Spiral Density Wave Theory

Spiral galaxies are one of the most captivating structures in astronomy, yet their nature is still not fully understood. Astronomers currently have two categories of theories that can explain this structure, depending on the environment of the galaxy, but a new study, accepted for publication in the Astrophysical Journal, suggests that one of these theories may be largely wrong.

For galaxies with nearby companions, astronomers have suggested that tidal forces may draw out spiral structure. However, for isolated galaxies, another mechanism is required in which galaxies form these structures without intervention from a neighbor. A possible solution to this was first worked out in 1964 by Lin & Shu in which they suggested that the winding structure is merely an illusion. Instead, these arms weren’t moving structures, but areas of greater density which remained stationary as stars entered and exited them similar to how a traffic jam remains in position although the component cars travel in and out. This theory has been dubbed the Lin-Shu density wave theory and has been largely successful. Previous papers have reported a progression from cold, HI regions and dust on the inner portion of the spiral arms, that crash into this higher density region and trigger star formation, making hot O & B class stars that die before exiting the structure, leaving the lower mass stars to populate the remainder of the disk.
One of the main questions on this theory has been the longevity of the overdense region. According to Lin & Shu as well as many other astronomers, these structures are generally stable over long time periods. Others suggest that the density wave comes and goes in relatively short-lived, recurrent patterns. This would be similar to the turn signal on your car and the one in front of you at times seeming to synch up before getting out of phase again, only to line up again in a few minutes. In galaxies, the pattern would be composed of the individual orbits of the stars, which would periodically line up to create the spiral arms. Teasing out which of these was the case has been a challenge.

To do so, the new research, led by Kelly Foyle from McMaster University in Ontario, examined the progression of star formation as gas and dust entered the shock region produced by the Lin-Shu density wave. If the theory was correct, they should expect to find a progression in which they would first find cold HI gas and carbon monoxide, and then offsets of warm molecular hydrogen and 24 μm emission from stars forming in clouds, and finally, another offset of the UV emission of fully formed and unobscured stars.

The team examined 12 nearby spiral galaxies, including M 51, M 63, M 66, M 74, M 81, and M 95. These galaxies represented several variations of spiral galaxies such as grand design spirals, barred spirals, flocculent spirals and an interacting spiral.

When using a computer algorithm to examine each for offsets that would support the Lin-Shu theory, the team reported that they could not find a difference in location between the three different phases of star formation. This contradicts the previous studies (which were done “by eye” and thus subject to potential bias) and casts doubt on long lived spiral structure as predicted by the Lin-Shu theory. Instead, this finding is in agreement with the possibility of transient spiral arms that break apart and reform periodically.

Another option, one that salvages the density wave theory is that there are multiple “pattern speeds” producing more complex density waves and thus blurs the expected offsets. This possibility is supported by a 2009 study which mapped these speeds and found that several spiral galaxies are likely to exhibit such behavior. Lastly, the team notes that the technique itself may be flawed and underestimating the emission from each zone of star formation. To settle the question, astronomers will need to produce more refined models and explore the regions in greater detail and in more galaxies.

UK and European Space Agencies Give a Go For Skylon Spaceplane

After 30 years of development, the UK and European space agencies have given a go for the Skylon Space-plane.
The Skylon, which is being developed at the Oxfordshire-based Reaction Engines in the UK, is an unpiloted and reusable spacecraft that can launch into Low Earth Orbit after taking off from a conventional runway.
Looking like something out of Star Wars, Skylon is a self contained, single stage, all in one reusable space vehicle. There are no expensive booster rockets, external fuel tanks or huge launch facilities needed.

The vehicle’s hybrid SABRE engines use liquid hydrogen combined with oxygen from the atmosphere at altitudes up to 26km and speeds of up to Mach 5, before switching over to on-board fuel for the final rocket powered stage of ascent into low Earth orbit.
The Skylon is intended to cut the costs involved with commercial activity in space, delivering payloads of up to 15 tons including satellites, equipment and even people into orbit at costs much lower than those that use expensive conventional rockets.

Once the spacecraft has completed its mission, it will re-enter Earth’s atmosphere and return to base, landing like an airplane on the same runway, making it a totally re-usable spaceplane, with a fast mission turn around.

