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 0027 ($z = 3.44$) 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 hitherto inaccessible CO (3-2) and HNC (4-3) lines, 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.

Figures (5)

• Figure 1: The Band 2 and 3 spectra. Extending beyond the Band 3 regime, the Band 2 data provide a deep view of spectral lines previously not available to ALMA, including the CO(3--2), HCN(4--3), HCO(4--3) and HNC(4--3) lines. The top part of the figure indicates bright emission lines (from Spilker2014Hagimoto2023) in the range of ALMA Band 2, with smaller fonts indicating undetected lines, while the larger fonts show the detected lines shown in detail in Figure \ref{['fig:linespectra']}. The bottom part shows, for good observing conditions (precipitable water vapour of 0.5 mm), the atmospheric transparency and observing windows of ALMA Band 2 and Band 3.
• Figure 2: Deep HST imaging on SPT 0027 (background) with both the ALMA Band 2 continuum imaging (red thick contours) and higher-resolution Band 7 continuum imaging (thin white contours). The beams of each of the ALMA observations are shown in the bottom left. The lensing morphology of SPT 0027 is suggested in the Band 2 imaging (as well as previous lower-resolution Band 7 analysis; Spilker2016), with the clumpy lensed nature of SPT 0027 appearing more apparent in the higher-resolution imaging.
• Figure 3: The spectral lines extracted from the Band 2 data. Per line, the left-hand side panel indicates the moment-0 map of the line (background and black thick contours), while the thin white contours indicate the continuum, and the yellow contours indicate the aperture used for the flux extraction. The right-hand side spectrum is shown as a function of velocity relative to the rest-frame, with the top $x$-axis indicating the observed frequency. The filled region indicates the velocity range used for generating the moment-0 map, and the bottom-right values for $N_{\rm beam}$ and $\sigma_{\rm cont}$ indicate the criteria for identifying the extraction contours in the data cube (i.e., the yellow contours in the left-hand side image).
• Figure 4: Redshift identification probability as a function of redshift ($0 \leq z \leq 8$) for three ALMA frequency configurations, evaluated against smoothed redshift distributions from the HerBS bakx18Bakx2020 and SPT reuter20 samples. Orange bars indicate the fraction of sources yielding a single detected spectral line, while blue bars indicate two or more detections. Hatched blue regions denote configurations where even a single line provides an unambiguous redshift identification; hatched orange regions indicate cases where redshift degeneracies persist given a 5$\sigma$ uncertainty in $z_\mathrm{phot}$. Grey dotted lines show the robust redshift fraction when [C i] emission is included, and black dashed lines show the improvement expected from refined photometric redshift uncertainties Bendo2023. The three panels correspond to left: the filled Band 3 tuning Weiss2013, which serves as the baseline, middle and right: the Band 2-based configurations that use the WSU with 8 (baseline) and 14 (goal) bandwidths. For the 8 GHz set-up, three tunings are used to cover a contiguous coverage between 107 GHz, while for the 14 GHz, two tunings are sufficient to cover the observed wavelengths between 67 and 115 GHz Carpenter2023. The Band 2 setup extends the accessible redshift space to lower redshifts compared to Band 3-only strategies, while simultaneously increasing the fraction of sources for which two or more lines are detected, directly resolving the redshift degeneracy without requiring follow-up observations. Band 2 enables a higher robust redshift fraction across both the HerBS and SPT redshift distributions.
• Figure 5: The equivalent width of an ALMA-detected spectral line can serve as a diagnostic, particularly in the WSU era. The velocity-integrated line flux compared to the adjacent dust continuum is much larger for [C ii] 158 $\mu{\rm m}$ and [O iii] 88 $\mu{\rm m}$ emission Bakx2024GoldenRatio, and thus blindly-found emission lines without associated dust continuum are likely from atomic fine-structure lines. Detections and upper-limits of emission line galaxies (black squares and arrows) are from Venemans2020 and Fudamoto2021, while the filled regions are from scaling relations by delooze14 and Hagimoto2023. The diagnostic capabilities of ALMA towards finding such line emitters will improve as larger bandwidths become available in the WSU era.