Transcript Document

Quasars:
Characteristics,
Observations,
Controversies, and
Spatial Distribution
Peter Leimbigler
David Rotenberg
Quasi-stellar radio objects
• The discovery of quasars in the late 1950s opened a new
window to the cosmos
• Quasars first described as ‘radio stars’ or ‘radio galaxies
without galaxies’
• are among the farthest and energetic objects in the
Universe (L > 1011L)
• Quasars can and have been used to deduce the structure
and conditions of the early Universe
• Quasars enable spectrographic mapping of otherwise
undetectable large-scale structure in the intervening space
• Spatial distribution of quasars is an active area of research
thanks to recent and extensive quasar surveys (e.g. 2dF,
Durham/AAO)
• Lively debate surrounds the nature of quasars
What Quasars are
• Thought to be the most luminous form of AGN
• Radio-loud quasars were discovered first (hence
‘radio object’); radio-quiet quasars (‘quasi-stellar
radio objects’, QSOs) were later found to be 10-30x
more common than radio-loud quasars
• Both radio-loud and radio-quiet quasars reside in
giant elliptical galaxies (from HST observations)
• in contrast, Seyfert nuclei (another type of AGN)
reside mostly in spirals
• “quasars are AGNs”  quasars are powered by
supermassive black holes (SMBHs)
Redshift
• Redshift z is the fractional increase in wavelength
of light
• mechanisms:
Solar spectrum
– Doppler
– gravitational
– Cosmological
–
spectrum of distant galaxy cluster
(Arp: non-cosmological?)
• The primary method of distance determination for
quasars
• Constitutes the final rung of the cosmic distance
ladder
The cosmic distance ladder
use for
quasars
More about redshift
• Doppler:
• relativistic Doppler:
lo = observed wavelength,
le = emitted wavelength
, where
More about redshift
• cosmological:
, for small
and large z, where R(t) is the scale length
(basically the size) of the Universe
• z = 5 quasar emitted its light when the Universe
was 1/6 its present size
• Cosmological redshift makes quasars appear to
recede; in fact, the whole coordinate system is
expanding, carrying them farther away and
redshifting their light as it travels through space
•Small yet vocal groups strongly oppose the use of redshifts as distance
indicators. The brunt of their arguments are aimed at refuting the validity
of the conclusions about the distance and age of quasars based on
redshift.
•If redshifts are not related to object distances as maintained by Hubble’s
law then the familiar concepts such as the expansion of the universe, and
modern cosmological theories would be falsified.
•Therefore the question of quasar distribution becomes elemental in
upholding current cosmological theories, including those centered
around the age, expansion and creation of our universe.
•One of the most influential contributors to the against the redshiftdistance law’s validity is Halton Arp, who claims to have observed
through his studies
Impossible Interactions?
Many of Arp’s claims rely on images of what
he interprets as galactic objects interacting one
another, despite the fact that they possess
spectra redshifted by different degrees. If in
fact redshifts are valid distance indicators,
then the vast distances between the galaxies
would make physical interaction impossible.
It is difficult to ascertain, if one ignores the
redshift-distance correlation what the relative
distances between galaxies are simply by visual
interpretation. Arp is scant when it comes to
determining the galaxies’ actual positions
relative on another.
Without clear data demonstrating that these such galaxies
are indeed interacting, of which there is seemingly none,
their existence can be explained by other methods fitting
with current theory. Such as the projection of more
distant galaxies onto those nearby, or seeming interactions
due to our particular line of sight. The photographs
supplied by Arp are of finite scientific value.
Preferential Distribution – Forbidden
Interactions Continued.
Arp also argues that quasars appear preferentially close to
bright or active galaxies. He supports this with photographic
evidence and imparts that the statistical likelihood of quasars
aligned nearby galaxies in this manner is on the order of 10 -6.
If this statistically likelihood is supported by a substantial
number of observations of such systems then the isotropic
distribution of quasars would be violated. However Arp’s
method have been strongly challenged.
-Searching for anomalies that depart from what would
otherwise be a regular, uniform distribution and taking note of
these and these alone seems contrary and counterproductive;
using ad hoc explanations to account for anomalies, as each
arisesNot only have claims of galaxies with different red-shift
interacting with one another been raised, but images have been
produced which suggest that quasars are interacting with
nearby galaxies, and that some are even embedded within
large host galaxies. If this were the case not only would it
invalidate the value of the redshift – distance relation, but
would also imply that quasars have completely different
origins then what has been stated by current theory.
If quasars are distant and ancient then the light that
they radiate would pass through myriad features on its
way to earth, some of which would be as ancient as the
quasars themselves. The interactions between the
quasar’s light and these features are recorded in their
spectra, which we can analyze and withdraw vast
information from.
According to current big bang theory, at the time when
quasars were thought to have formed, the universe was
a different environment. Referring to redshift as both a
distance and age indicator, the light from highly
redshifted quasars should show evidence of the
variance between the modern and ancient universe.
Ly α “Forest”
Gunn-Peterson Trough
Of particular interest is the contrast between
quasars of different redshift. The oldest
quasars, those of z =6 exhibit what is
referred to as the Gunn-Peterson trough a
depression between the hydrogen lines.
Whereas the Ly α forest is the product of
diffuse gas clouds, the Peterson trough is
thought to be caused by absorption by
neutral hydrogen, before the early universe
was completely ionized.
This precise fit into the cosmological model,
and big bang theory suggests that indeed
quasars are not only as distant, but as old as
redshift would indicate.
The question pertaining to the distribution of quasars in space is
still in debate by those who argue against upheld theory.
Although the consensus of astronomers is as of 2007 that:
quasars are indeed the most distant and ancient objects we have
so far observed, and have a nearly isotropic distribution, new
discoveries always arise to challenge this consensus.
As for a conclusion: is it possible that there are sources capable
of producing high redshift phenomenon without being extremely
distant?
Redshift Survey (2dF QZ, 1996-2002)
Distribution
• Quasars are observed in all directions of the
sky in a seemingly random distribution
• Clustering is not as strong as that of
galaxies
• Quasars are very rare near us: ~ 1 per GPc3,
or one quasar among millions of normal
galaxies
• Density increases rapidly with increasing z
The quasar epoch
Fit for data collected by Shaver et al. (1988)
Density distribution
Implications of distribution
• Quasars preferentially formed in the early
Universe and have died out today
• Stage of galaxy formation
• Galaxy collisions ‘feed’ SMBHs; perhaps galaxy
collisions were more common in the early
Universe
• Explanations for high-z falloff:
– selection effect: only bright high-z quasars are seen
– evolution: quasar cores take time to form
– both?
Redshift Survey (2dF QZ, 1996-2002)
For comparison: 2dF galaxy redshift survey