NOEMA millimeter reduced data cubes of HerBS-89a

The NOEMA data are complemented with Very Large Array data tracing the ^12^CO(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 (1.15um). Together with photometric data from the Gran Telescopio Canarias, we derive a photometric redshift of zphot=0.9^+0.3^_-0.5_ for the foreground lensing galaxy.

Modelling the lensing of HerBS-89a, we reconstruct the dust continuum (magnified by a factor {mu}~=5.0) and molecular emission lines (magnified by {mu}~4-5) in the source plane, which probe scales of ~0.1" (or 800pc). The ^12^CO(9-8) and H_2_O(2_02_-1_11_) 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_~10^7-9^cm^-3^ is revealed by the NH_2_ emission lines and the possible HCN(1_1_-1_0_) absorption line. HerBS-89a is a powerful star forming galaxy with a molecular gas mass of M_mol_=(2.1+/-0.4)x10^11^M_{sun}_, an infrared luminosity of L_IR_=(4.6+/-0.4)x10^12^L_{sun}_, and a dust mass of M_dust_=(2.6+/-0.2)x10^9^M_{sun}_, yielding a dust-to-gas ratio {delta}_GDR_~=80.

We derive a star formation rate SFR=614+/-59M_{sun}_/yr and a depletion timescale {tau}_depl_=(3.4+/-1.0)x10^8^years. The OH+ and CH+ absorption lines, which trace low (~100cm^-3^) density molecular gas, all have their main velocity component red-shifted by {DELTA}V~100km/s relative to the global CO reservoir. We argue that these absorption lines trace a rare example of gas inflow towards the center of a starburst galaxy, indicating that HerBS-89a is accreting gas from its surroundings.