An ALMA Survey of Submillimeter Galaxies in the Extended Chandra Deep Field South: The Redshift Distribution and Evolution of Submillimeter Galaxies

Abstract: We present the first photometric redshift distribution for a large unbiased sample of 870um selected submillimeter galaxies (SMGs) with robust identifications based on observations with the Atacama Large Millimeter Array (ALMA). In our analysis we consider 96 SMGs in the Extended Chandra Deep Field South, 77 of which have 4-19 band, optical-near-infrared, photometry. We model the Spectral Energy Distributions (SEDs) for these 77 SMGs, deriving a median photometric redshift of z=2.3+/-0.1. The remaining 19 SMGs have insufficient optical or near-infrared photometry to derive photometric redshifts, but a stacking analysis of IRAC and Herschel observations confirms they are not spurious. Assuming these sources have an absolute H-band magnitude distribution comparable to that of a complete sample of z~1-2 SMGs, we demonstrate that the undetected SMGs lie at higher redshifts, raising the median redshift for SMGs to z=2.5+/-0.2. More critically we show that the proportion of galaxies undergoing an SMG phase at z>3 is 35+/-5% of the total population. We derive a median stellar mass for SMGs of Mstar=(8+/-1)x10^10Mo, but caution that there are significant systematic uncertainties in our stellar mass estimate, up to x5 for individual sources. We compare our sample of SMGs to a volume-limited, morphologically classified sample of ellipticals in the local Universe. Assuming the star formation activity in SMGs has a timescale of ~100Myr we show that their descendants at z~0 would have a space density and M_H distribution which are in good agreement with those of local ellipticals. In addition the inferred mass-weighted ages of the local ellipticals broadly agree with the look-back times of the SMG events. Taken together, these results are consistent with a simple model that identifies SMGs as events that form most of the stars seen in the majority of luminous elliptical galaxies at the present day.

Photometric Redshift Distribution and Analysis of Submillimeter Galaxies in the Extended Chandra Deep Field South

The study by Simpson et al. investigates submillimeter galaxies (SMGs) within the Extended Chandra Deep Field South (ECDFS), offering a meticulous analysis of their photometric redshift distribution using ALMA observations. It presents the first comprehensive photometric redshift distribution for SMGs identified through robust submillimeter observations, addressing a lingering challenge of misidentification and incompleteness inherent in previous radio-submillimeter identification methods.

The analysis encompasses a sample of 96 SMGs detected via ALMA, with photometric data spanning 19 optical and infrared bands. The authors measured photometric redshifts for 77 of these SMGs, achieving a median redshift of ( z_{phot} = 2.3 \pm 0.1 ). The paper further discusses 19 SMGs with insufficient photometric coverage, performing stacking analyses that confirm these sources are not spurious. Through assumptions and statistical modeling around these faint SMGs, Simpson et al. propose that these are more likely positioned at higher redshifts, potentially modifying the median redshift to ( z_{phot} = 2.5 \pm 0.2 ).

Key findings include evidence of a significant tail at high redshift, with up to 35% of the SMGs potentially residing at ( z \ge 3 ). The derived photometric redshift distribution contrasts earlier radio-selected samples, such as Chapman et al. (2005), which demonstrated fewer SMGs at high redshifts. This discrepancy underscores the necessity of reevaluating SMGs' redshift distributions via pure submillimeter-imaging approaches.

The authors address limitations and potential biases introduced by observational methodologies, including concerns over blending and limited source resolution, offering a critical perspective on the interpretation of SMG characteristics and distributions. Their analysis suggests that SMGs are pivotal events in the formation of most stars in luminous elliptical galaxies observed today.

Furthermore, the implications of their findings extend to cosmic history, providing insights into early-type and elliptical galaxies' formation, particularly their stellar mass accumulation. With a mean stellar mass of ( M_{\star} = (8 \pm 1) \times 10^{10} \, M_{\odot} ), the study extrapolates the SMG space density, proposing a coherent evolutionary model aligning with the characteristics of local elliptical galaxies. This model supports the hypothesis of SMGs as progenitors of modern elliptical galaxies, marked by alignment in space density and mass-weighted stellar ages.

Simpson et al.'s work complements previous small-sample studies by offering a more statistically robust redshift distribution of SMGs, thereby refining our understanding of their role within galactic evolutionary pathways. The study highlights the dynamic interplay between star formation epochs and subsequent passive evolution, ultimately contributing to the broader discourse on galaxy formation and evolution—a testament to ALMA's capabilities in redefining our comprehension of submillimeter astrophysics.

Future directions could include expanding spectroscopic follow-up to further constrain potential secondary redshift solutions in SMG studies, as well as addressing the need for broader millimeter-wavelength surveys to corroborate these findings across varying cosmic environments.