<p>Regarding your original question about the "correct value for ellipticity", keep in mind that a galaxy has no objective ellipticity. The inserted image nicely shows that the light distribution of a real galaxy is way more complicated than a smooth profile with isophotes of identical ellipticity. In other words, the ellipticity will in fact be defined through the measurement process.</p>

<p>Flux-weighted second moments (which use the <em>pixel brightness</em> as weighting function) or adaptive moments (which use an elliptical <em>Gaussian</em> weighting function) are two ways of doing this. And there might be nothing wrong with the observed difference in ellipticity measurements, as the galaxy light profile is very different from a Gaussian!</p>

<p>The flux-weighted measurement is more sensitive to the ellipticity at inner radii, while the adaptive moments give more weight to the disk (assuming that the algorithms converge well).</p>

CC BY-SA 3.0
2016-02-16T12:29:10.560Z
2016-02-16T12:54:53.047Z
2016-02-16T12:54:53.047Z
mtewes
<p>I recommend using the axis ratio of the exponential fit, <code>expAB_r</code>.
You may want to check out:
<a href="https://www.sdss.org/dr12/algorithms/magnitudes/" rel="nofollow noreferrer">https://www.sdss.org/dr12/algorithms/magnitudes/</a>
<a href="http://egg.astro.cornell.edu/alfalfa/grads/hunt12/luke120626.html" rel="nofollow noreferrer">http://egg.astro.cornell.edu/alfalfa/grads/hunt12/luke120626.html</a></p>
<p>On the other hand, to determine the ellipticity, you also need the Stokes V parameter, i.e.</p>
<p><span class="math-container">$\tan{2\chi} = \dfrac{V}{\sqrt{Q^2+U^2}}$</span></p>
<p>Check out: <a href="https://en.wikipedia.org/wiki/Stokes_parameters" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Stokes_parameters</a></p>
<p>Unfortunately, the Stokes V parameter is nowhere to be found in SDSS. I understand that the ellipticity equation was derived from the information in the official website. However, I am not so sure how exactly are Q and U determined. By visual inspection, the ellipticity is indeed close to 0.5.</p>

CC BY-SA 4.0
2022-12-18T07:46:46.547Z
2022-12-18T07:46:46.547Z
cssstudent
<p>What is the best approach to determine the ellipticity of galaxies in the SDSS DR12. I have read this <a href="http://www.sdss.org/dr12/algorithms/classify/" rel="nofollow noreferrer">page</a>. Are those really good methods? </p>

<p>Do flux-weighted second moments (as given in the stokes parameters) really give the correct value for ellipticity? </p>

<p>Also I am not sure what table field they actually refer too (in the <a href="http://skyserver.sdss.org/dr12/en/help/browser/browser.aspx#&amp;&amp;history=description+Galaxy+V" rel="nofollow noreferrer">Galaxy</a> view) when talking about the method using the Adaptive Moments. For example where can I find the <code>m_rr_cc</code> value for the method described <a href="http://www.sdss.org/dr12/algorithms/classify/#photo_adaptive" rel="nofollow noreferrer">here</a>?</p>

<p><strong>Update</strong></p>

<p>I have calculated the ellipticity of a spiral galaxiy with two different methods using data from SDSS DR7. First I used the method using the stokes parameters and second I used the values for isoA and isoB. Both calculations where done in the <em>r-band</em>. </p>

<p>The data can be obtained with the following CAS Query:</p>

<pre><code>SELECT
    g.objid as objId,
    /*  stoke parameters*/
    q_r,u_r,
     /*  iso_a, iso_b */
    isoA_r,isoB_r
FROM Galaxy AS g
WHERE
    g.objid = 587722982832013381
</code></pre>

<p>yielding the result</p>

<pre><code>objId: 587722982832013381   
q_r : -0.1206308    
u_r: 0.002988584    
isoA_r: 169.3949    
isoB_r: 84.78848
</code></pre>

<p>The ellipticity by stoke parameters is given by</p>

<p>$$
e  = 1 - \frac{b}{a} = 1 - \frac{1 - \sqrt{Q^2 + U^2}}{1 + \sqrt{Q^2 + U^2}}.
$$</p>

<p>The ellipticity from isoA and isoB is very given by</p>

<p>$$
e  = 1 - \frac{isoB}{isoA}.
$$</p>

<p>But I get completely different results when I use isoA and isoB or the stoke parameters. I get:</p>

