# Galaxy formation

Gold Member

## Main Question or Discussion Point

When trying to work out how galaxies formed, there must have been some seed; some perturbations that started the ball rolling, These perturbations which collapsed gravitationaly to form seed galaxies must have been primordial in origin, where did they come from?

phinds
Gold Member
2019 Award
Tiny inhomogeneities in the early universe led to small differences in the density of regions of space after the surface of last scattering and that was enough to get the ball rolling. WHY there was any inhomogeneities in the early universe is beyond my knowledge. We see the remnants of it in the CMB, which shows inhomogeneities of 1 part in 100,000

Chronos
Gold Member
Keep in mind there was also a prodigious amount of dark matter in the primordial universe that influenced galaxy formation. Without dark matter, it is nearly impossible to even model early galaxy formation. You might get the odd star here and there, but, not dense populations. It is even considered possible DM fostered the formation of primordial SMBH, which seeded galaxy formation.

ChrisVer
Gold Member
WHY there was any inhomogeneities in the early universe is beyond my knowledge. We see the remnants of it in the CMB, which shows inhomogeneities of 1 part in 100,000
Inhomogeneities are there because of fluctuations in the metric during the inflation?

Gold Member
Does it make any difference if the seed galaxies were formed from inhomogeneites in a bouncing universe, where deconstruction was not complete?

mfb
Mentor
Does it make any difference if the seed galaxies were formed from inhomogeneites in a bouncing universe, where deconstruction was not complete?

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Drakkith
Staff Emeritus
Keep in mind there was also a prodigious amount of dark matter in the primordial universe that influenced galaxy formation. Without dark matter, it is nearly impossible to even model early galaxy formation. You might get the odd star here and there, but, not dense populations. It is even considered possible DM fostered the formation of primordial SMBH, which seeded galaxy formation.
Can you elaborate on why dark matter is required for early galaxy formation? Or link something that elaborates?

Chronos
Gold Member
CKH
http://arxiv.org/abs/0909.2021, Dark Matter and Galaxy Formation
At the time when the CMB originated, how were CDM and baryonic matter distributed?

The confusion I have with CDM helping to condense baryonic matter is that CDM has a very small ability to condense on it's own because there is no way that it can dissipate energy (it doesn't radiate).

Oh wait, except perhaps by having high velocity particles escape slightly denser areas, those areas can lose energy and condense. But that leaves all the high velocity CDM particles freely moving through neighboring denser regions and possibly heating those regions causing them to become less dense.

What happens? It's not obvious. Maybe it's not obvious for baryonic matter either. Radiation will travel and eventually get absorbed.

Oh wait, perhaps the expansion itself creates space for hot stuff to get lost by cooling.

Obviously there is no way to know for sure yet, but I believe that the hydrogen gas clouds in the early universe compressed then formed super massive black holes. Then the remaining gas that was orbiting the bh later compressed into stars due to the effects of the bh.

phinds
Gold Member
2019 Award
Obviously there is no way to know for sure yet, but I believe that the hydrogen gas clouds in the early universe compressed then formed super massive black holes. Then the remaining gas that was orbiting the bh later compressed into stars due to the effects of the bh.
May well have happened that way, but don't forget, dark matter was interspersed with regular matter as far as is known so the BHs would all be 80% dark matter (not that it matters since once inside the EH, it's irrelevant what kind of matter it was outside) ; I'm just pointing out that the BHs would NOT have formed primarily from hydrogen as you posit but from the same inhomogeneities (in ALL matter) that I mentioned in post #2

EDIT: It's possible that the DM might have kept passing through the nascent BH (the way it passes through the center of galaxie today) and not be as much of it as I'm thinking.

