<p>The first Google-hits suggest that <a href="https://en.wikipedia.org/wiki/Segue_2" rel="nofollow noreferrer">Segue 2</a> is the smallest spiral galaxy, but that is actually incorrect since it is the smallest <em>spherical</em> galaxy yet discovered. It has a radius of roughly 34 parsec or 110 light years. The original publication on that topic dates back to 2013, but it was again <a href="https://www.forbes.com/sites/startswithabang/2020/04/06/the-smallest-galaxy-ever-discovered-demonstrates-the-unequivocal-need-for-dark-matter/?sh=23c4f5a354a9" rel="nofollow noreferrer">in the news in April 2020</a>.</p>
<p>Then, there are lecture notes on a course <a href="http://www.ifa.hawaii.edu/%7Ecowie/ast626_dir/gal_lec17.pdf" rel="nofollow noreferrer"><em>Astronomy 626 Galaxies</em></a> which shows the following diagram:</p>
<p><a href="https://i.sstatic.net/81OIs.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/81OIs.png" alt="Radius vs. mass for galaxies" /></a></p>
<p>Also, the lecture notes claim:</p>
<blockquote>
<p>Typical values for the scale length [of spiral glaxies] are: <span class="math-container">$1 {\rm kpc} &lt; h_R &lt; 10 {\rm kpc}$</span></p>
</blockquote>
<p>I agree that this does not answer your question fully, but I got the impression that the lower bound should be around <span class="math-container">$1 {\rm kpc}$</span>.</p>
<h3>References</h3>
<ul>
<li><a href="https://www.nasa.gov/press/2014/september/hubble-helps-find-smallest-known-galaxy-containing-a-supermassive-black-hole" rel="nofollow noreferrer">https://www.nasa.gov/press/2014/september/hubble-helps-find-smallest-known-galaxy-containing-a-supermassive-black-hole</a></li>
<li><a href="https://www.treehugger.com/just-how-big-is-the-smallest-galaxy-in-the-universe-4862842" rel="nofollow noreferrer">https://www.treehugger.com/just-how-big-is-the-smallest-galaxy-in-the-universe-4862842</a></li>
<li><a href="http://www.sci-news.com/astronomy/article01147-segue-2-dwarf-galaxy.html" rel="nofollow noreferrer">http://www.sci-news.com/astronomy/article01147-segue-2-dwarf-galaxy.html</a></li>
<li><a href="https://iopscience.iop.org/article/10.1088/0004-637X/770/1/16" rel="nofollow noreferrer">Evan N. Kirby et. al: <strong>Segue 2: The least massive galaxy</strong></a></li>
<li><a href="https://physics.stackexchange.com/questions/148418/">https://physics.stackexchange.com/questions/148418/</a></li>
</ul>

CC BY-SA 4.0
2021-02-10T10:57:03.867Z
@ConnorGarcia I did not manage to figure out how to deduce a lower bound for the size of a spiral galaxy using the TFR, but I updated my question so that it hopefully deserves less downvotes.
B--rian
CC BY-SA 4.0
2021-02-10T11:48:03.460Z
The (baryonic) T-F relation applies to spiral *and* irregular galaxies; since the latter tend to be smaller than spirals, the T-F relation cannot reliably be used to estimate lower limits for *spiral* galaxies.
Peter Erwin
CC BY-SA 4.0
2020-12-20T00:59:10.040Z
2021-02-10T11:59:31.600Z
2021-02-10T11:59:31.600Z
B--rian
<p>The smallest observed spiral galaxies I can find are <a href="https://en.wikipedia.org/wiki/NGC_2976" rel="nofollow noreferrer">NGC 2976</a> and <a href="https://en.wikipedia.org/wiki/NGC_4605" rel="nofollow noreferrer">NGC 4605</a>, both with a diameter of 20 kly, but I don't have anything like an exhaustive source to search. I've also found a mention that the Small Magellanic Cloud (7 kly diameter) might have had more spiral structure before it was distorted by the Milky Way.</p>
<p>From looking at these galaxies, spiral structure seems to be less clear the smaller the galaxy is, which makes me think there might not be a clear lower limit. Still, I'm curious how small a galaxy with recognizable spiral structure could be, either based on observations or theory.</p>

CC BY-SA 4.0
2020-03-10T09:57:30.340Z
[NGC 6503](https://www.spacetelescope.org/images/heic1513a/) is only ~15 klyr across…
pela
CC BY-SA 4.0
2020-03-10T12:11:52.567Z
https://memeguy.com/photos/images/anyone-remember-orion-from-men-in-black-133059.jpg
userLTK
CC BY-SA 4.0
2020-04-11T22:35:12.790Z
https://en.wikipedia.org/wiki/Dwarf_spiral_galaxy 5kly, think this is what you were looking for.
Dkcash413
CC BY-SA 4.0
2020-04-12T09:56:09.360Z
Dkcash413 - The size mentioned in that article is 5 *kpc*, not 5 "kly".
Peter Erwin
CC BY-SA 4.0
2020-04-13T15:29:12.577Z
How are those "diameters" defined? Galaxies do not have sharp edges (in their stellar or gas distributions), so it's impossible to answer your question unless you can provide a specific metric (exponential scale length? half-light radius?).
Peter Erwin
CC BY-SA 4.0
2020-04-28T14:21:40.283Z
The difference between a large globular cluster at the fringes of our galaxy and a small dwarf galaxy are essentially ones of interpretation. Spiralisation is a different matter though - but given that we don't have a theory for galactic rotation backed by observational evidence (dark matter problem) - this might be moot.
adrianmcmenamin
CC BY-SA 4.0
2020-04-28T19:14:52.797Z
@adrianmcmenamin Am I right in guessing the answer might be that we don't know because models are lacking and small galaxies are hard enough to image that we maybe just wouldn't find tiny spirals if they existed?
Elukka
CC BY-SA 4.0
2020-12-25T08:08:52.980Z
5kpc is 16.5ly i.e. 20.000 times smaller than the milky way
bandybabboon
<galaxy><size><spiral-arms>
How small can a spiral galaxy be?
Elukka
# How small can a spiral galaxy be?

