Molecular gas budget of strongly magnified low-mass star-forming galaxies at cosmic noon

¹ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
² Las Campanas Observatory, Carnegie Institution of Washington, Raúl Bitrán 1200, La Serena, Chile
³ Núcleo de Astronomía de la Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
⁴ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
⁵ Department of Physics, University of Cincinnati, Cincinnati, OH 45221, USA
⁶ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago, Chile
⁷ STAR Institute, Quartier Agora – Allée du six Août, 19c, B-4000 Liège, Belgium
⁸ Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
⁹ Institute for Computational Cosmology, Durham University, South Road, Durham DH1 3LE, UK
¹⁰ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
¹¹ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla, 4059 Valparaíso, Chile

^⋆ Corresponding author; victoria.catan@uc.cl

Received: 14 August 2024
Accepted: 4 November 2024

Abstract

Aims. The aim of this study is to investigate the molecular gas content of strongly magnified low-mass star-forming galaxies (SFGs) around the cosmic noon period (z ∼ 2) through observations of carbon monoxide (CO) emission lines and dust continuum emission, both of which serve as tracers of molecular gas (H₂).

Methods. We observed 12 strongly lensed arcs with the Atacama Compact Array (ACA) to detect CO mid-J rotational transitions and dust continuum. Thanks to the strong lensing, we were able to probe the previously understudied low-mass regime. With a compiled set of observations, we recalibrated empirical relations between star formation rate density (Σ_SFR) and the CO line ratios. We derived galaxy properties using spectral energy distribution fitting (SED). We also performed galaxy stacking to combine faint signals. In all cases, molecular gas masses were estimated using both tracers.

Results. We detected CO emission in 3 of the 12 arcs and dust continuum emission in another 3. The obtained H₂ masses indicate that most of these galaxies (M_* < 10^10.7 M_⊙) have lower molecular gas fractions and shorter depletion times compared to expectations from established scaling relations at these redshifts. We explored several possible explanations for this gas deficit, including uncertainties in mass estimates, effects of low-metallicity environments, larger atomic gas reservoirs in low-mass systems, and the possibility that these represent low-mass analogs of main sequence starburst (MS SBs) galaxies that are undergoing sustained star formation due to gas compaction despite low overall gas fractions.

Conclusions. We conclude that these mass and metallicity regimes present a molecular gas deficit. Our results suggest that this deficit is likely due to a significant amount of atomic gas, which our stacking indicates is about 91% of the total gas. However, this estimation might be an upper limit, as the possibility remains that our galaxies contain CO-dark gas.