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17 votes

What would evidence of stars and galaxies significantly older than 13.8 billion years old look like? In what parts of space has it not been found?

Assuming the cosmological principle still holds, then we might expect that your own galaxy and those around us should contain evidence of stars/objects older than 13.8 billion years old. These might ...
ProfRob's user avatar
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17 votes

How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes?

The idea behind the paper (Shannon et al. 2013) that article is based on is to measure the gravitational wave background (GWB) produced by mergers of supermassive black holes, and determine which ...
HDE 226868's user avatar
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14 votes
Accepted

Why wouldn't an elliptical galaxy collapse back into a spiral galaxy?

If an elliptical galaxy is free of gas then it can't really "collapse" effectively, since there is no mechanism to dissipate the kinetic energy of the stars. In fact, once stars have formed ...
ProfRob's user avatar
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10 votes

How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes?

We know that black holes can gain mass other than merging with other black holes because we see high redshift quasars. The luminosity of quasars is caused by the accretion of mass into their central ...
ProfRob's user avatar
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6 votes
Accepted

Why aren't there blue elliptical galaxies or yellow spiral galaxies?

Galaxies consist of dark matter, stars, and gas. While gas is "collisional", i.e. it may interact hydrodynamically and cool, dark matter and, effectively, stars are collisionless. Hence, it ...
pela's user avatar
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5 votes
Accepted

Why do stars orbit much faster than gas in galaxies?

The velocity dispersion of giant molecular clouds in the vicinity of the Sun might be 30-40 km/s (this sounds too large to me actually), but their orbital speeds are of order 220 km/s, like the Sun. ...
ProfRob's user avatar
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4 votes
Accepted

Using cosmological model GADGET - 4

Whether there is a tool that allows to define ICs of your choice, mainly depends on what is your choice. If you want to start from cosmological initial conditions, there are various flags in the code ...
Prallax's user avatar
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3 votes

Is there a strict chronological order that determines the shape of galaxies?

Galaxies evolve from spiral to elliptical. The spirals are formed by patterns of new star formation in the disk surrounding the bulge, which contains mostly old stars. As galaxies run out of ...
James K's user avatar
  • 123k
3 votes
Accepted

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 ...
Jo Bovy's user avatar
  • 146
3 votes

Is the solar system the only survivor of cosmic evolution?

Leaving aside the incorrect notion that "all the phenomena are several thousand light years away", why would that happen? The universe appears to follow rules - the laws of physics. They are ...
ProfRob's user avatar
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2 votes
Accepted

Can the collapse of a gas cloud lead to an elliptical structure?

The shape of a collapsing cloud is influenced by various factors, including the initial shape, angular momentum, ability to dissipate heat, and sub-clumping processes. When a gas cloud collapses, it ...
eshaya's user avatar
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2 votes

What physical process can Metallicity(Z) of a galaxy tell us?

Yes, the main processes that create elements heavier than Helium all involve stars: Some elements are formed in stellar cores, some in neutron star collisions (but you can only get neutron stars from ...
James K's user avatar
  • 123k
1 vote

When dark energy became significant 6 billion years after the Big Bang was there anything else of significance going on in the universe?

The dark energy density doesn't strengthen. In the simplest model it stays exactly constant. What changes is the density of normal and dark matter, both of which decrease as the universe expands. 5 ...
ProfRob's user avatar
  • 153k
1 vote

Can the collapse of a gas cloud lead to an elliptical structure?

You said "Due to the conservation of angular momentum disk structures are more likely to form" . Well, there may not be any angular momentum in the first place, so disc structures can not ...
Thomas's user avatar
  • 3,484
1 vote

How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes?

tl;dr: A study reported a possible scale (in)variance of a tidal disruption event or TDE. Axios' Big and little black holes feed the same way has their own recent take on this question. What they ...
uhoh's user avatar
  • 30.5k
1 vote

Why aren't there blue elliptical galaxies or yellow spiral galaxies?

The main reason the late-type galaxies (spirals and irregulars) are blue is that the brightness contribution of the hotter stars (Main sequence O, B, and such) surpasses the contribution of colder, ...
eneguev's user avatar
  • 27

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