Determining The Galaxy Collision Rate

Galactic Wrecks Far from Earth: These images from NASA's Hubble Space Telescope's ACS in 2004 and 2005 show four examples of interacting galaxies far away from Earth. The galaxies, beginning at far left, are shown at various stages of the merger process. The top row displays merging galaxies found in different regions of a large survey known as the AEGIS. More detailed views are in the bottom row of images. (Credit: NASA; ESA; J. Lotz, STScI; M. Davis, University of California, Berkeley; and A. Koekemoer, STScI)


Big galaxies… Little galaxies… But how often do they meet? Thanks to information from some of the latest Hubble surveys, astronomers have been able to more closely estimate galaxy collision rates than ever before. Apparently those that have happened within the last eight to nine billion years have occurred somewhere in-between previous estimates.

When it comes to galaxy evolution, the collision rate is an indicator of how individual galaxies accumulated mass over time. While it’s pretty much a standard measurement, there’s a large margin with no information of how often it might have occurred in the very distant past. By taking a look at in deep-field surveys made by NASA’s Hubble Space Telescope, astronomers were able to get a general look – one that showed a merger rate of anywhere from 5 percent to 25 percent of those studied.

The science team, led by Jennifer Lotz of the Space Telescope Science Institute in Baltimore, Maryland, took a close look at galaxy interactions spaced over vast distances. This allowed the group to essentially study mergers which occurred at different times. What they found was larger galaxies had a merger rate of once every nine billion years, while smaller ones crashed up more often. When taking a look a dwarf galaxies compared to massive ones, the team found it happened three times more often than the rate for large galaxies.

“Having an accurate value for the merger rate is critical because galactic collisions may be a key process that drives galaxy assembly, rapid star formation at early times, and the accretion of gas onto central supermassive black holes at the centers of galaxies,” Lotz explains.

While there were past studies of galaxy mergers done with Hubble information, astronomers used a different method and came up with different results. “These different techniques probe mergers at different ‘snapshots’ in time along the merger process,” Lotz says. “It is a little bit like trying to count car crashes by taking snapshots. If you look for cars on a collision course, you will only see a few of them. If you count up the number of wrecked cars you see afterwards, you will see many more. Studies that looked for close pairs of galaxies that appeared ready to collide gave much lower numbers of mergers than those that searched for galaxies with disturbed shapes, evidence that they’re in smashups.”

To help determine how often the merger rate occurred with time, Lotz and her team had to know how long an encountered galaxy would appear disrupted. In order to get a good working model, the team used computer simulations and then mapped them compared to Hubble images of galaxy interactions. While this effort took a great deal of time, the team did their best to create every possible scenario – from a pair of galaxies with equal mass to disparate ones. They also took into account orbits, collisional events and even orientation. Of these studies, 57 different situations and 10 viewing angles were accounted for. “Viewing the simulations was akin to watching a slow-motion car crash,” Lotz says. These computer created scenarios followed the galaxies for 2 billion to 3 billion years, starting at the merger beginning and ending a billion years later when completed. “Our simulations offer a realistic picture of mergers between galaxies,” explains Lotz.

While it was easy enough to see what happens with a giant galaxy, it was a bit more difficult to observe what happens with diminutive ones. Observing a dwarf merger is far more difficult simply because they are so much more dim – but plentiful. “Dwarf galaxies are the most common galaxy in the universe,” Lotz says. “They may have contributed to the buildup of large galaxies. In fact, our own Milky Way galaxy had several such mergers with small galaxies in its recent past, which helped to build up the outer regions of its halo. This study provides the first quantitative understanding of how the number of galaxies disturbed by these minor mergers changed with time.”

However, studies of this type just don’t happen with a handful of photos. Lotz and the team had to compare the simulations with literally thousands of galaxy images taken from some of Hubble’s largest surveys, including the All-Wavelength Extended Groth Strip International Survey (AEGIS), the Cosmological Evolution Survey (COSMOS), and the Great Observatories Origins Deep Survey (GOODS), as well as mergers identified by the DEEP2 survey with the W.M. Keck Observatory in Hawaii. At the beginning they found a wide variety of merger rates, but ended up with about a thousand merger candidates. “When we applied what we learned from the simulations to the Hubble surveys in our study, we derived much more consistent results,” Lotz says.

