2021A&A...646A.122B

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 (1.15µm). 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 800pc). 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 ∼1kpc 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∼10^7–9cm^–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)x10¹¹M_☉, an infrared luminosity of L_IR=(4.6±0.4)x10¹²L_☉, and a dust mass of M_dust=(2.6±0.2)x10⁹M_☉, yielding a dust-to-gas ratio δ_GDR≃80. We derive a star formation rate SFR=614±59M_☉/yr and a depletion timescale τ_depl=(3.4±1.0)x10⁸ years. The OH⁺ and CH⁺ absorption lines, which trace low (∼100cm^–3) density molecular gas, all have their main velocity component red-shifted by ΔV∼100km/s 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.