Talk:Active galactic nucleus

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Radio galaxies?[edit]

There was a question in the article from At18; I moved it here:

I put infrared and radio galaxies here too, is it the right place?

Certainly better here than nowhere, work in progress ... -- looxix 18:58 Apr 18, 2003 (UTC)

On the above point, I think Radio galaxy should have it's own article, instead of being redirected to Active galaxy, since the two terms are not interchangable. Radio galaxies are a sub-category (as are quasars, blazars, Bl Lacs, etc..). Unless there's any objections, I can begin work on a Radio Galaxy page and un-redirect once it's reached a semi-mature status. Privong 18:42, 21 Apr 2005 (UTC)

Well, since this doesn't seem to have happened, would anyone object to me setting up a radio galaxy page? There is currently a lot of detail about radio galaxies in this article which obscures the basic point, while most other classes of AGN have their own entries. Mhardcastle 20:43, 1 February 2006 (UTC)

OK, it's there now... Mhardcastle 23:41, 5 February 2006 (UTC)

Starburst galaxies[edit]

Starburst galaxies aren't active galaxies (as the definition at the top of the article shows; they're just normal galaxies with a large amount of star formation).

Starbursts should really have their own entry, but I don't have time to do it now: I've just removed the misleading comments from the active galaxy page, and tidied it up. -- mjh, some time ago

  • Starburst galaxies are active galaxies, they just don't have AGN. This article should be at Active galactic nuclei instead. Don't you remember all the theories on how starburst galaxies could account for AGN type galaxies? They are active, but not single sourced, and they are bright, and spectacular, like AGN galaxies. 05:46, 6 Dec 2004 (UTC)

Synchrotron radiation[edit]

There were a couple of misleading statements about synchrotron radiation and high temperatures in the accretion disc. I think this was probably a confusion between the properties of the accretion disc and the jet that is sometimes seen; the jet is synchrotron radiation and can have a very high brightness temperature (nothing to do with actual temperature -- the temperature of the accretion disc is quite low). -- mjh May 23 2004.


I'm no expert but should "An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas." be clarified to a numerical proportion? Or is this impossible. A comparison, to a regular galaxy if possible, might help non-experts like me understand this better. Grox 10:07, 13 Feb 2005 (UTC)

There isn't a concrete numerical factor, but the AGN will generally outshine the host galaxy by ~1000x. Privong 18:22, 21 Apr 2005 (UTC)
This is only true for the most powerful AGN. Mhardcastle 20:43, 1 February 2006 (UTC)

New text under `Types of active galaxy'[edit]

As this stands it not only is factually wrong in several respects but duplicates (or in some cases contradicts) what's already there under `Optical spectra'. I'm tempted to delete the whole lot unless it gets a lot better before I look at this again Mhardcastle 18:54, 24 October 2006 (UTC)

Go for it. I was just attempting to clean up what was already there. The Seyfert definitions were reversed before I made the edits. Perhaps it might be better to remove the optical spectra section and instead include that in the 'Types of active galaxies' section. That might be a more logical way to approach it? Privong 11:49, 25 October 2006 (UTC)

Low-Ionization Nuclear Emission-line Region (LINER)[edit]

This article lacks a section on Low-Ionization Nuclear Emission-line Region (LINER)-type galaxies, which are supposedly the most common type of active nuclei galaxy. Any interest? Thanks. — RJH (talk) 20:03, 4 December 2006 (UTC)

Complete rewrite[edit]

Once I started editing this, I couldn't stop. Hope readers approve of the results. It could do with some diagrams and some more references in places. I think it's an improvement on the original, though. I'm afraid it still doesn't mention LINERs, but they should have their own article, which should then be linked here. I'm not enough of an expert on LINERs to write one from scratch. Mhardcastle 22:02, 24 December 2006 (UTC)

I added a "see also" link. -- Beland (talk) 02:10, 24 November 2007 (UTC)

H II nucleus is an AGN?[edit]

Is H II nucleus a type of AGN? I notice it in this paper: [ A SEARCH FOR "DWARF" SEYFERT NUCLEI. III. SPECTROSCOPIC PARAMETERS AND PROPERTIES OF THE HOST GALAXIES] These are not covered in the article presently. WilliamKF 01:59, 26 February 2007 (UTC)

