| Topic: |
Science > Physics |
| User: |
"John Schutkeker" |
| Date: |
13 Jan 2008 10:09:41 AM |
| Object: |
Quasars, AGN's & Core Black Holes |
Here is a very interesting article.
http://space.newscientist.com/article/dn13166-biggest-black-hole-in-the-
cosmos-discovered.html
It describes the measurements at Tuorla Observatory, Finland, of two
galactic core black holes orbiting each other, with one hole moving in
and out of the other's accretion disk at 12 year intervals. In my book,
that's extremely impressive.
However, the article states that scientists correctly understand what a
quasar is, and that a quasar is just an ordinary galactic core black
hole, eating stars and emitting energy as radiation. If that were true,
then AGN's would all be quasars, but we know that they're not, because
they're Seyfert Galaxies, which emit heavily in the radio band.
So the question remains, how does a quasar differ from an AGN?
Considering the Tuorla observation, I suppose that it is now clear that
a core hole powers a quasar, or this observation would simply be wrong.
But there must be an extraordinary difference between the fuel falling
into the hole, to make such an extraordinary difference in the observed
behaviors.
I was tempted to postulate that Valtonen's (the Tuorla astronomer's)
model of two orbiting holes, with one cycling through the accretion
disk, might answer the riddle of what powers a quasar, but I think
that's going too far. If that were the case, then it would be
observable in *all* quasars, and Valtonen's observation could be
repeated on several different quasars, to finally solve the riddle.
That's a Nobel Prize-worthy accomplishment, but it hasn't happened yet,
and I don't think that it will.
But if we can reliably state that both quasars and Seyfert galaxies are
powered by core holes, that means that the only difference between them
is the quantity of fuel falling in to the two. The core hole in a
Seyfert galaxy much have an extraordinarily thin supply of matter for
the power source, and the core hole in a quasar must have a very heavy
supply of matter to drive the emissions.
Thus the question now becomes, what are the differences between the
supplies of matter into the holes and/or their accretion disks, and why
does one accretion disk pump large volumes of matter into the hole,
while the other pumps small volumes of matter? And is there a third
kind of active galactic core that straddles the boundary between the
AGN's and quasars, and/or has a completely different "fuel supply" than
the two familiar kinds?
Either way, I believe that Valtonen's observation, if correct, should
have extremely profound implications for our understanding of active
galactic black holes, which is one of the most interesting of all the
cutting edge fields of "ultra high power" astrophysics.
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| User: "Steve Willner" |
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| Title: Re: Quasars, AGN's & Core Black Holes |
23 Jan 2008 03:46:34 PM |
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In article <Xns9A24718C9C33Alkajehoriuasldfjknak@207.115.33.102>,
John Schutkeker <jschutkeker@sbcglobal.net.nospam> writes:
http://space.newscientist.com/article/dn13166-biggest-black-hole-in-the-
cosmos-discovered.html
It describes the measurements at Tuorla Observatory, Finland, of two
galactic core black holes orbiting each other, with one hole moving in
and out of the other's accretion disk at 12 year intervals. In my book,
that's extremely impressive.
Yep!
However, the article states that scientists correctly understand what a
quasar is, and that a quasar is just an ordinary galactic core black
hole, eating stars and emitting energy as radiation. If that were true,
then AGN's would all be quasars, but we know that they're not, because
they're Seyfert Galaxies, which emit heavily in the radio band.
This is confused. The current picture is that Seyfert galaxies and
quasi-stellar objects (QSOs) are powered by the same physical process
(black hole accretion), differing only in luminosity. They are
called collectively "active galactic nuclei" or AGN.
Only a small fraction, perhaps 5%, of AGN are radio-bright. If they
have high luminosities, the radio-bright ones are called "quasars,"
though the term is often (ab)used to include all QSOs. Now that I
think about it, I don't think I've seen how the radio-bright fraction
changes with luminosity, but it's not wildly different for Seyferts
and QSOs. The reason some AGN are radio-bright and others aren't is
not known.
