Universe Today

Andromeda Galaxy taken in ultraviolet. Image credit: GALEX Click to enlarge
Astronomers have long believed that the Andromeda galaxy had a different upbringing from our own Milky Way, but now it seems we aren’t so different after all. An international team of researchers have completed a survey of the metal content in Andromeda’s halo, and found that it’s relatively metal poor – just like the Milky Way. If both galaxies have the same amount of metal in their halos, that means they probably evolved in similar ways; both got started half a billion years after the Big Bang and grew from a collection of protogalactic fragments.

For the last decade, astronomers have thought that the Andromeda galaxy, our nearest galactic neighbor, was rather different from the Milky Way. But a group of researchers have determined that the two galaxies are probably quite similar in the way they evolved, at least over their first several billion years.

In an upcoming issue of the Astrophysical Journal, Scott Chapman of the California Institute of Technology, Rodrigo Ibata of the Observatoire de Strasbourg, and their colleagues report that their detailed studies of the motions and metals of nearly 10,000 stars in Andromeda show that the galaxy’s stellar halo is “metal-poor.” In astronomical parlance, this means that the stars lying in the outer bounds of the galaxy are pretty much lacking in all the elements heavier than hydrogen.

This is surprising, says Chapman, because one of the key differences thought to exist between Andromeda and the Milky Way was that the former’s stellar halo was metal-rich and the latter’s was metal-poor. If both galaxies are metal-poor, then they must have had very similar evolutions.

“Probably, both galaxies got started within a half billion years of the Big Bang, and over the next three to four billion years, both were building up in the same way by protogalactic fragments containing smaller groups of stars falling into the two dark-matter haloes,” Chapman explains.

While no one yet knows what dark matter is made of, its existence is well established because of the mass that must exist in galaxies for their stars to orbit the galactic centers the way they do. Current theories of galactic evolution, in fact, assume that dark-matter wells acted as a sort of “seed” for today’s galaxies, with the dark matter pulling in smaller groups of stars as they passed nearby. What’s more, galaxies like Andromeda and the Milky Way have each probably gobbled up about 200 smaller galaxies and protogalactic fragments over the last 12 billion years.

Chapman and his colleagues arrived at the conclusion about the metal-poor Andromeda halo by obtaining careful measurements of the speed at which individual stars are coming directly toward or moving directly away from Earth. This measure is called the radial velocity, and can be determined very accurately with the spectrographs of major instruments such as the 10-meter Keck-II telescope, which was used in the study.

Of the approximately 10,000 Andromeda stars for which the researchers have obtained radial velocities, about 1,000 turned out to be stars in the giant stellar halo that extends outward by more than 500,000 light-years. These stars, because of their lack of metals, are thought to have formed quite early, at a time when the massive dark-matter halo had captured its first protogalactic fragments.

The stars that dominate closer to the center of the galaxy, by contrast, are those that formed and merged later, and contain heavier elements due to stellar evolution processes.

In addition to being metal-poor, the stars of the halo follow random orbits and are not in rotation. By contrast, the stars of Andromeda’s visible disk are rotating at speeds upwards of 200 kilometers per second.

According to Ibata, the study could lead to new insights on the nature of dark matter. “This is the first time we’ve been able to obtain a panoramic view of the motions of stars in the halo of a galaxy,” says Ibata. “These stars allow us to weigh the dark matter, and determine how it decreases with distance.”

In addition to Chapman and Ibata, the other authors are Geraint Lewis of the University of Sydney; Annette Ferguson of the University of Edinburgh; Mike Irwin of the Institute of Astronomy in Cambridge, England; Alan McConnachie of the University of Victoria; and Nial Tanvir of the University of Hertfordshire.

Original Source: Caltech News Release

Shock wave in Stephen’s Quintet captured by Spitzer. Image credit: NASA/JPL-Caltech. Click to enlarge
This photograph, taken by the Spitzer space telescope and a ground-based telescope in Spain, shows the Stephan’s Quintet galaxy cluster, with one of the largest shockwaves ever seen in the Universe. The green arc in the photograph is the point which two galaxies are colliding. There are actually 5 galaxies in this photograph, but two have been so beaten up, all that’s left are their bright centers. The galaxies are located 300 million light-years away in the Pegasus constellation.

