[Paper Review] Scientific requirements of ALMA, and its capabilities for key-projects: Extragalactic

C. De Breuck|arXiv (Cornell University)|Oct 29, 2004

Superconducting and THz Device Technology3 citations

TL;DR

This paper outlines ALMA's scientific requirements for extragalactic astronomy, focusing on detecting CO emission from Milky Way-like galaxies at z=3 and achieving 0.1" angular resolution. It demonstrates that ALMA’s sensitivity—driven by a 7000 m² collecting area, low system temperature, and advanced detectors—enables sub-arcsecond imaging and sub-24-hour detection of faint molecular lines, making it ideal for studying high-redshift submillimeter galaxies and their star formation processes.

ABSTRACT

The Atacama Large Millimeter Array (ALMA) consists of 64 antennas of 12m diameter that will initially observe in 4 frequency bands between 84 and 720 GHz with spatial resolutions down to 0.01" and velocity resolutions as fine as 0.05 km/s. These technical requirements are based on three primary science goals. We illustrate two of these requirements: (i) the ability to detect spectral line emission from a Milky-Way type galaxy at z=3, and (ii) the ability to provide precise images at an angular resolution of 0.1". Finally, we present a possible large extragalactic project with ALMA: molecular line studies of submm galaxies.

Motivation & Objective

• To define the technical requirements for ALMA to detect spectral line emission from a Milky Way-type galaxy at redshift z=3 within 24 hours.
• To establish the capability for high-resolution imaging (0.1") of extragalactic sources, ensuring precision down to 0.1% of peak brightness.
• To demonstrate ALMA's feasibility for large-scale extragalactic projects, particularly molecular line studies of submillimeter galaxies.
• To quantify the sensitivity gains from increased collecting area, improved detector noise, and polarization measurement.
• To support the design of the ALMA Science Operations Plan and software through a reference set of high-priority projects.

Proposed method

• Calculated flux density sensitivity using the interferometric sensitivity equation: ΔS = 2.6×10⁶ Tsys / (εa N D² √(Δν Δt)) in mJy.
• Assessed system temperature (Tsys) and aperture efficiency (εa) across ALMA’s frequency bands (35–850 GHz) at 50° elevation.
• Used the CO(1–0) luminosity of the Milky Way (L′CO = 5×10⁸ K km s⁻¹ pc²) to estimate expected flux density at z=3.
• Modeled atmospheric transmission at the Chajnantor site to determine usable frequency bands (initially bands 3, 6, 7, 9).
• Simulated detection of CO transitions across redshifts using ALMA’s 2×4 GHz bandwidth per band, enabling multi-line redshift confirmation.
• Evaluated the feasibility of detecting HCN lines (10× fainter than CO) and derived integration time estimates for representative samples.

Experimental results

Research questions

• RQ1 What collecting area is required for ALMA to detect CO emission from a z=3 Milky Way-like galaxy in less than 24 hours?
• RQ2 How does ALMA’s angular resolution of 0.1" enable precise imaging of extragalactic sources, especially in resolving kinematics and structures?
• RQ3 Can ALMA detect molecular lines in submillimeter galaxies with 1 mJy 850 μm flux density in less than 2 hours per source?
• RQ4 What is the expected sensitivity gain from combining increased collecting area, improved detector noise, and polarization measurement?
• RQ5 How can ALMA bypass the need for optical redshifts by detecting multiple CO transitions in a single observation?

Key findings

• ALMA requires a collecting area greater than 7000 m² to detect CO emission from a z=3 Milky Way-like galaxy within 24 hours, driven by the need to overcome a factor of 20–30 fainter flux in unlensed sources.
• With a system temperature of ~35 K at 35 GHz and aperture efficiency of 0.75, ALMA achieves a flux density sensitivity of ~1.5 mJy for a 1-hour integration in band 3.
• ALMA can detect 1 mJy 850 μm submillimeter galaxies in less than 2 hours per source, enabling a representative sample of 50 sources to be surveyed in 100 hours.
• High-resolution CO imaging (1 hour per source) will allow dynamical mass estimates and merger morphology studies, reducing time from 24+ hours for brighter sources.
• The HCN line, 10× fainter than CO, is detectable in approximately 10 hours per source, providing a tracer of dense gas in star-forming regions.
• ALMA’s 2×4 GHz bandwidth allows detection of at least one CO transition in three frequency settings between 90–116 GHz, enabling redshift confirmation without prior optical data.