About Our Project
Motivation
It is commonly known that there is a great diversity of galaxy properties and its strong associations with mass, environment, and time across the Universe. However, we have not reached a clear answer that can fully explain how these factors drive physical mechanisms of galaxy formation and evolution. To address this issue, mapping star-forming activities of galaxies in high-redshift clusters and fields is quite effective, which enables us to investigate mass-assembly histories of galaxies and those environmental dependence within large-scale structures.
Survey Strategy
"Mahalo-Subaru" project (MApping HAlpha and Lines of Oxygen with Subaru; "mahalo" is a Hawaiian word meaning "thanks") is designed particularly to investigate the environmental effects on star-forming activities in over-dense regions at a wide redshift range. We observed 14 clusters/proto-clusters at 0.4<z<3.8, and an un-biased blank fields (UDS and COSMOS), based on the unique sets of narrow-band filters on the two wide-field cameras, Suprime-Cam and MOIRCS on the Subaru 8.2m Telescope.
Narrow-band technique is very powerful to exhaustively find (proto-)cluster members. We can map line-emitting galaxies down to a certain flux limit over the entire FoV (right panel).
Following color-color selections (e.g. BzK selection) allow us to select more promising members located at the target redshift while contamination rates from the other emitters even without this process should be minor in high density regions (10–20 %).
Highlights
- Bottom-up growth of distant clusters
The bottom-up scenario is widely recognized as the framework of formation of large-scale structures in the Universe. High sampling densities of cluster members with the narrow-band mapping successfully represent the bottom-up structure growth of ancient clusters. For example, the youngest proto-cluster at z=2.53 among our targets, shows the clumpy substructure where the proto-cluster is made up of several over-dense groups rather than the massive single core (#8). Another protocluster at z=2.16 show a clear concentration of Hα emitters in the center and have filamentary structures in outer regions (#9). A massive cluster at z=0.41 is accompanied by several filaments and subgroups traced by Hα emitters around the X-ray detected center core shaped like octopus (#2).
- Inside-out propagation of star formation and quenching in cluster galaxies
Our observations identified that the peak of star-forming activity traced by line emitters is shifted from densest cluster cores to lower-density outskirts and filamentary outer structures with time from the redshift of 2.5 to 0.4, indicating the inside-out growth of clusters as widely demonstrated by the modern cosmological simulations. Interestingly, such propagation or shift is accompanied by the unique population, i.e. the red line emitters. They always coincide with the most active environment (#1,2,5,7,8,9), and are likely in the mode of dusty star formation or AGN hosts. We are considering that those are the key populations under the influence of environmental effects such as galaxy-galaxy interactions.
We are working on characterizing massive, red emitters with follow-up spectroscopic surveys at the multi-wavelength range. This enables identifications of physical phenomena such as galaxy mergers and AGN feedback perhaps occurring in the over-density regions discovered by our narrow-band mapping. In particular, we extensively carry out sub-mm and radio observations towards the (proto-)cluster galaxies, which address the environmental effects on star formation mode & efficiency in galaxies associated with massive structures at high-redshifts. Furthermore, we analyze spatially-resolved UV, stellar, and emission-line images of (proto-)cluster members with Hubble Space Telescope and AO-assisted Subaru observations.