<pre><code>ellipticity from stoke parameters: 0.2153498356
ellipticity from 1-(isoA/isoB): 0.4994626166
</code></pre>

<p>The results can be reproduced with the following python code:</p>

<pre><code>import math

def ellipticityStokes(q, u):
    e = 1 - ((1-math.sqrt((q**2) + (u**2)))/(1+math.sqrt((q**2) + (u**2))))
    return e

def ellipticityNormal(a, b):
    e = 1 - (b/a)
    return e

q = -0.1206308
u = 0.002988584 

isoA = 169.3949  
isoB = 84.78848

print(str(ellipticityStokes(q,u))) #0.2153498356239003
print(str(ellipticityNormal(isoA,isoB))) #0.4994626166431221
</code></pre>

<p>Looking at the image</p>

<p><img src="https://i.sstatic.net/gD8HT.jpg" width="200" height="200"></p>

<p>(and compare with this <a href="https://ned.ipac.caltech.edu/level5/Sept11/Buta/Buta5.html" rel="nofollow noreferrer">website from Buta</a>) I would say the value 0.4994626166 (from isoA and isoB) makes more sense. The value calculated from the stokes parameters is just wrong I think. </p>

<p>What is the mistake here? I would expect both calculations to yield similar results.</p>

<p>However for newer data releases isoA and isoB are not available any more because they seem not to trust those value? What should I do with newer data releases as DR12?</p>

CC BY-SA 3.0
2016-02-15T14:56:21.447Z
Are the Stoke Parameters the Q and U values?
MichaelJRoberts
CC BY-SA 3.0
2016-02-15T19:22:31.463Z
Yes. I have linked [this page](http://www.sdss.org/dr12/algorithms/classify/) with a description. Go to the section with title 'Ellipticities'.
Sjoerd222888
CC BY-SA 3.0
2016-02-15T19:24:11.747Z
And what does your SQL query give for the Q and U values?
MichaelJRoberts
CC BY-SA 3.0
2016-02-15T19:26:50.767Z
I have added the CasJobs query result to the question.
Sjoerd222888
CC BY-SA 3.0
2016-02-15T20:36:50.610Z
I make e from the Stoke parameters as 0.7847. I think you forgot to take the 1 -
MichaelJRoberts
CC BY-SA 3.0
2016-02-15T20:40:51.867Z
Perhaps discuss this disparity with the SDSS Helpdesk...they might advise you on the most accurate parameterisation of the ellipticity.
MichaelJRoberts
CC BY-SA 3.0
2016-02-15T22:04:22.473Z
@MichaelJRoberts I have added some python3 code to reproduce the results. I think the numbers I got are correct and I did not forget to take a 1-.
Sjoerd222888
CC BY-SA 3.0
2016-02-15T22:10:57.943Z
You're quite right! Turns out I must have forgotten to take the 1 minus!
MichaelJRoberts
<galaxy><observational-astronomy>
How to determine the ellipticity of galaxies in SDSS
Sjoerd222888
# How to determine the ellipticity of galaxies in SDSS

(Asked by: username_0 on 2016-02-14)

What is the best approach to determine the ellipticity of galaxies in the SDSS DR12. I have read this [page](http://www.sdss.org/dr12/algorithms/classify/). Are those really good methods?

Do flux-weighted second moments (as given in the stokes parameters) really give the correct value for ellipticity?

Also I am not sure what table field they actually refer too (in the [Galaxy](http://skyserver.sdss.org/dr12/en/help/browser/browser.aspx#&&history=description+Galaxy+V) view) when talking about the method using the Adaptive Moments. For example where can I find the `m_rr_cc` value for the method described [here](http://www.sdss.org/dr12/algorithms/classify/#photo_adaptive)?

**Update**

I have calculated the ellipticity of a spiral galaxiy with two different methods using data from SDSS DR7. First I used the method using the stokes parameters and second I used the values for isoA and isoB. Both calculations where done in the *r-band*.

