[Paper Review] The ALPINE-ALMA [CII] survey: Survey strategy, observations and sample properties of 118 star-forming galaxies at $4<z<6$

O. Le Fèvre, M. Béthermin+8 more|arXiv (Cornell University)|Oct 21, 2019

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy3 references13 citations

TL;DR

This paper presents the ALPINE-ALMA [CII] survey, a Large Program using ALMA to observe 118 star-forming galaxies at 4 < z < 6, focusing on [CII] 158µm emission and FIR continuum. With a 64% detection rate in [CII] at S/N > 3.5, the survey reveals a diverse population—40% mergers, 20% dispersion-dominated, 13% rotating discs—indicating that galaxy mergers are a dominant, not just bursty, process in normal main-sequence galaxies during early mass assembly.

ABSTRACT

The ALMA-ALPINE [CII] survey is aimed at characterizing the properties of a sample of normal star-forming galaxies (SFGs). The ALMA Large Program to INvestigate (ALPINE) features 118 galaxies observed in the [CII]-158$\mu$m line and far infrared (FIR) continuum emission during the period of rapid mass assembly, right after the end of the HI reionization, at redshifts of 4<z<6. We present the survey science goals, the observational strategy, and the sample selection of the 118 galaxies observed with ALMA, with an average beam minor axis of about 0.85 arcsec, or $\sim$5 kpc at the median redshift of the survey. The properties of the sample are described, including spectroscopic redshifts derived from the UV-rest frame, stellar masses, and star-formation rates obtained from a spectral energy distribution (SED) fitting. The observed properties derived from the ALMA data are presented and discussed in terms of the overall detection rate in [CII] and FIR continuum, with the observed signal-to-noise distribution. The sample is representative of the SFG population in the main sequence at these redshifts. The overall detection rate in [CII] is 64% for a signal-to-noise ratio (S/N) threshold larger than 3.5 corresponding to a 95% purity (40% detection rate for S/N>5). Based on a visual inspection of the [CII] data cubes together with the large wealth of ancillary data, we find a surprisingly wide range of galaxy types, including 40% that are mergers, 20% extended and dispersion-dominated, 13% compact, and 11% rotating discs, with the remaining 16% too faint to be classified. This diversity indicates that a wide array of physical processes must be at work at this epoch, first and foremost, those of galaxy mergers. This paper sets a reference sample for the gas distribution in normal SFGs at 4<z<6.

Motivation & Objective

• To characterize the [CII] and FIR properties of a representative sample of normal star-forming galaxies (SFGs) at 4 < z < 6, a critical epoch for galaxy mass assembly.
• To address the lack of statistically significant observations of normal SFGs (not extreme starbursts) at high redshift, which are biased toward high-SFR systems in prior surveys.
• To establish a reference sample for studying gas distribution and physical processes in early galaxies, especially in preparation for JWST and ELT observations.
• To classify galaxy morphologies and kinematics using ALMA [CII] data combined with multi-wavelength ancillary data, revealing the diversity of physical processes at work.
• To quantify the role of galaxy mergers in normal SFGs at z ~ 4.7, challenging the assumption that mergers are only linked to starburst activity.

Proposed method

• Target selection based on spectroscopic redshifts from UV rest-frame data and SED-fitting-derived SFRs to ensure galaxies lie on the main sequence at z ~ 4–6.
• ALMA observations in Band 6 (1.3 mm) to detect [CII] 158µm line emission and FIR continuum emission with a typical beam size of ~0.85′′ (~5 kpc at median z).
• Use of the De Looze et al. (2014) [CII] luminosity–SFR relation to predict detectability and select galaxies with L[CII] > 1.2 × 10⁸ L⊙.
• Creation of velocity-integrated [CII] flux maps and velocity channel maps to analyze spatial and kinematic structure.
• Morpho-kinematic classification of galaxies using visual inspection of [CII] data cubes, HST imaging (F814W), and ancillary data, resulting in four main classes: mergers, dispersion-dominated, rotating discs, and compact systems.
• Statistical analysis of detection rates, signal-to-noise distributions, and morphological fractions to assess the representativeness of the sample and the prevalence of physical processes.

Experimental results

Research questions

• RQ1 What is the [CII] detection rate for normal star-forming galaxies at 4 < z < 6, and how does it compare to theoretical expectations?
• RQ2 What is the morpho-kinematic diversity of normal SFGs at z ~ 4.7, and what does this imply about the dominant physical processes driving mass assembly?
• RQ3 How do the [CII] and FIR properties of these galaxies compare to those of local galaxies and high-SFR starbursts?
• RQ4 To what extent are galaxy mergers associated with main-sequence SFGs at z ~ 4.7, and what does this imply about the role of mergers in normal star formation?
• RQ5 How representative is the ALPINE sample of the broader population of SFGs at this redshift, and what is its utility for future studies with JWST and ELTs?

Key findings

• The overall [CII] detection rate is 64% for a signal-to-noise ratio (S/N) threshold > 3.5, corresponding to 95% purity, with a lower detection rate of 40% at S/N > 5.
• The FIR continuum detection rate is 21%, indicating that a significant fraction of the sample is also bright in submillimeter emission.
• 40% of the galaxies are classified as mergers, indicating that major and minor mergers are a dominant physical process even in normal SFGs on the main sequence at z ~ 4.7.
• 20% of the sample are extended and dispersion-dominated, 13.3% are rotating discs, and 10.7% are compact, with 16% too faint to classify, revealing a wide morpho-kinematic diversity.
• The presence of extended [CII] nebulae in some systems suggests large reservoirs of cold gas were available to fuel star formation shortly after reionization.
• The diversity of morphologies and kinematics implies that multiple physical processes—especially galaxy mergers—are actively shaping galaxy assembly at this early epoch, challenging the view that mergers are exclusive to starburst systems.