Talk:Quasar/Archive for 2008

Highest redshift quasar

The article says 'which is significant because it implies a maximum distance-more distant quasars should be easily observable if they existed' - is this actually the case and can anyone give a reference for that? The z=6.4 quasar was discovered relatively recently in Sloan. If it had been discovered some time ago then I think you could make a case that any more distant quasars would now have been found, but I'm not convinced you can say that now. EddEdmondson 10:40, 18 Jun 2004 (UTC)

I second the request for references. Please provide links to a few before reverting, as the request has been outstanding for a year.. --Christopher Thomas 16:58, 30 Mar 2005 (UTC)

Hi, I removed the bit about the highest redshift being significant. I'm an astronomer with the SDSS, the survey that formed the basis for finding these high-redshift quasars, and it's not significant. The only reason there is a maximum to the redshift of known quasars is that it's hard to find them using current telescopes and cameras and analysis techniques. The way the current high-redshift quasars are found is by looking for objects that have only been detected in the longest-wavelength 'z' filter of the 5 SDSS filter passbands u, g, r, i, z. Some of the z-band only detections are just image defects, some are cool stars, and a few of them are high-redshift quasars. They do not have any emission at shorter wavelengths because a) the Lyman-alpha emission line is redshifted into the z-band, and it has the shortest wavelength among the strong emission lines that quasars usually have, and b) because absorption by Hydrogen along the line of sight from us to the quasar is eating up flux. To find even higher-redshift quasars, you would need to extend this 'dropout' technique to even longer wavelengths; however, the quasars get fainter at higher redshifts, and the Lyman-alpha line moves to even longer wavelengths, so that you would need to use a camera that is similarly sensitive as the SDSS camera, but observes at longer wavelengths, and can cover a similar area as the SDSS (it plowed through a quarter of the night sky to find 6 quasars at z>6...). Such a camera doesn't exist yet.

So sometimes the reason that nothing is found is not that there's nothing there, but just that the something is hard to spot even if you know it's there. Higher-redshift quasars will be found eventually. More quasars at the same redshift will also be found eventually. Sebastian Jester

I've updated the quasar with the highest known redshift to be CFHQS J2329-0301, z=6.43, discovery published in December 2007. Apetre (talk) 02:42, 8 April 2008 (UTC)

I'm worried about wikipedia...

This is clearly very OT here... this is the same SDSS guy who wrote the comments about the highest-redshift objects. I checked back to see how the quasar entry had evolved. I want to say that I'm a bit shocked at the incoherence and inaccuracy of the article's content, it certainly has not improved. For example, who cares who thought which quasar was the most luminous if they later turned out to be less luminous? And besides, SDSS has found (or re-found) several quasars that are more luminous than 3C273.

If every wikipedia entry has the same ratio between "what's in the article" and "what an average person who has happened to study the subject matter knows" as the quasar entry, I'm very troubled about how useful the whole thing is. In other words, wikipedia is *a* source of information, but it can never be considered as an authoritative source, because I'll never know whether the latest edit made the entry more accurate, or less.

Of course, I could write a completely new entry that would represent what I think is important about quasars, but if someone else can just edit it back to what it is now, why bother?

Hence, I personally feel it's a waste of my time to contribute anything about quasars to wikipedia because I'd either have to risk that my contribution disappears completely, or have to check back once a week to make sure that nobody has added (obvious or subtle) nonsense. There are more efficient ways to educate the public about your speciality than allowing the public to alter your description of what you know. And there are better ways for the public to learn about quasars than going to wikipedia and hoping that some benevolent person in the know has removed the latest addition of nonsense.

So, please blast me for being off-topic, and off-spirit, but as far as I'm concerned, wikipedia is much ado about nonsense.

