elliptical galaxy Centaurus A
July 23, 2014

Hubble Probes The Dim Halo Of Centaurus A

April Flowers for redOrbit.com - Your Universe Online

The Hubble Space Telescope is a joint project between NASA and the European Space Agency (ESA) that launched in 1990. Since then, it has returned thousands of images from its orbital position above the atmosphere, helping scientists to shed light on some of the mysteries of the cosmos.

Hubble images prove that there is more to a galaxy than what first meets the eye. The telescope is able to look beyond the bright glow of a galaxy's center and swirling arms, and even the elliptical fuzz, to see the dim halo of stars sprawling away into space.

An important component of a galaxy, the halo preserves signatures of both its formation and evolution, and yet, we know very little about the haloes of galaxies beyond our own Milky Way. Exploring such faint, expansive fields of stars is challenging, and so far, astronomers have only managed to detect very few haloes around other galaxies.

Hubble's unique, space-based location, along with the very sensitive scientific instruments such as the Advanced Camera for Surveys and the Wide Field Camera 3, has allowed a team of astronomers to examine the halo surrounding the giant elliptical galaxy NGC 5128, called Centaurus A. The unprecedented distances covered by Hubble's instruments have led to the discovery that Centaurus A's halo extends farther into space than previously thought, and in an unexpected form.

"Tracing this much of a galaxy's halo gives us surprising insights into a galaxy's formation, evolution, and composition," said Marina Rejkuba of the European Southern Observatory in Garching, Germany, in a recent statement. “We found more stars scattered in one direction than the other, giving the halo a lopsided shape — which we hadn't expected!"

The scientists mapped a region approximately 450,000 light-years across — about 25 times further than the galaxy's radius — and 295,000 light-years in width — about 16 times further than the "effective radius" of the galaxy. For comparison, the visible portion of the Milky Way is around 120,000 light-years in diameter. The region mapped for this study extends across 4 degrees in the sky, effectively 8 times the apparent width of the Moon.

The astronomers found that the stars in the halo had an uneven distribution, and that they displayed surprising properties relating to the proportion of elements heavier than hydrogen and helium found in the gas that forms the stars. In the Milky Way and other nearby galaxies, the stars are typically very low in heavy elements. In Centaurus A, however, the stars are rich in heavy elements, even at the outermost locations examined by the team.

"Even at these extreme distances, we still haven't reached the edge of Centaurus A's halo, nor have we detected the very oldest generation of stars," added Laura Greggio of INAF, Italy. "This aged generation is very important. The larger stars from it are responsible for manufacturing the heavy elements now found in the bulk of the galaxy's stars. And even though the large stars are long dead, the smaller stars of the generation still live on and could tell us a great deal."

Scientists believe the reason large spiral galaxies like the Milky Way have small amounts of heavy elements is the mechanisms behind the formation and evolution of the galaxy. Such galaxies slowly pull in numerous small satellite galaxies, absorbing their stars. The presence of such heavy element rich stars in far-flung locations in Centaurus A's halo suggests that there was only a single merger with a large spiral galaxy which ejected stars from the spiral galaxy's disc to become Centaurus A's outer halo.

"Measuring the amount of heavy elements in individual stars in a giant elliptical galaxy such as Centaurus A is uniquely the province of Hubble — we couldn't do it with any other telescope, and certainly not yet from the ground," adds Rejkuba. “These kinds of observations are fundamentally important to understanding the galaxies in the Universe around us."

The findings of their study are to be published online this week in the Astrophysical Journal.