Galaxies

Dance of the galaxies

Alan Hale Published 6:31 a.m. MT Feb. 21, 2019

If you’re bored with the weather here on Earth, well let's take a look at what’s happening elsewhere in the solar system. Buzz60

This author has recently returned from a trip to Australia, where among other things he was able to re-acquaint himself with those regions of the sky that are permanently below New Mexico’s southern horizon. Among the sights within the southern hemisphere’s nighttime sky are two cloud-like objects that look like detached sections of the Milky Way; while these haveobviously been known for millennia by the indigenous peoples in the southern hemisphere, today they are usually referred to as the Magellanic Clouds, sonamed after the Portuguese explorer Ferdinand Magellan, whose around-the-world expedition circa 1520 recorded them. 

We now know that the Magellanic Clouds are separate galaxies nearby our own galaxy. The larger of the two – dubbed, for obvious reasons, the “Large Magellanic Cloud” (LMC for short) – is located some 160,000 light-years away from us, while the smaller one – the “Small Magellanic Cloud,” or SMC – is about 200,000 light-years away. Many of the features of our own galaxy, such as gas and dust clouds, star-forming regions, star clusters, hot giant stars, and so on, also appear in the Magellanic Clouds, and the study of these helps provide insights into the goings-on within our galaxy. In 1987 a supernova – an exploding giant star – appeared in the LMC, becoming in the process the closest appearance of such an object in almost four centuries, and completely revolutionizing our understanding of the supernova phenomenon.

Both Magellanic Clouds are considerably smaller than our galaxy, and they are often considered as being “companion” or “satellite” galaxies of our own. This brings to mind the questions of how long this situation has lasted, and what will happen to the Clouds in the distant future.

Recent studies, involving modern computer-modeling programs as well as data from the European Space Agency’s Gaia spacecraft (which has provided extremely precise positional measurements of well over one billion stars since its launch in 2013), havestarted to bring some clarity to these questions. Some very recently announced results suggest that the SMC, along with various other nearby “dwarf” galaxies that have been discovered within the relatively recent past, are in fact satellite galaxies of the LMC. This entire retinue of galaxies appears to be approaching our galaxy for the very first time.

This approach will continue for quite some time, until about 2 1Ž2 billion years from now when the LMC (and its companions) will collide with our galaxy and will merge with it. Such “galactic cannibalism” is how large galaxies like ours grow over time – indeed, at this very moment our galaxy is absorbing a “dwarf” galaxy in the constellation Sagittarius that wasn’t identified until just a quarter-century ago – but the merger with the LMC may well be one of the largest such events in our galaxy’s history (thus far, anyway).

A somewhat similar situation is playing out with the nearest large galaxy to ours, the Andromeda Galaxy – some 2.5 million light-years away – that is visible to the unaided eye from reasonably dark observing sites and which can be seen in our northwestern evening skies this time of year. Andromeda has many “companion” galaxies, the largest of which is located in the constellation of Triangulum (adjacent to the constellation of Andromeda); this galaxy can dimly be seen with the unaided eye from very dark rural sites.

The Triangulum and Andromeda galaxies are also approaching each other, and the recent studies indicate that this event is also happening for the first time. Unlike the situation between our galaxy and the LMC, however, Andromeda and Triangulum will apparently not come close enough to collide and merge (although there will obviously be significant gravitational effects between the two galaxies). Rather, they will likely pass by each other in about a billion years or so, although what may happen in the far distantfuture remains to be seen.

The most important element in this overall dance of the galaxies is the fact that our galaxy and Andromeda are approaching each other. It has been known for some time that our two galaxies will eventually merge and form one giant galaxy, and it has generally been believed that this event will take place about four billion years from now. However, recent analysis of the Gaia data suggests that there is a significant sideways motion of Andromeda with respect to our galaxy, and thus – at least initially – the two galaxies may slide past each other without actually colliding. But this only puts off the inevitable, as the two galaxies will indeed eventually merge, although it now appears that this will take place perhaps half a billion years later than originally thought.