Skylon has received approval from a European Space Authority panel tasked with evaluating the design. “No impediments or critical items have been identified for either the Skylon vehicle or the SABRE engine that are a block to further development,” the panel’s report concludes.
“The consensus for the way forward is to proceed with the innovative development of the engine which in turn will enable the overall vehicle development.”
The UK Space Agency says that Reaction Engines will carry out an important demonstration of the SABRE engine’s key pre-cooler technology later this summer.

End of the Road for Spirit Rover

In the wee hours of May 25, 2011 the scientists and engineers of the Mars Exploration Rover team will send the last command in attempt to contact the Spirit rover. Over the past year, they have sent over 1,200 commands and haven’t heard anything in reply from the stuck and likely frozen rover. “We have exhausted all the likely scenarios for contacting Spirit, and the likelihood of success is now practically zero,” said John Callas, Project Manager for the Mars Exploration Rover mission. “And at this point, the season is declining and we couldn’t do any of the planned science objectives even if we heard from her now. The Deep Space Network will occasionally listen for Spirit when resources permit, but we have decided not to do anything past the last commands that will done tonight.”

 Spirit, the plucky rover that landed on Mars on January 3, 2004, overcame many difficulties and endured way past her 90-day warranty. For nearly six years, she traveled long distances, climbed hills — something the rovers weren’t really designed to do — she roved and stopped at interesting rocks along the way, all the while beaming back the information she garnered, enlightening us all about the nature of Mars, past and present.
Spirit became embedded in soft Martian soil in May of 2009 and that was the beginning of the end. The team spent months planning for her extrication, and then months again attempting to drive her out, but they ran out of time and power in the approaching Martian winter. The team was unable to put the rover in a favorable position to catch rays of sunlight on her solar panels, and after another freezing, grueling winter, Spirit has now likely succumbed to the harsh environment on Mars.

“We drove it, literally, until its wheels came off and at the beginning of the mission, we never expected that would be the way this project would end up,” said Dave Lavery, MER program director at NASA Headquarters.

The last commands will be sent early on May 25, 0700 UTC, which is just after midnight at JPL in Pasadena, California.

So, this is it. This is end of the Spirit rover mission.

“We always knew we would get to this point,” Callas said during a teleconference with the press, “and really, that’s what we wanted to do, to utilize these rovers as much as possible and wear them out. We are here today because we really wore Spirit out. If on sol 90 (the 90th Martian day of the mission) someone would have said this was going to last another 6 years, we just wouldn’t have believed it.”

Multi-Planet Systems Common in Kepler Findings

Of the 1235 planetary candidates that NASA’s Kepler space telescope has found so far, 408 reside in multiple-planet systems – a growing trend that indicates planets do, in fact,  like company.

The systems observed also seem to behave quite differently than our own solar system. In particular many are flatter than ours; that is, the planets orbit their stars in more or less the same exact plane. This, of course, is what allows Kepler to see them in the first place… the planets have to transit their stars perpendicular to Kepler’s point of view in order for it to detect the oh-so-subtle change in brightness that indicates the likely presence of a planet. In our solar system there’s a variation in the orbital plane of some planets up to 7º – enough of a difference that an alien Kepler-esque telescope might very well not be able to spot all eight planets.

The reason for this relative placidity in exoplanet orbits may be due to the lack of gas giants like Jupiter in these systems. So far, all the multiple-planet systems found have planets smaller than Neptune.
Without the massive gravitational influence of a Jupiter-sized world to shake things up, these exosystems likely experience a much calmer environment – gravitationally speaking, of course.

“Most likely, if our solar system didn’t have large planets like Jupiter and Saturn to have stirred things up with their gravitational disturbances, it would be just as flat. Systems with smaller planets probably had a much more sedate history.”

– David Latham, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA

Systems containing large gas giants have also been found but they are not as flat as those without, and many smaller worlds are indeed out there… “probably including a lot of them comparable in size to Earth,” said planet-hunter Geoff Marcy of the University of California, Berkeley.

While multiple-planet systems were expected, the scientists on the Kepler team were surprised by the amount that have been discovered.

