Abstract
Galaxy clusters at high redshift are key targets for understanding matter assembly in the early Universe, yet they are challenging to locate. A sample of more than 2000 high-z candidate structures has been found using Planck’s all-sky submillimetre maps, and a sub-set of 234 have been followed up with Herschel-SPIRE, which showed that the emission can be attributed to large overdensities of dusty star-forming galaxies. As a next step, we need to resolve and characterise the individual galaxies giving rise to the emission seen by Planck and Herschel, and to find out whether they constitute the progenitors of present-day, massive galaxy clusters. Thus, we targeted the eight brightest Herschel-SPIRE sources in the centre of the Planck peak PLCK G073.4−57.5 using ALMA at 1.3 mm, and complemented these observations with multi-wavelength data from Spitzer-IRAC, CFHT-WIRCam in the J and Ks bands, and JCMT’s SCUBA-2 instrument. We detected a total of 18 millimetre galaxies brighter than 0.3 mJy within the 2.4 arcmin2 ALMA pointings, corresponding to an ALMA source density 8–30 times higher than average background estimates and larger than seen in typical “proto-cluster” fields. We were able to match all but one of the ALMA sources to a near infrared (NIR) counterpart. The four most significant SCUBA-2 sources are not included in the ALMA pointings, but we find an 8σ stacking detection of the ALMA sources in the SCUBA-2 map at 850 μm. We derive photometric redshifts, infrared (IR) luminosities, star-formation rates (SFRs), stellar masses (ℳ), dust temperatures, and dust masses for all of the ALMA galaxies. Photometric redshifts identify two groups each of five sources, concentrated around z ≃ 1.5 and 2.4. The two groups show two “red sequences”, that is similar near-IR [3.6] − [4.5] colours and different J − Ks colours. The majority of the ALMA-detected galaxies are on the SFR versus ℳ main sequence (MS), and half of the sample is more massive than the characteristic ℳ* at the corresponding redshift. We find that the z ≃ 1.5 group has total SFR = 840−100+120 M⊙ yr−1 and ℳ = 5.8−2.4+1.7 × 1011 M⊙, and that the z ≃ 2.4 group has SFR = 1020−170+310 M⊙ yr−1 and ℳ = 4.2−2.1+1.5 × 1011 M⊙, but the latter group is more scattered in stellar mass and around the MS. Serendipitous CO line detections in two of the galaxies appear to match their photometric redshifts at z = 1.54. We performed an analysis of star-formation efficiencies (SFEs) and CO- and mm-continuum-derived gas fractions of our ALMA sources, combined with a sample of 1 < z < 3 cluster and proto-cluster members, and observed trends in both quantities with respect to stellar masses and in comparison to field galaxies.
Highlights
Hierarchical clustering models of large-scale structure and galaxy formation predict that the progenitors of the most massive galaxies in today’s clusters are dusty star-forming galaxies (SFGs) at high redshift (z 2–3, e.g. Lilly et al 1999; Swinbank et al 2008)
Using the whole catalogue, the expected number of detections in 1.8 arcmin2 is 2.9 sources, broadly consistent with the average source counts we adopt from the literature, and giving a 6-times higher observed source density. It is beyond the scope of this paper to attempt a modelling of all the selection effects that enter into the final detection with ALMA, or a detailed comparison of the clustering we find within the photometric redshift distribution; one thing we can do is compare the simulated redshift distribution for all sources, renormalised to the observed number of sources with the observed distribution
We find that we see no significant difference, given the relatively large Poisson errors; in the z = 0.85–1.7 redshift bin 3.0 more sources are observed than expected in the renormalised distribution (9.9 times higher than the simulated numbers) and in the z = 1.7– 2.55 redshift bin 1.6 more sources are observed than expected when renormalised (8.0 times higher compared to simulated numbers)
Summary
Hierarchical clustering models of large-scale structure and galaxy formation predict that the progenitors of the most massive galaxies in today’s clusters are dusty star-forming galaxies (SFGs) at high redshift (z 2–3, e.g. Lilly et al 1999; Swinbank et al 2008). This selection is distinctly different to the Planck catalogue of cluster candidates detected via the Sunyaev–Zeldovich effect (Planck Collaboration XXVII 2016, the “PSZ2” catalogue) It targets the bright, far-infrared spectral energy distribution of dust heated by star formation, and selects predominantly rapidly growing galaxies. 234 of these submm peaks (chosen to have S /N > 4 at 545 GHz, as well as flux-density ratios S 857/S 545 < 1.5, and S 217 < S 353) were subsequently followed up with Herschel-SPIRE observations between 250 and 500 μm, and the half-arcminute (or better) resolution was capable of distinguishing between bright gravitational lenses and concentrations of clustered mm/submm galaxies around redshifts of 2–3 (Planck Collaboration Int. XXVII 2015). We note that ALMA source ID 16, which is on the edge of pointing field 7, has a recovered peak flux density of 0.59 ± 0.17 mJy beam−1, that is 3.5σ, and should be considered tentative, in spite of the match with a Spitzer source (cf. section and Fig. 3)
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