ethan@utastro.UUCP (Ethan Vishniac) (05/17/84)
To quote from Mr. Gwyn: >Although I am not convinced that the evidence that quasars are >remote is conclusive, this question does not seem to be to bear >directly on the question of which cosmology to choose. I think I'd better expand on the point I was trying to make. We see quasars in all directions at very large distances. The space density of quasars at large distances is very much larger than the local space density. In fact we see that in a large shell around us quasars were common at the point that they emitted the light that reaches us. If we just take the light travel time from them to us we see that quasars were evidently common when the oldest stars in our galaxy were just forming. This is a strong piece of circumstantial evidence that the universe as a whole has changed a lot since our galaxy was young. A steady-state cosmology would not seem to be compatible with this conclusion. "Just another Cosmic Cowboy" Ethan Vishniac {ut-sally,ut-ngp,kpno}!utastro!ethan Department of Astronomy University of Texas Austin, Texas 78712
gwyn@brl-vgr.ARPA (Doug Gwyn ) (05/17/84)
Thanks for the clarification about quasars. I would put it, "There are more highly red-shifted quasars than slightly red-shifted ones" or some such more neutral statement. Red-shift != distant so it should not be treated as the same when stating what is OBSERVED.
elt@astrovax.UUCP (Ed Turner) (05/18/84)
Ethan's arguement that there are more distant quasars than nearby ones (by factors of order 10^3 in fact) has usually been based on the association of redshift and distance as Doug suggests. Recently, however, it has been pointed out by M. Schmidt and R. Green that this effect can be established independently of the nature of the redshift (Ap. J. Lett. 1980 +/- 1yr I think). Their trick is to essentially compare the relative number of bright and faint quasars in the sky. Given merely the assumption that energy flux falls off like the inverse distance squared (i.e., energy conservation + geometry), they managed to show that either the total density of quasars or their typical luminosities must increase very rapidly with distance. This rather recent result puts a final nail (if one was needed) in any strict Steady State theory in my opinion. Ed Turner astrovax!elt
gwyn@brl-vgr.ARPA (Doug Gwyn ) (05/18/84)
Thanks for the additional quasar info, Ed. Is it correct to say that the observational evidence is: The apparent brightness distribution of quasars is much more peaked than would be expected if they all had the same intrinsic luminosity and were uniformly scattered through a 3-dimensional Euclidean space. ? I.e., are the observed quasars closer to all the same brightness than one might expect? I think this is equivalent to what you were saying in your note. It is tricky to express these things in purely observational terms... If I got this stated right, then again there is more than one possible theoretical interpretation. The conventional one is that there are a lot of really luminous quasars far away (i.e. far back in time, toward the "big bang"). An alternative for those who think quasars are nearby is that they are all about the same distance away (like, in a sphere about our galaxy) and have similar intrinsic luminosity (but different red-shifts, which is admittedly a bit suspect). Quasars associated with other galaxies would be hard to detect... Something like this actually happened with Cepheid variable stars. I have seen the distance scale of the universe grow by an order of magnitude as the result of mistaken assumptions about what was being seen being corrected. Two reasons for preferring the nearby theory of quasars to the cosmic one are: (a) The intrinsic luminosity is reasonable if they are nearby, but really hard to explain if they are far away; (b) The periods of pulsars are hard to explain for far-away pulsars, but admit of simpler explanations if the objects are nearby. Neither of these is a decisive argument (unless one can disprove the existence of neutron stars), but they make the nearby quasar theorists feel more comfortable with their position. The trouble with this cosmology business is that it is hard to directly check what the theories are telling us. The actual observed evidence is much more constrained than would be true for an experimental science. It would be great if a space probe could be sent out of the galaxy to see how things look from there (would settle the quasar question and 2.7oK radiation question), but it is not going to happen soon (even if it did, the answers would not be known soon). Much of what I have been saying in these discussions boils down to "just because the evidence agrees with a particular theory, that does not mean that that theory is correct, and it is important to distinguish between what we observe and our explanations of what we observe based on a particular theory". I hope this statement is acceptable...
wls@astrovax.UUCP (William L. Sebok) (05/19/84)
> Two reasons for preferring the nearby theory of quasars to the cosmic > one are: > (a) The intrinsic luminosity is reasonable if they are nearby, > but really hard to explain if they are far away; > (b) The periods of pulsars are hard to explain for far-away > pulsars, but admit of simpler explanations if the objects > are nearby. > Neither of these is a decisive argument (unless one can disprove the > existence of neutron stars), but they make the nearby quasar theorists > feel more comfortable with their position. What do pulsars have to do with this? Pulsars are entirely different objects than quasars. For example, quasars are distributed isotropically around the sky while pulsars are observed to be concentrated into the plane of our galaxy. Indeed, nobody (at least that I am aware of) is claiming that pulsars are any large distance away but instead at typical distances inside our galaxy, several to tens of kiloparsecs. -- Bill Sebok Princeton University, Astrophysics {allegra,akgua,burl,cbosgd,decvax,ihnp4,kpno,princeton,vax135}!astrovax!wls
sharp@kpnoa.UUCP (05/19/84)
I cannot let this go without one quick comment. Known quasars are definitely
NOT distributed uniformly around the sky, and there is a big debate about the
causes of this. Even on the most conservative view, quasars are clustered to
each other. The articles by Seldner&Peebles, by Nieto&Seldner, and by Bacon
&Nieto and references therein are essential reading. I can give full references
if needed, but at the moment it is not possible to say other than that quasar
searches have not yet been uniform over the sky. Whether there is something
other than selection effects involved must be a moot point.