Sebastian Jester

Please don't give up on it! Your changes are never lost, but saved in version control and someone discovering a messed-up page can read the previous edition, and you can "revert" it. It would be nice if you did check it once a month (not once a week), but you'll do that anyway when adding new discoveries on the subject, right? You can "watch" the page and have it noted that a change was made, so you don't have to actually keep reading it. That is how quality pages can exist. Długosz

So just checking back on the state of this wiki page... the fact that it's listed as being "controversial" means I just can't change my mind. Controversial amongst whom? The astronomically uneducated half of people who regularly edit this page... Who cares? SJ 22:28, 10 April 2008 (UTC) —Preceding unsigned comment added by 78.42.129.139 (talk)

I don't see anything "controversial" in this page (or any tags to that effect), except the very small (and I'd say, relevant) note in the introduction. I've cleaned up a few of the non-science links at the end. If you have some suggestions on how to make it better, please post them here, or make the changes. It could certainly be improved, but I'd say the article isn't too bad, as far as our current understanding of quasars goes. - Parejkoj (talk) 20:33, 11 April 2008 (UTC)

Current existence of quasars?

The article states that all the quasars that have been found have been very, very far away. Is it safe to assume, then, that all quasars are thus a relic of the early(ish) universe and no longer actively exist? --Jeffrey O. Gustafson - Shazaam! - <*> 09:20, 6 December 2006 (UTC)

"No longer exist" is a term that's hard to justify in a universe where you can see all the way back to the beginning. --ScienceApologist 12:54, 6 December 2006 (UTC)

Eh, yes and no. Although we can see it, something that happened ten billion years ago is not exactly happening now (as far as we know). Its subjective, I guess. In any event, are they a relic of the early universe and what is the earliest quasar we know of? --Jeffrey O. Gustafson - Shazaam! - <*> 21:12, 6 December 2006 (UTC)

The "earliest" quasar we know of is the one with the highest redshift. Since space and time are connected something that happened ten billion years ago "there" is not the same as ten billion years ago "here". --ScienceApologist

Jeffrey, "very, very far away" is a relative term. According to the article, the nearest quasar is 780 million light years away; the age of the Universe is ~13.7 billion years. Thus the nearest known quasar existed when the universe was ~96% of its current age. (There may be some error here if the two numbers I'm quoting assumed different cosmological parameters, but I think the point will stand.) I would say, then, that we have evidence of "recent" quasar activity on cosmological timescales. Why don't we see any closer (more recent) than that? Well, there just isn't that much volume of space at low redshift. If you wanted to prove that there's quasar activity at 99% of the current age of the universe, your survey would be limited to a relatively small distance from the earth, which may not happen to contain a quasar, just because they are fairly rare objects. You can't cover more volume in your survey without also increasing your lookback time. Hope that makes sense. (Note that quasar activity is becoming less common as time goes by, so that's part of the answer. But I don't think it's true that quasar activity is completely eliminated at current times.) -- Coneslayer 22:42, 6 December 2006 (UTC)

The current and accepted model for quasars associates them with the luminous centers of active galaxies. At large distances the luminosity of the quasar, which is on the order of tens to hundreds of galaxies, overpowers the brightness from the rest of its host galaxy (hence why it was considered a 'quasi-stellar' object instead of a galaxy). In general we don't see quasars short of z=.1, however we do see active galaxies, which means that below this redshift we can observe the host galaxy surrounding the AGN. If we can see the host, then the object is no longer classified as a quasar, but as an active galaxy, which is the proposed quasar engine. Essentially, the definition of a quasar limits them to larger distances. --stim 07:45, 15 December 2006 (UTC)

In a vein I think is similar to this subtopic, I'm confused by this statement: "Quasars also provide some clues as to the end of the Big Bang's reionization. The oldest quasars (redshift >~ 6) display a Gunn-Peterson trough and have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas." What does it really mean to say they are the "oldest". I understand that the high redshift means they are farther away, but doesn't that also mean that we are seeing them when they were very young. From our frame of reference they may not even be quasars anymore. Can they really be definitely said to be older than closer quasars? Perhaps the closer quasars have been burning just as long and are in fact older. --64.56.3.187 (talk) 01:44, 30 September 2008 (UTC)

