Explanations:

1
Explanations First, atoms are cooled in an
atomic trap. Then, at a certain moment,the trap
is switched off and the atomic cloud starts to
expand. A bit later a photograph of the cloud
is taken. From such a photograph it is possible
to reconstruct the velocity distribution of the
atoms (indeed, there is an obvious correlation
between the particle velocity and its position in
space, at the moment when the photograph was
taken). In the above picture such velocity
distributions are plotted. More specifically, the
z-axis shows the relative number of particles
with some given x and y-components of velocity.
The left picture (TgtTc) clearly shows a Gaussian
(Boltzmann) distribution. In the middle picture
one can see that small velocities have a more
narrow distribution and cannot be fitted by a
Gaussian these particles form the condensate. In
the right picture these non-Gaussian features
become more pronounced and the distribution gets
very narrow, which means that most of the
particles are in the condensate.