--
Nigel Sharp National Optical Astronomy Observatories
Tucson, Arizona (602) 325-9273
UUCP: {akgua,allegra,arizona,decvax,hao,ihnp4,lbl-csam,seismo}!noao!sharp
ARPA: noao!sharp@lbl-csam.arpawls@astrovax.UUCP (William L. Sebok) (05/20/84)
>> For example, quasars are distributed isotropically around the sky while >> pulsars are observed to be concentrated into the plane of our galaxy. >> Bill Sebok >I cannot let this go without one quick comment. Known quasars are definitely >NOT distributed uniformly around the sky, and there is a big debate about the >causes of this. Even on the most conservative view, quasars are clustered to >each other. The articles by Seldner&Peebles, by Nieto&Seldner, and by Bacon >&Nieto and references therein are essential reading. I can give full references >if needed, but at the moment it is not possible to say other than that quasar >searches have not yet been uniform over the sky. Whether there is something >other than selection effects involved must be a moot point. >-- Nigel Sharp Sorry, in the heat of the moment I forgot to phrase what I said carefully and use the proper hedge-words. I am aware that quasars are clustered to each other. A weaker statement is that there is no evidence (that I am aware of anyway) that quasars prefer to be found in the plane of our own galaxy. -- Bill Sebok Princeton University, Astrophysics {allegra,akgua,burl,cbosgd,decvax,ihnp4,kpno,princeton,vax135}!astrovax!wls
allan@noao.UUCP (05/21/84)
What is all this nonsense about pulsars periods and cosmology? Pulsars are situated in our galaxy and have nothing to do with the large scale structure of the universe. Peter (theories to go) Allan Kitt Peak National Observatory Tucson, Az
jlg@lanl-a.UUCP (05/24/84)
i I think that the original submitter of the pulsar-quasar mix-up was referring to the periodic variation in quarar magnitudes. I don't remember details but I think that this variation is somewhat irregular. The problem is that the approximate period of the variations is short (on the order of hours). No known mechanism (rotational or otherwise) can produce regular variations of period shorter that the light travel time across the object. Yet the diameter of quasars must be larger than a few light-hours if they are really far away. The thing is, quasars (at least one) is very far away indeed! There is an example of a galaxy (fairly distant itself) which acts as a gravitational lense and bends the light from a quasar behind it. Seems like the quasar has to be significantly further away than the galaxy for this effect.
gwyn@brl-vgr.ARPA (Doug Gwyn ) (05/25/84)
Yes, the original idea was that the pulsar period was too fast for sensible explanation. However, I am willing to drop that issue as not necessarily related to cosmology. I would like to see more information on this quasar that is known to be behind a galaxy due to "lens" effect. Are there multiple images? If so, is this explainable by a lens effect? If it can really be shown that only by being behind the galaxy can this be explained, then indeed at least one quasar is extra-galactic. What does Burbidge have to say about this one? etc. Nice to see another contributor to the discussion!
allan@noao.UUCP (05/25/84)
I certainly agree that quasars are far away, however, the problem with the timescale for quasars variability is not quite as bad as you seemed to imply. Certainly there is no way that a quasar can vary in brightest by a large amount on a time shorter than the light travel time across the source of the energy. The favourite models have the energy being generated close to a black hole, and so the minimum time is the light travel time across this distance. Those quasars that have been found to be variable do have times of variation that are longer than this minimum time, although there are some that are very close to the limit. There is a problem with some BL Lac objects though. Some of these (e.g. 3C446) have been found to vary faster than the minimum time allowed. The usual way to argue around this is to say that the source of the radiation is travelling towards us in a beam so that any variations appear to be compressed in time due to relativistic effects. There are problems with this model and it is not universally accepted, but at least it does explain the time variations. Peter (theories to go) Allan Kitt Peak National Observatory Tucson, Az
allan@noao.UUCP (05/25/84)
Yes there are multiple images of the lensed quasar. There are two images about 7 arcseconds apart on the sky and are about 17th magnitude. According to the theory, there has to be an odd number of images if a distributed object such as a galaxy is doing the bending. The idea is that there are three images, but that two of them are so close together that they appear as a single object. As well as the optical information on the double quasar, there are also radio maps which show the double structure. The two images have been seen to vary, but they do not vary together. There is the hope that by monitoring the quasars over many years that it will be possible to correlate the variability and so find the difference in the light travel time along the two paths to the quasar. Knowing this and the geometry of the situation, we can measure the TRUE distance to the quasar. If this can be done then it will give us the value of the Hubble constant since we know the redshift and the variations give the distance. The problem with this is that so far we are not too sure about the geometry as there is a cluster of galaxies associated with the lensing galaxy, and this cluster provides a significant lensing effect. There are (I think) 4 more examples of multiple quasars. PG1115+080 is a triple quasar, just like the theory predicts. Peter (theories to go) Allan Kitt Peak National Observatory Tucson, Az