A rare type of spiral galaxy was photographed in detail by Hubble

The photography was interrupted after 80 exposures, due to a malfunction in the Hubble survey camera, and will be resumed, it is hoped, after the last upgrade of the telescope, which is currently expected in May

The Hubble Space Telescope has sent a spectacular image of an unusual spiral galaxy in the Coma galaxy cluster. This image was created from data collected by the Advanced Survey Camera on Hubble. It reveals fine details in the galaxy, NGC 4921, as well as a rich background of more distant galaxies, all the way back to the early universe.

The Coma galaxy cluster, in the northern constellation Bernica Hair, is one of the largest galaxies in the nearby universe. The cluster, also known as Abel 1656, lies 320 million light-years from Earth and contains over a thousand galaxies. The brightest galaxy in the cluster, NGC 4921, seen here, was discovered in the late 18th century by astronomer William Herschel.

The galaxies in the rich cluster had many encounters and mergers between them and tended to turn spiral galaxies into ellipticals without much star formation activity. As a result, there are many more elliptical galaxies and slightly more spiral galaxies in the cluster than in quieter regions of the universe.
NGC 4921 is one of the rare examples of a spiral galaxy in the floor cluster, and it is also quite unusual - it is an example of an "anemic spiral galaxy" in which the rate of star formation in the arms is much less massive, and as a result, only a thin cloud of gas surrounds the galaxy in a ring, containing a few Bright, young blue stars that can be spotted thanks to Hubble's sharp eyesight. Most of the structure of the pale spirals in the outer parts of the galaxy is unusually smooth and gives the whole galaxy the appearance of a ghost galaxy reminiscent of a semi-transparent jellyfish.

Hubble's long exposure times and sharp vision also made it possible not only to photograph NGC 4921 in fine detail but also to see far behind it into the distant universe. From all directions and even through the galaxy itself, thousands of galaxies of all shapes, sizes and colors are visible. Many of the galaxies appear to have an irregular shape from before the division between spiral and elliptical galaxies.

The team led by Kam Cook of the Lawrence Livermore National Institute in California used Hubble to detect cupid variables in NGC 4921 with the goal of using them to measure the distance to Bernica hairs and thus the expansion rate of the universe. Unfortunately, they were unable to complete the observation due to the malfunction that occurred in 2007 with the Advanced Survey Camera, and they intend to continue the observation after the Hubble upgrade mission is completed.

The spectacular image was obtained by combining fifty different exposures through a yellow filter and another 30 exposures through a near infrared filter, using the wide field channel of Hubble's Advanced Survey Camera. The total exposure time was 17 hours and 10 hours respectively.

As we recall this week, NASA announced that the launch of the space shuttle Atlantis for mission STS-125 to upgrade the Hubble Space Telescope will take place on May 12 this year (2009).

For the announcement of the Space Telescope Science Center