<p>That depends on how accurate you want your answer.</p>
<p>The reason is that the angle <span class="math-container">$\theta$</span> spanned by a length <span class="math-container">$L = 1\,\mathrm{Mpc}$</span> depends on the distance <span class="math-container">$d$</span> of that length — in comoving coordinates, <span class="math-container">$\theta$</span> keeps decreasing with <span class="math-container">$d$</span>, just like a normal item, say a bicycle, looks smaller the farther away it is (curiously, in <em>physical</em> coordinates, this is not the case. Due to the finite speed of light and the expansion of the Universe, galaxies only look smaller out a certain distance, after which they start to look larger).</p>
<p>An <span class="math-container">$(L = 1\,\mathrm{Mpc})^2$</span> square spans <span class="math-container">$\theta_2 = 39''$</span> at a redshift of <span class="math-container">$z=2$</span>, and <span class="math-container">$\theta_3 = 32''$</span> at <span class="math-container">$z=3$</span>. The comoving distances are <span class="math-container">$d_2 = 5.3\,\mathrm{Gpc}$</span> and <span class="math-container">$d_3 = 6.5\,\mathrm{Gpc}$</span>, respectively.</p>
<p>So yes, <em>roughly</em> you can say that the comoving volume of an <span class="math-container">$A = 1\,\mathrm{Mpc}^2$</span> square between <span class="math-container">$z=2$</span> and <span class="math-container">$z=3$</span> is <span class="math-container">$V = A(d_3-d_2)$</span>. But read on.</p>
<h3>The calculation</h3>
<p>You mention a survey, so I assume you're not actually given an area, but a field of view (FOV). The problem is the same, just the other way; your FOV doesn't cover the same area at different redshifts.</p>
<p>So, here's the rigorous way to calculate it:</p>
<p>Let's say your FOV spans a solid angle <span class="math-container">$\Omega=\theta_\mathrm{RA}\times\theta_\mathrm{dec}$</span>, which measured in radians comprises a fraction <span class="math-container">$\Omega/4\pi$</span> of the whole sphere. The total, comoving volume to a comoving distance <span class="math-container">$d$</span> is just <span class="math-container">$V = 4\pi d^3/3$</span>, so the volume of the shell between <span class="math-container">$z=2$</span> and <span class="math-container">$z=3$</span> is
<span class="math-container">$$
V_\mathrm{2\rightarrow3} = V_3 - V_2 = \frac{4\pi}{3} \big(d_3^3 - d_2^3\big).
$$</span></p>
<p>The comoving volume spanned by your FOV is thus
<span class="math-container">$$
V = \frac{\Omega}{4\pi} V_\mathrm{2\rightarrow3},
$$</span>
or
<span class="math-container">$$
V = \frac{\Omega}{3} \big(d_3^3 - d_2^3\big).
$$</span></p>
<p><a href="https://i.sstatic.net/8vfQW.png" rel="noreferrer"><img src="https://i.sstatic.net/8vfQW.png" alt="comvol" /></a></p>
<h3>The error</h3>
<p>The difference between the two approaches increases with the difference between the two redshifts. With a FOV of, say, <span class="math-container">$\Omega = (\theta=32'')^2 = 1024\,\mathrm{arcsec}^2=2.4\times10^{-8}\,\mathrm{sr}$</span>, if you say that it spans an area of <span class="math-container">$A = 1\,\mathrm{Mpc}^2$</span> (which is only correct at <span class="math-container">$z=3$</span>), you'll get
<span class="math-container">$$
V_\mathrm{approx.} = 1\,\mathrm{Mpc}^2 \times (6508 - 5312)\,\mathrm{Mpc} = 1198\,\mathrm{Mpc}^3,
$$</span>
whereas with the correct formula you get
<span class="math-container">$$
V_\mathrm{true} = \frac{2.4\times10^{-8}}{3} \left( 6508^3 - 5312^3\right)\,\mathrm{Mpc}^3 = 1011\,\mathrm{Mpc}^3,
$$</span>
that is, 16% percent lower.</p>
<p>If, on the other hand, your FOV is <span class="math-container">$\Omega = (\mathbf{39}'')^2$</span>, and if you say that it spans an area of <span class="math-container">$A = 1\,\mathrm{Mpc}^2$</span> (which is only correct at <span class="math-container">$z=\mathbf{2}$</span>), then the true volume would be 1500 Mpc, i.e. 25% larger than your ~1200 Mpc.</p>
<p>Since the correct calculation isn't much harder than the approximation, I suggest you stick to the correct one.</p>
<h3>The Python way</h3>
<p>With the <code>astropy</code> module, just type</p>
<pre><code>from astropy.cosmology import Planck15
from astropy import units as u
theta_RA  = 32 * u.arcsec
theta_dec = 32 * u.arcsec
Omega     = (theta_RA * theta_dec).to(u.steradian).value # get rid of unit
d2        = Planck15.comoving_distance(2)
d3        = Planck15.comoving_distance(3)
V         = Omega/3 * (d3**3 - d2**3)
print(V)

1011.0148201494444 Mpc3
</code></pre>
<p>or</p>
<pre><code>V23  = Planck15.comoving_volume(3) - Planck15.comoving_volume(2)
V    = Omega/(4*pi) * V23
</code></pre>
<p>which gives the same result.</p>

