Milky Way Sidelined in Galactic Tug-of-War, Computer Simulation Shows

This plot shows the simulated gas distribution of the Magellanic System resulting from the tidal encounter between the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) as they orbit our home Milky Way Galaxy. The entire sky is plotted in galactocentric coordinates of longitude and latitude. The Magellanic Stream is the pronounced tail of material that stretches 150 degrees across the southern sky. The solid line shows the calculated path of the LMC and the dotted line is the path of the SMC. The color range from dark to light shows the density (lower to higher) of the hydrogen gas making up the Magellanic Stream and the Bridge that connects the two dwarf galaxies. (Credit: Plot by G. Besla, Milky Way background image by Axel Mellinger (used with permission))Magellanic Stream is an arc of hydrogen gas spanning more than 100 degrees of the sky as it trails behind the Milky Way's neighbor galaxies, the Large and Small Magellanic Clouds. Our home galaxy, the Milky Way, has long been thought to be the dominant gravitational force in forming the Stream by pulling gas from the Clouds.

A new computer simulation by Gurtina Besla (Harvard-Smithsonian Center for Astrophysics) and her colleagues now shows, however, that the Magellanic Stream resulted from a past close encounter between these dwarf galaxies rather than effects of the Milky Way.

"The traditional models required the Magellanic Clouds to complete an orbit about the Milky Way in less than 2 billion years in order for the Stream to form," says Besla. Other work by Besla and her colleagues, and measurements from the Hubble Space Telescope by colleague Nitya Kallivaylil, rule out such an orbit, however, suggesting the Magellanic Clouds are new arrivals and not long-time satellites of the Milky Way.

This creates a problem: How can the Stream have formed without a complete orbit about the Milky Way?

To address this, Besla and her team set up a simulation assuming the Clouds were a stable binary system on their first passage about the Milky Way in order to show how the Stream could form without relying on a close encounter with the Milky Way.

The team postulated that the Magellanic Stream and Bridge are similar to bridge and tail structures seen in other interacting galaxies and, importantly, formed before the Clouds were captured by the Milky Way.

"While the Clouds didn't actually collide," says Besla, "they came close enough that the Large Cloud pulled large amounts of hydrogen gas away from the Small Cloud. This tidal interaction gave rise to the Bridge we see between the Clouds, as well as the Stream."

"We believe our model illustrates that dwarf-dwarf galaxy tidal interactions are a powerful mechanism to change the shape of dwarf galaxies without the need for repeated interactions with a massive host galaxy like the Milky Way."

While the Milky Way may not have drawn the Stream material out of the Clouds, the Milky Way's gravity now shapes the orbit of the Clouds and thereby controls the appearance of the tail.

"We can tell this from the line-of-sight velocities and spatial location of the tail observed in the Stream today," says team member Lars Hernquist of the Center.

The paper describing this work has been accepted for publication in the October 1 issue of the Astrophysical Journal Letters.

Besla's co-authors were Nitya Kallivayalil (MIT Kavli Institute for Astrophysics & Space Research), Lars Hernquist, R. P. van der Marel (STScI), T.J. Cox (Carnegie Observatories) and D. Keres (Harvard-Smithsonian Center for Astrophysics).

The Outbursts of Fornax

A false color image of the radio-wavelength emitting lobes in the galaxy Fornax A. This hot gas spans a distance of over one million light years. A new paper by CfA astronomers suggests that they are caused by the collision of another galaxy with Fornax in which dust and gas from the neighbor ends up triggering jets from the black hole at the galaxy's nucleus. Credit: Ed Fomalont (NRAO) et al., VLA, NRAO, AUI, NSF

The galaxy Fornax A, at a distance of about 74 million light-years, is one of the nearest and brightest galaxies with giant radio lobes. These huge radio lobes -- they span a million light-years -- are immense reservoirs of hot gas glowing brightly at radio wavelengths. They are thought to have been generated by oppositely-pointed jets of particles streaming out from the galaxy's supermassive black hole.

A collision between Fornax and a smaller galaxy may have swept material towards the black hole, creating the jets that in turn so brightly illuminate the ambient material. Exactly how this process occurs, however, is not known, in part because the black hole is obscured deep inside the galaxy's nuclear region. We still do not know, for example, whether the outbursts are one-time events, as perhaps is suggested by the collision scenario, or whether they recur, and on what timescales.

