[Paper Review] The Extended Mapping Obscuration to Reionization with ALMA (Ex-MORA) Survey: 5$σ$ Source Catalog and Redshift Distribution

Arianna S. Long, Caitlin M. Casey+42 more|arXiv (Cornell University)|Aug 26, 2024

Astronomical Observations and Instrumentation4 citations

TL;DR

The Ex-MORA survey conducts a 2 mm blank-field ALMA survey covering 577 arcmin² in the COSMOS-Web field to identify high-redshift, dust-obscured galaxies at z > 3. By leveraging the negative K-correction at 2 mm, it successfully isolates 37 >5σ sources with a median redshift of ⟨z⟩ = 3.6⁺⁰.¹₋₀.², of which two-thirds are at z > 3 and nearly half at z > 4, demonstrating the effectiveness of 2 mm selection for probing the early Universe's obscured star formation.

ABSTRACT

One of the greatest challenges in galaxy evolution over the last decade has been constraining the prevalence of heavily dust-obscured galaxies in the early Universe. At $z>3$, these galaxies are increasingly rare, and difficult to identify as they are interspersed among the more numerous dust-obscured galaxy population at $z=1-3$, making efforts to secure confident spectroscopic redshifts expensive, and sometimes unsuccessful. In this work, we present the Extended Mapping Obscuration to Reionization with ALMA (Ex-MORA) Survey -- a 2mm blank-field survey in the COSMOS-Web field, and the largest ever ALMA blank-field survey to-date covering 577 arcmin$^2$. Ex-MORA is an expansion of the MORA survey designed to identify primarily $z>3$ dusty, star-forming galaxies while simultaneously filtering out the more numerous $z<3$ population by leveraging the very negative $K$-correction at observed-frame 2mm. We identify 37 significant ($>$5$σ$) sources, 33 of which are robust thermal dust emitters. We measure a median redshift of $\langle z angle = 3.6^{+0.1}_{-0.2}$, with two-thirds of the sample at $z>3$, and just under half at $z>4$, demonstrating the overall success of the 2mm-selection technique. The integrated $z>3$ volume density of Ex-MORA sources is $\sim1-3 imes10^{-5}$ Mpc$^{-3}$, consistent with other surveys of infrared luminous galaxies at similar epochs. We also find that techniques using rest-frame optical emission (or lack thereof) to identify $z>3$ heavily dust-obscured galaxies miss at least half of Ex-MORA galaxies. This supports the idea that the dusty galaxy population is heterogeneous, and that synergies across observatories spanning multiple energy regimes are critical to understanding their formation and evolution at $z>3$.

Motivation & Objective

• To address the challenge of identifying rare, dust-obscured galaxies at z > 3, which are difficult to detect due to contamination from more common z = 1–3 sources.
• To overcome the limitations of traditional selection methods by exploiting the negative K-correction at 2 mm to preferentially select high-redshift sources.
• To produce a robust, high-significance catalog of 5σ sources in a blank-field ALMA survey to enable statistical studies of early-universe galaxy evolution.
• To measure the redshift distribution and volume density of high-redshift dusty galaxies to test models of cosmic reionization and early star formation.
• To assess the completeness of alternative selection techniques for z > 3 dusty galaxies and advocate for multi-wavelength, synergistic surveys.

Proposed method

• Conducting a 2 mm continuum survey using ALMA in the COSMOS-Web field, covering 577 arcmin², the largest blank-field ALMA survey to date at this wavelength.
• Utilizing the negative K-correction at observed-frame 2 mm to suppress emission from lower-redshift (z < 3) dusty galaxies, thereby enhancing selection of z > 3 sources.
• Applying deep imaging and source extraction with a 5σ significance threshold to identify 37 robust sources in the 2 mm map.
• Measuring photometric redshifts using SED fitting with a combination of ALMA 2 mm data and multi-wavelength SEDs from UV to submillimeter bands.
• Estimating the volume density of z > 3 sources by integrating over the redshift distribution and survey volume.
• Comparing the Ex-MORA sample with results from other selection techniques (e.g., Wang et al. 2019; Talia et al. 2021) to assess completeness across different methods.

Experimental results

Research questions

• RQ1 What is the redshift distribution of 2 mm-selected sources in a blank-field ALMA survey, and how many lie at z > 3?
• RQ2 To what extent does 2 mm selection effectively isolate high-redshift dusty galaxies by suppressing lower-redshift contaminants?
• RQ3 How does the observed volume density of z > 3 dusty galaxies from Ex-MORA compare to theoretical models of the early Universe?
• RQ4 What fraction of the Ex-MORA source sample is recovered by other selection techniques at different redshifts?
• RQ5 What does the completeness of different selection methods imply for the need for multi-wavelength, synergistic surveys in probing the high-redshift dusty galaxy population?

Key findings

• The Ex-MORA survey identifies 37 significant (>5σ) sources in a 577 arcmin² 2 mm ALMA map, with 33 confirmed as robust thermal dust emitters.
• The median redshift of the sample is ⟨z⟩ = 3.6⁺⁰.¹₋₀.², with 66% of sources at z > 3 and 46% at z > 4, confirming the success of 2 mm selection for high-redshift targets.
• The integrated volume density of z > 3 sources is measured at ∼1–3 × 10⁻⁵ Mpc⁻³, consistent with the 'dust-poor' early Universe model.
• Other selection techniques (e.g., Wang et al. 2019; Talia et al. 2021; Barrufet et al. 2023; Gottumukkala et al. 2024) recover between 10% and 40% of the Ex-MORA sample, indicating significant incompleteness in existing methods.
• The results support the necessity of multi-wavelength, multi-instrument approaches—especially with ALMA, JWST, and VLA—to achieve a complete census of high-redshift dusty galaxies.
• The survey demonstrates that 2 mm continuum selection is a powerful and efficient method for isolating z > 3 dusty star-forming galaxies, even in the face of low source densities.