Close-up view of a luminous star-forming galaxy at z = 2.95^⋆

¹ Institut de Radioastronomie Millimétrique (IRAM), 300 Rue de la Piscine, 38400 Saint-Martin-d’Hères, France
e-mail: berta@iram.fr
² Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019, USA
³ Sorbonne Université, UPMC Université Paris 6 & CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98b Boulevard Arago, 75014 Paris, France
⁴ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The School of Natural Sciences, The University of Manchester, Manchester M13 9PL, UK
⁵ School of Physics and Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
⁶ Instituto Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁷ Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
⁸ University of Cape Town, Department of Astronomy, Private Bag X3, Rondebosch, 7701 Cape Town, South Africa
⁹ Department of Physics and Astronomy, University of the Western Cape, Private Bag X17, Bellville, 7535 Cape Town, South Africa
¹⁰ Istituto Nazionale di Astrofisica, Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
¹¹ European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001 Vitacura, Santiago, Chile
¹² Department of Astronomy, Cornell University, Space Sciences Building, Ithaca, NY 14853, USA
¹³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ Leiden University, Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ Division of Particle and Astrophysical Science, Graduate School of Science, Nagoya University, Aichi 464-8602, Japan
¹⁶ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
¹⁸ Aix-Marseille Université, CNRS & CNES, Laboratoire d’Astrophysique de Marseille, 38, Rue Frédéric Joliot-Curie, 13388 Marseille, France
¹⁹ University of California Irvine, Physics & Astronomy, FRH 2174, Irvine, CA 92697, USA
²⁰ School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
²¹ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²² Instituto Nacional de Astrofísica, Óptica y Electrónica, Astrophysics Department, Apdo 51 y 216, Tonantzintla, Puebla 72000, Mexico
²³ Instituto de Astrofísica e Ciãncias do Espaco, Tapada da Ajuda, Edifício Leste, 1349-018 Lisboa, Portugal
²⁴ Department of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
²⁵ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
²⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 22 October 2020
Accepted: 1 December 2020

Abstract

Exploiting the sensitivity of the IRAM NOrthern Extended Millimeter Array (NOEMA) and its ability to process large instantaneous bandwidths, we have studied the morphology and other properties of the molecular gas and dust in the star forming galaxy, H-ATLAS J131611.5+281219 (HerBS-89a), at z = 2.95. High angular resolution (0_.″3) images reveal a partial 1_.″0 diameter Einstein ring in the dust continuum emission and the molecular emission lines of ¹²CO(9−8) and H₂O(2₀₂ − 1₁₁). Together with lower angular resolution (0_.″6) images, we report the detection of a series of molecular lines including the three fundamental transitions of the molecular ion OH⁺, namely (1₁ − 0₁), (1₂ − 0₁), and (1₀ − 0₁), seen in absorption; the molecular ion CH⁺(1 − 0) seen in absorption, and tentatively in emission; two transitions of amidogen (NH₂), namely (2₀₂ − 1₁₁) and (2₂₀ − 2₁₁) seen in emission; and HCN(11 − 10) and/or NH(1₂ − 0₁) seen in absorption. The NOEMA data are complemented with Very Large Array data tracing the ¹²CO(1 − 0) emission line, which provides a measurement of the total mass of molecular gas and an anchor for a CO excitation analysis. In addition, we present Hubble Space Telescope imaging that reveals the foreground lensing galaxy in the near-infrared (1.15 μm). Together with photometric data from the Gran Telescopio Canarias, we derive a photometric redshift of z_phot = 0.9_−0.5^+0.3 for the foreground lensing galaxy. Modeling the lensing of HerBS-89a, we reconstruct the dust continuum (magnified by a factor μ ≃ 5.0) and molecular emission lines (magnified by μ ∼ 4 − 5) in the source plane, which probe scales of ∼0_.″1 (or 800 pc). The ¹²CO(9 − 8) and H₂O(2₀₂ − 1₁₁) emission lines have comparable spatial and kinematic distributions; the source-plane reconstructions do not clearly distinguish between a one-component and a two-component scenario, but the latter, which reveals two compact rotating components with sizes of ≈1 kpc that are likely merging, more naturally accounts for the broad line widths observed in HerBS-89a. In the core of HerBS-89a, very dense gas with n_H2 ∼ 10^7 − 9 cm⁻³ is revealed by the NH₂ emission lines and the possible HCN(11 − 10) absorption line. HerBS-89a is a powerful star forming galaxy with a molecular gas mass of M_mol = (2.1 ± 0.4) × 10¹¹ M_⊙, an infrared luminosity of L_IR = (4.6 ± 0.4) × 10¹² L_⊙, and a dust mass of M_dust = (2.6 ± 0.2) × 10⁹ M_⊙, yielding a dust-to-gas ratio δ_GDR ≈ 80. We derive a star formation rate SFR = 614 ± 59 M_⊙ yr⁻¹ and a depletion timescale τ_depl = (3.4 ± 1.0) × 10⁸ years. The OH⁺ and CH⁺ absorption lines, which trace low (∼100 cm⁻³) density molecular gas, all have their main velocity component red-shifted by ΔV ∼ 100 km s⁻¹ relative to the global CO reservoir. We argue that these absorption lines trace a rare example of gas inflow toward the center of a galaxy, indicating that HerBS-89a is accreting gas from its surroundings.