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

TL;DR

This study investigates 221 850 μm–selected SMGs in the SSA22 field by combining deep submillimeter and radio data with multiwavelength identifications and SED fitting using CIGALE. It derives an average, IR-luminosity–normalized SED (L_IR = 2.25 × 10^{12} L_⊙) showing cooler dust peaks (λ_peak ≈ 100 μm, T_peak ≈ 50 K) and substantial UV–NIR dispersion, with SMGs largely occupying the high-mass end of the star-forming main sequence (M_* ∼ 10^{11} M_⊙). Through reconstructed SFHs, the paper finds a downsizing trend where more massive systems form earlier and quench by z ≈ 1.5, and quantifies SMG contributions to the cosmic SFR density and stellar mass density (≈21–28%, peaking at z ≈ 2.5–3.5). The infrared–radio correlation parameter is measured as q_IR ≈ 2.37 and evolves as q_IR ∝ (1+z)^{-0.11}, suggesting a combination of AGN contributions and population differences at high redshift. Overall, the results illuminate mass- and redshift-dependent pathways for SMG mass assembly and their role in shaping cosmic star formation.

Abstract

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 $z=2.5-3.5$. We suggest that 850 $μ$m surveys may miss a population of faint, warm galaxies at $z \geq 1$-2. 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.

Figures (7)

• Figure 1: Panel (a): The SEDs of the 221 SMGs in the SSA22 deep field, normalized to the average infrared luminosity, and color-coded by dust attenuation $A_V$. The solid black line and grey shaded gray region represent the median SED and 1$\sigma$ dispersion, respectively. Panel (b): A comparison of the SSA22 SMG SEDs with those from other submillimeter surveys and local starburst galaxies and ULIRGs. Our results are consistent with the literature and exhibit colder dust temperatures than typical local ULIRGs or starbursts. However, the SED shape of SMGs more closely resembles that of local ULIRGs than that of local starbursts. Panel (c): SEDs of SMGs in different evolutionary stages. Quiescent SMGs display colder dust temperatures and prominent features associated with older stellar populations, while starbursting SMGs show stronger dust attenuation and warmer dust temperatures.
• Figure 2: The median SEDs for SMGs for groups categorized by different physical or observational properties. The number of sources in each group shown within brackets in the legend. The black dashed curve represents the median SED of the total sample, while vertical dotted lines indicate the peak of FIR emiision for each group's SED. Notably, the average SEDs within each group are not normalized to their infrared luminoisty.
• Figure 3: The average SED of SSA22 SMGs, along with the median contributions from various physical components. Different emission components–including stellar populations and dust features–are indicated with distinct colors and labels.
• Figure 4: The distribution of SMGs on the SFR-$M_\text{star}$ panel. The cyan solid line denotes the main sequence relation speagle2014 at the central value of the redshift range, with yellow and blue dashed lines representing the 1$\sigma$ scatter (0.2 dex). The outermost red and black solid lines delineate the 2.5$\sigma$ dispersion; SMGs within this boundary are considered main sequence galaxies, while those outside are classified as starburst or quiescent systems. Grey dots represent SMGs from ALMA surveys cunha2015dud2020hyun2023. It's important to note that classification actually requires calculations based on individual galaxy redshifts and masses. Purple and green circles represent counterparts identified through different methods (see Paper III in the series), showing consistent distributions of SMGs across methods. Yellow stars indicate SMGs within the redshift range of 3.09 $\pm$ 0.1, most of which remain on the main sequence or starburst phase.
• Figure 5: Left: The distribution of galaxies' offset ($\Delta$MS = log$_\text{10}$(SFR/SFR$_\text{MS}$) )from the main sequence against stellar mass. The horizontal dashed lines serve as a boundary between the main sequence and quiescent (or starburst) states. The vertical dashed line indicates a characteristic stellar mass $M_\text{star} \gtrsim 10^{11} M_\odot$; above this threshold, SMGs are predominantly quiescent or on the main sequence, while lower-mass systems tend to be starbursts or main-sequence galaxies. Circles and triangles denote counterparts identified through different methods (refer to Paper III in the series), showing broadly consistent distributions. Middle: Evolution of SMGs in different evolutionary stages with redshift. Right: Fraction of SMGs in different evolutionary stages, conditioned on exceeding a given luminosity threshold.