Caught in the act: discovery of a physical quasar triplet

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 Mg II emission line) are <1000 km s−1, suggesting that the quasars belong to the same physical structure. Broad-band near-infrared (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 of the European Southern Observatory and at the Calar Alto Observatory with the 3.5 m telescope of the Centro Astrónmico Hispano Alemán.