MUSE-ALMA Haloes XIII. Molecular gas in $z \sim 0.5$ HI-selected galaxies

TL;DR

The study targets molecular gas in HI-absorber–selected galaxies at $z\sim0.5$ using the MUSE-ALMA Haloes dataset, combining new ALMA Cycle 10 CO observations with extensive VLT/MUSE and HST ancillary data. It detects CO in 12 of 60 galaxies (20%), reaching $\sim$1.2 dex deeper in $M_{\text{H}_2}$ than prior absorber studies, and finds a dual behaviour in star formation efficiency: low-$M_{\text{H}_2}$ systems align with main-sequence scaling, while high-$M_{\text{H}_2}$ systems show suppressed star formation relative to their molecular reservoirs. The analysis reveals that CO detectability correlates with metallicity but not simply with absorber properties, and most CO-rich systems inhabit dense environments, suggesting that group dynamics and gas accretion influence molecular content. The results imply a substantial reservoir of CO-dark molecular gas in HI-selected systems and highlight the need for non-equilibrium, chemistry-aware simulations to capture the observed diversity and its role in the cosmic baryon cycle.

Abstract

We present new results from the MUSE-ALMA Haloes survey, covering 79 galaxies associated with strong HI absorption at redshift about 0.5. Our ALMA Cycle 10 observations add 39 systems to the initial 21, bringing the total to 60 galaxies. CO emission is detected in 9 new galaxies, and in 12 of 60 total, doubling the number of CO-emitting HI-selected galaxies and probing 1.2 dex deeper in molecular gas mass than previous studies. These galaxies span a wide range of stellar masses and metallicities. By comparing CO(2-1) and CO(3-2) properties with star formation rates and gas-phase metallicities from VLT/MUSE and HST, we find a dual behaviour in star formation efficiency: low-mass systems follow main-sequence scaling relations, while high-mass systems show suppressed star formation. This diversity indicates that HI absorbers trace both evolved and younger galaxies, providing a key step toward completing the baryon census at redshift about 0.5.

Figures (8)

• Figure 1: Distribution of CO-detections (blue) and non-CO-detections (light blue) on galaxies as a function of redshift (left) and stellar mass (middle). The right panel is plotted as a function of metallicity corresponding to multiple galaxies associated with the absorbers (Halley et al., in prep). This figure represents the MUSE-ALMA Haloes survey, including all the sources from the new ALMA Large Program and previous ALMA data (see Section \ref{['sec:alma_data']}). The middle and right panels have fewer numbers because only those with reliable $M_{\star}$ and [M/H] determinations are included. The median values for the detections are 0.6, 9.66 and $-1.23$ for the redshift, stellar mass and absorption metallicity, while the non-detections have median values of 0.5, 9.16, and $-1.84$. Overall, we report no clear correlation between the detection rate and any of these properties, suggesting that the detection of CO emission may be governed by a combination of global galaxy properties, rather than just one property alone.
• Figure 2: CO detections and spectral profiles of the new nine detected sources. Left panels show the moment zero maps with the $\pm$3, $\pm$4, and $\pm$5$\sigma$ level contours, while the right panels display the extracted CO spectra. The grey shaded regions around each spectrum represent the $\pm1\sigma$ RMS noise level estimate from the individual data cubes, and the blue shaded region indicates the velocity range used to create the moment maps. The final row, right figure, presents the rest-frame stacked spectrum of non-detections with $\pm1\sigma$ and $\pm2\sigma$ levels. A summary of the integrated CO fluxes and associated uncertainties for all detected sources is provided in Table \ref{['table:co_prop']}.
• Figure 3: Different $\alpha_{\text{CO}}$ models across varying metallicites. In this work, we adopt the model proposed by genzelCombinedCODust2015, shown in the blue points, which corresponds to the geometric mean of the prescriptions of bolattoCOtoH2ConversionFactor2013 and genzelMETALLICITYDEPENDENCECO2012, as indicated in Eq. \ref{['eq:alpha_co']}. As a reference, we also include the model proposed by accursoDerivingMultivariateACO2017, which would lead to higher values for $\alpha_{\text{CO}}$ for the galaxies in our sample. To prevent extrapolation into poorly constrained regions of parameter space, we impose an upper limit on $\alpha_{\text{CO}}$, set to the lowest emission metallicity measured in our sample, which leads to an $\alpha_{\text{CO}} \sim 15$.
• Figure 4: Left: Molecular gas mass ($M_{\text{H}_2}$) as a function of H i column density ($N_{\text{HI}}$). Blue circles represent our sample in comparison with previous studies on H i-absorption selected systems from kanekarMassiveAbsorptionselectedGalaxies2018 and klitschH2MolecularGas2021, shown in grey. Right: $N_{\text{HI}}$ and impact parameter, colour coded by the molecular gas masses. In this plot, we include all impact parameters for the sources in our sample, rather than limiting to the smallest $b$ as done in previous studies perouxSINFONIIntegralField2016hamanowiczMUSEALMAHaloesPhysical2020wengMUSEALMAHaloesVIII2022. The circles represent CO detections, and the triangle represents non-CO detections. We also include the median radial profiles for the neutral hydrogen column density from the simulations by voortCosmologicalSimulationsCircumgalactic2019 and nelsonResolvingSmallscaleCold2020 in the dashed and dotted lines, respectively. We report no clear correlation between the column density and impact parameter for different molecular gas masses.
• Figure 5: Left: Star formation rate (SFR) as a function of molecular gas mass ($M_{\text{H}_2}$). Right: Stellar mass ($M_{\star}$) as a function of molecular gas mass ($M_{\text{H}_2}$). Our sample is shown in blue circles, and the blue arrows represent a $3\sigma$ upper limit. The result from the stacked spectrum described in Sect. \ref{['subsec:stack']} is shown by the blue star. We also include the mean dust correction for SFR depicted by a large blue arrow on the corner of the left panel. For comparison, we include data from the xCOLD GASS survey saintongeXCOLDGASSComplete2017, which represents low-redshift galaxies, and the PHIBSS survey tacconiPHIBSSUnifiedScaling2018, in the redshift range $z\sim 0.3 - 1.2$. The solid line represents the molecular gas main sequence scaling relation from tacconiPHIBSSUnifiedScaling2018. We also included estimates from kanekarMassiveAbsorptionselectedGalaxies2018 and klitschH2MolecularGas2021 in both panels, labelled as other H i–selected systems. Our sources with $M_{\rm H_2}/M_{\odot }\lesssim10^{9.8}$ lie $\sim0.3$ dex above the expected SFR$-M_{\text{H}_2}$ relations for 'normal star-forming galaxies' at the same redshift range, while the systems with with $M_{\rm H_2}/M_{\odot }>10^{9.8}$ lie $\sim1.5$ dex below the relation. This trend suggests that H i–selected systems have a dual behaviour. Galaxies with low molecular gas masses form stars efficiently, following both the depletion timescales and the $M_{\star}-M_{\text{H}_2}$ scaling relations of main-sequence galaxies. In contrast, galaxies with high molecular gas masses show inefficient star formation and fall below the expected stellar mass growth, likely because they are still actively accreting gas from the intergalactic medium or interacting within group environments. These systems appear not yet to have reached the equilibrium conditions characteristic of main-sequence galaxies.