# Tag Info

9

"Peculiar velocity" is a fixed term and describes the velocity of an object relative to a defined rest frame. Astronomy has the problem that you need different methods to measure the 3D motion of an object. Therefor one often only gives the velocity within line-of-sight (from spectrographic data) or the perpendicular velocity as measured from ...

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The standard treatment can be found in (Binney & Tremaine 2008), but see also (Adams & Laughlin 1997) for a good treatment. The overall timescale for galactic evaporation is $$\tau_{evap}= 100\tau_{relax}\sim 10^{19}$$ years. The relaxation timescale $$\tau_{relax}=\frac{R}{v}\frac{N}{12 \ln(N/2)},$$ where $R$ is the size of the system, $v$ is the ...

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Galaxies move though space independently of the orientation of their axis of rotation. That this is true can be appreciated from the fact that their direction through space is relative; that is, in the reference frame of an observer that is passing the galaxy in its plane, it is moving like a frisbee, whereas in the reference frame of an observer who is ...

4

I think your recreation is essentially correct. For example, if you look at Figure 1 in that paper, you can see that the potential goes from positive to negative as a function of the azimuthal angle ("phase"). What you're missing is that the potential and density functions they define are perturbations, which are intended to be added to an axisymmetric ...

4

The short answer is "Yes, the orbits of satellite galaxies definitely show the influence of dark matter (or something like MOND, if you prefer)." For galaxies outside the Local Group, you can only measure their radial velocities from Doppler shifts (i.e., velocities along our line of sight to the galaxy), but you can still look at the distribution of these ...

4

The Canis Major dwarf galaxy is about 8 kpc from the Sun, but is only 8 degrees below the Galactic plane (and further out than the Sun). So it is about 42,000 light years from the Galactic centre and about 1150 light years below the plane. This almost within the disc of the Galaxy itself. The Galactic disc has a density that varies pseudo-exponentially in ...

3

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 hierarchical, bottom-up fashion. That is, small halos of gas and dark matter (DM) collapse first on small scales, and these halos then merge to form larger halos. ...

3

They know there's molecular gas because they observed emission from the CO molecule in two of the previously identified (atomic-hydrogen-emitting) gas clouds. From the paper: ... we targeted two objects (hereafter, MW-C1 and MW-C2), highlighted by red boxes in Fig. 1, in the 12CO(2 → 1) emission line at 230.538 GHz with the 12-m Atacama Pathfinder ...

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Black holes do not suck in matter any more than stars and planets do: an object in orbit would remain in stable orbit if nothing perturbed it. However, in the long run ($10^{19}$ years and more) interactions between stars will perturb their orbits, making many of them end up in the central black hole. An easy way of seeing this is to consider a random close ...

3

The spherical $(r, l, b)$ can be converted to rectangular $(U, V, W)$ as: \begin{align} U &= r \cos b \cos l \\ V &= r \cos b \sin l \\ W &= r \sin b \end{align} Using $(r, l, b) = (770 \pm 40 ~\text{kpc}, 121.2^\circ, -21.6^\circ)$, \begin{align} U &= -371 \mp 19 ~\text{kpc} \\ V &= ~~~ 612 \pm 32 ~\text{kpc} \\ W &= -283 \mp ...

2

Although the answer you link to doesn't mention irregulars, the answer applies to those as well: Star move around in the common gravitational potential created by everything in the galaxy, i.e. gas, stars, and, in particular, dark matter. This potential has a center, but there isn't necessarily anything exactly at this center. The stars then move on ...

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The standard cold-dark-matter model ("Lambda CDM") says that galaxy formation is seeded by initial density fluctuations in the dark matter and gas. The earliest galaxy formation will happen in the densest such fluctuations (denser = stronger gravity = faster collapse), which will be local overdensities within an initial, larger-scale overdensity. ...

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I think the answer to this isn't that Coma-type galaxy clusters are interesting, it's that galaxy clusters are interesting. Chiang et al. (2014) talks about the importance of studying protoclusters as a means of understanding how galaxies evolve over time. Galaxy clusters are extreme products of structure formation. They are ideal laboratories to study ...

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I conflated your question with "do satellite galaxies show the influence of dark matter in general...ie. Do they HAVE dark matter?", but I think I answered your question at the bottom regarding orbits and velocity dispersion as you mention. Tidal dwarf galaxies (TDGs) are typically observed to lack dark matter. TDGs are different from “regular” dwarf ...

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Proper motions are not velocities. They are angle traversed per unit time. They can be converted to tangential velocities (i.e. tangential to a line joining the star and the Sun) if the distance to the star is known, but you need additional information to get the line of sight (radial) velocity and hence a 3D velocity. Such velocities are with respect to ...

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