What’s next for Lotz and her team? It’s time to take a look at galaxy interactions that happened about 11 billion years ago. Their goal is to check out when star formation across the Universe reached its greatest as compared to the merger rate. Perhaps there might be a correlation between encounters and rapid star birth!

Original Story Source: Hubble Space Telescope News.

Red-Burning Galaxies… Let’s Get The Party Started!

An image illustrating the number density of galaxies estimated to be four billion light years from the Earth. Bright areas indicate high-density regions. The brightest region in the center corresponds to the main body of the CL0939 cluster. Red squares show the positions of the red -burning galaxies while the greenish-blue dots show the blue H? emitting galaxies. Evidently, the red burning galaxies avoid the central region of the cluster and concentrate in small groups located far away from it.


Utilizing the Subaru Telescope, a research team of astronomers from the University of Tokyo and the National Astronomical Society of Japan (NAOJ) used a wide-field image to take a look four billion years back in time. The object of their interest was a galaxy cluster, but what really took their fancy wasn’t the old matrons – it was the red star-forming galaxies hanging around the edges.

Just exactly what is a “red-burning galaxy”? Astronomers hypothesize they might be the transitional key between the young and old… and present at a party that shows dramatic evolution. It’s not the fact that such galaxies exist within galactic clusters, but why they seem to appear along the outskirts.

When galaxies first began forming under the weight of their own gravity some ten billion years ago, they either became part of big clusters or small groups. As they came together, they took on properties of their environment – just as party goers tend to group together where interests are similar. At a galactic get-together with high density, galaxies form into lenticular or elliptical, while the solitary wall flowers tend toward spiral structure. But exactly how they form and evolve is one of astronomy’s greatest enigmas.

A panoramic view of the CL0939+4713 cluster located 4 billion light years away from Earth. Images were captured with the Subaru Prime Focus Camera (Suprime-Cam), all of which are a composite of a B-band image (blue), a R-band image (green), and a z'-band image (red). Left 27 arcmin x 27 arcmin field of view. Top-right: Close-up view of the central cluster region, 2.5 arcmin x 2.5 arcmin field of view. Bottom-right: Example of the concentration of red-burning galaxies, which are marked with red squares.

To help solve the mystery, researchers are looking further back into the past. A research team led by Dr. Yusei Koyama used the Subaru Prime Focus Camera (Suprime-Cam) to carry out a panoramic observation targeting a relatively well-known rich cluster, CL0939+4713. By using a special filter that separates the hydrogen-alpha emission lline Koyama’s team members identified more than 400 galaxies showing a narrowband excess which could denote the star formation process. Strangely enough, it was these very galaxies that showed an impressive amount of red and were located in groups well away from the main body.

Needless to say, this opened the door to even more questions. Where did they come from and why are they concentrated in groups and not clusters? At this point, who knows? Astronomers are positive the “red-burning galaxies” get their properties from starbirth – not elderly populations. They also anticipate the main galaxy cluster will one day absorb these strays into the main body as well. How can they tell? Just like the party, the red-burning galaxies are already changing in relationship to their environment. Older galaxies that no longer have active star-forming regions seem to be increasing in the groups, exactly where the red-burners are most frequently found.

“This suggests that the red-burning galaxies are related to the increase in old galaxies, and that they are likely to be in a transitional phase from a younger to an older generation. The finding that such transitional galaxies are located most frequently within group environments shows that galaxy groups are the key environments for understanding how environment shapes the evolution of galaxies.” says the Subaru research team. “This should be an important and exciting step toward a more complete understanding of the environments shaping the galaxies in the present-day Universe.”

Party on, dudes…

Original Story Source: Subaru Telescope Press Release.

Better Late Than Never: Dwarf Galaxies Finally Come Together

Hickson 31 (Credit: NASA, ESA, and S. Gallagher (The University of Western Ontario), and J. English (University of Manitoba))

Have you heard of ‘living fossils’? The coelacanth, the ginko tree, the platypus, and several others are species alive today which seem to be the same as those found as fossils, in rocks up to hundreds of millions of years old.

