Scottish Research: Milky Way Galaxy is smaller than believed

The Milky Way is smaller than astronomers previously thought, according to new research by Dr Jorge Penarrubia of Scotland's Edinburgh University.

For the first time, scientists have been able to precisely measure the mass of the galaxy that contains our solar system.

Researchers have found that the Milky Way is approximately half the weight of a neighbouring galaxy, Andromeda, which has a similar structure to our own.

The Milky Way and Andromeda are the two largest in a region of galaxies which astronomers call the Local Group.

Scientists say that Andromeda's extra weight must be present in the form of dark matter, a little-understood invisible substance which makes up most of the outer regions of galaxies.

They estimate that Andromeda contains twice as much dark matter as the Milky Way, causing it to be twice as heavy.

Researchers say their work should help them learn more about how the outer regions of galaxies are structured. Their findings also provide further evidence in support of a theory which suggests that the universe is expanding.

Although both galaxies appear to be of similar dimensions, until now scientists had been unable to prove which is larger.

We always suspected that Andromeda is more massive than the Milky Way, but weighing both galaxies simultaneously proved to be extremely challenging. 

Our study combined recent measurements of the relative motion between our galaxy and Andromeda with the largest catalogue of nearby galaxies ever compiled to make this possible. - Dr Jorge Peñarrubia, Edinburgh University, School of Physics and Astronomy.

Previous studies were only able to measure the mass enclosed within both galaxies' inner regions. In this new study, researchers were also able to work out the mass of invisible matter found in the outer regions of both galaxies, and reveal their total weights. They say 90 per cent of both galaxies' matter is invisible.

An image of the Andromeda galaxy, Messier 31.

Credit: Malyshchyts Viktar / Fotolia

A team of scientists led by the University of Edinburgh used recently published data on the known distances between galaxies, as well as their velocities, to calculate the total masses of Andromeda and the Milky Way.

Journal Reference: Jorge Peñarrubia, Yin-Zhe Ma, Matthew G. Walker, and Alan McConnachie. A dynamical model of the local cosmic expansion. Monthly Notices of the Royal Astronomical Society,, 2014; 443: 2204-2222 DOI: 10.1093/mnras/stu879

Milky Way and Andromeda galaxies set to collide - update 2014

The Andromeda spiral galaxy. 

Image courtesy NASA.

A short time ago, in a galaxy very, very close by, a NASA satellite thought it detected a gamma ray burst in Andromeda.

It was a false alarm, but astronomers used the opportunity as a reminder that our galaxy and Andromeda are set for a head-on collision.

NASA's Swift satellite discovers and measures gamma-ray bursts, the most powerful explosions in the universe, and their afterglow in X-ray, optical, and ultraviolet wavelengths of light.

The space agency says the spacecraft is designed "with powerful telescopes and quick reflexes to capture gamma-ray bursts as they flash and leave a lingering afterglow."

On Wednesday, astronomers and astrophiles alike buzzed with the news that Swift's equipment had captured a powerful flash of gamma rays, believed to be coming from the Andromeda Galaxy.

Those who watch the sky were caught up in the possibility that a clashing pair of neutron stars or a bright X-ray source was acting up a mere 2.5 million light-years away, NBC News reported.

But, alas, the excitement was short-lived. By the time more raw data came in and the initial information was re-analyzed, the project team realized what they thought was a giant burst of radiation was not the dense remnants of dead stars crashing together.

"We... do not believe this source to be in outburst. Instead, it was a serendipitous constant source in the field of view of a BAT subthreshold trigger," Swift team member Kim Page wrote in a NASA message.

The reason scientists (and the Twittersphere) were in such a tizzy was because, "The Andromeda galaxy, known in astronomical parlance as M31, holds a special place in our own future," the New York Times wrote.

NASA astronomers announced in 2012 "with certainty" that the next major cosmic event to hit the Milky Way will quite literally hit the Milky Way.

Our galaxy will have a "titanic collision" with neighbouring Andromeda. Not only that, the Triangulum galaxy, M33, is likely to join in on the action, causing a three-galaxy pile-up (and could even merge with the other two), scientists said in a statement.

The Milky Way and Andromeda are the dominant members of a small family of galaxies called the Local Group. Family ties bind the bevy together in the form of their mutual gravity.

