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Yusei Koyama

Publications and source records attributed to Yusei Koyama.

At least 37 records · Page 2Linked to original sources
arXiv2024

New insights into the role of AGNs in forming the cluster red sequence

As a considerable investment of time from various telescope facilities were dedicated toward studying the Spiderweb protocluster at $z=2.2$, it so far remains one of the most extensively studied protocluster. We report here the latest results in this field, adding a new dimension to previous research on cluster formation at high redshift. Previous studies have reported a significant overdensity ($δ\sim10$) of massive H$α$ (+ [Nii]) -emitting galaxies in 3700 comoving Mpc$^3$. Many of these were previously considered to be dusty, actively star-forming galaxies, given their rest-frame optical and infrared features. However, this study argues that a third of them are more likely to be "passively-evolving" galaxies with low-luminosity active galactic nuclei (AGNs) rather than star-forming galaxies, given the multi-wavelength spectral energy distribution (SED) fitting including an AGN component. For their SED-based star formation rates to be valid, bulk of their H$α$ + [Nii] emission should come from the central AGNs. This difference in interpretation between this work and past studies, including ours, is particularly supported by the recent deep Chandra X-ray observation. Furthermore, we have spectroscopically confirmed a quiescent nature for one of these AGNs, with its multiple stellar absorption lines but also low ionisation emission lines. This important update provides new insights into the role of AGNs in forming the cluster red sequence observed in the present-day universe.

astro-ph.GA↗
arXiv2024

The ALMaQUEST Survey XII: Dense Molecular Gas as traced by HCN and HCO$^{+}$ in Green Valley Galaxies

We present ALMA observations of two dense gas tracers, HCN(1-0) and HCO$^{+}$(1-0), for three galaxies in the green valley and two galaxies on the star-forming main sequence with comparable molecular gas fractions as traced by the CO(1-0) emissions, selected from the ALMaQUEST survey. We investigate whether the deficit of molecular gas star formation efficiency (SFE$_{\rm mol}$) that leads to the low specific star formation rate in these green valley galaxies is due to a lack of dense gas (characterized by the dense gas fraction $f_{\rm dense}$) or the low star formation efficiency of dense gas (SFE$_{\rm dense}$). We find that SFE$_{\rm mol}$ as traced by the CO emissions, when considering both star-forming and retired spaxels together, is tightly correlated with SFE$_{\rm dense}$ and depends only weakly on $f_{\rm dense}$. The specific star formation rate (sSFR) on kpc scales is primarily driven by SFE$_{\rm mol}$ and SFE$_{\rm dense}$, followed by the dependence on $f_{\rm mol}$, and is least correlated with $f_{\rm dense}$ or the dense-to-stellar mass ratio ($R_{\rm dense}$). When compared with other works in the literature, we find that our green valley sample shows lower global SFE$_{\rm mol}$ as well as lower SFE$_{\rm dense}$ while exhibiting similar dense gas fractions when compared to star-forming and starburst galaxies. We conclude that the star formation of the 3 green valley galaxies with a normal abundance of molecular gas is suppressed mainly due to the reduced SFE$_{\rm dense}$ rather than the lack of dense gas.

astro-ph.GA↗
arXiv2023

Enhanced star formation and metallicity deficit in the USS 1558-003 forming protocluster at z=2.53

