[Paper Review] The ALMA Frontier Fields survey I: 1.1 mm continuum detections in Abell 2744, MACSJ0416.1-2403 and MACSJ1149.5+2223

Jorge González-López, F. E. Bauer+22 more|CaltechAUTHORS (California Institute of Technology)|Jul 13, 2016

Galaxies: Formation, Evolution, Phenomena64 references21 citations

TL;DR

This study presents the first ALMA Frontier Fields survey detecting 12 dusty star-forming galaxies at 1.1 mm in three massive galaxy clusters, using deep submillimeter imaging and strong gravitational lensing to probe the faint sub-mJy population. The results show that lensing-corrected sizes of these faint sources are consistent with brighter DSFGs but with larger dispersion, suggesting a significant fraction may be more extended than their brighter counterparts.

ABSTRACT

Dusty star-forming galaxies are among the most prodigious systems at high redshift (z>1), characterized by high star formation rates and huge dust reservoirs. The bright end of this population has been well characterized in recent years, but considerable uncertainties remain for fainter dusty star-forming galaxies, which are responsible for the bulk of star formation at high redshift and thus play a key role in galaxy growth and evolution. In this first paper of our series, we describe our methods for finding high redshift faint dusty galaxies using millimeter observations with ALMA. We obtained ALMA 1.1 mm mosaic images for three strong-lensing galaxy clusters from the Frontier Fields survey. The 2'x2' mosaics overlap with the deep HST WFC3/IR footprints and encompass the high magnification regions of each cluster. The combination of extremely high ALMA sensitivity and the magnification power of these clusters allows us to systematically probe the sub-mJy population of dusty star-forming galaxies over a large surveyed area. We present a description of the reduction and analysis of the ALMA continuum observations for the galaxy clusters Abell 2744 (z=0.308), MACSJ0416.1-2403 (z=0.396) and MACSJ1149.5+2223 (z=0.543), for which we reach observed rms sensitivities of 55, 59 and 71 $μ$Jy beam$^{-1}$ respectively. We detect 12 dusty star-forming galaxies at S/N>5.0 across the three clusters, all of them presenting coincidence with NIR detected counterparts in the HST images. None of the sources fall close to the lensing caustics, thus they are not strongly lensed. The observed 1.1 mm flux densities for the total sample of galaxies range from 0.41 to 2.82 mJy, with observed effective radii spanning <0.05" to 0.37"$\pm$0.21". The lensing-corrected sizes of the detected sources appear to be in the same range as those measured in brighter samples, albeit with possibly larger dispersion.

Motivation & Objective

• To systematically probe the faint sub-mJy population of dusty star-forming galaxies (DSFGs) at high redshift using ALMA's deep 1.1 mm continuum observations.
• To leverage strong gravitational lensing from three Frontier Fields galaxy clusters (Abell 2744, MACSJ0416.1-2403, MACSJ1149.5+2223) to enhance sensitivity and detect fainter sources.
• To characterize the intrinsic physical properties—especially flux densities and effective radii—of these high-redshift DSFGs after correcting for lensing magnification.
• To assess the spatial co-location of submillimeter and near-infrared emission to validate source associations and astrometric consistency.
• To investigate whether the faint DSFG population exhibits structural differences (e.g., size distribution) compared to brighter, well-studied DSFGs.

Proposed method

• Obtained 1.1 mm continuum mosaic images using ALMA over ~2′ × 2′ fields centered on three strong-lensing galaxy clusters, overlapping with deep HST WFC3/IR data.
• Achieved observed rms sensitivities of 55, 59, and 71 μJy beam⁻¹ for Abell 2744, MACSJ0416.1-2403, and MACSJ1149.5+2223, respectively, with sensitivity variations due to observational conditions and shadowing.
• Identified 12 sources with S/N ≥ 5.0 using natural-weighted maps, all of which have associated HST F160W counterparts.
• Fitted 2D Gaussian models in the uv-plane using the uvmcmcfit code to estimate total flux densities and angular sizes at 1.1 mm for each detected source.
• Applied lensing magnification models (from CATS, Merten & Zitrin, Sharon, Williams, GLAFIC, and Diego groups) assuming z = 2 for sources without spectroscopic redshifts to derive intrinsic (demagnified) fluxes and sizes.
• Assessed astrometric offsets between ALMA and HST positions, finding a typical scatter of ~0.1′′ and a potential ~0.13′′ offset in Declination for A2744, suggesting real spatial offsets in some cases.

Experimental results

Research questions

• RQ1 What is the intrinsic size distribution of faint dusty star-forming galaxies (sub-mJy sources) at high redshift, after correcting for gravitational lensing magnification?
• RQ2 How do the physical properties (flux density, size) of sub-mJy DSFGs compare to those of brighter, previously studied DSFGs?
• RQ3 To what extent is the submillimeter emission spatially coincident with near-infrared emission in these high-redshift sources?
• RQ4 Are there significant spatial offsets between the peak of submillimeter and optical emission in individual sources, and what might this imply about obscured star formation?
• RQ5 What is the sensitivity and dynamic range of ALMA 1.1 mm observations in the high-magnification regions of Frontier Fields clusters, and how does it enable detection of faint DSFGs?

Key findings

• The survey detected 12 dusty star-forming galaxies at 1.1 mm with S/N ≥ 5.0 across the three clusters, all with clear HST F160W counterparts.
• Observed 1.1 mm flux densities ranged from 0.411 to 2.816 mJy, with observed effective radii between ~0.′′05 and 0.′′37 ± 0.′′21.
• After correcting for lensing magnification (μ ≈ 1.5–4.2), the intrinsic (demagnified) sizes of the detected sources are consistent with those of brighter DSFGs, though with a larger dispersion.
• Approximately two-thirds of the sources have demagnified sizes that are a factor of ≥1.6 larger than the average size of brighter DSFGs, suggesting a potential population of more extended faint sources.
• The astrometric offset between ALMA and HST positions is small on average (~0.1′′), indicating good co-location of submillimeter and optical emission, though a possible ~0.13′′ offset in Declination was noted in the A2744 field.
• A2744-ID02 shows a significant spatial offset of ~0.′′5 (≈4 kpc in the source plane) between peak submillimeter and near-infrared emission, indicating possible spatially resolved, highly obscured star formation.