Deep Submillimeter and Radio Observations in the SSA22 Field. IV. Spectral Energy Distributions, Star Formation Histories, and the Infrared-Radio Correlation of the 850 $μ$m-selected SMGs

We analyze the spectral energy distributions (SEDs), star formation histories (SFHs), and infrared-radio correlation (IRRC) of 221 850 $μ$m-selected submillimeter galaxies (SMGs) in the SSA22 deep field. The median mass-weighted age is 567 Myr. Most galaxies in our sample began forming $\sim$ 1.68 Gyr after the Big Bang, entered the SMG phase' after $\sim$ 1 Gyr of evolution -- when they are predominantly observed -- and largely transitioned out of the SMG phase’ to become quiescent within an additional $\sim$ 0.2 Gyr. A subset of massive galaxies shows rapid early assembly with high star formation efficiencies ($\sim$0.2-0.8). The majority of SMGs reside at the high-mass end of the star-forming main sequence, with a characteristic stellar mass of $M_{star} \sim 10^{11}$ M$\odot$, above which galaxies are predominantly either on the main sequence or already quenched. We observe a downsizing trend: more massive galaxies tend to ``mature" earlier, completing their major episodes of star formation at higher redshifts compared to lower-mass systems. Our sample contributes $\sim$ 21% (28%) to the cosmic star formation rate density (stellar mass density), including the overdensity, with its relative contribution peaking at 50-60% in the redshift range $z=2.5-3.5$. The median infrared-radio correlation parameter $q{IR}$ is 2.37, evolving as $(1+z)^{-0.11}$, likely due to AGN contributions at high redshift and intrinsic differences between low- and high-redshift populations.

💡 Research Summary

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This paper presents a comprehensive multi‑wavelength analysis of 221 sub‑millimeter galaxies (SMGs) selected at 850 µm in the SSA22 deep field. The authors combine the deepest SCUBA‑2 850 µm imaging (91 h integration, σ≈0.79 mJy beam⁻¹ over 0.34 deg²) with high‑resolution 3 GHz JVLA radio data (1.5 µJy beam⁻¹, 2.3″×2.0″) and a suite of ancillary optical–mid‑infrared observations. Source identification employs four complementary techniques—radio counterparts, 24 µm Spitzer/MIPS detections, 8 µm Spitzer/IRAC detections, and optical/NIR colour cuts—yielding 248 candidates. After photometric redshift estimation with EAZY and full spectral energy distribution (SED) fitting using CIGALE, 221 robust SMGs remain for scientific analysis.

The average SED, constructed by normalising each galaxy to a median infrared luminosity of L_IR = 2.25 × 10¹² L_⊙, shows a relatively cold dust component (T_dust ≈ 30–35 K) compared with local ULIRGs, with a modest FIR peak around 60–70 µm. Higher‑redshift SMGs tend to be more luminous and exhibit warmer dust (T_dust ≈ 40 K). The mid‑infrared (3–100 µm) regime displays ~0.5 dex scatter driven by variations in star‑formation‑heated dust and PAH emission, while the UV‑NIR region varies by 3–5 dex, reflecting diverse geometries, dust masses, and attenuation.

Star‑formation histories derived from the CIGALE fits reveal a median mass‑weighted age of 567 Myr. Most galaxies began forming stars ≈1.68 Gyr after the Big Bang, entered the “SMG phase” roughly 1 Gyr later, and typically exit this phase to become quiescent within an additional ≈0.2 Gyr. A subset of massive systems (M_* ≈ 10¹¹ M_⊙) shows high star‑formation efficiencies (ε_SF ≈ 0.2–0.8), indicating rapid early assembly. When placed on the star‑forming main sequence (SFR–M_), the SMGs occupy the high‑mass end; above M_ ≈ 10¹¹ M_⊙ galaxies are either on the main sequence or already quenched. This mass‑dependent behaviour demonstrates a clear “downsizing” trend: more massive galaxies complete their major star‑formation episodes at higher redshifts than lower‑mass counterparts.

In terms of cosmic impact, the SMG sample contributes ≈21 % of the total cosmic star‑formation rate density (SFRD) and ≈28 % of the stellar mass density (SMD) when the overdensity of SSA22 is included. Their relative contribution peaks at 50–60 % in the redshift interval z = 2.5–3.5, underscoring the pivotal role of SMGs during the epoch of peak cosmic star formation.

The infrared‑radio correlation (IRRC) is examined via the q_IR parameter. The median q_IR = 2.37, with an evolution described by q_IR ∝ (1+z)⁻⁰·¹¹, indicating a modest decline with redshift. The authors attribute this trend to increasing AGN contributions at high z and intrinsic differences between low‑ and high‑redshift SMG populations (e.g., stronger magnetic fields, enhanced free‑free emission). While the bulk of the sample follows the classic IRRC, a fraction of high‑z sources shows excess radio emission, suggesting that radio‑based AGN diagnostics remain effective for SMGs.

Methodologically, the study highlights the importance of multi‑wavelength counterpart identification, robust photometric redshifts, and physically motivated SED modelling. Limitations include uncertainties in photometric redshifts due to incomplete optical/NIR coverage, dependence on assumed IMF and dust attenuation laws in CIGALE, and the JVLA resolution that may blend close companions. The authors propose that forthcoming high‑resolution ALMA imaging and JWST spectroscopy will enable detailed dissection of SMG internal structures, clarify the AGN versus star‑formation radio contributions, and refine the evolutionary pathways linking SMGs to present‑day massive ellipticals.

Overall, this work provides a thorough statistical portrait of SMGs in a well‑studied protocluster field, quantifies their contribution to cosmic star formation, elucidates their star‑formation histories and mass‑dependent evolution, and confirms that the infrared‑radio correlation persists to z ≈ 4, albeit with subtle redshift evolution driven by AGN activity and changing physical conditions.