Monthly Archives: March 2011

Young Universe with an Old Galaxy Cluster

ESA’s XMM-Newton Satellite Discovers Most distant, Mature Galaxy Cluster

The universe, as seen by those who study its earliest ages, shows much more interesting structure in

its early history than scientists used to think.  According to the most widely accepted theory about how the universe grew, the infant cosmos began with small objects and grew steadily larger by a process called “accretion.” This simply means that smaller objects coalesced together under the influence of their mutual gravitational pull. The cosmos was populated with ever-growing objects.  This is why

galaxy clusters, the largest cosmic structures that are bound together by gravity, are the last things to form. They take a lot of material and a strong gravitational field to hold together.

It’s not clear exactly when the first gravitationally bound galaxy clusters appeared, and so astronomers are focusing their attention, and the instruments of such satellites as the XMM-Newton observatory, to study them. iven that galaxy clusters are scarce, and even more so at high redshifts where they are still taking shape, these observations are challenging. “This is why discovering a galaxy cluster at a redshift of 2 feels like unearthing a rare and valuable gem,” says Raphael Gobat from the Commissariat à l’Énergie Atomique (CEA), in France, who led an extensive study that revealed what appears to be the most distant galaxy cluster detected yet.

The cluster, called CL J1449+0856, is seen as it was when the universe was only about a quarter of its current age –around 3 billion years old. In contrast to other structures observed in the young universe, this object is already in its prime, as is evident from its diffuse X-ray emission and evolved population of galaxies. This shows that fully-grown galaxy clusters were already in place this early in cosmic history.

This multi-wavelength study is based on observations performed with a number of ground- and space-based observatories: infrared data from NASA’s Spitzer Space Telescope; optical and near-infrared data from the Subaru Telescope of the National Astronomical Observatory of Japan, the Very Large Telescope of the European Southern Observatory, the William Herschel Telescope, the NASA/ESA Hubble Space Telescope and the W.M. Keck Observatory; X-ray data from ESA’s XMM-Newton and NASA’s Chandra X-ray observatories.

The x-ray data from both observatories exhibit detections that correspond to the cluster, however, only the XMM-Newton detection is associated with the diffuse intra-cluster gas, which is key to the findings presented in this paper, whereas the Chandra detection is associated with one of the galaxies belonging to the cluster.

The diffuse x-ray emission is a key signature of the cluster’s advanced evolutionary state. “Only galaxy clusters that have had time to fully develop, collapsing under the influence of their own gravity, are visible in x-rays,” explains Alexis Finoguenov from the Max-Planck-Institut für extraterrestrische Physik (MPE), in Germany, co-author of the paper in which the result is presented. The x-ray emission originates from the hot intra-cluster gas: subject to the cluster’s gravitational potential, the gas is compressed and heated to temperatures of over 10 million Kelvin, and shines at x-ray wavelengths.

This fully mature galaxy cluster is located just over 1o billion light-years away, which is a record-breaking distance for such an object.

For more details about this remarkable distant galaxy cluster, visit XMM-Newton’s web page.

Dust and Distance in a Nearby Galaxy

ESO Studies the Dusty Disk of NGC 247

Astronomers using the MPG/ESO 2.2-meter telescope located at La Silla Observatory in Chile, have taken a detailed image of a nearby galaxy that turns out to be closer than they thought. The galaxy, as seen from our point of view, is highly tilted, causing us to see it almost edge-on. That tilt caused astronomers to overestimate its distance from us.

NGC 247 is part of the Sculptor Group, a collection of galaxies associated with the Sculptor Galaxy (NGC 253). Putting a precise value on how far away this galaxy and its group is difficult.  Astronomers use a variety of methods, but to measure the distance from the Earth to a nearby galaxy, they rely on a type of variable star called a Cepheid to act as a distance marker. Cepheids are very luminous stars, whose brightness varies at regular intervals. The time taken for the star to brighten and fade can be plugged into a simple mathematical relation that gives its intrinsic brightness. When compared with the measured brightness this gives the distance. However, this method isn’t foolproof, as astronomers think this period–luminosity relationship depends on the composition of the Cepheid.

Another problem arises from the fact that some of the light from a Cepheid may be absorbed by dust en route to Earth, making it appear fainter, and therefore further away than it really is. This is a particular problem for NGC 247 with its highly inclined orientation, as the line of sight to the Cepheids passes through the galaxy’s dusty disk. To determine a more exact distance, astronomers have to take into account the thickness of that disk, particularly since this galaxy is tilted. The light from Cepheids will pass through more dust before it is detected by instruments.

In a study called the Araucaria Project,  a team of astronomers has reported that NGC 247 is more than a million light-years closer to the Milky Way than was previously thought, bringing its distance down to just over 11 million light-years.

This high-resolution image also reveals large numbers of the galaxy’s component stars, which appear are clearly resolved. The many glowing pink clouds of hydrogen indicate regions of active star formation, which are scattered throughout the loose and ragged spiral arms.  In addition to features in NGC 247, the image also shows numerous galaxies shining from far beyond. In the upper right of the picture three prominent spirals form a line and still further out, far behind them, many more galaxies can be seen, some shining right through the disc of NGC 247.

March Stargazing in Our Night Sky

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Springtime stargazing is almost upon northern hemisphere stargazers in the month of March. For viewers in the southern hemisphere, the skies of autumn are fast approach.  In this month’s episode of “Our Night Sky”, we look at a few of the many beautiful starry sights available this time of year. We begin with Jupiter, the bright, star-like object in the west after sunset.  It’s really a planet, blazing there in reflected light from the Sun.  Next, we turn our gaze to the east and look for Saturn, just rising a couple of hours after sunset.

The constellations Gemini, Cancer, and Leo are harbingers of the change of seasons. They each have their treasures, so look at them through binoculars or a small telescope as you do your skyviewing this month.

In the southern hemisphere, the stars Sirius and Canopus point to a pair of fuzzy blobs that are really companion galaxies to the Milky Way.  All of this and more are highlighted in the March episode of “Our Night Sky.” Check it out!  You can also use [ this handy chart] as a guide to viewing some of the stars and constellations up during March.

March 2011
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