"Young" Galaxies

Question from a Wikipedia newbie: The introductory paragraph mentions that Quasars are found in "young" galaxies, but I'm not certain I know what is meant by "young" here. Are young galaxies ones that existed relatively shortly after the big bang, or is youth rather an initial step in the evolution of a galaxy? Is there some property of older galaxies that prohibit Quasar activity, or does the lack of a Quasar itself mark a galaxy as being old? (I'm guessing that we're saying Quasars were a feature of the universe back when there was more matter available to feed the black holes. If so, it would seem that this is more a property of the early universe as a whole, rather than a property of individual galaxies; I'd think that if even an old galaxy somehow bumped into a sufficiently large cloud of gas, it could generate Quasar-level output while consuming it...) Thanks! Jpietrzak (talk) 00:55, 15 February 2008 (UTC)

Question

How can we see something 28 billion light years away in a 13.7 billion year old universe? Is there proper motion involved or is this a mistake? —Preceding unsigned comment added by 70.135.192.119 (talk) (transported to the talk page by —Animum (talk) 16:34, 18 February 2008 (UTC))

See these threads from last month in Talk:Universe: 1 2. -- BenRG (talk) 12:50, 20 February 2008 (UTC)

28 vs 17.4

I changed the distance to the most distant known quasar from 13 to 28 billion light years, and Fivemack then changed that to 17.4. I admit to knowing nothing about this quasar; all I did was plug the given redshift into Ned Wright's cosmology calculator, click "Flat", and copy the comoving radial distance that it spat out back into the article. Where did the 17.4 Gly figure come from? -- BenRG (talk) 02:06, 21 February 2008 (UTC)

Someone changed it back to 13. I changed it to 28 again pending a response here. I hope this cycle doesn't continue... -- BenRG (talk) 15:54, 25 February 2008 (UTC)

Based on present day theory, the quasar was about 13 billion ly away when it emitted the light we see, it is now about 28 billion ly away due to expansion. So both answers are correct based on what you mean by 'distance away'. Perhaps it could be made a little clearer. PhySusie (talk) 17:47, 25 February 2008 (UTC)

As reasonable as that sounds, it's not correct. Light doesn't really travel at c in cosmology. In terms of straight-line metric distance at a constant cosmological time, the quasar is about 28 billion ly away now and was about (28 billion ly)/(1+z) = 3.8 billion ly away when it emitted the light. There is something called the light-travel-time distance, defined as c times the elapsed time, but it's really just an odd way of stating an elapsed time. I've been replacing LTT distances with present-day metric distances in Wikipedia articles because I think metric distance is the only physically sensible distance measure; it's only with respect to the metric distance that the universe is homogeneous and Hubble's law holds. The metric distance at the time of light emission makes some physical sense, but it doesn't always increase with increasing redshift. The CMBR is only around 40 million light years away by that metric, when really it should be farther than everything else we see. -- BenRG (talk) 00:00, 26 February 2008 (UTC)

Shouldn't the article mention that we kind of have a problem when we find something that is 28 billion ly away in a universe that is estimated to be only about 13.7 billion ly old? Isn't this a current problem in cosmology? --KarlFrei (talk) 11:56, 4 March 2008 (UTC)

No. There is no "problem" with cosmology here. The issue is simply that the question "how far away is light emitter X" doesn't have a single, simple answer when the distance to X has changed substantially between the time the light left X and the time you detected it. See Distance measures (cosmology) for possible interpretations of "distance"; note the plot at the bottom, and see that some measures can be larger than lookback time. By analogy, consider an ant walking at constant speed on a rubber band, which is being continuously stretched. By the time he gets from one end to the other, the length of the rubber band will be greater than the distance that the ant walked (basically, because some of the expansion took place behind him). -- Coneslayer (talk) 20:25, 4 March 2008 (UTC)

See also Observable_universe#Misconceptions, specifically the "13.7 billion years" bullet. -- Coneslayer (talk) 20:53, 4 March 2008 (UTC)

