arXiv subjects

Tadayuki Kodama

Publications and source records attributed to Tadayuki Kodama.

At least 109 records · Page 6Linked to original sources

arXiv2017

Evolutionary phases of gas-rich galaxies in a galaxy cluster at z=1.46

We report a survey of molecular gas in galaxies in the XMMXCS J2215.9-1738 cluster at $z=1.46$. We have detected emission lines from 17 galaxies within a radius of $R_{200}$ from the cluster center, in Band 3 data of the Atacama Large Millimeter/submillimeter Array (ALMA) with a coverage of 93 -- 95 GHz in frequency and 2.33 arcmin$^2$ in spatial direction. The lines are all identified as CO $J$=2--1 emission lines from cluster members at $z\sim1.46$ by their redshifts and the colors of their optical and near-infrared (NIR) counterparts. The line luminosities reach down to $L'_{\rm CO(2-1)}=4.5\times10^{9}$ K km s$^{-1}$ pc$^2$. The spatial distribution of galaxies with a detection of CO(2--1) suggests that they disappear from the very center of the cluster. The phase-space diagram showing relative velocity versus cluster-centric distance indicates that the gas-rich galaxies have entered the cluster more recently than the gas-poor star-forming galaxies and passive galaxies located in the virialized region of this cluster. The results imply that the galaxies have experienced ram-pressure stripping and/or strangulation during the course of infall towards the cluster center and then the molecular gas in the galaxies at the cluster center is depleted by star formation.

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arXiv2017

Rotating starburst cores in massive galaxies at z=2.5

We present spatially resolved ALMA observations of the CO J=3-2 emission line in two massive galaxies at z=2.5 on the star-forming main sequence. Both galaxies have compact dusty star-forming cores with effective radii of Re=1.3 kpc and Re=1.2 kpc in the 870 um continuum emission. The spatial extent of star-forming molecular gas is also compact with Re=1.9 kpc and Re=2.3 kpc, but more extended than the dust emission. Interpreting the observed position-velocity diagrams with dynamical models, we find the starburst cores to be rotation-dominated with the ratio of the maximum rotation velocity to the local velocity dispersion of v/sigma=7.0 (v=386 km/s) and v/sigma_0=4.1 (v=391 km/s). Given that the descendants of these massive galaxies in the local universe are likely ellipticals with v/sigma nearly an order of magnitude lower, the rapidly rotating galaxies would lose significant net angular momentum in the intervening time. The comparisons among dynamical, stellar, gas, and dust mass suggest that the starburst CO-to-H2 conversion factor of alpha_CO=0.8 Msun/(K km/s/pc2) is appropriate in the spatially resolved cores. The dense cores are likely to be formed in extreme environments similar to the central regions of local ultraluminous infrared galaxies. Our work also demonstrates that a combination of medium-resolution CO and high-resolution dust continuum observations is a powerful tool for characterizing the dynamical state of molecular gas in distant galaxies.

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arXiv2017

An imperfectly passive nature: Bright sub-millimeter emission from dust-obscured star formation in the z=3.717 "passive" system, ZF20115

The identification of high-redshift massive galaxies with old stellar populations may pose challenges to some models of galaxy formation. However, to securely classify a galaxy as quiescent, it is necessary to exclude significant ongoing star formation, something that can be challenging to achieve at high redshift. In this letter, we analyse deep ALMA/870um and SCUBA-2/450um imaging of the claimed "post-starburst" galaxy ZF-20115 at z=3.717 that exhibits a strong Balmer break and absorption lines. The far-infrared imaging reveals a luminous starburst located 0.4+/-0.1 arcsec (~3kpc in projection) from the position of the rest-frame ultra-violet/optical emission, with an obscured star-formation rate of 100 Mo/yr. This star-forming component is undetected in the rest-frame ultraviolet but contributes significantly to the lower angular resolution photometry at restframe wavelengths >3500A, significantly complicating the determination of a reliable stellar mass. Importantly, in the presence of dust obscuration, strong Balmer features are not a unique signature of a post-starburst galaxy and are indeed frequently observed in infrared-luminous galaxies. We conclude that the ZF20015 system does not pose a challenge to current models of galaxy formation and that deep sub-/millimeter observations are a prerequisite for any claims of quiescence. The multi-wavelength observations of ZF20115 unveil a complex system with an intricate and spatially-varying star-formation history. ZF20115 demonstrates that understanding high-redshift obscured starbursts will only be possible with multi-wavelength studies that include high-resolution observations, available with the JWST, at mid-infrared wavelengths.