CfA graduate student Lauranne Lanz, together with CfA astronomers Christine Jones, Bill Forman, Matt Ashby, Ralph Kraft, and Ryan Hickox, combined the radio images of Fornax A with infrared images from the Spitzer Space Telescope and X-ray images from the Chandra X-ray Observatory and the XMM-Newton X-ray satellite to help answer some of the puzzles around these dramatic lobes.

By carefully modeling and then subtracting the starlight seen in the Spitzer images, the scientists were able to reveal excess dust emission around the nuclear region, and moreover show that the dust lies in two irregular arcs sweeping across about ten thousand light-years. The existence of that infrared dust emission is puzzling: Fornax A is a type of galaxy thought to be relatively dust-free.

When combined with X-ray and radio information for the same locations, however, the dust enabled the astronomers to construct a more complete picture of what is going on. Most likely the galaxy collided about 400 million years ago with a gas-rich neighbor -- one having 2-3 times the gas in Fornax A itself -- but with many fewer stars (perhaps only 10%). The dust comes from the neighboring galaxy; the collision also helped to funnel gas from the neighbor and trigger the outburst from the black hole.

The geometry of the nuclear region, and the presence of two X-ray cavities, further suggest that unless there are some nuclear regions still undetected in the radio, which is a possibility, there were likely to have been not one but two outbursts during the past 400 million years that contributed to the structures observed. The new paper helps to unravel the mysterious origins of these dramatic radio lobes, and also highlights the importance of multiwavelength observations.

Comet and Earth to have rare close encounter

Comet Hartley 2 should be easily visible this month using small telescopes, binoculars

Typically during the courseof a year about a dozen comets will come within the range of amateur telescopes. Most quietly come and go with little fanfare, but during the upcoming weeks one rather small comet will be making an unusually close approach to the Earth.

On Oct. 20, Comet Hartley 2 will pass just over 11 million miles from Earth. During October it should be easily visible in small telescopes, binoculars and from sites with dark enough skies even with the naked eye.

Comets are composed of rock, dust, water ice and frozen gases such as carbon monoxide, carbon dioxide, methane and ammonia.

Because of their low mass, comet nuclei do not become spherical under their own gravity as larger bodies in space do, and thus have irregular shapes. They are often popularly described as "dirty snowballs," though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath the crust.

In addition to its close pass of Earth, Comet Hartley 2 will be visited in early November by the Deep Impact spacecraft, which already had a previous encounter with Comet Tempel 1 in 2005.

Hartley's discovery

Back on March 15, 1986, astronomer Malcolm Hartley discovered a new comet on photographic images taken at the U.K. Schmidt Telescope Unit at Siding Spring, Australia.

At the time Hartley discovered it, the newfound comet was an exceedingly faint object, with just a hint of a tail; it was about 25,000 times dimmer than the faintest stars that can be seen with the naked eye. After further images were obtained over the next several days, Hartley announced his discovery to the Central Bureau for Astronomical Telegrams in Cambridge, Mass.

This was Hartleys second discovery of a comet and so was designated as Comet Hartley 2. Orbital calculations by Brian Marsden of the Smithsonian Astrophysical Observatory in Cambridge, Mass., indicated that Hartley 2 had already made its closest approach to the sun nine months earlier and that at that time it was too close to the sun's glare to have been detected.

Marsden also calculated that the comet made a close approach to Jupiter during 1982. The comet takes roughly 6-1/2 years to circle the sun and it has since been observed again in 1991, 1997 and 2004.

Rare close encounter

This fall, Comet Hartley 2 will again be passing through the inner solar system, reaching its closest point to the sun (called perihelion) on Oct. 28 at a distance of 98.4 million miles.

And while en route to the sun, it will also make a very close approach to the Earth. In fact, at 3 p.m. ET on Oct. 20, the comet will be at its closest point to our planet at a distance of 11.2 million miles.

It's quite unusual for any comet to approach this close to Earth. Such an event only happens on average perhaps three or four times a century.

If this comet were reasonably large, it would likely put on a very spectacular show around the time of its closest approach.

Halley's Comet passed a similar distance from Earth in the year 1066 and was described in ancient records as appearing "like a moon" as well as being depicted in the Bayeux Tapestry, a long embroidered piece of cloth that depicts the Norman conquest of England.

