Universe Today
In 2012, the
Hubble Space Telescope Frontier Fields
program (aka. Hubble Deep Fields Initiative 2012) officially kicked off. The purpose of this project was to study the faintest and most distant galaxies in the Universe using the
gravitational lensing
technique, thus advancing our knowledge of early galaxy formation. By 2017, the Frontier Field program wrapped up, and the hard work of analyzing all the data it collected began.
One of the more interesting finds within the Frontier Fields data has been the discovery of low mass galaxies with high star formation rates. After examining the "parallel fields" for
Abell 2744 and MACS J0416.1-2403
- two galaxy clusters studied by the program - a pair of astronomers noted the presence of what they refer to as "
Little Blue Dots
" (LBDs), a finding which has implications for galaxy formation and globular clusters.
The study which details their findings recently appeared online under the title "
Little Blue Dots in the Hubble Space Telescope Frontier Fields: Precursors to Globular Clusters?
". The study team consisted of Dr. Debra Meloy Elmegreen - a professor of astronomy at
Vassar College
- and Dr. Bruce G. Elmegreen, an astronomer with the IBM Research Division at the
T.J. Watson Research Center
in Yorktown Heights.
[caption id="attachment_138083" align="aligncenter" width="580"]
The galaxy clusters Abell 2744, MACS J0416.1-2403, MACS J0717.5+3745, MACS J1149.5+2223, Abell S1063, Abell 370. Credit: NASA, ESA, STScI, and the HFF team
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To put it simply, the Frontier Fields program used the Hubble Space Telescope to observe six massive galaxy clusters at optical and near-infrared wavelengths - with its
Advanced Camera for Surveys
(ACS) and
Wide Field Camera 3
(WFC3), respectively. These massive galaxies were used to magnify and stretch images of remote galaxies located behind them which were otherwise too faint for Hubble to see directly (aka. gravitational lensing).
While one of these Hubble cameras would look at a galaxy cluster, the other would simultaneously view an adjacent patch of sky. These adjacent patches are known as "parallel fields", otherwise faint regions that provide some of the deepest looks into the early Universe. As Dr. Bruce Elmegreen told Universe Today via email:
This six galaxy clusters used for the sake of the project included Abell 2744, MACS J0416.1-2403 and their parallel fields, the latter of which were the focal point in this study. These and the other clusters were used to find galaxies that existed just 600 to 900 million years after the Big Bang. These galaxies and their respective parallels had already been
cataloged using computer algorithms that
automatically found galaxies in the images and determined their properties.
[caption id="attachment_138086" align="aligncenter" width="580"]
Images of the MACS J0416.1–2403 and Abell 2744 galaxy clusters, taken as part of the Hubble Frontier Fields program. Credit: NASA/ESA/HST Frontier Fields team (STScI)
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As the research duo go on to explain in their study, recent large-scale deep surveys have enabled studies of smaller galaxies at higher redshifts. These include "green peas" - luminous, compact and low mass galaxies with high specific star formation rates - and even lower-mass "blueberries", small starburst galaxies that are a faint extension of the green peas that also show intense rates of star formation.
Using the aforementioned catalogues, and examining the parallel fields for Abell 2744 and MACS J0416.1-2403, the team went looking for other examples of low-mass galaxies with high star formation rates. The purpose of this was to measure the properties of these dwarf galaxies, and to see if any of their positions accorded with where globular clusters are known to have formed.
What they found was what they referred to as "Little Blue Dots" (LBSs), which are even lower-mass versions of "blueberries". As Dr. Debra Elmegreen told Universe Today via email:
[caption id="attachment_138097" align="aligncenter" width="580"]
Close-up view of a HFF parallel field, showing a LBD (upper right). Credit: Debra & Bruce Elmegreen
[/caption]
These galaxies didn't show structure, so Debra and Bruce stacked the images of galaxies into 3 different ranges of redshift (which worked out to about 20 galaxies each) to create deeper images. "Still they showed no structure or faint extended outer disk," said Debra, "so they are at the limit of resolution, with average sizes of 100-200 parsecs (about 300-600 light years) and masses of a few million times the mass of our sun."
In the end, they determined that within these LBDs, star formation rates were very high. They also noted that these dwarf galaxies were very young, being less than 1% the age of the Universe at the time that they were observed.
"So the tiny galaxies just formed,: said Bruce, "and their star formation rates are high enough to account for the globular clusters, maybe one in each LBD, when the star bursts in them wind down after a few tens of million years."
Debra and Bruce Elmegreen are no strangers to high redshift galaxies. Back in 2012, Bruce published
a paper
that suggested that the globular clusters that orbit the Milky Way (and most other galaxies) formed in dwarf galaxies during the early Universe. These dwarf galaxies would have since been acquired by larger galaxies like our own, and the clusters are essentially their remnants.
Globular clusters are essentially massive star clusters that orbit around the Milky Way Halo.
They are typically around 1 million Solar masses and are made up of stars that are very old - somewhere on the order of 10 to 13 billion years. Beyond the Milky Way, many
appear in common orbits and in the Andromeda Galaxy, some even appear connected by a stream of stars.
[caption id="attachment_138098" align="aligncenter" width="580"]
The three groups of LBDs, where the dwarf galaxies were stacked based on their redshift. Credit: Debra & Bruce Elmegreen
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As Bruce explained, his is a compelling argument for the theory that globular clusters formed from dwarf galaxies in the early Universe:
This study has many implications for our understanding of how the Universe evolved, which was the chief aim of the Hubble Frontier Fields program. By examining objects in the early Universe, and determining their properties, scientists are able to determine how the structures that we are familiar with today - i.e. stars, galaxies, clusters, etc. - truly came from.
These same studies also allow scientists to make educated guesses about where the Universe is going and what will become of those same structures millions or even billions of years from now. In short, knowing where we've been lets us predict where we are headed!
Further Reading: arXiv