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arXiv2017

Direct evidence for Ly$α$ depletion in the protocluster core

We have carried out a panoramic Ly$α$ narrowband imaging with Suprime-Cam on Subaru towards the known protocluster USS1558--003 at $z=2.53$. Our previous narrowband imaging at near-infrared has identified multiple dense groups of H$α$ emitters (HAEs) within the protocluster. We have now identified the large-scale structures across a $\sim$50 comoving Mpc scale traced by Ly$α$ emitters (LAEs) in which the protocluster traced by the HAEs is embedded. On a smaller scale, however, there are remarkably few LAEs in the regions of HAE overdensities. Moreover, the stacking analyses of the images show that HAEs in higher-density regions show systematically lower escape fractions of Ly$α$ photons than those of HAEs in lower-density regions. These phenomena may be driven by the extra depletion of Ly$α$ emission lines along our line of sight by more intervening cold circumgalactic/intergalactic medium and/or dust existing in the dense core. We also caution that all the past high-$z$ protocluster surveys using LAEs as the tracers would have largely missed galaxies in the very dense cores of the protoclusters where we would expect to see any early environmental effects.

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arXiv2017

Similarities and uniqueness of Ly$α$ emitters among star-forming galaxies at z=2.5

We conducted a deep narrow-band imaging survey with the Subaru Prime Focus Camera on the Subaru Telescope and constructed a sample of Ly$α$ emitters (LAEs) at z=2.53 in the UDS-CANDELS field where a sample of H$α$ emitters (HAEs) at the same redshift is already obtained from our previous narrow-band observation at NIR. The deep narrow-band and multi broadband data allow us to find LAEs of stellar masses and star-formation rates (SFRs) down to $\gtrsim$$10^8$ M$_\odot$ and $\gtrsim$0.2 M$_\odot$/yr, respectively. We show that the LAEs are located along the same mass-SFR sequence traced by normal star-forming galaxies such as HAEs, but towards a significantly lower mass regime. Likewise, LAEs seem to share the same mass--size relation with typical star-forming galaxies, except for the massive LAEs, which tend to show significantly compact sizes. We identify a vigorous mass growth in the central part of LAEs: the stellar mass density in the central region of LAEs increases as their total galaxy mass grows. On the other hand, we see no Ly$α$ line in emission for most of the HAEs. Rather, we find that the Ly$α$ feature is either absent or in absorption (Ly$α$ absorbers; LAAs), and its absorption strength may increase with reddening of the UV continuum slope. We demonstrate that a deep Ly$α$ narrow-band imaging like this study is able to search for not only LAEs but also LAAs in a certain redshift slice. This work suggests that LAEs trace normal star-forming galaxies in the low-mass regime, while they remain as a unique population because the majority of HAEs are not LAEs.

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arXiv2016

Bulge-forming galaxies with an extended rotating disk at z~2

We present 0".2-resolution Atacama Large Millimeter/submillimeter Array observations at 870 um for 25 Halpha-seleced star-forming galaxies (SFGs) around the main-sequence at z=2.2-2.5. We detect significant 870 um continuum emission in 16 (64%) of these SFGs. The high-resolution maps reveal that the dust emission is mostly radiated from a single region close to the galaxy center. Exploiting the visibility data taken over a wide $uv$ distance range, we measure the half-light radii of the rest-frame far-infrared emission for the best sample of 12 massive galaxies with logM*>11. We find nine galaxies to be associated with extremely compact dust emission with R_{1/2,870um}<1.5 kpc, which is more than a factor of 2 smaller than their rest-optical sizes, R_{1/2,1.6um}=3.2 kpc, and is comparable with optical sizes of massive quiescent galaxies at similar redshifts. As they have an exponential disk with Sersic index of n=1.2 in the rest-optical, they are likely to be in the transition phase from extended disks to compact spheroids. Given their high star formation rate surface densities within the central 1 kpc of Sigma SFR1kpc=40 Msol/yr/kpc^2, the intense circumnuclear starbursts can rapidly build up a central bulge with Sigma M*1kpc>1e10 Msol/kpc^2 in several hundred Myr, i.e. by z~2. Moreover, ionized gas kinematics reveal that they are rotation-supported with an angular momentum as large as that of typical SFGs at z=1-3. Our results suggest bulges are commonly formed in extended rotating disks by internal processes, not involving major mergers.

