Are Little Blue Dots in the Hubble Frontier Fields Precursors to Globular Clusters?

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.

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

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:

“The purpose of the HFF program is to take deep images of 6 regions of the sky where there are clusters of galaxies, because these clusters magnify background galaxies through the gravitational lens effect. In this way, we can see further than just with direct imaging of the sky alone. Many galaxies have been studied using this magnification technique. The clusters of galaxies are important because they are big mass concentrations which make strong gravitational lenses.”

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.

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)

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:

“When I was examining the images (there are about 3400 galaxies detected in each field), I noticed occasional galaxies that appeared as little blue dots, which was very intriguing because of Bruce’s previous theoretical work on dwarf galaxies. The published catalogs included redshifts and star formation rates and masses for each galaxy, and it turns out the little blue dots are low mass galaxies with very high star formation rates for their mass.”

Close-up view of a HFF parallel field, showing a LBD (upper right). Credit: Debra & Bruce Elmegreen

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.

The three groups of LBDs, where the dwarf galaxies were stacked based on their redshift. Credit: Debra & Bruce Elmegreen

As Bruce explained, his is a compelling argument for the theory that globular clusters formed from dwarf galaxies in the early Universe:

“This suggests that the metal-poor globular clusters are the dense remnants of little galaxies that got captured by bigger galaxies, like the Milky Way, and ripped apart by tidal forces. This idea for the origin of halo globular clusters goes back several decades… It would be only the metal-poor one that are like this, which are about half the total, because dwarf galaxies are metal poor compared to big galaxies, and they were also more metal poor 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

Dark Matter: Hot Or Not?

For almost a century, astronomers and cosmologists have postulated that space is filled with an invisible mass known as “dark matter”. Accounting for 27% of the mass and energy in the observable universe, the existence of this matter was intended to explain all the “missing” baryonic matter in cosmological models. Unfortunately, the concept of dark matter has solved one cosmological problem, only to create another.

If this matter does exist, what is it made of? So far, theories have ranged from saying that it is made up of cold, warm or hot matter, with the most widely-accepted theory being the Lambda Cold Dark Matter (Lambda-CDM) model. However, a new study produced by a team of European astronomer suggests that the Warm Dark Matter (WDM) model may be able to explain the latest observations made of the early Universe.

But first, some explanations are in order. The different theories on dark matter (cold, warm, hot) refer not to the temperatures of the matter itself, but the size of the particles themselves with respect to the size of a protogalaxy – an early Universe formation, from which dwarf galaxies would later form.

Diagram showing the Lambda-CBR universe, from the Big Bang to the the current era. Credit: Alex Mittelmann/Coldcreation
Diagram showing the Lambda-CBR universe, from the Big Bang to the the current era. Credit: Alex Mittelmann/Coldcreation

The size of these particles determines how fast they can travel, which determines their thermodynamic properties, and indicates how far they could have traveled – aka. their “free streaming length” (FSL) – before being slowed by cosmic expansion. Whereas hot dark matter would be made up of very light particles with high FSLs, cold dark matter is believed to be made up of massive particles that have a low FSL.

Cold dark matter has been speculated to take the form of Massive Compact Halo Objects (MACHOs) like black holes, or a class of undiscovered heavy particles – i.e. Weakly-Interacting Massive Particles (WIMPs), and axions. The widely-accepted Lambda-CDM model is based in part of the theory that dark matter is “cold”.

As cosmological explanations go, it is the most simple and can account for the formation of galaxies or galaxy cluster formations. However, there remains some holes in this theory, the biggest of which is that it predicts that there should be many more small, dwarf galaxies in the early Universe than we can account for.

In short, the existence of dark matter as massive particles that have low FSL would result in small fluctuations in the density of matter in the early Universe – which would lead to large amounts of low-mass galaxies to be found as satellites of galactic halos, and with large concentrations of dark matter in their centers.

Illustration of the depth by which Hubble imaged galaxies in prior Deep Field initiatives, in units of the Age of the Universe. The goal of the Frontier Fields is to peer back further than the Hubble Ultra Deep Field and get a wealth of images of galaxies as they existed in the first several hundred million years after the Big Bang. Note that the unit of time is not linear in this illustration. Illustration Credit: NASA and A. Feild (STScI)
Illustration of the depth by which Hubble imaged galaxies in prior Deep Field initiatives, in units of the Age of the Universe. The goal of the Frontier Fields is to peer back further than the Hubble Ultra Deep Field. Credit: NASA and A. Feild (STScI)

Naturally, the absence of these galaxies might lead one to speculate that we simply haven’t spotted these galaxies yet, and that IR surveys like the Two-Micron All Sky Survey (2MASS) and the Wide-field Infrared Survey Explorer (WISE) missions might find them in time.

