[Paper Review] The Science Case for ALMA Band 2 and Band 2+3

G. A. Fuller, A. Avison+19 more|arXiv (Cornell University)|Feb 7, 2016

Spectroscopy and Laser Applications5 references6 citations

TL;DR

This paper advocates for the development of ALMA Band 2 (67–90 GHz) and a combined Band 2+3 receiver (67–116 GHz) to enable high-sensitivity, wideband observations of cold molecular gas in star-forming regions and distant galaxies. The enhanced frequency coverage and increased bandwidth (up to 16 GHz) would dramatically improve redshift determination efficiency, enable detection of deuterated molecules and prebiotic species like glycine in cold cores, and allow simultaneous monitoring of time-variable emissions from solar system bodies.

ABSTRACT

We discuss the science drivers for ALMA Band 2 which spans the frequency range from 67 to 90 GHz. The key science in this frequency range are the study of the deuterated molecules in cold, dense, quiescent gas and the study of redshifted emission from galaxies in CO and other species. However, Band 2 has a range of other applications which are also presented. The science enabled by a single receiver system which would combine ALMA Bands 2 and 3 covering the frequency range 67 to 116 GHz, as well as the possible doubling of the IF bandwidth of ALMA to 16 GHz, are also considered.

Motivation & Objective

• To address the limitations of current ALMA bands in redshift surveys by extending low-frequency coverage to 67 GHz.
• To enable the study of the lowest-energy transitions of deuterated molecules in cold, dense molecular gas, which trace the coldest and most quiescent phases of star formation.
• To facilitate the detection of prebiotic molecules like glycine in cold prestellar cores by accessing strong, low-frequency transitions in Band 2+3.
• To improve the efficiency and completeness of extragalactic spectral surveys by enabling contiguous, wideband coverage with reduced need for multiple tunings.
• To support time-domain studies of variable emissions from comets and planetary bodies by allowing simultaneous observation of multiple molecular transitions.

Proposed method

• Propose a single receiver cartridge covering 67–116 GHz (Band 2+3) to replace separate Band 2 and Band 3 receivers.
• Leverage improved receiver technology to achieve full, contiguous spectral coverage across the 16 GHz IF bandwidth, reducing gaps and overlap issues.
• Use atmospheric transmission models (e.g., zenith 2 mm PWV) to optimize frequency selection and ensure high transparency in Band 2.
• Model molecular line intensities and excitation conditions in cold sources (e.g., L1544) and hot cores to predict detectability of glycine and other species.
• Simulate multi-transition observations of volatile species (e.g., SO, SO2, HNC, HC3N) in comets and planetary atmospheres to assess the benefits of simultaneous wideband sampling.
• Compare the performance of current ALMA basebands (2×3.75 GHz) with a proposed 2×4.0 GHz configuration to quantify improvements in spectral coverage and survey efficiency.

Experimental results

Research questions

• RQ1 How would extending ALMA’s low-frequency coverage to 67 GHz improve redshift determination completeness for high-redshift galaxies?
• RQ2 What is the detectability of deuterated molecules in cold, quiescent gas using Band 2, and how do they trace the coldest phases of star formation?
• RQ3 Can ALMA Band 2+3 enable the first unambiguous detection of glycine in cold prestellar cores like L1544?
• RQ4 How does a 16 GHz IF bandwidth improve the efficiency and completeness of extragalactic spectral surveys compared to current 7.5 GHz basebands?
• RQ5 To what extent can simultaneous multi-transition observations in Band 2+3 improve the characterization of time-variable emissions from comets and planetary atmospheres?

Key findings

• A Band 2+3 receiver would reduce the redshift desert from 0.37–0.99 and 1.74–2.00 to just 0.72–0.99, increasing redshift survey completeness to nearly 100% for z > 1.
• The brightest glycine lines in Band 2+3 (below 80 GHz) have predicted peak intensities >8 mK, making them strong targets for detection in cold cores like L1544.
• Glycine emission in cold cores (T ~ 10 K) is strongest in Band 2+3 due to low-energy transitions, while in hot cores (T > 100 K), these lines are weaker by factors of 3–15 due to inefficient population of low-lying levels.
• With 8 GHz per sideband, Band 2+3 enables simultaneous observation of multiple SO, SO2, HNC, and HC3N transitions, improving constraints on excitation and abundance variations.
• The proposed 2×4.0 GHz baseband configuration would reduce the number of required tunings from 10 to 5 compared to the current 2×3.75 GHz setup, significantly improving survey efficiency.
• The combination of extended low-frequency coverage and increased bandwidth would allow unambiguous redshift determination for >95% of dusty star-forming galaxies at z > 2, compared to ~70% with Band 3 alone.