Tracing large-scale structure at high redshift with Lyman-α emitters: the effect of peculiar velocities

¹ Dipartimento di Astronomia, Università di Trieste, via Tiepolo 11, 34131 Trieste, Italy e-mail: monaco@ts.astro.it
² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching by München, Germany
³ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁴ University of Aarhus, Ny Munkegade, 8000 Århus C, Denmark
⁵ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
⁶ Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
⁷ University of Antwerp, Campus Drie Eiken, Universiteitsplein 1, 2610 Antwerp, Belgium

Received: 17 December 2004
Accepted: 17 May 2005

Abstract

We investigate the effect of peculiar velocities on the redshift space distribution of galaxies, and we focus in particular on Lyα emitters. We generate catalogues of dark matter (DM) halos and identify emitters with halos of the same co-moving space density (). We decompose the peculiar velocity field of halos into streaming, gradient and random components, and compute and analyse these as a function of scale. Streaming velocities are determined by fluctuations on very large scales, strongly affected by sample variance, but have a modest impact on the interpretation of observations. Gradient velocities are the most important as they distort structures in redshift space, changing the thickness and orientation of sheets and filaments. Random velocities are typically below or of the same order as the typical observational uncertainty on the redshift. We discuss the importance of these effects for the interpretation of data on the large-scale structure as traced by Lyα emitters (or similar kinds of astrophysical high-redshift objects), focusing on the induced errors in the viewing angles of filaments. We compare our predictions of velocity patterns for Lyα emitters to observations and find that redshift clumping of Lyα emitters, as reported for instance in the fields of high-redshift radio galaxies, does not allow to infer whether an observed field is sampling an early galaxy overdensity.