Many large galaxies, such as the Milky Way, are thought to have lots of satellite galaxies too dim to see. They are dominated by “dark matter,” which astronomers say makes up 85 percent of all matter in the universe but so far remains undetected. Chakrabarti, a post-doctoral fellow and theoretical astronomer at the University of California, Berkeley, has developed a way to find “dark” satellite galaxies by analyzing the ripples in the hydrogen gas distribution in spiral galaxies. Planet X was predicted — erroneously — more than 100 years ago based on perturbations in the orbit of Neptune. Earlier this year, Chakrabarti used her mathematical method to predict that a dwarf galaxy sits on the opposite side of the Milky Way from Earth, and that it has been unseen to date because it is obscured by the intervening gas and dust in the galaxy’s disk. One astronomer has already applied for time on the Spitzer Space Telescope to look in infrared wavelengths for this hypothetical Galaxy X.
This animation shows the distribution of the dark matter, obtained from a numerical simulation, at a redshift z~2, or when the Universe was about 3 billion years old. The first frame displays the continuous distribution of dark matter particles, showing the typical wispy structure of the cosmic web, with a network of sheets and filaments that developed out of tiny fluctuations in the early Universe. The second frame provides a simplified view of the complex network of dark matter structure according to the so-called halo model, a statistical approach used to describe the distribution of dark matter on both large and small scales. Within this framework, the dark matter distribution is viewed as an ensemble of discrete objects, the dark matter halos, corresponding to the densest knots of the cosmic web. The last frame highlights the dark matter halos (shown in yellow) that represent the most efficient cosmic sites for the formation of galaxies. Only halos with a mass above a certain threshold can trigger the ignition of intense bursts of star formation, thus creating a starburst galaxy. According to the latest measurements achieved with Herschel, the minimum mass needed by a halo for a starburst galaxy to form within it is 3 x 10^11 times that of the Sun. (via ESA Portal - Herschel finds less dark matter but more stars - images)
Astronomers have used Hubble to spot what they think is the furthest and one of the very earliest galaxies ever seen in the Universe. Candidate galaxy UDFj-39546284 appears as a faint red blob at the end of this zoom into an ultra deep field exposure taken with the NASA/ESA Hubble Space Telescope. Based on the object’s colour, astronomers believe that its light has taken 13.2 billion years to reach us. Spectroscopic confirmation that this is indeed the most distant galaxy ever seen is expected to come from the NASA/ESA/CSA James Webb Space Telescope, which is planned for launch later this decade.
The core of Omega Centauri. This globular cluster is the core of a dwarf galaxy that has been eaten by the Milky Way billions of years ago. Only the central black hole and the stars that orbit it remain, so this is an isolated “bulge” of a galaxy. The stars in this cluster are only 0.1 light years away from each other.