Dust at cosmic dawn: unveiling the onset of dust and obscured star formation at z 8

The gist Dust at cosmic dawn ALMA2040 reveals how next-generation submillimeter interferometry will unveil the onset of dust and obscured star formation at high redshifts by providing unprecedented sensitivity and multi-band characterization for galaxies at z > 8.

Scientific context and open questions

Dust fundamentally shapes the evolution of galaxies and their multi-wavelength appearance because it profoundly alters the (sub)millimeter wavelength regime, which is uniquely capable of weighing these dust reservoirs, revealing the total extent of galaxy evolution that remains hidden behind them. With current facilities, however, doing so at z > 8 is largely out of reach. Understanding the origin and abundance of dust, as well as the importance of obscured star formation in the early Universe, requires pushing the detection and characterization of dusty galaxies deep into the epoch of cosmic dawn (here defined as z > 8, effectively the current ‘dusty frontier’). This requires a transformational (sub)millimeter facility capable of answering the following fundamental questions:

** When and how do the earliest dusty galaxies arise? **

** What is the nature of early dust, and where in galaxies does it reside? **

** What fraction of cosmic star formation remains obscured at cosmic dawn? **

When and how do the earliest dusty galaxies arise?

How, and how rapidly, dust is produced in the early Universe remains a key open question. The first dust is thought to originate from core-collapse supernovae (SNe; Todini & Ferrara 2001), though it remains unclear whether they alone account for the entirety of the earliest dust reservoirs. In the local Universe, dust production through Asymptotic Giant Branch (AGB) stars and growth in the interstellar medium (‘grain growth’) are believed to be the dominant channels (e.g., De Looze et al. 2020; Galliano et al. 2021). However, at high redshift, AGB dust production is limited by both timescale and metallicity: more massive AGB stars (> ∼ 3 M⊙) evolve quickly but produce little dust at low metallicity, while lower-mass stars form dust largely independent of metallicity, but only after several hundred Myr (Nanni et al. 2014; Schneider & Maiolino 2024). At z > ∼ 8, AGB stars are therefore expected to play a limited role in dust production. ALMA observations of (UV-luminous and generally massive) galaxies at 6 <∼ z <∼ 8 have revealed dust emission in a significant fraction of these systems (Watson et al. 2015; Tamura et al. 2019; Inami et al. 2022). Subsequent multi-band follow-up has enabled improved constraints on their dust masses (e.g., Bakx et al. 2021), which, combined with stellar mass measurements, provide a first-order diagnostic of their dust production pathways. The resulting dust-to-stellar mass ratios of Md/M⋆ ∼ 10−3 − 10−2 (e.g., Witstok et al. 2022; Algera et al. 2024) suggest either particularly high SN yields, an early onset of rapid AGB enrichment, or a significant contribution from grain growth (e.g., Popping et al. 2017; Di Cesare et al. 2023; Choban et al. 2025). Disentangling the relative importance of these channels requires additional ancillary data beyond stellar mass alone. Metallicity, in particular, offers a key diagnostic (e.g., Remy-Ruyer et al. ´ 2014; De Vis et al. 2019; Geesink et al. 2026; Tailor et al. 2026): while dust production via SNe alone is expected to yield a dust-to-gas (DTG) ratio that scales roughly linearly with metallicity (i.e., a constant dust-to-metal ratio, DTM), the onset of efficient grain growth is predicted to produce a much steeper, super-linear increase in DTG above a characteristic metallicity threshold of Z > ∼ 0.1 − 0.2 Z⊙ (e.g., Asano et al. 2013; Choban et al. 2024). With the advent of the JWST, highredshift constraints on the metallicity-dependence of the DTG and DTM ratios are now possible at z > ∼ 2.5. While statistics of dusty galaxies with metallicity information remain limited, the first constraints suggest that metal-rich (Z > ∼ 0.2 Z⊙) galaxies in the Epoch of Reionization host substantial dust reservoirs but are located (slightly) below the local DTG-metallicity scaling relation (Heintz et al. 2025; Umehata et al. 2025; Algera et al. 2026b; Rowland et al. 2026b; Pollock et al. 2026). To probe what are likely ‘pristine’ SN-produced dust reservoirs, it is essential to constrain the dust masses of ultra-high-redshift (z > ∼ 10) and/or metal-poor (Z < ∼ 0.1 Z⊙) galaxies, where AGB production and grain growth should both be inefficient. Beyond z = 8, however, MACS0416- Y1 at z = 8.31 (Tamura et al. 2019; Bakx et al. 2020) has remained the only known dusty galaxy since its discovery in 2019, even though JWST indicates the presence of dust-attenuated galaxies at even earlier times (Donnan et al. 2025; Mitsuhashi et al. 2026a). Several recent deep ALMAbased searches at z > 8 have come up empty-handed (Algera et al. 2026a; Bakx et al. 2026; Mitsuhashi et al. 2026b), suggesting a rapid transition in the typical dust contents of galaxies between z ∼ 8 − 9, the origin of which remains heavily debated (e.g., Ferrara et al. 2023; Burgarella et al. 2025; Narayanan et al. 2026; Sommovigo et al. 2026). Current ALMA upper limits on individual galaxies suggest Md/M⋆ <∼ 10−3 − 10−2.5 (Figs. 1 and 3), though with the caveat that the exact limits depend sensitively on the assumed dust temperature. At face value, these imply supernova yields below ySN <∼ 0.1 − 0.3 M⊙ SN−1 given a standard Chabrier IMF and no further dust production/destruction mechanisms (Michałowski 2015). However, to constrain SN yields and reveal the importance of grain growth at high redshift, much deeper observations across multiple bands are essential, probing down to Md/M⋆ <∼ 10−4 (equivalent to ySN <∼ 10−2 M⊙ SN−1). As quantified below, ALMA2040 will deliver these constraints even in typical, low-mass galaxies (M⋆ ∼ 108.5−9 M⊙) at cosmic dawn.

