ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: The Infrared Excess of UV-selected z=2-10 galaxies as a function of UV-continuum Slope and Stellar Mass

Abstract: We make use of deep 1.2mm-continuum observations (12.7microJy/beam RMS) of a 1 arcmin^2 region in the Hubble Ultra Deep Field to probe dust-enshrouded star formation from 330 Lyman-break galaxies spanning the redshift range z=2-10 (to ~2-3 Msol/yr at 1sigma over the entire range). Given the depth and area of ASPECS, we would expect to tentatively detect 35 galaxies extrapolating the Meurer z~0 IRX-beta relation to z>~2 (assuming T_d~35 K). However, only 6 tentative detections are found at z>~2 in ASPECS, with just three at >3sigma. Subdividing z=2-10 galaxies according to stellar mass, UV luminosity, and UV-continuum slope and stacking the results, we only find a significant detection in the most massive (>10^9.75 Msol) subsample, with an infrared excess (IRX=L_{IR}/L_{UV}) consistent with previous z~2 results. However, the infrared excess we measure from our large selection of sub-L* (<10^9.75 Msol) galaxies is 0.11(-0.42)(+0.32) and 0.14(-0.14)(+0.15) at z=2-3 and z=4-10, respectively, lying below even an SMC IRX-beta relation (95% confidence). These results demonstrate the relevance of stellar mass for predicting the IR luminosity of z>~2 galaxies. We furthermore find that the evolution of the IRX-stellar mass relationship depends on the evolution of the dust temperature. If the dust temperature increases monotonically with redshift (as (1+z)^0.32) such that T_d~44-50 K at z>=4, current results are suggestive of little evolution in this relationship to z~6. We use these results to revisit recent estimates of the z>~3 SFR density. One less obvious implication is in interpreting the high Halpha EWs seen in z~5 galaxies: our results imply that star-forming galaxies produce Lyman-continuum photons at twice the efficiency (per unit UV luminosity) as implied in conventional models. Star-forming galaxies can then reionize the Universe, even if the escape fraction is <10%.

• The paper quantifies the evolution of the IRX–β relation, with only six tentative detections (three above 3σ) against an expected 35 based on local predictions.
• It employs deep ALMA observations of 330 Lyman-break galaxies in the HUDF, categorizing them by stellar mass, UV luminosity, and continuum slope.
• The study shows that massive galaxies (M > 10^9.75 M⊙) display significant infrared excess, while lower-mass galaxies exhibit minimal dust obscuration affecting SFR estimates.

Overview of the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field

The paper "ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: The Infrared Excess of UV-selected $z=2$-10 galaxies as a function of UV-continuum Slope and Stellar Mass" by Bouwens et al. seeks to quantify the dust-enshrouded star formation processes in high-redshift galaxies. Using the Atacama Large Millimeter/submillimeter Array (ALMA) observations, the study examines 330 Lyman-break galaxies (LBGs) across a redshift range from $z=2$ to $z=10$ within a 1 arcmin$^2$ field in the Hubble Ultra Deep Field (HUDF).

Key Results and Analysis

The researchers anticipated detecting 35 galaxies based on extrapolations from the Meurer $z\sim0$ IRX-$\beta$ relation if applied to $z\geq2$ galaxies, assuming a dust temperature $T_d\sim35$ K. Contrary to these expectations, only six detections were tentatively identified in the ALMA deep field data at $z\gtrsim2$, with three being statistically significant above $3\sigma$. This discrepancy highlights the potential evolution in the IRX-$\beta$ relationship at these redshifts.

Further analysis that categorized galaxies into subsamples based on stellar mass, UV luminosity, and UV-continuum slope indicated significant infrared excess primarily in galaxies with stellar masses greater than $10^{9.75}$ $M_{\odot}$. The infrared excess (IRX) for these massive galaxies aligns with previous $z\sim2$ results. However, for sub-$L^*$ galaxies with lower masses, the measured IRX is underwhelming, suggesting a dependency of IRX on stellar mass rather than UV continuum slope.

Implications and Theoretical Considerations

This research provides substantial insight into the importance of stellar mass in predicting the infrared luminosity of $z\gtrsim2$ galaxies. The findings imply a possible evolution in the infrared excess-stellar mass relationship which may be contingent on dust temperature. The study suggests that if dust temperatures increase with redshift, the IRX-stellar mass relationship shows little evolution up to $z\sim6$.

The results have profound implications for understanding the star formation rate density (SFRD) at high redshifts. The lack of significant dust emission from a large portion of the observed LBG population suggests that the star formation, particularly in less massive galaxies at $z>3$, is less obscured by dust. This conclusion is crucial for modeling galaxy evolution and the cosmic star formation history, as it implies that analyses solely based on rest-UV observations might not severely underestimate the SFRD at these epochs.

Speculative Directions and Future Prospects

This study raises important questions about the evolution of dust properties in early galaxies. The potential increase in dust temperature with redshift, as inferred, requires further investigation. Upcoming ALMA observations and Synergistic work with future JWST observations will be critical to confirm these trends and explore the physics regulating dust heating in high-redshift galaxies. Enhanced constraints could ultimately refine models of cosmic reionization and galaxy formation.

Overall, the Bouwens et al. study provides a nuanced understanding of the infrared characteristics of high-redshift galaxies and paves the way for further exploration of the role of dust in cosmic evolution.