galaxy luminosity function

Suppose you took a fair census of all the galaxies in a big patch of the universe and asked a simple question: how many are bright, how many are dim, how many in between? The answer, plotted as the number of galaxies at each luminosity, is the galaxy luminosity function — a population pyramid for galaxies.

The answer has a characteristic shape, well described by a formula called the Schechter function. There are enormously many faint, low-luminosity galaxies — dwarfs vastly outnumber giants — and their numbers keep rising toward the dim end. But toward the bright end the count falls off sharply: above a certain characteristic luminosity (called L-star, the 'knee' of the curve, roughly the luminosity of a galaxy like the Milky Way), galaxies become rapidly rarer, and truly giant galaxies are exceedingly scarce. So the universe makes a great many small galaxies and only a handful of behemoths.

The luminosity function matters because its shape is a direct test of galaxy formation theory. The steep cutoff at the bright end and the shallower-than-expected rise at the faint end are clues about feedback: energy from supernovae and stellar winds suppresses star formation in small galaxies, while feedback from supermassive black holes (active nuclei) caps the growth of the biggest ones. Matching the predicted to the observed luminosity function — and its cousin the stellar mass function — is one of the key benchmarks any successful model of galaxy evolution must pass, and it also feeds into counting up the total light and stellar mass in the cosmos.