Hi, mongolia-ga:
The short answer is that quasars are very mysterious and galaxies are
only slightly mysterious, due to the fact that we live in one, have
mapped the 3D locations of over 100,000 of them, and estimates of the
number in the visible universe run easily into the billions:
[How many Galaxies are there?]
http://www.faqs.org/faqs/astronomy/faq/part8/section-4.html
[Quasar -- Wikipedia]
http://en.wikipedia.org/wiki/Quasar
"A quasar (from quasi-stellar radio source) is an astronomical object
that looks like a star in optical telescopes (i.e. it is a point
source), and has a very high redshift. The general consensus is that
this high redshift is cosmological, the result of Hubble's law, which
implies that quasars must be very distant and must emit more energy
than dozens of normal galaxies."
There is more evidence that at least some quasars are indeed small
objects (in astronomical terms!) because these have been observed to
change brightness over short time spans. Because the speed of light
limits how quickly a change can propagate, the natural interpretation
of this is that the quasars are small. Other interpretations are
conceivable however; see here for one:
[Quasar J1819+3845 -- Wikipedia]
http://en.wikipedia.org/wiki/J1819_plus_3845
The number of observed quasars is several thousand, according to this
fairly old page from NASA:
[Number of Quasars]
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/981222a.html
It is conceivable that these are small "active galaxies" or more
plausibly super-massive black holes that flare on for a limited period
of time due to encounters with surrounding material (based on the
enormous of amount of energy being produced in compact regions). Not
all quasars are strong radio sources; about 10% of them are "radio
quiet".
But it is fair to say that we know quasars are not simply "ordinary"
galaxies, even very early ones, on account of their intrinsic
brightness, esp. in the high-energy gamma ray spectrum.
It is possible that current research on gamma ray bursts will produce
insights on new mechanisms for production of such titanic amounts of
energy:
[The Swift Gamma-Ray Burst Mission]
http://swift.gsfc.nasa.gov/docs/swift/swiftsc.html
See also this recent discovery:
[Discovery of new type of dust leads to new quasar ideas]
http://www.physorg.com/news2483.html
regards, mathtalk-ga |
Clarification of Answer by
mathtalk-ga
on
04 May 2005 19:32 PDT
On the specific issue of whether our Milky Way "home" galaxy was once
a quasar, see these links:
[Milky Way as a Quasar]
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/040407a.html
[Quasar Birth]
http://www.earthsky.com/shows/showsmore.php?t=20040122
"Perhaps 10 billion years ago, when the universe was 3.7 billion years
young, the Milky Way and many of its neighboring galaxies were
quasars.
"Astronomers who hold this view believe that the Milky Way quasar
might have shut down when it ran out of things to eat."
The point is made, however, that we have yet to observe a quasar
starting up or shutting down, and until one is, the idea that many
galaxies would appear as quasars in an early (or intermittent) phase
will remain speculative.
regards, mathtalk-ga
|
Clarification of Answer by
mathtalk-ga
on
05 May 2005 05:42 PDT
Let me try to address the individual questions raised:
1. So do all Galaxies start out as quasars? Or are there galaxies
which just started out as Galaxies? At what point does a quasar become
"just a galaxy"?
The most conventional theory about a quasar is that it is an "active
galactic nucleus" (AGN) in which an accretion disk forms around a
black hole near the center of a spiral galaxy:
[Quasars]
http://www.astro.umd.edu/education/astro/actgal/quasars.html
[Power Source for Active Galaxies and Quasars]
http://www.astronomynotes.com/galaxy/s14.htm
Certainly there are galaxies which lack AGN now and presumably from
their inception. It is thought that most if not all large spiral
galaxies with a central bulge (like our Milky Way) harbor a
super-massive black hole there. But observations of the neighboring
M33 spiral galaxy, which lacks a central bulge, suggest that if it has
a black hole, it is not so massive as the black hole in the Milky Way:
[Black Holes: Most Galaxies May Not Have One]
http://www.space.com/scienceastronomy/astronomy/no_blackhole_010719.html
[Rutgers' black hole discovery:
The first galaxy without one or the smallest black hole yet?]
http://www.eurekalert.org/pub_releases/2001-07/rtsu-rbh071601.php
A more recent discovery of a galaxy with a "small" black hole is reported here:
[NGC 4395's pint-size black hole]
http://www.astronomy.com/asy/default.aspx?c=a&id=2905
The accretion disk for a black hole that powers a quasar must be a
remarkably compact object, no bigger than our solar system roughly.
Modeling of the dynamics suggests that early in a galaxy's history,
the matter in the disk is maintained by infalls from stars passing
through the disk (after being perturbed into "radial" orbits with
respect to the black hole through interactions with other stars in the
dense galactic center). It's a delicate affair as the black hole must
be super-massive to prevent the intense radiation produced from simply
blowing away the disk, and at the same time the disk's size is
restricted.