“We didn’t anticipate that we would find so many multiple-transit systems. We thought we might see two or three. Instead, we found more than 100,” said Latham.

A total of 171 multiple-planet systems have been found so far… with many more to come, no doubt!

Announced yesterday at the American Astronomical Society conference in Boston, these findings are the result of only the first four months of Kepler’s observations. There will be another news release next summer but in the meantime the team wants time to extensively research the data.

“We don’t want to get premature information out. There’s still a lot of analysis that needs to be done.”

– Kepler principal investigator William Borucki

Carina Nebula: Pumping More Than Just Iron

We are all just star stuff… But when it comes to the elements produced by a star, it just doesn’t get any heavier than iron. So how do more exotic elements come into existence? Try the Great Cosmic Recycler – supernova. Its energy disperses newly synthesized materials right into the interstellar neighborhood where an enriched generation of stars begin life again.

 The beautiful Carina Nebula may very well be a literal supernova factory. Encompassing a large field of 1.4 square degrees, Chandra made of a mosaic of 22 individual pointings. In total, the image represents 1.2 million seconds – or nearly two weeks – of Chandra observing time. In addition, multi-wavelength data, such as infrared observations from the Spitzer Space Telescope and the Very Large Telescope (VLT), were then added to the mix to reveal that the supernova process has already begun. Clues, such as the lack of bright x-ray sources from Trumpler 15, suggest its massive stars have already been destroyed. In addition, six candidate neutron stars – instead of just one – provide additional evidence that supernova activity is gearing up in Carina.


But stellar destruction isn’t the only evidence Chandra has found. A new population of young massive stars has also been detected… potentially doubling the number of known young, massive stars which are usually destined to be destroyed later in supernova explosions. In the composite image, they appear as bright X-ray sources scattered across the x-ray emission like freckles on a child’s face. But what really holds our interest is the infamous Eta Carinae – a massive, unstable star on the brink of extinction.
Thanks to this latest research, we now know it’s not alone…

Australian Student Uncovers the Universe’s Missing Mass

Not since the work of Fritz Zwicky has the astronomy world been so excited about the missing mass of the Universe. His evidence came from the orbital velocities of galaxies in clusters, rotational speeds, and gravitational lensing of background objects. Now there’s even more evidence that Zwicky was right as Australian student – Amelia Fraser-McKelvie – made another breakthrough in the world of astrophysics.
Working with a team at the Monash School of Physics, the 22-year-old undergraduate Aerospace Engineering/Science student conducted a targeted X-ray search for the hidden matter and within just three months made a very exciting discovery. Astrophysicists predicted the mass would be low in density, but high in temperature – approximately one million degrees Celsius. According to theory, the matter should have been observable at X-ray wavelengths and Amelia Fraser-McKelvie’s discovery has proved the prediction to be correct.

Dr Kevin Pimbblet from the School of Astrophysics explains: “It was thought from a theoretical viewpoint that there should be about double the amount of matter in the local Universe compared to what was observed. It was predicted that the majority of this missing mass should be located in large-scale cosmic structures called filaments – a bit like thick shoelaces.”


Up until this point in time, theories were based solely on numerical models, so Fraser-McKelvie’s observations represent a true break-through in determining just how much of this mass is caught in filamentary structure. “Most of the baryons in the Universe are thought to be contained within filaments of galaxies, but as yet, no single study has published the observed properties of a large sample of known filaments to determine typical physical characteristics such as temperature and electron density.” says Amelia. “We examine if a filament’s membership to a supercluster leads to an enhanced electron density as reported by Kull & Bohringer (1999). We suggest it remains unclear if supercluster membership causes such an enhancement.”
Still a year away from undertaking her Honors year (which she will complete under the supervision of Dr Pimbblet), Ms Fraser-McKelvie is being hailed as one of Australia’s most exciting young students… and we can see why!

New Arm Embraces Milky Way

Some sixteen decades ago, Lord Rosse was the first to point out spiral structure in distant “nebula”… and today astrophysicists Thomas Dame and Patrick Thaddeus are discovering it closer to home. Our Milky Way Galaxy was believed to only have six spiral arms, but their research has revealed an outer extension of the Scutum-Centaurus arm from the inner galaxy.

