“What powers galaxies?" has become one of those banal questions in astronomy. Somehow we always end up there and one wonders how close are we to the answer. Nevertheless, as basic as the question might sound, it is undeniably one of the most important ones to answer. Afterall, energy production is what makes the Universe tick, so to speak.
My work in the last few years has focused on understanding the emission from AGN in relation to the global energy budget of luminous galaxies, from a statistical rather than a physical perspective. The competition between the primary energy production mechanisms in these sources has been a topic of contention for decades and the reasons are twofold: 1) most of them, if not all, host powerful AGN and 2) they are very dusty. The combination of these characteristics makes it challenging to disentangle the contribution of stars and AGN to their total energy budget, yet it is only by doing so that we can make progress in truly understanding how (large) galaxies evolve.
I believe that we have recently managed to get much closer to the answer, partly by challenging well-established preconceptions regarding the AGN spectral energy distribution and partly by looking for clues in the galaxy and AGN luminosity functions. This will be the focus of my talk, while also highlighting the implications of our results on typical galaxy population measurements, such as number densities, star-formation rates, etc.. At the end, and given the information at hand, I will be able to answer the question “What powers the most luminous galaxies at all redshifts?”, but also outline the outstanding gaps in our knowledge regarding these systems.