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There are some milestone objects on different scales of the universe.

  • The star is a notable feature on a solar system scale
  • The supermassive black hole is a feature on a scale of a galaxy
  • ??

The tricky part is that I am looking for something localizable. Great attractors, galactic filaments, and other easily findable objects are features alright, but they are, unsurprisingly, very big and hardly bounded.

Are there any rare objects that appear only on a Hubble volume scale (a proper volume of $\sim (c/H_0)^3$), but have definable boundaries?

EDIT: The lifetime of these objects should be at least millions of years.

EDIT': Red spirals fit the kind of objects I am looking for, but unfortunately, they are not rare enough. As @ProfRob pointed out they are only two orders of magnitude less frequent than usual galaxies.

I think that as long as an object is more frequent than a quasar it is too common.

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    $\begingroup$ How rare? How long lasting? For example quasars are several orders of magnitude more rare than galaxies, yet there are still about a million of them known. They also are (on a galactic scale) short-lived, the black hole will be active for a while (and form a quasar) then quiet. So the Andromedeans may not be seeing the same quasars that we see, since they are a couple of million light years from us. Many rare objects are rare because they are short lived (and so only visible in a "light shell") Do these "count"? $\endgroup$
    – James K
    Aug 2, 2023 at 16:49
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    $\begingroup$ A nice example might by voørwerps. I think only seven are known, but again they are probably short-lived and only visible from particular perspectives. $\endgroup$
    – James K
    Aug 2, 2023 at 16:50
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    $\begingroup$ The two examples given (stars and SMBHs) represent the centers of highly inhomogeneous systems. The universe that we see is homogeneous, so we have no reason to think there's a center. There may be objects with number density of order $H^3$, but they won't be "milestones" in the same way. $\endgroup$
    – Sten
    Aug 2, 2023 at 16:57
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    $\begingroup$ @JamesK, quasars are a good fallback option, I thought that if nothing else would come up, large quasar groups are the objects I am looking for. They are localized and are relatively rare compared to galaxies. They live long enough for my purposes: millions of years are ok. $\endgroup$
    – FrogOfJuly
    Aug 2, 2023 at 17:09
  • $\begingroup$ Ultraluminous quasars are almost there at around 10 per Hubble volume in the present-day universe. Whether they "live" (as quasars) very long is another matter - presumably not, which is partly why they are rare. $\endgroup$
    – ProfRob
    Aug 4, 2023 at 15:37

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The space density of ultraluminous quasars (those with luminosities greater than $5\times 10^{13}L_\odot$) in the local universe is $\sim 10^{-9}$ (Mpc)$^{-3}$ (see for example Caplar et al. (2015) and the plot below).

Space density of quasars vs redshift

The local Hubble volume of $(c/H_0)^3$ is $\sim 8 \times 10^{10}$ (Mpc)^3, so there would only be $\sim 100$ ultraluminous quasars in the local Hubble volume.

However, there is a population of even more luminous quasars in the distant universe, at least to $\sim 10^{16}L_\odot$ (see the plot below from Shen et al. 2020). There is ONE rare example at $z<1$, which is what you are looking for. The quasar SMSS J114447.77-430859.3 at $z=0.83$ (just about in the local Hubble volume) has a luminosity of $\sim 2\times 10^{14}L_\odot$ and is claimed to be "the most luminous QSO in the last 9 Gyr" Kammoun et al. 2023). This fits the bill if it is long-lived enough.

Quasar luminosity functions at various redshifts

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  • $\begingroup$ Sounds very nice. Do you have a nice plot with the qusar luminocity distribution? I followed the linkes, but as I understand plots in these papers are much more specific $\endgroup$
    – FrogOfJuly
    Aug 4, 2023 at 16:27
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    $\begingroup$ @FrogOfJuly Pictures added. You can see what the estimated space density is at $z \sim 0$ and then in the second plot you can see that there are even higher luminosity quasars seen at high redshifts - where their comoving space density is higher - but there is AFAIK only one known quasar with $L > 5\times 10^{47}$ erg/s with $z<0.9$. $\endgroup$
    – ProfRob
    Aug 4, 2023 at 17:03

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