Thank you for your explanation. So perhaps we could add a line to the article saying that this does not contradict the estimated age of the universe, with a link to those misconceptions? I am sure I am not the only one who thought this (I hope). --KarlFrei (talk) 09:48, 5 March 2008 (UTC)

Oops, this has already been done. Never mind! --KarlFrei (talk) 13:38, 5 March 2008 (UTC)

The line currently present in the article "(NB there are some subtleties in distance definitions in cosmology, so that distances greater than 13.7 billion light-years, or even greater than 27.4 = 2*13.7 light-years, can occur.)" is wrong. It isn't a subtlety in 'distance definitions', it is because spacetime is highly curved on cosmological scales. This is also explained in http://en.wikipedia.org/wiki/Observable_universe --Anonymous wikipedia reader, 17:31, 29 july 2008

Impossible Distance Cited

When astronomers speak of distances to objects in the universe they speak of them in terms of the age of the light that they are receiving. The Universe being only approximately 14 billion years old, could not have observable objects with light 28 billion years old. —Preceding unsigned comment added by Quidproquo2004 (talk • contribs) 04:54, 2 April 2008 (UTC)

Actually, the distance across the universe is on the order of 90billion light years. In the case of the big bang, the universe was (maybe still is?) expanding faster than the speed of light. Physicists now say that it's ok for something to recede faster than light, but approaching FTL is still forbidden.--MaizeAndBlue86 (talk) 11:29, 2 April 2008 (UTC)

Please see KarlFrei's question immediately above yours. It's essentially the same concern. -- Coneslayer (talk) 12:56, 2 April 2008 (UTC)

(You could also see the question immediately above that one, which is also essentially the same. Groundhog Day was two months ago, but you wouldn't know it here...) -- Coneslayer (talk) 13:01, 2 April 2008 (UTC)

It's a bit scary how many people have been visibly confused by this change to the article. For every reader who edits the article or posts a question here there must be dozens or hundreds who have the same reaction but don't make an edit to show it. Someone also asked about it at the reference desk, spawning this discussion, for which I actually drew a 3D visual aid, reproduced to the right. The brown line is the earth, the yellow line is the quasar, the orange line is the distance to the quasar right now (28 billion light years), and the red line is the path of the light from the quasar to us. The grid lines mark off billions of years and billions of comoving light years. The red line "moves at the speed of light", meaning that it always makes a 45-degree angle with the local grid lines. The way that space seems to curve around into a circle is an artifact of the embedding with no physical significance, and I trimmed it short of a full circle to emphasize that. The pictures are numerically accurate (I calculated the shape from the WMAP data) and you can verify by visual inspection that all of the distances and times come out right. I think these diagrams and the explanation are far too much of a digression for this article, but maybe they'll help someone here on the talk page. -- BenRG (talk) 10:18, 16 April 2008 (UTC)

Good answer im one of those people. I never edit scientific articles I know far to little about the topics (stick to my own background areas) but it did certainly seem a big contradiction. Thanks for the explaination much appreciated. Kurtk60 (talk) 23:45, 9 December 2008 (UTC)

brightest objects just happen to be the farthest away?

Is this because Quasars formed more frequently during the early universe? If so, some more explanatory text about it might be good, including why they formed more frequently in the early universe.

Also, I think I saw figures about the distance of the average Quasar, but I don't recall the distance to the average normal galaxy. beefman (talk) 19:08, 3 September 2008 (UTC)

New article can be linked

You may be able to link to Sołtan argument at this article. 24.199.92.132 (talk) 08:27, 28 September 2008 (UTC)

Quasars presently existing

Do any quasars presently exist in our universe? Or are all the quasars gone? YouthoNation (talk) 14:42, 28 September 2008 (UTC)

"Power source"

The text currently shows: "The release of gravitational energy by matter falling towards a massive black hole is the only process known that can produce such high power continuously."

I looked around (including Gravitational wave#Sources of gravitational waves) and I see no mention of it, therefore, I placed a {fact} tag on it for your kind review. Cheers, -BatteryIncluded (talk) 01:35, 22 October 2008 (UTC)