Reconciling dwarf galaxies with dark matter

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Andrew Wetzel.

Dwarf galaxies are enigmas wrapped in riddles. Although they are the smallest galaxies, they represent some of the biggest mysteries about our universe. While many dwarf galaxies surround our own Milky Way, there seem to be far too few of them compared with standard cosmological models, which raises a lot of questions about the nature of dark matter its role in galaxy formation. New theoretical modeling work from Andrew Wetzel, who holds a joint fellowship between Carnegie Caltech, offers the most accurate predictions to date about the dwarf galaxies in the Milky Way’s neighborhood. Wetzel achieved this by running the highest-resolution most-detailed simulation ever of a galaxy like our Milky Way. His findings, published by The Astrophysical Journal Letters, help to resolve longstanding debates about how these dwarf galaxies formed.

One of the biggest mysteries of dwarf galaxies has to do with dark matter, which is why scientists are so fascinated by them.

“Dwarf galaxies are at the nexus of dark matter science,” Wetzel said.

Dark matter makes up a quarter of our universe. It exerts a gravitational pull, but doesn’t seem to interact with regular matter–like atoms, stars, us–in any other way. We know it exists because of the gravitational effect it has on stars gas dust. This effect is why it is key to understanding galaxy formation. Without dark matter, galaxies could not have formed in our universe as they did. There just isn’t enough gravity to hold them together without it.

The role of dark matter in the formation of dwarf galaxies has remained a mystery. The standard cosmological model has told us that, because of dark matter, there should be many more dwarf galaxies out there, surrounding our own Milky Way, than we have found. Astronomers have developed a number of theories for why we haven’t found more, but none of them could account for both the paucity of dwarf galaxies their properties, including their mass, size, density.

As observation techniques have improved, more dwarf galaxies have been spotted orbiting the Milky Way. But still not enough to align with predictions based on standard cosmological models.

So scientists have been honing their simulation techniques in order to bring theoretical modeling predictions observations into better agreement. In particular, Wetzel his collaborators worked on carefully modeling the complex physics of stellar evolution, including how supernovae–the fantastic explosions that punctuate the death of massive stars–affect their host galaxy.

With these advances, Wetzel ran the most-detailed simulation of a galaxy like our Milky Way. Excitingly, his model resulted in a population of dwarf galaxies that is similar to what astronomers observe around us.

As Wetzel explained: “By improving how we modeled the physics of stars, this new simulation offered a clear theoretical demonstration that we can, indeed, understthe dwarf galaxies we’ve observed around the Milky Way. Our results thus reconcile our understanding of dark matter’s role in the universe with observations of dwarf galaxies in the Milky Way’s neighborhood.”

Despite having run the highest-resolution simulation to date, Wetzel continues to push forward, he is in the process of running an even higher-resolution, more-sophisticated simulation that will allow him to model the very faintest dwarf galaxies around the Milky Way.

“This mass range gets interesting, because these ‘ultra-faint’ dwarf galaxies are so faint that we do not yet have a complete observational census of how many exist around the Milky Way. With this next simulation, we can start to predict how many there should be for observers to find,” he added.

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NASA sees 2 landfalls for Hurricane Newton in Mexico

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Credits: NASA Goddard MODIS Rapid Response Team

On Sept. 7 at 5:35 a.m. EDT (935 UTC) when NASA's Aqua satellite looked at Newton the center had moved onto the mainlof Mexico. Strongest storms (purple) with coldest cloud top temperatures appeared south of the center at that time.

Credits: NASA JPL, Ed Olsen

NASA’s Terra Aqua satellites caught Hurricane Newton’s two landfalls in Mexico. The MODIS or Moderate Resolution Imaging Spectroradiometer instrument aboard NASA’s Terra satellite captured a visible image of Hurricane Newton after its center had made landfall in the southern part of Baja California, Mexico. MODIS provided an image of the storm at 2:25 p.m. EDT (18:25 UTC) that showed the cloud-filled center of circulation over the Baja, the eastern quadrant of the storm extending over the mainlof western Mexico.

