The MUSE eXtremely Deep Field

Classifying the spectral shapes of Lyα-emitting galaxies

¹ Observatoire de Genève, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² ESO Vitacura, Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago de Chile, Chile
³ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400 Austin, TX 78712-1205, USA
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 NL-9700 AV Groningen, The Netherlands
⁵ National Astronomical Observatory of Japan (NAOJ), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ Department of Physics, ETH Zürich, Wolfgang-Pauli-Strasse 27, Zürich 8093, Switzerland
⁷ Institute of Science and Technology Austria (IST Austria), Am Campus 1, Klosterneuburg, Austria
⁸ Univ. Lyon, Univ. Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁹ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
¹⁰ Leibniz-Institut fur Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹² Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
¹³ JWST Australian Data Centre, Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Victoria 3122, Australia
¹⁴ Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse, CNRS, UPS, CNES, 31400 Toulouse, France

^⋆ Corresponding author; eloise.vitte@gmail.com

Received: 17 April 2024
Accepted: 21 November 2024

Abstract

Context. The hydrogen Lyman-alpha (Lyα) line, the brightest rest-frame ultraviolet line of high-redshift galaxies, exhibits a large variety of shapes, which is due to factors at different scales, from the interstellar medium to the intergalactic medium (IGM).

Aims. The aim of this work is to provide a systematic inventory and classification of the spectral shapes of Lyα emission lines to better understand the general population of high-redshift Lyα emitting galaxies (LAEs).

Methods. Using the unprecedentedly deep data from the MUSE eXtremely Deep Field (MXDF; up to 140 hour exposure time), we selected 477 galaxies observed in the ∼2.8−6.6 redshift range, 15 of which have a systemic redshift from nebular lines. We developed a method to classify Lyα emission lines in four spectral and three spatial categories by combining a pure spectral analysis with a narrow-band image analysis. We measured spectral properties, such as the peak separation and the blue-to-total flux ratio for the double-peaked galaxies.

Results. To ensure a robust sample for statistical analysis, we define two unbiased subsets, inclusive and restrictive, by applying thresholds for signal-to-noise ratio, peak separation, and Lyα luminosity, yielding a final unbiased sample of 206 galaxies. Our analysis reveals that between 32% and 51% of the galaxies exhibit double-peaked profiles, with peak separations ranging from 150 km s⁻¹ to nearly 1600 km s⁻¹. The fraction of double-peaked galaxies seems to evolve dependently with the Lyα luminosity, while we do not see a severe decrease in this fraction with redshift, which is expected given the IGM attenuation at high redshift. An artificial increase in the number of double-peaked galaxies at the highest redshifts may cause the observation of a plateau instead of a decrease. A notable number of these double-peaked profiles show blue-dominated spectra, suggesting unique gas dynamics and inflow characteristics in some high-redshift galaxies. The consequent fraction of blue-dominated spectra needs to be confirmed by obtaining new systemic redshift measurements. Among the double-peaked galaxies, 4% are spurious detections, that is, the blue and red peaks do not come from the same spatial location. Around 20% out of the 477 sources of the parent sample lie in a complex environment, meaning there are other clumps or galaxies at the same redshift within a distance of 30 kpc.

Conclusions. Our results suggest that the double-peaked LAE fraction may trace the evolution of IGM attenuation, but the faintest galaxies must be observed at high redshift. We also need more data to confirm the trend seen at low redshift. In addition, it is crucial to obtain secure systemic redshifts for LAEs to better constrain the nature of the Lyα double-peaked lines. Statistical samples of double-peaked and triple-peaked galaxies are a promising probe of the evolution of the physical properties of galaxies across cosmic time.