So the question remains, how does a quasar differ from an AGN?
A quasar is a radio-bright, high-luminosity AGN. (Some people would
drop the requirement that it be radio-bright.)
But there must be an extraordinary difference between the fuel falling
into the hole, to make such an extraordinary difference in the observed
behaviors.
Yes, luminosity is generally thought to be the _product_ of black
hole mass and accretion rate, though there are indications that the
luminous efficiency is not the same for all objects. Untangling this
is very much a topic of ongoing research.
But if we can reliably state that both quasars and Seyfert galaxies are
powered by core holes, that means that the only difference between them
is the quantity of fuel falling in to the two.
Also the black hole mass, but yes, that's the basic picture. Black
hole mass is thought to correlate very well with the mass of the
stellar bulge (and not the disk!) of each galaxy. Presumably this
says the assembly of bulges and black holes was intimately related.
Thus the question now becomes, what are the differences between the
supplies of matter into the holes and/or their accretion disks, and why
does one accretion disk pump large volumes of matter into the hole,
while the other pumps small volumes of matter?
Good question. Maybe some day we'll figure it out. The fuel supply
presumably has a lot to do with the local galactic structure around
the black hole, but details are not known. There may very well be
two or more "modes" of accretion, resulting in different
efficiencies. Most likely, accretion is episodic in nature. For
example, the black hole at the center of the Milky Way currently has
a very low accretion rate, but it may have had a much higher rate in
the past.
And is there a third
kind of active galactic core that straddles the boundary between the
AGN's and quasars, and/or has a completely different "fuel supply" than
the two familiar kinds?
As noted above, AGN is the generic term. There's no sharp boundary
between Seyferts and QSOs but rather a continuous distribution of
luminosities.
--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)
.
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| User: "John Schutkeker" |
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| Title: Re: Quasars, AGN's & Core Black Holes |
26 Jan 2008 01:48:05 PM |
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(Steve Willner) wrote in
news:fn8cjq$njh$1@registered.motzarella.org:
In article <Xns9A24718C9C33Alkajehoriuasldfjknak@207.115.33.102>,
John Schutkeker <jschutkeker@sbcglobal.net.nospam> writes:
http://space.newscientist.com/article/dn13166-biggest-black-hole-in-
th
e- cosmos-discovered.html
It describes the measurements at Tuorla Observatory, Finland, of two
galactic core black holes orbiting each other, with one hole moving
in and out of the other's accretion disk at 12 year intervals. In my
book, that's extremely impressive.
Yep!
However, the article states that scientists correctly understand what
a quasar is, and that a quasar is just an ordinary galactic core
black hole, eating stars and emitting energy as radiation. If that
were true, then AGN's would all be quasars, but we know that they're
not, because they're Seyfert Galaxies, which emit heavily in the
radio band.
This is confused. The current picture is that Seyfert galaxies and
quasi-stellar objects (QSOs) are powered by the same physical process
(black hole accretion), differing only in luminosity. They are
called collectively "active galactic nuclei" or AGN.
Thank you for clearing that up. :)
Only a small fraction, perhaps 5%, of AGN are radio-bright.
I don't think that's right. AFAIK, all AGN's (quasars and Seyferts) are
radio bright. If it's not radio bright, we can't know it's an AGN, and
we'll just assume that it's an "ordinary" galaxy.
If they
have high luminosities, the radio-bright ones are called "quasars,"
though the term is often (ab)used to include all QSOs.
I'm not aware of any QSO's that aren't quasars.
Now that I
think about it, I don't think I've seen how the radio-bright fraction
changes with luminosity, but it's not wildly different for Seyferts
and QSOs. The reason some AGN are radio-bright and others aren't is
not known.
That's pretty much what I was saying. Considering the distance to
quasars, their luminosity must be sooo much larger than Seyferts that it
suggests a different process is happening in the core. If Seyferts are
powered by standard accretion disks made up of torn up stars, then
quasars must be powered by some exotic behavior or source of matter.
