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Thread: Where have all the quasars gone?

  1. #1
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    Where have all the quasars gone?

    As far as I know, there are no quasars within half a lighteon of the Milky Way, although there are millions in the observable universe. Why did they go out of style? Did the SMBHs "eat" all the available matter near them? Did the SMBHs get too big for the light of the accretion disk to escape them? What happened?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

  2. #2
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    What's a lighteon?

    Part of it's a sampling problem. Quasars are rare, so the chance of one appearing in the small volume of space we count as being "nearby" is small.
    And part of it's the fact that there's a characteristic epoch for Active Galactic Nuclei--it takes a while for them to turn on after galaxies start forming, and then they use up their fuel and turn off again. The peak density of AGNs per unit volume of space therefore occurs about a cosmological redshift of 2 or 3, IIRC.

    Grant Hutchison

  3. #3
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    Quote Originally Posted by grant hutchison View Post
    What's a lighteon?
    Took me a sec but I assume it's 109 lightyears.
    Last edited by PetersCreek; 2020-Aug-06 at 09:29 PM. Reason: exponent oops
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  4. #4
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    Quote Originally Posted by PetersCreek View Post
    Took me a sec but I assume it's 109 lightyears.
    Thanks. Google doesn't seem to recognize it as a thing.

    Grant Hutchison

  5. #5
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    It is possible that M87's core might look like a blazar to us if it were aimed at us. Is it a quasar? Maybe a very dim one, but it is close.
    Forming opinions as we speak

  6. #6
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    Both effects are seen - rarity for luminous AGN, and actual evolution in the volume density of powerful AGN (in comoving coordinates, that is, factoring out cosmic expansion to compare fractions of galaxies). This had been a major focus of studies trying to make sure they have known incompleteness across a large redshift range so we can understand what really changes. Handwavily, we see an AGN when a supermassive black hole is accreting, so running out of surrounding gas will shut it down. Paradoxically, once one reaches a high enough mass, it may also be harder for it to show the accretion signatures - instead of tidally disrupting most kinds of stars and giving accretion fireworks, the star slips past the event horizon before being tidally shredded. Lower-luminosity AGN change their space density less than the most luminous ones, but what that translates to in whether they are in comparable galaxies over cosmic time is much less clear.

    I hasten to add that everything is more complicated - AGN evidently turn on and off a lot, so instead of "luminous AGN" we might more precisely think "AGN spending a larger fraction of its time in a luminous phase" or something. And galaxies may accrete more gas across cosmic time than we used to think, so comparing the host galaxies to isolate a single outcome is a slippery thing.

  7. #7
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    What is the definition of a "quasar"?

  8. #8
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    It's whatever the writer decides to call a "quasar", I'm afraid. There's no official Handbook of Astronomical Terminology which defines it. In general, people tend to use "quasar" to refer to the most luminous AGN, but as far as I know, there's no set limit in luminosity.

  9. #9
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    Without expansion, the volume within 1 Gly of Sun is 1*4/3*pi cubic Gly, and volume within 13 Gly of Sun would be 13ˇ3=2197*4/3*pi Gly.
    With expansion, what would be the volume ratio?

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