As for the Triangulum galaxy, it may eventually become a “satellite” galaxy of the merger between us and Andromeda. Perhaps it may eventually itself merge with us some billions of years after that, or it may get flung away from our combined galaxy altogether. There isn’t enough information available right now that can tell us which scenario is more likely.

Due to the changes that will be taking place within our sun over the intervening billions of years, Earth will have become uninhabitable long before these events play out. But if any far-future descendants of ours are still around somewhere, they will see goings-on in whatever skies they might have that are rather different from those we see nowadays.

Alan Hale is a professional astronomer who resides in Cloudcroft. He is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is http://www.earthriseinstitute.org.

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02

New Night Sky Map Reveals 'Hundreds Of Thousands' Unknown Galaxies With 15 Million More Forecast

Galaxy cluster Abell 1314 in the constellation Ursa Major in a distance of approximately 460 million light years. The LOFAR observations reveal radio emission from high-speed cosmic electrons (marked in red) resulting from collisions with other galaxy clusters. The overlay onto an optical image also shows hot X-ray gas (marked in grey) from observations with the Chandra satellite.Amanda Wilber/LOFAR Surveys Team

Data on just 2% of the night sky has revealed hundreds of thousands of previously undetected galaxies.

It's the first result from a major new radio sky survey of the night sky by Low-Frequency Array (LOFAR) telescope, a European network of radio antenna.

The data comes from a survey of 25% of the northern hemisphere's night sky at low radio frequencies, which revealed three hundred thousand sources, almost all of which are galaxies in the distant Universe. However, the astronomers behind the discovery ultimately expect to discover 15 million more radio sources after their survey of the entire northern hemisphere night sky is complete. Many of those sources are likely to be as-yet-undiscovered galaxies.

What is radio astronomy?

Radio astronomy is the detection of radio waves emitted by celestial objects. The radio signals collected by the LOFAR telescope have traveled billions of light years before reaching Earth.

LOFAR station Effelsberg, shown from 50 meters above ground.W. Reich/MPIfR

What is LOFAR?

Operated by ASTRON in the Netherlands, the LOFAR telescope is a network of radio antennas in the Netherlands, Germany, the UK, France, Sweden, Poland, Ireland and Italy, all connected by high-speed fiber optic cables. The signals of the combined 100,000+ antennas are then processed by supercomputers to effectively create a virtual radio telescope dish measuring 1,900 kilometers in diameter.

LOFAR is a forerunner of the Square Kilometre Array (SKA), which will be the largest and most sensitive radio telescope in the world destined for Western Australia and South Africa.

Artist’s impression of the full Square Kilometre Array at night featuring all four elements. The low-frequency aperture array antennas (bottom right), and precursor ASKAP dishes (background right) will be located in Western Australia. The SKA-mid (front left) dishes and precursor MeerKAT dishes (background left) will be located in South Africa, with some remote stations in other African partner countries.SKA Organisation

Why is this important?

All of this new data will allow astronomers to study the evolution of galaxies in unprecedented detail. “This sky map will be a wonderful scientific legacy for the future," says Carole Jackson, Director General of ASTRON. Where do black holes come from? Why do clusters of galaxies produce radio emission when they merge? Is the space between galaxies completely magnetic? The incredible accuracy of LOFAR's measurements and low-frequency radio sky maps are already shedding light on these questions.

What happens next?

This is just the beginning. These first results from only the first two percent of the sky survey were published by an international team of more than 200 astronomers from 18 countries. Describing the survey and its first results took 26 research papers in a special issue of the scientific journal Astronomy & Astrophysics. The same astronomers now hope to make sensitive high-resolution images of the entire northern sky. “Just imagine some of the discoveries we may make along the way," said Jackson. One expectation is the discovery of the first massive black holes that formed when the Universe was very young.