CC BY-SA 4.0
2021-06-16T14:09:42.300Z
+1 for great explanation and showing the coding option!
Daddy Kropotkin
CC BY-SA 4.0
2021-06-16T15:42:05.783Z
Thanks for the detailed explanation!
Framazu
CC BY-SA 4.0
2021-06-16T19:33:34.647Z
@Framazu You're welcome :)
pela
CC BY-SA 4.0
2021-06-16T13:25:10.703Z
2021-06-16T15:11:03.130Z
2021-06-16T15:11:03.130Z
pela
<p>Suppose I have data from an astronomical survey at redshifts in the range <span class="math-container">$z = [2,3]$</span>. Suppose that, on average in this range, the data covers an area on the sky of <span class="math-container">$A=1$</span> <span class="math-container">$\mathrm{Mpc}^2$</span>. How would I calculate the comoving volume covered by the survey? Is it as simple as calculating the comoving distance from <span class="math-container">$z=2$</span> to <span class="math-container">$z=3$</span> and multiplying by the average area? That is: <span class="math-container">$V_C = A(D_C(3)-D_C(2))$</span>?</p>

<observational-astronomy><cosmology><distances><redshift>
Comoving Volume Calculation
Framazu
# Comoving Volume Calculation

(Asked by: username_0 on 2021-06-16)

Suppose I have data from an astronomical survey at redshifts in the range $z = [2,3]$. Suppose that, on average in this range, the data covers an area on the sky of $A=1$ $\mathrm{Mpc}^2$. How would I calculate the comoving volume covered by the survey? Is it as simple as calculating the comoving distance from $z=2$ to $z=3$ and multiplying by the average area? That is: $V\_C = A(D\_C(3)-D\_C(2))$?

**Tags:** observational-astronomy, cosmology, distances, redshift

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## 1 answers

### Answer by username_1 on 2021-06-16. Score: 8 | Accepted answer

That depends on how accurate you want your answer.
The reason is that the angle $\theta$ spanned by a length $L = 1\,\mathrm{Mpc}$ depends on the distance $d$ of that length — in comoving coordinates, $\theta$ keeps decreasing with $d$, just like a normal item, say a bicycle, looks smaller the farther away it is (curiously, in physical coordinates, this is not the case. Due to the finite speed of light and the expansion of the Universe, galaxies only look smaller out a certain distance, after which they start to look larger).
An $(L = 1\,\mathrm{Mpc})^2$ square spans $\theta_2 = 39''$ at a redshift of $z=2$, and $\theta_3 = 32''$ at $z=3$. The comoving distances are $d_2 = 5.3\,\mathrm{Gpc}$ and $d_3 = 6.5\,\mathrm{Gpc}$, respectively.
So yes, roughly you can say that the comoving volume of an $A = 1\,\mathrm{Mpc}^2$ square between $z=2$ and $z=3$ is $V = A(d_3-d_2)$. But read on.
The calculation
You mention a survey, so I assume you're not actually given an area, but a field of view (FOV). The problem is the same, just the other way; your FOV doesn't cover the same area at different redshifts.
So, here's the rigorous way to calculate it:
Let's say your FOV spans a solid angle $\Omega=\theta_\mathrm{RA}\times\theta_\mathrm{dec}$, which measured in radians comprises a fraction $\Omega/4\pi$ of the whole sphere. The total, comoving volume to a comoving distance $d$ is just $V = 4\pi d^3/3$, so the volume of the shell between $z=2$ and $z=3$ is
$$
V_\mathrm{2\rightarrow3} = V_3 - V_2 = \frac{4\pi}{3} \big(d_3^3 - d_2^3\big).
$$
The comoving volume spanned by your FOV is thus
$$
V = \frac{\Omega}{4\pi} V_\mathrm{2\rightarrow3},
$$
or
$$
V = \frac{\Omega}{3} \big(d_3^3 - d_2^3\big).
$$

The error
The difference between the two approaches increases with the difference between the two redshifts. With a FOV of, say, $\Omega = (\theta=32'')^2 = 1024\,\mathrm{arcsec}^2=2.4\times10^{-8}\,\mathrm{sr}$, if you say that it spans an area of $A = 1\,\mathrm{Mpc}^2$ (which is only correct at $z=3$), you'll get
$$
V_\mathrm{approx.} = 1\,\mathrm{Mpc}^2 \times (6508 - 5312)\,\mathrm{Mpc} = 1198\,\mathrm{Mpc}^3,
$$
whereas with the correct formula you get
$$
V_\mathrm{true} = \frac{2.4\times10^{-8}}{3} \left( 6508^3 - 5312^3\right)\,\mathrm{Mpc}^3 = 1011\,\mathrm{Mpc}^3,
$$
that is, 16% percent lower.
If, on the other hand, your FOV is $\Omega = (\mathbf{39}'')^2$, and if you say that it spans an area of $A = 1\,\mathrm{Mpc}^2$ (which is only correct at $z=\mathbf{2}$), then the true volume would be 1500 Mpc, i.e. 25% larger than your ~1200 Mpc.
Since the correct calculation isn't much harder than the approximation, I suggest you stick to the correct one.
The Python way
With the astropy module, just type
from astropy.cosmology import Planck15
from astropy import units as u
theta_RA  = 32 * u.arcsec
theta_dec = 32 * u.arcsec
Omega     = (theta_RA * theta_dec).to(u.steradian).value # get rid of unit
d2        = Planck15.comoving_distance(2)
d3        = Planck15.comoving_distance(3)
V         = Omega/3 * (d3**3 - d2**3)
print(V)

1011.0148201494444 Mpc3

or
V23  = Planck15.comoving_volume(3) - Planck15.comoving_volume(2)
V    = Omega/(4*pi) * V23

which gives the same result.

> **Comments:**
> **username_2**: +1 for great explanation and showing the coding option!  
> **username_0**: Thanks for the detailed explanation!  
> **username_1**: @Framazu You're welcome :)  

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