Likewise, if quasars are powered by standard accretion disks, made of
torn up stars, then Seyferts must be powered by a very diffuse supply of
matter, perhaps simple interstellar dust and gas. I would hold this up
as the most likely explanation, however, I'm not convinced that simple
interstellar gas and dust can exist in densely populated galactic cores,
without getting swept up into ordinary stars.
Thus the riddle remains, why are quasars and Seyferts so substantially
different, even though they're both AGN's, which are all believed to be
powered by black hole accretion disks?
So the question remains, how does a quasar differ from an AGN?
A quasar is a radio-bright, high-luminosity AGN. (Some people would
drop the requirement that it be radio-bright.)
But there must be an extraordinary difference between the fuel
falling into the hole, to make such an extraordinary difference in
the observed behaviors.
Yes, luminosity is generally thought to be the _product_ of black
hole mass and accretion rate, though there are indications that the
luminous efficiency is not the same for all objects. Untangling this
is very much a topic of ongoing research.
But if we can reliably state that both quasars and Seyfert galaxies
are powered by core holes, that means that the only difference
between them is the quantity of fuel falling in to the two.
Also the black hole mass, but yes, that's the basic picture. Black
hole mass is thought to correlate very well with the mass of the
stellar bulge (and not the disk!) of each galaxy. Presumably this
says the assembly of bulges and black holes was intimately related.
Thus the question now becomes, what are the differences between the
supplies of matter into the holes and/or their accretion disks, and
why does one accretion disk pump large volumes of matter into the
hole, while the other pumps small volumes of matter?
Good question. Maybe some day we'll figure it out.
Thank you. Then let's ask a second question - what observation could we
make that would allow us to figure it out? Seyferts are much closer
than quasars, so it makes more sense to try to make observations on
them. AFAIK, the best hypothesis we've got is that Seyferts have a
diffuse supply of matter for the accretion disk. Would there be any
located close enough to us to try to confirm or disprove that
hypothesis?
The fuel supply
presumably has a lot to do with the local galactic structure around
the black hole, but details are not known. There may very well be
two or more "modes" of accretion, resulting in different
efficiencies. Most likely, accretion is episodic in nature.
I'm a little skeptical of this, because I've never before heard talk
about accretion disks that appear and disappeared, which would be
required by an "episodic" AGN, even if the episode were to last hundreds
of thousands of years.
But if could be true, because if a Seyfert accretion disk contained, for
instance, only the matter of a single star, it could eventually be
depleted, if more stars didn't fall in.
Incidentally, this reminds me of another idea that I've had, about
quasars. Since they're several billion years away, they're also several
billion years in the past, in the history of the entire cosmos. That
suggests to me that there might be a process happening in them that only
occurred early in the history of the cosmos, due to severely different
conditions, way back then.
Back then, perhaps, galactic concentrations of matter were much more
dense, and stars proportionately larger, as with the new breed of stars
just recently discovered - Population III, which supposedly have
hundreds of solar masses.
To me this suggests that Seyfert black holes might have ordinary
accretion disks made up of reasonable numbers of ordinary
Hertzprung-Russell stars. Quasars may still have ordinary accretion
disks, but filled with such a humongously greater quantity of matter,
due to local increases in density, that it accounts for the extreme
illumination.
For
example, the black hole at the center of the Milky Way currently has
a very low accretion rate, but it may have had a much higher rate in
the past.
AFAIK, there is no accretion rate at all, but is there any direct
evidence that there ever was one? Of course the theories predict it,
but the theories are notoriously incomplete. For instance, one thing
I'd worry about in such a case, would be radiation pressure blowing
ordinary gas and dust clouds clean out of the galaxy. Perhaps even
light bodies, like Oort bodies and rocky planets, although I'm uncertain
about that.
And is there a third
kind of active galactic core that straddles the boundary between the
AGN's and quasars, and/or has a completely different "fuel supply"
than the two familiar kinds?
As noted above, AGN is the generic term. There's no sharp boundary
between Seyferts and QSOs but rather a continuous distribution of
luminosities.
