The FourStar Galaxy Evolution Survey (ZFOURGE) aims to establish an observational benchmark of galaxy properties at z > 1 by deriving 1-2% accurate redshifts of ∼60,000 galaxies at 1 < z < 3.

Most normal L∗ galaxies during this crucial epoch are too faint for spectroscopy, which has forced us to rely on inaccurate broadband photometric redshifts in the past. In response, we have equipped FourStar with innovative “medium-bandwidth” filters from 1 − 1.8μm, which enable accurate and robust redshifts to z = 3.5.

These observations allow us to observe complete galaxy samples down to low mass at z > 1, explore the role of mass and environment in galaxy transformations, measure galaxy scaling relations and the shape of the stellar mass function to z = 3, to find luminous galaxies at z = 6-9, and to identify high-redshift 1.5 < z < 2.5 (proto)clusters.

Scientific goals

  1. What causes galaxies to stop star formation?
    ZFOURGE can identify quiescent galaxies from their Balmer/4000A break to z = 3.5. We will correlate the strength of the break with stellar mass, surface density (compactness), AGN activity, and local galaxy density. If quiescence is driven by any of these properties, correlations with SFR should exist.
  2. Galaxy scaling relations and the stellar mass function to z = 3.5.
    The evolution of the mass- and luminosity functions, the size-mass relation, the SFR-mass relation, and the color-mass and color-magnitude relations, provide insight in how galaxies acquire their gas, convert it into stars, and grow by merging. ZFOURGE provides accurate redshifts, thus remove the main source of uncertainty in these and other measurements, reducing both statistical and systematic errors in the SMF by a factor
    of > 3.
  3. The role of environment.
    The morphology-density relation is a classic result of galaxy evolution, stating that massive, quiescent early-type galaxies are more abundant in clusters than in the field. ZFOURGE is expected to resolve the local galaxy density field and detect large scale structures at z > 1, such as high-redshift groups and (proto)clusters. The sample is sufficiently large to determine the relation between galaxy properties and environment at fixed mass and redshift, allowing us disentangle the two effects.
  4. The high-redshift universe z = 6−10.
    One of the quests of galaxy formation studies is to trace back the evolutionary history of galaxies directly to the birth of the first population of stars. ZFOURGE opens up the exciting possibility of identifying the brightest z ~ 8 galaxies, by selecting galaxies that are not detected in the J1-filter, but clearly detected in the J2 + J3-bands. We expect to find 3 − 9 galaxies at z ~ 8 per deep FourStar pointing, placing strong constraints of the shape of the luminosity function and providing extremely compelling candidates for spectroscopic follow up.
  5. Additional science with ZFOURGE.
    Beyond the core science, many other projects are made possible by the deep medium-band photometry and accurate redshifts:
    • identifying massive evolved galaxies z > 3.5
    • mapping the correlation of galaxies to dark matter at z > 2 through galaxy clustering studies
    • confirming several high-redshift 1.5 < z < 2.5 (proto)clusters
    • measuring accurate U V −slopes of galaxies at z = 5 − 7
    • efficient and reliable pre-selection for deep spectroscopic follow up