29:50 Modern Astronomy
Fall 1999
Lecture 27 ...October 29, 1999
Radio Galaxies and Quasars II

Be sure to look at the latest batch of homework problems. There will be more next week.

I would like to wrap up and summarize what I discussed last time about the properties of radio galaxies and quasars.
(1) All galaxies are receding from us. This is indicated by the use of the Doppler Effect. The wavelengths at which we measure absorption lines in all galaxies are shifted to the red (``redshifted'') with respect to the wavelengths we measure in the lab. The equation is

Astronomers use the variable z to indicate the speed of recession divided by the speed of light. This formula is accurate as long as .
(2) Hubble's Law says that the more distant a radio galaxy, the faster it is receding.

where km/sec/Mpc.

An example was calculated for the radio galaxy 3C79, with v = 76800 km/sec, z = 0.264, and a distance of 1100 Megaparsecs.
(3) Quasars were discovered as star-like objects on photographs of the sky, whose spectra showed enormously redshifted spectra lines. If one applied the mathematics above to them, you found very distant, very luminous objects.

The redshifts of quasars range from about 0.10 for the nearest ones (closer than many radio galaxies) to . Obviously these are extremely distant objects.

We now know that the quasars are indeed at the distances indicated by Hubble's Law, and the starlike appearance is due to the fact that a brilliant source of light at the centers of these galaxies drowns out the light from the rest of the galaxy.

(4) The histogram of Quasar redshifts shows a maximum around , with a dropoff on either side. The dropoff on the low redshift (relatively nearby) side is interpreted as Death of the Quasars. They were very common 5 to 10 billion years ago, but there are few around now. Since they were some sort of phenomenon in the centers of galaxies which are still around, presumably many galaxies harbor quasar corpses. The dropoff on the high redshift side is interpreted as the Turn-on Time for the quasars. At these distances, we are seeing quasars as they were billions of years ago, when the universe was young. If one goes far enough out, the ``lookback time'' is back to an extremely early time before galaxies and quasars had formed.
Blackboard diagram of z distribution for quasars.
Figure 24-3 from book.
Figure 24-4 from book.

A quasar or galaxy with a redshift of 4.4 is being observed at a time when the universe was only 8 % as old as it is now. This would have been about 1 billion years after the Big Bang.

Galaxies have been observed with redshifts about to 5.7.

(6) What is going on in Quasars and Radio Galaxies? What is the source of their energy. To quote from the textbook, ``Quasar luminosities equivalent to 100 average galaxies are common; luminosities as great as 10,000 galaxies have been observed.

As I mentioned last time, at radio wavelengths Quasars can be times as luminous as the Milky Way at radio wavelengths. What is the power source?

Answer: Massive Black Holes. In contrast to stellar black holes with 3 - 15 , these black holes would have at least , and might have as much as .

Why black holes? Their accretion disks are super-efficient energy generators.
Transparency with Black Hole accretion disk.

We saw earlier in the class that matter can be converted to energy according to

In the proton-proton cycle which powers the Sun, about 0.7 % of the mass of the hydrogen is converted to energy in the process of nuclear fusion. Calculation indicate that about 20 % of a mass which spirals in through a black hole accretion disk would be converted to energy. Black Hole accretion disks are the most efficient energy generators known to operate on a large scale in the universe.

(6) What evidence is there for Massive Black Holes in galactic nuclei? Answer: plenty. We can begin here at home in the Milky Way, where recent observations have shown the presence of a Black Hole with a mass of 2.6 million solar masses.
Title page from Andrea Ghez paper last December.

In the nearby spiral galaxy M106, astronomers have seen the evidence for a Black Hole with a mass of 39 million solar masses. In the giant elliptical M87, there is excellent evidence for a black hole at the very center of the galaxy.
Images of M87.

Many more examples exist, and the number is increasing all the time. It seems likely that all major galaxies have massive black holes at their centers, which are the relic of the quasar-era Black Holes. In some cases like M87, this quasar is still partially functioning.
Birth of a Quasar

Large Scale Structures in the Universe

(7) ``We are now at the point, when thinking about the universe as a whole, of considering galaxies like atoms in a gas.''

The question at the moment is whether galaxies are distributed uniformly, like atoms in a gas, or whether they are clumped into even bigger entities, clusters of galaxies or even galaxies of galaxies.

Clusters of Galaxies certainly exist. The Virgo Cluster is the nearest of them. The Abell catalog of Virgo-like clusters, which appeared in 1958, contained 2700 clusters of galaxies.
Map of Abell Clusters.
(8) We can repeat the question as to whether the clusters are uniformly distributed or are organized into clumps.
Illustration of uniform and clumped distribution of points.

To answer this, you need measurements of distances to galaxies. You can do that if you measure the distance to a whole bunch of galaxies. There are many programs to measure the distances to very many galaxies. At the present, we have such data on 11000 galaxies in the northern celestial hemisphere, and 12000 in the southern. The results are extremely interesting.
Figure 25.27 from book.
Figure 25.29

These surveys show the existence of vast Voids in space, which sometimes are 100 Megaparsecs on a side. At the present it is not known if these are completely empty of all matter, or simply are places where galaxies have not formed.

(9) ``More data are needed''. To better determine the nature of these voids, we have to have more galaxies, covering a much bigger volume of space. At the present time a project called the Sloan Digital Sky Survey is underway with a dedicated large telescope. During the 5 year project, the goal is to measure the distances to one million galaxies and 100000 Quasars. This will give us a much clearer view of these voids.

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