Caught in the Act: Discovery of a Physical Quasar Triplet

TL;DR

This paper reports the discovery of a rare triplet of quasars at redshift 1.51 within a small projected distance, indicating a potential early-stage massive structure, and rules out gravitational lensing as the cause.

Contribution

First discovery of a physical quasar triplet at z~1.51, providing insights into early massive structure formation and quasar clustering.

Findings

Quasar triplet located within 200 kpc projected distance.

Velocity differences among quasars are less than 1000 km/s.

No evidence of gravitational lensing or galaxy clusters nearby.

Abstract

We present the discovery of a triplet of quasars at z~1.51. The whole system is well accommodated within 25 arcsec (i.e., 200 kpc in projected distance). The velocity differences among the three objects (as measured through the broad MgII emission line) are less than 1000 km/s, suggesting that the quasars belong to the same physical structure. Broad band NIR images of the field do not reveal evidence of galaxies or galaxy clusters that could act as a gravitational lens, ruling out the possibility that two or all the three quasars are multiple images of a single, strongly lensed source. QQQ J1519+0627 is the second triplet of quasars known up to date. We estimate that these systems are extremely rare in terms of simple accidental superposition. The lack of strong galaxy overdensity suggests that this peculiar system is harboured in the seeds of a yet-to-be-formed massive structure. Based on observations collected at the La Silla Observatory with the New Technology Telescope (NTT) of the European Southern Observatory (ESO) and at the Calar Alto Observatory with the 3.5m telescope of the Centro Astron\'omico Hispano Alem\'an (CAHA).