* Astronomy

Members of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) team, have used the Spitzer infrared space telescope to look at the dust-drenched plane of our galaxy. When they did this, the galaxy's obscuring clouds of gas and dust became transparent, revealing approximately 100 new star clusters, each containing tens to hundreds of stars.

According to lead investigator Emily Mercer, a graduate student at Boston University, Mass., the new clusters will tell astronomers a great deal about the structure of the Milky Way and star formation within the galaxy.

"These little guys were quite hard to find. The discovery required sophisticated computer sifting of GLIMPSE data and careful inspection of the Spitzer images" - Emily Mercer.

In the past, our galaxy wasn't so quick to give up its stellar secrets. Because we sit inside its flat, spiral disk, most of the galaxy appears as a thick blurry band of light that stretches across the sky. Many of the stars in this galactic plane cannot be detected with visible-light or ultraviolet telescopes. That's because the cool clouds of dust and gas that hover around the galaxy's centre and make up galactic spiral arms block their starlight from our view.
Two-thirds of the new star clusters were discovered through a computer method developed by Mercer and her advisor, Dr. Dan Clemens, also of Boston University. They used an algorithm to automatically sift through the GLIMPSE data for clusters. The rest were found using the traditional method of visually scrutinizing images for star clusters.


New star cluster in the constellation Aquila.
The new cluster is seen in the centre of the red nebula, or star-forming cloud, as the grouping of small blue, yellow, and green stars. The wisps of red are organic molecules within the dust which have been illuminated by nearby star formation. Green indicates the presence of hot hydrogen gas. Blue predominantly reveals older stars. The bright white arc located to the lower left side of the central star cluster shows the area where a massive star is forming.


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The picture is a 4-channel false-colour composite, showing emission from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange) and 8.0 microns (red).

Mercer also found that there are nearly twice as many star clusters in the southern galactic plane, the portion of the galactic plane visible from the Earth's Southern Hemisphere, as in the northern galactic plane. She suspects that this observation may help astronomers map the location of the Milky Way's spiral arms.

"Emily has done a great job. Her computer method for finding clusters has proved to be the most successful automated effort to date" - Dr. Dan Clemens.

Both Clemens and Mercer are members of the multi-institutional GLIMPSE team, which is led by Dr. Edward Churchwell of the University of Wisconsin, Madison. The group was selected to survey the galactic plane with Spitzer's infrared array camera in November 2000 as part of Spitzer's Legacy program. So far, more than 30 million stars in the inner Milky Way have already been catalogued by GLIMPSE, and the team expects to identify more than 50 million stars by the end of the project.

"By making the galactic plane transparent, Spitzer opens a new door for astronomers to study the Milky Way. Some of the most interesting science likely to come out of this project will be serendipitous discoveries, which will open up entirely new avenues of inquiry" - Edward Churchwell.

New research has produced the most accurate distance measurement ever made of the Milky Way's Perseus spiral arm.
The Perseus spiral arm has been found to be twice as close to Earth as some previous estimates had suggested.

The Milky Way is made up of four main arms .

"However, our view from the interior makes it difficult to determine its spiral structure" - Ye Xu, Shanghai Astronomical Observatory in China.

Measuring the distance to the spiral arms can be particularly tricky. This is because astronomers can only measure the speed of an astronomical object in terms of how fast it is moving towards or away from the Earth. Comparing this speed to theoretical models, which assume the objects travel on circular paths around the centre of the galaxy, allows astronomers to deduce the object's distance from Earth.
Using this technique astronomers had previously estimated the distance to Perseus at more than 13,000 light years. But other researchers using a method that compares the apparent brightness of massive, young stars with estimates of their intrinsic brightness, arrived at half that distance.

The new technique - 100 times more accurate than the other two - has shown that the Perseus arm is just 6400 light years from Earth.

Ye Xu used the Very Long Baseline Array (VLBA), a system of 10 radio dishes - each spanning 25 metres - that are scattered from Hawaii to the Caribbean Sea.

They focused on a star-forming region called W3OH inside the Perseus arm. Bright, young stars in the region heat methanol vapour in gas clouds around them, which in turn emits radio waves in what are called "masers".
Over the course of a year the team tracked the masers at five intervals , and determined their distance by "triangulating" their observed positions from different points along Earth's orbit, with an accuracy of 10 micro-arcseconds.


Position(2000): RA = 02 27 03.8192 Dec = 61 52 25.230.

"We used our changing vantage point to form one leg of a triangle. Then, measuring the change in angle of the source as the Earth orbits the Sun, we can calculate the source's distance by simple trigonometry." - Mark Reid, astronomer at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, US.

W3OH is at a distance of 1.953 ±0.04 kpc. This resolves a long-standing difference between kinematic distances (about 4 kpc) and O-star luminosity distances (about 2.2 kpc) to this region of the Perseus arm. The O-star luminosity distances are correct and this region is strongly kinematically anomalous.
They found that W3OH is not moving in a perfectly circular orbit but instead follows an elliptical path, as if drawn along the Perseus spiral arm.

"It seems to be indicating that the spiral arms may have a higher density than previously guessed. We hope to use such data to better understand how spiral arms form" - Mark Reid.

The proper motion of W3OH, relative to extragalactic sources, is a peculiar motion of 22~km/s directed inward and counter to Galactic rotation. This is in qualitative agreement with spiral density wave theory.

The team will now use the VLBA to measure the distances to a dozen star-forming regions spread across several of the Milky Way's spiral arms.

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The Milky Way in Stars and Dust
Credit Serge Brunier

-- Edited by Blobrana at 14:40, 2005-10-04

In an article to be published in the Astrophysical Journal, Bélanger et al. present the results of a detailed analysis of approximately 1900 hours of observations of the galactic centre, obtained with Integral since the launch of the spacecraft in October 2002.

The IBIS/ISGRI imager on the Integral observatory detected for the first time a hard X-ray source, IGRJ17456–2901, located within 1 arcminute of Sagittarius A* (Sgr A* - the black hole residing at the centre of our Galaxy) over the energy range 20–100 keV.

Two years and an effective exposure of 4.7×10^6 s have allowed for obtaining more stringent positional constraints on this high-energy source and the construction of its spectrum in the energy range 20–400 keV.

This central source near Sgr A* appears not to be a point source as previously thought, but likely is a diffuse, but compact, source. The observations by Bélanger et al. also show that the source is faint, but persistent with no detected variability.

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The galactic centre as observed with IBIS/ISGRI onboard INTEGRAL. The image covers a region of about 2.5 ° x 1.5 °.
For this image a total of 7 × 10^6 second observation time was used, giving an effective exposure time at the position of Sgr A* of 4.7 × 106 seconds. The colours represent significances ranging from 3 sigma (black) to greater than 60 sigma (white). The contours mark significance levels from 9.5 to 75 linearly.
Credits: G. Bélanger (CEA Saclay) et al.


By combining the ISGRI spectrum together with the total X-ray spectrum corresponding to the same physical region around Sgr A* from XMM-Newton data, and collected during part of the gamma-ray observations, Bélanger et al. have also constructed the first accurate wide band high-energy spectrum for the central arcminutes of the Galaxy.

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