Astronomers release most complete ultraviolet-light survey of nearby galaxies

This image shows the galaxy NGC 6744, about 30 million
light-years away. It is one of 50 galaxies observed as part
of the Hubble Space Telescope’s Legacy ExtraGalactic UV
Survey (LEGUS), the sharpest, most comprehensive
ultraviolet-light survey of star-forming galaxies in the
nearby Universe, offering an extensive resource for
understanding the complexities of star formation and galaxy
evolution. The image is a composite using both ultraviolet
light and visible light, gathered with Hubble’s Wide Field
Camera 3 and Advanced Camera for Surveys. Credit: NASA,
ESA, and the LEGUS team

Capitalizing on the unparalleled sharpness and spectral range
of NASA’s Hubble Space Telescope, an international team of
astronomers is releasing the most comprehensive,
high-resolution ultraviolet-light survey of nearby
star-forming galaxies.


The researchers combined new Hubble observations with archival
Hubble images for 50 star-forming spiral and dwarf in the local universe, offering a large
and extensive resource for understanding the complexities of
and . The project, called the Legacy
ExtraGalactic UV Survey (LEGUS), has amassed star catalogs for
each of the LEGUS galaxies and cluster catalogs for 30 of the
galaxies, as well as images of the galaxies themselves. The
data provide detailed information on young, massive and star clusters, and how their
environment affects their development.

“There has never before been a star cluster and a stellar
catalog that included observations in ultraviolet light,”
explained survey leader Daniela Calzetti of the University of
Massachusetts, Amherst. “Ultraviolet light is a major tracer of
the youngest and hottest star populations, which astronomers
need to derive the ages of stars and get a complete stellar
history. The synergy of the two catalogs combined offers an
unprecedented potential for understanding star formation.”

How stars form is still a vexing question in astronomy. “Much
of the light we get from the universe comes from stars, and yet
we still don’t understand many aspects of how stars form,” said
team member Elena Sabbi of the Space Telescope Science
Institute in Baltimore, Maryland. “This is even key to our
existence—we know life wouldn’t be here if we didn’t have a
star around.”

The research team carefully selected the LEGUS targets from
among 500 galaxies, compiled in ground-based surveys, located
between 11 million and 58 million light-years from Earth. Team
members chose the galaxies based on their mass, star-formation
rate, and abundances of elements that are heavier than hydrogen
and helium. The catalog of ultraviolet objects collected by
NASA’s Galaxy Evolution Explorer (GALEX) spacecraft also helped
lay the path for the Hubble study.

UGCA 281 is a blue compact dwarf galaxy located in the
constellation of Canes Venatici. Within it, two giant star
clusters appear brilliant white and are swaddled by greenish
hydrogen gas clouds. These clusters are responsible for most
of the recent star formation in UGCA 281; the rest of the
galaxy is comprised of older stars and appears redder in
colour. The reddish objects in the background are background
galaxies that appear through the diffuse dwarf galaxy. The
image is a composite using both ultraviolet light and visible
light, gathered with Hubble’s Wide Field Camera 3 and
Advanced Camera for Surveys. Credit: NASA, ESA, and the LEGUS
team

The team used Hubble’s Wide Field Camera 3 and the Advanced
Camera for Surveys over a one-year period to snap visible- and
ultraviolet-light images of the galaxies and their most massive
young stars and star clusters. The researchers also added
archival visible-light images to provide a complete picture.

The star cluster catalogs contain about 8,000 young clusters
whose ages range from 1 million to roughly 500 million years
old. These stellar groupings are as much as 10 times more
massive than the largest clusters seen in our Milky Way galaxy.

The star catalogs comprise about 39 million stars that are at
least five times more massive than our Sun. Stars in the
visible-light images are between 1 million and several billion
years old; the youngest stars, those between 1 million and 100
million years old, shine prominently in ultraviolet light.

The Hubble data provide all of the information to analyze these
galaxies, the researchers explained. “We also are offering
computer models to help astronomers interpret the data in the
star and cluster catalogs,” Sabbi said. “Researchers, for
example, can investigate how star formation occurred in one
specific galaxy or a set of galaxies. They can correlate the
properties of the galaxies with their star formation. They can
derive the star-formation history of the galaxies. The
ultraviolet-light images may also help astronomers identify the
progenitor stars of supernovas found in the data.”

One of the key questions the survey may help astronomers answer
is the connection between star formation and the major
structures, such as spiral arms, that make up a galaxy.

