The formation of galaxies through gravitational
collapse of dark matter is not an easy problem to understand. A good model
for galaxy formation has to take into account all the observed features of
real galaxies. For example, it seems that many disc galaxies contain a black
hole in their center, but others do not \cite{Galx-bh}. Typical galaxies are
spiral, elliptical or dwarf galaxies (irregular galaxies may be galaxies still
evolving). In most spiral and elliptical galaxies the luminous matter extends
to \sim 10-30kpc, and the total content of matter (including dark matter)
is of the order of 10^{10}-10^{12}M_{\odot }, with about 10 times more dark
matter than luminous one. The central density profile of the dark matter in
galaxies should not be cusp \cite{Smooth}. Even though the luminous matter
represents only a small fraction of the total amount of matter in galaxies,
it plays an important role in galaxy formation and stability \cite{DMCQG}.
On the other hand, it is still not well established if the mass of the central
black hole and the mass of the halo are correlated \cite{correlated}, etc.
There are some ideas in this respect when dealing with a scalar field. It
is known that the final stage of a collapsed scalar field could be a massive
object made of scalar field particles in quantum coherent states, like boson
stars (for a complex scalar field) or oscillatons (for a real scalar field)
\cite{seidel91,seidel94,franky,luis}. It is thus important to investigate
whether the scalar field would collapse to form structures of the size of
galaxies and provide the correct properties of any galactic dark matter candidate,
like growing rotation curves and appropriate dark matter distribution functions.
Also, we need to know which are the conditions that must be imposed on the
scalar field particles.
Our main aim has been to give a plausible scenario for galaxy formation under
the scalar field dark matter (SFDM) hypothesis. Through a gravitational cooling
process \cite{seidel91,seidel94}, a cosmological fluctuation of the scalar
field collapses to form a compact oscillaton by ejecting part of the field.
The key idea consists precisely in assuming that such final object could distribute
as galactic dark matter does \cite{franky}. The final configuration then should
consist of a central object (a core), i.e. an oscillaton, surrounded by a
diffuse cloud of scalar field, both formed at the same time due to the same
collapse process. For details see gr-qc/0110102.
The results of the numerical simulations are as follows. Essentially, we have
found three different types of behavior for the scalar field collapse. In
the first case, a generic feature is that scalar field distributions with
an initial mass slightly larger than the critical mass collapse very violently
and form a black hole. In the second type of behavior, fluctuations with an
initial mass significantly smaller than the critical mass can not form stable
oscillatons: the scalar field is completely ejected out as the system evolves
\cite{futuro}. The third behavior corresponds to a case where a fraction of
the initial density is spread out, leaving an oscillating object that appears
to be stable. This situation happens in a narrow window of initial conditions,
between 0.05-1\times M_{crit} \cite{futuro}.
In this simulation we show the evolution in time of the collapse
of an oscillaton (halo of a galaxy).
After the halo has formed, the luminous matter comes to the
regions where the dark matter concentrates. But, for different initial fluctuations
of the luminous matter, it will form different kind of galaxies. Here we show
two exemples of simulations from two different initial conditions for the formation
of the luminoues matter in the scalar field dark matter halo formed in the
previus one. Observe the formation of bars and spiral arms, something that is a challenge for all DM models and seems to be so natural in the Scalar Field Dark Matter model. (The simulatins are big, so, make click in the pictures and please
wait a moment).
