Deep Submillimeter and Radio Observations in the SSA22 Field. III. Multiwavelength Identifications and Properties of the 850 $μ$m-selected Submillimeter Galaxies

We present a multiwavelength analysis of 850 $μ$m-selected SMGs (deblended S${\rm 850}\gtrsim$ 1mJy) in the SSA22 field, where our deepest JCMT/SCUBA-2 observations reach a sensitivity of $σ{850}\sim$ 0.80mJy beam$^{-1}$. Using multiple identification methods, we have identified 248 deblended SMG candidates for 192 SCUBA-2 sources. The average multiplicity of SCUBA-2 sources is $\sim$26%, with brighter sources exhibiting higher multiplicity. After applying quality cuts based on SED fitting reliability, our final sample comprises 221 SMGs associated with 186 SCUBA-2 sources. The SSA22 SMGs have a median infrared luminosity of (2.25$\pm$0.25) $\times$10$^{12}$ L${\odot}$, with $\sim$ 63% ($\sim$ 8%) of the sample classified as ULIRGs (HLIRGs). The median redshift of the sample is $z = 2.00 \pm 0.08$, while optically faint galaxies exhibit higher median redshift ($\sim 2.20 \pm 0.17$). The comoving volume density of SMGs increases by a factor of $\sim 6$ at $z \lesssim 4$, plateauing at $\sim$ 1.78-3.16 $\times$ 10$^{-5}$ cMpc$^{-3}$ over $z \sim$ 1-3 (including the overdensity). The significant overdensity of SMGs within large-scale structures demonstrates their reliability as tracers of cosmic structure formation at high redshift. The median stellar mass and SFR of our SMG sample are $(1.55 \pm 0.22) \times 10^{11}$ M$\odot$ and $166 \pm 25$ M$\odot$ yr$^{-1}$, respectively. We observe a clear ``downsizing" signature: after cosmic noon ($z \lesssim 2$), massive SMGs exhaust their gas reservoirs and transition to quiescence, while lower-mass SMGs continue forming stars and dominate the cosmic SFR density. The sample has a median dust mass of (1.95 $\pm$ 0.14) $\times$ 10$^{9}$ M${\odot}$. The dust fraction ($ M_{\text{dust}}/M_{\text{star}}$) has a median value of (1.4 $\pm$ 0.2) $\times$ 10$^{-2}$. The median $A_V$ of SMGs is 3.09$\pm$0.07mag.

💡 Research Summary

This paper presents a comprehensive multi‑wavelength study of 850 µm‑selected submillimeter galaxies (SMGs) in the well‑known SSA22 field, leveraging the deepest JCMT/SCUBA‑2 map to date (σ≈0.80 mJy beam⁻¹ over 0.34 deg²). By combining the SCUBA‑2 data with extensive ancillary observations—including VLA 3 GHz radio, Spitzer/MIPS 24 µm, Herschel PACS/SPIRE, IRAC 8 µm, and deep optical/NIR imaging from CFHT, Subaru, and UKIRT—the authors construct a robust prior catalog and apply the “super‑deblending” technique (Liu et al. 2018) to overcome the coarse resolution and source confusion inherent to single‑dish submillimeter surveys.

From 192 SCUBA‑2 detections, 248 SMG candidates are identified, corresponding to an average multiplicity of ~26 % (i.e., roughly one‑quarter of the single‑dish sources split into multiple physical galaxies). The multiplicity rises with 850 µm flux, reaching ~40 % for sources brighter than 5 mJy, confirming that bright submillimeter peaks often comprise several distinct galaxies. After imposing quality cuts based on the reliability of spectral energy distribution (SED) fitting, the final sample contains 221 SMGs associated with 186 SCUBA‑2 sources.

Physical properties are derived via full‑SED fitting with CIGALE, which simultaneously fits UV–radio photometry while enforcing energy balance. This approach yields consistent estimates of photometric redshifts, infrared luminosities, stellar masses, star‑formation rates (SFRs), dust masses, and visual attenuation (A_V). The median infrared luminosity is L_IR = (2.25 ± 0.25) × 10¹² L_⊙, placing 63 % of the sample in the ULIRG regime and 8 % in the hyper‑luminous (HLIRG) regime. The redshift distribution peaks at z = 2.00 ± 0.08; optically faint SMGs (i > 25) have a slightly higher median redshift of ~2.20. Stellar masses average (1.55 ± 0.22) × 10¹¹ M_⊙, while SFRs average 166 ± 25 M_⊙ yr⁻¹, indicating that most SMGs lie on or above the star‑forming main sequence at these epochs. Dust masses are (1.95 ± 0.14) × 10⁹ M_⊙, giving a dust‑to‑stellar mass ratio of (1.4 ± 0.2) × 10⁻². The median visual extinction is A_V = 3.09 ± 0.07 mag, with a notable decline beyond z ≈ 4, suggesting a transition to less dust‑obscured star formation in the early universe.

The comoving number density of SMGs rises by a factor of ~6 up to z ≈ 4 and then plateaus at 1.8–3.2 × 10⁻⁵ cMpc⁻³ between z ≈ 1–3, even after accounting for the known overdensity at z ≈ 3.09 associated with the SSA22 proto‑cluster. This pronounced clustering demonstrates that SMGs are reliable tracers of high‑redshift large‑scale structure, especially in overdense environments.

A clear “downsizing” trend emerges: at z < 2, massive SMGs (M_* ≳ 10¹¹ M_⊙) show a rapid decline, consistent with gas depletion and quenching, whereas lower‑mass SMGs (M_* ∼ 10¹⁰ M_⊙) continue vigorous star formation and dominate the cosmic SFR density at later times. This evolutionary picture aligns with the notion that SMGs represent a transitional phase for the progenitors of massive elliptical galaxies, with the most massive systems exhausting their fuel early and evolving into quiescent systems.

The study highlights the power of combining deep single‑dish submillimeter maps with high‑resolution radio/infrared priors and sophisticated deblending algorithms to obtain a near‑complete census of SMGs, including those lacking optical counterparts. The authors discuss the implications for models of galaxy formation, emphasizing the role of SMGs in metal and dust enrichment, gas accretion, and the build‑up of massive dark‑matter halos. They also note that upcoming facilities such as the 15‑m XSMT, JWST, and continued ALMA surveys will enable even finer dissection of SMG sub‑structures, gas dynamics, and AGN activity, further refining our understanding of how these dusty powerhouses shape cosmic evolution.