Are you sure about that? AFAIK, there is at least a sharp boundary in
distance, since quasars are so distant that they have fairly large
redshifts. As I understand it, that also translates to a fairly sharp
discontinuity in luminosity. Of course a graph would be nice, to verify
what I just said. You wouldn't happen to know of an on-line one, would
you? How close are the nearby Seyferts, anyhow?
.
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| User: "Odysseus" |
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| Title: Re: Quasars, AGN's & Core Black Holes |
26 Jan 2008 05:02:39 PM |
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In article <Xns9A3196969AD3Aalsfaskldfjaklsdfj@207.115.17.102>,
John Schutkeker <jschutkeker@sbcglobal.net.nospam> wrote:
willner@cfa.harvard.edu (Steve Willner) wrote in
news:fn8cjq$njh$1@registered.motzarella.org:
<snip>
Only a small fraction, perhaps 5%, of AGN are radio-bright.
I don't think that's right. AFAIK, all AGN's (quasars and Seyferts) are
radio bright. If it's not radio bright, we can't know it's an AGN, and
we'll just assume that it's an "ordinary" galaxy.
What about those that produce intense high-frequency (X-ray and gamma)
emissions, or have visible polar jets? Are they all radio-bright as well?
--
Odysseus
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| User: "John Schutkeker" |
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| Title: Re: Quasars, AGN's & Core Black Holes |
26 Jan 2008 05:28:33 PM |
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Odysseus <odysseus1479-at@yahoo-dot.ca> wrote in
news:odysseus1479-at-2DCA2F.16023926012008@news.telus.net:
In article <Xns9A3196969AD3Aalsfaskldfjaklsdfj@207.115.17.102>,
John Schutkeker <jschutkeker@sbcglobal.net.nospam> wrote:
willner@cfa.harvard.edu (Steve Willner) wrote in
news:fn8cjq$njh$1@registered.motzarella.org:
<snip>
Only a small fraction, perhaps 5%, of AGN are radio-bright.
I don't think that's right. AFAIK, all AGN's (quasars and Seyferts)
are radio bright. If it's not radio bright, we can't know it's an
AGN, and we'll just assume that it's an "ordinary" galaxy.
What about those that produce intense high-frequency (X-ray and gamma)
emissions, or have visible polar jets? Are they all radio-bright as
well?
I believe so, but if they aren't, then there *is* a third kind of AGN,
because quasars and Seyferts are both radio emitters. I need to read
upon this more.
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| User: "Steve Willner" |
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| Title: Re: Quasars, AGN's & Core Black Holes |
31 Jan 2008 01:21:51 PM |
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SW> Only a small fraction, perhaps 5%, of AGN are radio-bright.
In article <Xns9A3196969AD3Aalsfaskldfjaklsdfj@207.115.17.102>,
John Schutkeker <jschutkeker@sbcglobal.net.nospam> writes:
I don't think that's right. AFAIK, all AGN's (quasars and Seyferts) are
radio bright. If it's not radio bright, we can't know it's an AGN, and
we'll just assume that it's an "ordinary" galaxy.
Take a look at Fig 1 of Elvis et al. (1994 ApJS 95, 1). "Radio-quiet"
doesn't mean "no radio emission at all," but there's a fairly sharp
distinction between the "loud" and "quiet" types amounting to about
two orders of magnitude.
Only a small fraction of AGN are discovered by radio emission these
days. More common are optical emission lines, UV excess, X-ray
emission, and mid-infrared colors. As for the existence of
radio-quiet QSOs, you might want to look at, for example, at the BQS
sample of QSOs (Schmidt & Green, 1983 ApJ, 269, 352). That was a
UV-selected sample, and only a few are radio-bright. (I haven't
counted how many.)
I'm not aware of any QSO's that aren't quasars.
As a matter of terminology, 'quasar' means "quasi-stellar _radio_
source," at least to us old pedants. :-) Not everyone, even among
professionals, is careful to follow this usage, I'm afraid. (Schmidt
& Green don't, for example.)
