Ionised gas kinematics and dynamical masses of z ≳ 6 galaxies from JADES/NIRSpec high-resolution spectroscopy

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: degraaff@mpia.de
² Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
³ Department of Astronomy and Astrophysics University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 96054, USA
⁴ Kavli Institute for Particle Astrophysics and Cosmology and Department of Physics, Stanford University, Stanford, CA 94305, USA
⁵ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
⁶ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
⁷ Centro de Astrobiología (CAB), CSIC–INTA, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁸ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁹ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
¹⁰ School of Physics, University of Melbourne, Parkville, 3010 VIC, Australia
¹¹ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
¹² Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
¹⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 AA Leiden, The Netherlands
¹⁶ Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
¹⁷ Department of Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218, USA
¹⁸ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁹ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
²⁰ Department for Astrophysical and Planetary Science, University of Colorado, Boulder, CO 80309, USA
²¹ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villafranca del Castillo, Madrid, Spain
²² NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
²³ NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada

Received: 18 August 2023
Accepted: 18 December 2023

Abstract

We explore the kinematic gas properties of six 5.5 < z < 7.4 galaxies in the JWST Advanced Deep Extragalactic Survey (JADES), using high-resolution JWST/NIRSpec multi-object spectroscopy of the rest-frame optical emission lines [OIII] and Hα. The objects are small and of low stellar mass (∼1 kpc; M_* ∼ 10^7 − 9 M_⊙), less massive than any galaxy studied kinematically at z > 1 thus far. The cold gas masses implied by the observed star formation rates are about ten times higher than the stellar masses. We find that their ionised gas is spatially resolved by JWST, with evidence for broadened lines and spatial velocity gradients. Using a simple thin-disc model, we fit these data with a novel forward-modelling software that accounts for the complex geometry, point spread function, and pixellation of the NIRSpec instrument. We find the sample to include both rotation- and dispersion-dominated structures, as we detect velocity gradients of v(r_e)∼100 − 150 km s⁻¹, and we find velocity dispersions of σ₀ ∼ 30 − 70 km s⁻¹ that are comparable to those at cosmic noon. The dynamical masses implied by these models (M_dyn ∼ 10^9 − 10 M_⊙) are higher than the stellar masses by up to a factor 40, and they are higher than the total baryonic mass (gas + stars) by a factor of ∼3. Qualitatively, this result is robust even if the observed velocity gradients reflect ongoing mergers rather than rotating discs. Unless the observed emission line kinematics is dominated by outflows, this implies that the centres of these galaxies are dominated by dark matter or that star formation is three times less efficient, leading to higher inferred gas masses.