If you look at the galaxy ESO 137-001 in visible light, you can see why it’s considered an example of a “jellyfish” galaxy. Blue ribbons of young stars dangle from the galaxy’s disk like cosmic tentacles. If you look at the galaxy in X-ray light, however, you will find a giant tail of hot gas streaming behind the galaxy. After launch, NASA’s James Webb Space Telescope will study ESO 137-001 to learn how the gas is being removed from the galaxy, and why stars are forming within that gaseous tail.
The newly forming stars in the tail are mysterious because processes common in large groups of galaxies should make it difficult for new stars to emerge. Most galaxies live in groups — for example, the Milky Way is a member of the Local Group, which also contains galaxies like Andromeda and the Triangulum spiral. Some galaxies reside in much larger gatherings of hundreds or even thousands of galaxies known as a galaxy cluster. The “jellyfish” galaxy ESO 137-001 is part of a cluster called Abell 3627.
A galaxy cluster isn’t just galaxies surrounded by empty space. The realm between the galaxies is filled with hot, tenuous gas. For galaxies living in the cluster or a wandering galaxy that gets pulled in by the cluster’s gravity, that gas acts like a headwind. That wind can remove gas and dust from the hapless galaxy in a process known as “ram pressure stripping.”
As a result, ram pressure stripping can slow star formation in the affected galaxy. Galaxies need gas to form stars. Eventually, all galaxies run out of gas and star formation stops. Ram pressure stripping can hasten that end.
This is one reason why galaxies in clusters stop forming new stars sooner than their relatives outside of clusters. But, the mechanisms involved are still mysterious.
“Both gas and dust are getting stripped off, but how much and what happens to the stripped material and the galaxy itself are still open questions,” said Stacey Alberts of the University of Arizona, a co-investigator on the project.
A star formation mystery
ESO 137-001 is a spiral galaxy similar in size to the Milky Way, and slightly less massive. Its tail extends across 260,000 light-years of space, almost three times the galaxy’s width. Galactic tails like this are difficult to spot because they are so tenuous. Surprisingly, stars seem to be forming in this tail.
Webb will target sites of star formation at different points along the tail: close to the galaxy, in the middle, and near the end of the tail. Since material at the tail’s end was removed before material close to the galaxy, astronomers can learn how the stripping process changed over time and how that affected conditions to form new stars.
Researchers aren’t sure how stars are able to form at all within the tail since the stripping process should have heated the gas. “We think it’s hard to strip off a molecular cloud that’s already forming stars because it should be tightly bound to the galaxy by gravity. Which means either we’re wrong, or this gas got stripped off and heated up, but then had to cool again so that it could condense and form stars,” explained Alberts.
“Telling these two scenarios apart is one of the things we want to get at,” she added.
Mid-infrared completes the puzzle
The team will examine ESO 137-001 using Webb’s Mid-Infrared Instrument (MIRI). MIRI observes infrared light at wavelengths of 5 to 28 microns, a range known as the mid-infrared. MIRI’s observations will provide 50 times more spatial detail and 20 times better spectral detail than previous work by other infrared observatories.
MIRI is sensitive to light emitted from hydrogen molecules as well as chemical elements like sulfur and oxygen. MIRI also will detect more complex, sooty molecules of carbon and hydrogen known as polycyclic aromatic hydrocarbons (PAHs), which are signposts of star formation. In addition to learning about the composition of gas and dust within these star-forming regions, astronomers will measure the physical conditions of the gas like temperature and density.
The team will combine the new Webb observations with existing data in visible light, X-rays, and at longer far-infrared wavelengths to get a more complete picture of ESO 137-001 and its environment. “Each different wavelength gives you a piece of the puzzle,” said Alberts.
Credits: NASA and J. Olmsted (STScI)
Ultimately, astronomers want to learn more about how stars came to form in the tail. They also want to understand how gas is being stripped from the galaxy, how much is being stripped, and how efficiently it’s being removed. This will provide clues to the eventual fate of ESO 137-001 and the question of whether ram pressure stripping will shut down star formation, leaving behind a dead relic filled with aging, red stars.
The observations described here will be taken as part of Webb’s Guaranteed Time Observation (GTO) program. The GTO program provides dedicated time to the scientists who have worked with NASA to craft the science and instrument capabilities of Webb throughout its development.
The James Webb Space Telescope will be the world’s premier space science observatory when it launches in 2021. Webb will solve mysteries of our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international project led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.
This Earth Day, NASA celebrates a new perspective of life found on our planet, motivated by our search for life and habitable worlds in our solar system and beyond, a search in which Webb will be deeply instrumental. Check out the jellyfish-themed poster: https://go.nasa.gov/EarthPoster2019
For more information about Webb, visit: www.nasa.gov/webb
By Christine Pulliam
Space Telescope Science Institute, Baltimore, Md.