These six images represent the variety of star-forming
regions in nearby galaxies. The galaxies are part of the
Hubble Space Telescope’s Legacy ExtraGalactic UV Survey
(LEGUS), the sharpest, most comprehensive ultraviolet-light
survey of star-forming galaxies in the nearby universe. The
LEGUS survey combines new Hubble observations with archival
Hubble images for 50 nearby star-forming spiral and dwarf
galaxies, offering a large and extensive resource for
understanding the complexities of star formation and galaxy
evolution. Astronomers are releasing the star catalogs for
each of the LEGUS galaxies and cluster catalogs for 30 of the
galaxies, as well as images of the galaxies themselves. The
catalogs provide detailed information on young, massive stars
and star clusters, and how their environment affects their
development. The six images consist of two dwarf galaxies
(UGC 5340 and UGCA 281) and four large spiral galaxies (NGC
3368, NGC 3627, NGC 6744, and NGC 4258). The images are a
blend of ultraviolet light and visible light from Hubble’s
Wide Field Camera 3 and Advanced Camera for Surveys. All of
the galaxies are undergoing vigorous star and star-cluster
formation. One of the goals of LEGUS is to sample
star-forming regions across each galaxy. Because the galaxies
are relatively close to Earth, Hubble can resolve individual
stars. The most intense and most recent star birth in the
dwarf galaxies is concentrated away from the center. In UGC
5340, a pocket of rapid star birth appears in the lower right
corner, and may have been triggered by a gravitational
interaction with an unseen companion galaxy. Star formation
is present across the entire body of UGC 5340, and the
relatively young stars are responsible for the galaxy’s
blue-white color. In UGCA 281, two giant star clusters appear
brilliant white and are swaddled by greenish hydrogen gas
clouds. These clusters are responsible for most of the recent
star formation in UGCA 281; the rest of the galaxy is
comprised of older stars and appears redder in color than UGC
5340. The reddish objects in the images of the dwarf galaxies
are background galaxies that appear through these diffuse
objects. In the spiral galaxies, a wave of star formation is
occurring along the dark filaments that make up the spiral
arms. The fledgling stars illuminate the surrounding hydrogen
gas, making the stars appear pink. Star birth begins at the
inner spiral arms and moves outward. The milky white regions
in the center of these galaxies represent the glow of
countless numbers of stars. The star clusters in these
galaxies range in age from 1 million to roughly 500 million
years old. These stellar groupings are as much as 10 times
more massive than the largest clusters seen in our Milky Way
galaxy. The galaxies’ stars that can be detected in the
images range from the size of our Sun to more than 100 times
our Sun’s mass. They are between 1 million and several
billion years old. The six galaxies are between 19 million
and 42 million light-years from Earth. They were observed
between January 2014 and July 2014. Credit: NASA, ESA, and
the LEGUS team

“When we look at a spiral galaxy, we usually don’t just see a
random distribution of stars,” Calzetti said. “It’s a very
orderly structure, whether it’s spiral arms or rings, and
that’s particularly true with the youngest stellar populations.
On the other hand, there are multiple competing theories to
connect the individual stars in individual star clusters to
these ordered structures.

“By seeing galaxies in very fine detail—the star clusters—while
also showing the connection to the larger structures, we are
trying to identify the physical parameters underlying this
ordering of stellar populations within galaxies. Getting the
final link between gas and star formation is key for
understanding galaxy evolution.”

Team member Linda Smith of the European Space Agency (ESA) and
the Space Telescope Science Institute, added: “We’re looking at
the effects of the environment, particularly with , and how their survival is linked
to the environment around them.”

The LEGUS survey will also help astronomers interpret views of
galaxies in the distant universe, where the ultraviolet glow
from young stars is stretched to infrared wavelengths due to
the expansion of space. “The data in the star and catalogs of these nearby galaxies will
help pave the way for what we see with NASA’s upcoming infrared
observatory, the James Webb Space Telescope, developed in
partnership with ESA and the Canadian Space Agency (CSA),”
Sabbi said.

Webb observations would be complementary to the LEGUS views.
The space observatory will penetrate dusty stellar cocoons to
reveal the infrared glow of infant stars, which cannot be seen
in visible- and images. “Webb will be able to
see how star formation propagates over a galaxy,” Sabbi
continued. “If you have information on the gas properties, you
can really connect the points and see where, when, and how star
formation happens.”

Explore further:

Hubble scopes out a galaxy of stellar birth

More information: legus.stsci.edu/

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