Anne’s Picture of the Day: Spiral Galaxy NGC 4402

read more at Anne’s Astronomy News http://annesastronomynews.com/

April 15, 2013

NGC 4402, an edge-on spiral galaxy in Virgo

Image Credit: NASA/ESA and Wikisky

NGC 4402 is an edge-on spiral galaxy of about 55 thousand light-years across, located some 55 million light-years away from Earth in the constellation of Virgo, while it is receding from us at approximately 232 kilometers per second. It is a member of the relatively nearby Virgo Cluster of galaxies. In 1976 the supernova SN 1976B was observed in this galaxy.

This image shows striking visual evidence for a galaxy being stripped bare of its star-forming material by its violent ongoing encounter with the hot gas in the center of a galaxy cluster.

An extremely hot X-ray-emitting gas known as the intra-cluster medium lurks between galaxies within clusters. As the galaxy moves toward the center of the cluster (located out of the image toward the bottom left), it experiences a “wind” from the hot intra-cluster medium, which can reach temperatures of millions of degrees. This hot wind strips out the much cooler gas and dust in the galaxy, the material for new stars, and once this gas is stripped, the galaxy can no longer form new stars and becomes ‘dead’.

There are at least four key lines of evidence for declaring that this so called “ram-pressure stripping” process is cleaning out this galaxy:

• First, the dust disk appears to be truncated, meaning that the light from stars extends out well beyond where gas and dust is observed. Since stars are born in clouds of gas and dust, this suggests that some of the material must have been stripped from the galaxy after the stars were born.
• Second, the dusty disk appears to be “bowed” upward; that is, it has been bent by the wind blowing from the southeast (from the lower left of the image).
• Third, it appears that the light emitted by the north side of the stellar disk has been reddened and dimmed by dust that has been pushed up in front of it by the pressure of the cluster gas. Simultaneously, the dust to the south of the disk has been removed, revealing young blue stars glowing behind it.
• Finally, some of the most unusual features of NGC 4402 are the linear filaments of dust to the south of its main disk. These remarkable filaments originate in clumps that appear to be the densest remnants of the now displaced disk of the galaxy.

This extremely disruptive ram pressure stripping process is believed to be a major influence on the evolution of galaxies and their star-forming ability over time, especially in very dense regions of the Universe like galaxy clusters.

But, the filaments of NGC 4402 are also being stripped away by the hot galaxy cluster wind. This strips away the outer layers of the clouds; the dust from these layers is then pushed away. In one case (the eastern or leftmost filament), we can see that the wind has either triggered star formation toward the tip of one of these dense clumps or exposed an already-existing star forming region.

The bright blue clusters of young stars in the bottom left region of the galaxy’s disk is further evidence of recently triggered star formation.

This image was taken with the Advanced Camera for Surveys on the Hubble Space Telescope using optical and infrared filters.
The text has been partially obtained of the National Optical Astronomy Observatory/Association of Universities for Research in Astronomy (NOAO/AURA)

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