RELICS Science

High-Redshift Galaxies

Hubble's WFC3 has revealed ~2,000 z~6–11 candidates in the universe's first billion years, but only a small fraction have been bright enough (H<25.5) for detailed follow-up study and spectroscopic confirmation. The highest redshift searches (z~9-12) have yielded fewer candidates than expected, leaving luminosity functions highly uncertain while hinting at accelerated evolution in the first 600 Myr. We propose a more efficient search for brightly observed high-redshift galaxies by targeting 46 fields exceptionally lensed by 41 galaxy clusters lacking HST infrared imaging. By leveraging the Planck SZ galaxy cluster catalog and archival ACS imaging, we will efficiently deliver 40–200 z~9–12 candidates placing strong new constraints on luminosity functions with minimal cosmic variance uncertainty.

RELICS will efficiently yield more z~9–12 galaxies than larger blank field surveys (left). These will be bright enough for detailed study and will constrain currently uncertain luminosity functions (right):

RELICS will also deliver ~170 z~8 candidates including ~20 brighter than H<25.5. Follow-up spectroscopy will confirm z>8 with Lyα, constrain the patchy nature of reionization, and detect redder UV spectral lines such as CIII]1909Å. ALMA follow-up will reveal dusty evolved populations in the early universe.

Examples of confirmations and properties measured in brightly observed high-redshift galaxies:

Galaxy Cluster Mass Scaling Relations

Joint lensing + X-ray + SZ analyses will improve currently uncertain mass scaling relations and tighten limits on the dark matter particle cross section. Well calibrated mass estimates are critical to realizing the full potential of future missions such as eROSITA, which anticipates detecting 100,000 clusters / groups out to z ~ 1.3.

Left: Lensing reveals mass (blue) and X-rays reveal gas (red) in the Baby Bullet cluster merger. Gas particles collide and get left behind, unlike dark matter particles. Right: RELICS will help calibrate SZ mass estimates, which currently underestimate the most massive clusters.


Supernovae will further constrain progenitor properties and provide important new empirical tests for lens modeling. Type Ia supernovae with known brightnesses are providing critical tests of our lens model magnification estimates. And multiply-imaged supernovae are testing our model estimates of time delays between the images.

Supernova Refsdal lensed to form multiple images by the Frontier Fields cluster MACS1149: