The Hunt for Jellyfish Galaxies in the Frontier Fields

Jellyfish galaxies, exotic galaxies with “tentacles” made of stars and gas, appear as though they are swimming through space. So far, astronomers studying the Frontier Fields have found several of these strange galaxies, and they are currently combing through the mountains of data to find even more.

Sometimes also known as “parachute galaxies” or “comet galaxies,” jellyfish galaxies form when spiral galaxies collide with galaxy clusters. When the cold gas from an approaching spiral hits the hot gas from a galaxy cluster, the stars continue on, but the collision blasts the cold gas out of the galaxy in trailing tails, or “tentacles.” Bursts of stars form in these streamers, sparked by the shock of cold gas hitting hot gas. The tentacles, with their knots of newborn stars, trace the path of the colliding, compressed gas. Eventually, these jellyfish galaxies are thought to settle into elliptical galaxies.

Three examples of jellyfish galaxies in the Frontier Fields. In each image, the telltale, trailing “tentacles” of stars and gas are present. The left and right galaxies are from galaxy cluster Abell 2744. The middle galaxy resides in galaxy cluster Abell S1063.

Some examples of jellyfish galaxies in the Frontier Fields. In each image, note the telltale, trailing “tentacles” of stars and gas. The left and right galaxies are from galaxy cluster Abell 2744. The middle galaxy resides in galaxy cluster Abell S1063.

Jellyfish galaxies are sometimes also seen in less massive groups of galaxies. Their characteristic shape is, however, usually much more pronounced for spirals falling into massive galaxy clusters, because the gas they encounter there is denser, and because they move faster due to the stronger gravitational pull of the cluster. The higher speed results in a more energetic collision that, in turn, increases the pressure that strips the infalling galaxy of its cold gas and triggers widespread star formation.

Astronomers have studied similar interactions in detail in nearby galaxy clusters but do not fully understand the much more violent process that creates jellyfish galaxies in very massive clusters. If the cold galactic gas is stripped very quickly these collisions could be the primary way by which spiral galaxies are transformed into ellipticals. Unfortunately, because the phenomenon is over so quickly, it is very difficult to observe. One expert on jellyfish galaxies—Dr. Harald Ebeling of the Institute for Astronomy at the University of Hawaii—explains that this is why astronomers are looking at extremely massive clusters, such as those in the Frontier Fields, in their search for a large sample of these galaxies.

Aside from helping to explain why elliptical galaxies are so common in the universe, jellyfish galaxies capture the process of galaxy/gas collisions in action. Their trailing, star-forming tentacles may also explain the presence of “orphan” stars that do not belong to any galaxy.

The work to uncover the secrets of the Frontier Fields goes on. Stay tuned for more exciting news on jellyfish galaxies and other oddities as scientists continue to study the vast amount of data collected in the Frontier Fields.

A Deep View Down Broadway

Abell 2744 Parallel Deep Field from the Hubble Frontier Fields Project Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

Abell 2744 Parallel Deep Field from the Hubble Frontier Fields Project
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

[Note: this blog post also appears on the Hubble’s Universe Unfiltered blog.]

One of the more philosophical concepts that astronomers have to deal with on an everyday basis is the commingling of space and time in astronomical images.

The underlying idea is straightforward. The speed of light is finite. Light from a star or nebula or galaxy takes a measurable amount of time to cross the space between it and us. Hence, the light we see now left that object at some previous time. We view astronomical objects as they were in the past. As I like to say, looking out in space is also looking back in time.

The implications of this maxim are considerable, especially in dealing with the deep field images from Hubble (see the accompanying image of the Abell 2744 Parallel deep field). Such images contain a wonderful assortment of galaxies, with a few stars here and there. Each object is at a different position in space, both in the two-dimensional sense of a different position within the image and in the three-dimensional sense of being at a different distance from Earth. Further, objects at different distances are seen at different times in the past. Hence, astronomers must examine these deep field images in four-dimensional space-time.

Tackling the expanse of space and time in these images can be mind-boggling. We’ll start with the stars, which are easier to understand. All the stars are local, within our Milky Way galaxy. These stars are generally hundreds to thousands of light-years away. The light we observe today might have left the star while the pyramids of Egypt were being built. Because stars don’t change appreciably on scales of thousands of years, stars in deep fields are just like stars in other astronomical images.

The galaxies, however, stretch much farther into space. The nearest are many millions of light-years away, while the most distant are around ten billion light-years away. Galaxies don’t change much on million-year timescales. For example, it takes over 200 million years for our Sun to orbit once within our galaxy. Even though the light may have left a galaxy when dinosaurs first started to dominate our planet, the same galaxy would look similar today. Thus, the nearby galaxies in these images are comparable to local galaxies.

Given billions of years, however, galaxies do change, and these deep field images provide compelling evidence. Distant galaxies do not have the standard spiral and elliptical shapes. They are often elongated, have bright spots of star formation, and are much smaller in size. We see galaxies as they were before the Sun, Earth, and the solar system formed. We study the development of galaxies over time to see how they form and grow. The perplexing point is that, for any given galaxy in the image, there is no distinct visual indicator of its distance in space or time. The layers of the universe are jumbled together across the image, and it is a grand puzzle of cosmology to sort them out.

The usual method to determine distances, and therefore times, is to measure the cosmological redshift of each galaxy. That concept has been discussed in a Frontier Fields blog post by Dr. Brandon Lawton: “Light Detectives: Using Color to Estimate Distance”. Thus, I’d like to take this essay in a different direction.

The Manhattan Deep Field

When discussing the cosmic mixture of space-time with an artist visiting from Spain, I happened upon a novel idea for a human-centric analogy.

Imagine you are in New York City, specifically Times Square in Manhattan. You look down Broadway to the southern end of the island about 4 miles away. If the speed of light were extremely slow, traveling only one mile per century, what would you see?

Each mile down Broadway would represent one hundred years of New York’s history. Each block would be 5 to 10 years earlier in the development of the metropolis.

A quarter of a mile away, the southern end of the theater district would appear as it did in the early 1990s when “Miss Saigon” came to Broadway. Only a few blocks farther would be the disco era and the civil unrest of the 1960s, then the World War II years and the Great Depression.

The Empire State Building, about a mile away, would vanish, as it was not built until 1931. At a similar distance, Madison Square Garden would be seen hosting heavyweight boxing matches in its original building, before the demolition and re-construction in the late 1920s.

Progressing another mile down Broadway to Union Square would travel back past the Civil War, Tammany Hall politics, economic growth fostered by the Erie Canal, and Alexander Hamilton’s original run on the New York stage.

The mile beyond to the SoHo district progresses through the times of New York as the capital of the United States, the Revolutionary War, the founding of Columbia University, and the importation of slaves by the Dutch West Indies Company.

The final mile to Battery Park leads through the colonial era alternately dominated by Dutch or English foreign powers, past the garrison of Fort Amsterdam, to the island’s Native American roots and the initial explorations by Henry Hudson.

A “slow speed of light” view from Times Square would lay out the entire history of the city of New York in a single view. The commingling of space and time would make it the historian’s exceptional equivalent of the astronomer’s standard observation: a deep view down Broadway.

This idea of a time-warped view of New York provides an analogy to what Hubble uncovers: the history of galaxies compressed and jumbled within each deep field. Perhaps it can help you to look at these images from that requisite four-dimensional perspective. These deep field images are truly a trip down memory lane.

Celebrating Hubble’s 25th Anniversary

In April, Hubble will celebrate a quarter-century in space. The telescope, launched into orbit in 1990, has become one of NASA’s most beloved and successful missions, its images changing our understanding of the universe and taking root in our cultural landscape. Hubble pictures have not only expanded our scientific knowledge, they have altered the way we imagine the cosmos to appear.

pillars 1

Hubble took its iconic “Pillars of Creation” image of these star-forming clouds of gas and dust in the Eagle Nebula in 1995. Credit: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University)

Hubble’s prolonged success has been a testament to its innovative design, which allowed it to be periodically updated by astronauts with new equipment and improved cameras. Hubble  has been able, to an extent, to keep up with technological changes over the past 25 years. The benefits are evident when comparing the images of the past and present.

pillars 2

This new image of the Eagle Nebula’s “Pillars of Creation” was taken in 2014 to launch Hubble’s year-long celebration of its 25th anniversary. The image was captured with Wide Field Camera 3, an instrument installed on the telescope in 2009. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Hubble’s new instruments — specifically, the near-infrared capabilities of Wide Field Camera 3 — are what makes the Frontier Fields project possible. The faint infrared light of the most distant, gravitationally lensed galaxies sought in the Frontier Fields project would be beyond the reach of Hubble’s earlier cameras. Frontier Fields highlights Hubble’s continuing quest to blaze new trails in astronomy — and pave the path for the upcoming Webb Space Telescope — so it makes sense that its imagery is included in a collection of 25 of Hubble’s significant images, specially selected for the anniversary year.

The immense gravity in this foreground galaxy cluster, Abell 2744, warps space to brighten and magnify images of far-more-distant background galaxies as they looked over 12 billion years ago, not long after the big bang.  This is the first of the Frontier Fields to be imaged.

