A Bar Within a Bar

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Credit & Copyright: Thorsten Lisker (U. Basel), Victor P. Debattista (U. Washington), Ignacio Ferreras (KCL), Peter Erwin (MPE), STScI

More than half of all spiral galaxies have a bar in their center. Amongst the better known barred galaxies are M83, NGC 1300, NGC 1365 and our own Milky Way Galaxy. Because bars are not always present, Hubble needed a tuning fork to classify galaxies. Some galaxies may even have two bars, a large scale bar and a smaller nuclear bar, and are then referred to as double-barred. Nuclear bars have been known since the 1970s, but only with HST has it been possible to search for them systematically because of their small sizes. As a result, we now know that about one third of barred galaxies host a nuclear bar inside their main bar. Nuclear bars have been thought to be one way in which gas can be funneled down to the centers of galaxies to feed the monster black holes lurking there. This double-barred galaxy, GOODS J033230.93-273923.7, was discovered by us using the wealth of data collected by the Hubble Space Telescope's Advanced Camera for Surveys. It is the furthest known example of a double-barred galaxy, at a distance of 2.3 billion light years. The nuclear bar, which is 5000 light years across, is surrounded by small spiral arms and embedded within the main bar, which is 46,000 light years from end to end. This galaxy and a similar object found at a somewhat lower distance demonstrate once again the capabilities of HST.