NASA sought to assure the world that the sun and Earth are unlikely to be hit by stars or planets from Andromeda because of the vast emptiness of the two galaxies.

So Earth, they said, should easily survive what will be a 1.9 million kilometer per hour (1.2 million mile per hour) galactic merger. Even at that speed, the event would take about 2 billion years.

"It's like a bad car crash in galaxy-land," Roeland van der Marel, an astronomer with the Space Telescope Science Institute in Baltimore, which operates Hubble, told the media.

Scientists said in 2012 the collision would take place in 4 billion years. But the New York Times reported Wednesday, “Recent measurements with the Hubble Space Telescope have confirmed that they will hit head on in about two billion years.”

Council of Giants: Astronomers map out Earth's place in the universe

This is a diagram showing the brightest galaxies within 20 million light years of the Milky Way, as seen from above. 

The largest galaxies, here shown in yellow at different points around the dotted line, make up the "Council of Giants." 

Credit: Marshall McCall / York University

We live in a galaxy known as the Milky Way – a vast conglomeration of 300 billion stars, planets whizzing around them, and clouds of gas and dust floating in between.

Though it has long been known that the Milky Way and its orbiting companion Andromeda are the dominant members of a small group of galaxies, the Local Group, which is about 3 million light years across, much less was known about our immediate neighbourhood in the universe.

Marshall McCall

Now, a new paper by York University Physics & Astronomy Professor Marshall McCall, published today in the Monthly Notices of the Royal Astronomical Society, maps out bright galaxies within 35-million light years of the Earth, offering up an expanded picture of what lies beyond our doorstep.

"All bright galaxies within 20 million light years, including us, are organized in a 'Local Sheet' 34-million light years across and only 1.5-million light years thick," says McCall.

"The Milky Way and Andromeda are encircled by twelve large galaxies arranged in a ring about 24-million light years across – this 'Council of Giants' stands in gravitational judgment of the Local Group by restricting its range of influence."

This is a diagram showing the brightest galaxies within 20 million light years of the Milky Way, this time viewed from the side. 

Credit: Marshall McCall / York University

McCall says twelve of the fourteen giants in the Local Sheet, including the Milky Way and Andromeda, are "spiral galaxies" which have highly flattened disks in which stars are forming.

The remaining two are more puffy "elliptical galaxies", whose stellar bulks were laid down long ago.

This movie illustrates the positions of the nearby galaxies, including those in the ‘Council of Giants’, in three dimensions. Credit: Marshall McCall / York University

Intriguingly, the two ellipticals sit on opposite sides of the Council. Winds expelled in the earliest phases of their development might have shepherded gas towards the Local Group, thereby helping to build the disks of the Milky Way and Andromeda.

McCall also examined how galaxies in the Council are spinning. He comments: "Thinking of a galaxy as a screw in a piece of wood, the direction of spin can be described as the direction the screw would move (in or out) if it were turned the same way as the galaxy rotates."

"Unexpectedly, the spin directions of Council giants are arranged around a small circle on the sky. This unusual alignment might have been set up by gravitational torques imposed by the Milky Way and Andromeda when the universe was smaller."

More Information: A Council of Giants - M.McCall, mnras.stu199

Did Andromeda crash into the Milky Way 10 billion years ago?

A schematic diagram showing how the Andromeda Galaxy (at bottom right) collided with the Milky Way (at the intersection of the axes) 10 billion years ago, moved out to a maximum distance of more than 3 million light years and is now approaching our Galaxy once again. 

The yellow line shows the track of Andromeda with respect to the Milky Way. 

Credit: Fabian Lueghausen / University of Bonn

For many years scientists have believed that our Galaxy, the Milky Way, is set to crash into its larger neighbour, the Andromeda Galaxy, in about 3 billion years' time and that this will be the first time such a collision has taken place.

But now a European team of astronomers led by Hongsheng Zhao of the University of St Andrews propose a very different idea; that the two star systems collided once before, some 10 billion years ago and that our understanding of gravity is fundamentally wrong.

Remarkably, this would neatly explain the observed structure of the two galaxies and their satellites, something that has been difficult to account for until now.

 Dr Zhao will present the new work at the RAS National Astronomy Meeting in St Andrews on Thursday 4 July.

The Milky Way, made up of about 200 billion stars, is part of a group of galaxies called the Local Group.

