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 zphot = 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 nH2 ∼ 107 − 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 Mmol = (2.1 ± 0.4) × 1011 M⊙, an infrared luminosity of LIR = (4.6 ± 0.4) × 1012 L⊙, and a dust mass of Mdust = (2.6 ± 0.2) × 109 M⊙, yielding a dust-to-gas ratio δGDR ≈ 80. We derive a star formation rate SFR = 614 ± 59 M⊙ yr−1 and a depletion timescale τdepl = (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.
Highlights
In the last two decades, surveys in the far-infrared and submillimeter wavebands have opened up a new window for our understanding of the formation and evolution of galaxies, revealing a population of massive, dust-enshrouded galaxies forming stars at enormous rates in the early Universe
We describe the new data obtained on HerBS89a, outline the properties derived from the NOrthern Extended Millimeter Array (NOEMA) 1 mm highangular resolution observations for both the continuum and the molecular emission and absorption lines (Sect. 3.1), and present the results on the 12CO(1−0) emission line measured with the Very Large Array (VLA) (Sect. 3.2)
The molecular ion CH+ is detected in the ground transition, CH+(1−0); its profile is dominated by an absorption line that has a width similar to those of the OH+ absorption lines and at the same red-shifted velocity; in addition, a weak and broad (∼850 km s−1) emission component is likely present, of which the extreme red and blue wings are detected at low signal-to-noise ratio (S/N) at either side of the absorption line
Summary
In the last two decades, surveys in the far-infrared and submillimeter wavebands have opened up a new window for our understanding of the formation and evolution of galaxies, revealing a population of massive, dust-enshrouded galaxies forming stars at enormous rates in the early Universe (see, e.g., Blain et al 2002; Carilli & Walter 2013; Casey et al 2014; Hodge & da Cunha 2020). Based on their estimated locations relative to the LCO(1−0) versus ∆V(CO) relationship of Harris et al (2012), several of the sources are inferred to be gravitationally amplified, while a number of them appear to belong to the rare class of hyper-luminous infrared galaxies (Neri et al.2020) One of these galaxies, H-ATLAS J131611.5+281219 (hereafter HerBS-89a), at z = 2.9497, displays a very strong 2 mm continuum (with a flux density S 159 GHz = 4.56 ± 0.05 mJy) and CO emission lines that are the broadest of the entire sample with a line width (FWHM) of ∆V ∼ 1100 km s−1.
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