An ALMA survey of submillimetre galaxies in the Extended Chandra Deep Field South: High resolution 870um source counts (1210.0249v2)

Abstract: We report the first counts of faint submillimetre galaxies (SMG) in the 870-um band derived from arcsecond resolution observations with the Atacama Large Millimeter Array (ALMA). We have used ALMA to map a sample of 122 870-um-selected submillimetre sources drawn from the (0.5x0.5)deg^2 LABOCA Extended Chandra Deep Field South Submillimetre Survey (LESS). These ALMA maps have an average depth of sigma(870um)~0.4mJy, some ~3x deeper than the original LABOCA survey and critically the angular resolution is more than an order of magnitude higher, FWHM of ~1.5" compared to ~19" for the LABOCA discovery map. This combination of sensitivity and resolution allows us to precisely pin-point the SMGs contributing to the submillimetre sources from the LABOCA map, free from the effects of confusion. We show that our ALMA-derived SMG counts broadly agree with the submillimetre source counts from previous, lower-resolution single-dish surveys, demonstrating that the bulk of the submillimetre sources are not caused by blending of unresolved SMGs. The difficulty which well-constrained theoretical models have in reproducing the high-surface densities of SMGs, thus remains. However, our observations do show that all of the very brightest sources in the LESS sample, S(870um)>12mJy, comprise emission from multiple, fainter SMGs, each with 870-um fluxes of <9mJy. This implies a natural limit to the star-formation rate in SMGs of <10^3 M_Sun/yr, which in turn suggests that the space densities of z>1 galaxies with gas masses in excess of ~5x10^10 M_Sun is <10^-5 Mpc^-3. We also discuss the influence of this blending on the identification and characterisation of the SMG counterparts to these bright submillimetre sources and suggest that it may be responsible for previous claims that they lie at higher redshifts than fainter SMGs.

• The paper demonstrates that high-resolution ALMA data can de-blend bright submillimetre sources into multiple, fainter components.
• It resolves 122 LABOCA-LESS sources to an accuracy of ~1.5″, revising previous source counts and establishing a natural star formation rate ceiling.
• These findings challenge existing galaxy evolution models by showing that bright-end SMG counts often result from overlapping fainter emissions rather than single, extreme starbursts.

High-Resolution 870 µm Source Counts from the ALMA/LESS Survey

The paper provides a meticulous analysis of 870 µm submillimetre galaxy (SMG) source counts derived from high-resolution observations with the Atacama Large Millimeter Array (ALMA) in the Extended Chandra Deep Field South (ECDFS). Leveraging ALMA's capabilities, the researchers present unambiguous identifications of SMGs, offering significant improvements over previous single-dish surveys.

Methodology and Findings

The paper employs ALMA to observe 122 sources identified from the LABOCA ECDFS Submillimetre Survey (LESS) at 870 µm, offering approximately three times greater sensitivity and significantly higher angular resolution. This methodology effectively resolves the confusion arising from blending effects seen in single-dish surveys. The sample reaches an average depth of ~0.4 mJy, substantially advancing the resolution to an FWHM of ~1.5", an improvement of over an order of magnitude compared to LABOCA's 19" resolution.

Key Results

1. Resolution of Blended Sources: ALMA's high resolution reveals that the brightest single-dish sources (S870µm > 12 mJy) often consist of several fainter SMGs rather than singular, intense emissions. This finding is pivotal, as it implies that previous bright-end SMG counts were inflated due to unresolved blending.
2. Submillimetre Galaxy Counts: The ALMA-derived galaxy counts challenge some of the predictions of theoretical models that struggle to reproduce the high surface densities of SMGs. The paper verifies that the majority of submillimetre source counts align closely with previous lower-resolution counts, while providing evidence that very bright sources consist of multiple components.
3. Star Formation Rate Limits: The research suggests a natural star-formation rate ceiling in SMGs, pegging it at approximately 1000 M⊙ yr^-1. This conclusion hints that massive gas reservoirs in galaxies (exceeding ~5 x 10^10 M⊙) are rare, with space densities of less than 10^-5 Mpc^-3 at z > 1.
4. Implications for Cosmological Models: By disentangling the contributions of individual SMGs, this work challenges models that necessitate modifications to starburst prescriptions, such as non-standard Initial Mass Functions (IMFs), for explaining the submillimetre source counts.

Implications and Future Directions

The discrepancy noted for the bright-end count has profound implications for understanding the characteristics and evolution of high-redshift galaxies. The multiplicity and distribution of SMGs highlight the necessity for high-resolution surveys to accurately interpret the cosmic infrared background and its contributors. Moreover, resolving bright source blending clarifies confusion in associated redshift and star formation rate estimates, aligning observed properties with expected theoretical distributions without necessitating exotic star formation scenarios.

The transition from single-dish to interferometer-based maps illustrates an exciting future for submillimetre astronomy, providing a more reliable dataset for testing galaxy formation and evolution theories. Going forward, the implications suggest refined models are needed to tie the new observational constraints with the evolutionary processes occurring in the early universe. Enhanced completeness across flux ranges will be crucial in building a coherent narrative of these intriguing galaxies' roles in the star-formation history of the cosmos.

In conclusion, this ALMA-based analysis refines our understanding of SMG characteristics, sets new benchmarks for observational astronomy, and poses significant questions for galaxy evolution models. Through the direct resolution of individual galaxy components, research like this steers theoretical and observational astrophysics towards increasingly accurate representations of our universe.