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I found the picture below in an answer to the post "Was the Milky Way ever a quasar". The picture is published on a website belonging to "NASA/IPAC Extragalactic Database". In the answer that has 16 "thumbs up" it says:

A quasar is simply an active galactic nucleus (AGN) that is viewed from a particular angle; see the picture below, in which quasars are labeled "QSO". This is really a remarkable figure because historically all of the names in the figure were thought to correspond to different types of objects, when really they all refer to the same thing!

AGN

There is another answer to the question "Difference between quasar and Active Galactic Nuclei?" where someone states that a quasars:

main property is that it's galaxy axis points towards the earth and this way we receive its light and radio signals. That's why they are one of the most energetic AGN.

This answer has gotten six thumbs up. However, there is no reference to any source backing up that claim so it seems like private opinion.

  1. Does the picture above represent the mainstream view among astronomers today or is it more like a private opinion of someone?

  2. According to Wikipedia, ten percent of all observed galaxies are classified as Seyfert galaxies. Would we perceive all of those as "quasars" if we happend to observe them from the right angle? (Maybe there is also a luminosity requirement)

  3. According to Wikipedia Markarian 231 is the closest quasar at a distance of 581 million light years. Also according to Wikipedia, the Circinus Galaxy is the closest Seyfert galaxy, only 13 million light years away. Would we perceive there to be many quasars closer than 581 million light years away, if we just happend to view the active galaxy from the right angle?

  4. Could someone please show me the scientific investigations proving that the galactical planes of galaxies perceived to host quasars always lies within a certain range of observational angles as viewed from the Earth? These kind of investigations should be available if the claim that whether or not an active galaxy is perceived to be a quasar is almost totally depending on observation angle has a real backing and is not just speculation.

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    $\begingroup$ Possible duplicate of Difference between quasar and Active Galactic Nuclei? $\endgroup$
    – user24157
    Commented Apr 6, 2019 at 15:00
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    $\begingroup$ I did read that question before I posted. Note that someone answered about quasars "Its main property is that it's galaxy axis points towards the earth and this way we receive its light and radio signals. That's why they are one of the most energetic AGN. " There is no source to back up that statement provided there either. Have there been an investigation that galaxies perceived to host quasars always are observed from a particular range of angles? $\endgroup$
    – Agerhell
    Commented Apr 7, 2019 at 10:30
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    $\begingroup$ I believe Quasars are far more luminous than typical AGNs as they are accreting material at a far higher rate, so if the Circinus galaxy was aligned with us it wouldn't be classified as a Quasar. However, Quasar isn't a precisely defined term. $\endgroup$
    – MichaelB76
    Commented Apr 8, 2019 at 8:44
  • $\begingroup$ Yes, I think this is the modern consensus $\endgroup$
    – Agile_Eagle
    Commented Jun 1, 2020 at 13:28

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There is total confusion in the answers you refer to.

Quasars were first defined because of their star-like appearance - quasi-stellar objects or QSOs.

The fact that they are Galaxy-sized objects with point-like appearances means they are very luminous, dominated by central nuclear emission and far away (as confirmd by their high redshifts).

It was later recognised that quasars are just the high-luminosity end of the AGN zoo.

Quasars can also be viewed at a variety of angles and have distinct properties as a result. So there are radio-quiet and radio-loud quasars, broad absorption line (BAL) quasars and optically violently variable (OVV) quasars. The distinction between these types is thought to be dominated by viewing angle, mass of the central black hole and the accretion rate. The nomenclature can also vary depending on what wavelength region you are talking about (radio, optical, X-ray etc.).

These objects have lower luminosity counterparts amongst AGNs - FR I and FR II AGNs, Seyfert I and IIs and blazars (or BL Lac objects) - again, whose differences are largely explained by viewing angle.

I don't think these days it is helpful to think of quasars and AGNs as different objects. They are actually similar and it is just a question of the scale of the accretion process going on at their centres that is different and covers a continuum rather than a bimodal distribution.

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The consensus view is that Quasars are a type of AGN, and depending on precisely what meaning you put in "Quasar" it can also mean it is strictly a type defined by viewing angle.

I think the consensus view also holds that viewing angle is the dominating factor for the typology, but there are other factors and it might spark a huge bikeshed debate about what is the most important factor.

Anyway, the precise naming of the various types of AGN is a HUGE mess, mostly for the historical reason that AGNs where independent rediscovered in various different wavebands before anyone managed to come up with a unified description. For an impression of just how big the mess is, look at table 1 in this paper that lists several AGN classes: https://arxiv.org/pdf/1707.07134.pdf

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    $\begingroup$ I'd sort of go along with this, but generally speaking the name quasar is applied to much more luminous objects than a typical Seyfert galaxy for example. $\endgroup$
    – ProfRob
    Commented Aug 19, 2020 at 16:05

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