Astrophysicists often theorise that most of the mass of the Local Group is invisible, made of so-called dark matter.

Most cosmologists believe that across the whole universe, this matter outweighs 'normal' matter by a factor of five.

The dark matter in both Andromeda and the Milky Way then makes the gravitational pull between the two galaxies strong enough to overcome the expansion of the cosmos, so that they are now moving towards each other at around 100 km per second, heading for a collision 3 billion years in the future.

But this model is based on the conventional model of gravity devised by Newton and modified by Einstein a century ago, and it struggles to explain some properties of the galaxies we see around us.

Dr Zhao and his team argue that at present the only way to successfully predict the total gravitational pull of any galaxy or small galaxy group, before measuring the motion of stars and gas in it, is to make use of a model first proposed by Prof. Mordehai Milgrom of the Weizmann Institute in Israel in 1983.

This modified gravity theory (Modified Newtonian Dynamics or MOND) describes how gravity behaves differently on the largest scales, diverging from the predictions made by Newton and Einstein.

Dr Zhao (University of St Andrews) and his colleagues have for the first time used this theory to calculate the motion of Local Group galaxies.

Their work suggests that the Milky Way and Andromeda galaxies had a close encounter about 10 billion years ago.

If gravity conforms to the conventional model on the largest scales then taking into account the supposed additional pull of dark matter, the two galaxies would have merged.

More information: A preprint of the the paper, entitled "Local Group timing in Milgromian dynamics. A past Milky Way-Andromeda encounter at z>0.8", is available from arxiv.org/abs/1306.6628

Numerous Hydrogen Clouds discovered lurking among our galactic neighbours

This combined graphic shows new, high-resolution GBT imaging (in box) of recently discovered hydrogen clouds between M31 (upper right) and M33 (bottom left). 

Credit: Bill Saxton, NRAO/AUI/NSF

In a dark, starless patch of intergalactic space, astronomers have discovered a never-before-seen cluster of hydrogen clouds strewn between two nearby galaxies, Andromeda (M31) and Triangulum (M33).

The researchers speculate that these rarefied blobs of gas—each about as massive as a dwarf galaxy—condensed out of a vast and as-yet undetected reservoir of hot, ionized gas, which could have accompanied an otherwise invisible band of dark matter.

The astronomers detected these objects using the National Science Foundation's Green Bank Telescope (GBT) at the National Radio Astronomy Observatory (NRAO) in Green Bank, W.Va. The results were published in the journal Nature.

Spencer Wolfe

"We have known for some time that many seemingly empty stretches of the Universe contain vast but diffuse patches of hot, ionized hydrogen," said Spencer Wolfe of West Virginia University in Morgantown.

"Earlier observations of the area between M31 and M33 suggested the presence of colder, neutral hydrogen, but we couldn't see any details to determine if it had a definitive structure or represented a new type of cosmic feature."

"Now, with high-resolution images from the GBT, we were able to detect discrete concentrations of neutral hydrogen emerging out of what was thought to be a mainly featureless field of gas."

Astronomers are able to observe neutral atomic hydrogen, which is referred to as HI (H and the Roman numeral one), because of the characteristic signal it emits at radio wavelengths, which can be detected by radio telescopes on Earth.

Though this material is abundant throughout the cosmos, in the space between galaxies it can be very tenuous and the faint signal it emits can be extremely difficult to detect.



The animation demonstrates the difference in resolution from the original Westerbork Radio Telescope data (Braun & Thilker, 2004) and the finer resolution imaging of GBT, which revealed the hydrogen clouds between M31 and M33. 

Credit: Bill Saxton, NRAO/AUI/NSF.

A little more than a decade ago, astronomers had the first speculative hints that a previously unrecognized reservoir of hydrogen lay between M31 and M33.

The signal from this gas, however, was too faint to draw any firm conclusions about its nature, origin, or even certain existence.

Last year, preliminary data taken with the GBT confirmed that there was indeed hydrogen gas, and a lot of it, smeared out between the galaxies.

These preliminary observations, however, lacked the necessary sensitivity to see any fine-grain structure in the gas or deduce whence it came and what it signified.

The most likely explanation at the time was that a few billion years earlier, these two galaxies had a close encounter and the resulting gravitational perturbations pulled off some wispy puffs of gas, leaving a tenuous bridge between the two.

