What I think I need to know is:
1) What is the current accepted shape of the Milky Way galaxy, and its sub-parts, such as core, halo, etc? Some documentation shows a sphere for dark halo, but other documentation shows models suggest some disc shape.
2) What percent of total mass is found in each sub-part, such as core, halo, etc? If this number isn't actually available for the Milky Way, what reference for another galaxy would be appropriate to use as a stand-in until we know?
3) Recent reports suggest that the mass of the entire galaxy is 1.5 Trillion solar masses. Does the mass of dark matter, which might make up 90% of this mass in the dark halo, affect objects within the halo the same as any other lit mass? For instance, does a particle of dark matter pull objects just like a neutron of equal mass?
4) What is the current accepted density of each subpart? Is this somewhat standard for barred spiral and spiral galaxies? Are globular clusters counted in any sub-part, or do they need to be factored separately? I would prefer to have them separate.
5) Can some sub-parts be rounded to homogeneity, or are there very important filamentous regions that must be accounted for? I read that the spiral arms are barely more dense (max 5%) than the region medium in which they exist.
6) Relative to a central black hole, within a relatively circular total circumference of the galaxy, and another reference point in the plane, what is the approximate location of the Sun in the overall X-Y plane of the galactic disk? I'd like for the other reference point defining the general plane to be maybe a globular cluster or important mass feature I need to calculate anyway.
7) I also want to make a map of mass-induced gravitational effect on earth, relative to a person standing anywhere on Earth.
If I model each subpart and define its density in a 3D shape, and then trace a ray through each shape beginning at Earth and ending at the limits of the dark halo, I want to sum the gravitational effect of each section at each of any number of spherical coordinates. Is it sufficient to run an integral though each subsection based on distance and average sub-part density until I reach the next sub-part?
So if my ray passes through one part, the total would be integral of start to end of part, and the formula within the integral would be the gravitational effect associated with mass at the point on the ray. The sum of each sub-part integral should give the total gravitational effect from edge of galaxy to the point of earth, correct? I mean just from mass alone, not from other force interplay, or mass-like effects of photons. I realize almost all mass in the galaxy has zero effect on Earth, or nearly zero, but I need the sum for my model. The sum of that many not-quite-zeros is not zero.
If I do it this way, should I just calculate at an interval of kpc and sum the parts, or take the real integral return?
8) How do I take the Earth as a reference system and project the external mass map or gravity map as a sky? I know the sun is tilted relative to galactic center, and the Earth is tilted relative to the sun.
Assume I have a map of the Milky Way with the equator being the galactic plane, projected onto a sphere (skybox), what angle(s) do I need to apply to a skybox to make it accurate from Earth? Another example: If I have created a mass or gravity map as a sphere with the Earth at the center of the sphere, ignoring daily rotation of the Earth what angle do I rotate the sphere to make the galactic plane match what we see in the sky?
9) A basic calculation suggests that the general projection of the milky way stays generally in the same 1% arc of the sky for 640k years as we move around the galactic center (230Ma orbit time/360°). Is this relatively reasonable? Is a linear account of our changing sky acceptable, or are we much closer to the galactic center at some point of our transit, causing the projection of the Milky Way to change faster on the other side of the galactic orbit?
I love references. So far all the galaxy shape astrometry text and images I've looked at give zero references. Example: https://content-calpoly-edu.s3.amazonaws.com/evolution/1/images/3universe/0MilkyWay/Milky-Way-structure-table.jpg.
Any help with any of the sub-questions, or even just the method itself, is very much appreciated.
For your Q8, to map the correct relative inclinations and intersections of the axes/planes/systems, you just need to know the celestial coordinates (RA & Dec) of their 3 "North poles":
- Earth's spin axis: North Celestial Pole, Dec = +90° [by definition]
- Earth's orbital plane about the Sun: North Ecliptic Pole, RA = 18h 00m [by definition]; Dec = +66° 33′.
- Milky Way's Galactic plane: North Galactic Pole, RA = 2h 49m; Dec = +27.4°.
For your Q8, to map the correct relative inclinations and intersections of the axes/planes/systems, you just need to know the celestial coordinates (RA & Dec) of their 3 "North poles":
- Earth's spin axis: North Celestial Pole, Dec = +90° [by definition]
- Earth's orbital plane about the Sun: North Ecliptic Pole, RA = 18h 00m [by definition]; Dec = +66° 33′.
- Milky Way's Galactic plane: North Galactic Pole, RA = 2h 49m; Dec = +27.4°.
Excellent, those are much better numbers than my guesses of approx 60 and 30.
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