One of the most incredible things about studying the universe is that when we use powerful
telescopes to look out into space, we are also looking back in time.
The distances between celestial objects are so incredibly vast, that the light they emit,
even though it travels at the speed of light, which is the fastest speed possible, it still
takes many years to get to us.
And in the case of objects outside the Milky Way galaxy, it takes millions or even billions
of years to get to us.
This puts a fundamental limit on how far we can see, because given that the universe began
approximately 13.8 billion years ago, and then factoring in for the expansion of the
universe over its lifetime, it is impossible for us to see farther than approximately 46.5
billion light years away, as light can’t have been traveling for longer than the universe
has existed, a limitation that does not change even though the universe has expanded significantly
during that time, which changes the wavelength of that light in predictable ways.
The finite length of time into the past for the birth of the universe, or more specifcally
the recombination era that occurred a few hundred thousand years into the lifetime of
the universe, producing light as we know it, is the limitation that determines the radius
of the observable universe and therefore what we can see with telescopes, although we don’t
know precisely how big the universe is as a whole.
But the fact remains that if we use powerful telescopes to look at objects that are many
billions of light years away, which we have been able to do for a few decades now, we
are looking at the first large-scale structures that developed in the early universe, and
we get to look at them today.
It’s as though we could learn about the history of the earth simply by digging a deep
hole in the ground and finding progressively older photographs of the earth as we go.
This fortunate situation gives us the incredible opportunity to learn about galaxy formation
in the early universe.
When we peer this incredibly far out into the universe, we see objects called quasars,
which is short for quasi-stellar objects, abbreviated as QSO.
These were named as such because when they were first observed by early radio astronomy
in the 1950s, we weren’t quite sure what they were, since they appeared as faint, star-like objects.
However, upon determining the extremely high red shift values for these objects, we began
to realize that these are actually extremely luminous but very distant active galactic
nuclei, abbreviated as AGN.
Essentially, quasars are supermassive black holes, of millions or even billions of solar
masses, surrounded by an accretion disk of gas, often producing enormous jets of material
that are ejected from near the surface of the black hole, perpendicular to the plane
of rotation.
This gas is racing around the black hole, which makes it very hot and bright, so bright
in fact that a quasar is thousands of times more luminous than a galaxy like our Milky Way.
That’s what first made them so confusing, as given their extreme distance, the apparent
luminosity is closer to what would be expected for a much closer object.
But now we understand that these are supermassive black holes that essentially serve as seeds
for brand new galaxies to form, given the immense gravitational pull of the black hole.
This makes sense in the context of galactic evolution, since there is a limited time after
galaxy formation that an accretion disk can exist, as the gas near the black hole will
eventually dissipate and the galaxy will become more ordinary.
For this reason, most of the quasars we see must have formed in the early universe, on
average about ten billion years ago, most of which can be found within what is referred
to as a host galaxy.
To date we have only found one quasar with no host galaxy, which is being studied to
find additional information about the mechanism of galaxy formation, such as the interesting
proposal that quasar jets can stimulate galaxy formation, which will then merge with the
quasar, given its supermassive status and the immense gravitational pull that results.
But nevertheless, the very farthest of quasars we have found represent some of the first
large-scale structures that ever formed, with the oldest forming when the universe was only
690 million years old, as the universe was emerging from the so-called dark ages, and
its host galaxy must have been one of the first to ever form in the early universe.
Such quasars from the very early universe are typically surrounded by a giant gas halo
extending many thousands of light years from the supermassive black hole, consisting of
cool and dense but glowing hydrogen gas, shown here in blue.
These halos sustain the growth of the black hole.
Comprehensive studies of quasars have revealed such bright, gaseous halos on a large proportion
of the quasars, indicating that this is a common feature.
Once again, for a few decades after their discovery, there was some controversy as to
what quasars were, but key evidence came by the way of X-ray astronomy, spectral analysis,
and especially due to gravitational lensing, which we learned about in the previous tutorial.
Sometimes the light from a quasar is bent around a foreground galaxy such that it appears
as four images, which is called an Einstein cross.
In addition, more detailed analysis can arrive when we distinguish between macrolensing and
microlensing.
Macrolensing is the type of gravitational lensing we have been discussing, where the
lensing object is very large, like a galaxy, which allows us to observe very distant objects.
This is the phenomenon that would produce an Einstein cross.
But individual stars within the foreground galaxy can produce additional magnification
which we call microlensing, which operates by the same principles, just on a smaller scale.
As the stars move in the lensing galaxy, the microlensing magnification will change, and
this causes a flicker in the brightness of a quasar that can be measured and studied,
in order to gather additional information about the object.
These days, we can employ an interesting strategy to study these distant quasars, and that is
to link multiple telescopes around the world to perform a technique called Very Long Baseline
Interferometry, or VLBI.
This allows multiple telescopes to act as a single telescope, thus enabling for sharper
observations.
Current studies are analyzing the distribution of quasars over billions of light years within
large-scale structures, as well as details regarding alignments between their spin axes
and that of the group they are embedded within.
Hopefully, a study of quasars will yield new information that will propel the forefront
of astrophysics, and help us better understand the universe as a whole.