How much gas and dust is in the $z=5.7$ Lyman Break Galaxy HZ10? An ALMA Band 10 to 4 and JWST/NIRSpec study of its interstellar medium

TL;DR

This study quantifies the gas and dust contents of the $z=5.65$ Lyman Break Galaxy HZ10 by integrating ALMA Band 10 and Band 4 continuum data with archival CO and [C II] observations, JWST metallicity, and dynamical mass constraints. The authors derive a dust mass of $\log(M_\text{dust}/M_\odot)=8.0\pm0.1$, a dust temperature of $T_\text{dust}=37_{-5}^{+6}$ K, and an infrared luminosity $\log(L_\text{IR}/L_\odot)=12.4\pm0.1$, corresponding to $\text{SFR}\approx304\,M_\odot\,\text{yr}^{-1}$. They calibrate the [C II] to total ISM mass with $\alpha_{\rm [CII]}^{\rm ISM}=39^{+50}_{-25}\,M_\odot\,L_\odot^{-1}$ and find that HZ10-C+E is gas-rich with $M_{\rm gas}/M_\star\sim2$, while the dust-to-gas and dust-to-metal ratios place it below local relations, implying inefficient ISM dust growth or enhanced destruction at this epoch. The work demonstrates a powerful ALMA–JWST synergy for disentangling baryonic components in early galaxies and sets the stage for larger samples to calibrate gas tracers and study dust build-up in the early universe.

Abstract

A complete overview of the stellar, gas and dust contents of galaxies is key to understanding their assembly at early times. However, an estimation of molecular and atomic gas reservoirs at high redshift relies on various indirect tracers, while robust dust mass measurements require multi-band far-infrared continuum observations. We take census of the full baryonic content of the main-sequence star-forming galaxy HZ10 at $z=5.65$, a unique case study where all necessary tracers are available. We present new ALMA Band 10 ($λ_\mathrm{rest}=50μ$m) and Band 4 ($300μ$m) observations towards HZ10, which combined with previously taken ALMA Band 6 through 9 data ($70-200μ$m) constrains its dust properties. We complete the baryonic picture using archival high-resolution [CII] observations that provide both a dynamical mass and molecular and atomic gas mass estimates, a JVLA CO(2-1)-based molecular gas mass, and JWST metallicity and stellar mass measurements. We detect continuum emission from HZ10 in Bands 10 and 4 at the $3.4-4.0σ$ level, and measure a dust temperature of $T_\mathrm{dust} = 37_{-5}^{+6}$K and dust mass $\log(M_\mathrm{dust}/M_\odot) = 8.0 \pm 0.1$. Leveraging the dynamical constraints, we infer its total gas budget, and find that commonly used [CII]-to-H$_2$ and [CII]-to-HI conversions overpredict the gas mass relative to the dynamical mass. For this reason, we derive a [CII]-to-total ISM mass (atomic + molecular) conversion factor, which for HZ10 corresponds to $α_\mathrm{[CII]}^\mathrm{ISM} = 39^{+50}_{-25}M_\odot L_\odot^{-1}$. We also find that HZ10 falls below the local scaling relation between dust-to-gas ratio and metallicity, suggesting inefficient ISM dust growth. These results demonstrate a powerful synergy between ALMA and JWST in disentangling the baryonic components of early galaxies, paving the way for future studies of larger samples.

Figures (6)

