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Star formation in the outer regions of the early type galaxy NGC 4203

NGC 4203 is a nearby early-type galaxy surrounded by a very large, low-column-density HI disc. In this paper we study the star formation efficiency in the gas disc of NGC 4203 by using the UV, deep optical imaging and infrared data. We confirm that the HI disc consists of two distinct components: an inner star forming ring with radius from $\sim$ 1 to $\sim$ 3 R$_{eff}$, and an outer disc. The outer HI disc is 9 times more massive than the inner HI ring. At the location of the inner HI ring we detect spiral-like structure both in the deep $g'-r'$ image and in the 8 $μ$m $Spitzer$-IRAC image, extending in radius up to $\sim$ 3 R$_{eff}$. These two gas components have a different star formation efficiency likely due to the different metallicity and dust content. The inner component has a star formation efficiency very similar to the inner regions of late-type galaxies. Although the outer component has a very low star formation efficiency, it is similar to that of the outer regions of spiral galaxies and dwarfs. We suggest that these differences can be explained with different gas origins for the two components such as stellar mass loss for the inner HI ring and accretion from the inter galactic medium (IGM) for the outer HI disc. The low level star formation efficiency in the outer HI disc is not enough to change the morphology of NGC 4203, making the depletion time of the HI gas much too long.