SW> The reason some AGN are radio-bright and others aren't is
SW> not known.
That's pretty much what I was saying. Considering the distance to
quasars, their luminosity must be sooo much larger than Seyferts that it
suggests a different process is happening in the core.
Both nearby QSOs and distant Seyferts are known, though the latter
require very deep surveys to be detected, and the former are rare.
Thus the riddle remains, why are quasars and Seyferts so substantially
different, even though they're both AGN's, which are all believed to be
powered by black hole accretion disks?
As I mentioned earlier, the main difference seems to be one of
accretion rate and secondarily black hole mass. There _may_ also be
differences in the physics of the accretion disk, but that's far from
clear at the moment.
Thus the question now becomes, what are the differences between the
supplies of matter into the holes and/or their accretion disks, and
why does one accretion disk pump large volumes of matter into the
hole, while the other pumps small volumes of matter?
Good question. Maybe some day we'll figure it out.
Thank you. Then let's ask a second question - what observation could we
make that would allow us to figure it out?
If I could give a simple answer to that, I'd be writing an observing
proposal, not messing around on Usenet. I think the complicated
answer is more and better multi-wavelength surveys in order to
compare AGN properties _statistically_ across a wide range of
wavelengths. One very important survey is AEGIS
http://aegis.ucolick.org/index.html , but it isn't sufficient by
itself.
Seyferts are much closer
than quasars, so it makes more sense to try to make observations on
them. AFAIK, the best hypothesis we've got is that Seyferts have a
diffuse supply of matter for the accretion disk. Would there be any
located close enough to us to try to confirm or disprove that
hypothesis?
I don't know what you mean by "diffuse supply of matter." There are
plenty of Seyferts that show a great deal of extinction, presumably
from matter within 100 pc of the nucleus, but this is far outside the
accretion disk.
SW> Most likely, accretion is episodic in nature.
I'm a little skeptical of this, because I've never before heard talk
about accretion disks that appear and disappeared, which would be
required by an "episodic" AGN, even if the episode were to last hundreds
of thousands of years.
That's about the right time frame (maybe a few million years would be
better), and episodic accretion is a pretty standard speculation
nowadays. Look at it this way: every galactic black hole had to be
accreting in the past, but most are quiescent today. Also, the
accretion rates needed are hundreds to thousands of solar masses per
year for QSOs; those cannot be sustained for more than several
million years. So we know accretion can turn off. Why shouldn't it
turn back on at least some of the time, fueled by a galactic merger
for example? My "most likely" was an overstatement of the current
evidence, but I don't see why episodic accretion shouldn't occur.
Incidentally, this reminds me of another idea that I've had, about
quasars. Since they're several billion years away, they're also several
billion years in the past, in the history of the entire cosmos. That
suggests to me that there might be a process happening in them that only
occurred early in the history of the cosmos, due to severely different
conditions, way back then.
The peak of accretion activity was at redshift 2 to 3. (This
summarizes about 30 years of many people's work in one sentence!)
Back then, perhaps, galactic concentrations of matter were much more
dense, and stars proportionately larger, as with the new breed of stars
just recently discovered - Population III, which supposedly have
hundreds of solar masses.
Population 3, if it existed at all, was at redshift >6. There's not
much difference between stars at z=3 and stars now. Populations were
generally younger and metallicities lower but not all that much.
SW> the black hole at the center of the Milky Way currently has
SW> a very low accretion rate, but it may have had a much higher rate in
SW> the past.
AFAIK, there is no accretion rate at all, but is there any direct
evidence that there ever was one?
How did the black hole form, if not by accretion? Also, the stars
orbiting the black hole must occasionally fall in, and _something_
must produce the observed radio emission. As you say, though, the
accretion rate _today_ might very well be zero.
one thing
I'd worry about in such a case, would be radiation pressure blowing
ordinary gas and dust clouds clean out of the galaxy. Perhaps even
light bodies, like Oort bodies and rocky planets, although I'm uncertain
about that.