The data can be obtained with the following CAS Query:

```
SELECT
    g.objid as objId,
    /*  stoke parameters*/
    q_r,u_r,
     /*  iso_a, iso_b */
    isoA_r,isoB_r
FROM Galaxy AS g
WHERE
    g.objid = 587722982832013381

```

yielding the result

```
objId: 587722982832013381   
q_r : -0.1206308    
u_r: 0.002988584    
isoA_r: 169.3949    
isoB_r: 84.78848

```

The ellipticity by stoke parameters is given by

$$
e = 1 - \frac{b}{a} = 1 - \frac{1 - \sqrt{Q^2 + U^2}}{1 + \sqrt{Q^2 + U^2}}.
$$

The ellipticity from isoA and isoB is very given by

$$
e = 1 - \frac{isoB}{isoA}.
$$

But I get completely different results when I use isoA and isoB or the stoke parameters. I get:

```
ellipticity from stoke parameters: 0.2153498356
ellipticity from 1-(isoA/isoB): 0.4994626166

```

The results can be reproduced with the following python code:

```
import math

def ellipticityStokes(q, u):
    e = 1 - ((1-math.sqrt((q**2) + (u**2)))/(1+math.sqrt((q**2) + (u**2))))
    return e

def ellipticityNormal(a, b):
    e = 1 - (b/a)
    return e

q = -0.1206308
u = 0.002988584 

isoA = 169.3949  
isoB = 84.78848

print(str(ellipticityStokes(q,u))) #0.2153498356239003
print(str(ellipticityNormal(isoA,isoB))) #0.4994626166431221

```

Looking at the image

![](https://i.sstatic.net/gD8HT.jpg)

(and compare with this [website from Buta](https://ned.ipac.caltech.edu/level5/Sept11/Buta/Buta5.html)) I would say the value 0.4994626166 (from isoA and isoB) makes more sense. The value calculated from the stokes parameters is just wrong I think.

What is the mistake here? I would expect both calculations to yield similar results.

However for newer data releases isoA and isoB are not available any more because they seem not to trust those value? What should I do with newer data releases as DR12?

**Tags:** galaxy, observational-astronomy

> **Comments:**
> **username_1**: Are the Stoke Parameters the Q and U values?  
> **username_0**: Yes. I have linked [this page](http://www.sdss.org/dr12/algorithms/classify/) with a description. Go to the section with title 'Ellipticities'.  
> **username_1**: And what does your SQL query give for the Q and U values?  
> **username_0**: I have added the CasJobs query result to the question.  
> **username_1**: I make e from the Stoke parameters as 0.7847. I think you forgot to take the 1 -  
> **username_1**: Perhaps discuss this disparity with the SDSS Helpdesk...they might advise you on the most accurate parameterisation of the ellipticity.  
> **username_0**: @username_1 I have added some python3 code to reproduce the results. I think the numbers I got are correct and I did not forget to take a 1-.  
> **username_1**: You're quite right! Turns out I must have forgotten to take the 1 minus!  

---

## 2 answers

### Answer by username_2 on 2016-02-16. Score: 2

Regarding your original question about the "correct value for ellipticity", keep in mind that a galaxy has no objective ellipticity. The inserted image nicely shows that the light distribution of a real galaxy is way more complicated than a smooth profile with isophotes of identical ellipticity. In other words, the ellipticity will in fact be defined through the measurement process.

Flux-weighted second moments (which use the *pixel brightness* as weighting function) or adaptive moments (which use an elliptical *Gaussian* weighting function) are two ways of doing this. And there might be nothing wrong with the observed difference in ellipticity measurements, as the galaxy light profile is very different from a Gaussian!

The flux-weighted measurement is more sensitive to the ellipticity at inner radii, while the adaptive moments give more weight to the disk (assuming that the algorithms converge well).


---

### Answer by username_3 on 2022-12-18. Score: 0

I recommend using the axis ratio of the exponential fit, `expAB_r`.
You may want to check out:
<https://www.sdss.org/dr12/algorithms/magnitudes/>
<http://egg.astro.cornell.edu/alfalfa/grads/hunt12/luke120626.html>

On the other hand, to determine the ellipticity, you also need the Stokes V parameter, i.e.

$\tan{2\chi} = \dfrac{V}{\sqrt{Q^2+U^2}}$

Check out: <https://en.wikipedia.org/wiki/Stokes_parameters>

Unfortunately, the Stokes V parameter is nowhere to be found in SDSS. I understand that the ellipticity equation was derived from the information in the official website. However, I am not so sure how exactly are Q and U determined. By visual inspection, the ellipticity is indeed close to 0.5.


---