Presumably the quasars that are powered in this fashion shut down when
the accretion process fails to maintain the black hole's appetite.
This is then taken to explain why all the observed quasars are
cosmologically distant and therefore very early processes.
2. Or are there galaxies which have aspects of quasars but are not
considered quasars?
There are several kinds of things that are like quasars in some respects:
[Seyfert Galaxies]
http://www.astro.umd.edu/education/astro/actgal/seyfert.html
"Seyfert Galaxies are spiral galaxies with unusually bright, tiny
cores that fluctuate in brightness."
Neutron stars are known to sometimes have compact accretion disks
(without being massive enough to be a quasar-like power source):
[neutron star explosion reveals inner accretion disk]
http://www.nasa.gov/centers/goddard/news/topstory/2004/0220stardisk.html
Also the acronym QSO (quasi-stellar object), though it was originally
the phrase abbreviated by quasar, is sometimes used to refer to
radio-quiet versions. Sometimes the distinction is made explicit by
labelling QSO's as "radio-loud" or "radio-quiet".
3. Also have astronomers been able to see individual stars in quasars?
No. A quasar appears to be "like" a star in being a single "point" of light.
4. Or observe the normal dust clouds which one sees in an Galaxy?
Dust clouds that surround a quasar may give important clues to their
nature (see an earlier link I posted on this), but we do not see dust
clouds "in" a quasar in the same sense in which one sees dust clouds
in a galaxy. For a lovely picture of crossed dust clouds, apparently
in the heart of the Whirlpool Galaxy, see the last part of the link to
"Power Source for Active Galaxies and Quasars" above.
5. And what are the key physical differences between a galaxy and a quasar?
Galaxies are big and quasars are small (but this doesn't rule out the
possibility that quasars are the "active galactic nuclei" of some
galaxies, possibly even a typical stage of evolution for spiral
galaxies with a central bulge.
The luminosity of a quasar is from 10 to 1,000 times the output of a
typical galaxy.
Quasars are sometimes observed to have jets which have velocities
incredibly close to the speed of light:
[Quasar Jets Create Cosmic Pileups]
http://www.space.com/scienceastronomy/astronomy/chandra_pileup_001108.html
which is one of the lines of evidence that supports an accretion disk
(or torus) as the inner geometry of a quasar.
regards, mathtalk-ga
|
Request for Answer Clarification by
mongolia-ga
on
09 May 2005 15:14 PDT
Mathtalk
some very interesting websites which do indeed try to address aspects
of my questions.
However I am still somewhat confused and have to say at this point that
RNT20 's comment has done the most to CLARIFY the picture for me.
Lets use RNT20's comment as a basis for going forwards and therefore I
am gong to make some assumptions:
1. All galaxies (to the best of our knowledge) contain Black holes at
their centres.
2. The black hole may be active or quiet
- In the case of our own galaxy it is quiet. In the case of a Seyfert
galaxy (as one example) it is active. If the Black hole is active ,
then it is referred to as a AGN (Active Galactic Nucleii)
3 If a galaxy contains an active black hole , then the active black hole
and phenomena directly related to the Black Hole (i.e. Accretion
disk and Jet stream) are referred as to the quasar. The quasar itself
is hosted by the Galaxy (per rnt20's comment)
4. Seyfert Galaxies and BL LAC objects, are galaxies which contain AGN's
and therefore these galaxies host quasars.
Now let us look at part of your answer:
>3. Also have astronomers been able to see individual stars in quasars?
>No. A quasar appears to be "like" a star in being a single "point" of light.
>4. Or observe the normal dust clouds which one sees in an Galaxy?
>Dust clouds that surround a quasar may give important clues to their
>nature (see an earlier link I posted on this), but we do not see dust
>clouds "in" a quasar in the same sense in which one sees dust clouds
>in a galaxy. For a lovely picture of crossed dust clouds, apparently
>in the heart of the Whirlpool Galaxy, see the last part of the link to
>"Power Source for Active Galaxies and Quasars" above.
Given that quasars such as 3C273, 3C9, and 3C48 have now been known
for some time (i.e. since the early sixties) I find it very
surprising that no evidence of the hosting Galaxy (both in terms of
Stars and Interstellar Gas)
has been detected. Does this mean that the quasar overwhelms other
aspects of the galaxy. (Also remember telescopes have improved
enormously since the 1960's)
Also if we were to observe 3C273 and 3C48 in more recent history (lets
assume we were observing from a galaxy which was "only" 10 million
light years away)
what exactly would we see: a normal galaxy like our own? A Seyfert
type galaxy? or perhaps "nothing" (i.e. they are big black holes
wandering aimlessly in space with nothing associated with them?)
