<p>You want to use NED (NASA Extragalactic Database) rather than SIMBAD, since the former has more galaxy- and cluster-specific information.</p>

<p>I'm going to assume you have a reasonably accurate position for your galaxy as well as its redshift (in km/s). The basic idea is to identify candidate galaxy clusters which are close enough to your galaxy on the sky <em>and</em> in redshift so that the galaxy could potentially be a member of one of them.</p>

<ol>
<li><p>Determine what angular radius $R$ corresponds to 1 or 2 megaparsecs at the redshift of your galaxy (e.g., using <a href="http://www.astro.ucla.edu/~wright/CosmoCalc.html" rel="nofollow">Ned Wright's Cosmology Calculator</a>)</p></li>
<li><p>Do a search in NED for Galaxy Clusters within <em>R</em> of the coordinates of your galaxy. If your galaxy has a name, you can skip the coordinate lookup and use the <a href="http://ned.ipac.caltech.edu/forms/nearname.html" rel="nofollow">"Near name" search form</a>; if you only have coordinates, there's a <a href="http://ned.ipac.caltech.edu/forms/nearposn.html" rel="nofollow">"Near position" search form</a>.</p>

<p>A. Put the estimated angular radius $R$ (converted to arc minutes) in the "Search Radius" box;</p>

<p>B. Set the "Selection in Redshift" form to "Between" and give it lower and upper limits of $V_{\rm gal} - 2000$ and $V_{\rm gal} + 2000$ in km/s, where $V_{\rm gal}$ is the redshift (in km/s) of your galaxy;</p>

<p>C. Under "Selection by Object Type", select "Galaxy Clusters" in the "Classified Extragalactic Objects" list.</p></li>
</ol>

<p>That should give you a list of galaxy clusters within the specified radius of your galaxy which are at a similar redshift. Note that the same cluster may come up multiple times under different names (i.e., from different cluster catalogs), but it should be clear from the coordinates and the redshifts which are the same clusters.</p>

<p>Now the question is whether your galaxy is plausibly a member of a given candidate cluster or not. Ideally you should find some measurement of each cluster's velocity dispersion $\sigma$; if the galaxy's redshift isn't within $\pm 3 \sigma$ of the cluster's redshift $V_{\rm cluster}$, then you can ignore that cluster, since it's very unlikely that a galaxy with that relative velocity with respect to the cluster is actually gravitationally bound to the cluster.</p>

<p>You can't really do better than a probabilistic assessment: roughly how likely it is that the galaxy is a member of a given cluster. Galaxies which are members of a cluster are more likely to be close to the center (on the sky); member galaxies closer to the center can have a higher range of relative velocities, while member galaxies further away should have velocities closer to the cluster redshift.</p>

<p>A quick example: Is NGC 4889 ($V_{\rm gal} = 6495$ km/s) a member of a cluster? At its redshift, 1 Mpc = 33 arc minutes; if we search on NED for galaxy clusters within 33 arcmin of NGC 4889 with redshift limits of 4500-8500 km/s, we get 6 results, 4 of which are just the Coma Cluster in different catalogs, with $V_{\rm cluster} \sim 6900$ km/s. Since the various distances ("Separ. arcmin") translate to $\sim$ 100-300 kpc from the cluster center, and since the Coma cluster has a velocity dispersion of $\sim$ 1000 km/s and the difference between NGC 4889 and the cluster is only $\sim$ 400 km/s, it's a very good bet that this galaxy is a member of the Coma Cluster. (It's actually one of the two cD galaxies in the cluster center.)</p>

CC BY-SA 3.0
2016-10-16T15:07:24.097Z
2016-10-16T16:15:40.000Z
2016-10-16T16:15:40.000Z
Peter Erwin
<p>First: You can use all galaxies in the line of sight of the cluster to construct their color-magnitude diagram. Those galaxies on the Red Sequence are more likely to be in the cluster (not always true).</p>

<p>Second: You should be able to determine their redshift, and compare with the mean cluster redshift. To be able to make cut, see next step.</p>

<p>Third: You should run some algorithms in order to see whether the redshifts/velocities you have are within the clusters mean velocity dispersion (usually in the clustercentric-velocity space), as used in these papers for example: 
<a href="https://arxiv.org/pdf/1601.06080.pdf" rel="nofollow">https://arxiv.org/pdf/1601.06080.pdf</a><br>
<a href="http://adsabs.harvard.edu/full/1996ApJ...473..670F" rel="nofollow">http://adsabs.harvard.edu/full/1996ApJ...473..670F</a></p>

CC BY-SA 3.0
2016-10-16T10:29:06.187Z
2016-10-16T10:29:06.187Z
Rob
<p>Your question is not specific, so I'm afraid the answer is general too.</p>

<p>Using Simbad/NED check to see what observations/survey data exists for you galaxies. Almost all large galaxy surveys, and a lot of smaller ones, have corresponding "environment catalogues" where the galaxies are associated to their corresponding group. By cross-matching the coordinates (ra,dec) and redshift (or v) of your galaxies with the members of groups and clusters in one or more of these catalogues you should be able to find a match.</p>

CC BY-SA 3.0
2016-08-25T01:45:31.710Z
2016-08-25T01:45:31.710Z
smashbro
<p>I am trying to find out how to find out if a certain galaxy is located inside a cluster. For the famous ones this is quite easy (sometimes even on wikipedia!), but I haven't been able to do it for some less-known galaxies. I have tried to look them up on Simbad, but I couldn't find it written down anywhere (even for the ones I know to be in clusters). Also generic "googling" has not helped me. Do you have any suggestions?</p>

