Three-dimensional stacking as a line intensity mapping statistic

¹ California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
² Center for Cosmology and Particle Physics, Department of Physics, New York University, 726 Broadway, New York, NY 10003, USA
³ Department of Physics, Southern Methodist University, Dallas, TX 75275, USA
⁴ Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
⁵ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, N-0315 Oslo, Norway
⁶ Département de Physique Théorique, Université de Genève, 24 Quai Ernest-Ansermet, CH-1211 Genève 4, Switzerland
⁷ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
⁸ Department of Physics, University of Toronto, 60 St. George Street, Toronto, ON M5S 1A7, Canada
⁹ David A. Dunlap Department of Astronomy, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
¹⁰ Department of Physics, University of Miami, 1320 Campo Sano Avenue, Coral Gables, FL 33146, USA
¹¹ Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

^⋆ Corresponding author: ddunne@astro.caltech.edu

Received: 26 March 2025
Accepted: 8 August 2025

Abstract

Line intensity mapping (LIM) is a growing technique that measures the integrated spectral line emission from unresolved galaxies over a three-dimensional region of the Universe. Although LIM experiments ultimately aim to provide powerful cosmological constraints via auto-correlation, many LIM experiments are also designed to take advantage of overlapping galaxy surveys, thus enabling joint analyses of two datasets. We introduce a flexible simulation pipeline that can generate mock galaxy surveys and mock LIM data simultaneously for the same population of simulated galaxies. Using this pipeline, we explore a simple joint analysis technique: three-dimensional co-addition (stacking) of LIM data on the positions of galaxies from a traditional galaxy catalogue. We test how the output of this technique reacts to changes in experimental design of both the LIM experiment and the galaxy survey, its sensitivity to various astrophysical parameters, and its susceptibility to common systematic errors. We find that an ideal catalogue for a stacking analysis targets as many high-mass dark matter halos as possible. We also find that the signal in a LIM stacking analysis originates almost entirely from the large-scale clustering of halos around the catalogue objects rather than the catalogue objects themselves. While stacking is a sensitive and conceptually simple way to achieve a LIM detection, thus providing a valuable way to validate a LIM auto-correlation detection, it will likely require a full cross-correlation to achieve further characterisation of the galaxy tracers involved, as the cosmological and astrophysical parameters we explore here have degenerate effects on the stack.