We use K-band multi-object near-infrared spectroscopy with Keck/MOSFIRE to search for environmental imprints on the gas properties of 27 narrow-band selected H$α$ emitters (HAEs) across the three major clumps of the assembling USS1558--003 protocluster at $z=2.53$. We target the H$α$ and [NII]$λ$6584 emission lines to obtain star-formation rates (SFR) and gas-phase oxygen abundances for our sources, confirming the membership of 23 objects. HAEs belonging to this protocluster display enhanced SFRs with respect to the main sequence of star formation at the same cosmic epoch. This effect is more prominent for low-mass galaxies ($\mathrm{\log M_*/M_\odot<10.0}$), which may be experiencing a vigorous phase of mass assembly shortly after they were formed. We compute the individual and stacked gas-phase metallicities for our sources finding a metallicity deficit for low-mass objects when compared against the field mass-metallicity relation and the massive Spiderweb protocluster at $z=2.16$. These results suggest that HAEs within USS1558--003 may be less evolved than those in the Spiderweb protocluster. Finally, we explore the gas metallicity - gas fraction relation for a small sample of five galaxies with CO(3-2) molecular gas information. Assuming our objects are in equilibrium, we obtain a relatively wide range of mass loading factors ($\mathrm{λ=0.5-2}$) matching field samples at the cosmic noon but in contrast with our previous results in the Spiderweb protocluster. We speculate that these discrepancies between protoclusters may be (partly) driven by differences in their current dynamical and mass assembly stages, hinting at the co-evolution of protoclusters and their galaxy populations at $2<z<3$.

astro-ph.GA↗
arXiv2023

Spatial extent of molecular gas, dust, and stars in massive galaxies at z=2-2.5 determined with ALMA and JWST

We present the results of 0.6"-resolution observations of CO J=3-2 line emission in 10 massive star-forming galaxies at z=2.2-2.5 with the Atacama Large Millimeter/submillimeter Array (ALMA). We compare the spatial extent of molecular gas with those of dust and stars, traced by the 870 $μ$m and 4.4 $μ$m continuum emissions, respectively. The average effective radius of the CO emission is 1.75$\pm$0.34 kpc, which is about 60 percent larger than that of the 870 $μ$m emission and is comparable with that of the 4.4 $μ$m emission. Utilizing the best-fit parametric models, we derive the radial gradients of the specific star-formation rate (sSFR), gas depletion timescale, and gas-mass fraction within the observed galaxies. We find a more intense star-formation activity with a higher sSFR and a shorter depletion timescale in the inner region than in the outer region. The central starburst may be the primary process for massive galaxies to build up a core. Furthermore, the gas-mass fraction is high, independent of the galactocentric radius in the observed galaxies, suggesting that the galaxies have not begun to quench star formation. Given the shorter gas depletion timescale in the center compared to the outer region, quenching is expected to occur in the center first and then propagate outward. We may be witnessing the observed galaxies in the formation phase of a core prior to the forthcoming phase of star formation propagating outward.

astro-ph.GA↗
arXiv2023

COALAS II. Extended molecular gas reservoirs are common in a distant, forming galaxy cluster

This paper presents the results of 475 hours of interferometric observations with the Australia Telescope Compact Array towards the Spiderweb protocluster at \(z=2.16\). We search for large, extended molecular gas reservoirs among 46 previously detected CO(1-0) emitters, employing a customised method we developed. Based on the CO emission images and position-velocity diagrams, as well as the ranking of sources using a binary weighting of six different criteria, we have identified 14 robust and 7 tentative candidates that exhibit large extended molecular gas reservoirs. These extended reservoirs are defined as having sizes greater than 40 kpc or super-galactic scale. This result suggests a high frequency of extended gas reservoirs, comprising at least \(30 \%\) of our CO-selected sample. An environmental study of the candidates is carried out based on N-th nearest neighbour and we find that the large molecular gas reservoirs tend to exist in denser regions. The spatial distribution of our candidates is mainly centred on the core region of the Spiderweb protocluster. The performance and adaptability of our method are discussed. We found 13 (potentially) extended gas reservoirs located in nine galaxy (proto)clusters from the literature. We noticed that large extended molecular gas reservoirs surrounding (normal) star-forming galaxies in protoclusters are rare. This may be attributable to the lack of observations low-J CO transitions and the lack of quantitative analyses of molecular gas morphologies. The large gas reservoirs in the Spiderweb protocluster are a potential source of the intracluster medium seen in low redshift Virgo- or Coma-like galaxy clusters.

astro-ph.GA↗
arXiv2023

Environmental impacts on the rest-frame UV size and morphology ofstar-forming galaxies at $z\sim2$

