ALMA Band 2 line survey of a z = 3.44 clumpy strongly lensed submillimetre galaxy

ABSTRACT I present the first molecular line survey of the strongly lensed submillimetre galaxy SPT-S J002706-5007.4 ($z = 3.44$) using the new Atacama Large Millimeter/submillimeter Array (ALMA) Band 2 receivers (67–116 GHz), whose commissioning completes ALMA’s full (sub-)millimetre frequency coverage. The broad spectral coverage from 76 to 111 GHz of the observations simultaneously accesses a large suite of molecular and atomic emission lines. I report the novel detections of the lines that were hitherto inaccessible at $z = 3.44$, CO(J = 3–2) and HNC(J = 4–3), as well as detections of previously observed CO(J = 4–3) transitions, the neutral carbon line [C i]($^{3}P_{1}$–$^{3}P_{0}$), HCN(J = 5–4), HCO$^{+}(J$ = 5–4), and HNC(J = 5–4), with fluxes in line with previous observations. The CO spectral line energy distribution and [C i]/CO line ratios indicate highly excited, dense molecular gas with a strong far-ultraviolet radiation field. The dense gas fraction is estimated at $17 \pm 9$ per cent, consistent with other dusty star-forming galaxies selected from wide-area surveys. High-resolution Band 7 continuum imaging reveals a clumpy lensed morphology, with star-forming clumps contributing 30–50 per cent of the total emission. With multiple CO lines accessible across a wide redshift range, ALMA Band 2 is uniquely positioned as the premier tool for robust spectroscopic redshifts at Cosmic Noon and beyond ($z \sim 1$–6), a capability that will be further enhanced by the Wideband Sensitivity Upgrade’s full-band coverage in fewer tunings.