The epoch of star formation for th e most massive galaxies Sarah Brough (AAO)

Executive Summary  1. Massive galaxies are an important test of models of galaxy formation.  2. We need to understand observationally how massive galaxies form.  3. Current models predict that massive galaxies formed quickly at early times.  4. ALMA can observe massive galaxies in the epoch they form stars.  5. We can use this information to test the current models.

 In the standard model of galaxy formation more massive galaxies assemble from mergers of smaller galaxies  The most massive galaxies are the extreme limit of this process. They form an important test of any simulation of galaxy formation. Their formation and growth need to be understood observationally.

Massive Red Monsters Credit: S Croom

HOW DID THEY GET TO BE SO MASSIVE? de Lucia & Blaizot % stars formed50% mass assembled

 Thomas et al. (2010)

Where does ALMA come in?  During the periods when galaxies formed their stars, they are brighter in the millimetre than in the optical/UV due to the re-processing of optical/UV by interstellar dust.  Also, the characteristic spectral energy distribution of galaxies means that the increasing distance is balanced by the increasing intrinsic luminosity. The consequence is that the galaxy’s flux density is approximately independent of redshift.  The top row shows the number of low and high redshift (z>1.5) galaxies expected from a simulated deep ALMA observation.  The bottom row shows that with an optical image, such as the Hubble Deep Field, most of the detections are of galaxies with z<1.5. Credit: ALMA Science Portal

 These properties mean that we can observe large numbers of massive galaxies in their star-forming phase for the first time.  These observations will directly establish when the epoch of star formation is and its duration.  Also: what the stars form from and where (in multiple clumps that merge or single clumps or?)  This information can then be fed back to test the current models to see how well we really understand galaxy formation since the Big Bang.