We report the measurement of rest-frame UV size and morphology of H$α$-emission-selected star-forming galaxies (HAEs) in four protoclusters at z $\sim$ 2 (PKS 1138-262, USS 1558-003, PHz G237.0+42.5, and CC 2.2) using archival Hubble Space Telescope Advanced Camera Surveys (HST/ACS) F814W data. We compare the measurement of 122 HAEs in protoclusters detected by HST/ACS to a coeval comparison field sample of 436 HAEs. We find the size distributions of protocluster and field HAEs are similar with typical half-light radius of $\sim$ 2.5 kpc. At fixed stellar mass, there is no significant difference between HAE in protocluster and in field, which is also supported by stacking analyses. This result suggests that the environment does not significantly affect the size of galaxies during the star-forming phase at this epoch. Based on Sérsic index and non-parametric morphologies, HAE morphologies in both environments at $z\sim2$ in rest-frame UV are consistent with disk-like star-forming galaxies, although we also find $29\% \pm 4\%$ HAEs have peculiar or disturbed morphologies. The fraction of disturbed galaxies is higher in protocluster environment, with $39 \pm 8 \%$ protocluster HAEs showing disturbed morphologies, compared to $26\pm4\%$ in the comparison field. The apparent disturbed morphologies are correlated with higher star-formation activity and may be caused by either in situ giant clumps or mergers.

astro-ph.GA↗
arXiv2023

GREX-PLUS Science Book

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA's strategic L-class mission to be launched in the 2030s. Its primary sciences are two-fold: galaxy formation and evolution and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a 1.2 m primary mirror aperture telescope cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2-8 $μ$m wavelength band and a high resolution spectrometer with a wavelength resolution of 30,000 in the 10-18 $μ$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high resolution spectrometer aims to identify the location of the water ``snow line'' in proto-planetary disks. Both instruments will provide unique data sets for a broad range of scientific topics including galaxy mass assembly, origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy for exoplanet atmosphere, planetary atmosphere in the Solar system, and so on.

astro-ph.CO↗
arXiv2023

EMPRESS. IX. Extremely Metal-Poor Galaxies are Very Gas-Rich Dispersion-Dominated Systems: Will JWST Witness Gaseous Turbulent High-z Primordial Galaxies?

We present kinematics of 6 local extremely metal-poor galaxies (EMPGs) with low metallicities ($0.016-0.098\ Z_{\odot}$) and low stellar masses ($10^{4.7}-10^{7.6} M_{\odot}$). Taking deep medium-high resolution ($R\sim7500$) integral-field spectra with 8.2-m Subaru, we resolve the small inner velocity gradients and dispersions of the EMPGs with H$α$ emission. Carefully masking out sub-structures originated by inflow and/or outflow, we fit 3-dimensional disk models to the observed H$α$ flux, velocity, and velocity-dispersion maps. All the EMPGs show rotational velocities ($v_{\rm rot}$) of 5--23 km s$^{-1}$ smaller than the velocity dispersions ($σ_{0}$) of 17--31 km s$^{-1}$, indicating dispersion-dominated ($v_{\rm rot}/σ_{0}=0.29-0.80<1$) systems affected by inflow and/or outflow. Except for two EMPGs with large uncertainties, we find that the EMPGs have very large gas-mass fractions of $f_{\rm gas}\simeq 0.9-1.0$. Comparing our results with other H$α$ kinematics studies, we find that $v_{\rm rot}/σ_{0}$ decreases and $f_{\rm gas}$ increases with decreasing metallicity, decreasing stellar mass, and increasing specific star-formation rate. We also find that simulated high-$z$ ($z\sim 7$) forming galaxies have gas fractions and dynamics similar to the observed EMPGs. Our EMPG observations and the simulations suggest that primordial galaxies are gas-rich dispersion-dominated systems, which would be identified by the forthcoming James Webb Space Telescope (JWST) observations at $z\sim 7$.

astro-ph.GA↗
arXiv2022

EMPRESS. VIII. A New Determination of Primordial He Abundance with Extremely Metal-Poor Galaxies: A Suggestion of the Lepton Asymmetry and Implications for the Hubble Tension

