Galaxy Evolution

Galaxies emit light at all wavelengths from X rays to radio. The dominant energy sources are stars. Starlight is emitted at ultraviolet, optical and near-infrared wavelengths, but may be absorbed by dust (stellar ash) mixed in the gas in the disks of galaxies, and it is then re-emitted at longer wavelengths, at mid-infrared, far-infrared, or sub-mm wavelengths. One of the important discoveries of the past 20 years has been the recognition that approximately half the light emitted by stars comes out at these longer wavelengths. Black holes, especially very massive black holes in the nuclei of galaxies, are also an important source of energy, in the form of light emitted from the hot accretion disks surrounding the black holes, and mechanical energy input from jets. A full understanding of galaxies requires an explanation of how the structures we see today came about. The goal of observers is to compile a detailed description of galaxies at all wavelengths, and at different stages in cosmic history (by looking further away). Work in this area is concentrated on two aspects. First, using the Herschel telescope to study the reprocessed light re-emitted at far infrared wavelengths. Second studying the Universe at early times, when it was only a few % of its current age, by identifying quasars of very-high redshift.