(Asked by: username_0 on 2020-03-10)

The smallest observed spiral galaxies I can find are [NGC 2976](https://en.wikipedia.org/wiki/NGC_2976) and [NGC 4605](https://en.wikipedia.org/wiki/NGC_4605), both with a diameter of 20 kly, but I don't have anything like an exhaustive source to search. I've also found a mention that the Small Magellanic Cloud (7 kly diameter) might have had more spiral structure before it was distorted by the Milky Way.

From looking at these galaxies, spiral structure seems to be less clear the smaller the galaxy is, which makes me think there might not be a clear lower limit. Still, I'm curious how small a galaxy with recognizable spiral structure could be, either based on observations or theory.

**Tags:** galaxy, size, spiral-arms
> **Comments:**
> **username_1**: [NGC 6503](https://www.spacetelescope.org/images/heic1513a/) is only ~15 klyr across…  
> **username_2**: https://memeguy.com/photos/images/anyone-remember-orion-from-men-in-black-133059.jpg  
> **username_3**: https://en.wikipedia.org/wiki/Dwarf_spiral_galaxy 5kly, think this is what you were looking for.  
> **username_4**: Dkcash413 - The size mentioned in that article is 5 *kpc*, not 5 "kly".  
> **username_4**: How are those "diameters" defined? Galaxies do not have sharp edges (in their stellar or gas distributions), so it's impossible to answer your question unless you can provide a specific metric (exponential scale length? half-light radius?).  
> **username_5**: The difference between a large globular cluster at the fringes of our galaxy and a small dwarf galaxy are essentially ones of interpretation. Spiralisation is a different matter though - but given that we don't have a theory for galactic rotation backed by observational evidence (dark matter problem) - this might be moot.  
> **username_0**: @adrianmcmenamin Am I right in guessing the answer might be that we don't know because models are lacking and small galaxies are hard enough to image that we maybe just wouldn't find tiny spirals if they existed?  
> **username_6**: 5kpc is 16.5ly i.e. 20.000 times smaller than the milky way  

---

## 1 answers

### Answer by username_7 on 2020-12-20. Score: 1

The first Google-hits suggest that [Segue 2](https://en.wikipedia.org/wiki/Segue_2) is the smallest spiral galaxy, but that is actually incorrect since it is the smallest *spherical* galaxy yet discovered. It has a radius of roughly 34 parsec or 110 light years. The original publication on that topic dates back to 2013, but it was again [in the news in April 2020](https://www.forbes.com/sites/startswithabang/2020/04/06/the-smallest-galaxy-ever-discovered-demonstrates-the-unequivocal-need-for-dark-matter/?sh=23c4f5a354a9).

Then, there are lecture notes on a course [*Astronomy 626 Galaxies*](http://www.ifa.hawaii.edu/%7Ecowie/ast626_dir/gal_lec17.pdf) which shows the following diagram:

[![Radius vs. mass for galaxies](https://i.sstatic.net/81OIs.png)](https://i.sstatic.net/81OIs.png)

Also, the lecture notes claim:

> Typical values for the scale length [of spiral glaxies] are: $1 {\rm kpc} < h\_R < 10 {\rm kpc}$

I agree that this does not answer your question fully, but I got the impression that the lower bound should be around $1 {\rm kpc}$.

### References

* <https://www.nasa.gov/press/2014/september/hubble-helps-find-smallest-known-galaxy-containing-a-supermassive-black-hole>
* <https://www.treehugger.com/just-how-big-is-the-smallest-galaxy-in-the-universe-4862842>
* <http://www.sci-news.com/astronomy/article01147-segue-2-dwarf-galaxy.html>
* [Evan N. Kirby et. al: **Segue 2: The least massive galaxy**](https://iopscience.iop.org/article/10.1088/0004-637X/770/1/16)
* <https://physics.stackexchange.com/questions/148418/>

> **Comments:**
> **username_7**: @ConnorGarcia I did not manage to figure out how to deduce a lower bound for the size of a spiral galaxy using the TFR, but I updated my question so that it hopefully deserves less downvotes.  
> **username_4**: The (baryonic) T-F relation applies to spiral *and* irregular galaxies; since the latter tend to be smaller than spirals, the T-F relation cannot reliably be used to estimate lower limits for *spiral* galaxies.  

---

