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

Rüdiger Kneissl , Maria del Carmen Polletta , Clement Martinache , Ryley Hill , Benjamin Clarenc , Herve Dole (1) , Nicole P. H. Nesvadba , Douglas Scott , Matthieu Bethermin (2) , Brenda Frye , Martin Giard (3) , Guilaine Lagache (2) , Ludovic Montier (3)

1 IAS - Institut d'astrophysique spatiale
2 LAM - Laboratoire d'Astrophysique de Marseille
3 IRAP - Institut de recherche en astrophysique et planétologie

Galaxy clusters at high redshifts 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 have been found using Planck's all-sky submillimetre maps, and a subset 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. However, the individual galaxies giving rise to the emission seen by Planck and Herschel have not yet been resolved nor characterized, so we do not know whether they constitute the progenitors of present-day, massive galaxy clusters. In an attempt to address this, we targeted the eight brightest Herschel-SPIRE sources in the centre of the Planck peak G073.4−57.5 using ALMA at 1.3 mm, and complemented these observations with multi-wavelength data from Spitzer-IRAC, CFHT-WIRCam J, K, and JCMT's SCUBA-2 instrument. We detect a total of 18 millimetre galaxies brighter than 0.3 mJy in 2.4 arcmin 2. The ALMA source density is 8–30 times higher than average background estimates, and larger than seen in typical " proto-cluster " fields. We are able to match all but one ALMA sources to their NIR counterparts. The most significant (four) 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, IR luminosities, star-formation rates, stellar masses, dust temperatures, and dust masses for all the ALMA galaxies; the photometric redshifts are concentrated around z 1.5 and the near-IR colours show a " red " sequence, while the star-formation rates indicate that one third of the galaxies are " starbursts, " the others being main-sequence star-forming galaxies. Serendipitous CO line detections of two of the galaxies appear to match their photometric redshifts, with z = 1.54. We thus find that the Planck peak G073.4−57.5 contains a proto-cluster candidate at z = 1.54, and one of the richest regions of ALMA sources known. The ALMA-detected members are massive galaxies on the main-sequence relation and account for a total star-formation rate of at least 1000 M yr −1 .

Domaines: Astrophysique galactique [astro-ph.GA]; Cosmologie et astrophysique extra-galactique [astro-ph.CO]

Rüdiger Kneissl, Maria del Carmen Polletta, Clement Martinache, Ryley Hill, Benjamin Clarenc, et al.. Using ALMA to resolve the nature of the early star-forming large-scale structure PLCK G073.4−57.5. 2018. ⟨hal-01789113⟩