CC BY-SA 3.0
2016-10-16T12:39:25.667Z
By "in" do you mean "currently present within the boundaries of" or "gravitationally bound to"? The former is fairly straightforward, the latter is more difficult since it requires a determination of the peculiar velocity of the galaxy with respect to the cluster and a mass estimate of the cluster.
Sean Lake
CC BY-SA 3.0
2016-10-16T16:40:20.487Z
Or do you just mean whether it's an optical coincidence??
Fattie
<galaxy><data-analysis><galaxy-cluster>
How to find out if a galaxy is in a specific cluster
Ahmed Mahroubi
# How to find out if a galaxy is in a specific cluster

(Asked by: username_0 on 2016-08-24)

I am trying to find out how to find out if a certain galaxy is located inside a cluster. For the famous ones this is quite easy (sometimes even on wikipedia!), but I haven't been able to do it for some less-known galaxies. I have tried to look them up on Simbad, but I couldn't find it written down anywhere (even for the ones I know to be in clusters). Also generic "googling" has not helped me. Do you have any suggestions?

**Tags:** galaxy, data-analysis, galaxy-cluster

> **Comments:**
> **username_1**: By "in" do you mean "currently present within the boundaries of" or "gravitationally bound to"? The former is fairly straightforward, the latter is more difficult since it requires a determination of the peculiar velocity of the galaxy with respect to the cluster and a mass estimate of the cluster.  
> **username_2**: Or do you just mean whether it's an optical coincidence??  

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## 3 answers

### Answer by username_3 on 2016-10-16. Score: 2

You want to use NED (NASA Extragalactic Database) rather than SIMBAD, since the former has more galaxy- and cluster-specific information.

I'm going to assume you have a reasonably accurate position for your galaxy as well as its redshift (in km/s). The basic idea is to identify candidate galaxy clusters which are close enough to your galaxy on the sky *and* in redshift so that the galaxy could potentially be a member of one of them.

1. Determine what angular radius $R$ corresponds to 1 or 2 megaparsecs at the redshift of your galaxy (e.g., using [Ned Wright's Cosmology Calculator](http://www.astro.ucla.edu/~wright/CosmoCalc.html))
2. Do a search in NED for Galaxy Clusters within *R* of the coordinates of your galaxy. If your galaxy has a name, you can skip the coordinate lookup and use the ["Near name" search form](http://ned.ipac.caltech.edu/forms/nearname.html); if you only have coordinates, there's a ["Near position" search form](http://ned.ipac.caltech.edu/forms/nearposn.html).

   A. Put the estimated angular radius $R$ (converted to arc minutes) in the "Search Radius" box;

   B. Set the "Selection in Redshift" form to "Between" and give it lower and upper limits of $V\_{\rm gal} - 2000$ and $V\_{\rm gal} + 2000$ in km/s, where $V\_{\rm gal}$ is the redshift (in km/s) of your galaxy;

   C. Under "Selection by Object Type", select "Galaxy Clusters" in the "Classified Extragalactic Objects" list.

That should give you a list of galaxy clusters within the specified radius of your galaxy which are at a similar redshift. Note that the same cluster may come up multiple times under different names (i.e., from different cluster catalogs), but it should be clear from the coordinates and the redshifts which are the same clusters.

Now the question is whether your galaxy is plausibly a member of a given candidate cluster or not. Ideally you should find some measurement of each cluster's velocity dispersion $\sigma$; if the galaxy's redshift isn't within $\pm 3 \sigma$ of the cluster's redshift $V\_{\rm cluster}$, then you can ignore that cluster, since it's very unlikely that a galaxy with that relative velocity with respect to the cluster is actually gravitationally bound to the cluster.

You can't really do better than a probabilistic assessment: roughly how likely it is that the galaxy is a member of a given cluster. Galaxies which are members of a cluster are more likely to be close to the center (on the sky); member galaxies closer to the center can have a higher range of relative velocities, while member galaxies further away should have velocities closer to the cluster redshift.

A quick example: Is NGC 4889 ($V\_{\rm gal} = 6495$ km/s) a member of a cluster? At its redshift, 1 Mpc = 33 arc minutes; if we search on NED for galaxy clusters within 33 arcmin of NGC 4889 with redshift limits of 4500-8500 km/s, we get 6 results, 4 of which are just the Coma Cluster in different catalogs, with $V\_{\rm cluster} \sim 6900$ km/s. Since the various distances ("Separ. arcmin") translate to $\sim$ 100-300 kpc from the cluster center, and since the Coma cluster has a velocity dispersion of $\sim$ 1000 km/s and the difference between NGC 4889 and the cluster is only $\sim$ 400 km/s, it's a very good bet that this galaxy is a member of the Coma Cluster. (It's actually one of the two cD galaxies in the cluster center.)


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### Answer by username_4 on 2016-10-16. Score: 1

First: You can use all galaxies in the line of sight of the cluster to construct their color-magnitude diagram. Those galaxies on the Red Sequence are more likely to be in the cluster (not always true).

Second: You should be able to determine their redshift, and compare with the mean cluster redshift. To be able to make cut, see next step.

Third: You should run some algorithms in order to see whether the redshifts/velocities you have are within the clusters mean velocity dispersion (usually in the clustercentric-velocity space), as used in these papers for example:
<https://arxiv.org/pdf/1601.06080.pdf>  
<http://adsabs.harvard.edu/full/1996ApJ...473..670F>


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### Answer by username_5 on 2016-08-25. Score: 0

Your question is not specific, so I'm afraid the answer is general too.

Using Simbad/NED check to see what observations/survey data exists for you galaxies. Almost all large galaxy surveys, and a lot of smaller ones, have corresponding "environment catalogues" where the galaxies are associated to their corresponding group. By cross-matching the coordinates (ra,dec) and redshift (or v) of your galaxies with the members of groups and clusters in one or more of these catalogues you should be able to find a match.


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