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

TL;DR

This paper introduces methods and a tool for identifying robust redshifts in the (sub)mm regime, evaluates current practices, and optimizes future observations to improve spectroscopic redshift measurements, especially with ALMA.

Contribution

It provides a graphical method and code for identifying robust redshifts, assesses the efficiency of different (sub)mm practices, and offers strategies to optimize future redshift searches.

Findings

Robust redshifts can be identified with single or multiple emission lines.

The code helps determine redshift ambiguities even with multiple lines.

2 mm observations are crucial for redshift determination in the z=2-4 range.

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.