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

TL;DR

The study tackles identifying and characterizing 850 μm-selected SMGs in the SSA22 field by combining deep SCUBA-2 imaging, JVLA 3 GHz data, and wide multiwavelength coverage. It uses a multi-method counterpart identification strategy with super-deblending for FIR/mm flux deconfusion and full SED fitting with CIGALE to derive photometric redshifts and physical properties. The resulting sample includes 221 SMGs associated with 186 SCUBA-2 sources, with a median infrared luminosity of $(2.25 \pm 0.25) \times 10^{12} L_\odot$ and a median redshift of $z = 2.00 \pm 0.08$; about $63\%$ are ULIRGs and $8\%$ HLIRGs, with median SFR $166 \pm 25$ M$_{\odot}$ yr$^{-1}$ and median stellar mass $(1.55 \pm 0.22) \times 10^{11}$ M$_{\odot}$. The analysis shows downsizing, with massive SMGs quenching after $z \lesssim 2$ while lower-mass SMGs continue forming stars and dominating the cosmic SFR density, and reveals significant overdensities of SMGs tracing large-scale structure in SSA22.

Abstract

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.

Figures (5)

• Figure 1: The gray circular base map represents the SCUBA-2 850 $\hbox{$\mu$m}$ map ($\sim$ 0.34 deg$^2$), with the identified region shown in dark grey. The black dashed line demarcates the deep survey region ($\sigma_\text{850}$$\leqslant$ 1 mJy). The multiwavelength data is depicted with colorful dashed lines in the SSA22 field. The blue lines delineate the footprint of the VLA 3 GHz map, and the yellow rectangle indicates the region of the optical/NIR dataset. The FIR/millimeter images encompass the Herschel/SPIRE data and the AzTEC/ASTE data, effectively covering the entire SCUBA-2 observed region. The Spitzer data consisted of both the deeper enhanced products (SEIP) and the shallower SpIES images. The latter provides complete coverage of the SSA22 field. The colorful points represent SMGs identified using different methods, with each color matching that of the corresponding identification band coverage region.
• Figure 2: Left panel: The 850 $\hbox{$\mu$m}$ flux density map of the SSA22 deep field observed with JCMT/SCUBA-2, covering an circular region of approximately 20$\hbox{$^\prime$}$ in radius (cyan circle). The majority of the "SSA22-Sb1" region yamada2012 lies within the SCUBA-2 footprint (green rectangle). Right panel: The corresponding rms noise map. Our analysis focuses on areas where the noise level is $\leqslant$ 2 mJy, encompassing $\sim 0.34$ deg$^2$. The deepest region reaches an rms of 0.79 mJy beam$^{-1}$, indicated by the innermost white isocontour, with additional contours at 1.0 and 1.5 mJy beam$^{-1}$. White dots mark submillimeter sources detected at $\geqslant$ 3.5$\sigma$ significance.
• Figure 3: An example of the source extraction procedure implemented with PYBDSF on the JVLA 3 GHz map. Panel (a) shows a portion of the island map, centered on SSA22.0000 and SSA22.0070, with Gaussian fits overlaid. Each island, bounded by 3$\sigma$ pixels, is marked with "+" symbol. Pixels within islands were fitted with Gaussian functions, where colored ellipses represent Gaussian fits to peak pixels above 5$\sigma$ significance, with different colors indicating distinct sources. The two white circles indicate SCUBA-2 beam sizes, marking the extent of SCUBA-2 sources SSA22.0000 and SSA22.0070. Panel (b) displays the Gaussian modeling of radio sources performed by the pipeline. Panel (c) presents the output flagging map, where sources with Gaussian FWHM values exceeding their island size when multiplied by the flag_maxsize_fwhm parameter (default=0.5) are flagged and subsequently excluded from the final catalog.
• Figure 4: A window within the central region of the SSA22 deep field. The base image displays the false-color map composed of g-r-i bands, with white contours indicating submillimeter emission at 4.5, 7, 10$\sigma$ significance, and cyan contours showing 3 GHz emission at 5 and 30$\sigma$ levels. Due to the tight correlation between infrared and radio emission, submillimeter sources are commonly associated with radio counterparts. Robust SMG identifications typically appear as red point sources or are undetected in the optical/near-infrared bands.
• Figure 5: An example of the source extraction procedure implemented with SExtractor, centered on SSA22.0000 and SSA22.0070. The two dashed circles indicate the SCUBA-2 beams for these sources, each with a radius of 7$\hbox{$^{\prime\prime}$}$. The panels show, form left to right and top to bottom: the background map, the filtered image, the detection image, the segmentation map, the photometric aperture, and the residual image after source subtraction.