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arXiv2016

SXDF-ALMA 2-arcmin^2 Deep Survey: Stacking of Rest-Frame Near-Infrared Selected Objects

We present stacking analyses on our ALMA deep 1.1 mm imaging in the SXDF using 1.6 μm and 3.6 μm selected galaxies in the CANDELS WFC3 catalog. We detect a stacked flux of ~0.03-0.05 mJy, corresponding to LIR < 10^11 Lsun and a star formation rate (SFR) of ~ 15 Msun/yr at z = 2. We find that galaxies brighter in the rest-frame near-infrared tend to be also brighter at 1.1 mm, and galaxies fainter than m[3.6um] = 23 do not produce detectable 1.1 mm emission. This suggests a correlation between stellar mass and SFR, but outliers to this correlation are also observed, suggesting strongly boosted star formation or extremely large extinction. We also find tendencies that redder galaxies and galaxies at higher redshifts are brighter at 1.1 mm. Our field contains z ~ 2.5 H-alpha emitters and a bright single-dish source. However, we do not find evidence of bias in our results caused by the bright source. By combining the fluxes of sources detected by ALMA and fluxes of faint sources detected with stacking, we recover a 1.1 mm surface brightness of up to 20.3 +/- 1.2 Jy/deg, comparable to the extragalactic background light measured by COBE. Based on the fractions of optically faint sources in our and previous ALMA studies and the COBE measurements, we find that approximately half of the cosmic star formation may be obscured by dust and missed by deep optical surveys, Much deeper and wider ALMA imaging is therefore needed to better constrain the obscured cosmic star formation history.

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arXiv2016

Enhanced Star Formation of Less Massive Galaxies in a Proto-Cluster at z=2.5

We investigate a correlation between star-formation rate (SFR) and stellar mass for Halpha emission line galaxies (HAEs) in one of the richest proto-clusters ever known at z~2.5, USS 1558-003 proto-cluster. This study is based on a 9.7-hour narrow-band imaging data with MOIRCS on the Subaru telescope. We are able to construct a sample, in combination with additional H-band data taken with WFC3 on Hubble Space Telescope (HST), of 100 HAEs reaching the dust-corrected SFRs down to 3 Msun/yr and the stellar masses down to $10^{8.0}$ Msun. We find that while the star-forming galaxies with >$10^{9.3}$ Msun are located on the universal SFR-mass main sequence irrespective of the environment, less massive star-forming galaxies with <$10^{9.3}$ Msun show a significant upward scatter from the main sequence in this proto-cluster. This suggests that some less massive galaxies are in a starburst phase, although we do not know yet if this is due to environmental effects.

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arXiv2016

SXDF-ALMA 2 Arcmin^2 Deep Survey: Resolving and Characterizing the Infrared Extragalactic Background Light Down to 0.5 mJy

We present a multi-wavelength analysis of five submillimeter sources (S_1.1mm = 0.54-2.02 mJy) that were detected during our 1.1-mm-deep continuum survey in the SXDF-UDS-CANDELS field (2 arcmin^2, 1sigma = 0.055 mJy beam^-1) using the Atacama Large Millimeter/submillimeter Array (ALMA). The two brightest sources correspond to a known single-dish (AzTEC) selected bright submillimeter galaxy (SMG), whereas the remaining three are faint SMGs newly uncovered by ALMA. If we exclude the two brightest sources, the contribution of the ALMA-detected faint SMGs to the infrared extragalactic background light is estimated to be ~ 4.1^{+5.4}_{-3.0} Jy deg^{-2}, which corresponds to ~ 16^{+22}_{-12}% of the infrared extragalactic background light. This suggests that their contribution to the infrared extragalactic background light is as large as that of bright SMGs. We identified multi-wavelength counterparts of the five ALMA sources. One of the sources (SXDF-ALMA3) is extremely faint in the optical to near-infrared region despite its infrared luminosity (L_IR ~ 1e12 L_sun or SFR ~ 100 M_sun yr^{-1}). By fitting the spectral energy distributions (SEDs) at the optical-to-near-infrared wavelengths of the remaining four ALMA sources, we obtained the photometric redshifts (z_photo) and stellar masses (M_*): z_photo ~ 1.3-2.5, M_* ~ (3.5-9.5)e10 M_sun. We also derived their star formation rates (SFRs) and specific SFRs (sSFRs) as ~ 30-200 M_sun yr^{-1} and ~ 0.8-2 Gyr^{-1}, respectively. These values imply that they are main-sequence star-forming galaxies.

