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

TL;DR

This paper presents the first molecular line survey of a strongly lensed galaxy at z=3.44 using ALMA Band 2, revealing new molecular detections and insights into the galaxy's dense, excited gas and morphology.

Contribution

It demonstrates ALMA Band 2's capability to detect previously inaccessible molecular lines at high redshift, expanding tools for studying early universe galaxies.

Findings

Detected new molecular lines CO (3-2) and HNC (4-3) at z=3.44.

Indicated highly excited, dense molecular gas with strong UV radiation.

Revealed a clumpy, star-forming morphology in the 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 (3-2) and HNC (4-3), as well as detections of previously-observed CO (4-3) transitions, the neutral carbon line [CI], HCN (5-4), HCO$^{+}$ (5-4), and HNC (5-4), with fluxes in line with previous observations. The CO spectral line energy distribution and [CI]/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.