However, it's all going to take some time. After all, LOFAR's first installment is the largest astronomical data collection so far and would have taken centuries to process on a regular computer. “LOFAR produces enormous amounts of data," says Cyril Tasse, Observatoire de Paris – Station de radioastronomie à Nançay, France. "We have to process the equivalent of ten million DVDs of data."

Wishing you clear skies and wide eyes

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03

Radio Survey Maps Hundreds of Thousands of New Galaxies

The LOFAR survey, based in The Netherlands, has released a bonanza of new sources. And with only 2% of the sky covered so far, this is only the beginning.

A unique sky survey at low radio frequencies has turned up more than 300,000 new sources, many of which are radio galaxies and quasars in the distant universe. And there’s much more to come: The new discoveries, published in a special issue of Astronomy & Astrophysics, are based on just 2% of the eventual survey volume.

The LOFAR image (orange) of galaxy cluster Abell 1314 in Ursa Major is superimposed on an inverted visible-light image of the region (grayscale). The radio emission comes from past collisions with other clusters. Rafaël Mostert / LOFAR Surveys Team / Sloan Digital Sky Survey DR13

“For the first time, we now have high-quality images of the radio sky at these low frequencies,” says Huub Röttgering (Leiden University, The Netherlands), principal investigator of the LOFAR Surveys Key Science Project. “That’s the real breakthrough.”

The Low Frequency Array (LOFAR), operated by ASTRON, Netherlands Institute for Radio Astronomy, is a novel instrument, consisting of some 100,000 simple antennas, grouped in 50 stations and connected to a central supercomputer through fiber optics. Most of the LOFAR stations are located in the Netherlands, but 12 of them are spread all over Europe, from Ireland to Poland and from Sweden to France.

Eventually, the Low-frequency Two-meter Sky Survey (LoTSS) will map the whole northern sky, but the first data release covers just 424 square degrees, centered on the handle of the Big Dipper. A total of 58 “radio pointings,” each 8 hours long, have been carried out at frequencies between 120 and 168 megahertz, yielding enough data to fill 10 million DVDs.

According to Röttgering, the biggest problem has been removing the effects of ionospheric turbulence, the radio equivalent of stars twinkling at visible wavelengths. Through clever mathematical tricks, scientists have sharpened the data to see details 6 arcseconds across.

The new catalog has 325,694 radio sources, accompanied by 58 high-resolution mosaic radio images. For about 70% of the sources, visible-light counterparts were found in existing data from the Sloan and Pan-STARRS surveys, giving astronomers rough distance estimates to the objects.

The radio waves from the galaxies that LOFAR has found are primarily generated by electrons spiraling along magnetic field lines that thread jets coming from the galaxies’ central black holes. Once these electrons have been spiraling for a while, they slow down and emit lower-frequency radio waves, so LOFAR is seeing jet activity on much longer timescales than existing higher-frequency radio surveys.

One of the LOFAR stations in the northeast of The Netherlands.ASTRON

One intriguing result is that all relatively massive galaxies exhibit jet activity close to their cores, which suggests that their central black holes are feeding more or less continuously. The new observations should shed light on the evolution of supermassive black holes over cosmic time. “Eventually, we hope to find the very first supermassive black holes in the history of the universe,” says Röttgering.

The 26 papers in Astronomy & Astrophysics, authored by more than 200 scientists from 18 countries, also cover LOFAR data on galaxy clusters. According to Annalisa Bonafede (University of Bologna, Italy), one surprising result is that even isolated clusters that aren’t interacting with their neighbors still produce radio emission from charged particles speeding among the gas between galaxies, though the emissions are at a very low level.

The future Square Kilometre Array (SKA) has a low-frequency segment based in Australia, which will be even more sensitive than LOFAR, but it won’t see details as sharply. According to Röttgering, LOFAR’s high spatial resolution (it may discern details up to 0.5 arcseconds across under ideal circumstances) will be unsurpassed for a long time to come.