An early phase of environmental effects on galaxy properties unveiled by near-infrared spectroscopy of protocluster galaxies at z > 2

Abstract

This work presents the results from our near-infrared spectroscopy of narrow-band selected Hα emitters (HAEs) in two rich overdensities (PKS 1138-262 at z=2.2 and USS 1558-003 at z = 2.5) with the Multi-Object Infrared Camera and Spectrograph (MOIRCS) on the Subaru telescope. These protoclusters are promising candidates for the most massive class of galaxy clusters seen today (Paper I). The confirmed HAEs in the protoclusters at z > 2 show high excitation levels as characterized by much higher [Oiii]/ Hβ or [Oiii]/ Hα line ratios than those of general galaxies at low-z. Such a high excitation level may not only be driven by high specific star formation rates (sSFRs) and lower gaseous metallicities, but also be contributed by some other effects. We investigate the environmental dependence of gaseous metallicities by comparing the HAEs in the protoclustrers with those in the general field at similar redshifts. We find that the gaseous metallicities of protocluster galaxies are more chemically enriched than those of field galaxies at a given stellar mass in the range of M ⋆ 10 11 M ⊙ . This can be attributed to many processes, such as intrinsic (or nature) effects, external (or nurture) effects, and/or some systematic sampling effects. The intrinsic (nature) effect leads to the advanced stage of 'down-sizing' galaxy evolution in protoclusters. On the other hand, the external (nurture) effects include the recycling of chemically enriched gas due to the higher pressure of intergalactic medium and/or stripping of outer gas in the reservoir in protoclusters. We also find that the offset of the mass-metallicity relation in dense environment becomes larger at higher redshifts. This can be naturally understood by the fact that the inflow/outflow rates in star-forming galaxies are much higher at higher redshifts. Therefore, the environmental dependence of such 'feeding' and 'feedback' mechanisms in galaxy formation is probably playing major roles in producing the offset of the mass-metallicity relation for the protocluster galaxies at z > 2.

A Radio-to-mm Census of Star-forming Galaxies in Protocluster 4C23.56 at Z = 2.5: Gas Mass and Its Fraction Revealed with ALMA

“…However, this iscontroversial given several contradictory cases, such as higher metallicity (e.g., Steidel et al 2014;Shimakawa et al 2015), a flat mass-metallicity relation (thus, higher metallicity in lower mass regime, e.g., Kulas et al 2013), and the same mass-metallicity relation as fields (e.g., Tran et al 2015) at z∼2. Therefore, we stick to the general comprehension of the stellar mass-metallicity relation.…”

The dependence of the mass–metallicity relation on large-scale environment

“…At higher redshifts, the dependence of metallicity on environment is not clear. Kulas et al (2013) and Shimakawa et al (2015) both report that proto-cluster galaxies at z ∼ 2 exhibit metallicity enhancements compared to field galaxies at those redshifts; Valentino et al (2015) report opposite trends. The samples at high redshifts are small and may be subject to selection effects and systematic measurement uncertainties.…”

A SYSTEMATIC SURVEY OF PROTOCLUSTERS AT z ∼ 3–6 IN THE CFHTLS DEEP FIELDS

“…Besides ordinary galaxies, very rare objects, such as Lyα blobs, submillimeter galaxies (SMGs), and active galactic nuclei (AGNs), are also frequently discovered in highdensity environments (Lehmer et al 2009;Digby-North et al 2010;Tamura et al 2010;Matsuda et al 2011). Some contradictory results in the mass-metallicity relation have been revealed among protoclusters at the same redshift (Kulas et al 2013; Kacprzak et al 2015;Shimakawa et al 2015;Valentino et al 2015). Furthermore, Cucciati et al (2014) have found a large amount of cold gas surrounding a z = 2.9 protocluster, which may serve as the reservoir for significant future star formation.…”