But as the international research team – which includes astronomers from the Astronomical Observatory of Rome (INAF), the Italian Space Agency Science Data Center and the Paris Observatory – another possibility is that dark matter is neither hot nor cold, but “warm” – i.e. consisting of middle-mass particles (also undiscovered) with FSLs that are roughly the same as objects big as galaxies.

As Dr. Nicola Menci – a researcher with the INAF and the lead author of the study – told Universe Today via email:

“The Cold Dark Matter particles are characterized by low root mean square velocities, due to their large masses (usually assumed of the order of >~ 100 GeV, a hundred times the mass of a proton). Such low thermal velocities allow for the clumping of CDM even on very small scales. Conversely, lighter dark matter particles with masses of the order of keV (around 1/500 the mass of the electron) would be characterized by larger thermal velocities, inhibiting the clumping of DM on mass scales of dwarf galaxies. This would suppress the abundance of dwarf galaxies (and of satellite galaxies) and produce shallow inner density profiles in such objects, naturally matching the observations without the need for a strong feedback from stellar populations.”

In other words, they found that the WDM could better account for the early Universe as we are seeing it today. Whereas the Lambda-CDM model would result in perturbations in densities in the early Universe, the longer FSL of warm dark matter particles would smooth these perturbations out, thus resembling what we see when we look deep into the cosmos to see the Universe during the epoch of galaxy formation.

For the sake of their study, which appeared recently in the July 1st issue of The Astrophysical Journal Letters, the research team relied on data obtained from the Hubble Frontier Fields (HFF) program. Taking advantage of improvements made in recent years, they were able to examine the magnitude of particularly faint and distant galaxies.

The large blue light is a lensing galaxy in the foreground, called SDP81, and the red arcs are the distorted image of a more distant galaxy. By analyzing small distortions in the red, distant galaxy, astronomers have determined that a dwarf dark galaxy, represented by the white dot in the lower left, is companion to SDP81. The image is a composite from ALMA and the Hubble. Image: Y. Hezaveh, Stanford Univ./ALMA (NRAO/ESO/NAOJ)/NASA/ESA Hubble Space Telescope
Composite image showing the lensing galaxy SDP81. The red arcs are the distorted image of a more distant galaxy, about 12 billion light years away. Credit: Y. Hezaveh, Stanford Univ./ALMA (NRAO/ESO/NAOJ)/NASA/ESA/Hubble Space Telescope

As Menci explained, this is a relatively new ability which the Hubble Space Telescope would not have been able to do a few years ago:

“Since galaxy formation is deeply affected by the nature of DM on the scale of dwarf galaxies, a powerful tool to constraint DM models is to measure the abundance of low-mass galaxies at early cosmic times (high redshifts z=6-8), the epoch of their formation. This is a challenging task since it implies finding extremely faint objects (absolute magnitudes M_UV=-12 to -13) at very large distances (12-13 billion of light years) even for the Hubble Space Telescope.

“However, the Hubble Frontier Field program exploits the gravitational lensing produced by foreground galaxy clusters to amplify the light from distant galaxies. Since the formation of dwarf galaxies is suppressed in WDM models – and the strength of the suppression is larger for lighter DM particles – the high measured abundance of high-redshift dwarf galaxies (~ 3 galaxies per cube Mpc) can provide a lower limit for the WDM particle mass, which is completely independent of the stellar properties of galaxies.”

The results they obtained provided strict constraints on dark matter and early galaxy formation, and were thus consistent with what HFF has been seeing. These results could indicate that our failure to detect dark matter so far may have been the result of looking for the wrong kind of particles. But of course, these results are just one step in a larger effort, and will require further testing and confirmation.

Artist's impression of dark matter surrounding the Milky Way. (ESO/L. Calçada)
Artist’s impression of dark matter surrounding the Milky Way. (ESO/L. Calçada)

Looking ahead, Menci and his colleagues hope to obtain further information from the HFF program, and hopes that future missions will allow them to see if their findings hold up. As already noted, these include infrared astronomy missions, which are expected to “see” more of the early Universe by looking beyond the visible spectrum.

“Our results are based on the abundance of high-redshift dwarfs measured in only two fields,” he said. “However, the HFF program aims at measuring such abundances in six independent fields. The operation of the James Webb Space Telescope in the near future – with a lensing program analogous to the HFF –  will allow us to pin down the possible mechanisms for the production of WDM particles, or to rule out WDM models as alternatives to CDM,” he said. “

For almost a century, dark matter has been a pervasive and elusive mystery, always receding away the moment we think we about to figure it out. But the  deeper we look into the known Universe (and the farther back in time) the more we are able to learn about the its evolution, and thus see if they accord with our theories.

Further Reading: The Astrophysical Journal Letters, AAS Nova