Largest Catalog Of Neighboring Galaxies Unveiled Today As Part Of The LEGUS Survey

Based on old and new Hubble data, this is the first-ever survey to include star and galaxy observations in UV light.


Based on old and new Hubble data, this is the first-ever survey to include star and galaxy observations in UV light.

NASA just announced the completion of the Legacy ExtraGalactic UV Survey (LEGUS) or, as the agency describes it, “the sharpest, most comprehensive ultraviolet-light survey of star-forming galaxies in the nearby universe.”

In fact, this elaborate project represents the first galactic survey performed in UV light, says Daniela Calzetti of the University of Massachusetts, Amherst, who led the international project.

“There has never before been a star cluster and a stellar catalog that included observations in ultraviolet light.”

The LEGUS survey draws on the recent observations of NASA’s Hubble Space Telescope, coupled with images from Hubble’s extensive archive. The project showcases 50 spiral and dwarf galaxies in the vicinity of the Milky Way, all of them veritable stellar nurseries.

The vast LEGUS collection includes “star catalogs for each of the LEGUS galaxies and cluster catalogs for 30 of the galaxies, as well as images of the galaxies themselves,” NASA revealed in today’s news release.

The comprehensive LEGUS survey is comprised of two distinct catalogs, one that features 8,000 stellar clusters from representative galaxies located between 11 million and 58 million light-years from our planet, and another one focused on 39 million stars that outweigh our sun by at least five times.

“The synergy of the two catalogs combined offers an unprecedented potential for understanding star formation,” says Calzetti.

Using Hubble’s Wide Field Camera 3 and the Advanced Camera for Surveys, the international team of astronomers observed the galaxies for a period of one year and took visible-light and ultraviolet-light snapshots of their biggest young stars and stellar clusters.

The photo collage below represents six varieties of star-forming regions in different galaxies neighboring the Milky Way.

Photo collage of six star-forming galaxies neighboring the Milky Way.
Six representative images of stellar nurseries in two dwarf galaxies (UGC 5340 and UGCA 281) and four large spiral galaxies (NGC 3368, NGC 3627, NGC 6744, and NGC 4258). ESA/LEGUS team / NASA

As Calzetti explains, the survey’s UV light observations, which were helped along the way by the ultraviolet object collection that NASA previously gathered with its Galaxy Evolution Explorer spacecraft, enable astronomers to gauge the age of stars and study how and when they sparked into existence.

This is because ultraviolet light can help track down the youngest and the most incandescent stars, the astronomer points out.

The stars cataloged in the LEGUS project vary in age between 1-million and several billion-years-old; those that were imaged in UV light only go up to 100-million-years-old and shine the brightest.

Meanwhile, the star clusters detailed in the new survey are 1 million to about 500 million years old and are 10 times larger than the Milky Way’s biggest clusters.

Aside from the data on these galaxies’ stars and star clusters, the LEGUS survey could provide important information about “the connection between star formation and the major structures, such as spiral arms, that make up a galaxy,” NASA points out.

“When we look at a spiral galaxy, we usually don’t just see a random distribution of stars,” says Calzetti.

“It’s a very orderly structure, whether it’s spiral arms or rings, and that’s particularly true with the youngest stellar populations,” notes the astronomer.

By grasping the correlation between star birth and galactic gas, astronomers might finally be able to piece together the puzzle of galaxy evolution.

Her colleague Elena Sabbi, from the Space Telescope Science Institute in Baltimore, Maryland, says that the newly-released LEGUS survey “will help pave the way” for the upcoming James Webb Space Telescope, an infrared observatory that NASA is currently developing together with the European and the Canadian space agencies.

According to Sabbi, the UV images included in the survey could help pinpoint which stars gave rise to the supernovas revealed in the data.

As an added bonus, the project also provides computer models that make it easier for astronomers to interpret the information in the two catalogs.

These models allow researchers to “investigate how star formation occurred in one specific galaxy or a set of galaxies” and “correlate the properties of the galaxies with their star formation,” in order to “derive the star-formation history of the galaxies,” says Sabbi.