The Atmospheric Infrared Sounder or AIRS instrument that flies aboard NASA’s Aqua satellite analyzed Newton in infrared light at Sept. 7 at 5:35 a.m. EDT (935 UTC) after its center had moved onto the mainlof Mexico. Infrared light provides temperature data. Coldest cloud top temperatures exceeded minus 63 degrees Fahrenheit (minus 53 degrees Celsius) around the center. Storms with temperatures that cold are high in the troposphere NASA research has shown they have the ability to generate heavy rain.

By 11 a.m. EDT on Sept. 7 NOAA’s National Hurricane Center (NHC) said .Newton continues to weaken over northern Sonora, Mexico high wind flash flood watches in effect for portions of southeastern Arizona southwestern New Mexico.

At that time the center of Tropical Storm Newton was located near 30.2 degrees north latitude 111.3 degrees west longitude. That puts the center of Newton about 80 miles (130 km) north-northwest of Hermosillo, Mexico about 135 miles (215 km) south of Tucson, Arizona.

Newton is moving toward the north-northeast near 18 mph (30 kph), this motion is expected to continue until Newton dissipates tonight. On the forecast track, the center of Newton will move into southeastern Arizona during the afternoon of Sept. 7. The estimated minimum central pressure is 994 millibars.

Maximum sustained winds have decreased to near 50 mph (85 kph) with higher gusts. Continued rapid weakening is forecast, Newton should weaken to a tropical depression over southeastern Arizona by this evening dissipate overnight.

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NASA science flights study effect of summer melt on Greenlice sheet

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NASA

Operation IceBridge, NASA’s airborne survey of polar ice, is flying in Greenlfor the second time this year, to observe the impact of the summer melt season on the ice sheet. The IceBridge flights, which began on August 27 will continue until September 16, are mostly repeats of lines that the team flew in early May, so that scientists can observe changes in ice elevation between the spring late summer. “Earlier in IceBridge’s history, we only surveyed the elevation of these glaciers once a year,” said Joe MacGregor, IceBridge’s deputy project scientist a glaciologist with NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “But these glaciers experience the climate year-round. Now we’re starting to complete the picture of what happens to them as the year goes on, especially after most of the summer melting has already occurred, so we can measure their cumulative response to that melt.”

The image above, taken during a high-priority flight that IceBridge carried on Aug. 29, shows Helheim Glacier, with its characteristic wishbone-shaped channels, as seen from about 20,000 feet in the sky. Helheim is one of Greenland’s largest fastest-melting glaciers. During the first week of the summer lice campaign, IceBridge has also flown over glaciers along Greenland’s northwest, southeast southwest coasts, also over lines that the Ice, Cloud, lElevation Satellite (ICESat) flew over Greenlduring its 2003-2009 period of operations, to observe how ice elevation has evolved since then. Future flights will cover critical areas in central southern Greenland, such as the world’s fastest glacier, Jakobshavn Isbræ.

For this short, end-of-summer campaign, the IceBridge scientists are flying aboard an HU-25A Guardian aircraft from NASA’s Langley Research Center in Hampton, Virginia. The Guardian is a version of an early-generation Falcon 20 business jet, modified for service with the US Coast Guard later acquired by NASA. It The plane carries a laser instrument that measures changes in the ice elevation, a high-resolution camera system to image the surface, an instrument to infer the surface temperature. Due to the Guardian’s limited range, the flights will be shorter (3.5 hours long) than the 8-hour missions carried during IceBridge’s spring Arctic campaign, but the team expects to fly twice a day whenever possible.

