How to Illuminate a Dark Galaxy with the Help of a Black Hole

How do galaxies around us form their stars? In our current picture of galaxy formation these bright islands filled with stars should have begun their life from tiny density fluctuations of the rarefied primordial gas that fills the Universe. As this gas collapses due to gravity, it becomes denser, allowing the formation of stars. 

However, until the first stars form the galaxies do not emit any light in the visible range. Therefore this epoch of their life is very difficult to study. These "dark" phases of galaxy formation are still the most mysterious part of a galaxy's life. How long do they last? Are there galaxies that were never able to form stars?

So far, searches with radio telescopes for "dark" galaxies in the local Universe around our own Milky Way have not been successful. Due to the faintness of the gas radio emission, this technique does not allow us to search the distant, early Universe where the probability to find such objects may be higher.

In this lecture, Dr. Cantalupo will discuss very recent searches for "dark" galaxies with a new method that allows us to detect these elusive objects at extremely large distances from us, corresponding to the first billion years in the history of the Universe. The idea is very simple: if these galaxies do not emit light, we will "shine" a bright light on them. The brightest sources of light in the Universe are produced by galaxies with supermassive black holes, called "quasars".  Similar to a fluorescent material, when the gas in the "dark" galaxies is illuminated by a quasar it becomes visible at a very particular frequency or color. Dr. Cantalupo will show how searching for this specific light around a quasar has produced the first sample of "dark" galaxies ever detected.

Speaker: Dr. Sebastiano Cantalupo, Lick Observatory, UCSC