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Cosmic "Dig" Reveals Vestiges
of the Milky Way's Building Blocks
Description: Globular Star Cluster
Position (J2000): RA: 17h 48m 05s Dec: -24° 46' 48.0"
Distance: 15,000- 25,000 light years
Apparent magnitude: 12.8
Radius: 3.9 ly
Image Credit: ESO/F. Ferraro
Release Date: November 25, 2009
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Peering through the thick dust clouds of our galaxy's "bulge" (the myriads of stars surrounding its center), and revealing an amazing amount of detail, a team of astronomers has unveiled an unusual mix of stars in the stellar grouping known as Terzan 5. Never observed anywhere in the bulge before, this peculiar "cocktail" of stars suggests that Terzan 5 is in fact one of the bulge's primordial building blocks, most likely the relic of a proto-galaxy that merged with the Milky Way during its very early days.
"The history of the Milky Way is encoded in its oldest fragments, globular clusters and other systems of stars that have witnessed the entire evolution of our galaxy," says Francesco Ferraro from the University of Bologna, lead author of a paper appearing in the journal Nature (Nature, 2009, vol. 462, p. 483-486). "Our study opens a new window on yet another piece of our galactic past."
Like archaeologists, who dig through the dust piling up on top of the remains of past civilizations and unearth crucial pieces of the history of mankind, astronomers have been gazing through the thick layers of interstellar dust obscuring the bulge of the Milky Way and have unveiled an extraordinary cosmic relic.
The target of the study is the star cluster Terzan 5. The new observations show that this object, unlike all but a few exceptional globular clusters, does not harbor stars which are all born at the same time - what astronomers call a "single population" of stars. Instead, the multitude of glowing stars in Terzan 5 formed in at least two different epochs, the earliest probably some 12 billion years ago and then again 6 billion years ago.
"Only one globular cluster with such a complex history of star formation has been observed in the halo of the Milky Way: Omega Centauri," says team member Emanuele Dalessandro. "This is the first time we see this in the bulge."
The galactic bulge is the most inaccessible region of our galaxy for astronomical observations: only infrared light can penetrate the dust clouds and reveal its myriads of stars. "It is only thanks to the outstanding instruments mounted on ESO's Very Large Telescope," says co-author Barbara Lanzoni, "that we have finally been able to 'disperse the fog' and gain a new perspective on the origin of the galactic bulge itself."
A technical jewel lies behind the scenes of this discovery, namely the Multi-conjugate Adaptive Optics Demonstrator (MAD), a cutting-edge instrument that allows the VLT to achieve superbly detailed images in the infrared. Adaptive optics is a technique through which astronomers can overcome the blurring that the Earth's turbulent atmosphere inflicts on astronomical images obtained from ground-based telescopes; MAD is a prototype of even more powerful, next-generation adaptive optics instruments.
Through the sharp eye of the VLT, the astronomers also found that Terzan 5 is more massive than previously thought: along with the complex composition and troubled star formation history of the system, this suggests that it might be the surviving remnant of a disrupted proto-galaxy, which merged with the Milky Way during its very early stages and thus contributed to form the galactic bulge.
"This could be the first of a series of further discoveries shedding light on the origin of bulges in galaxies, which is still hotly debated," concludes Ferraro. "Several similar systems could be hidden behind the bulge's dust: it is in these objects that the formation history of our Milky Way is written."
Terzan 5 is a heavily obscured globular cluster belonging to the bulge (the central star concentration) of the Milky Way galaxy. It was one of six globulars discovered by French astronomer Agop Terzan in 1968 and was initially labeled Terzan 11. The cluster was cataloged by the Two-Micron Sky Survey as IRC-20385. It is situated in the Sagittarius constellation in the direction of the Milky Way's center. Terzan 5 probably follows an unknown complicated orbit around the center of the galaxy, but currently it moves towards the Sun with the speed of around 90 km/s.
The absolute magnitude of Terzan 5 is at least MV=-7.5, meaning that its luminosity is about 100,000 times more than that of the Sun, and is close to the average absolute magnitude for galactic globular clusters. The small core of Terzan 5 has one of the highest star densities in the galaxy. Its volume mass density exceeds 106 M?/pc3, while its volume luminosity density exceeds 105.5 L?/pc3, where M? and L? are the Sun's mass and luminosity, respectively. The cluster also has one of the highest metallicities among Milky Way's globular clusters-[Fe/H]=-0.21.
In 2009 it was discovered that Terzan 5 consists of at least two generations of stars with ages of 12 and 6 billion years and slightly different metallicities, possibly indicating that it is the core of a disrupted dwarf galaxy, not a true globular cluster. There are only a few other globular clusters in Milky Way that contain stars with different ages. Among them are M54 and Omega Centauri, the former being the core of Sagittarius Dwarf Elliptical Galaxy. The cluster also contains around 1300 core helium burning horizontal branch (HB) stars, including at least one RR Lyrae variable star.