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Witnessing galaxy assembly in an extended z~3 structure.

Fumagalli, M. and Mackenzie, R. and Trayford, J. and Theuns, T. and Cantalupo, S. and Christensen, L. and Fynbo, J.P.U. and Moller, P. and O'Meara, J. and Prochaska, J.X and Rafelski, M. and Shanks, T. (2017) 'Witnessing galaxy assembly in an extended z~3 structure.', Monthly notices of the Royal Astronomical Society., 471 (3). pp. 3686-3698.


We present new observations acquired with the Multi Unit Spectroscopic Explorer instrument on the Very Large Telescope in a quasar field that hosts a high column density damped Lyα absorber (DLA) at z ≈ 3.25. We detect Lyα emission from a nebula at the redshift of the DLA with line luminosity (27 ± 1) × 1041 erg s− 1, which extends over 37 ± 1 kpc above a surface brightness limit of 6 × 10−19 erg s− 1 cm− 2 arcsec− 2 at a projected distance of 30.5 ± 0.5 kpc from the quasar sightline. Two clumps lie inside this nebula, both with Lyα rest-frame equivalent width > 50 Å and with relative line-of-sight velocities aligned with two main absorption components seen in the DLA spectrum. In addition, we identify a compact galaxy at a projected distance of 19.1 ± 0.5 kpc from the quasar sightline. The galaxy spectrum is noisy but consistent with that of a star-forming galaxy at the DLA redshift. We argue that the Lyα nebula is ionized by radiation from star formation inside the two clumps, or by radiation from the compact galaxy. In either case, these data imply the presence of a structure with size ≫50 kpc inside which galaxies are assembling, a picture consistent with galaxy formation in groups and filaments as predicted by cosmological simulations such as the EAGLE simulations.

Item Type:Article
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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2017. The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.
Date accepted:19 July 2017
Date deposited:27 July 2017
Date of first online publication:26 July 2017
Date first made open access:No date available

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