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Shelling a galaxy

Submitter: Erwin de Blok
Description: The picture shows an optical image of the galaxy NGC 4414. Recently this galaxy was observed with the Westerbork telescope (as part of the HALOGAS survey) to obtain the ultra-deep maps of the neutral hydrogen. In parallel we have also been obtaining deep optical images of the HALOGAS galaxies. NGC 4414 threw up some surprises! The black and white picture shows (in negative) the high-contrast, deep image. Superimposed in colour is the "normal contrast" picture from the Sloan Digital Sky Survey. We can immediately see that the stellar disk of NGC 4414 is much more extended than the colour image suggests. Also, surrounding the galaxy we see a "shell", a sharply-defined arc of starlight. Computer simulations have shown that such shells can form when a small dwarf galaxy gets disrupted by a larger galaxy. The shell is just a part of the remnant of the dwarf galaxy. The neutral hydrogen observations are currently being analysed, and will be shown in a future AJDI... (Deep image obtained by Maria Patterson from the HALOGAS collaboration at Kitt Peak Observatory, USA).
Copyright: HALOGAS collaboration
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