The primordial He abundance $Y_\mathrm{P}$ is a powerful probe of cosmology. Currently, $Y_\mathrm{P}$ is best determined by observations of metal-poor galaxies, while there are only a few known local extremely metal-poor ($<0.1 Z_\odot$) galaxies (EMPGs) having reliable He/H measurements with HeI$λ$10830 near-infrared (NIR) emission. Here we present deep Subaru NIR spectroscopy for 10 EMPGs. Combining the existing optical data, He/H values of 5 out of the 10 EMPGs are reliably derived by the Markov chain Monte Carlo algorithm. Adding the existing 3 EMPGs and 51 moderately metal-poor ($0.1-0.4 Z_\odot$) galaxies with reliable He/H estimates, we obtain $Y_\mathrm{P}=0.2370^{+0.0034}_{-0.0033}$ by linear regression in the $\mathrm{(He/H)}-\mathrm{(O/H)}$ plane, where we increase the number of EMPGs from 3 to 8 anchoring He/H of the most metal-poor gas in galaxies. Although our $Y_\mathrm{P}$ measurement and previous measurements are consistent, our result is slightly ($\sim 1σ$) smaller due to our EMPGs. With our $Y_\mathrm{P}$ and the existing primordial deuterium $D_\mathrm{P}$ measurement, we constrain the effective number of neutrino species $N_\mathrm{eff}$ and the baryon-to-photon ratio $η$ showing $\gtrsim 1-2σ$ tensions with the Standard Model and Planck Collaboration et al. (2020). Motivated by the tensions, we allow the degeneracy parameter of electron-neutrino $ξ_e$ to vary as well as $N_\mathrm{eff}$ and $η$. We obtain $ξ_e = 0.05^{+0.03}_{-0.02}$, $N_\mathrm{eff}=3.11^{+0.34}_{-0.31}$, and $η\times10^{10}=6.08^{+0.06}_{-0.06}$ from the $Y_\mathrm{P}$ and $D_\mathrm{P}$ measurements with a prior of $η$ taken from Planck Collaboration et al. (2020). Our constraints suggest a lepton asymmetry and allow for a high value of $N_\mathrm{eff}$ within the $1σ$ level, which could mitigate the Hubble tension.

astro-ph.CO↗
arXiv2022

CO($J$=1-0) mapping survey of 64 galaxies in the Fornax cluster with the ALMA Morita array

We conduct a $^{12}$C$^{16}$O($J$=1-0) (hereafter CO) mapping survey of 64 galaxies in the Fornax cluster using the ALMA Morita array in cycle 5. CO emission is detected from 23 out of the 64 galaxies. Our sample includes dwarf, spiral and elliptical galaxies with stellar masses of $M_{\rm star}\sim10^{6.3-11.6}$~M$_\odot$. The achieved beam size and sensitivity are $15''\times8''$ and $\sim12$~mJy~beam$^{-1}$ at the velocity resolution of $\sim10$~km~s$^{-1}$, respectively. We study the cold-gas (molecular- and atomic-gas) properties of 38 subsamples with $M_{\rm star}>10^9$~M$_\odot$ combined with literature HI data. We find that: (1) the low star-formation (SF) activity in the Fornax galaxies is caused by the decrease in the cold-gas mass fraction with respect to stellar mass (hereafter, gas fraction) rather than the decrease of the SF efficiency from the cold gas; (2) the atomic-gas fraction is more heavily reduced than the molecular-gas fraction of such galaxies with low SF activity. A comparison between the cold-gas properties of the Fornax galaxies and their environmental properties suggests that the atomic gas is stripped tidally and by the ram pressure, which leads to the molecular gas depletion with an aid of the strangulation and consequently SF quenching. Pre-processes in the group environment would also play a role in reducing cold-gas reservoirs in some Fornax galaxies.

astro-ph.GA↗
arXiv2022

Signs of environmental effects on star-forming galaxies in the Spiderweb protocluster at z=2.16

