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 $4 \lesssim z \lesssim 9$ 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 $132 \, \rm km \, s^{-1}$ and $57 \, \rm km \, s^{-1}$, and are physically identical, but $524$ Myr apart: starbursts $\lesssim 50$ Myr old, metal-poor ($0.2 \, \rm Z_\odot$), dust-free, dynamically cold, and as bright as the LRDs themselves in the rest-frame UV. The LRD in Waltz hosts a $\approx 3 \times 10^{6} \, \rm M_\odot$ 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 $5 \times 10^{10} \, \rm M_\odot$. 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.