[Paper Review] GOODS-ALMA: 1.1 mm galaxy survey - I. Source catalogue and optically dark galaxies

Maximilien Franco, D. Elbaz+37 more|Figshare|Mar 1, 2018

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy161 references56 citations

TL;DR

A wide 69 arcmin^2 ALMA 1.1 mm survey in GOODS-South detects 20 robust sources (0.7 mJy cutoff) with >80% purity; includes 4 HST-dark galaxies and a median redshift of z~2.92 and median stellar mass ~1.1e11 Msun.

ABSTRACT

We present a 69 arcmin$^2$ ALMA survey at 1.1mm, GOODS-ALMA, matching the deepest HST-WFC3 H-band part of the GOODS-South field. We taper the 0"24 original image with a homogeneous and circular synthesized beam of 0"60 to reduce the number of independent beams - thus reducing the number of purely statistical spurious detections - and optimize the sensitivity to point sources. We extract a catalogue of galaxies purely selected by ALMA and identify sources with and without HST counterparts down to a 5$σ$ limiting depth of H=28.2 AB (HST/WFC3 F160W). ALMA detects 20 sources brighter than 0.7 mJy in the 0"60 tapered mosaic (rms sensitivity =0.18 mJy/beam) with a purity greater than 80%. Among these detections, we identify three sources with no HST nor Spitzer-IRAC counterpart, consistent with the expected number of spurious galaxies from the analysis of the inverted image; their definitive status will require additional investigation. An additional three sources with HST counterparts are detected either at high significance in the higher resolution map, or with different detection-algorithm parameters ensuring a purity greater than 80%. Hence we identify in total 20 robust detections. Our wide contiguous survey allows us to push further in redshift the blind detection of massive galaxies with ALMA with a median redshift of $z$=2.92 and a median stellar mass of M$_{\star}$ = 1.1 $ imes 10^{11}$M$_\odot$. Our sample includes 20% HST-dark galaxies (4 out of 20), all detected in the mid-infrared with IRAC. The near-infrared based photometric redshifts of two of them $z\sim$4.3 and 4.8) suggest that these sources have redshifts $z$>4. At least 40% of the ALMA sources host an X-ray AGN, compared to 14% for other galaxies of similar mass and redshift. The wide area of our ALMA survey provides lower values at the bright end of number counts than single-dish telescopes

Motivation & Objective

• Motivate the study of dust-obscured star formation in massive high-redshift galaxies without prior source bias.
• Construct an unbiased, wide-area ALMA 1.1 mm source catalogue in GOODS-South.
• Quantify the contribution of optically dark (HST-dark) galaxies to the ALMA population.
• Characterize redshift and stellar-mass distributions of ALMA-detected galaxies in a well-studied field.

Proposed method

• Design a 69 arcmin^2 ALMA mosaic at 1.1 mm with a tapered synthesized beam of 0.60 arcsec to optimize point-source detection and reduce spurious identifications.
• Use CASA for calibration and imaging; apply a 0.60″ beam taper to create a uniform mosaic for robust source extraction.
• Detect sources in the dirty map with Blobcat, using thresholds based on local RMS noise to achieve >80% purity.
• Construct noise (RMS) maps via k-sigma clipping to characterize local sensitivity variations (median RMS ≈ 0.176 mJy/beam).
• Cross-match ALMA detections with multiwavelength data (HST CANDELS/UVIS, Spitzer IRAC, MIPS, Herschel, VLA radio, Chandra X-ray) to identify counterparts and assess HST-dark status.

Experimental results

Research questions

• RQ1 What is the number and nature of 1.1 mm sources detected in an unbiased 69 arcmin^2 GOODS-South ALMA survey?
• RQ2 What fraction of ALMA-detected galaxies are HST-dark, and what are their properties?
• RQ3 What are the redshift and stellar-mass distributions of ALMA-detected galaxies in GOODS-South, and do they host AGN?
• RQ4 How do the observed 1.1 mm number counts compare to previous (sub)millimeter surveys when probing bright-end populations?
• RQ5 What is the role of ALMA-detected galaxies in the broader context of dust-obscured star formation at high redshift?

Key findings

• 20 robust ALMA 1.1 mm detections at S/N > 5 (in 0.60″ tapered map) with a purity >80%.
• Three additional high-significance detections appear in higher-resolution maps or with alternate detection parameters, bringing the robust total to 20 sources.
• Median redshift of the sample is z = 2.92 with a median stellar mass of M* = 1.1 × 10^11 M⊙.
• Around 20% (4/20) of the sources are HST-dark, detected in Spitzer–IRAC mid-infrared data.
• At least 40% of the ALMA sources host an X-ray AGN, higher than the ~14% for other galaxies of similar mass and redshift.
• The wide-area survey yields lower bright-end number counts compared with single-dish surveys affected by confusion.