GA-NIFS: Multiphase analysis of a star-forming galaxy at z  ∼  5.5

¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
² Departamento de Astronomía, Universidad de Concepción, Barrio Universitario, Concepción, Chile
³ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, E-28850 Madrid, Spain
⁴ University of Oxford, Department of Physics, Denys Wilkinson Building, Keble Road, Oxford OX13RH, United Kingdom
⁵ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
⁶ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁷ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁸ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁹ European Space Agency, c/o STScI, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁰ INAF – Osservatorio Astrofisco di Arcetri, largo E. Fermi 5, 50127 Firenze, Italy
¹¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
¹² Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto F.no (Firenze), Italy

^⋆ Corresponding author; eleonora.parlanti@sns.it

Received: 29 July 2024
Accepted: 1 February 2025

Abstract

In this study, we present a detailed multiphase analysis of HZ4, a main-sequence star-forming galaxy at z ∼ 5.5, known for being a turbulent rotating disk and having a detection of a [C II] outflow in the ALMA observations. We exploited JWST/NIRSpec observations in the integral field spectroscopy mode with low- and high-spectral resolution which allow us, for the first time, to spatially resolve the rest-frame UV and optical emission of the galaxy to investigate the galaxy properties. In particular, the high-resolution dataset allowed us to study the kinematics of the ionized gas phase, and the conditions of the interstellar medium, such as the excitation mechanism, dust attenuation, and metallicity. The lower spectral-resolution observations allowed us to study the continuum emission and infer the stellar populations’ ages and properties. Our findings suggest that HZ4 is a galaxy merger rather than a rotating disk as previously inferred from lower-resolution [C II] data. The merger is associated with an extended broad, blueshifted emission, potentially indicative of an outflow originating from a region of intense star formation and extending up to 4 kpc. In light of these new observations, we reanalyzed the ALMA data to compare the multiphase gas properties. If we interpret the broad components seen in [C II] and [O III]λ5007 Å as outflows, the neutral and ionized components are co-spatial, and the mass loading factor of the ionized phase is significantly lower than that of the neutral phase, aligning with trends observed in multiphase systems at lower redshifts. Nonetheless, additional observations and larger statistical samples are essential to determine the role of mergers and outflows in the early Universe and to clarify the origin of the broad emission components observed in this system.