Looks to me as though they are using 'HII nucleus' to mean 'nucleus where the spectrum is consistent with that of an HII region', i.e. a source whose emission lines are powered by star formation. Look at the start of S3. Mhardcastle 08:53, 26 February 2007 (UTC)

Effective Gravitomagnetic Explanation for Jets[edit]

The statement in the article about unknown explanations for accretition disc jet properties should be amended. Reva Kay Williams, University of Florida, has provided an explanation for the immense power and features of relativistic jets. She developed a proof of Roger Penrose's mechanism from gravitomagnetism for extracting energy and momentum from rotating Kerr black holes. Her model shows that the Lense-Thirring effect (frame dragging) produced by a spinning black hole can account for the observed high energies and luminosities of quasars and active galactic nuclei; the collimated jets about the polar axis; and asymmetrical jets (relative to the orbital plane). They are inherent properties of Kerr black holes. Those properties can be derived without resorting to magnetohydrodynamics.

  • Gariel, J.; MacCallum, M. A. H.; Marcilhacy, G.; Santos, N. O. (2007, February 23). Kerr geodesics, the Penrose process and jet collimation by a black hole. Preprint arXiv: gr-qc/0702123v1.
  • Penrose, R. (1969). Gravitational collapse: The role of general relativity. Nuovo Cimento Rivista, Numero Speciale 1, 252-276.
  • Williams, R. K. (1995, May 15). Extracting x rays, Ύ rays, and relativistic e-e+ pairs from supermassive Kerr black holes using the Penrose mechanism. Physical Review, 51(10), 5387-5427.
  • Williams, R. K. (2001, October 15). Collimated energy-momentum extraction from rotating black holes in quasars and microquasars using the Penrose mechanism. AIP Conference Proceedings, 586, 448-453.
  • Williams, R. K. (2004, August 20). Collimated escaping votical polar e-e+ jets intrinsically produced by rotating black holes and Penrose processes. The Astrophysical Journal, 611, 952-963.
  • Williams, R. K. (2005). Gravitomagnetic field and Penrose scattering processes. Annals of the New York Academy of Sciences, 1045, 232-245.

I believe some of the above should be considered for incorporation with this article. Tcisco 06:36, 21 April 2007 (UTC)

As the article makes clear at present, the problem is not that there are no models for jet generation -- the problem is that there are many models and no observational way to distinguish between them. I don't see anything in the references above that would require that statement to be changed. An article that presented all the models and the arguments for and against would be interesting, but this article just needs to summarize the situation, and the summary at the moment is accurate (unlike the grossly inaccurate state of e.g. relativistic jet). Mhardcastle 07:29, 21 April 2007 (UTC)

Merger proposal[edit]

The stub content of Galaxy AGN classification is a subset of what is contained in `Types of active galaxy'. No content needs to be merged; I just am not too sure about dealing with pages that link to the redundant stub. (talk) 18:41, 8 September 2008 (UTC)

Redirect to the relevant part of this article? Mhardcastle (talk) 19:43, 8 September 2008 (UTC)
Agree - merge and add redirect. Actually I don't think there is much worth merging Codec (talk) 06:57, 9 September 2008 (UTC)

ㅡㅡ —Preceding unsigned comment added by (talk) 03:04, 20 March 2009 (UTC)

This article has severe mistakes, inaccuracies and unsupported claims - in serious need of revision[edit]


I'm working on giving a lecture about the AGN-Star Formation Connection and I was hoping to hunt down some nice images, but I started reading part of this article and couldn't stop since I kept stumbling upon inaccuracies. For instance, the statements with regards to LINERs are totally inaccurate. Whoever wrote this needs to clean it up and read Luis Ho's 2008 review on the subject of low-luminosity AGN (LLAGN).

"(LINERs. . . ) As the name suggests, these systems show only weak nuclear emission-line regions, and no other signatures of AGN emission. It is debatable whether all such systems are true AGN (powered by accretion on to a supermassive black hole). If they are, they constitute the lowest-luminosity class of radio-quiet AGN. Some may be radio-quiet analogues of the low-excitation radio galaxies (see below)."