We use multi-object near-infrared (NIR) spectroscopy with VLT/KMOS to investigate the role of the environment in the evolution of the ionized gas properties of narrow-band selected H$α$ emitters (HAEs) in the Spiderweb protocluster at $z=2.16$. Based on rest-frame optical emission lines, H$α$ and [NII]$λ$6584, we confirm the cluster membership of 39 of our targets (i.e. 93% success rate), and measure their star-formation rates (SFR), gas-phase oxygen abundances and effective radius. We parametrize the environment where our targets reside by using local and global density indicators based on previous samples of spectroscopic and narrow-band cluster members. We find that star-forming galaxies embedded in the Spiderweb protocluster display SFRs compatible with those of the main sequence and morphologies comparable to those of late-type galaxies at $z=2.2$ in the field. We also report a mild gas-phase metallicity enhancement ($0.6\pm0.3$ dex) at intermediate stellar masses. Furthermore, we identify two UVJ-selected quiescent galaxies with residual H$α$-based star formation and find signs of extreme dust obscuration in a small sample of SMGs based on their FIR and H$α$ emission. Interestingly, the spatial distribution of these objects differs from the rest of HAEs, avoiding the protocluster core. Finally, we explore the gas fraction-gas metallicity diagram for 7 galaxies with molecular gas masses measured by ATCA using CO(1-0). In the context of the gas-regulator model, our objects are consistent with relatively low mass-loading factors, suggesting lower outflow activity than field samples at the cosmic noon and thus, hinting at the onset of environmental effects in this massive protocluster.

astro-ph.GA↗
arXiv2022

High-resolution ALMA study of CO (2-1) line and dust continuum emissions in cluster galaxies at z = 1.46

We present new Atacama Large Millimeter/submillimeter Array (ALMA) results obtained from spatially resolved CO $J$=2-1 line ($0.4''$ resolution) and 870 $μ$m continuum ($0.2''$ resolution) observations of cluster galaxies in XMMXCS J2215.9-1738 at $z=1.46$. Our sample comprises 17 galaxies within $\sim0.5$ Mpc ($0.6R_{200}$) of the cluster center, all of which have previously been detected in the CO $J$=2-1 line at a lower resolution. The effective radii of both the CO $J$=2-1 line and 870 $μ$m dust continuum emissions are robustly measured for nine galaxies by modeling the visibilities. We find that the CO $J$=2-1 line emission in all of the nine galaxies is more extended than the dust continuum emission by a factor of $2.8\pm1.4$. We investigate the spatially resolved Kennicutt-Schmidt (KS) relation in two regions within the interstellar medium of the galaxies. The relation for our sample reveals that the central region ($0<r<R_{e,{\rm 870μm}}$) of galaxies tends to have a shorter gas depletion timescale, i.e., a higher star formation efficiency, compared to the extended region ($R_{e,{\rm 870μm}}<r<R_{e,{\rm CO}}$). Overall, our result suggests that star formation activities are concentrated inside the extended gas reservoir, possibly resulting in the formation of a bulge structure. We find consistency between the ALMA 870 $μ$m radii of star-forming members and the Hubble Space Telescope/1.6 $μ$m radii of passive members in a mass-size distribution, which suggests a transition from star-forming to passive members within $\sim0.5$ Gyr. In addition, no clear differences in the KS relation nor in the sizes are found between galaxies with and without a close companion.

astro-ph.GA↗
arXiv2022

Star Formation Properties of Sloan Digital Sky Survey BOSS Void Galaxies in the Hyper Suprime-Cam Survey