ESA Herschel Image: Andromeda's Colourful Rings

The ring-like swirls of dust filling the Andromeda galaxy stand out colourfully in this new image from the Herschel Space Observatory, a European Space Agency mission with important NASA participation.

The glow seen here comes from the longer-wavelength, or far, end of the infrared spectrum, giving astronomers the chance to identify the very coldest dust in our galactic neighbor.

These light wavelengths span from 250 to 500 microns, which are a quarter to half of a millimeter in size.

Herschel's ability to detect the light allows astronomers to see clouds of dust at temperatures of only a few tens of degrees above absolute zero.

These clouds are dark and opaque at shorter wavelengths. The Herschel view also highlights spokes of dust between the concentric rings.

The colours in this image have been enhanced to make them easier to see, but they do reflect real variations in the data. The very coldest clouds are brightest in the longest wavelengths, and coloured red here, while the warmer ones take on a bluish tinge.

These data, together with those from other observatories, reveal that other dust properties, beyond just temperature, are affecting the infrared color of the image.

Clumping of dust grains, or growth of icy mantles on the grains towards the outskirts of the galaxy, appear to contribute to these subtle color variations.

These observations were made by Herschel's Spectral and Photometric Imaging Receiver (SPIRE) instrument. 

The data were processed as part of a project to improve methods for assembling mosaics from SPIRE observations.

Light with a wavelength of 250 microns is rendered as blue, 350-micron is green, and 500-micron light is red. Color saturation has been enhanced to bring out the small differences at these wavelengths.

Image credit: ESA/NASA/JPL-Caltech/NHSC

Australian Astronomers: Milky Way and Andromeda On Its Way To Become Old, Stagnant Galaxies

The Milky Way and Andromeda galaxies are in transition from being young and star-forming into old and stagnant galaxies

The Australian astronomers, led by Simon Mutch, of the Swinburne University of Technology in Melbourne, sought to determine the colour the Milky Way and the nearby spiral Andromeda galaxy.

They found out that instead of the typical blue or red color which are signs of young, active galaxies, Milky Way and Andromeda are in a state of green.

"Green galaxies are commonly thought to represent galaxies which are undergoing the transition from being young, dynamic, energetic, star-forming blue galaxies to being old, lethargic red galaxies," Mutch told SPACE.com.

The astronomer added that the two galaxies will likely be unable to produce an active galactic nucleus or AGN, which is among the brightest radio signals in the universe and can be seen across great distances.

"Our finding that both the Milky Way and Andromeda are green suggests that there will be little cold gas left in both these galaxies when they merge sometime in the next 5 billion years or so," Mutch said.

The new color "provides us with an interesting open question as to what exactly is causing the Milky Way and Andromeda to be running out of fuel for producing new stars," he added.

The study was published in the Astrophysical Journal (July edition).

NASA WiSE Vision: The Magnificent and Beautiful Andromeda Galaxy

The immense Andromeda galaxy, also known as Messier 31 or simply M31, is captured in full in this new image from NASA's Wide-field Infrared Survey Explorer, or WISE. The mosaic covers an area equivalent to more than 100 full moons, or five degrees across the sky.

WISE used all four of its infrared detectors to capture this picture (3.4- and 4.6-micron light is colored blue; 12-micron light is green; and 22-micron light is red). Blue highlights mature stars, while yellow and red show dust heated by newborn, massive stars.

Andromeda is the closest large galaxy to our Milky Way galaxy, and is located 2.5 million light-years from our sun. It is close enough for telescopes to spy the details of its ringed arms of new stars and hazy blue backbone of older stars.

Also seen in the mosaic are two satellite galaxies, known as M32, located just a bit above Andromeda to the left of center, and the fuzzy blue M110, located below the center of the great spiral arms. These satellites are the largest of several that are gravitationally bound to Andromeda.

The Andromeda galaxy is larger than our Milky Way and contains more stars, but the Milky Way is thought to perhaps have more mass due to its larger proportion of a mysterious substance called dark matter.

Both galaxies belong to our so-called Local Group, a collection of more than 50 galaxies, most of which are tiny dwarf systems. In its quest to map the whole sky, WISE will capture the entire Local Group.

Image credit: NASA/JPL-Caltech/UCLA