Confirming Herschel Candidate Protoclusters from ALMA/VLA CO Observations

Abstract

ALMA 870 µm continuum imaging has uncovered a population of blends of multiple dusty starforming galaxies (DSFGs) in sources originally detected with the Herschel Space Observatory. However, their pairwise separations are much smaller that what is found by ALMA follow-up of other single-dish surveys or expected from theoretical simulations. Using ALMA and VLA, we have targeted three of these systems to confirm whether the multiple 870 µm continuum sources lie at the same redshift, successfully detecting 12 CO(J = 3-2) and 12 CO(J = 1-0) lines and being able to confirm that in the three cases all the multiple DSFGs are likely physically associated within the same structure. Therefore, we report the discovery of two new gas-rich dusty protocluster cores (HELAISS02, z = 2.171 ± 0.004; HXMM20, z = 2.602 ± 0.002). The third target is located in the well known COSMOS overdensity at z = 2.51 (named CL J1001+0220 in the literature), for which we do not find any new secure CO(1-0) detection, although some of its members show only tentative detections and require further confirmation. From the gas, dust, and stellar properties of the two new protocluster cores, we find very large molecular gas fractions yet low stellar masses, pushing the sources above the main sequence, while not enhancing their star formation efficiency. We suggest that the sources might be newly formed galaxies migrating to the main sequence. The properties of the three systems compared to each other and to field galaxies may suggest a different evolutionary stage between systems.

On the Gas Content, Star Formation Efficiency, and Environmental Quenching of Massive Galaxies in Protoclusters at z ≈ 2.0–2.5

“…While some proto-clusters show most of their members with gas fractions in agreement with coeval field galaxies, as those presented in this work (see also , there is clear evidence that other structures have individual systems with large gas masses (e.g. Tadaki et al 2019;Gómez-Guijarro et al 2019), as discussed above. Discriminating between if these gas-rich systems exist preferentially in overdense environments or if they represent only outliers of the general population (note that this kind of outliers also exists in the field) requires deeper observations of complete samples, even deeper than the ones presented here.…”

“…Most of the remaing sources, and all the galaxies with M ⋆ 10 10 M ⊙ , have gas fractions (or upper limits) in agreement with the field relation. Similarly, Gómez-Guijarro et al (2019) found some galaxies with large gas fractions (all of them with M ⋆ 6 × 10 10 M ⊙ ) despite showing SFEs consistent with the field. Nevertheless, most of these gas-rich galaxies lie above the main sequence, and therefore, might be more representative of the starburst population.…”

“…Their detections are plotted in Figure 4 (gold triangles), where it can be seen that their values are in agreement with our measurements. Figure 4 also shows the results from Tadaki et al (2019) and Gómez-Guijarro et al (2019) who observed galaxies in several overdensities within z = 2.17 − 2.60. Beside the longer gas depletion timescales found by Tadaki et al (2019) for some galaxies with 10.5 < log(M ⋆ /M ⊙ ) < 11.0, they also found sources with high gas mass fractions (although they represent 50% of all the galaxies within this stellar mass range).…”

“…The derived gas mas fraction of most of the detected galaxies are in good agreement with those measured for field galaxies at similar redshifts and with the field scaling relation (gray shaded region, Scoville et al 2016), implying that these sources do not show enhanced gas masses. For comparison, results from studies on other proto-clusters are also included (gold, gray, and green triangles for , Gómez-Guijarro et al 2019, and Tadaki et al 2019, showing both galaxies with enhanced gas fractions and galaxies in agreement with the field (in addition to several non-detections). Interestingly, in the systems studied in this work, there are several massive non-detected galaxies for which the estimated stacked upper limits on their gas mass lie significantly below the expected field relation (large blue and red downward arrows), resembling those measured for passive quiescent galaxies and suggesting that they are likely transitional galaxies soon-to-be quenched (see discussion in §4.3).…”

“…Although cluster-to-cluster variations, reflecting different evolutionary stages (e.g. Shimakawa et al 2018;Gómez-Guijarro et al 2019) are expected, it is still unclear if the bulk of the population in the aforementioned cluster also have enhanced gas fractions. Unfortunately, deeper ALMA spectroscopic observations would require several hours of on-source time per target, making spectroscopic observations of larger samples prohibitive.…”