I have never met such an idea... Can anyone comment?
We can observe that most galaxies have a massive black holes inside them.So from that I think we can conclude that black holes are creating galaxies.
Most but not all galaxies appear to have black holes at their center.
In general, galaxies are thought to form from uneven clumps in clouds of intergalactic gas.
It is possible that a black hole could act as a seed for such formation. However, in that case the black hole itself must have already formed from something, so that creates a "chicken and egg" problem. One idea which was recently put forward was that jets from a quasar (powered by a black hole) or active galactic nucleus might trigger formation of new nearby galaxies, but so far the evidence for this is not clear.
From a distance the gravitational pull of a black hole is essentially the same as that of the equivalent amount of normal matter, so it is unlikely to be anything more than a trigger to start formation.
For many galaxies the mass of the black hole and certain measures of the size of the galaxy seem to be related by a fairly simple law, which suggests either that the size of the black hole is determined by the size of the galaxy or vice versa. A partial explanation of this might be that if an active galactic nucleus becomes very bright, the radiation pressure can drive off the interstellar gas, which could slow down any further star formation and prevent the galaxy from continuing to grow.
ıs this means we need such a galatic structure to create a massive black hole ? ,Or just an alone massive star could create a black hole ?
I don't think this is known at present.
My feeling is that a black hole which is large enough to trigger galaxy formation would need a concentration of mass many times that of a single star, and that would only be found either within a galaxy or perhaps in region of space being disturbed by jets from an existing quasar or similar object. If that is the case, then early galaxies would have had to form without such "seeds" and might have been somewhat different in structure.
The main alternative is that somehow supermassive black holes were able to form by collapse directly from intergalactic gas in the early universe.
This sounds nice.
I am sceptical, the supermassive black hole at the center of the Milky Way is very light in comparison to the galaxy's total mass. The seeds for galaxy formation are usually believed to be the gravitational wells created by density variations of dark matter.
So dark matter creates a galaxy structure,And that structure creates a black hole ?
No one has ever witnessed a real black hole as I understand. Correct? Of course, all the evidences indicate such an object does exist. But what if it is just the gravity of all stars that center of a galaxy (quasar) consists of creates these spirals of matter?
There are plenty of observations that show large masses, as shown by the orbits of things around them, being confined within small regions of space, regions not much larger than the Schwarzschild radius associated with the mass. Observations also show things falling into such regions and nothing coming back out. The only kind of object we know of that can account for such observations is a black hole.
The observations don't just show a lot of gravity--i.e., a lot of mass. They show a lot of mass confined within a small region of space. The latter is why we think there are supermassive black holes at the centers of most galaxies.
For some quasars we also have another line of evidence: their brightness can vary significantly on short time scales, on the order of a month. This implies that whatever object is producing the light can only be a light-month or less in size, otherwise the variation in brightness could not be coordinated on that time scale. But a light-month is a small distance in galactic terms, so again this points to a large mass being confined in a small region of space.
My understanding is that it's currently not known how these supermassive BHs were formed. The difficulties are:
* stellar mass BH can't grow fast enough to have many millions of solar masses in only a few billions of years. Even in a dense stellar cluster, say like globular cluster, stars very rarely collide and "get eaten" by a BH - this is even more rare than stars colliding with other stars.
* maybe SMBHs form by direct collapse of very large gas/dust clouds? IIRC attempts to model this numerically show that this results in star formation - the cloud inevitably fragments into smaller subclouds, which become stars. Does not seem to work.
We do not know the probability distribution for primordial density fluctuations in the early universe so the likely number and size of any primordial black holes is, at best, a guess. I think it is fair to say the environment then in effect vastly differed from that now prevailing. PBH's remain on the table mainly because it is incredibly difficult to otherwise explain the abundance and size of supermassive black holes in the current universe. The case for them has strengthened significantly in the past few years, e.g.; https://arxiv.org/abs/1603.08522, First Identification of Direct Collapse Black Hole Candidates in the Early Universe in CANDELS/GOODS-S.
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