A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z  = 3.5

The origin of the rest-optical emission of compact, red, high-redshift sources known as little red dots (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs constitute the densest stellar systems in the Universe. However, alternative models suggest active galactic nuclei (AGN) may instead power the rest-optical continuum. Here, we present JWST/NIRSpec, NIRCam, and MIRI observations from the RUBIES and PRIMER programs of The Cliff : a bright LRD at z = 3.55 with an exceptional Balmer break, twice as strong as that of any high-redshift source previously observed. The spectra also reveal broad hydrogen (H α FWHM ∼ 1500 km s −1 ) and He I emission, but no significant metal lines. We…

• NS
  National Science Foundation
• NA
  National Aeronautics and Space Administration

  Awards: 5-03127, NAS 5-03127, NAS5-26555, HST-HF2-51515.001-A
• ST
  Space Telescope Science Institute

  Awards: NAS5-26555, 5-03127, 26555, HST-HF2-51515.001-A, NAS 5-03127
• NR
  National Research Foundation
• SN
  Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung

  Awards: 26555, 200020_207349
• DG
  Danmarks Grundforskningsfond
• DZ
  Deutsches Zentrum für Luft- und Raumfahrt
• BF
  Bundesministerium für Wirtschaft und Energie
• SF
  Staatssekretariat für Bildung, Forschung und Innovation