Ok - the first sentence actually gives no information about what a LINER actually is. You should explain which emission lines/emission line ratios are examined in order to classify the nucleus of a galaxy as such. The statement that there are "no other signatures of AGN emission" is totally wrong! LLAGN (including LINERs) are almost never silent in the radio and can be readily detected with sufficiently deep and high resolution observations (the recently upgraded VLA - now called the EVLA - can now reach down to rms noises of 10s of micro-Janskys for continuum observations in relatively short times due to expanded bandwidths, for instance . . . I have GBs of data of LINER detections!). In the X-ray regime, Chandra can pick out the compact, hard X-ray point sources which are also indicative of LLAGN (again, including LINERs). There are also other wavelengths which are promising which I am not an expert in (though I know that Luis Ho explains them thoroughly in his 2008 review) as well as methods such as determining the q-value from the radio-FIR relation or correlating the 5 GHz data, hard X-ray data and dynamical SMBH mass in log space (the fundamental plane of black hole activity). Are LINERs really the lowest-luminosity class of radio-quiet AGN? Where did you read that (unsupported claim - or if you did reference one I would suspect it was never actually read)? Radio-loudness is kind of a fuzzy parameter anyways - some people define it relative to optical luminosity and others chose to define it against X-ray luminosity and still others may use a different point of comparison. You have to define your convention when doing astronomy - even coordinates are not consistent from one observer's project to that of another observer!

Also, you have not considered the intriguing class of LLAGN known as "Transition Objects" - these are galaxies with nuclear optical emission line characteristics intermediate between (star-forming) HII regions and LINERs. And what about Type I versus Type II (you mention the distinction for Seyferts but not LINERs)? And LIERS (low-ionization emission regions that are not coincident with the nucleus)?

The summary table is pretty much useless, there are so many mistakes and things left out (and you have totally unsubstantiated claims in there!). Really - Quasars only have "some" jets (um, no - ask any radio astronomer and they will be sort of horrified)? What does "some" even mean? What fraction? And Seyfert galaxies most certainly can have jets! I have data sitting on my laptop of a very bright, beautiful Seyfert galaxy (NGC 3665) that I have just re-imaged with the EVLA at 1.4 GHz that is JET-DOMINATED in the radio! You have to define an observing regime, and really, if you're thinking optically then you're not getting the most bang for your buck (radio observations are critical for AGN studies).

Also, how can you say that a starburst isn't variable? You didn't state a TIMESCALE. If a starburst has depleted the cold gas in a Gyr, that could be considered "variable" - why not. Yes, timescales are important when you start to talk about variability. Different types of AGN display variability on different timescales (another cute little tip-off to what you're dealing with). Why can't a Seyfert be radio-loud? I'm pretty sure there are radio-lound Seyferts (especially with the definition of radio-loud being so loose). You can't just put "yes" and "no" in columns when you are doing astronomy - everything is a percentage and there are ALWAYS some oddball outliers no matter what the norm is. I suggest completely removing that table since it is only spreading false information.

I would like to eventually help fix up this article and create something accurate from someone in the field with a graduate degree, but I don't have time to delve into the full scope of the subject of AGN at this time (I specialize in LLAGN - so my knowledge of FRI/FRII objects, quasars, blazars, etc. isn't so detailed . . . it was the inaccurate statements about LINERs that caught my eye at first).

I don't know how this got a B rating - this needs to be reclassified as a stub with the "needs attention from an expert" notice at the top. Hopefully I'll have time to fix this up soon.

Kris1284x (talk) 09:52, 2 September 2011 (UTC)

I agree that it needs some work, and some more recent citations. But the statements about LINERs are correct. Whether or not all LINERs are AGN is still a major point of contention (we should cite Sarzi et al. 2010), and neither Chandra-detected X-rays nor deep radio detections are a guarantee that there's an accreting SMBH at low energy levels (say, L_x ~< 10^40 erg/s), as central star cluster SN remnants and X-ray binaries can produce that. I agree that Ho's LINERs probably host accreting SMBHs, but if they are placed at higher redshifts, like a typical SDSS main sample galaxy (z ~ 0.1), distinguishing an AGN-driven LINER from a shock or XRB-driven LINER becomes much harder, if not impossible.
As to your comments on radio, I agree that the table could be more quantitative, but I think it's broadly correct. Only ~10% or quasars are radio loud, and not all have jets (not even most, e.g. work by Best or Filho). Starbursts are not variable on human timescales (we could make a footnote labeling that column as such). Few Seyferts have strong radio emission (I agree saying None do is incorrect, but it's few). I think the article should have some sort of table like this, as there is a very wide range of terms that all describe galaxies with accreting supermassive black holes, and most of these labels were applied to observed objects long before AGN unification, or even before the SMBH hypothesis. This table is trying to describe properties across a broad range of observations. Maybe a separate table that shows the empirical classifications for each object, although that's sort of what the current table does.
We should probably mention the optical transition/composite class, though what exactly those objects are is a really big question. It is, after all, a purely empirical classification. Some are pure starformers, some are weak Seyferts with current star formation, some are LINERs with recent star formation. etc.
I recommend you be BOLD, and then we can talk about your changes.
- Parejkoj (talk) 14:27, 2 September 2011 (UTC)
First of all, your comment about the person who wrote the article and your addressing them as 'you' betrays some ignorance about the way Wikipedia works. No one person wrote the article, and the idea that nobody with a graduate degree in the field has looked at it since it's existed is laughable. As it happens, the article's current form is based on a complete rewrite that I did in late 2006, though I'm not responsible either for the text on LINERs or the table. If you look me up on ADS (it's not hard) you should see that I have some qualifications to do this (i.e. a PhD and 18 years' experience). So have many of the other contributors. I'd recommend that you don't wrap your comments up in gratuitous abuse -- they'll be better received.
Secondly, and most importantly, the way to fix a Wikipedia article is not to rant and rave in the comments but to edit. I fully support what Parejkoj has said above. Edit what you think is wrong, and if it's an improvement, it'll be fixed. The article has always needed more references and more detail (though not infinite amounts more: think of the target audience: this is why there are not detailed numerical descriptions of every criterion used) and if you have the time to provide them, that will be great.
On specific science points:
  • On radio-loudness: the question is whether an object has energetically significant radio emission. *No* galaxy is radio-silent. If a LINER had strong radio emission, comparable to that of low-luminosity radio galaxies, it would, ipso facto, be classed as a radio galaxy. I have seen Luis Ho stand up and claim with a straight face that M87 is a LLAGN (though its jet power exceeds 10^44 erg/s). The article prefers to treat such objects as radio galaxies since that is what they are called by the majority of people who work on them. The ones that are not treated as radio galaxies then, by definition, have *weak* (not absent) radio emission.
  • The article says, and has said since my revision in 2006, that some Seyferts have weak jets. The one-word fix to make the table reflect this, since made by Parejkoj, would have been easier than typing a paragraph of comment! A radio-loud Seyfert would, by definition, be classed as a narrow-line radio galaxy (Seyfert 2) or broad-line radio galaxy (Seyfert 1).
  • Quasars come in radio-loud and radio-quiet flavours. Only some quasars (the radio-loud ones) have energetically significant jets. You may be thinking of the 'quasar/QSO' distinction used by some people, but the article doesn't use this terminology.
  • I don't think starbursts should be in the table at all: they're not AGN. But I think we had this argument when they were inserted.
  • The table is there (again, I didn't put it there, but I don't think it's a bad idea in principle) to try to show the broad boundaries of what are fundamentally artificial categories of AGN classifications. There will always be oddball objects, because nature doesn't actually care about our classification schemes. But what we want is for the reader to come away with an idea of what the characteristics of typical object of type X are likely to be. Therefore the maximum level of detail required is 'yes', 'no', or 'sometimes'. Details should be provided in the text.
Mhardcastle (talk) 07:56, 3 September 2011 (UTC)

Unified model image[edit]

Wouldn't be useful an image like this? I was planning on creating something like that myself, but it won't be fantastic. Does anyone know a good free image? In case not, what should I highlight in the image apart from the obvious (accretion disk and jets)? I was thinking about removing the whole viewing angle story, since it already has its section and image. What do you all think? Heinerj (talk) 22:51, 15 May 2015 (UTC)

I've been waiting for someone to create a newer version of that plot for a long time: there are a few different versions of that, all made in the mid/late 90s. It's not inaccurate, but it certainly could be made nicer. - Parejkoj (talk) 05:50, 16 May 2015 (UTC)
What are you thinking about? What should be included\excluded or could be better? Heinerj (talk) 20:43, 17 May 2015 (UTC)