# What percentage of galaxies rotate with trailing arms?

I am looking for the percentage of galaxies that rotate with trailing arms. I know that only a small percentage of galaxies rotate with leading arms and would like to know whether there are any results that determine the percentage of spiral galaxies that rotate with tailing arms.

I am interested in publications in this topic.

Note: So far I have found the following:

Do you know of any newer publication on this question? More recent than Pasha & Smirnov (1982) where the sample is quite small?

• I don't know the exact fraction, but it's most definitely very large. Leading arms are rare, and thought to be the result of past merging or at least some tidal encounter. Have a look at Buta et al. (2003).
– pela
Jan 14 '16 at 14:14
• Do you mean trailing arms? Jan 16 '16 at 1:28
• Yes, I just fixed the typo. Jan 18 '16 at 6:46
• There are some interesting videos by Deepskyvideos on youtube that may point towards some sources you could use. Mar 15 '16 at 11:24
• If you click on your links go to ads and find out which paper cites your paper you usually find more recent publications of relevance? Jun 2 '18 at 8:06

## 1 Answer

What percentage of galaxies rotate with trailing arms?

I am looking for the percentage of galaxies that rotate with trailing arms.

...

Do you know of any newer publication on this question? More recent than Pasha & Smirnov (1982) where the sample is quite small?

I think the best answer is that exact numbers are very difficult to determine.

Take for example page 5 of "Arm classifications for spiral galaxies" (1987) by D. M. Elmegreen and B. G. Elmegreen, cited over 200 times.

Affiliation: IBM Thomas J. Watson Research Center, Yorktown Heights, NY

Publication: Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 314, March 1, 1987, p. 3-9.

Page 5:

"II. THE CATALOGS

a) Selection Criteria

Table 2 includes all 708 spiral galaxies listed in the Second Reference Catalogue of Bright Galaxies - [3rd] Edition link (A. de Vaucouleurs, G. de Vaucouleurs, and H. G. Corwin 1976), with declinations $\delta \gt -35°$, inclination-corrected radii at 25 mag arcsec$^{-2}$, R$_{25}$, greater than $1^\prime$, and inclinations less than 60°; 654 galaxies could be classified, 54 galaxies which met the criteria but were too faint, had no details (were overexposed on the POSS or were too early in Hubble type to distinguish structure) or were too inclined for reliable classification are also included in Table 2, but with designations f n, or i, respectively, instead of arm classes.

...

b) Self-Consistency Checks

Most galaxies in Table 2 were classified at least 3 times, separated by intervals of several months, as a consistency check. They were classified from both paper and glass copies of the POSS. In the de Vaucouleurs sample, ~10% of the galaxies were given different arm classes during independent examinations on the paper prints. An 8% variation occurred when the glass images were subsequently examined.

...

For the high-resolution galaxies listed in Table 3, again ~10% of the arm classes changed upon reclassification. In these cases the large amount of detail presented confusion; irregular features were given more or less weight at different times of classification. Arm classifications become increasingly difficult for very small, faint galaxies. The cutoff of $1 ^\prime$ radius for the galaxies in Table 2 represents a minimum practical size for classification from the POSS.

As you can see, even with expert examination multiple times of the same data, determination of exact numbers with any accuracy is impossible at the present time. A guess is "most of them", an exception example is: NGC 4622 (also called a backward galaxy).

• When you make astronomical observations you never have exact numbers (except maybe if you count the number of telescopes in an observatory). Every number has an uncertainty so just saying we can't give precise number is just an answer to almost every astronomical question. But in this case of course the uncertainty is really high. And of course galaxies can be too inclined for classification. But this is not really a big problem because we assume isotropy and can therefore accept the number we find for the galaxies we can classify. But of course a lower sample size reduces the meaningfulness. Jul 24 '18 at 7:25
• Most of this paper use human classification which is also problematic, did you know human have a bias when classifying spiral direction (Land et al. 2008)? Jul 24 '18 at 7:30