• Figure 1: The new ALMA observations of HZ10 presented in this work. (Left): ALMA Band 10 contours (orange) on top of the ALMA Band 9 continuum imaging presented in villanueva2024. The centroids of the central (C) and western (W) components as identified via [C ii] emission in telikova2024 are annotated. We moreover show the center of the eastern component (E) identified in JWST/NIRSpec by jones2024, although it is fully blended with HZ10-C at the resolution of the ALMA observations. (Right): ALMA Band 4 contours (red) on top of the ALMA Band 9 map. Both cutouts span $4"\times4"$, and beam sizes are shown in the lower left corner. Contours are placed at $\pm1\sigma$ intervals starting from $\pm2\sigma$ with negative contours being dashed. Band 10 (4) continuum emission is detected at the $3.4\sigma$ ($4.0\sigma$) level. We extract the Band 10 and 4 flux densities of the full HZ10 system in the dashed apertures, avoiding a likely noise spike to the south-east in the Band 10 map. We extract the Band 10 flux density of HZ10-C+E in the smaller circular aperture.
• Figure 2: (Left): Composite image of HZ10 combining ALMA continuum observations from Band 4 (red), Band 7 (green), and Band 9 (blue). The beam sizes for each ALMA band are shown at the bottom, and the physical scale is indicated on the right. (Right): Our fiducial optically thin modified blackbody fits to the multi-band ALMA photometry of HZ10 (black circles and blue shading) and HZ10-C+E (open squares and dashed grey shading). The central wavelength of the Band 10 measurement for HZ10-C+E is shifted by $+10\,\mu\mathrm{m}$ for visual clarity. All ALMA bands used in this work are showing through the vertical shading, with those used in the composite image shaded in the same color.
• Figure 3: (Left) Total mass budget of HZ10 as a function of the CO-to-H$_2$ ($\alpha_{\rm CO}$) and [CII]-to-HI ($\beta_{\rm HI}$) conversion factors. The blue line represents the case where $M_{\rm bar} - M_{\star} - M_{\rm dust} - M_{\rm HI} - M_{\rm mol} = 0$. The shaded regions indicate the forbidden region where this expression is less than zero. The green and red lines correspond to typical values of $\beta_{\rm HI}$ and $\alpha_{\rm CO}$, respectively, commonly assumed in the literature for different galaxy conditions. For the molecular gas, these include: MW-like $\alpha_{\rm CO}$bolatto2013, ULIRG-like or merger-like $\alpha_{\rm CO}$downes1998tacconi2008, $\alpha_{\rm CO}$ from a serra simulated Lyman-break galaxy at $z=6$vallini2018, and $\alpha_{\rm CO}$ based on the dust temperature and metallicity of HZ10-C+E following magnelli2012 and bolatto2013, respectively. For the atomic hydrogen gas, we include the calibration by heintz21 and the mass-to-light ratio observed within one effective radius in nearby star-forming galaxies deblok2016. (Right) Similar to the left panel, but here the x-axis represents the [CII]-to-H$_2$ ($\alpha_{\rm [CII]}$) conversion factor. The $\alpha_{\rm [CII]}$ calibrations by zanella18, madden2020, and rizzo21 are included.
• Figure 4: The dust-to-gas (DtG; upper panel) and dust-to-metal (DtM; bottom panel) ratios of HZ10-C+E (orange star) as a function of metallicity. For the dust mass, we use our fiducial optically thin model, while the gas mass of HZ10-C+E is determined as the difference of its baryonic mass -- inferred through kinematic modeling by telikova2024 -- and stellar mass mitsuhashi2024. The metallicity of HZ10-C+E was recently inferred through JWST/NIRSpec IFU observations by jones2024. We compare to a similar dynamically-inferred DtG and DtM for the $z=7.31$ galaxy REBELS-25 rowland2024rowland2025algera2025, while at Cosmic Noon we leverage ALMA-based dust and gas mass measurements from shapley2020 and popping2023. The open pentagons represent [C ii]-based measurements from algera2025. Finally, we overlay several local relations remyruyer2014devis2019galliano2021 as well as model tracks at $z\approx6$popping2017vijayan2019mauerhofer2025. Despite its metal-enriched nature, we find that HZ10-C+E falls below the local scaling relations, suggesting either a limited efficiency of ISM dust growth in HZ10, or efficient dust destruction.
• Figure 5: Six-band ALMA continuum maps of HZ10 ($4"\times4"$). Contours are drawn at $2,3,4,5,7$ and $10\sigma$, after which they continue in steps of $5\sigma$. Negative contours are dashed. The synthesized beam of each observation is shown as a filled ellipse, and the Band 9 contours and synthesized beams are shown in white in each panel for reference. The apertures used for extracting the flux density of the full HZ10 system and -- where possible -- that of HZ10-C+E are shown as dashed and solid lavender regions, respectively.