Astronomers get closer look at star-forming regions in ancient galaxy protoclusters

Tokyo: Astronomers have recently found new details about star formation in the ancient galaxy protoclusters, it has been revealed.

Ongoing studies of distant galaxy protoclusters using the Multi-Object Infrared Camera and Spectrograph (MOIRCS) instrument on the Subaru Telescope has given astronomers a closer look at the characteristics of star-forming regions in galaxies in the early universe.

A team of astronomers from the National Astronomical Observatory of Japan (NAOJ) and SOKENDAI (Graduate University of Advanced Studies, Japan) are tracking velocity structures and gaseous metallicities in galaxies in two protoclusters located in the direction of the constellation Serpens.

These appear around the radio galaxies PKS 1138-262 (at a redshift of 2.2, Figure 1) and USS 1558-003 (at a redshift of 2.5). The clusters appear as they would have looked 11 billion years ago, and the team concluded that they are in the process of cluster formation that has led to present-day galaxy clusters.

Based on the MOIRCS data, the team estimated that both protoclusters have a weight of about 10 caret 14 solar masses. These follow the typical mass growth history of the today's most massive clusters, such as the 'Coma Cluster.' That makes the two protoclusters ideal laboratories for exploring early phases of galaxy formation in a unique clustered environment.

The metallicity of the gases in the protocluster galaxies was studied using multiple spectral lines emitted from them. The result showed their gaseous metallicities are chemically enriched compared with those of galaxies in the general fields. Metals (elements heavier than hydrogen and helium) are created in the interiors of stars as they evolve and then released into surrounding gas through supernova explosions or stellar winds (often referred to as chemical evolution.)

The difference in gaseous metallicity between protoclusters and general fields suggests that star-formation histories and/or gas inflow/outflow processes should be different in the protocluster regions.

The results also suggested that galaxy formation has already been influenced by environmental conditions in the era that star-formation activities are the most active across the universe. This would be an early phase of strong environmental effects seen in the present galaxy clusters.

The research team concluded that the metallicity excess in the protocluster regions results from unique phenomena occurring in the cluster environment. The PI of this research, Mr. Rhythm Shimakawa of NAOJ and SOKENDAI, was determined to continue studying the detailed physical properties of individual forming galaxies in the protoclusters to find clear evidence that proves this hypothesis.