“We have identified a spiral arm lying beyond the Outer Arm in the first Galactic quadrant ~15 kpc from the Galactic center.” says Dame and Thaddeus. “One of the detections was fully mapped to reveal a large molecular cloud with a radius of 47 pc and a molecular mass of ~50,000 M. At a mean distance of 21 kpc, the molecular gas in this arm is the most distant yet detected in the Milky Way.

The new arm appears to be the continuation of the Scutum–Centaurus Arm in the outer Galaxy, as a symmetric counterpart of the nearby Perseus Arm.”

Over the last 50 years, many models of our galaxy have been proposed – revealing a pleasing, duo-symmetry. However, finding evidence to prove these theories has been a bit more elusive. Since we cannot observe ourselves, seeing spiral structure on the far side of the galaxy is problematic – hidden by near-side emission at the same velocity. But these researchers didn’t stop. The new arm was found as a result of attempts to follow the Sct–Cen Arm past its tangent.

“The new arm was largely overlooked in existing 21 cm surveys probably because it lies mainly out of the Galactic plane, its Galactic latitude steadily increasing with longitude as it follows the warp in the distant outer Galaxy.” says Dame. “In the first quadrant the only prominent HI spiral feature in the outer Galaxy is the well-known Outer Arm, a feature also well traced by CO. However, at 3 degrees above the plane one sees instead the new arm as a prominent linear feature running roughly parallel to the locus of the Outer Arm but shifted to more negative velocities.”

Is our smoothly constructed galaxy indeed a mirror image of itself? This new evidence suggests the Scutum-Centaurus arm embraces the entire Milky Way – forming a symmetrical, star-forming counterpart to the galaxy’s other arm, Perseus. “Confirmation of the present feature as the ”Outer Sct-Cen Arm” will require a great deal of new data from several telescopes and much observing time over an extended period.” says Thaddeus. “Key steps toward confirming the proposal include, as mentioned, tracking Sct–Cen in the fourth quadrant and, even harder, tracking the Perseus Arm from the point where it passes inside the solar circle near longitude 50 degrees to its putative origin at the far end of the bar.”

Mapping the findings of galactic data on atomic hydrogen gas isn’t going to happen overnight… and even more discoveries and clarifications could be revealed in the future. “The Galactic symmetry suggested by the present work and clearly demonstrated by the identification of the Far 3-kpc Arm a few years ago, coupled with evidence for a global two-armed spiral pattern in the old stars, and, indeed, with the discovery of the bar itself, all hint at Galactic spiral structure that is both simpler and more amenable to study than had long been assumed. As emphasized here, much work remains, but aided by greatly improved distances from forthcoming astrometric surveys, a reasonably complete picture of our Galaxy’s spiral pattern may be achieved over the next decade.”

GSAT 8 Successful Launch from French Guiana - ISRO

Today India's advanced communication satellite, GSAT-8, was successfully launched at 02:08 hrs IST today (May 21, 2011) by the Ariane-V launch vehicle of Arianespace from Kourou. French Guiana.

Ariane V placed GSAT-8 into the intended Geosynchronous Transfer Orbit (GTO) of 35,861 km apogee and 258 km perigee, with an orbital inclination of 2.503 deg with respect to equator.

ISRO's Facility takes control of the Satellite :
ISRO's Master Control Facility (MCF) at Hassan in Karnataka acquired the signals from GSAT-8 satellite immediately after the injection. Initial checks on the satellite have indicated normal health of the satellite. The satellite was captured in three-axis stabilization mode. 

What Next ?
Preparations are underway for the firing of 440 Newton Liquid Apogee Motor (LAM) during the third orbit of the satellite on May 22, 2011 at 03:58 hrs IST as a first step towards taking the satellite to its geostationary orbital home

Basically there are two main payload in this mission :-
1. Communication Payload:
24 Ku-band transponders each with 36 MHz usable bandwidth employing 140 WTWTAs in order to cover entire Indian mainland including Andaman Nicobar has been placed.

2. Navigation Payload :
Two-channel GAGAN payload operating in LI and L5 bands provides Satellite-based Navigation services with accuracy and integrity required for civil aviation applications over Indian Air Space.