Dwarf Galaxy Holmberg IX: This loose collection of stars is actually a dwarf irregular galaxy, called Holmberg IX. It resides just off the outer edge of M81, a large spiral galaxy in Ursa Major. This image was taken with Hubble's Advanced Camera for Surveys in early 2006. Holmberg IX is of the so-called Magellanic type of galaxy, as its size and irregularity in structure are similar to the Small Magellanic Cloud, a neighbour to our own Milky Way. Holmberg IX was first discovered by astronomer Sidney van den Bergh in 1959, and catalogued as DDO 66. The galaxy received its "Holmberg IX" naming when it was discussed in Eric Holmberg's study of groups of galaxies ten years later. It is suspected that the dwarf galaxy was created as a result of a galactic interaction between M81 and neighbouring galaxy M82. Of the more than 20,000 stars that can be resolved in this Hubble image, only about 10% are considered to be old stars with ages of billions of years. The rest are thought to be young stars with ages of only 10 - 200 million years. Due to the Advanced Camera for Surveys' resolution in this image, astronomers have noted that the old and the young stars have distinct spatial distributions which might be related to their origin. Simulations predict that the triplet M81, M82, and nearby NGC 3077 had a close passage 200-300 million years ago. This close encounter may have triggered the newer star formation that has occurred in Holmberg IX. The bluish-white fuzz in the space surrounding M81 and Holmberg IX is new star formation triggered by gravitational interactions between the two galaxies. There are many low mass galaxies that form stars in nearby space. While none of these are as dominated by recently produced stars as Holmberg IX, they might be related to the same family. By understanding how Holmberg IX was formed, scientists hope to understand their role as building blocks of large galaxies.

Name: Holmberg IX, M81, M82
Type: Local Universe : Galaxy : Type : InteractingLocal Universe : Galaxy : Size : Dwarf
Category: Galaxies
Constellation: Ursa Major
Distance: 12 million light years
Release date: 8 January 2008, 20:00
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Acknowledgment: D. de Mello (Catholic University of America and GSFC)

Hubble maps dark matter web in a large galaxy cluster: These images reveal the distribution of dark matter in the supercluster Abell 901/902, composed of hundreds of galaxies. The image in the center shows the entire supercluster. Astronomers assembled this photo by combining a visible-light image of the supercluster taken with the MPG/ESO 2.2-meter telescope in La Silla, Chile, with a dark matter map derived from observations with the NASA/ESA Hubble Space Telescope. The magenta-tinted clumps represent a map of the dark matter in the cluster. Dark matter is an invisible form of matter that accounts for most of the Universe's mass. The image shows that the supercluster galaxies lie within the clumps of dark matter. Hubble cannot see the dark matter directly. Astronomers inferred its location by analyzing the effect of so-called weak gravitational lensing, where light from more than 60,000 galaxies behind Abell 901/902 is distorted by intervening matter within the cluster. Researchers used the observed, subtle distortion of the galaxies' shapes to reconstruct the dark matter distribution in the supercluster. The image was assembled by combining a visible-light image of the supercluster with a map of the dark matter distribution. The Hubble study pinpointed four main areas in the supercluster where dark matter has pooled into dense clumps. These areas match the location of hundreds of galaxies that have experienced a violent history in their passage from the outskirts of the supercluster into these dense regions. The four close-up images flanking the central photo are Hubble views of the four dense clumps of matter. To make this image, astronomers superimposed the dark matter map over a Hubble visible-light image of the supercluster galaxies. The images are part of the Space Telescope Abell 901/902 Galaxy Evolution Survey (STAGES), which covers one of the largest patches of sky ever observed by the Hubble telescope. The area surveyed is so wide that it took 80 Hubble images to cover the entire field. Hubble's Advanced Camera for Surveys made the observations in June and July 2005 and in January 2006.

Name: Abell 901, Abell 902
Type: Early Universe : Galaxy : Grouping : SuperclusterEarly Universe : Cosmology : Phenomenon : Dark Matter
Category: Cosmology
Distance: z=0.17 (redshift)
Release date: 10 January 2008, 16:00
Credit: Credit for the Hubble images: NASA, ESA, C. Heymans (University of British Columbia, Vancouver), M. Gray (University of Nottingham, U.K.), M. Barden (Innsbruck), and the STAGES collaboration Credit for the ground-based image: ESO, C. Wolf (Oxford University, U.K.), K. Meisenheimer (Max-Planck Institute for Astronomy, Heidelberg), and the COMBO-17 collaboration