I think you might want to recheck your figures. Small dust grains,
maybe, and maybe they could even drag gas with them. Large dust
grains or anything bigger, no way.
How close are the nearby Seyferts, anyhow?
NGC 1068 or 4151 are a couple of the closest Seyferts; maybe there
are others even closer. (M81 has a low-luminosity AGN, though it's
not generally classified as a Seyfert because the luminosity is so
low.) I'm not sure about the closest QSO, maybe redshift of 0.1 or
so. That the distance is so large reflects the rarity of luminous
AGN in today's universe; they were much more common in the past.
Your other question, which I've snipped, was asking about the
"luminosity function" of AGN. There is a huge literature on that
subject, covering a wide range of wavelengths, AGN selection
criteria, and redshift ranges. (The luminosity function changes with
redshift; the most luminous AGN were _far_ more common in the past
than they are today.) One good overview based on Sloan data is at
http://adsabs.harvard.edu/abs/2004ApJ...613..109H
(Download the "archive e-print" if the refereed paper is not yet
public.) References cited in that paper will be a good place to
start looking for further answers. Just don't expect to learn all
there is to know on the subject in a few minutes.
--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)
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| User: "Steve Willner" |
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| Title: Re: Quasars, AGN's & Core Black Holes |
01 Feb 2008 12:32:39 PM |
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Thus the question now becomes, what are the differences between the
supplies of matter into the holes and/or their accretion disks, and
why does one accretion disk pump large volumes of matter into the
hole, while the other pumps small volumes of matter?
In article <fnt74f$bu2$1@registered.motzarella.org>, I suggested more
and better observations -- my natural prejudice, given that I'm an
observer myself. It's only fair to add that better theory would help
a lot. What's needed is a general relativistic treatment of
magnetohydrodynamics, and I don't expect that will be simple, to put
it mildly.
--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)
.
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| User: "Agent Smith" |
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| Title: Re: Quasars, AGN's & Core Black Holes |
01 Feb 2008 06:56:56 PM |
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(Steve Willner) wrote in
news:fnvok7$7op$1@registered.motzarella.org:
Thus the question now becomes, what are the differences between
the supplies of matter into the holes and/or their accretion
disks, and why does one accretion disk pump large volumes of
matter into the hole, while the other pumps small volumes of
matter?
In article <fnt74f$bu2$1@registered.motzarella.org>, I suggested more
and better observations -- my natural prejudice, given that I'm an
observer myself. It's only fair to add that better theory would help
a lot. What's needed is a general relativistic treatment of
magnetohydrodynamics, and I don't expect that will be simple, to put
it mildly.
Take the Red Line down to MIT, and drop by the office of Jeff Freidberg,
who is the chief MHD guru at their Plasma Fusion Canter. If anyone can
give you the names of people who will be interested, he can. America's
fusion program keeps getting butchered, and by now there are probably a
lot of plasma physics people who need some juicy ideas to work on.
Here's a link to his page; he used to be the department chairman, and
he's a **really** nice guy. He's a Celtics fan. :)
http://www.psfc.mit.edu/people_new/faculty/jf.html
Actually, the Red Line drops off in Kendall Square, which is quite far
from his office, so the best way to find him would be to drive down Mass
Ave. and take a right on Albany Street. That's the last street before
the railroad tracks and adjacent to the giant green nuclear reactor. Ya
can't miss it. Parking should be no trouble at all, because there's a
lot just a couple of hundred yards past his building, and I don't think
you need a tag to park there.
I'd look at the problem myself, but I'm already swamped. But just for
fun, maybe I'll trundle Jackson out to a coffee house tomorrow and see
if anything looks obvious.
I assume that you're on campus at Harvard, right? When I was in
Cambridge, I foolishly neglected my interest in astronomy, and never
made the effort to find the notorious Harvard-Smithsonian Institute for
Astrophysics, where I assume you work. That and I was a dumb kid. Now
I'm a dumb adult. ;(
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| User: "Agent Smith" |
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| Title: Re: Quasars, AGN's & Core Black Holes |
01 Feb 2008 07:09:20 PM |
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(Steve Willner) wrote in
news:fnvok7$7op$1@registered.motzarella.org:
Thus the question now becomes, what are the differences between
the supplies of matter into the holes and/or their accretion
disks, and why does one accretion disk pump large volumes of
matter into the hole, while the other pumps small volumes of
matter?
