Probing the influence of the protocluster environment on galaxy morphology at z = 2.23

TL;DR

The study investigates whether the protocluster environment at $z \sim 2.23$ alters galaxy morphology by measuring Sérsic indices for 151 H$\alpha$ emitters in two MAMMOTH protoclusters (BOSS1244, BOSS1542) using HST/WFC3 F160W data and comparing to a coeval field built from CANDELS. It finds that, for star-forming HAEs, protocluster morphologies largely resemble field galaxies, with only a modest, statistically insignificant internal morphology-density trend in BOSS1244 and none in BOSS1542. A notable population of multi-peak, potentially merging or clumpy systems (primarily with $n < 1.5$) suggests pathways to future early-type populations, while a central pile-up of strongly bulge-dominated galaxies near the BCG and nearby quasars in BOSS1244 hints at a possible global-environment effect and an early-forming cluster core. Overall, quiescent populations and a strong morphology-density signal are not yet evident in these HAEs, indicating that morphological transformation may lag quenching or depend on a broader sample across mass and redshift; larger surveys and JWST-era data are needed to clarify environmental roles in high-$z$ morphology evolution.

Abstract

As galaxies evolve in dense cluster and protocluster environments, they interact and quench their star formation, which gradually transforms the galaxy population from star-forming galaxies to quiescent galaxies. This transformation is identifiable by observing galaxy colors and can be seen in the morphological transformation of late-type galaxies into early-type galaxies, which creates the morphology-density relation seen when comparing populations in clusters to co-eval field galaxies. However, high-z (z > 2) galaxy morphology studies are hindered by the high angular resolution necessary to characterize morphology. We present a study of HST WFC3 F160W observations of protoclusters from the MAMMOTH survey (BOSS1244 and BOSS1542) at z ~ 2.23 with populations of previously identified HAEs. By measuring the Sersic index of 151 HAEs, we look for the early morphological transformation of star-forming galaxies in these well-studied, large, non-virialized protoclusters, which we believe are precursors of present-day clusters. We find the morphology of the populations of star-forming protocluster galaxies does not differ from the co-eval field. However, we identify a population of clumpy, potentially merging galaxies, which could lead to an increase in the population of early-type galaxies in these structures. Additionally, in BOSS1244, which has two previously identified massive quiescent galaxies including a BCG, we find an abundance of early-type galaxies near both the BCG and two co-eval high-z quasars. Although we find a strong similarity between the morphology of field and protocluster galaxies, the population of early-type star-forming galaxies surrounding the spectroscopically confirmed quiescent BCG in BOSS1244, something not seen in BOSS1542, may point to differences in the evolutionary state of these co-eval protoclusters and be a sign of an early forming cluster core in BOSS1244.

Figures (14)

• Figure 1: The HST coverage of the MAMMOTH protoclusters in our sample. In each panel, we present our HST WFC3 F160W images. The overlaid red circular regions denote the HAEs. The overlaid cyan circular regions denote HAEs that were spectroscopically confirmed in Shi2021 and fall within the larger HST footprint (a small number of them fall outside the coverage region shown). In the left panel, the galaxies that were spectroscopically confirmed via HST grism observations are overlaid in magenta squares. Similarly, the quiescent galaxies identified in Shi2024 are shown in blue squares. In both BOSS1244 and BOSS1542, the SDSS-identified co-eval quasars in each field are shown in purple squares. Some of these quasars were previously identified as HAEs. In both BOSS1244 and BOSS1542, the densest regions are found in the lower right quadrant with HST coverage. The total area shown for BOSS1244 and BOSS1542 is $\sim$ 12$\arcmin$$\times$ 17$\arcmin$.
• Figure 2: Examples of image cutouts of HST WFC3 F160W detected HAEs in BOSS1244 (top row) and BOSS1542 (bottom row). Early-type galaxies are shown in the left two columns and late-type galaxies are shown in the right two columns. The Sérsic index of each galaxy is included at the top of each cutout. Each cutout is 2$\farcs$88 $\times$ 2$\farcs$88. The size of the FWHM of the HST WFC3 F160W PSF is shown as the black circle in the bottom of each cutout.
• Figure 3: Examples of image cutouts of HST WFC3 F160W detected multi-peak galaxies identified in BOSS1244 (top row) and BOSS1542 (bottom row). Each cutout is 2$\farcs$88 $\times$ 2$\farcs$88. The size of the FWHM of the HST WFC3 F160W PSF is shown as the black circle in the bottom of each cutout. These galaxies were detected as a singular galaxy with Source Extractor/Galapagos, but show multiple bright peaks, potentially indicative of a merging and/or clumpy galaxy.
• Figure 4: The SFR as a function of stellar mass for our samples of HAEs from BOSS1244 and BOSS1542 overplotted against the co-eval sample of field galaxies with SFRs and stellar masses from Osborne2024 in the CANDELS fields. The color of the co-eval field sample galaxy represents the likelihood that a given galaxy is at $z$ = 2.246 $\pm$ 0.02 based on the photometric redshifts from Kodra2023. We overplot an estimate of the star formation main sequence at $z$ = 2.23 based on the analysis in Popesso2023 in green. Because the star formation main sequence is calibrated using a Kroupa IMF, we convert the star formation rates and stellar masses to reflect the Chabrier IMF used in Osborne2024. Although we measure the likelihood that all galaxies, regardless of the median redshift estimate are at $z$ = 2.246, in the above plot, we only include those galaxies with a median photometric redshift (or spectroscopic redshift) of 2 $<$$z$$<$ 3. The vertical line represents the 90$\%$ stellar mass completeness limit for BOSS1244 and BOSS1542 (10$^{10.3}$ M$_{\odot}$) based on the analysis in Liu2023. The horizontal line shows our estimate of the SFR completeness limit (20 M$_{\odot}$/yr).
• Figure 5: The morphology-density relation in two MAMMOTH protoclusters, BOSS1244 (in red) and BOSS1542 (in black). The weighted CANDELS field is shown in blue and the CANDELS stellar mass and SFR sample (CANDELS M$_{*}$ & SFR lim) is shown in magenta. The top panel shows all detected galaxies brighter than our magnitude threshold (m$_{F160W}$$\leq$ 24.5), while the bottom panel shows all detected galaxies brighter than m*+1, where m* is the modeled magnitude of an L$^{*}$ galaxy at $z$ = 2.246 estimated using EzGal Mancone2012 (i.e., m$_{F160W}$$\leq$ 23.36 magnitudes). In each plot, the density measurement is for the entire sample. The value of the fraction of galaxies with $n$$\geq$ 2 for each sample is measured by bootstrapping over the measured errors in Sérsic index over 1000 iterations. We find evidence for an internal morphology-density trend in both protoclusters among all galaxies, but no evidence for an enhancement of the early-type fraction relative to the field. However, we do find a difference among the brighter galaxies in each protocluster, where the denser regions of BOSS1542 have fewer early-type massive galaxies.