High-z Sudoku: A diagnostic tool for identifying robust (sub)mm redshifts

Abstract

We present methods to (i) graphically identify robust redshifts using emission lines in the (sub)mm regime, (ii) evaluate the capabilities of different (sub)mm practices for measuring spectroscopic redshifts, and (iii) optimise future (sub)mm observations towards increasing the fraction of robust redshifts. Using this publicly-available code (https://github.com/tjlcbakx/redshift-search-graphs), we discuss scenarios where robust redshifts can be identified using both single- and multiple-line detections, as well as scenarios where the redshift remains ambiguous, even after the detection of multiple lines. Using the redshift distribution of (sub)mm samples, we quantify the efficiencies of various practices for measuring spectroscopic redshifts, including interferometers, as well as existing and future instruments specifically designed for redshift searches. Finally, we provide a method to optimise the observation strategy for future (sub)mm spectroscopic redshift searches with the Atacama Large Millimetre/submillimetre Array, where 2 mm proves indispensable for robust redshifts in the z = 2 - 4 region.

9 pages, 8 figures, resubmitted and implemented the comments of the referee

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• Bright Extragalactic ALMA Redshift Survey (BEARS) III: Detailed study of emission lines from 71 Herschel targets
• The Bright Extragalactic ALMA Redshift Survey (BEARS) II: Millimetre photometry of gravitational lens candidates
• z-GAL -- A NOEMA spectroscopic redshift survey of bright Herschel galaxies: [I] Overview
• The LMT 2 millimeter receiver system (B4R). II. Science demonstration observations toward Orion-KL/OMC-1
• The LMT 2 Millimeter Receiver System (B4R). I. Overview and Results of Science Demonstration
• Hiding behind a curtain of dust: Gas and dust properties of an ultra-luminous strongly-lensed z = 3.75 galaxy behind the Milky Way disk