The Protogalactic Family
Like protoplanets (P 1) and protostars (S 1), galaxies must be born. Understanding their birth process is likewise fraught with difficulties and, even more than in astronomy’s other two Kingdoms, hampered by extreme distance. The details of galaxy formation remain one of the great unsolved problems of astrophysics, and therefore a dynamic field of study exists, informed by observation, theory, and simulations. The observations, both ground- and space-based, range from nearby mature galaxies and extremely distant young galaxies present within a few hundred million years of the Big Bang. Theoretical modeling and simulations have grown increasingly important over the last three decades, to such an extent that in some cases even some astronomers cannot tell the difference between simulation and reality. Nonetheless, the problem is complicated because the different structural components of galaxies may have been formed at different times. For example, bulges of spiral galaxies likely formed first, followed by disks gradually assembling around them. And different classes of galaxies likely did not form the same way as spirals. In a dynamic interplay, observers and modelers now feed on each other’s results, and both await the results of the James Webb Space Telescope which will look back to the beginning of the universe.