4 ms·

So I have a question about that.. I thought it would only have an accretion disk if it was active. So what gives? I guess I'm wrong? What constitutes "active" i

So I have a question about that.. I thought it would only have an accretion disk if it was active. So what gives? I guess I'm wrong? What constitutes "active" if not "stuff is falling in"?

raattgift 4y ago
Unexpected question from someone whose nickname starts with "quasar"! :-) Active in the sense of (from <https://en.wikipedia.org/wiki/Active_galactic_nucleus https://en.wikipedia.org/wiki/Active_galactic_nucleus>) "much higher than normal ... excess non-stellar emission" from the region very close to the central black hole. Our central parsec is simply dim in practically every wavelength compared to its enclosing central bulge. An active galactic nucleus in our galaxy would be very noticeable to the naked human eye: it would "light up" (with far ultraviolet to gamma radiation) lots of interstellar material that, having been heated, would glow very brightly in the reds and oranges. The luminosity of AGNs and quasars (especially luminous AGNs) is mostly from matter-matter collisions between gas and dust on intersecting geodesics (a fancy sort of friction that becomes enormous in the material nearest the black hole), with a contribution from daughter products of those collisions. There isn't much matter circulating close to Sgr A*. It's "starving". In the panel they tried to put it in terms of a human diet: if you were eating like Sgr A* is, scaled down to human size, you'd be eating a grain of rice every few million years. Good luck keeping your body heat up on that diet. :-) In the past -- tens of thousands to many millions of years ago -- there might have been a lot of matter circulating around Sgr A*, being swept up into luminous jets, which generated the Fermi bubbles <https://en.wikipedia.org/wiki/Galactic_Center#Gamma-_and_X-ray_emitting_Fermi_bubbles https://en.wikipedia.org/wiki/Galactic_Center#Gamma-_and_X-r...>. That's an area of current research: do central black holes eventually go radio-quiet in general? Is there a active-quiet-active-quiet cycle in central black holes in general? Or is this all an ultraviolet herring with the Fermi bubbles being produced by some mechanism that doesn't involve (or only very weakly depends upon) Sgr A*? ETA: I forgot to re-read my own (grandparent) comment. The "luminous structures nearby" are not just an accretion disc. Jets count, too. Also anything that they strike (molecular gas clouds, for instance) will tend to become luminous in some part of the spectrum (sometimes this results in "frustrated lobes", sometimes "light echoes" or ionization echoes from flares).
- quasarj 4y ago
  Thanks for the reply! And yes, given my name I thought I should know more about it. Especially since this was not in line with my previous understanding.

So I have a question about that.. I thought it would only have an accretion disk if it was active. So what gives? I guess I'm wrong? What constitutes "active" if not "stuff is falling in"?

raattgift 4y ago

Unexpected question from someone whose nickname starts with "quasar"! :-) Active in the sense of (from <https://en.wikipedia.org/wiki/Active_galactic_nucleus https://en.wikipedia.org/wiki/Active_galactic_nucleus>) "much higher than normal ... excess non-stellar emission" from the region very close to the central black hole. Our central parsec is simply dim in practically every wavelength compared to its enclosing central bulge. An active galactic nucleus in our galaxy would be very noticeable to the naked human eye: it would "light up" (with far ultraviolet to gamma radiation) lots of interstellar material that, having been heated, would glow very brightly in the reds and oranges. The luminosity of AGNs and quasars (especially luminous AGNs) is mostly from matter-matter collisions between gas and dust on intersecting geodesics (a fancy sort of friction that becomes enormous in the material nearest the black hole), with a contribution from daughter products of those collisions. There isn't much matter circulating close to Sgr A*. It's "starving". In the panel they tried to put it in terms of a human diet: if you were eating like Sgr A* is, scaled down to human size, you'd be eating a grain of rice every few million years. Good luck keeping your body heat up on that diet. :-) In the past -- tens of thousands to many millions of years ago -- there might have been a lot of matter circulating around Sgr A*, being swept up into luminous jets, which generated the Fermi bubbles <https://en.wikipedia.org/wiki/Galactic_Center#Gamma-_and_X-ray_emitting_Fermi_bubbles https://en.wikipedia.org/wiki/Galactic_Center#Gamma-_and_X-r...>. That's an area of current research: do central black holes eventually go radio-quiet in general? Is there a active-quiet-active-quiet cycle in central black holes in general? Or is this all an ultraviolet herring with the Fermi bubbles being produced by some mechanism that doesn't involve (or only very weakly depends upon) Sgr A*? ETA: I forgot to re-read my own (grandparent) comment. The "luminous structures nearby" are not just an accretion disc. Jets count, too. Also anything that they strike (molecular gas clouds, for instance) will tend to become luminous in some part of the spectrum (sometimes this results in "frustrated lobes", sometimes "light echoes" or ionization echoes from flares).

quasarj 4y ago
Thanks for the reply! And yes, given my name I thought I should know more about it. Especially since this was not in line with my previous understanding.