<p>The space density of ultraluminous quasars (those with luminosities greater than <span class="math-container">$5\times 10^{13}L_\odot$</span>) in the local universe is <span class="math-container">$\sim 10^{-9}$</span> (Mpc)<span class="math-container">$^{-3}$</span> (see for example <a href="https://arxiv.org/abs/1411.3719" rel="nofollow noreferrer">Caplar et al. (2015)</a> and the plot below).</p>
<p><a href="https://i.sstatic.net/l8O5a.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/l8O5a.jpg" alt="Space density of quasars vs redshift" /></a></p>
<p>The local Hubble volume of <span class="math-container">$(c/H_0)^3$</span> is <span class="math-container">$\sim 8 \times 10^{10}$</span> (Mpc)^3, so there would only be <span class="math-container">$\sim 100$</span> ultraluminous quasars in the local Hubble volume.</p>
<p>However, there is a population of <em>even</em> more luminous quasars in the distant universe, at least to <span class="math-container">$\sim 10^{16}L_\odot$</span> (see the plot below from <a href="https://academic.oup.com/mnras/article/495/3/3252/5841290" rel="nofollow noreferrer">Shen et al. 2020</a>). There is ONE rare example at <span class="math-container">$z&lt;1$</span>, which is what you are looking for. The quasar SMSS J114447.77-430859.3 at <span class="math-container">$z=0.83$</span> (just about in the local Hubble volume) has a luminosity of <span class="math-container">$\sim 2\times 10^{14}L_\odot$</span> and is claimed to be &quot;<em>the most luminous QSO in the last 9 Gyr</em>&quot; <a href="https://academic.oup.com/mnras/article/522/4/5217/7100972" rel="nofollow noreferrer">Kammoun et al. 2023</a>). This fits the bill if it is long-lived enough.</p>
<p><a href="https://i.sstatic.net/XG9hi.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/XG9hi.jpg" alt="Quasar luminosity functions at various redshifts" /></a></p>

CC BY-SA 4.0
2023-08-04T16:27:31.120Z
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
FrogOfJuly
CC BY-SA 4.0
2023-08-04T17:03:42.913Z
@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 $z<0.9$.
ProfRob
CC BY-SA 4.0
2023-08-04T15:55:01.590Z
2023-08-04T17:01:39.520Z
2023-08-04T17:01:39.520Z
ProfRob
<p>There are some milestone objects on different scales of the universe.</p>
<ul>
<li>The star is a notable feature on a solar system scale</li>
<li>The supermassive black hole is a feature on a scale of a galaxy</li>
<li>??</li>
</ul>
<p>The tricky part is that I am looking for something localizable. Great attractors, galactic filaments, and <a href="https://en.wikipedia.org/wiki/List_of_largest_cosmic_structures" rel="nofollow noreferrer">other easily findable objects</a> are features alright, but they are, unsurprisingly, very big and hardly bounded.</p>
<p>Are there any rare objects that appear only on a <a href="https://en.wikipedia.org/wiki/Hubble_volume" rel="nofollow noreferrer">Hubble volume</a> scale (a proper volume of <span class="math-container">$\sim (c/H_0)^3$</span>), but have definable boundaries?</p>
<p>EDIT: The lifetime of these objects should be at least millions of years.</p>
<p>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.</p>
<p>I think that as long as an object is more frequent than a quasar it is too common.</p>

CC BY-SA 4.0
2023-08-04T15:37:24.513Z
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.
ProfRob
CC BY-SA 4.0
2023-08-04T15:50:32.290Z
@ProfRob, how does one distinguish between ultraluminous quasars and the usual ones? Is there a gap between them, or is it just the top N quasars by luminosity?
FrogOfJuly
CC BY-SA 4.0
2023-08-04T15:56:11.377Z
No gap, just a diminishing number of more and more luminous objects. The most luminous example was published just this month,
ProfRob
CC BY-SA 4.0
2023-08-02T16:49:09.153Z
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"?
James K
CC BY-SA 4.0
2023-08-02T16:50:37.847Z
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.
James K
CC BY-SA 4.0
2023-08-02T16:57:58.760Z
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.
Sten
CC BY-SA 4.0
2023-08-02T17:09:51.420Z
@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.
FrogOfJuly
<universe><observable-universe>
What natural astronomical object is rare on a Hubble volume scale?
FrogOfJuly
# What natural astronomical object is rare on a Hubble volume scale?

(Asked by: username_0 on 2023-08-02)

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](https://en.wikipedia.org/wiki/List_of_largest_cosmic_structures) are features alright, but they are, unsurprisingly, very big and hardly bounded.

Are there any rare objects that appear only on a [Hubble volume](https://en.wikipedia.org/wiki/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 @username_1 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.

**Tags:** universe, observable-universe

> **Comments:**
> **username_2**: 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"?  
> **username_2**: 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.  
> **username_3**: 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.  
> **username_0**: @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.  
> **username_1**: 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.  
> **username_0**: @username_1, how does one distinguish between ultraluminous quasars and the usual ones? Is there a gap between them, or is it just the top N quasars by luminosity?  
> **username_1**: No gap, just a diminishing number of more and more luminous objects. The most luminous example was published just this month,  

---

## 1 answers

### Answer by username_1 on 2023-08-04. Score: 2 | Accepted answer

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)](https://arxiv.org/abs/1411.3719) and the plot below).

[![Space density of quasars vs redshift](https://i.sstatic.net/l8O5a.jpg)](https://i.sstatic.net/l8O5a.jpg)

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](https://academic.oup.com/mnras/article/495/3/3252/5841290)). 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](https://academic.oup.com/mnras/article/522/4/5217/7100972)). This fits the bill if it is long-lived enough.

[![Quasar luminosity functions at various redshifts](https://i.sstatic.net/XG9hi.jpg)](https://i.sstatic.net/XG9hi.jpg)

> **Comments:**
> **username_0**: 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  
> **username_1**: @username_0 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 $z<0.9$.  

---

