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The hidden arms of M94
KEITH COOPER
ASTRONOMY NOW
Posted: 14 January 2010


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This amazing ultra-deep picture of M94, just 15.1 million light years away, is shedding light on the hidden nature of this spiral galaxy by uncovering two new spiral arms that nobody realised were even there.

R Jay GaBany’s enhanced image of M94, showing the extended spiral arms. Visit his website at http://www.cosmotography.com.

Believe it or not this picture wasn’t taken by the Hubble Space Telescope or the Very Large Telescope in Chile, but by amateur R Jay GaBany using a 0.5-metre Ritchey–Chrètien telescope and an SBIG STL-11100 CCD at the Blackbird Remote Observatory in New Mexico, USA. GaBany, one of the finest deep imagers in the world, first photographed M94 back in 2006, capturing is compact inner disc with a ghostly ring of stars encircling it. Stellar rings are not so unusual, and can be formed after a galaxy has experienced a direct hit with another, smaller galaxy, creating the ring like a stone generating ripples in a pond.

GaBany’s picture captured the attention of a team of astronomers at the Astrophysical Institute of the Canary Islands led by Dr David Martinez–Delgado, as well as astronomers at Cambridge and Cardiff universities. They noticed tantalising hints of structure in the ring and so, suspecting some other phenomenon was at work, they recruited GaBany to join their team to figure out what secrets M94 was hiding.

The multiwavelength view of M94, clearly showing the spiral arms in ultraviolet and far infrared light. Click here for larger version. Image: Spitzer Legacy Programme/GALEX Nearby Galaxy Survey/R Jay GaBany.

To their surprise, by enhancing GaBany’s original image and adding data taken at ultraviolet wavelengths by NASA’s Galaxy Evolution Explorer, infrared data from the Two Micron All Sky Survey, 2MASS and the Spitzer Space Telescope, and extra optical data from the Sloan Digital Sky Survey, they discovered that M94 didn’t have an outer ring at all. Instead, it had an outer disc formed from two giant spiral arms protruding from the main body of the galaxy. Furthermore, the inner part of the Galaxy – the bit that we see when we look through our telescopes – wasn’t a disc at all, but a so-called oval distortion. These lens-shaped features are, for want of a better term, ‘fat bars’. Galactic bars are produced from instabilities in the disc of a galaxy, and we see many examples in the Universe, including our own Milky Way Galaxy (see our related news story here). The bar in M94 is much wider than most, and we have seen in other galaxies and simulations that oval distortions not only lead to a set of inner spiral arms, but they can also disrupt the outer regions of the galaxy, creating the huge and faint spiral arms that the team found.

R Jay GaBany’s original, un-enhanced image of M94 taken in 2006 and showing the spiral arms as merely a ghostly ring of glowing light.

These spiral arms can barely be discerned in optical light, requiring lots of image enhancement to bring them out in GaBany’s picture. On the other hand they stand out brilliantly in ultraviolet and infrared wavelengths, which trace hot young stars and glowing clouds of star-forming gas and dust in the arms. In total, these ‘hidden’ arms contribute almost a quarter of the total stellar mass of M94, and contribute up to 15 percent of new stars in the galaxy. In fact, given the amount of star-forming material on hand in the outer spiral arms, this region is the most efficient in producing stars anywhere in M94. Despite that, in all this time we never realised the arms were there.

For Jay GaBany, it has been an experience never to forget. “Over a period of three and a half years, I continued to enhance my image and assist the team as they probed this galaxy in infrared and ultraviolet wavelengths. Working with data from spacecraft was a learning experience that I still cherish,” he exudes. The team’s work is published in The Astrophysical Journal, and you can see Jay GaBany’s fantastic work at his website, http://www.cosmotography.com.

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