A small, faint comet

Unfortunately, Hartley 2 is a very small and intrinsically faint comet. Observations by the Spitzer Space Telescope in August 2008 showed that the comet's nucleus has a diameter of just 0.7 miles. Its nucleus is only about one tenth the size of Halley's Comet and perhaps only one thirtieth that of Comet Hale-Bopp.

Nonetheless, Hartley 2 will come close enough to us to become dimly visible to the unaided eye during early to mid October.

Astronomers use magnitude to define the brightness of sky objects; the lower the magnitude, the brighter the object. The brightest stars are zero or first magnitude, while the faintest stars are sixth magnitude.

Current expectation is that the comet will reach a peak magnitude of perhaps 4.4 at the time of its closest approach. However, around that time the comet will probably appear very large in overall apparent size, perhaps similar to the apparent size of the full moon.

As a result, much of the comet's brightness will be "spread out" over that area of the sky. So visually to the eye under a dark sky it will appear not as a sharp star-like image, but more like a dim, circular patch of light.

Binoculars or a small low-power telescope will provide a somewhat more pleasing view: a dim, circular, grayish-blue ball of light with a star-like condensation (the nucleus) at the center.

If the comet develops a tail of any kind, it likely will be of the gaseous variety very thin, faint and narrow, giving the comet the appearance of what comet expert John Bortle likens to "an apple on a stick."

Where to find it
As October begins, Comet Hartley 2 will be in the constellation of Cassiopeia, which at dusk will be positioned halfway up in the northeast sky; through Oct. 5 it will be passing below and to the right of the famous "W"-shaped formation composed of five bright stars.

The comet then moves into the constellation of Perseus on Oct. 6 and before dawn on the morning of Oct. 8, it will be situated only 0.7 degrees below and to the right of the famous Double Star Cluster. The cluster supposedly marks the sword handle of Perseus and is often touted as one of the most impressive star clusters in the entire sky.

On the morning of Oct. 10, the comet will appear to almost touch the 4th-magnitude star, Eta Persei. On Oct. 17, it will enter the boundaries of the constellation Auriga, and on the morning of Oct. 18, Hartley 2 will be about 1.2 degrees above and to the right of the star Epsilon Aurigae and 3 degrees below and to the right of

Scientists Surprised by Rapid Changes in Space Structure

Photo: NASA

NASA's Interstellar Boundary Explorer (IBEX)

Scientists working on NASA's Interstellar Boundary Explorer, or IBEX, satellite have unveiled a new map of the heliosphere. That's the region at the edge of the solar system, where the charged particles from our Sun meet interstellar space. The latest findings add a new layer of mystery to an already puzzling phenomenon.

When IBEX was launched two years ago, scientists expected its map of the heliosphere to show a relatively smooth region more than 15 billion kilometers from Earth.

The first map delivered by IBEX last year showed a dense structure that looks like a ribbon cutting across the sky, with an especially intense "hot spot" in the middle of it. That was puzzling enough.

Now, IBEX principal investigator David McComas of the Texas-based Southwest Research Institute says the latest map shows significant changes in the ribbon in virtually the blink of an astronomical eye.

"Today, we've announced that we observed that this ribbon actually varies on incredibly short time scales, six months," he told reporters in a telephone news conference.

"We didn't understand where the ribbon came from in the first place; it was completely unlike any of the predictions that had come before it. It's even more confounding now, to know that this structure can change on such very short time scales."

McComas explained that the changes over only six months are especially baffling because the two major influences on the heliosphere - the Sun and space - vary at much longer intervals.

"Once we saw a structure, we didn't think we'd see variations on such short time scales in the structure. The time scales for the internal forces for the heliosphere is the solar cycle, 11 years, and for outside [interstellar space], it's tens of thousands of years."

Another IBEX scientist, Nathan Schwadron of the University of New Hampshire, said in the briefing that one reason to try to understand what's going on is that the heliosphere, which in a sense is the outer boundary of the solar system, helps shield us from hazardous interstellar radiation.

"It turns out that these boundaries are actually protective. They protect us from the fairly high energy and dangerous radiation called galactic cosmic rays that pervades the galaxy," Schwadron said.

Understanding changes in that protective boundary could be particularly important in the future, as space flights take humans on long-distance voyages around the solar system.