Magnitude limits for active galactic nuclei

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Summary:

How powerful is a galactic nucleus to be called "active"? "quasar"?
Milky Way centre cannot be seen because of Great Rift, but it is known as a loud radio source Sagittarius A since 1930s.
What would be the absolute magnitude of Sagittarius A if it could be seen? Visual magnitude?

Not all galaxies have loud centres. Large Magellanic Cloud does not show a sign of a powerful point radio source. Neither does Triangulum.

Yet despite Sgr A being one of the loudest radio objects in sky, Milky Way is not regarded as a radio galaxy, nor a Seyfert galaxy.

What is the magnitude threshold above which a galactic nucleus is classified as active and below which, passive?

Also, what is the magnitude threshold above which a galactic nucleus is a quasar and below which, a Seyfert galaxy?
 

Answers and Replies

  • #2
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Hmm I suggest you relook over these questions and what you want from us as there is a fair amount of nuance given the dynamic nature of Active Galactic Nuclei (AGN) and there are also still unresolved questions regarding how they form and evolve relative to their host galaxies.

In regarding what defines a galaxy as active nor not is based on whether we see clear AGN activity or not yes but this isn't somthing you would be able to see by eye. AGN are characterized spectrally through a broad category of characteristics that suggest material is infalling into an accretion disk around a compact object. What these are varies by perspective and structure of the AGN activity for example active Jets produce radio emission, reverse compton scattering of an accretion disk results in hard X-Ray signatures. In practice you aren't going to usually be able to tell the difference between an AGN and a quiescent(not passive) galaxy as the features which distinguish the characteristsics aren't somthing our eyes can pick up on so there is no point in even trying to define a magnitude.


It may be tempting to think of AGN as intrinsically different and persistent features but evidence strongly suggests things aren't so simple.

First there is plenty of evidence of AGN variability as activity and brightness of an AGN change over time with the transition between quiescent and active and vice versa can be surprisingly abrupt. On time scales of decades galaxies which before had appeared quiescent can later become AGN either because activity around the black hole ramped up or because previously obscuring material no longer blocks the galaxies emission etc.. Additionally sometimes you can get bursts of activity that can arise suddenly through processes like tidal disruption events or somthing iteracting with the accretion disk etc. which add another dimension to the picture. It is now generally thought galaxies transition between AGN phases slowly through feed back processes of cold gas that forms new stars and or falls into the central black hole driving supernovae and or AGN activity which heat the gas either disrupting it through ionizing radiation or through powerful outflows.

Because of this galaxies we typically define as AGN are those which were caught during a phase of sustained AGN activity over the timespan of observations as these more sustained pulses of activity spanning thousands to millions of years if not longer are more stable/persistent in their AGN properties and thus more readily observed and identified by astronomers.

In more recent years however I think it has become more apparent that AGN are really a phase within a larger feedback cycle that regulates galaxy star formation history and evolution as we have been able to observe rarer more transient events and activity.


As such I don't think you can define AGN activity by magnitude at least not for a typical AGN as the features which define AGN aren't directly relatable to brightness or luminosity. The question just doesn't make any sense.


The distinction between a Seyfert galaxy and a Quasar is subtle Quasars were a phenomenon more prevalent in the early universe as far as I can tell there really doesn't seem to be an absolute distinction rather a quasar galaxy is a galaxy where the light from the galaxy is outshined by the AGN meaning if you want to pick out the light of the galaxy you have to block out the AGN at the center similar to how you need to block out a star to view any of its exoplanets. In this sense you might be able to describe this in terms of the ratio of the galaxy's absolute magnitude and the AGN's magnitude with some ratio of the two serving to define when it has become a quasar but without a sensitive telescope capable of resolving the galaxy you are just going to image a very distant point-like light source.

Please try and reformat whatever questions you have into a more appropriate language I suggest looking up AGN or Active Galactic Nuclei to get a better feel for what they are and why your questions didn't make sense.

The goal here is to help you learn to ask the right questions
 
  • #3
1,515
177
Hmm I suggest you relook over these questions and what you want from us as there is a fair amount of nuance given the dynamic nature of Active Galactic Nuclei (AGN) and there are also still unresolved questions regarding how they form and evolve relative to their host galaxies.

In regarding what defines a galaxy as active nor not is based on whether we see clear AGN activity or not yes but this isn't somthing you would be able to see by eye. AGN are characterized spectrally through a broad category of characteristics that suggest material is infalling into an accretion disk around a compact object. What these are varies by perspective and structure of the AGN activity for example active Jets produce radio emission, reverse compton scattering of an accretion disk results in hard X-Ray signatures. In practice you aren't going to usually be able to tell the difference between an AGN and a quiescent(not passive) galaxy as the features which distinguish the characteristsics aren't somthing our eyes can pick up on so there is no point in even trying to define a magnitude.
In other words, many active galactic nuclei are dim in visible light but loud in radio or X-rays. Correct?
It may be tempting to think of AGN as intrinsically different and persistent features but evidence strongly suggests things aren't so simple.

First there is plenty of evidence of AGN variability as activity and brightness of an AGN change over time with the transition between quiescent and active and vice versa can be surprisingly abrupt. On time scales of decades galaxies which before had appeared quiescent can later become AGN either because activity around the black hole ramped up or because previously obscuring material no longer blocks the galaxies emission etc.. Additionally sometimes you can get bursts of activity that can arise suddenly through processes like tidal disruption events or somthing iteracting with the accretion disk etc. which add another dimension to the picture. It is now generally thought galaxies transition between AGN phases slowly through feed back processes of cold gas that forms new stars and or falls into the central black hole driving supernovae and or AGN activity which heat the gas either disrupting it through ionizing radiation or through powerful outflows.

Because of this galaxies we typically define as AGN are those which were caught during a phase of sustained AGN activity over the timespan of observations as these more sustained pulses of activity spanning thousands to millions of years if not longer are more stable/persistent in their AGN properties and thus more readily observed and identified by astronomers.

In more recent years however I think it has become more apparent that AGN are really a phase within a larger feedback cycle that regulates galaxy star formation history and evolution as we have been able to observe rarer more transient events and activity.


As such I don't think you can define AGN activity by magnitude at least not for a typical AGN as the features which define AGN aren't directly relatable to brightness or luminosity. The question just doesn't make any sense.
Ask it then this way: what features define a galactic nucleus as "quiescent" rather than "active"? Assuming that the nucleus is not silent - Sagittarius A is one of the loud radio sources in sky, so what criteria define Sagittarius A as "quiescent" rather than "active"?
Please try and reformat whatever questions you have into a more appropriate language I suggest looking up AGN or Active Galactic Nuclei to get a better feel for what they are and why your questions didn't make sense.

The goal here is to help you learn to ask the right questions
I looked up, and I did not find AGN discussed in context of the "quiescent" Sagittarius A. Which is why I am asking.
 

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