In article <fnt74f$bu2$1@registered.motzarella.org>, I suggested more
and better observations -- my natural prejudice, given that I'm an
observer myself. It's only fair to add that better theory would help
a lot. What's needed is a general relativistic treatment of
magnetohydrodynamics, and I don't expect that will be simple, to put
it mildly.
Wait a minute, maybe my idea about meeting Freidberg isn't so bright.
For some reason, I didn't see the word "general" when you wrote "general
relativistic," and I don't expect that the fusion people will know
anything at all about general relativity. To get them onboard, you'd
probably have to put on a major sales operation.
I don't know anything about GR either, so maybe I won't take Jackson to
the coffee house tomorrow. There's just too damn many textbooks to
read, which I guess that's what they mean when they say "embarrassment
of riches."
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| User: "malibu" |
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| Title: Re: Quasars, AGN's & Core Black Holes |
31 Jan 2008 03:10:21 PM |
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On Jan 31, 1:21 pm, (Steve Willner) wrote:
SW> Only a small fraction, perhaps 5%, of AGN are radio-bright.
In article <Xns9A3196969AD3Aalsfaskldfjakls...@207.115.17.102>,
John Schutkeker <jschutke...@sbcglobal.net.nospam> writes:
I don't think that's right. AFAIK, all AGN's (quasars and Seyferts) are
radio bright. If it's not radio bright, we can't know it's an AGN, and
we'll just assume that it's an "ordinary" galaxy.
Take a look at Fig 1 of Elvis et al. (1994 ApJS 95, 1). "Radio-quiet"
doesn't mean "no radio emission at all," but there's a fairly sharp
distinction between the "loud" and "quiet" types amounting to about
two orders of magnitude.
Only a small fraction of AGN are discovered by radio emission these
days. More common are optical emission lines, UV excess, X-ray
emission, and mid-infrared colors. As for the existence of
radio-quiet QSOs, you might want to look at, for example, at the BQS
sample of QSOs (Schmidt & Green, 1983 ApJ, 269, 352). That was a
UV-selected sample, and only a few are radio-bright. (I haven't
counted how many.)
I'm not aware of any QSO's that aren't quasars.
As a matter of terminology, 'quasar' means "quasi-stellar _radio_
source," at least to us old pedants. :-) Not everyone, even among
professionals, is careful to follow this usage, I'm afraid. (Schmidt
& Green don't, for example.)
SW> The reason some AGN are radio-bright and others aren't is
SW> not known.
That's pretty much what I was saying. Considering the distance to
quasars, their luminosity must be sooo much larger than Seyferts that it
suggests a different process is happening in the core.
Both nearby QSOs and distant Seyferts are known, though the latter
require very deep surveys to be detected, and the former are rare.
Thus the riddle remains, why are quasars and Seyferts so substantially
different, even though they're both AGN's, which are all believed to be
powered by black hole accretion disks?
As I mentioned earlier, the main difference seems to be one of
accretion rate and secondarily black hole mass. There _may_ also be
differences in the physics of the accretion disk, but that's far from
clear at the moment.
Thus the question now becomes, what are the differences between the
supplies of matter into the holes and/or their accretion disks, and
why does one accretion disk pump large volumes of matter into the
hole, while the other pumps small volumes of matter?
Good question. Maybe some day we'll figure it out.
Thank you. Then let's ask a second question - what observation could we
make that would allow us to figure it out?
If I could give a simple answer to that, I'd be writing an observing
proposal, not messing around on Usenet. I think the complicated
answer is more and better multi-wavelength surveys in order to
compare AGN properties _statistically_ across a wide range of
wavelengths. One very important survey is AEGIShttp://aegis.ucolick.org/index.html, but it isn't sufficient by
itself.
