<p>I was recently doing a project on calculating ellipticities of galaxies using SDSS data. I downloaded the stokes parameter values (q and u) from data</p>

<p><span class="math-container">$e  = 1 - \frac{b}{a} = 1 - \frac{1 - \sqrt{Q^2 + U^2}}{1 + \sqrt{Q^2 + U^2}} $</span>
 and for few values of stokes parameters, the value of ellipticity goes above 1.
<a href="https://i.sstatic.net/L7sGr.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/L7sGr.png" alt="enter image description here"></a></p>

<p>What could this possibly mean? Could this mean the data of random galaxies I've taken has irregular agalaxies in it and that's the reason for ellipticity being greater than 1? I would like to hear your opinions and answers.</p>

CC BY-SA 4.0
2019-08-18T21:40:15.927Z
@mistertribs When it comes to galaxy morphology, ["Stokes parameters" has a different meaning](http://www.sdss3.org/dr9/algorithms/classify.php#photo_stokes) than the typical one.
HDE 226868
CC BY-SA 4.0
2019-08-18T23:31:23.210Z
The ellipticity cannot be greater than 1, for any kind of galaxy. (Or equal to 1, really, as that would require an infinitely thin disk.)
Peter Erwin
CC BY-SA 4.0
2019-08-19T19:50:33.247Z
It's not the ellipticity in your data that's greater than 1, it's the raw Stokes parameter, which is allowed to be greater than one, or less than zero. Pop that into the equation and you'll see that you cannot get out a number greater than 1.
Ingolifs
CC BY-SA 4.0
2019-08-20T02:40:17.750Z
@Ingolifs I did try inputting the values into the equation.Ellipiticity turns out to be greater than one.
Krishna K
CC BY-SA 4.0
2019-08-20T02:44:39.810Z
@PeterErwin I totally agree with you. But what could be the reason for ellipiticty to be like this. I checked calculations and my prodecure many times. Shall I just ignore these values and continue uing rest of the values in doing the project?
Krishna K
CC BY-SA 4.0
2019-08-20T15:13:17.393Z
@SaikrishnaK My guess would be that some of the "Stokes parameter" values are bad (in the link in HDE 22686's comment, there is mention of the Stokes parameters not being very reliable when the S/N is low). E.g., the "q_r" value in the line you highlighted. So I'd just ignore those sources.
Peter Erwin
<observational-astronomy><galaxy><astrophysics><galaxy-cluster>
What is the meaning of ellipticity of a galaxy being more than 1?
Krishna K
# What is the meaning of ellipticity of a galaxy being more than 1?

(Asked by: username_0 on 2019-08-18)

I was recently doing a project on calculating ellipticities of galaxies using SDSS data. I downloaded the stokes parameter values (q and u) from data

$e = 1 - \frac{b}{a} = 1 - \frac{1 - \sqrt{Q^2 + U^2}}{1 + \sqrt{Q^2 + U^2}} $
and for few values of stokes parameters, the value of ellipticity goes above 1.
[![enter image description here](https://i.sstatic.net/L7sGr.png)](https://i.sstatic.net/L7sGr.png)

What could this possibly mean? Could this mean the data of random galaxies I've taken has irregular agalaxies in it and that's the reason for ellipticity being greater than 1? I would like to hear your opinions and answers.

**Tags:** observational-astronomy, galaxy, astrophysics, galaxy-cluster

> **Comments:**
> **username_1**: @mistertribs When it comes to galaxy morphology, ["Stokes parameters" has a different meaning](http://www.sdss3.org/dr9/algorithms/classify.php#photo_stokes) than the typical one.  
> **username_2**: The ellipticity cannot be greater than 1, for any kind of galaxy. (Or equal to 1, really, as that would require an infinitely thin disk.)  
> **username_3**: It's not the ellipticity in your data that's greater than 1, it's the raw Stokes parameter, which is allowed to be greater than one, or less than zero. Pop that into the equation and you'll see that you cannot get out a number greater than 1.  
> **username_0**: @username_3 I did try inputting the values into the equation.Ellipiticity turns out to be greater than one.  
> **username_0**: @PeterErwin I totally agree with you. But what could be the reason for ellipiticty to be like this. I checked calculations and my prodecure many times. Shall I just ignore these values and continue uing rest of the values in doing the project?  
> **username_2**: @SaikrishnaK My guess would be that some of the "Stokes parameter" values are bad (in the link in HDE 22686's comment, there is mention of the Stokes parameters not being very reliable when the S/N is low). E.g., the "q_r" value in the line you highlighted. So I'd just ignore those sources.  

---

## 0 answers