Abell 2744, the first of the Frontier Fields to be imaged, is part of Hubble’s 25th anniversary collection of top images. The immense gravity of the foreground galaxy cluster warps space to brighten and magnify images of far-more-distant background galaxies. Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)


The 25th birthday is a significant milestone, so Hubble is throwing a year-long celebration, with events happening in communities and online throughout 2015. Last week, Tony Darnell hosted a discussion of the beauty and scientific relevance of the Hubble 25th anniversary images, one of the many anniversary-themed Hubble Hangouts he’ll be doing as the months go on. To keep an eye on upcoming events, see the images, and learn about the science, visit our special 25th anniversary website,

Frontier Fields Finds Faint Light of Homeless Stars

The Frontier Fields’ project has detected the glow of about 200 billion freely drifting stars within the massive galaxy cluster Abell 2744. The stars were dragged from their home galaxies by gravitational tides during collisions and interactions over the course of 6 billion years.

As many as six Milky Way-sized galaxies were torn apart in the cluster. The light of the outcast stars is believed to contribute to 10 percent of the cluster’s brightness, though that light is quite faint because the density of the stars is low. The combination of depth and multiwavelength observations provided by the Frontier Fields program makes this study of such dim stars possible.

The total starlight of galaxy cluster Abell 2744 is depicted here in blue in this Frontier Fields image. Not all the starlight is contained within the galaxies, which appear as blue-white objects. A portion of the light comes from stars that have been pulled from their galaxies and now drift untethered within the cluster. Credit: NASA, ESA, M. Montes (IAC), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

The stars are rich in heavy elements such as oxygen, carbon, and nitrogen, which means they formed from material released by earlier generations of stars. The presence of these elements indicates that the stars likely came from galaxies with similar mass and metallicity to our own Milky Way galaxy, which have the ability to sustain ongoing star formation and thus build populations of such chemically enriched stars. Elliptical galaxies are low in star formation while dwarf galaxies lack the kind of constant star formation that would be essential.

This discovery indicates that a significant fraction of the stars that would otherwise end up in these galaxies is being stripped out in the merger process. Astronomers intend to look for the light of such estranged stars in the remainder of the Frontier Fields galaxy clusters.

Gravitational Lensing in Action

In my previous blog post, Visual “Proof” of General Relativity, I discussed how gravitational lensing demonstrates the effects of Einstein’s theory of general relativity in a direct, visual manner. Images created by gravitational lenses show features that are not possible in Newton’s version of gravity.

Although seeing general relativity with your own eyes is kinda awesome, there’s one unsatisfying aspect: you only see the result, not the process. Since you don’t know exactly what those galaxies looked like before the gravitational lensing, it is hard to fully appreciate the magnitude of the distortions. We have no on/off switch for the mass of the galaxy cluster to be able to examine the un-lensed image and compare against the lensed one.


A simulation of gravitational lensing by a cluster of galaxies (click on image for larger version). The galaxies of cluster Abell 2744 (left) are inserted into the Hubble Ultra Deep Field (right) to produce the combined image with gravitational lensing (center).

But we can demonstrate the process of gravitational lensing through scientific visualization. The images above show a simulation of gravitational lensing by a galaxy cluster. On the left is an image of only the galaxies that belong to galaxy cluster Abell 2744; all of the foreground and background objects have been removed. On the right is a deep field image of galaxies. In the center is a simulation of how the galaxies of Abell 2744 would distort the galaxy images in the deep field.

By carefully comparing galaxy images between the right and center panels, one can see how the un-lensed galaxies transform to their distorted lensed versions.  The elongated streaks and arcs in the center image generally come from compact, ellipse-shaped galaxies in the right image. But not all galaxies are changed, a fact easily seen by examining the larger, yellow galaxy in the lower right.

The explanation comes from the details of the simulated lensing. The deep field used above is a portion of the Hubble Ultra Deep Field (HUDF), and includes only galaxies for which we have a good measure of their distance. Using those distances and the distance to Abell 2744, we were able to place the galaxies of Abell 2744 at their correct positions within the deep field. HUDF galaxies which are closer than the galaxy cluster would not be lensed, and appear the same in the right and center images. Only those galaxies behind the cluster were transformed by the simulated lensing. Thus, the central image provides a proper simulation of what would be seen if Abell 2744 suddenly wandered across the sky and ended up in the middle of the HUDF.

I note that all of the background galaxies were combined into a single image at a set distance behind the cluster for simplicity. The full, and rather tedious, 3D calculation could have been performed, but was deemed unlikely to provide a significant visual difference for a public-level illustration. I further note that it is an occupational hazard of being a scientist that one feels compelled to provide such full-disclosure details.

The really difficult challenge is to do the reverse of this simulation. Start with an image of gravitational lensing and then work out the mass distribution of the galaxy cluster from the distribution of streaks and arcs. But, hey, no one said being an astrophysicist was easy.

In the final part of this series of blog posts, I’ll provide a more down-to-earth example of gravitational lensing.