James Webb Space Telescope Feed Post

GA-NIFS: The core of an extremely massive proto-cluster at the Epoch of Reionization probed with JWST/NIRSpec

Left: Image generated from the NIRSpec R100 datacube, integrating over the spectral range 1.1-1.4 µm. The color bar is in units of 10-19 erg s-1cm-2 per spatial pixel. Right: HST image obtained with WFC3 and F125W filter resampled at the pixel size used with NIRSpec (i.e., 0.050 ''). The inset zooms in on the region around the emission peak used to align the NIRSpec cube (red isophotes) with the image represented with an unsaturated color code. Here, and in all the spectral maps presented in this work, the (0,0) corresponds to coordinates a = 03h 11m 33s.248 and d = –58º 23’ 33”.24 (white cross). In the image the peak of the lens galaxy is at relative coordinates (–0.64 '', –0.29 ''). The green lines indicate the border of the NIRSpec IFU FoV. The black and white circles in the bottom-left corners of the panels represent the FWHW of the respective PSFs. Abstract: The SPT0311-58 system resides in a massive dark matter halo at z ~ 6.9. It hosts two dusty galaxies (E and W) with a combined star formation rate (SFR) of ~3500 Msun/yr. Its surrounding field exhibits an overdensity of sub-mm sources, making it a candidate proto-cluster. We use spatially-resolved spectroscopy provided by the JWST/NIRSpec Integral Field Unit (IFU) to probe a field of view (FoV) ~ 17 x 17 kpc^2 around this object, with a spatial resolution ~ 0.5 kpc. These observations have revealed ten new galaxies at z ~ 6.9, characterised by dynamical masses spanning from ~10^9 to 10^10 Msun and a range in radial velocities of ~ 1500 km/s, in addition to the already known E and W galaxies. The implied large number density, and the wide spread in velocities, indicate that SPT0311-58 is at the core of a proto-cluster, immersed in a very massive dark matter halo of ~ 6 x 10^12 Msun. We study the dynamical stage of the system and find that it likely is not fully virialized, although most of the galaxies are gravitationally bound to the halo. The galaxies exhibit a great diversity of properties. We derive their ongoing Halpha-based unobscured SFR, and find that its contribution to the total SF varies significantly across the galaxies in the system. Their ionization conditions range from those typical of galaxies at similar redshift recently studied with JWST to those found in lower redshift objects. The metallicity spans more than 0.8 dex across the FoV, reaching in some cases nearly solar values. The detailed IFU spectroscopy of the E galaxy reveals that it is actively assembling its stellar mass, showing a metallicity gradient (~ 0.1 dex/kpc) that can be explained by accretion of low metallicity gas from the intergalactic medium. The kinematic maps indicate departures from regular rotation, high turbulence, and a possible pre-collision minor merger. (Abridged)