Finally I am going to takee issue with one of the things one of your
websites has indicated:
www.earthsky.com/shows/showsmore/.php?t=20040122
>The nearest known quasar is also the first discovered by humans. 3C
273lies >about 2 billion light-years away in the Spring constellation
Virgo, just >beyond the giant Virgo Cluster of galaxies. Found in the
1960s, it produces a >hundred times more light that our Milky Way.
Because it's closer than most >quasars, it appears to be the
brightest. Amateur astronomers can glimpse 3C >273 through small
telescopes as a faint star.
>3C 273 is unusual among quasars for being so near to us in space and
time. >Some believe that 3C 273 might be a quasar "reborn" in the
manner described >above - or perhaps it is telling us something new
about how quasars operate.
An earlier question i submitted (question Id 196236) appears to
refute the assertion that 3c273 is the closest quasar and indicates
that it is 3C405.
I know this is a bit for you to chew on but if you could clarify some of my
questions I would be delighted.
PS RNT20 if you are looking in,I would also be delighted to hear
anything you may wish to add.
Regards
Mongolia
|
Clarification of Answer by
mathtalk-ga
on
11 May 2005 14:37 PDT
Hi, mongolia-ga:
You have indeed given me a good bit to chew on, and I wish to be
thorough in as much as I have a great deal to learn as well about
these mysterious objects.
Regarding the issue of what quasar is nearest to us, I think most
astronomers would not classify 3C 405, aka Cygnus A, as a quasar, but
rather as an extraordinarily intense radio galaxy. When "viewed" in
radio frequencies, it appears that the most intense radio signals come
from two widely separated lobes rather than from a compact region.
Of course it may be helpful to relax terminology as observational
evidence points to identification of formerly disparate phenomena as
parts or perspectives on a single underlying reality. The jets that
are observed coming from some quasars and from some AGN surely
suggests a similarity to BL Lacertae objects. But I'm unwilling to
expand the definition of quasar so far as to include unusually intense
radio galaxies per se.
As I work toward clarifying some the additional questions you raise,
please have a look at this article by Bill Keel:
[Quasar Astronomy Forty Years On]
http://www.astr.ua.edu/keel/agn/quasar40.html
which is but one page in a site devoted to Quasars and Active Galactic
Nuclei (see the link at bottom of page "back" to the broader topic and
site outline).
regards, mathtalk-ga
|
Clarification of Answer by
mathtalk-ga
on
09 Jun 2005 18:34 PDT
A "notebook" which compares and contrasts the spatial and "z"
(redshift) distributions of quasars, active galactic nuclei, and BL
Lac objects is here:
[Notebook "Quasars and Active Galactic Nuclei"]
http://www.ioffe.rssi.ru/astro/QC/notebook.htm
which is an adjunct to a discussion of large-scale nonuniformities in
the distribution of matter in the universe here:
[Space-time distribution of matter in the Universe]
http://www.ioffe.rssi.ru/astro/QC/std_00.htm
At any rate the apparent distribution of quasars peaks markedly at
redshifts between 2 and 2.5, while the peak for AGN is below 0.1 and
that for BL Lac objects is around 0.2.
Perhaps the best known advocate of an unconventional interpretation
for the large redshift values that are associated with quasars is
Halton "Chip" Arp. A nice paper of this genre is:
[Further Evidence that Some Quasars Originate in Nearby Galaxies: NGC3628]
http://web.infinito.it/utenti/a/armenzano.oss/9907219_-_Further_Evidence_that_Some_Quasars_Originate_in_Ne.pdf
a 1999 paper by Flesch and Arp. His theory is that at least some
quasars are ejected from active galaxies, and in a number of papers he
has presented sometimes striking observational evidence to associate
quasars or suspected quasars with galaxies that appear to be in
proximity (without containing them).
Arp's own site is here and presents the theory much better than I could:
[The website of Halton Arp]
http://www.haltonarp.com/?Page=Abstracts&ArticleId=1
"It is argued here that, empirically, the quasars are ejected from
active galaxies. They evolve to lower redshift with time, fragmenting
at the end of their development into clusters of low luminosity
galaxies. The cluster galaxies can be at the same distance as their
lower redshift parents because they still retain a component of their
earlier, quasar intrinsic redshift."
Still the size distinction holds true: quasars are point-like,
galaxies have extended appearance, even at cosmological distances.
What is different is Arp's characterization of at least some quasars
as ejecta, and his theorizing of the redshift association with quasars
as being of non-cosmological origin (ie. not at the vast distances
that bespeak of an exceptionally early age of the universe).
regards, mathtalk-ga
|
