B Discover 4 Milky Way Arms: Older Galaxies?

[TEFLing]

https://Earth'sky.org/space/new-map-confirms-four-milky-way-arms

...four spiral arms. The arms are where most stars in the galaxy are born. They contain most of the galaxy’s gas and dust, the raw ingredients for new stars.

Two of the arms, called Perseus and Scutum-Centaurus, seem to be more prominent and jam-packed with stars, while the Sagittarius and Outer arms have as much gas as the other two arms but not as many stars.​

The "minor" arms (Sagittarius & Outer) are more tightly wound. How can one spiral galaxy have more than one type of arm winding, aren't spirals classified according to a single degree of arm winding?

Are the more tightly wound arms, which evidently lack as many bright young stellar nurseries, older structures?

 

[phinds]

Not sure this helps, but spiral arms are not permanent structures. They are an artifact of density waves, as stars move in and out of the arms.

 

[TEFLing]

[1a] https://www.space.com/29631-milky-way-map-pins-down-our-galaxy-s-arms.html
[1b] https://Earth'sky.org/space/new-map-confirms-four-milky-way-arms
[2] http://www.sci-news.com/astronomy/science-milky-way-galaxy-four-spiral-arms-01649.html

Over-plotted all the raw data I could find for star forming regions, raw data doesn't cleanly match the "artist's impression" as much as I personally would prefer:

 

[TEFLing]

https://physics.stackexchange.com/questions/195602/how-do-we-know-what-the-milky-way-looks-like

with molecular clouds indicated via 21cm:

 

[TEFLing]

Actual raw data of 21cm radiation from Molecular Clouds is much messier, like M101, obvious "swirling whorl" of dust-lanes, yet without any grandly designed arms.

Hence, our position in the "Orion Spur" arm segment is unsurprising, the whole whirling maelstrom of dust-lanes in the MWG disk is much more complex that the artist impression "cartoon" publicized widely on the web.

Obviously the artist's impression is better for outreach, yet the actual data is more complicated:

References:
https://astronomy.stackexchange.com...y-early-21-cm-map-of-the-milky-way-oort-et-al
https://www.researchgate.net/publication/228773700_Observations_of_Milky_Way_at_21_cm_Wavelength_at_the_Bosscha_Observatory

 

[Ratman]

Interesting to see the actual radio data, it could be interesting to compare it with Gaia's star map. But what causes the "shadow" in the direction of Galactic anticenter? Orion molecular cloud complex?

 

[TEFLing]

Interesting to see the actual radio data, it could be interesting to compare it with Gaia's star map. But what causes the "shadow" in the direction of Galactic anticenter? Orion molecular cloud complex?

think that's the solar system ecliptic plane, but great question

 

[stefan r]

The data would be more funky at 0 and 180 degrees. It is co-rotating. The distance of a pixel is determined by the blue/red shift of the gas emission. At 180 the shift is just the random motion of the gas and does not indicate distance.

 

[TEFLing]

From raw data:

Offer that the Milky Way more resembles Andromeda (M31) or NGC 2841

 

[alantheastronomer]

Spiral structure in disk galaxies is not an exact science. See the work of Elmgreen and Elmgreen, Vassar College, on "flocculent spiral galaxies" or look at images of them on the Hubble website to see the wide variations among spirals.

 

[TEFLing]

lemme add one more thing :)

Hard to tell what's happening within the Solar radius of 8kpc

However, found some data on 21cm emission at large radii up to 25kpc... suggests our galaxy more resembles M101 "Pinwheel Galaxy", and that like that major spiral, our galaxy is also ~50kpc across