We utilize the Hyper Suprime-Cam (HSC) Wide Survey to explore the properties of galaxies located in the voids identified from the Baryon Oscillation Spectroscopic Survey (BOSS) up to z~0.7. The HSC reaches i~25, allowing us to characterize the void galaxies down to 10$^{9.2}$ solar mass. We find that the revised void galaxy densities, when including faint galaxies in voids defined by bright galaxies, are still underdense compared to the mean density from the entire field. In addition, we classify galaxies into star-forming, quiescent, and green valley populations, and find that void galaxies tend to have slightly higher fractions of star-forming galaxies under the mass and redshift control, although the significance of this result is only moderate (2$σ$). However, when we focus on the star-forming population, the distribution of the specific star formation rate (sSFR) of void galaxies shows little difference from that of the control galaxies. Similarly, the median sSFR of star-forming void galaxies is also in good agreement with that of the star-forming control galaxies. Moreover, the effective green valley fraction of void galaxies, defined as the number of green valley galaxies over the number of nonquiescent galaxies, is comparable to that of the control ones, supporting the suggestion that void and control galaxies evolve under similar physical processes and quenching frequencies. Our results thus favor a scenario of the galaxy assembly bias.

astro-ph.GA↗
arXiv2021

Environmental dependence of gas properties in a protocluster at z~2.5

In a protocluster USS1558-003 at z=2.53, galaxies in the dense cores show systematically elevated star-forming activities than those in less dense regions. To understand its origin, we look into the gas properties of the galaxies in the dense cores by conducting deep 1.1mm observations with Atacama Large Millimeter/submillimeter Array (ALMA). We detect interstellar dust continuum emissions from 12 member galaxies and estimate their molecular gas masses. Comparing these gas masses with our previous measurements from CO(3-2) line, we infer that the latter might be overestimated. We find that the gas to stellar mass ratios of the galaxies in the dense cores tend to be higher (at M* ~ 10^{10} M_sun) where we see the enhanced star-forming activities), suggesting that such large gas masses can sustain their high star-forming activities. However, if we compare the gas properties of these protocluster galaxies with the gas scaling relations constructed for field galaxies at similar cosmic epoch, we find no significant environmental difference at the same stellar mass and star formation rate. Although both gas mass ratios and star-forming activities are enhanced in the majority of member galaxies, they appear to follow the same scaling relation as field galaxies. Our results are consistent with the scenario that the cold gas is efficiently supplied to protocluster cores and to galaxies therein along surrounding filamentary structures, which leads to the high gas mass fractions and thus the elevated star-formation activities, but without changing the star formation law.

astro-ph.GA↗
arXiv2021

What determines the HI gas content in galaxies?: morphological dependence of the HI gas fraction across M*-SFR plane

We perform a stacking analysis of the HI spectra from the Arecibo Legacy Fast ALFA (ALFALFA) survey for optically-selected local galaxies from the Sloan Digital Sky Survey (SDSS) to study the average gas fraction of galaxies at fixed stellar mass ($M_*$) and star formation rate (SFR). We first confirm that the average gas fraction strongly depends on the stellar mass and SFR of host galaxies; massive galaxies tend to have a lower gas fraction, and actively star-forming galaxies show higher gas fraction, which is consistent with many previous studies. Then we investigate the morphological dependence of the HI gas mass fraction at fixed $M_*$ and SFR to minimize the effects of these parameters. We use three morphological classifications based on parametric indicator (Sérsic index), non-parametric indicator (C-index), and visual inspection (smoothness from the Galaxy Zoo 2 project) on the optical image. We find that there is no significant morphological dependence of the HI gas mass fraction at fixed $M_*$ and SFR when we use C-index. In comparison, there exists a hint of diminishment in the HI gas mass fraction for "smooth" galaxies compared with "non-smooth" galaxies. We find that the visual smoothness is sensitive to the existence of small-scale structures in a galaxy. Our result suggests that even at fixed $M_*$ and SFR, the presence of such small-scale structures (seen in the optical image) is linked to their total HI gas content.

astro-ph.GA↗
arXiv2021

A phase-space view of cold-gas properties of Virgo-cluster galaxies: multiple quenching processes at work?

