The nearby region of the Universe, known as the Local Group and is located at the edge of what
is known as the the Virgo supercluster of galaxies. The use of Cepheid variables is limited to
within the volume of space outlined by Virgo system. Thus, the distances to nearby galaxies does
not measure the true Hubble flow of the expanding Universe, but rather the gravitational infall
into Virgo.
In order to determine Hubble's constant, we must measure the velocity of galaxies much farther
away then the Local Group or the Virgo supercluster. But, at these distances we cannot see
Cepheid stars, so we determine the total luminosity of the galaxy by the Tully-Fisher method, the
last leg of the distance scale ladder.
The Tully-Fisher relation is basically a plot of mass versus luminosity of a galaxy. Its not
surprising that luminosity and mass are correlated since stars make up basically most of a
galaxy's mass and all of the light. Missing mass would be in the form of gas, dust and dark
matter.
The key parameter for this last leg of the distance scale are the calibrating galaxies to the
Tully-Fisher relation, i.e. the galaxies where we know both the total luminosity from Cepheid
distances and the total luminosity from the Tully-Fisher relation.
There is currently a strong debate on the value of the Hubble's constant fueled by new data from
HST Cepheid studies of nearby galaxies. The community is divided into two schools of thought;
1) the old school which proposes a value for Hubble's constant around 50 to agree with the ages
of the oldest stars in our Galaxy, and 2) a newer, and larger school which finds a higher Hubble's
constant of 75. This higher value poses a problem for modern cosmology in that the age of the
Universe from Hubble's constant is less than the age of the oldest stars as determined by nuclear
physics.
So the dilemma is this, either something is wrong with nuclear physics or something is wrong
with our understanding of the geometry of the Universe. One possible solution is the introduction
of the cosmological constant, once rejected as unnecessary to cosmology, it has now grown in
importance due to the conflict of stellar ages and the age of the Universe.
Quasars:
Quasars are the most luminous objects in the Universe. The typical quasar emits 100 to 1000
times the amount of radiation as our own Milky Way galaxy. However, quasars are also variable
on the order of a few days, which means that the source of radiation must be contained in a
volume of space on a few light-days across. How such amounts of energy can be generated in
such small volumes is a challenge to our current physics.
Quasars were originally discovered in the radio region of the spectrum, even though they emit
most of their radiation in the high energy x-ray and gamma-ray regions. Optical spectra of the
first quasars in the 1960's showed them to be over two billion light-years away, meaning two
billion years into the past as well.
Over a thousand quasars have been discovered, most having redshifts greater than 10 billion
light-years away. The number density of quasars drops off very fast, such that they are objects
associated with a time when galaxies were young.
The large amount of radio and x-ray emission from quasars gives them similar properties to the
class of what are called active galaxies, such as Seyfert galaxies, originally recognized by the
American astronomer Carl K. Seyfert from optical spectra. Seyfert galaxies have very bright
nuclei with strong emission lines of hydrogen and other common elements, showing velocities of
hundreds or thousands of kilometers per second, where the high energy emission is probably due
to a Galactic mass black hole at the galaxies core (for example, NGC 4261 shown below). The
idea is that quasars are younger, and brighter, versions of Seyfert galaxies.
HST imaging showed that quasars are centered in the middle of host galaxies, giving more
support to the idea that the quasar phenomenon is associated with Galactic mass black holes in
the middle of the host galaxies. Since a majority of the host galaxies are disturbed in appearance,
the suspicion is that colliding galaxies cause stars and gas to be tidally pushed into the black hole
to fuel the quasar.
This process would explain the occurrence of quasars with redshift. In the far distant past there
were no galaxies, so no sites for quasars. In the early phases of galaxy formation, the galaxy
density was high, and there were many collisions producing many quasars. As time passed, the
number of collisions decreased as space expanded and the number of quasar also dropped.
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