This false-color composite image of the Stephan’s Quintet galaxy cluster clearly shows one of the largest shock waves ever seen (green arc), produced by one galaxy falling toward another at over a million miles per hour. It is made up of data from NASA’s Spitzer Space Telescope and a ground-based telescope in Spain.

Four of the five galaxies in this image are involved in a violent collision, which has already stripped most of the hydrogen gas from the interiors of the galaxies. The centers of the galaxies appear as bright yellow-pink knots inside a blue haze of stars, and the galaxy producing all the turmoil, NGC7318b, is the left of two small bright regions in the middle right of the image. One galaxy, the large spiral at the bottom left of the image, is a foreground object and is not associated with the cluster.

The titanic shock wave, larger than our own Milky Way galaxy, was detected by the ground-based telescope using visible-light wavelengths. It consists of hot hydrogen gas. As NGC7318b collides with gas spread throughout the cluster, atoms of hydrogen are heated in the shock wave, producing the green glow.

Spitzer pointed its infrared spectrograph at the peak of this shock wave (middle of green glow) to learn more about its inner workings. This instrument breaks light apart into its basic components. Data from the instrument are referred to as spectra and are displayed as curving lines that indicate the amount of light coming at each specific wavelength.

The Spitzer spectrum showed a strong infrared signature for incredibly turbulent gas made up of hydrogen molecules. This gas is caused when atoms of hydrogen rapidly pair-up to form molecules in the wake of the shock wave. Molecular hydrogen, unlike atomic hydrogen, gives off most of its energy through vibrations that emit in the infrared.

This highly disturbed gas is the most turbulent molecular hydrogen ever seen. Astronomers were surprised not only by the turbulence of the gas, but by the incredible strength of the emission. The reason the molecular hydrogen emission is so powerful is not yet completely understood.

Stephan’s Quintet is located 300 million light-years away in the Pegasus constellation.

This image is composed of three data sets: near-infrared light (blue) and visible light called H-alpha (green) from the Calar Alto Observatory in Spain, operated by the Max Planck Institute in Germany; and 8-micron infrared light (red) from Spitzer’s infrared array camera.

Original Source: Spitzer Space Telescope

Artist’s illustration represents tightly-wound magnetic field confining jet. Image credit: NRAO/AUI/NSF. Click to enlarge
Radio astronomers have uncovered a dying star with twin jets of material confined by a powerful magnetic field. The star is located about 8,500 light-years away from Earth in the constellation of Aquila, and it’s in the process of forming a planetary nebula. Many stars like this produce elongated nebulae, where the star’s outer envelope is pushed away and channeled into tight jets. The jets come out in a corkscrew shape, which means that the star is slowly rotating.

Molecules spewed outward from a dying star are confined into narrow jets by a tightly-wound magnetic field, according to astronomers who used the National Science Foundation’s Very Long Baseline Array (VLBA) radio telescope to study an old star about 8,500 light-years from Earth.

The star, called W43A, in the constellation Aquila, is in the process of forming a planetary nebula, a shell of brightly-glowing gas lit by the hot ember into which the star will collapse. In 2002, astronomers discovered that the aging star was ejecting twin jets of water molecules. That discovery was a breakthrough in understanding how many planetary nebulae are formed into elongated shapes.

“The next question was, what is keeping this outpouring of material confined into narrow jets? Theoreticians suspected magnetic fields, and we now have found the first direct evidence that a magnetic field is confining such a jet,” said Wouter Vlemmings, a Marie Curie Fellow working at the Jodrell Bank Observatory of the University of Manchester in England.

“Magnetic fields previously have been detected in jets emitted by quasars and protostars, but the evidence was not conclusive that the magnetic fields were actually confining the jets. These new VLBA observations now make that direct connection for the very first time,” Vlemmings added.

By using the VLBA to study the alignment, or polarization, of radio waves emitted by water molecules in the jets, the scientists were able to determine the strength and orientation of the magnetic field surrounding the jets.

“Our observations support recent theoretical models in which magnetically-confined jets produce the sometimes-complex shapes we see in planetary nebulae,” said Philip Diamond, also of Jodrell Bank Observatory.