Seyferts are much closer
than quasars, so it makes more sense to try to make observations on
them. AFAIK, the best hypothesis we've got is that Seyferts have a
diffuse supply of matter for the accretion disk. Would there be any
located close enough to us to try to confirm or disprove that
hypothesis?
I don't know what you mean by "diffuse supply of matter." There are
plenty of Seyferts that show a great deal of extinction, presumably
from matter within 100 pc of the nucleus, but this is far outside the
accretion disk.
SW> Most likely, accretion is episodic in nature.
I'm a little skeptical of this, because I've never before heard talk
about accretion disks that appear and disappeared, which would be
required by an "episodic" AGN, even if the episode were to last hundreds
of thousands of years.
That's about the right time frame (maybe a few million years would be
better), and episodic accretion is a pretty standard speculation
nowadays. Look at it this way: every galactic black hole had to be
accreting in the past, but most are quiescent today. Also, the
accretion rates needed are hundreds to thousands of solar masses per
year for QSOs; those cannot be sustained for more than several
million years. So we know accretion can turn off. Why shouldn't it
turn back on at least some of the time, fueled by a galactic merger
for example? My "most likely" was an overstatement of the current
evidence, but I don't see why episodic accretion shouldn't occur.
Incidentally, this reminds me of another idea that I've had, about
quasars. Since they're several billion years away, they're also several
billion years in the past, in the history of the entire cosmos. That
suggests to me that there might be a process happening in them that only
occurred early in the history of the cosmos, due to severely different
conditions, way back then.
The peak of accretion activity was at redshift 2 to 3. (This
summarizes about 30 years of many people's work in one sentence!)
Back then, perhaps, galactic concentrations of matter were much more
dense, and stars proportionately larger, as with the new breed of stars
just recently discovered - Population III, which supposedly have
hundreds of solar masses.
Population 3, if it existed at all, was at redshift >6. There's not
much difference between stars at z=3 and stars now. Populations were
generally younger and metallicities lower but not all that much.
SW> the black hole at the center of the Milky Way currently has
SW> a very low accretion rate, but it may have had a much higher rate in
SW> the past.
AFAIK, there is no accretion rate at all, but is there any direct
evidence that there ever was one?
How did the black hole form, if not by accretion? Also, the stars
orbiting the black hole must occasionally fall in, and _something_
must produce the observed radio emission. As you say, though, the
accretion rate _today_ might very well be zero.
one thing
I'd worry about in such a case, would be radiation pressure blowing
ordinary gas and dust clouds clean out of the galaxy. Perhaps even
light bodies, like Oort bodies and rocky planets, although I'm uncertain
about that.
I think you might want to recheck your figures. Small dust grains,
maybe, and maybe they could even drag gas with them. Large dust
grains or anything bigger, no way.
How close are the nearby Seyferts, anyhow?
NGC 1068 or 4151 are a couple of the closest Seyferts; maybe there
are others even closer. (M81 has a low-luminosity AGN, though it's
not generally classified as a Seyfert because the luminosity is so
low.) I'm not sure about the closest QSO, maybe redshift of 0.1 or
so. That the distance is so large reflects the rarity of luminous
AGN in today's universe; they were much more common in the past.
Your other question, which I've snipped, was asking about the
"luminosity function" of AGN. There is a huge literature on that
subject, covering a wide range of wavelengths, AGN selection
criteria, and redshift ranges. (The luminosity function changes with
redshift; the most luminous AGN were _far_ more common in the past
than they are today.) One good overview based on Sloan data is athttp://adsabs.harvard.edu/abs/2004ApJ...613..109H
(Download the "archive e-print" if the refereed paper is not yet
public.) References cited in that paper will be a good place to
start looking for further answers. Just don't expect to learn all
there is to know on the subject in a few minutes.
--
Steve Willner Phone 617-495-7123 s
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)
Another small observation:
*all* of these quasars with their 'billions of
times the energy' are far, far away.
Boy, we sure are lucky none are close to us!!
John
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