Now combined results from the Hubble Space Telescope, Spitzer, Galaxy Evolution Explorer (GALEX), and Swift show that there are ‘living galaxy fossils’ in our own backyard!

Hubble: red, yellow-green, and blue; Spitzer: orange; GALEX: purple

Hickson Compact Group 31 is one of 100 compact galaxy groups catalogued by Canadian astronomer Paul Hickson; the recent study of them – led by Sarah Gallagher of The University of Western Ontario in London, Ontario – shows that the four dwarf galaxies in it are in the process of coming together (or ‘merging’ as astronomers say).

Such encounters between dwarf galaxies are normally seen billions of light-years away and therefore occurred billions of years ago. But these galaxies are relatively nearby, only 166 million light-years away.

New images of this foursome by NASA’s Hubble Space Telescope offer a window into the universe’s formative years when the buildup of large galaxies from smaller building blocks was common.

Astronomers have known for decades that these dwarf galaxies are gravitationally tugging on each other. Their classical spiral shapes have been stretched like taffy, pulling out long streamers of gas and dust. The brightest object in the Hubble image is actually two colliding galaxies. The entire system is aglow with a firestorm of star birth, triggered when hydrogen gas is compressed by the close encounters between the galaxies and collapses to form stars.

The Hubble observations have added important clues to the story of this interacting group, allowing astronomers to determine when the encounter began and to predict a future merger.

“We found the oldest stars in a few ancient globular star clusters that date back to about 10 billion years ago. Therefore, we know the system has been around for a while,” says Gallagher; “most other dwarf galaxies like these interacted billions of years ago, but these galaxies are just coming together for the first time. This encounter has been going on for at most a few hundred million years, the blink of an eye in cosmic history. It is an extremely rare local example of what we think was a quite common event in the distant universe.”

In other words, a living fossil.

Everywhere the astronomers looked in this group they found batches of infant star clusters and regions brimming with star birth. The entire system is rich in hydrogen gas, the stuff of which stars are made. Gallagher and her team used Hubble’s Advanced Camera for Surveys to resolve the youngest and brightest of those clusters, which allowed them to calculate the clusters’ ages, trace the star-formation history, and determine that the galaxies are undergoing the final stages of galaxy assembly.

The analysis was bolstered by infrared data from NASA’s Spitzer Space Telescope and ultraviolet observations from the Galaxy Evolution Explorer (GALEX) and NASA’s Swift satellite. Those data helped the astronomers measure the total amount of star formation in the system. “Hubble has the sharpness to resolve individual star clusters, which allowed us to age-date the clusters,” Gallagher adds.

Hubble reveals that the brightest clusters, hefty groups each holding at least 100,000 stars, are less than 10 million years old. The stars are feeding off of plenty of gas. A measurement of the gas content shows that very little has been used up – further proof that the “galactic fireworks” seen in the images are a recent event. The group has about five times as much hydrogen gas as our Milky Way Galaxy.

“This is a clear example of a group of galaxies on their way toward a merger because there is so much gas that is going to mix everything up,” Gallagher says. “The galaxies are relatively small, comparable in size to the Large Magellanic Cloud, a satellite galaxy of our Milky Way. Their velocities, measured from previous studies, show that they are moving very slowly relative to each other, just 134,000 miles an hour (60 kilometers a second). So it’s hard to imagine how this system wouldn’t wind up as a single elliptical galaxy in another billion years.”

Adds team member Pat Durrell of Youngstown State University: “The four small galaxies are extremely close together, within 75,000 light-years of each other – we could fit them all within our Milky Way.”

Why did the galaxies wait so long to interact? Perhaps, says Gallagher, because the system resides in a lower-density region of the universe, the equivalent of a rural village. Getting together took billions of years longer than it did for galaxies in denser areas.

Source: HubbleSite News Release. Gallagher et al.’s results appear in the February issue of The Astronomical Journal (the preprint is arXiv:1002.3323)

Youngsters Caught Gorging – on Gas

Typical massive galaxy at z=1.1 (left: V, I (Hubble); right: CO 3-2 mm emission (IRAM); copyright MPE/IRAM)

Galaxies long, long ago were very fecund; they gave birth to stars at a rate at least ten times what we see today.