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arXiv2016

Ultra-deep Ks-band Imaging of the Hubble Frontier Fields

We present an overview of the "KIFF" project, which provides ultra-deep Ks-band imaging of all six of the Hubble Frontier Fields clusters Abell 2744, MACS-0416, Abell S1063, Abell 370, MACS-0717 and MACS-1149. All of these fields have recently been observed with large allocations of Directors' Discretionary Time with the HST and Spitzer telescopes covering 0.4 < lambda < 1.6 microns and 3.6--4.5 microns, respectively. VLT/HAWK-I integrations of the first four fields reach 5-sigma limiting depths of Ks~26.0 (AB, point sources) and have excellent image quality (FWHM ~ 0."4). Shorter Keck/MOSFIRE integrations of the MACS-0717 (MACS-1149) field better observable in the north reach limiting depths Ks=25.5 (25.1) with seeing FWHM ~0."4 (0."5). In all cases the Ks-band mosaics cover the primary cluster and parallel HST/ACS+WFC3 fields. The total area of the Ks-band coverage is 490 arcmin^2. The Ks-band at 2.2 microns crucially fills the gap between the reddest HST filter (1.6 micron ~ H-band) and the IRAC 3.6 micron passband. While reaching the full depths of the space-based imaging is not currently feasible from the ground, the deep Ks-band images provide important constraints on both the redshifts and the stellar population properties of galaxies extending well below the characteristic stellar mass across most of the age of the universe, down to, and including, the redshifts of the targeted galaxy clusters (z < 0.5).

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arXiv2016

The nature of H-alpha star-forming galaxies at z~0.4 in and around Cl 0939+4713: the environment matters

Cluster star-forming galaxies are found to have an excess of Far-Infrared emission relative to H-alpha (Ha), when compared to those in the field, which could be caused by intense AGN activity, dust and/or declining star formation histories. Here we present spectroscopic observations of Ha emitters in the Cl 0939+4713 (Abell 851) super-cluster at z=0.41, using AF2+WYFFOS on the WHT. We measure [OII], Hbeta (Hb), [OIII], Ha and [NII] for a sample of 119 Ha emitters in and around the cluster. We find that 17+-5% of the Ha emitters are AGN, irrespective of environment. For star-forming galaxies, we obtain Balmer decrements, metallicities and ionisation parameters with different methods, individually and by stacking. We find a strong mass-metallicity relation at all environments, with no significant dependence on environment. The ionisation parameter declines with increasing stellar mass for low-mass galaxies. Ha emitters residing in intermediate environments show the highest ionisation parameters (along with high [OIII]/Ha and high [OIII]/[OII] line ratios, typically twice as large as in the highest and lowest densities), which decline with increasing environmental density. Dust extinction (A$_{Hα}$) correlates strongly with stellar mass, but also with environmental density. Star-forming galaxies in the densest environments are found to be significantly dustier (A$_{Hα}$~1.5-1.6) than those residing in the lowest density environments (A$_{Hα}$~0.6), deviating significantly from what would be predicted given their stellar masses.

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arXiv2016

Galaxy Infall by Interacting with its Environment: a Comprehensive Study of 340 Galaxy Clusters

To study systematically the evolution on the angular extents of the galaxy, ICM, and dark matter components in galaxy clusters, we compiled the optical and X-ray properties of a sample of 340 clusters with redshifts $<0.5$, based on all the available data with the Sloan Digital Sky Survey (SDSS) and {\it Chandra}/{\it XMM-Newton}. For each cluster, the member galaxies were determined primarily with photometric redshift measurements. The radial ICM mass distribution, as well as the total gravitational mass distribution, were derived from a spatially-resolved spectral analysis of the X-ray data. When normalizing the radial profile of galaxy number to that of the ICM mass, the relative curve was found to depend significantly on the cluster redshift; it drops more steeply towards outside in lower redshift subsamples. The same evolution is found in the galaxy-to-total mass profile, while the ICM-to-total mass profile varies in an opposite way. We interpret that the galaxies, the ICM, and the dark matter components had similar angular distributions when a cluster was formed, while the galaxies travelling interior of the cluster have continuously fallen towards the center relative to the other components, and the ICM has slightly expanded relative to the dark matter although it suffers strong radiative loss. This cosmological galaxy infall, accompanied by an ICM expansion, can be explained by considering that the galaxies interact strongly with the ICM while they are moving through it. The interaction is considered to create a large energy flow of $10^{44-45}$ erg $\rm s^{-1}$ per cluster from the member galaxies to their environment, which is expected to continue over cosmological time scales.

