Formation of a galaxy

Formation of a disk galaxy in a CDM universe with cosmological constant. The panels zoom in onto a disk galaxy in the process of formation at the epoch where the virtual universe of the simulation was "only" one and a half billion years old. The panels show (clock-wise from the top left) regions of 36 kiloparsec, 72 kiloparsec, 144 kiloparsec, 288 kiloparsec, respectively. The young galaxy forms in a high-density peak in which several large-scale filaments of matter intersects. These filaments deliver a fresh supply of gas and dark matter to the galaxy. The acretting gas fuels very active star formation, while accreted dark matter and smaller galaxies lead to the rapid growth of the galaxy's mass.

In the top panels, showing the smallest regions, you can see a young spiral disk. The disk is forming stars very actively with regions of star formation concentrated towards the densest regions near the plane of the disk (the brightest blue) and so the distribution of stars is also disk-like. Unlike gas, however, the stars can be "heated" by frequent collisions and mergers with other galaxies common at these early epochs. Therefore, the stellar disk is somewhat thicker than the gas disk. The distribution of stars in the disk is shown in the two panels (face-on and edge-on view) below. The stellar particles were color-coded according to their age, so that white-colored stars are the youngest and red are the oldest. You can see that the youngest stars concentrate towards the plane of the disk and the central regions, while the old stars have a more extended distribution and form what astronomers call a "bulge". Our Milky Way galaxy has a bulge quite similar to the bulge of red stars you see on these pictures.