We investigate the cold-gas properties of massive Virgo galaxies ($>10^9$ M$_\odot$) at $<3R_{200}$ ($R_{200}$ is the radius where the mean interior density is 200 times the critical density) on the projected phase-space diagram (PSD) with the largest archival dataset to date to understand the environmental effect on galaxy evolution in the Virgo cluster. We find: lower HI and H$_2$ mass fractions and higher star-formation efficiencies (SFEs) from HI and H$_2$ in the Virgo galaxies than the field galaxies for matched stellar masses; the Virgo galaxies generally follow the field relationships between the offset from the main sequence of the star-forming galaxies [$Δ$(MS)] with gas fractions and SFEs but slightly offset to lower gas fractions or higher SFEs than field galaxies at $Δ({\rm MS})< 0$; lower gas fractions in galaxies with smaller clustocentric distance and velocity; lower gas fractions in the galaxies in the W cloud, a substructure of the Virgo cluster. Our results suggest the cold-gas properties of some Virgo galaxies are affected by their environment at least at $3 R_{200}$ maybe via strangulation and/or pre-processes and HI and H$_2$ in some galaxies are removed by ram pressure at $<1.5 R_{200}$. Our data cannot rule the possibility of the other processes such as strangulation and galaxy harassment accounting for the gas reduction in some galaxies at $<1.5 R_{200}$. Future dedicated observations of a mass-limited complete sample are required for definitive conclusions.

astro-ph.GA↗
arXiv2021

A Planck-selected dusty proto-cluster at z=2.16 associated with a strong over-density of massive H$α$ emitting galaxies

We discovered an over-density of H-alpha-emitting galaxies associated with a Planck compact source in the COSMOS field (PHzG237.0+42.5) through narrow-band imaging observations with Subaru/MOIRCS. This Planck-selected dusty proto-cluster at z=2.16 has 38 H-alpha emitters including six spectroscopically confirmed galaxies in the observed MOIRCS 4'x7' field (corresponding to ~2.0x3.5~Mpc^2 in physical scale). We find that massive H-alpha emitters with log(M*/Msun)>10.5 are strongly clustered in the core of the proto-cluster (within ~300-kpc from the density peak of the H-alpha emitters). Most of the H-alpha emitters in this proto-cluster lie along the star-forming main sequence using H-alpha-based SFR estimates, whilst the cluster total SFR derived by integrating the H-alpha-based SFRs is an order of magnitude smaller than those estimated from Planck/Herschel FIR photometry. Our results suggest that H-alpha is a good observable for detecting moderately star-forming galaxies and tracing the large-scale environment in and around high-z dusty proto-clusters, but there is a possibility that a large fraction of star formation could be obscured by dust and undetected in H-alpha observations.

astro-ph.GA↗
arXiv2020

Dust, gas, and metal content in star-forming galaxies at $z\sim3.3$ revealed with ALMA and Near-IR spectroscopy

We conducted sub-millimeter observations with the Atacama Large Millimeter/sub-millimeter Array (ALMA) of star-forming galaxies at $z\sim3.3$, whose gas-phase metallicities have been previously measured. We investigate the dust and gas contents of the galaxies at $z\sim3.3$ and study how galaxies are interacting with their circumgalactic/intergalactic medium at this epoch by probing their gas mass fractions and gas-phase metallicities. Single-band dust continuum emission tracing dust mass and the relation between the gas-phase metallicity and gas-to-dust mass ratio are used to estimate the gas masses. The estimated gas mass fractions and depletion timescales are $f_{\rm gas}=$ 0.20-0.75 and $t_{\rm dep}=$ 0.09-1.55 Gyr, respectively. Although the galaxies appear to tightly distribute around the star-forming main sequence at $z\sim3.3$, both quantities show a wider spread at a fixed stellar mass than expected from the scaling relation, suggesting a large diversity of fundamental gas properties among star-forming galaxies apparently on the main sequence. Comparing gas mass fraction and gas-phase metallicity between the star-forming galaxies at $z\sim3.3$ and at lower redshifts, star-forming galaxies at $z\sim3.3$ appear to be more metal-poor than local galaxies with similar gas mass fractions. Using the gas regulator model to interpret this offset, we find that it can be explained by a higher mass-loading factor, suggesting that the mass-loading factor in outflows increases at earlier cosmic times.

astro-ph.GA↗