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arXiv2015

SXDF-ALMA 1.5 arcmin^2 deep survey. A compact dusty star-forming galaxy at z=2.5

We present first results from the SXDF-ALMA 1.5 arcmin^2 deep survey at 1.1 mm using Atacama Large Millimeter Array (ALMA). The map reaches a 1sigma depth of 55 uJy/beam and covers 12 Halpha-selected star-forming galaxies at z = 2.19 or z=2.53. We have detected continuum emission from three of our Halpha-selected sample, including one compact star-forming galaxy with high stellar surface density, NB2315-07. They are all red in the rest-frame optical and have stellar masses of log (M*/Msun)>10.9 whereas the other blue, main-sequence galaxies with log(M*/Msun)=10.0-10.8 are exceedingly faint, <290 uJy (2sigma upper limit). We also find the 1.1 mm-brightest galaxy, NB2315-02, to be associated with a compact (R_e=0.7+-0.1 kpc), dusty star-forming component. Given high gas fraction (44^{+20}_{-8}% or 37^{+25}_{-3}%) and high star formation rate surface density (126^{+27}_{-30} Msun yr^{-1}kpc^{-2}), the concentrated starburst can within less than 50^{+12}_{-11} Myr build up a stellar surface density matching that of massive compact galaxies at z~2, provided at least 19+-3% of the total gas is converted into stars in the galaxy centre. On the other hand, NB2315-07, which already has such a high stellar surface density core, shows a gas fraction (23+-8%) and is located in the lower envelope of the star formation main-sequence. This compact less star-forming galaxy is likely to be in an intermediate phase between compact dusty star-forming and quiescent galaxies.

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arXiv2015

Predicting dust extinction properties of star-forming galaxies from H-alpha/UV ratio

Using star-forming galaxies sample in the nearby Universe (0.02<z<0.10) selected from the SDSS (DR7) and GALEX all-sky survey (GR5), we present a new empirical calibration for predicting dust extinction of galaxies from H-alpha-to-FUV flux ratio. We find that the H-alpha dust extinction (A(Ha)) derived with H-alpha/H-beta ratio (Balmer decrement) increases with increasing H-alpha/UV ratio as expected, but there remains a considerable scatter around the relation, which is largely dependent on stellar mass and/or H-alpha equivalent width (EW(Ha)). At fixed H-alpha/UV ratio, galaxies with higher stellar mass (or galaxies with lower EW(Ha)) tend to be more highly obscured by dust. We quantify this trend and establish an empirical calibration for predicting A(Ha) with a combination of H-alpha/UV ratio, stellar mass and EW(Ha), with which we can successfully reduce the systematic uncertainties accompanying the simple H-alpha/UV approach by ~15-30%. The new recipes proposed in this study will provide a convenient tool for predicting dust extinction level of galaxies particularly when Balmer decrement is not available. By comparing A(Ha) (derived with Balmer decrement) and A(UV) (derived with IR/UV luminosity ratio) for a subsample of galaxies for which AKARI FIR photometry is available, we demonstrate that more massive galaxies tend to have higher extra extinction towards the nebular regions compared to the stellar continuum light. Considering recent studies reporting smaller extra extinction towards nebular regions for high-redshift galaxies, we argue that the dust geometry within high-redshift galaxies resemble more like low-mass galaxies in the nearby Universe.

