THE GALAXIES

NATURE OF GALAXIES

• One 19th C theory -- Our galaxy is an "island universe"
  • our galaxy contains all the stars in the universe; outside the galaxy is essentially nothing.
  • The "spiral nebulae" (first observed in 18th C, e.g. Messier) are small whorls of gas within our own galaxy

• Another point of view (Immanuel Kant in 18th C, influenced by T. Wright) -- The "spiral nebulae" are very distant and comparable in size to the Milky Way: "island universes" in their own right

Visual observation was unable to distinguish the stars in other galaxies. As photographic techniques improved, it became possible to make photographs of other galaxies: the "spiral nebulae" were then thought to be regions of solar system formation. The key problem: How to determine distance to galaxies, and thus their size?

Answered after discovery of Cepheid variables. The galaxies are very distant (Andromeda, M31, the nearest large galaxy, is 600 kpc away) and comparable in size to the Milky Way. It is estimated that there are 100 billion galaxies in the Universe.

NORMAL GALAXIES

Hubble classification scheme, 1924. Evolutionary diagram for the galaxies?

spirals (S) and barred spirals (SB). Examples: Milky Way, Andromeda.

• disk, bulge, halo, like the Milky Way
• disk contains gas, dust and new stars, like the Milky Way
• subclassified (a-c) according to how well developed the spiral arms are
  • Those with large bulges have tight, nearly circular spiral arms
  • Those with medium bulges have more open arms
  • Those with small bulges have loose, poorly defined arms

Elliptical galaxies (E)

• vary in size from giant (10⁶ pc, trillions of stars) to dwarf (10³ pc, a million stars)
• have no disk, dust or gas: essentially all halo. (Some giant ellipticals are exceptional in that they contain gas and dust: these might be normal ellipticals that have collided and merged with a spiral companion)
• subclassified (0-7) by how elongated they are

An intermediate type, an elliptical with a thin disk, has the category S0 or SB0. The ellipticals and spirals can be put together in a continuum called the "tuning fork" diagram.

Irregulars (Irr). Examples: Large and Small Magellanic Clouds, Sagittarius Dwarf Galaxy

• Some irregulars are like misshapen spirals with lots of gas and dust
• Some irregulars may be the result of a collision between normal galaxies, or the result of an explosion in a normal galaxy

The most common kinds of galaxies are dwarf irregulars and dwarf ellipticals

FORMATION OF GALAXIES

• Formation of galaxies begins after Big Bang.
• Different from formation of stars in that galaxies often collide, or even merge.
• Galaxies are probably built up by mergers (contrast breakup of clouds in star formation). E.g. the Milky Way is currently colliding with the Sagittarius Dwarf Galaxy

Evidence for collisions between galaxies:

• Collisions may help explain the formation of spiral structures.
• Some galaxies are observed to have double cores, perhaps from mergers
• lots of recent images from HST

DISTANCES TO THE GALAXIES -- the next rung on the cosmic distance ladder

1. Tully-Fisher relationship -- using known distance to nearby galaxies, establish a relationship between mass and luminosity
   • Mass can be deduced from galactic rotation curves.
   • From mass, deduce luminosity
   • By comparing observed brightness and luminosity, distance can be determined.
2. Type I supernovae as "standard candles."

These two methods together extend the distance scale out to 200 Mpc (millions of parsecs), but don't completely agree.

HUBBLE'S LAW -- last rung on the cosmic distance ladder

1912 Vesto Slipher observes that almost all "spiral nebulae" have redshifted spectra: they are receding from the Milky Way

1920's Edwin Hubble observes that on the average, the recession velocity of a galaxy is proportional to its distance

• Universe is expanding
• Hubble's law is only true on average; each galaxy has its own random velocity ("peculiar velocity") added to the Hubble velocity; however, for very distant galaxies, the Hubble velocity is much greater than the peculiar velocity

Hubble's law: recessional velocity=Hubble's constant x distance

Hubble's constant H₀ is not very well known. It lies between 60-90 km/s/Mpc. See HST results on observations of Cepheids in distant galaxies.

Distance -- using Hubble's law, distance can be deduced from measured recession velocity (redshift)

BEYOND THE GALACTIC SCALE

Clusters of galaxies

Galaxies form gravitationally bound groups such as:

• the Local Group, a group of about 20 galaxies including the Milky Way and Andromeda
• the Virgo Cluster, a group of about 2500 galaxies in the direction of the constellation Virgo.

Superclusters

A cluster of clusters; example: the Local Supercluster, 100 Mpc across, containing many tens of thousands of galaxies

MASS OF GALAXIES; MISSING MASS PROBLEM CONT'D

mass of galaxies deduced from their gravitational effects

• mass of nearby galaxies can be deduced from their rotation curves
• Periods of binary (or larger) systems of galaxies cannot be measured, because of the long time periods involved; but we can measure their separation and their velocities at the present time. This is sufficient to tell us the average mass of large groups of galaxies (if they are gravitationally bound)

• galaxies contain 3-10 times as much mass as we can detect by other means
• clusters of galaxies contain 10-100 times the amount of detectable mass

Conclusion: up to 90% of the mass of the universe has yet to be detected by us.

(Some intergalactic gas is observed to exist between the galaxies, but not enough to solve the missing mass problem. No gas has yet been detected outside the clusters.)

STRUCTURE BEYOND SUPERCLUSTER SCALE

Since 1981, three-dimensional plots of galactic positions show that the Universe has great big empty spaces in it, like bubbles (the galaxies are scattered along the surfaces of the bubbles). Structure at this scale must have formed at time of Big Bang: provides stringent test of theories of formation of Universe.

Largest structure known to date: the Great Wall, 70 Mpc x 200 Mpc

ACTIVE GALAXIES AND QUASARS

The most distant objects tend to be more luminous than nearby galaxies Common features:

• high luminosity
• nonstellar spectra (lots of IR and radio emission)
• energy output varies with time
• often exhibit jets or other signs of explosive activity
• optical spectra indicate rapid internal motion

Active Galaxies

Seyfert galaxies emit tremendous amounts of radio energy. They show a small very bright core, whose brightness varies with time over the course of a year--implying that the size of the core is less than a light-year.

Radio galaxies emit tremendous amounts of energy at radio wavelength. Some of them have huge lobes, which appear to be jets of material ejected from the galaxy.

Quasars or QSO's (quasi-stellar objects)

If the redshift of quasars can be interpreted using Hubble's law,

• at 240 to 3800 Mpc, they are the most distant objects in the Universe
• light from the most distant quasar takes over 10 billion years to reach Earth: we see them as they were in early Universe
• If they are so far away, they must be incredibly luminous, 1000 times more luminous than the brightest galaxies.

The best explanation of the power of active galaxies and and the even more incredibile power of quasars is that they are powered by accretion disks surrounding giant black holes at the centers of galaxies. New results from HST appear to confirm the theory that quasars are associated with the cores of young galaxies.

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