Far-infrared Herschel SPIRE spectroscopy of lensed starbursts reveals physical conditions of ionized gas.

The most intensively star-forming galaxies are extremely luminous at far-infrared (FIR) wavelengths, highly obscured at optical and ultraviolet wavelengths, and lie at |$z$| ≥ 1–3. We present a programme of FIR spectroscopic observations with the SPIRE FTS, as well as photometric observations with PACS, both on board Herschel, towards a sample of 45 gravitationally lensed, dusty starbursts across |$z$| ∼ 1–3.6. In total, we detected 27 individual lines down to |$3\, \sigma$|⁠, including nine [C  ii ] 158  |$\mu$| m lines with confirmed spectroscopic redshifts, five possible [C  ii ] lines consistent with their FIR photometric redshifts, and in some individual sources a few [O  iii ] 88  |$\mu$| m, [O  iii ] 52  |$\mu$| m, [O  i ] 145  |$\mu$| m, [O  i ] 63  |$\mu$| m, [N  ii ] 122  |$\mu$| m and OH 119  |$\mu$| m (in absorption) lines. To derive the typical physical properties of the gas in the sample, we stack all spectra weighted by their intrinsic luminosity and by their 500 |$\mu$| m flux densities, with the spectra scaled to a common redshift. In the stacked spectra, we detect emission lines of [C  ii ] 158  |$\mu$| m, [N  ii ] 122  |$\mu$| m, [O  iii ] 88  |$\mu$| m, [O  iii ] 52  |$\mu$| m, [O  i ] 63  |$\mu$| m and the absorption doublet of OH at 119  |$\mu$| m, at high fidelity. We find that the average electron densities traced by the [N  ii ] and [O  iii ] lines are higher than the average values in local star-forming galaxies and ULIRGs, using the same tracers. From the [N  ii ]/[C  ii ] and [O  i ]/[C  ii ] ratios, we find that the [C  ii ] emission is likely dominated by the photodominated regions (PDR), instead of by ionized gas or large-scale shocks. [ABSTRACT FROM AUTHOR]