An order-of-magnitude leap in continuum sensitivity with ALMA2040

The left panel of Figure 2 shows the order-of-magnitude increase in sensitivity that ALMA2040 will deliver compared to ALMA – currently the most sensitive facility at these wavelengths.

1. Improved sensitivity for detecting early dust emission: ALMA2040 is set to change this, allowing it to "routinely detect dust emission from galaxies at z > 8. This capability enables the detection of dust masses down to log(Md/M⊙) ∼ 3.9 − 5.4 across z ∼ 8 − 15 in just 5 h on-source in Band 7, which is an order of magnitude deeper than ALMA can achieve in the same time."

2. Constraining early dust production pathways: The system can disentangle the relative importance of these channels by measuring metallicity, as it allows distinguishing between mechanisms where grain growth is efficient above a certain threshold: "the onset of efficient grain growth is predicted to produce a much steeper, super-linear increase in DTG above a characteristic metallicity threshold of Z > ∼ 0.1 − 0.2 Z⊙."

3. Mapping dust properties on sub-kpc scales: The system will enable resolving dust emission on sub-kpc spatial scales, allowing researchers to map directly where it is being produced and study the impact of geometry on attenuation curves by combining ALMA2040's sensitivity with rest-UV/optical measurements.

4. Constraining obscured star formation fraction: The system can constrain ρSFR,obsc(z) without assuming a single (evolving) dust temperature, allowing it to determine if these galaxies are truly dust-free, or how much higher the intrinsic UV luminosities that models must explain really are.

Although less than 1 percent of the baryonic mass budget of a galaxy, cosmic dust profoundly alters the multi-wavelength appearance of galaxies across cosmic time. The (sub)millimeter wavelength regime is uniquely capable of weighing these dust reservoirs, and revealing the total extent of galaxy evolution that remains hidden behind them. With current facilities, however, doing so at z > 8 is largely out of reach. ALMA2040, a next-generation (sub)millimeter interferometer, is set to change this. Through an order-of-magnitude increase in sensitivity compared to ALMA, ALMA2040 is a transformational facility that will be able to routinely detect dust emission from galaxies at z > 8 and, through simultaneous multi-band characterization, accurately measure their dust masses, temperatures, and infrared luminosities. Combined with ancillary metallicity measurements -- including from the far-infrared lines detected by ALMA2040 itself -- this will establish the relative roles of supernovae and grain growth in building up dust in the earliest galaxies. Simultaneously, ALMA2040 will measure the fraction of star formation that is obscured by dust at cosmic dawn, and thereby the extent to which rest-UV and optical surveys miss early galaxy build-up. Without a 10 times upgrade to ALMA -- comparable to the leap from the VLT to the ELT -- we will not be able to capitalize on the momentous advances enabled by JWST, and will remain blind to the cold and obscured component of galaxy formation in the z > 8 Universe.