PeterDonis
Mentor
2019 Award
It's possible that the DM might have kept passing through the nascent BH (the way it passes through the center of galaxie today)
DM passes through other matter today because it doesn't interact with it. But passing through a BH would require the DM to travel faster than light (in order to escape the hole's horizon once it was inside). That's not possible.

phinds
Gold Member
2019 Award
DM passes through other matter today because it doesn't interact with it. But passing through a BH would require the DM to travel faster than light (in order to escape the hole's horizon once it was inside). That's not possible.
Yes, but I didn't SAY it would pass through a BH, 'cause I know better. I said it might pass through the NASCENT black hole (and thus not make up as much of the BH as I first stated it would).

mfb
Mentor
Obviously there is no way to know for sure yet, but I believe that the hydrogen gas clouds in the early universe compressed then formed super massive black holes. Then the remaining gas that was orbiting the bh later compressed into stars due to the effects of the bh.
Science is not about beliefs. Are there publications supporting this idea?

PeterDonis
Mentor
2019 Award
I said it might pass through the NASCENT black hole (and thus not make up as much of the BH as I first stated it would)
Ah, ok, so by "NASCENT black hole" you meant "a region where a black hole is going to form but hasn't formed yet", so the DM can still pass through and escape. Sorry for the misunderstanding on my part.

Chronos
Gold Member
DM appears resistant to capture by black holes according to http://arxiv.org/abs/1002.0553, An upper limit to the central density of dark matter haloes from consistency with the presence of massive central black holes

mfb
Mentor
DM appears resistant to capture by black holes
I don't see where this conclusion comes from. We know upper limits on the amount of dark matter falling into the black holes - so what? Dark matter particles can fall into black holes like regular particles. They just don't form an accretion disk.

Chronos
Gold Member
DM cannot shed kinetic energy so it does not spiral in like baryonic matter. Assuming that is not in dispute, I don't quite grasp your point.

mfb
Mentor
I would not call something "resistant to capture" if it does fall in, just at a significantly lower rate.

Chronos
Gold Member
The choice of words may be arguable, but, it does contrast the behavior of baryonic matter and DM being attracted by a black hole. BM will shed kinetic energy via friction, whereas DM cannot shed kinetic energy. The slowed BM occupies a lower orbit that continues to decay via friction until it eventually falls out of orbit and is eaten. The faster DM has no means to slow hence is largely immune to being eaten. That is not to say DM can never eaten by a black hole, merely that it is a rare occurrence.

CKH
Lately papers have been published concerning a resolution of the core/cusp problem raised by DM halos. The solution put forth is that supernova blast waves can drag DM away from the core (through gravitational interaction with the density waves). Perhaps similar processes involving non-uniform accretion of matter around a BH could drag some DM into SMBHs.

Another possibility is "evaporative cooling" that I think may account for the ability of DM to clump on it's own. The particles may exchange momentum through gravitational interaction. Those with the highest velocities escape leaving those that remain a bit cooler. This would likely be a very slow cooling process, nothing comparable to radiating normal matter.

Might there be any special, detectable effect of the consumption of DM by a BH? CDM particles are expected to be heavy.

TumblingDice
Gold Member
I would not call something "resistant to capture" if it does fall in, just at a significantly lower rate.
In common terminology, resistant doesn't imply 100% of the time. A water-resistant watch is not water-proof. Wrinkle-resistent clothes, drug resistant virus, etc... The term doesn't mean "always" or "never". In this case, "immune to capture" would imply that DM never falls in.

Dark matter particles can fall into black holes like regular particles. They just don't form an accretion disk.
Using "like" to compare how DM reacts to how 'regular particles' react is a bit of a gloss over. DM would need to be aimed right "on target" (or very close) to the BH or it will slip through any accretion disk and pass right by the BH.

mfb
Mentor
Okay I won't argue about words, this does not lead to anything.
Using "like" to compare how DM reacts to how 'regular particles' react is a bit of a gloss over. DM would need to be aimed right "on target" (or very close) to the BH or it will slip through any accretion disk and pass right by the BH.
Its angular momentum has to be smaller than $x c R_s$ where x is a prefactor of the order of 1 (I think 2 for a non-rotating black hole, it gets a function of the orientation for rotating black holes).
If it is cold (cold enough to keep in a galaxy), this corresponds to an impact parameter a thousand times larger than the Schwarzschild radius of the black hole. Still tiny compared to the galaxy, but significantly larger than the black hole.

Chronos