Surveys
• Survey to about 200 Mpc (made in 1980s)
  • On very large scales the galaxies are not distributed randomly
  • "Bubbles" of galaxies surround unpopulated regions known as ( voids)
  • the Great Wall is one of the largest known structure in the universe
• Recent Survey to about 1000 Mpc
  • Numerous voids and "Great Wall-like" filaments
  • Largest structures about 200 Mpc

Quasar Absorption Lines
• Quasars send light from the largest distances of the universe
• The principal wavelength (Lyman-alpha) is redshifted
• During the journey, the light is absorbed by the intervening materials, each at a different redshift
• Resulting spectrum, the "Lyman-Alpha Forest"
  • Each absorption line reveals a foreground gas cloud which the light passed through on its journey from the quasar to Earth

Quasar Mirages
• Binary quasar - two quasars with identical redshifts and spectra
  • Designated AC114 and located about 7 billion light years (2 billion pc) away
  • These two blobs of light have striking symmetry
  • Rather than twins, the two large blobs are images of the same object, created by a gravitational lens
  • The lensing galaxy itself is probably not visible in this image
    • the two objects near the center of the frame are thought to be unrelated galaxies in a foreground cluster
• "Einstein Cross", a multiple-imaged quasar
  • Hubble view, spanning only a couple of arc seconds, showing four separate images of the same quasar produced by an intervening galaxy

Mapping Dark Matter
• Blue arcs
  • Created by the gravitational lensing of the intervening galaxy which deflects and distorts the light from background galaxies
  • Measuring the distortion yields the mass of the intervening galaxy
• Dark-Matter Map
  • View of the sky with fuzzy blue specs from much more distant background galaxies
  • Dark matter distribution derived from light from background galaxies
• Cluster Collision
  • Bullet cluster, 1 billion pc away
  • dark matter in blue, X-ray emitting gas
  • separation of dark matter and gas important confirmation of theory of dark matter

Summary

• Masses of galaxies are determined from rotation curves, binary motions, and galaxy cluster movement
• More than 90% of the mass in the universe is dark matter
  • Stellar dark matter can only explain a small percentage
• Evolution of galaxies procedes through a complex sequence, characterized by collisions and mergers
  • Large galaxies are formed by the merger of smaller ones
  • Mergers of spiral galaxies likely leads to elliptical galaxies
• Quasars, active galaxies, and normal galaxies probably represent an evolutionary process
  • Supermassive black holes are common
• Galaxy clusters gather into superclusters, arranged on the surface of enormous bubbles surrounding low-density regions known as voids
• Quasars provide one means of probing the matter of the universe over large look-back times