Environmental Regulation of Dust and Star Formation Unveiled by Subaru Dual Narrow-band Imaging: Degree-scale Balmer Decrement Mapping across a z = 0.9 Supercluster

TL;DR

This study probes how environment shapes dust content and star formation in a $z \sim 0.9$ supercluster by innovatively mapping the Balmer decrement using dual narrow-band imaging with Subaru. By combining NB1244 (Hα + [N II]) and NB921 (Hβ) data, along with broad-band photometry and archival spectroscopy, the authors derive dust extinction $E(B-V)_{neb}$, stellar masses, and dust-corrected SFRs for 94 emission-line galaxies across CL1604. They find a robust positive correlation between dust extinction and stellar mass in star-forming systems, with most galaxies near the star-forming main sequence, while the most massive members in dense environments are redder and often sub-main-sequence, indicating ongoing quenching or transitional states. The results suggest an inside-out growth pattern in overdense regions and reveal that, on a broad level, dust extinction does not exhibit a strong environmental dependence, though marginal trends in cluster cores warrant further investigation. The work demonstrates the feasibility and value of Balmer-decrement measurements from wide-field narrow-band imaging and sets the stage for future wide-field spectroscopic follow-ups like Subaru PFS to disentangle internal and environmental quenching processes.

Abstract

We present results from a dual narrow-band imaging survey targeting the CL1604 supercluster at z = 0.9 using the Subaru Telescope. By combining the NB921 filter on HSC and the NB1244 filter on SWIMS, we can detect redshifted H$α$ and H$β$ emission lines from the supercluster. This unique technique allows us to measure both star formation rates and dust extinction for a sample of 94 emission-line galaxies across the supercluster. We find that dust extinction, estimated from the Balmer decrement (H$α$/H$β$ ratio), increases with stellar mass in star-forming galaxies, whereas relatively quiescent systems exhibit comparatively low extinction. Among galaxies with intermediate masses ($10^{8.5} < M_* < 10^{10.5}\,M_\odot$), the dust-corrected H$α$-based star formation rates align with the main sequence at this epoch. More massive galaxies, however, deviate from this relation, exhibit redder colors, and reside predominantly in higher-density environments. Although stellar mass, SFR, and galaxy color are clearly influenced by environment, we detect no strong, systematic environmental dependence of dust extinction for the whole sample.

Figures (12)

• Figure 1: Field of view for narrow-band imaging with SWIMS NB1244 (magenta rectangle) and HSC NB921 (blue circle). The J-band imaging (the corresponding broad-band of SWIMS NB1244) was conducted with MOIRCS, except for field C1, which was observed using SWIMS (dashed rectangle). The area with archival IRAC coverage is indicated by the red rectangle (see Section \ref{['sec:spitzer']}). The background green contours represent galaxy surface density derived from sources within the redshift slice $z=0.88$–$0.91$, including both spectroscopically confirmed galaxies (Section \ref{['sec:spec']}) and the dual emitters identified in this study. Galaxy density is computed using Gaussian kernel density estimation. We note that ${\rm H\beta}$ emitters selected solely based on HSC NB921 imaging are not included in the density estimation.
• Figure 2: Redshift distribution of spectroscopically confirmed galaxies (see Section \ref{['sec:spec']}) within the filter coverage of six Subaru pointings, overlaid with the transmission curves of the NB921 and NB1244 narrow-band filters (arbitrarily scaled). The corresponding velocity distribution, centered at $z=0.895$, is also shown. The narrow-band filters effectively capture the redshift peak at $z=0.895$ over a considerable velocity range.
• Figure 3: (Left) Spatial distribution of emission-line galaxies within our narrow-band redshift coverage, color-coded according to their $r-J$ colors. The quiescent galaxies are defined by the rest-frame $UVJ$ diagram and selected from the subsample with IRAC coverage (Section \ref{['sec:spitzer']}). The yellow crosses mark the cluster/group centers as defined by Lemaux12. (Right) Spectroscopically confirmed galaxies from the ORELSE survey Lemaux12 and Hayashi19. The shaded regions correspond to the filter effective width coverage for NB1244 (orange) and NB921 (blue).
• Figure 4: Narrow–broad band color–magnitude diagram used to select ${\rm H\alpha}$ emitters. The color excess is calculated using MAG_APER. The gray dashed lines represent the 5$\sigma$ limiting magnitudes in the J and NB1244 bands. The gray solid curve and the line indicate the 3$\sigma$ excess in J - NB1244 colors and the J - NB1244 = 0.2 cut, respectively. Orange crosses represent galaxies brighter than the narrow-band limiting magnitude. Galaxies meeting our selection criteria (see Section \ref{['sec:sample']}) are shown as blue points with error bars representing magnitude/color uncertainties.
• Figure 5: (Left) Predicted color tracks at $z = 0.9$ and $z = 1.5$ based on the Kodama1999 model, illustrating variations in the bulge-to-total ratio from 1 to 0 along the curves. The final sample of 94 galaxies is shown as filled circles, with orange circles representing narrow-band selected emitters and gray circles representing narrow-band selected emitters confirmed with spectroscopically redshifts. Error bars indicate magnitude uncertainties. (Right) Photometric redshift distribution of the final sample.