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Old April 9th 18, 05:27 AM posted to sci.astro.research
jacobnavia
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Default Galaxy evolution

Le 08/04/2018 Ã* 22:16, Phillip Helbig (undress to reply) a écritÂ*:
In article , jacobnavia
writes:

Heavy stars are produced by the galaxy, converting cold gas into black
holes or heavy neutron stars.

Supernova explosions aren't symmetric, most stars receive a "kick" when
transforming into a black hole or a neutron star.


As eons pass, the dead stars accumulate either at the center of the
galaxy or in a diffuse halo of invisible matter around the galaxy.

This invisible mass can't explain the sorely needed black matter?


No, since it would mean a baryonic density higher than that obtained by
other arguments.


Yes, of course that would mean a higher baryon density... Black holes
and small cool white dwarfs are undetectable by most scopes.

Also, such a population would be detectable via
microlensing.


Sure, but do we have clear data in that direction?

An hitherto unknown population of black holes near the center has been
discovered. Halo black holes are MUCH more difficult to detect, because
the volume of the space to look at is much bigger and the objects are
small and do not emit any radiation...

Couldn't a symmetrical population of dead and invisible halo stars make
for the modified gravity we see?


No; see above.


Above says: "higher baryonic density". Yes, what about higher baryonic
density then?

Problem is, we are speaking about an unknown quantity, an exact
cartography of the galaxy, that is mostly unknown to us.

Satellites have measured the exact positions and speeds of thousands of
stars. What is the shape of the gravity field then?

At least for the part of the galaxy that is known in detail, are
simulations impossible that try to emulate those movements in a galactic
field, reading general rules from the sample?

[[Mod. note -- These black holes are more massive than most stars,
so dynamical friction (
https://en.wikipedia.org/wiki/Dynami...uitive_account
) causes them to gradually sink towards the center of the galaxy.


In theory yes, but how long does it take?

Because anyway, the masses of the stars were interacting with the
gravity field of the galaxy during all their lives. They shrink to
invisible objects but their masses go on circling the center as before.

The kick they receive when implosing could have (and surely does)
consequences, since it gives the star a speed relative to the center
either away or towards the center.

As the article in wikipedia says, halo stars do get slowed down and stay
in the galaxy. But far away from it, depending on, the vector of the
kick they receive when implosing, or the interactions with other stars,
whatever, many reasons could lead to a star having a very wide orbit.

This means that there may be a high density of black holes near the
center of the galaxy.


Yes, but do they stay there a long time? The central black hole must
have acquired its mass somehow...

The central black hole receives all black holes falling from above.

Others, wander in the halo regions, high above the center.

But, because these black holes are concentrated
at the center of the galactic, they can't explain flat galaxy rotation
curves. To explain those (without MOND) dark matter must be widely
distributed throughout the galaxy.
-- jt]]



Anything that doesn't emit radiation visible to us is black. A lot of
stuff could be black to our senses (and scopes) but be there nonetheless.

Normal stuff, what I think you call "baryonic". Yes, I would say the
normal stuff density could be much higher than what we think.

Right. Actually, to explain flat rotation curves with dark matter, most
of the dark matter must be outside the visible galaxy.


Huge clouds of very cold gas could exist without anyone noticing it.
Transparent and with no emissions, those clouds would be quite invisible.

The first space scopes are just as old as Hubble... Nothing really. We
have still not a lot of hard data about the cartography of the milky
way, our own galaxy.

As our scopes improve and our view of the stuff increases, dark matter
will become visible...