A warm ultra-luminous infrared galaxy just 600 million years after the Big Bang

TL;DR

The study directly constrains the dust properties of a z = 8.3 LBG, MACS0416_Y1, via ALMA Band 9 observations, revealing a warm dust component with Td ≈ 90 K and an infrared luminosity of LIR ≈ 1.0 × 10^12 L⊙, placing it in the ULIRG regime. A single modified black-body fit to multi-band continuum data (Bands 3–9) with CMB heating corrections yields Md ≈ 1.4 × 10^6 M⊙ and IRX ≈ 1.4, while the dust and UV emission are spatially related but not perfectly coextensive, implying sub-200 pc dust–star separation. The high dust temperature minimizes the required dust mass and suggests substantial obscured star formation (≈173 M⊙ yr^-1), with an obscured fraction ≈ 93%, contributing significantly to the early cosmic star-formation budget. The results support a picture where warm, low-metallicity ISM environments at the end of the reionization era can host intense, dust-obscured star formation that is partly unresolved by current UV/optical surveys, emphasizing the importance of (sub)mm observations for a complete census of early galaxy growth.

Abstract

We present an Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 continuum detection ($3.3 σ$) of MACS0416_Y1 that confirms the suspected warm dust (91$^{+62}_{-35}$ K) of this Lyman-Break Galaxy (LBG) at $z = 8.3$ with $\log_{10} M_{\ast}/$M$_{\odot} = 9.0 \pm 0.1$. A modified black-body fit to the ALMA Bands 3 through 9 data of MACS0416_Y1 finds an intrinsic infrared luminosity of 1.0$^{+1.8}_{-0.6} \times{} 10^{12}\ \mathrm{L_{\odot}}$, placing this UV-selected LBG in the regime of Ultra Luminous Infrared Galaxies (ULIRGs). Its luminous but modest dust reservoir (1.4$^{+1.3}_{-0.5} \times{} 10^{6}\ \mathrm{M_{\odot}}$) is co-spatial to regions with a UV-continuum slope $β_{\rm UV} \approx -1.5$ as seen by James Webb Space Telescope (JWST) imaging. Although this implies some dust obscuration, the JWST photometry implies less obscured star formation than seen in the complete characterization by ALMA, implying some spatial separation of dust and stars on scales below 200 pc, i.e., smaller than those probed by JWST and ALMA. This source is an extreme example of dust-obscured star formation contributing strongly to the cosmic build-up of stellar mass, which can only be revealed through direct and comprehensive observations in the (sub)mm regime.

Figures (10)

• Figure 1: The dust continuum of MACS0416-Y1 in all (tentatively) detected Bands, namely 5, 6, 7, 8, and 9 -- covering rest-frame wavelengths 160 to 45$\mu$m. For Band 9, the 0.5 arcsec $uv$-tapered dust continuum is shown. The poststamps are 3 by 3 arcseconds, with the background image showing the continuum overlaid, together with white contours indicating the $2, 3, 4, 5 \sigma$ emission. The beam is shown in the bottom-left corner, and the scalebar is shown in the bottom-right corner in the right-hand side poststamp.
• Figure 2: Three modified black-bodies are fitted to the data of Y1 (blue stars for novel data, and black circles for archival data, with upper limits in grey circles), with different considerations of the $\beta_{\rm d}$ slope. The blue line and fill indicates the best-fit results of a free $\beta_{\rm d}$ fit, while the red and orange lines indicate fits with fixed $\beta_{\rm d}$ of 2 and 1.5, respectively. The best-fit temperatures (listed in Table \ref{['tab:data']}) range between 72 and 94 K. The dashed lines show the new [C ii]-based dust temperature using the methodology of Sommovigo2021 in black (see Appendix for details) and the dust scaling relation from Fudamoto2022DustTemperaturesgrey.
• Figure 3: The infrared luminosity as a function of redshift of Y1 (light blue star) is compared against those of individual and stacked $z > 8$ sources that remain undetected in deep sub-mm observations (dark red triangles; Bakx et al. in prep.), as well as similar Lyman-break galaxies at $z = 5 - 15$ (orange pentagons; Laporte2019faisst2020Harikane2020Sugahara2022ApJ...935..119SMitsuhashi2024Serenade and Bakx et al. in prep.), UV-detected galaxies targeted in the ALMA Large Programs ALPINE and REBELS (blue, with upper limits in light blue squares; lefevre2019bethermin2020Bouwens2022REBELSInami2022), quasar-companion galaxies (purple pluses; venemans2020Bakx2024QSOcompanions), and dusty star-forming galaxies (red triangles; reuter20ismail2023zbendo2023). A model predicting the brightest dusty galaxy from a $0.2$ square degree survey size, assuming $T_{\rm d} = 50$K and a dust-to-stellar mass ratio of 0.01 (blue line; Behroozi2018). Note that the warm dust temperature of Y1 causes its high luminosity with a more modest dust mass of $\sim 1.4 \times 10^6$M$_{\odot}$.
• Figure 4: Dust temperature of galaxies and samples as a function of redshift. The dust temperature estimate for Y1 is shown in light blue, and is compared to previous estimates based on [C ii] Sommovigo2021 and on a clumpy dust distribution geometric estimate Fudamoto2022DustTemperatures in grey. Dust temperatures obtained for lower-redshift sources at $z < 4$ are shown in grey points Schreiber2018, and for higher-redshift galaxies in blue squares (faisst2020Harikane2020Sugahara2022ApJ...935..119SWitstok2023DustAlgera2024Algera2024R25). Dust temperature evolution based on stacked SEDs is shown in red and green pluses, which are shown to increase linearly with redshift up to $z = 6$Schreiber2018bethermin2015bethermin2020. The best-fit from Schreiber2018 is shown in a red line, while the average physically-motivated dust temperature relation from Sommovigo2022REBELS is shown in a blue line. Prior to the Band 9 data, only lower limits were available on Y1. The other axis indicates the change in infrared luminosity compared to 50 K, indicating the strong dependence of infrared luminosity on dust temperature, and the greyed-out region indicates the CMB temperature at a given redshift.
• Figure 5: Obscured fraction of the star formation against the observed absolute UV magnitude ($M_{\rm UV}$-$f_{\rm obs}$). Y1 lies above the relation at a lower absolute UV-magnitude than previous stacked results. The increasing hues of green downward triangles, triangles, squares, circles, diamonds, and pentagons are the results at $z\sim4.5$, 5, 5.5, 6.7 and 7 Fudamoto2020Mitsuhashi2024CRISTALBowler2024Algera2023Mitsuhashi2024Serenade, respectively. The scaling relations from Whitaker2017Fudamoto2020 are shown in solid and dashed lines, respectively.