The mission of Operation IceBridge is to collect data on changing polar lsea ice maintain continuity of measurements between ICESat missions. The original ICESat mission ended in 2009, its successor, ICESat-2, is scheduled for launch in 2018. For more about Operation IceBridge to follow the current campaign, visit: http://www.nasa.gov/icebridge

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The supernova that wasn’t: A tale of 3 cosmic eruptions

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Nathan Smith/UA NASA

This animated view of Hubble Space Telescope images taken between 1993 2014 reveals how much the mass ejections from Eta Carinae have moved outward into space, some at a speed of 2 million miles per hour. The outermost ejecta visible in this image stem from previously unknown eruptions predating the Great Eruption of the 1840s.

Kiminki et al./NASA

Vectors illustrating the observed proper motions of 792 features in the ejecta of Eta Carinae. The arrows are color-coded by the date of ejection from the central star. Until now, only one eruption was known (red arrows). Blue green arrows mark previous eruptions (mid-13th mid-16th centuries, respectively).

Kiminki et al./NASA

1800s, astronomers surveying the night sky in the Southern Hemisphere noticed something strange: Over the course of a few years, a previously inconspicuous star named Eta Carinae grew brighter brighter, eventually outshining all other stars except Sirius, before fading again over the next decade, becoming too dim to be seen with the naked eye. What had happened to cause this outburst? Did 19th-century astronomers witness some strange type of supernova, a star ending its life in a cataclysmic explosion?

“Not quite,” says Megan Kiminki, a doctoral student in the University of Arizona’s Department of Astronomy Steward Observatory. “Eta Carinae is what we call a supernova impostor. The star became very bright as it blew off a lot of material, but it was still there.”

Indeed, in the mid-20th century Eta Carinae began to brighten again.

The aftermath of the “Great Eruption” of the mid-1800s, which is now readily visible through a small telescope if you happen to be in the Southern Hemisphere, made Eta Carinae a celebrity among objects in the universe known for their bizarre beauty. An hourglass-shaped, billowing cloud of glowing gas dust enshrouds the star its companion. Known as the Homunculus nebula, the cloud consists of stellar material hurled into space during the Great Eruption, drifting away at 2 million miles per hour.

By carefully analyzing images of Eta Carinae taken with NASA’s Hubble Space Telescope, Kiminki her team were surprised to discover that the Great Eruption was only the latest in a series of massive outbursts launched by the star system since the 13th century. Published in the journal Monthly Notices of the Royal Astronomical Society, the paper was co-authored by Nathan Smith, associate professor in the UA’s Department of Astronomy, Megan Reiter, who obtained her Ph.D. from the same department last year is now a postdoctoral fellow at the University of Michigan.

The expansion rate of gas that was far outside the Homunculus indicated that it was moving slowly must have been ejected centuries before the observed 19th-century brightening. In fact, the motions of the outer material point to two separate eruptions in the mid-13th mid-16th centuries.

For scientists trying to piece together what makes star systems such as Eta Carinae tick, the findings are like the stereotypical smoking gun in a detective story.

“From the first reports of its 19th-century outburst up to the most recent data obtained with advanced capabilities on modern telescopes, Eta Carinae continues to baffle us,” Smith says. “The most important unsolved problem has always been the underlying cause of its eruption, now we find that there were multiple previous eruptions. This is a bit like reconstructing the eruption history of a volcano by discovering ancient lava flows.”

Although the glowing gases of the Homunculus nebula prevent astronomers from getting a clear look at what’s inside, they have figured out that Eta Carinae is a binary system of two very massive stars that orbit each other every 5.5 years. Both are much bigger than our sun at least one of them is nearing the end of its life.

“These are very large stars that appear very volatile, even when they’re not blowing off nebulae,” Kiminki says. “They have a dense core very fluffy envelopes. If you replaced our sun by the larger of the two, which has about 90-100 solar masses, it could very well extend into the orbit of Mars.”

Because the Homunculus nebula is such an iconic visually stunning object, it has been a popular target of astronomical observations. A total of eight images, taken over the course of two decades with Hubble, turned out to be a treasure trove for Kiminki her co-authors.

