Infrared Legacy

 

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Messier 51, also known as the "Whirlpool Galaxy" or as NGC 5194, is located about 37 million light-years away in the constellation Canes Venatici. This Spitzer Space Telescope image is a four-color composite of invisible infrared light, showing emissions from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange) and 8.0 microns (red). These wavelengths are roughly 10 times longer than those seen by the human eye. The Spitzer image reveals strange structures bridging the gaps between the dust-rich spiral arms, and traces the dust, gas and stellar populations in both the bright spiral galaxy and its companion.

The light seen in the image originates from very different sources. At shorter infrared wavelengths (3.6 to 4.5 microns), the light comes mainly from stars. This starlight fades at longer wavelengths (5.8 to 8.0 microns), where we see the glow from clouds of interstellar dust. This dust consists mainly of a variety of carbon-based organic molecules known collectively as polycyclic aromatic hydrocarbons. Wherever these compounds are found, there will also be dust granules and gas, which provide a reservoir of raw materials for future star formation.

Particularly puzzling are the large number of thin filaments of red emission seen between the arms of the large spiral galaxy. In contrast to the beady nature of the dust emission seen in the arms themselves, these spoke-like features are thin and regular, and prevalent in the gaps all over the face of the galaxy.

Also of interest is the contrast in the distributions of dust and stars between the spiral and its faint companion. While the spiral is rich in dust, bright in the longer infrared wavebands, and actively forming new stars, its blue companion shows little infrared emission and hosts an older stellar population. The spectacular whirlpool structure and star formation in Messier 51 are thought to be triggered by an ongoing collision with its companion. Understanding the impact on star formation by the interaction of galaxies is one of the goals of these observations.

The Spitzer observations of M51 are part of a large 500-hour science project, known as the Spitzer Infrared Nearby Galaxy Survey, which will comprehensively study 75 nearby galaxies with infrared imaging and spectroscopy. From these data, astronomers will probe the physical processes connecting star formation to the properties of galaxies. This information will provide a vital foundation of data, diagnostic tools, and astrophysical inputs for understanding the distant universe, ultraluminous galaxies, and the formation and evolution of galaxies.

Image credit: NASA/JPL-Caltech/R. Kennicutt (Univ. of Arizona)