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arXiv2015

Environmental dependence of polycyclic aromatic hydrocarbon emission at z~0.8. Investigation by observing the RX J0152.7-1357 with AKARI

We study the environmental dependence of the strength of polycyclic aromatic hydrocarbon (PAH) emission by AKARI observations of RX J0152.7-1357, a galaxy cluster at z=0.84. PAH emission reflects the physical conditions of galaxies and dominates 8 um luminosity (L8), which can directly be measured with the L15 band of AKARI. L8 to infrared luminosity (LIR) ratio is used as a tracer of the PAH strength. Both photometric and spectroscopic redshifts are applied to identify the cluster members. The L15-band-detected galaxies tend to reside in the outskirt of the cluster and have optically green colour, R-z'~ 1.2. We find no clear difference of the L8/LIR behaviour of galaxies in field and cluster environment. The L8/LIR of cluster galaxies decreases with specific-star-formation rate divided by that of main-sequence galaxies, and with LIR, consistent with the results for field galaxies. The relation between L8/LIR and LIR is between those at z=0 and z=2 in the literature. Our data also shows that starburst galaxies, which have lower L8/LIR than main-sequence, are located only in the outskirt of the cluster. All these findings extend previous studies, indicating that environment affects only the fraction of galaxy types and does not affect the L8/LIR behaviour of star-forming galaxies.

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arXiv2015

Galaxy formation at z > 3 revealed by narrow-band selected [OIII] emission line galaxies

We present the physical properties of [OIII] emission line galaxies at z>3 as the tracers of active galaxies at 1Gyr before the peak epoch at z~2. We have performed deep narrow-band imaging surveys in the Subaru/XMM-Newton Deep Survey Field with MOIRCS on the Subaru Telescope and have constructed coherent samples of 34 [OIII] emitters at z=3.2 and 3.6, as well as 107 H$α$ emitters at z=2.2 and 2.5. We investigate their basic physical quantities, such as stellar masses, star formation rates (SFRs), and sizes using the publicly available multi-wavelength data and high resolution images by the Hubble Space Telescope. The stellar masses and SFRs show a clear correlation known as the "main sequence" of star-forming galaxies. It is found that the location of the main sequence of the [OIII] emitters at z=3.2 and 3.6 is almost identical to that of the H$α$ emitters at z=2.2 and 2.5. Also, we investigate their mass-size relation and find that the relation does not change between the two epochs. When we assume that the star-forming galaxies at z=3.2 grow simply along the same main sequence down to z=2.2, galaxies with $M_* = 10^{9}$-$10^{11} M_{\odot}$ increase their stellar masses significantly by a factor of 10-2. They climb up the main sequence, and their star formation rates also increase a lot as their stellar masses grow. This indicates that star formation activities of galaxies are accelerated from z>3 towards the peak epoch of galaxy formation at z~2.

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arXiv2015

Correlation between star formation activity and electron density of ionized gas at z=2.5

In the redshift interval of $2<z<3$, the physical conditions of the inter-stellar medium (ISM) in star-forming galaxies are likely to be different from those in the local Universe because of lower gaseous metallicities, higher gas fractions, and higher star formation activities. In fact, observations suggest that higher electron densities, higher ionization parameters, and harder UV radiation fields are common. In this paper, based on the spectra of H$α$-selected star-forming galaxies at $z=2.5$ taken with Multi-Object Spectrometer for InfraRed Exploration (MOSFIRE) on Keck-1 telescope, we measure electron densities ($n_e$) using the oxygen line ratio ( [OII]$λλ$3726,3729), and investigate the relationships between the electron density of ionized gas and other physical properties. As a result, we find that the specific star formation rate (sSFR) and the surface density of SFR ($Σ_\mathrm{SFR}$) are correlated with the electron density at $z=2.5$ for the first time. The $Σ_\mathrm{SFR}-n_e$ relation is likely to be linked to the star formation law in HII regions (where star formation activity is regulated by interstellar pressure). Moreover, we discuss the mode of star formation in those galaxies. The correlation between sSFR and $Σ_\mathrm{SFR}$ suggests that highly star-forming galaxies (with high sSFR) tend to be characterized by higher surface densities of star formation ($Σ_\mathrm{SFR}$) and thus higher $n_e$ values as well.

astro-ph.CO↗

arXiv2015

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

This work presents the results from our near-infrared spectroscopy of narrow-band selected H$α$ emitters (HAEs) in two rich over-densities (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]/H$β$ or [OIII]/H$α$ 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 protoclusters 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$_\star\lesssim10^{11}$ M$_\odot$. This can be attributed to many processes, such as intrinsic (or nature) effects, external (or nurture) effects, and/or some systematic sampling effects. 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 in/out-flow rates in star forming galaxies are much higher at higher redshifts. Therefore the environmental dependence of such "feeding" and "feedback" mechanisms in galaxy formation are probably playing major roles in producing the offset of the mass-metallicity relation for the protocluster galaxies at $z>2$.

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