National Aeronautics and Space Administration
New Star Birth in an
Ancient Elliptical Galaxy
Taken from:
Hubble 2010: Science Year in Review
Produced by NASA Goddard Space Flight Center
and the Space Telescope Science Institute.
The full contents of this book include Hubble science articles, an overview of
the telescope, and more. The complete volume and its component sections are
available for download online at:
www.hubblesite.org/hubble_discoveries/science_year_in_review
HUBBLE 2010: SCIENCE YEAR IN REVIEW
New Star Birth in an Ancient Elliptical Galaxy
Elliptical galaxies appear as large but relatively featureless collections of yellow to reddish stars that are billions of years old.
Unlike spiral galaxies, they lack the dust and gas that serve as raw materials for making new stars. However, recent groundbased spectroscopic analysis of the elliptical galaxy NGC 4150 yielded tantalizing clues that stars were forming in its core,
and therefore warranted closer investigation.
Astrophysicists Mark Crockett and Sugata Kaviraj, both currently working in the United Kingdom, led a study of NGC 4150
using Hubble. They were further motivated to make the observation by a growing body of evidence collected from both
ground- and space-based telescopes, suggesting that new star formation was unexpectedly occurring in many elliptical
galaxies. Ground-based observatories had initially captured a blue glow in these galaxies. Then, satellites such as the Galaxy
Evolution Explorer—which detects ultraviolet light—confirmed that the blue glow came from hot, fledgling stars.
Crockett and Kaviraj imaged the core of NGC 4150 in near-ultraviolet light with Hubble’s Wide Field Camera 3. Their
observation revealed streamers of dust and gas as well as young clusters of blue stars that are significantly less than a billion
years old. The young clusters trace a ring around the galaxy’s center that is approximately 1,300 light-years across. Long
strands of dust are silhouetted against the yellowish core, which is composed of older stars.
From an analysis of the stars’ colors, Crockett and his team calculated that star formation started in NGC 4150 approximately
a billion years ago, a comparatively recent event in cosmological history. The galaxy’s star birth has slowed since then.
Astronomers are therefore seeing this galaxy after the major starburst occurred. The most massive stars formed in the burst
have already ended their lives as core-collapse supernovas. The youngest stars are only a few hundred million years old. By
comparison, most of the stars in the galaxy are around 10 billion years old.
Hubble reveals fresh star birth in the ancient elliptical galaxy NGC 4150, located approximately 44 million light-years away. The new data
bolsters the emerging view that ancient elliptical galaxies like NGC 4150 harbor a significant amount of recent, merger-driven star formation.
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In the wide-view Hubble image taken in visible and near-infrared
wavelengths, NGC 4150 looks like a typical elliptical galaxy.
The dark strands of dust in the center, however, provide tentative
evidence of a recent galaxy merger. The inset image in
visible and near-ultraviolet light is a magnified view of the
star-birth activity inside the galaxy’s core. The inset has a
higher contrast to enhance the absorption features and the
blue star-forming regions.
The details of these findings support the emerging
view that most elliptical galaxies have young stars
as a result of mergers with smaller, neighboring
galaxies. The observations also support the more
general theory that galaxies built themselves up over billions of years through multiple collisions with other galaxies.
Astronomers believe that a merger with a small, gas-rich galaxy around a billion years ago supplied NGC 4150 with the
fuel necessary to form new stars. The scarcity of metals in the young stars—elements heavier than hydrogen and helium—
suggests the galaxy that merged with NGC 4150 was also metal poor. This points to a merger with a small dwarf galaxy, one
with approximately one-twentieth the mass of NGC 4150. The encounter that started the star birth would have been similar
to our Milky Way assimilating the small nearby galaxy called the Large Magellanic Cloud.
Minor mergers like this one are thought by astronomers to be more common than interactions between larger, more massive
galaxies. Compared with major mergers, collisions between a large galaxy and a small one are as much as 10 times more
frequent. Major collisions are easier to see because they create distorted galaxies, long streamers of gas, and dozens of
young star clusters. Smaller interactions are more difficult to detect because they leave relatively little trace.
Elliptical galaxies, however, present an ideal environment for studying minor galactic mergers. When observed in visible
light, these ancient galaxies have a reddish hue caused by the dominance of old, red stars. Hot, young stars—in contrast—
primarily emit ultraviolet and blue light. This allows astronomers to scan for ultraviolet emissions and identify mergers
based on areas of new star formation. At least a third of all elliptical galaxies observed in the ultraviolet spectrum manifest
these starburst regions.
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The merger of NGC 4150 with its galactic neighbor appears to be a nearby example of a more common occurrence in the
early universe. Astronomers plan to examine other elliptical galaxies with Hubble. By locating additional star birth sites, they
hope to gain a more nuanced understanding of subtle mergers and their impact on galaxy formation over time.
Further Reading
Andrews, Bill. “What Are Galaxies Trying to Tell Us?” Astronomy 39, no. 2 (February 2011): 24–29.
Collins, Hugh. “Cannibal Galaxy Eats Its Neighbor to Stay Young,” AOL News, November 20, 2010.
http://www.aolnews.com/2010/11/20/cannibal-galaxy-eats-its-neighbor-to-stay-young/.
Crockett, M., et al. “Anatomy of a Post-starburst Minor Merger: A Multi-wavelength WFC3 Study of NGC 4150.” The Astrophysical
Journal 727, no. 2 (February 1, 2011): 115.
“Old Galaxy Still Giving Birth to Baby Stars.” Space.com, November 30, 2010.
http://www.space.com/9610-galaxy-giving-birth-baby-stars.html.
Dr. Mark Crockett is an astrophysicist working on galaxy evolution studies at the University of Oxford, United
Kingdom. His work is focused on early-type systems and especially the role that galaxy mergers play in their
formation and evolution. Dr. Crockett is originally from Kilrea in County Londonderry, Northern Ireland. He
studied physics at Queen’s University Belfast before obtaining a PhD in astrophysics from the same institution
in 2009. His doctoral research focused on the detection and characterization of core-collapse supenova
progenitors—the stars that end their lives in colossal supernova explosions—and he is still involved in this
work today.
Dr. Sugata Kaviraj is an astrophysicist working at Imperial College London and the University of Oxford. His
research focuses on deciphering the formation and evolution of the most massive galaxies over cosmic time. He
has made important contributions to the understanding of early-type galaxies, extra galactic star clusters, and
the role of energetic feedback from supernovas and active galactic nuclei in the evolution of their host systems.
Born in New Delhi, India, he was educated in the United Kingdom, completing an undergraduate degree at
Imperial College London and a doctorate at the University of Oxford. He currently holds fellowships from
Imperial College London, the Royal Commission for the Exhibition of 1851, and Worcester College, Oxford.
In 2011, he was awarded the Royal Astronomical Society’s Winton Capital Award for excellence in research.
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