During their “normal” lives, stars similar to our Sun are powered by the nuclear fusion of hydrogen atoms in their cores. As they near the end of their lives they begin to blow off their outer atmospheres and eventually collapse down to a white dwarf star about the size of Earth. Intense ultraviolet radiation from the white dwarf causes the gas thrown off earlier to glow, producing a planetary nebula. Astronomers believe that W43A is in the transition phase that will produce a planetary nebula. That transition phase, they say, is probably only a few decades old, so W43A offers the astronomers a rare opportunity to watch the process.

While the stars that produce planetary nebulae are spherical, most of the nebulae themselves are not. Instead, they show complex shapes, many elongated. The earlier discovery of jets in W43A showed one mechanism that could produce the elongated shapes. The latest observations will help scientists understand the mechanisms producing the jets.

The water molecules the scientists observed are in regions nearly 100 billion miles from the old star, where they are amplifying, or strengthening, radio waves at a frequency of 22 GHz. Such regions are called masers, because they amplify microwave radiation the same way a laser amplifies light radiation.

The earlier observations showed that the jets are coming out from the star in a corkscrew shape, indicating that whatever is squirting them out is slowly rotating.

Vlemmings and Diamond worked with Hiroshi Imai of Kagoshima University in Japan. The astronomers reported their work in the March 2 issue of the scientific journal Nature.

The VLBA is a system of ten radio-telescope antennas, each with a dish 25 meters (82 feet) in diameter and weighing 240 tons. From Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands, the VLBA spans more than 5,000 miles, providing astronomers with the sharpest vision of any telescope on Earth or in space. Dedicated in 1993, the VLBA has an ability to see fine detail equivalent to being able to stand in New York and read a newspaper in Los Angeles.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release

Model image of cepheid L Carinae. Image credit: Click to enlarge
The European Southern Observatory’s Very Large Telescope Interferometer has uncovered three Cepheid variable stars surrounded by a cocoon of hot gas. Cepheids are known to pulse in brightness at a regular rate, and used by astronomers to calculate relatively nearby distances. As a Cepheid pulses, the velocity of its photosphere changes dramatically. It could be that this envelope is stellar material left behind as the star grows and shrinks.

Using ESO’s Very Large Telescope Interferometer (VLTI) at Cerro Paranal, Chile, and the CHARA Interferometer at Mount Wilson, California, a team of French and North American astronomers has discovered envelopes around three Cepheids, including the Pole star. This is the first time that matter is found surrounding members of this important class of rare and very luminous stars whose luminosity varies in a very regular way. Cepheids play a crucial role in cosmology, being one of the first “steps” on the cosmic distance ladder.

The southern Cepheid L Carinae was observed with the VINCI and MIDI instrument at the VLTI, while Polaris (the Pole Star) and Delta Cephei (the prototype of its class) were scrutinised with FLUOR on CHARA, located on the other side of the equator. FLUOR is the prototype instrument of VINCI. Both were built by the Paris Observatory (France).

For most stars, the observations made with the interferometers follow very tightly the theoretical stellar models. However, for these three stars, a tiny deviation was detected, revealing the presence of an envelope.

“The fact that such deviations were found for all three stars, which however have very different properties, seems to imply that envelopes surrounding Cepheids are a widespread phenomenon”, said Pierre Kervella, one of the lead authors.

The envelopes were found to be 2 to 3 times as large as the star itself. Although such stars are rather large – about fifty to several hundreds of solar radii – they are so far away that they can’t be resolved by single telescopes. Indeed, even the largest Cepheids in the sky subtend an angle of only 0.003 arc second. To observe this is similar to viewing a two-storey house on the Moon.

Astronomers have thus to rely on the interferometric technique, which combines the light of two or more distant telescopes, thereby providing the angular resolution of a unique telescope as large as the separation between them. With the VLTI, it is possible to achieve a resolution of 0.001 arc second or less.

“The physical processes that have created these envelopes are still uncertain, but, in analogy to what happens around other classes of stars, it is most probable that the environments were created by matter ejected by the star itself”, said Antoine Merand, lead-author of the second paper describing the results.

Cepheids pulsate with periods of a few days. As a consequence, they go regularly through large amplitude oscillations that create very rapid motions of its apparent surface (the photosphere) with velocities up to 30 km/s, or 108 000 km/h! While this remains to be established, there could be a link between the pulsation, the mass loss and the formation of the envelopes.

Original Source: ESO News Release