Association of cold gas, massive galaxies, and AGNs in a filamentary protocluster traced by triple narrow-band imaging

TL;DR

The paper investigates the HS1700+64 protocluster at $z=2.30$ using triple narrow-band imaging to map Ly$\alpha$, H$\alpha$, and [O III] emitters across two filamentary structures. It demonstrates that H$\alpha$ emitters cluster at filament intersections, while Ly$\alpha$ emitters tend to avoid dense filaments, consistent with cold gas accretion fueling star formation along filaments and at cores. The study also reveals that massive, evolved galaxies (DRGs) align with filaments and that compact-star-forming systems concentrate along these structures, with AGN candidates primarily located in intermediate-density regions. These results underscore the critical role of large-scale filaments in channeling cold gas to galaxies and driving early cluster mass assembly, and showcase a powerful, imaging-based approach for probing gas, star formation, and AGN activity in protoclusters.

Abstract

We investigate galaxy populations in the HS1700+64 protocluster at $z=2.30$, characterized by two prominent linear filaments traced by spatially extended Ly$α$ blobs. We conducted a wide area mapping of emission line galaxies across the protocluster using the unique combination of three matched narrow-band filters, corresponding to Ly$α$, H$α$, and [OIII] emission lines at $z=2.30$. We find that H$α$ emitters are strongly clustered at the intersection of the filaments, suggesting a protocluster core. In contrast, Ly$α$ emitters tend to avoid the dense region and the filaments, likely due to the resonant scattering of Ly$α$ photons by HII gas and/or enhanced dust attenuation in galaxies associated with these structures. These findings support a scenario in which cold gas flows via filaments and to the core, fed by the cold-stream mode accretion in the early phase of protocluster assembly, and promoting active star formation there. Further evidence of the scenario comes from the alignment of massive, evolved galaxies in those filaments traced by distant red galaxies, suggesting accelerated galaxy growth in the filaments in the early Universe. This study clearly shows observationally that accelerated galaxy formation takes place not only in the protocluster core but also in the associated surrounding filamentary structure. This underscores the critical role of large-scale filaments in efficiently accumulating the cold gas and channeling it to galaxies therein and to the protocluster core. Such vigorous gas assembly facilitates star formation activity and drives galaxy growth in the early stage of cluster formation.

Figures (23)

• Figure 1: Filter response functions in redshift space of each NB filter; MOIRCS/Br$\gamma$ (red), SWIMS/NB2167 (orange), SWIMS/NB1653 (green), and Palomar/NB4010 (blue). The dashed histogram represents the redshift distribution of the spectroscopically confirmed cluster member galaxies identified by Keck Baryonic Structure Survey (KBSS) Rudie2012Steidel2014Strom2017.
• Figure 2: The colour--magnitude diagram for HS1700 protocluster at $z=2.30$. The blue points show the emitters with more than 3-sigma colour excesses in K$_{\rm s}$$-$Br$\gamma$, or K$_{\rm s}$$-$NB2167, or K$_{\rm s}$$-$NB1653, respectively. Br$\gamma$ and NB2167 filters can select HAEs at $z=2.30$ while NB1653 can select O3Es at $z=2.30$.
• Figure 3: The top panels show the model tracks on the colour -- colour diagrams. The solid lines show the galaxies at $2.2<z<2.4$. The dashed lines show the low-$z$ galaxies ($0<z<1.6$) and the dashed and dotted lines show the high-$z$ galaxies ($3.2<z<5.0$). We set the boundaries so that we can pick out emission line galaxies at $z=2.3$ with reference to these model tracks and spectroscopic data (black dashed line). These lines are derived from model templates of star-forming and quiescent galaxies available in the EAZY code Brammer2008. The bottom panels show our NB emitters. Circles represent the selected emitters at $z=2.3$. Grey circles represent the emitters at different redshifts. The enclosed squares are confirmed emitters from spectroscopy or paired NB filters.
• Figure 4: The sky coverages of our MOIRCS and SWIMS imaging observations of the protocluster HS1700 overlaid on the spatial distributions of NB-selected emitters. A larger figure of the spatial distribution of galaxy populations will appear in Figure \ref{['fig:dist_hs1700']}. Blue and pink lines enclose our MOIRCS and SWIMS coverages, respectively. The background colour map represents the mean projected distance (equivalent to local density) of HAE galaxies ($\bar{b}_{\mathrm{5th,HAEs}}$), with values ranging from 0 to 400 kpc. Black cross marks represent LABs. HAEs and O3Es are marked with red points and grey open diamonds. Other symbols represent black points for NBEs (HAE, O3E, Pa$\beta$ emitters, etc.), and open squares for spectroscopic confirmed emitters. The minimum $\bar{b}_{\mathrm{5th,HAEs}}$ corresponds to 76.7 kpc, located at RA = 17:00:48.678, Dec = +64:15:41.96.
• Figure 5: The dust-corrected SFR -- $M_{\star}$ relation (star-forming main-sequence diagram) of HAEs in three protoclusters, namely, HS1700 (blue; this work), USS1558 Daikuhara2024, Spiderweb Daikuhara2024, and those in the general field COSMOS Daikuhara2024. The upper panel shows a projected histogram of stellar masses, while the right panel shows a projected histogram of dust-corrected SFRs.