The Dance of the Little Red Dots: Spectroscopy of Two Blue Companions at z = 3.9 and 5.5 Reveals Young, Metal-Poor Starbursts Merging with Their LRDs within a Few Hundred Myr
Abstract
The Little Red Dots (LRDs) are compact, red sources, abundant at 4 ≲ z ≲ 9 and likely powered by massive black holes. Photometric studies have found blue companions around them, but none has yet been confirmed spectroscopically. This Letter presents the only two LRD-companion pairs with spectroscopy of both members in the literature: "Waltz" at z = 3.9 and "Tango" at z = 5.5, with projected separations of $3.6$ and $2.5$ kpc. Both companions lie at the redshift of their LRD, within 132 km s−1 and 57 km s−1, and are physically identical, but $524$ Myr apart: starbursts ≲ 50 Myr old, metal-poor (0.2 Z⊙), dust-free, dynamically cold, and as bright as the LRDs themselves in the rest-frame UV. The LRD in Waltz hosts a ≈ 3 × 106 M⊙ black hole accreting at half the Eddington rate, a factor of a few above the birth mass of a direct collapse black hole, and is embedded in gas as metal-poor as its companion. Orbital modeling shows that Tango is bound in 81% of the allowed orbits and that Waltz is bound if its host halo exceeds 5 × 1010 M⊙. Both blue companions then merge with their respective LRD within a few hundred Myr. We conclude that we are observing what heavy seeding requires: a young black hole beside the metal-poor starburst that the synchronized-pair scenario predicts, in the last steps of a cosmic dance that ends in a merger, and in a galaxy seeded by its black hole.
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BibTeX
@misc{pacucci2026dance,
title = {{The Dance of the Little Red Dots: Spectroscopy of Two Blue Companions at z = 3.9 and 5.5 Reveals Young, Metal-Poor Starbursts Merging with Their LRDs within a Few Hundred Myr}},
author = {Fabio Pacucci and Pierluigi Rinaldi},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.18968},
eprint = {2610.18968},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.18968},
}
RIS
TY - UNPB
TI - The Dance of the Little Red Dots: Spectroscopy of Two Blue Companions at z = 3.9 and 5.5 Reveals Young, Metal-Poor Starbursts Merging with Their LRDs within a Few Hundred Myr
AU - Fabio Pacucci
AU - Pierluigi Rinaldi
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.18968
UR - https://arxiv.org/abs/2610.18968
AB - The Little Red Dots (LRDs) are compact, red sources, abundant at 4 ≲ z ≲ 9 and likely powered by massive black holes. Photometric studies have found blue companions around them, but none has yet been confirmed spectroscopically. This Letter presents the only two LRD-companion pairs with spectroscopy of both members in the literature: "Waltz" at z = 3.9 and "Tango" at z = 5.5, with projected separations of $3.6$ and $2.5$ kpc. Both companions lie at the redshift of their LRD, within 132 km s^−1 and 57 km s^−1, and are physically identical, but $524$ Myr apart: starbursts ≲ 50 Myr old, metal-poor (0.2 Z_⊙), dust-free, dynamically cold, and as bright as the LRDs themselves in the rest-frame UV. The LRD in Waltz hosts a ≈ 3 × 10^6 M_⊙ black hole accreting at half the Eddington rate, a factor of a few above the birth mass of a direct collapse black hole, and is embedded in gas as metal-poor as its companion. Orbital modeling shows that Tango is bound in 81% of the allowed orbits and that Waltz is bound if its host halo exceeds 5 × 10^10 M_⊙. Both blue companions then merge with their respective LRD within a few hundred Myr. We conclude that we are observing what heavy seeding requires: a young black hole beside the metal-poor starburst that the synchronized-pair scenario predicts, in the last steps of a cosmic dance that ends in a merger, and in a galaxy seeded by its black hole.
ER -