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Study Reveals the Growth of Massive Galaxies in the Early Universe


The Fiber-Multi-Object Spectrograph (FMOS). © Subaru Telescope.

Over nine billion years ago, galaxies provided a nurturing and orderly environment for the birth of new stars at remarkable rates. A study, undertaken as part of the Cosmic Evolution Survey (COSMOS), shows that even at this early time, there were signs of mature galaxies that were surrounded by dust and included elements heavier than hydrogen and helium.  

A team of astronomers, including several from the IfA, used the Fiber-Multi-Object Spectrograph (FMOS) mounted on the Subaru Telescope on Maunakea for this work. They sought to answer to the question, “What was the Universe like when it was maximally forming its stars?” according to John Silverman of the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) in Japan, who serves as the principal investigator of the FMOS-COSMOS project. COSMOS’s research is designed to examine how the environment of large-scale structures influenced the formation and evolution of galaxies over cosmic time. Determining whether the individual properties of galaxies, such as their rate of growth, are connected to the larger-scale environment helps scientists understand what factors in the early Universe shaped the current form of local galaxies. To do this, the team has been carrying out an intensive program of research using FMOS on Subaru to map the distribution of about 2,000 galaxies when the Universe was only 4 billion years old.  

FMOS typically acquires spectra from 200 galaxies simultaneously. Being able to capture so many objects in such a wide field of view is useful for a range of purposes, from studying galaxy evolution and the variation within the galaxy environment to investigating star-forming regions, cluster formation, and cosmology. FMOS provides unprecedented views of the distant Universe by using fiber optic cables to collect the light of multiple objects over an area of the sky equal to that spanned by our full Moon and also by using a built-in filter to remove unwanted bright light from the warm night sky.

“FMOS has clearly revolutionized our ability to study how galaxies form and evolve across cosmic time,” said David Sanders, the principal investigator of the FMOS-COSMOS project at the IfA. “It is currently the most powerful instrument we have to study the large numbers of objects needed to understand galaxies of all sizes, shapes, and masses—from the largest ellipticals to the smallest dwarfs. We are extremely fortunate that the Kavli IPMU-IfA collaboration is giving us this unique opportunity to study the distant Universe in such exquisite detail.”

The team has concluded that star formation during the epoch studied was 20 times greater than it is now. They also learned that the galaxies observed with FMOS have significantly lower levels of chemically enriched gas in their interstellar medium than galaxies of the same mass in the Universe near Earth. This finding supports the view that galaxies that have room to grow accumulate pristine gas that fuels their intense star formation. Larger amounts of dust and metal content indicate that the more massive galaxies 4 billion years ago were similar to later, fully mature local galaxies that have stopped star formation.

The FMOS-COSMOS survey has reached the halfway mark toward its goal of observing approximately 2,000 galaxies to map the large-scale structure. Future efforts with FMOS may expand the areal coverage, and the team may also use complementary instruments on other telescopes that can cover other parts of the spectrum or have deeper penetrating power but are limited by a small area of coverage. Such complementarity may allow FMOS to detect the first structures that likely evolved into the massive clusters of galaxies that we see today.

In addition to Sanders, former IfA astronomer Lisa Kewley (now at Australian National University), IfA graduate students Jabran Zahid and Jason Chu, and IfA alumna Jeyhan Kartaltepe are working on the project.