Appearances can be deceiving. The spiral M 90 and the irregular galaxy IC 3583 are close together on the sky, as seen in the image above, but they are in fact well-separated from one another. While M 90 is one of the largest and brightest spirals in the Virgo cluster, at a distance of about 60 million light-years from us, IC 3583 is actually outside the cluster, lying some
20 million light-years in the foreground . The pair form the 76th entry in Arp's 1966 Atlas of Peculiar Galaxies, and had long been thought to be gravitationally interacting, however their large separation makes that seem unlikely.
Also in contrast to my suggestive framing of the image, M 90 is not in hot pursuit of IC 3593. The irregular galaxy and the Virgo cluster as a whole are moving away from us at similar speeds, in excess of 1000 km/sec, while M 90 is moving in our direction at about 230 km/sec, due to its gravitational interaction with the cluster's enormous mass, and
may eventually escape the cluster .
M 90 has very smooth spiral arms that lack the knotted star-forming regions of more typical spirals, and is sometimes referred to as an "anemic" galaxy. It has had much of its gas content stripped away by ram pressure, which is created by its
motion through the intra-cluster medium .
This image is the result of of about 10 hours of integration, divided roughly equally between luminance and colour (RGB), taken over four nights between April 7 and April 14. The plate scale is 0.62"/pixel, and the field-of view is about 26'x20'.
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