44
votes
Accepted
Are there any stars that orbit perpendicular to the Milky Way's galactic plane?
The Sun and most of the other stars are in the bulging disk of the Milky Way galaxy, but about 1% of the galaxy's stellar mass is in the galactic halo. The halo also includes 50 globular clusters and ...
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21
votes
Are there any stars that orbit perpendicular to the Milky Way's galactic plane?
Most stars in the galaxy are in the disc, but there is also a population in the galactic halo, these are in orbits at essentially random inclinations to the disc. There will be some that orbit ...
- 116k
18
votes
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Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?
The friends-of-friends (FOF) algorithm (Huchra & Geller 1982; Press & Davis 1982, Davis et al. 1985) for finding groups of particles can be used for various numerical problems, not just ...
- 37.1k
8
votes
Accepted
What is the mean separation between a star and its closest neighbor in the Milky Way halo?
I did some rough calculations, and 100 light-years doesn't seem to be a bad guess. If we assume that the average mass of a halo star is $\sim0.3M_{\odot}$, as would be expected for a typical IMF, and ...
- 36k
6
votes
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The contribution of components like dark matter, stars and gas to the mass of a galaxy
This is a rather complex question, for several reasons.
First, galaxies come in many variations, regarding such diverse
properties as mass, morphology, and environment.
Second, different ...
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5
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Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?
A quick search brought me to a book by Houjun Mo, et al. Galaxy Formation and Evolution which says
For example, the frequently used friends-of-friends (FOF) algorithm defines halos as structures ...
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4
votes
What object(s) are furthest above/below the galactic plane?
Bochanski et al. (2014; "The Most Distant Stars in the Milky Way") reported the discovery of two stars -- ULAS J001535.72+015549.6 and ULAS J074417.48+253233.0 -- found to be M giants at ...
- 16.4k
4
votes
Accepted
How do disk-shaped galaxies then form at the sites of spherical galactic halos? Do the halo stars start orbiting or just sit in place?
The key to understanding this is that dark matter and, effectively, stars are collisionless, whereas gas is collisional.
Structure formation
As written in the second quote, structure forms in a ...
- 37.1k
3
votes
Accepted
Why is the boundary of friends-of-friends (FOF) halo corresponding to iso-density contour?
If you have enough particles to sample your field, then a given (local) density corresponds to the same mean distance between your particles. The FOF algorithm keeps linking particles to a halo until ...
- 37.1k
3
votes
What object(s) are furthest above/below the galactic plane?
Globular clusters orbit in the Galactic halo, and so are good candidates for being far above the disc. The globular cluster PSO J174.0675-10.8774 is the furthest, 470,000 light years distant, well ...
- 116k
3
votes
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Evaluating Potentials using Galpy
For future reference, I believe the issue here was that you didn't instantiate the potential. That is, you probably tried to do
...
- 146
2
votes
Accepted
Is it possible to observe Lyman-continuum emission from extragalactic objects?
I think the short answer is "You can't observe $z=0$ LyC leakers, but you can observe $z\sim0$ LyC leakers."
No z = 0 leakers…
Measuring the Lyman continuum (LyC) — i.e. photons capable of ...
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2
votes
Accepted
How to build an adjacency matrix of stars connectivity from their physical kpc distances?
Computationally, a brute force algorithm where you calculate $r_{ij}$ for each pair $(i,j)$ and set $A_{ij}$ if it is smaller than the cut-off distance runs in $O(N^2)$ time. While this was much too ...
- 14.4k
1
vote
Accepted
What does it mean for cold clouds to be in pressure equilibrium with a diffuse hot medium?
Pressure equilibrium means that the pressure of the hot component and the pressure of the cold component are equal at their interface, so that neither expands nor contracts relative to the other. The ...
- 16.4k
1
vote
What is the mean separation between a star and its closest neighbor in the Milky Way halo?
About 300 light years
Wolfram has previously informed me that if our planet is as big as a flea (1MM), the sun is as big as a pigeon (11CM), the average distance of milky way halo stars is 25000 KM, ...
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