Motion of spiral arms of galaxies

I want to find out in what period the spiral arms of
galaxies similar to ours rotate. I am *not* interested
in how fast the matter rotates, but the appearance of
the spiral arms which is quite different.

I understand that one idea is that the appearance of arms
is caused by pressure waves making new stars light up.
In that case, the questions needed to answer this question a

1. What is the velocity of the pressure waves?

2. What is the typical linear distance between spiral arms
measured in the direction of the pressure wave fronts?

Thanks in advance for any replies.

--
Ray Tomes
http://ray.tomes.biz/
http://www.cyclesresearchinstitute.org/

"RT" == Ray Tomes writes:

RT I want to find out in what period the spiral arms of galaxies
RT similar to ours rotate. I am *not* interested in how fast the
RT matter rotates, but the appearance of the spiral arms which is
RT quite different.

Right. I think that the term you want is "pattern speed."

RT I understand that one idea is that the appearance of arms is
RT caused by pressure waves making new stars light up. In that case,
RT the questions needed to answer this question a

RT 1. What is the velocity of the pressure waves?

RT 2. What is the typical linear distance between spiral arms
RT measured in the direction of the pressure wave fronts?

ADS is your friend. I entered "pattern speed 'milky way'" into the
abstract/keywords section and got 40 hits. A quick survey suggests
that the pattern speed in the Milky Way is about 20 km/s/kpc. At the
Sun's distance (~ 8 kpc), that suggests a pattern speed of about 160
km/s.

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