The original goal of the team’s observing program was to measure proper motions of stars protostellar jets — fast streams of matter ejected by young stars during formation — in the Carina Nebula, but the same data also provided a powerful way to measure the motion of debris ejected by Eta Carinae itself.

“As I was aligning the images, I noticed that the one that Eta Carinae in it was more difficult to align,” Kiminki says. “We can only use objects as alignment points that aren’t moving, I thought, ‘Wow, a lot of this stuff is really moving.’ And then we decided to take a closer look.”

By aligning the multi-epoch images of the nebula, the team was able to track the movement of more than 800 blobs of gas Eta Carinae had ejected over time derived a likely ejection date for each. The analyses showed that the Homunculus nebula the observed 19th-century brightening tell only part of the story. Measuring the speed with which wisps of ejected material expoutward into space revealed that they must have resulted from two separate eruptions that occurred about 600 300 years before the Great Eruption of the 19th century.

In addition to having a separate origin in time, the older material also showed a very different geometry from the Homunculus, where material was ejected out from the star’s poles appears symmetric about its rotation axis.

“We found one of the prior eruptions was similarly symmetric, but at a totally different angle from the axis of the Great Eruption,” Kiminki explains. “Even more surprising was that the oldest eruption was very one-sided, suggesting two stars were involved, because it would be very unlikely for one star to blow material out toward just one side.”

Though perplexing, the findings are a big step forward for astronomers trying to understwhat causes the frequent outbursts.

“We don’t really know what’s going on with Eta Carinae,” Kiminki says. “But knowing that Eta Carinae erupted at least three times tells us that whatever causes those eruptions must be a recurring process, because it wouldn’t be very likely that each eruption is caused by a different mechanism.”

“Even though we still have not figured out the underlying physical mechanism that caused the 19th-century eruption, we now know that it isn’t a one-time event,” Smith says. “That makes it harder to understand, but it is also a critical piece of the puzzle of how very massive stars die. Stars like Eta Carinae apparently refuse to go quietly into the night.”

Eta Carinae’s eruptions provide unique insights into the last unstable phases of a very massive star’s life. Researchers who study supernovae have identified a subclass of supernova explosions that appear to suffer violent eruptions shortly before they finally explode. Smith notes that Eta Carinae might be our nearest example of this.

Because it takes light 7,000 years to travel from Eta Carinae to Earth, much could have happened in the meantime, Kiminki says. “Eta Carinae may have gone supernova by now, we wouldn’t know until 7,000 years from now.”

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NASA sees Hermine’s twin towers

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Credits: NASA/Owen Kelley

In order for Hermine or any other tropical depression, to intensify there must be a pathway for heat energy from the ocean surface to enter the atmosphere. For Hermine, the conduit may have been one of the two “hot towers” that the Global Precipitation Measurement mission or GPM core satellite observed on Aug. 31 at 4:09 p.m. EDT (2009 UTC). A hot tower is a tall convective cell in which updrafts are strong enough to lift precipitation as high as the top of the troposphere. Updrafts this strong mean that a lot of moisture is condensing in this storm cell as the rain forms. The closer a hot tower is to the low-pressure center of a tropical depression, the more of the heat energy released in that tower is likely to “energize” the tropical depression, leading to a deepening of the low pressure center, ultimately leading to an acceleration of the winds that circle the low-pressure center.

Two “hot towers” were seen south east of the center. “One of them was closer to the low pressure center, so it is more likely to be intimately involved with the strengthening of Hermine,” said researcher Owen Kelley of NASA’s Goddard Space Flight Center in Greenbelt, Marylwho created a 3-D image of the storm using GPM data.

“Repeated observations of cloud-top heights by geosynchronous satellites showed that these two towers were persistent features, lasting approximately 9 to 12 hours, from the evening of Aug. 31 into the morning of Sept. 1. Research performed by NASA scientists suggests that vigorous convection that persists this long can eject enough energy into the atmosphere to influence the fate of hurricanes other tropical systems,” Kelley said.

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