One of the more unexpected discoveries from the James Webb Space Telescope (JWST) is the emergence of a significant population of compact, high-redshift sources at z > 4 known as "Little Red Dots" (LRDs). These objects exhibit extremely small physical sizes (≲100 pc) and unusual spectral energy distributions (SEDs), combining blue rest-frame UV continuum with red rest-frame optical colors. Despite the interest that they sparked, their true nature remains a mystery. Leading interpretations suggest they could be either dusty, massive (≳1010 M⊙) starburst galaxies—posing a challenge to current models of early galaxy formation efficiency—or faint, heavily obscured active galactic nuclei (AGNs), 2–3 orders of magnitude less luminous than typical quasars. In the latter scenario, LRDs could offer key insights into black hole seeding and the early growth of supermassive black holes (SMBHs).

In this talk, I will review key results from the first three years of JWST/NIRCam, MIRI, and NIRSpec observations of LRDs, emphasizing findings from the CEERS, SMILES, and CAPERS surveys, and discuss the implications for their interpretation as star-forming galaxies or obscured AGNs. While red optical continua and broad permitted lines point toward a dusty Type I AGN origin, other features—such as blue UV slopes and modest MIRI colors—complicate that interpretation. Meanwhile, the lack of ALMA detections, even in stacked data, challenges the dusty star-forming galaxy scenario. These conflicting results suggest that LRDs may not fit cleanly into existing categories. Recent models propose more exotic explanations, such as AGNs embedded in unusually dense gas or extended dusty structures that obscure or scatter light in complex ways—challenging our current understanding of early black hole and galaxy co-evolution.

Speaker: Guillermo Barro (University of the Pacific) 

The 2025 Spring Colloquium talks are held on Wednesdays at 3:00 PM. This colloquium is hosted by STScI and will be held as an in-person and virtual event.

You may join in person at STScI’s John N. Bahcall Auditorium or virtually on STScI's Live Science Events Facebook page.

Please direct questions or comments to contact above. The 2025 Spring Colloquium Committee members are: Nestor Espinoza (STScI), Joel Green (STScI), Nick Indriolo (STScI), Elena Manjavacas (STScI), Namrata Roy (JHU), Kevin Schlaufman (JHU), Ethan Vishniac (JHU).