The Halo Around Centaurus A

This image shows the stunning elliptical galaxy Centaurus A. Recently, astronomers have used the NASA/ESA Hubble Space Telescope to probe the outskirts of this galaxy to learn more about its dim halo of stars.

The galaxy's halo of stars has been found to reach much further than expected, extending across 4 degrees in the sky – equivalent to eight times the apparent width of the Moon, or almost twice the width of this image.

This image is a composite of images from Digitized Sky Survey 2 (DSS2), the MPG/ESO 2.2-meter telescope, and the NASA/ESA Hubble Space Telescope's Advanced Camera for Surveys (ACS).

The areas probed by Hubble are identified in the annotated version of this image. Hubble's unique capabilities allowed astronomers to analyze the faint stars in the halo.

Image credit: ESA/Hubble, NASA, Digitized Sky Survey, MPG/ESO; acknowledgement: Davide de Martin

Note: For more information, see Hubble Traces the Halo of a Galaxy More Accurately Than Ever Before and Hubble Traces the Halo of a Galaxy More Accurately Than Ever Before.

Virtual Guide to the Milky Way

This virtual journey shows the different components that make up our home galaxy, the Milky Way, which contains about a hundred billion stars.

It starts at the black hole at the center of the Milky Way and with the stars that orbit around it, before zooming out through the central Galactic Bulge, which hosts about ten billion stars.

The journey continues through a younger population of stars in the stellar disc, home to most of the Milky Way's stars, and which is embedded in a slightly larger gaseous disc. Stars in the disc are arranged in a spiral arm pattern and orbit the center of the Galaxy.

The discs and bulge are embedded in the stellar halo, a spherical structure that consists of a large number of globular clusters – the oldest population of stars in the Galaxy – as well as many isolated stars. An even larger halo of invisible dark matter is inferred by its gravitational effect on the motions of stars in the Galaxy.

Looking at a face-on view of the Galaxy we see the position of our Sun, located at a distance of about 26,000 light-years from the Galactic Center.

Finally, the extent of the stellar survey conducted by ESA’s Hipparcos mission is shown, which surveyed more than 100,000 stars up to 300 light-years away from the Sun. In comparison, ESA’s Gaia survey will study one billion stars out to 30,000 light-years away.

Video credit: ESA

CID 1711 and CID 3083: Close Encounters of the Galactic Kind

Astronomers have used a large survey to test a prediction that close encounters between galaxies can trigger the rapid growth of supermassive black holes. Key to this work was Chandra's unique ability to pinpoint actively growing black holes through the X-rays they generate.

The researchers looked at 562 pairs of galaxies ranging in distances from about 3 billion to 8 billion light years from Earth. They found that the galaxies in the early stages of an encounter with another were more likely than isolated, or "lonelier" galaxies to have actively growing black holes in their cores.

These two composite images show a sample of the pairs of galaxies that are undergoing close encounters in the survey. In these images, the data from NASA's Chandra X-ray Observatory are shown in purple and Hubble Space Telescope data are in gold. In both images, the point-like X-ray source near the center is generated by gas that has been heated to millions of degrees as it falls toward a supermassive black hole located in the middle of its host galaxy. The other faint X-ray emission may be caused by hot gas associated with the pair of galaxies.

The authors of the study estimate that nearly one-fifth of all moderately active black holes are found in galaxies undergoing the early stages of an interaction. This leaves open the question of what events are responsible for fueling the remaining 80% of growing black holes. Some of these may involve the late stages of mergers between two galaxies. Less violent events such as gas falling in from the halo of the galaxy, or the disruption of small satellite galaxies are also likely to play an important role.

The survey used in this research is called the Cosmic Evolution Survey (COSMOS), which covers two square degrees on the sky with observations from several major space-based observatories including Chandra and Hubble. Accurate distance information about the galaxies was also derived from optical observations with the European Southern Observatory's Very Large Telescope. The researchers compared a sample of 562 galaxies in pairs with 2726 solo galaxies to come to their conclusions.

Photo credit: X-ray: NASA/CXC/IPMU/J.Silverman et al; Optical: NASA/STScI/Caltech/N.Scoville et al.

The Leo Triplet

Triplet of bright galaxies in the constellation of Leo (The Lion), together with a multitude of fainter objects: distant background galaxies and much closer Milky Way stars. The image hints at the power of the VST and OmegaCAM for surveying the extragalactic Universe and for mapping the low brightness objects of the galactic halo. This image is a composite created by combining exposures taken through three different filters. Light that passed through a near-infrared filter was colored red, light in the red part of the spectrum is colored green, and green light is colored magenta.

Photo credit: ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute

Notes: For more information, see VST Looks at the Leo Triplet — and Beyond. The galaxy to the left is NGC 3628; opposite it to the right is Messier 66, and in the top right corner is Messier 65.