Why? Was there more stuff around then, to make stars? Or were galaxies back then more efficient at star-making? Or something else??

Dr. Linda Tacconi, from Germany’s Max-Planck-Institut für extraterrestrische Physik, led an international team of astronomers to find out why … and the answer seems to be that young galaxies were stuffed to the gills with gas.

“We have been able, for the first time, to detect and image the cold molecular gas in normal star forming galaxies, which are representative of the typical massive galaxy populations shortly after the Big Bang,” said Dr Tacconi.

The challenging observations yield the first glimpse how galaxies, or more precisely the cold gas in these galaxies, looked a mere 3 to 5 billion years after the Big Bang (equivalent to a cosmological redshift z~2 to z~1). At this age, galaxies seem to have formed stars more or less continuously with at least ten times the rate seen in similar mass systems in the local Universe.

It is now reasonably well-established that galaxies formed from proto-galaxies, which themselves formed in local over-densities, dominated by cold dark matter – dark matter halos – where the newly neutral hydrogen and helium collected and cooled. Through collisions and mergers, and some on-going gas accretion, the proto-galaxies formed young galaxies, a few billion years after the Big Bang – in short, hierarchical formation.

The Plateau de Bure millimetre interferometer in the southern French Alps. Copyright: IRAM

Detailed observations of the cold gas and its distribution and dynamics hold a key role in disentangling the complex mechanisms responsible for turning the first proto-galaxies into modern galaxies, such as the Milky-Way. A major study of distant, luminous star forming galaxies at the Plateau de Bure millimeter interferometer has now resulted in a breakthrough by having a direct look at the star formation “food”. The study took advantage of major recent advances in the sensitivity of the radiometers at the observatory to make the first systematic survey of cold gas properties (traced by a rotational line of the carbon monoxide molecule) of normal massive galaxies when the Universe was 40% (z=1.2) and 24% (z=2.3) of its current age. Previous observations were largely restricted to rare, very luminous objects, including galaxy mergers and quasars. The new study instead traces massive star forming galaxies representative of the ‘normal’, average galaxy population in this mass and redshift range.

“When we started the programme about a year ago”, says Dr. Tacconi, “we could not be sure that we would even detect anything. But the observations were successful beyond our most optimistic hopes. We have been able to demonstrate that massive normal galaxies at z~1.2 and z~2.3 had five to ten times more gas than what we see in the local Universe. Given that these galaxies were forming gas at a high rate over long periods of time, this means that gas must have been continuously replenished by accretion from the dark matter halos, in excellent agreement with recent theoretical work.”

Another important result of these observations is the first spatially resolved images of the cold gas distribution and motions in several of the galaxies. “This survey has opened the door for an entirely new avenue of studying the evolution of galaxies,” says Pierre Cox, the director of IRAM. “This is really exciting and there is much more to come.”

“These fascinating findings provide us with important clues and constraints for next-generation theoretical models that we will use to study the early phases of galaxy development in more detail,” says Andreas Burkert, specialist for star formation and the evolution of galaxies at Germany’s Excellence Cluster Universe. “Eventually these results will help to understand the origin and the development of our Milky Way.”

About the EGS 1305123 image: Spatially resolved optical and millimeter images of a typical massive galaxy at redshift z=1.1 (5.5 billion years after the Big Bang). The left image was taken with the Hubble Space Telescope in the V- and I-optical bands, as part of the AEGIS survey of distant galaxies. The right image is an overlay of the CO 3-2 emission observed with the PdBI (red/yellow colors) superposed on the I-image (grey). For the first time these observations clearly show that the molecular line emission and the optical light from massive stars trace a massive, rotating disk of diameter ~60,000 light years. This disk is similar in size and structure as seen in z~0 disk galaxies, such as the Milky Way. However, the mass of cold gas is in this disk is about an order of magnitude larger than in typical z~0 disk galaxies. This explains why high-z galaxies can form continuously at about ten times the rate of typical z~0 galaxies.

Sources: Max Planck Institute for Extraterrestrial Physics, Tacconi et al. (2010), Nature 463, 781 (preprint: arXiv:1002.2149)