ALMA-ALPINE [CII] survey: The sub-kpc morphology of three main sequence galaxy systems at z ∼ 4.5 revealed by ALMA^⋆

¹ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
e-mail: Toby.devereaux@studenti.unipd.it
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Valparaíso, Chile
⁴ Université de Strasbourg, CNRS, Observatoire Astronomique 392 de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁵ Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
⁶ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
⁷ IPAC, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁸ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁰ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
¹¹ University of Wisconsin, 475 N Charter Str., Madison, WI, USA
¹² University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, MA 01003-9305, USA
¹³ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
¹⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁵ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla, 4059 Valparaíso, Chile
¹⁶ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
¹⁷ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
¹⁸ Department of Physics and Astronomy, University of California Davis, One Shields Avenue, Davis, CA 95616, USA
¹⁹ Gemini Observatory, NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA
²⁰ Cosmic Dawn Center (DAWN), Jagtvej 128, 2200 Copenhagen N, Denmark
²¹ DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
²² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
²³ Departamento de Astronomía, Universidad de La Serena, La Serena, Chile
²⁴ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, La Serena, Chile
²⁵ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Valparaíso, Chile
²⁶ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
²⁷ National Centre for Nuclear Research, ul. Pasteura 7, 02-093 Warsaw, Poland

Received: 6 November 2023
Accepted: 4 March 2024

Abstract

Context. Going from a redshift of 6 down to nearly 4, galaxies grow rapidly from low-mass galaxies towards the more mature types of massive galaxies seen at cosmic noon. Growth via gas accretion and mergers undoubtedly shape this evolution, however, there is considerable uncertainty at present over the contribution of each of these processes to the overall evolution of galaxies. Furthermore, previous characterisations of the morphology of galaxies in the molecular gas phase have been limited by the coarse resolution of earlier observations.

Aims. In this work, we utilise new high-resolution ALMA [CII] observations to analyse three main sequence (MS) galaxy systems at a redshift of z ∼ 4.5 and at resolutions of up to 0.15″. This approach enables us to investigate the morphology and kinematics on a kpc scale and understand the processes at play as well as the classifications of galaxies at high resolution. Thanks to this unique window, we are able to gain insights into the molecular gas of MS galaxies undergoing mass assembly in the early Universe.

Methods. We used intensity and velocity maps, position-velocity diagrams, and radial profiles of [CII] in combination with dust continuum maps to analyse the morphology and kinematics of the three systems.

Results. In general, we find that the high-resolution ALMA data reveal more complex morpho-kinematic properties. For one galaxy in our sample, we identified interaction-induced clumps, demonstrating the profound effect that mergers have on the molecular gas in galaxies, which is consistent with what has been suggested by recent simulations. One galaxy that was previously classified as dispersion-dominated turned out to show two bright [CII] emission regions, which could either be classified as merging galaxies or massive star-forming regions within the galaxy itself. The high-resolution data for the other dispersion dominated object also revealed clumps of [CII] that had not been identified previously. Within the sample, we might also detect star-formation powered outflows (or outflows from active galactic nuclei) that appear to be fuelling diffuse gas regions and enriching the circumgalactic medium. The new high-resolution ALMA data we present in this paper reveal that the galaxies in our sample are much more complex than they previously appeared in the low-resolution ALPINE data. In particular, we find evidence of merger induced clumps in the galaxy DC8187, along with signs of merging components for the other two objects. This may be evidence that the number of mergers at high redshift are significantly underestimated at present.