/sci/ - Science & Math

I don't get it. Why do they say they have proof for 2 arms or 4 arms when the only thing they have are distance estimates for the dots in pic related. They don't line up with any of the projected arms in the *artistic representation* and nobody could draw any spiral pattern through these dots.

>>6714825
Didn't they like compare it with other galaxies?

>>6714825
Not all the data points will be in the arms. There are many degrees of spiral galaxies, the main arms aren't the only place you will find bright stars. A thousand solar luminosity star will live long enough to see most of a rotation. There is evidence from both star formation regions seen in radio and pulsar dispersion measures that tell us about the arms.

That said both your pictures show rather good agreement with arms both major and minor, and the data points.

>>6714745

The simple fact of the matter is we don't really know. Much like trying to guess the shape of a forest when you're standing in the middle of it, there's only so much you can derive from our position in the galaxy. Astronomers have made a pretty good educated guess, but we're not likely to ever know the details.

>>6714825
a simple factor analysis would fix your problem.

>>6714745
I did my dissertation on measuring the mass of the Milky Way, where I measured the radial velocity of the gas at different parts of the galaxy. I found that the rotation curve I measured dipped up and down quite periodically. This was talked about in previous studies and was very likely caused by gaps in the density of gas where I was looking, i.e. there was no gas in that area, corresponding to a lack of a spiral arm. Although this doesn't PROVE spiral arms exist, it was definitely a way of implying that they existed.

All in all, trying to answer your question here
>>6714825
>They don't line up with any of the projected arms in the *artistic representation* and nobody could draw any spiral pattern through these dots.
I'm quite sure we didn't extrapolate spiral arms from the STARS, we discovered spiral arms from the density of gas throughout the galaxy. Though I may be wrong and I can't be bothered to actually do any research to confirm my suspicions.

>>6714892
Is the rotation curve really flat for the outer galaxy? And what about the dip at 10 kpc?

>>6714958
forgot pic

What about this?

http://galaxymap.org/drupal/blog/1

>>6714892
>does dissertation on measuring the mass of MW
>not sure what spiral arms represent

>>6714958
My project could only get me results up to the Solar radius from the galactic centre (~8kpc) because I was using a radio telescope, so I can't confirm the dip at 10kpc or actually the flat rotation curve, but my results showed a rotation curve exactly as your picture shows up to 8kpc. The dips I described in >>6714892 aren't the same as the dips in your picture at 3kpc and 10kpc but I found dips at about 3.5kpc, 4.5kpc, 6kpc and 7.5kpc which sort of correspond to the OP picture.
Galaxies do have flat rotation curves though, huge amounts of research have found this.

>>6714983
1. I know what spiral arms are.
2. Spiral arms played absolutely no part in my determining the mass of the Milky Way, I'm completely missing your point.

>>6714892
if i get this right areas where the spiral arm "exists" the mass within it is circling faster around the galactic core, and areas that aren't on board the arm are moving slower?

>>6715003
That's what is implied by what my results showed, however by reading into past research I found that the reason it dipped was simply because the lack of hydrogen gas at that point I was looking at meant I was recording the velocity of a different part of the galaxy. It's hard to explain but basically it means the dips weren't actually showing a different velocity, they were simply recording the velocity of the wrong thing.

>>6715003
No, everything moves at similar speeds. Stuff farther away from us *appears* to be moving faster away from or towards us. If mass (clouds in this case) were evely distributed along the galactic disk, you should detect stuff moving at all kinds of radial velocities. But in reality you only get groups of clouds moving at specific speeds at specific points along the disk.

>>6715032

>>6715033
>>6715032
>>6714892
that kinda makes me want to read your dissertation

>>6715044
Dissertation guy here. These >>6715033 >>6715032 aren't me, my graphs didn't look anywhere near as tasty as those.