Using ALMA to resolve the nature of the early star-forming large-scale structure PLCK G073.4−57.5

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 K s bands, and JCMT's SCUBA-2 instrument. We detected a total of 18 millimetre galaxies brighter than 0.3 mJy within the 2.4 arcmin 2 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 8 σ 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 − K s 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 +120 −100 M yr −1 and M = 5.8 +1.7 −2.4 × 10 11 M , and that the z 2.4 group has SFR = 1020 +310 −170 M yr −1 and M = 4.2 +1.5 −2.1 × 10 11 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. A dedicated Herschel (Pilbratt et al. 2010) follow-up programme with the SPIRE instrument for 234 Planck targets (Planck Collab. XXXVII 2015) found a significant excess of 'red' sources (where red means S 350 / S 250 > 0.7 and S 500 / S 350 > 0.6, which is consistent with z > ∼ 2 SFGs), in comparison to reference SPIRE fields. Assuming a single com...

Spectroscopic observations of PHz G237.01+42.50: A galaxy protocluster at z = 2.16 in the Cosmos field

“…Spectroscopic observations, necessary to confirm the protocluster nature of these sources, have been obtained only for three PHz, PHz G95.5−61.6, resulting in the identification of two potential structures, one at z 2 with six members, and a second one at z 1.7 with three members (Flores-Cacho et al 2016), and PHz G237.01+42.50, whose observations are presented here. A third PHz, PLCK G073.4−57.5, has been observed with ALMA, yielding two CO line detections at the same redshift, z = 1.54 (Kneissl et al 2019). The paucity of spectroscopic observations is due to the complication of identifying the counterpart to the sub-mm sources, the need of data over wide enough areas to cover the extension of the PHz, and the difficulty of obtaining redshifts for galaxies at z ∼ 2, in particular, for dusty star-forming galaxies (DSFGs; Casey et al 2017).…”

Is there enough star formation in simulated protoclusters?

“…Data from cosmic microwave background experiments, for example, the all-sky Planck survey Planck Collaboration XXVII ( 2015) and Planck Collaboration XXXIX (2016), are particularly useful for this purpose. This technique has uncovered protoclusters such as PHz G95.5-61.6 (Flores-Cacho et al 2016, z 2), PHz G073.4-57.5 (Kneissl et al 2019, z = 1.5-2.4), and SPT2349-56 (Miller et al 2018;Hill et al 2020, z = 4.3); indeed, the South Pole Telescope (SPT) survey has uncovered nine protocluster candidates to date at z = 3-7 (Wang et al 2020).…”

Megaparsec-scale structure around the protocluster core SPT2349–56 at z = 4.3

“…Distant proto-clusters are excellent laboratories for studying not only the details of cluster formation, but also galaxy evolution and star formation, since these processes are likely undergoing their most active phase at this epoch. A number of proto-clusters have been discovered beyond redshifts of 2, typically through their rest-frame optical emission, which traces unobscured stellar light (e.g., Steidel et al 2000;Shimasaku et al 2003;Steidel et al 2005;Venemans et al 2007;Chiang et al 2015;Dey et al 2016;Harikane et al 2019), or as overdensities of submm galaxies (SMGs), which probes their rest-frame far-infrared emission and traces star formation (e.g., Tamura et al 2009;Chapman et al 2009;Dannerbauer et al 2014;Chiang et al 2015;Flores-Cacho et al 2016;Umehata et al 2015;Casey et al 2015;Hung et al 2016;Oteo et al 2018;Lacaille et al 2019;Kneissl et al 2019). However, comparing these systems to current simulations is challenging due to their very low number density, which requires large simulated cosmological volumes, and because they contain very massive galaxies with high gas and stellar densities that require significant resolution to simulate accurately.…”