Sizes and Stellar Masses of the Little Red Dots Imply Immense Stellar Densities

Abstract

The ``Little Red Dots'' (LRDs) are red and compact galaxies detected in JWST deep fields, mainly in the redshift range $z=4-8$. Given their compactness and the inferred stellar masses in the hypothesis that LRDs are starburst galaxies, the implied stellar densities are immense. This Research Note uses an extensive catalog of LRDs from the PRIMER and the COSMOS-Web surveys to investigate these densities. We find a median (upper limit) on the effective radius of $80$ pc, which leads to median (lower limit) values of the core density of $\sim 10^4{\mathrm{M}}_{\odot }\mathrm{p}{\mathrm{c}}^{-3}$, and individual densities as high as $\sim 10^8{\mathrm{M}}_{\odot }\mathrm{p}{\mathrm{c}}^{-3}$, which is $\sim 10$ times higher than the density necessary for runaway collisions to take place. For $\sim 35\%$ of the LRDs investigated, the lower limits are higher than the highest stellar densities observed in any system in any redshift range.

Accepted on Research Notes of the AAS. This is a concise note (3 pages, 1 figure) regarding the inferred stellar densities for 475 LRDs from the PRIMER and the COSMOS-Web surveys

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• Lonely Little Red Dots: Challenges to the AGN-nature of little red dots through their clustering and spectral energy distributions
• Little Red Dots are Tidal Disruption Events in Runaway-Collapsing Clusters
• Connecting the Dots: UV-Bright Companions of Little Red Dots as Lyman-Werner Sources Enabling Direct Collapse Black Hole Formation
• JWST Discovery of a High-Redshift Tidal Disruption Event Candidate in COSMOS-Web
• Mysteries of Capotauro: investigating the puzzling nature of an extreme F356W-dropout
• Direct Collapse Black Hole Candidates from Decaying Dark Matter
• Evolution of Size, Mass, and Density of Galaxies Since Cosmic Dawn
• The implications of overmassive black holes at $z>5$ for quasar and black hole growth
• Back to basics: Little Red Dots as galaxies and dust-obscured AGNs in a synthetic NIRCam sky simulated with L-GalaxiesBH
• Tidal disruption event rates across cosmic time: forecasts for LSST, Roman, and JWST and their constraints on the supermassive black hole mass function
• Mass and Spin Growth of Very Massive Stars in Star Clusters Potentially Associated with Little Red Dots
• Rapid sinking and efficient mergers of supermassive black holes in compact high-redshift galaxies
• Massive star clusters detected by JWST as natural birth places to form intermediate-mass black holes