ALMA Lensing Cluster Survey: Molecular Gas Properties of Line-Emitting Galaxies from a Blind Survey

TL;DR

This study uses ALMA Lensing Cluster Survey data to perform a blind search for millimeter line emitters in 33 galaxy clusters, exploiting gravitational lensing to detect intrinsically fainter molecular gas reservoirs. A LineSeeker-based pipeline identifies seven robust line emitters (one [CII] at $z=6.071$, four CO lines at $z oughly 0.8$–1.1, and two ambiguous identifications) with four counterpartless candidates; CH emission is serendipitously detected toward M0553-C190 at $z=1.14$, marking the first CH detection from a galaxy found in a blind survey. Gas masses derived from CO, [CII], and [CI], corrected for lensing, reach about 1 dex below previous deep surveys like ASPECS, and most emitters lie on the star-forming main sequence with depletion times of order $\tau_{dep} \sim 1$ Gyr, though substantial systematic uncertainties persist in line IDs and conversion factors. The results largely support the survival of canonical scaling relations across $z \lesssim 6$, while the CH detection hints at AGN-influenced chemistry in high-redshift environments; several unidentified candidates point to a potentially hidden population of faint or dust-obscured galaxies awaiting deeper follow-up.

Abstract

We present results of a blind search for line-emitting galaxies using ALMA Lensing Cluster Survey data. We detected seven line emitters, one of which is [C\,{\sc ii}] at $z = 6.071$, four are CO at $z = 0.8$--1.1, and the remaining two are possibly CO or [C\,{\sc i}] within photometric redshift ranges. Three of the four CO emitters are multiple images of the same galaxy. Compared to previous line-emitter searches in ALMA deep fields, our sample probes molecular gas masses $\sim$1 dex below the lower bound, thanks to gravitational lensing (typically $μ\sim 4$, up to $\sim$30 in extreme cases). Most emitters are located in a region similar to normal star-forming galaxies in the star formation rate (SFR) versus molecular gas mass plane. To reduce dependence on SFR and stellar mass, we analyzed the molecular gas fraction and depletion timescale as a function of distance from the star-formation main sequence. We found that most emitters broadly follow the scaling relations from previous studies, consistent within the intrinsic scatter. In addition, we serendipitously detected the CH $N = 1$, $J = 3/2 \rightarrow 1/2$ $Λ$-doublet transition from one CO emitter at $z = 1.142$, representing the first detection of CH from an individual galaxy at cosmological distances through a blind survey. The CH/CO column density ratio of $\sim$$10^{-4}$ is comparable to that of local AGN-host galaxies, suggesting that CH traces molecular gas associated with AGN activity, possibly irradiated by X-rays.

Figures (10)

• Figure 1: The relationship between the observed frequency and redshift of emission lines expected for high-redshift galaxies. The shaded yellow region represents the frequency ranges observed by ALCS.
• Figure 2: Multi-wavelength image cutouts ($5" \times 5"$) of the line emitters. From left to right: ALMA velocity-integrated intensity, ALMA 1.2 mm continuum, HST/F814W, HST/F160W, IRAC/ch1, and IRAC/ch2. The rightmost images are $100" \times 100"$ cutouts of Herschel/SPIRE 350 $\mu$m. The ALMA beam size is shown in the bottom left of the ALMA images. The spectroscopic redshifts are shown in the leftmost images of the line emitters with counterparts. The contours indicate the intensity at $3\sigma$, $4\sigma$, and $5\sigma$ levels. R0600-C13 and M0553-C303 are not covered by HST imaging. M0553-C190, M0553-C249, and M0553-C133 correspond to multiple lensed images of the same galaxy.
• Figure 3: Continuum-subtracted spectra of line emitters with counterparts. The error bars indicate the rms noise level measured in an emission-free region of the map for each channel. The spectra are shown with a velocity resolution of $60~\mathrm{km}~\mathrm{s}^{-1}$. The profiles are obtained by extracting the spectra at the location of the peak position identified in the moment 0 images. The yellow shaded area corresponds to the velocity range used to obtain the moment-0 images used in Figure \ref{['fig:img']}. The red curve shows the spectral fitting result.
• Figure 4: Continuum-subtracted spectra of line emitter candidates without counterparts at other wavelengths. The error bars indicate the rms noise level measured in an emission-free region of the map for each channel. The spectra are shown with a velocity resolution of $60~\mathrm{km}~\mathrm{s}^{-1}$. The profiles are obtained by extracting the spectra at the location of the peak position identified in the moment-0 image. The orange shaded area corresponds to the velocity range used to obtain the moment-0 images used in Figure \ref{['fig:img']}. The red curve shows the spectral fitting result.
• Figure 5: Steller mass vs SFR estimated from the SED modeling diagram for ALCS sources (including three sources in 2024arXiv240609890T), ASPECS sources Aravena_2019, PHIBSS1/2 CO sources Tacconi_20132018ApJ...853..179T, ALPINE sources 2020AA...643A...2B, and REBELS sources 2024AA...682A..24A. Solid lines represent the main sequence of star-forming galaxies at $z = 1$, 3, and 6.