16762

The recently discovered galaxy BST 1047+1156 is the most diffuse star-forming galaxy found to date. Located in the Leo I galaxy group (D=11 Mpc) comma its extremely diffuse nature tests models of both galaxy formation and star formation at the lowest densities. BST1047.s extraordinarily blue optical colors and UV emission both signal a recent episode of star formation in the object comma even though its HI column density is well below that in which star formation typically occurs. Exactly how BST1047 formed remains uncleardoublePoint it may be either a long-lived but extremely diffuse LSB galaxy caught in a tidal interaction comma or a recently spawned tidal dwarf caught in the throes of disruption. Either scenario places BST1047 at the extreme limits of our understanding of galaxy evolution processes. To resolve the question of BST1047.s origins comma we propose deep F606W-F814W imaging to construct a color magnitude diagram of its resolved stellar population and constrain the ages and metallicities of its stars. This will cleanly differentiate between the two formation scenarios comma as the presence of old red giant branch stars would rule out the young tidal dwarf model. A purely young stellar population would mark BST1047 as a disrupting tidal dwarf comma the first such object known comma providing a new link between tidal interactions and dwarf galaxy formation and disruption. We will also place BST1047 on the well-determined stellar mass-metallicity relationship for normal star forming galaxies comma providing another important test of its origins. Finally comma we will use the BST1047.s populations to probe the spatial distribution of recent and past star formation in this anomolous object.

近期发现的星系BST 1047+1156是迄今已知最弥散的恒星形成星系。该星系位于狮子座I星系群（距离=11百万秒差距，D=11 Mpc），其极端弥散特性对星系形成模型及低密度环境下的恒星形成模型均构成挑战。BST 1047异常蓝的光学颜色与紫外辐射均表明其近期发生过恒星形成事件，尽管其中性氢(HI)柱密度远低于恒星形成通常发生的阈值。BST 1047的形成机制尚不明确，它可能是长期存在但极端弥散的低表面亮度星系（LSB galaxy），正处于潮汐相互作用中；也可能是近期形成的潮汐矮星系，正处于瓦解的阵痛之中。无论哪种情形，BST 1047都处于我们对星系演化过程认知的极端边界。为解决BST 1047的起源问题，我们建议采用深度F606W-F814W成像观测，构建其解析恒星族（resolved stellar population）的颜色星等图，并约束其恒星的年龄与金属丰度。这将清晰区分两种形成场景：若存在年老红巨星分支恒星，则可排除年轻潮汐矮星系模型；若为纯年轻恒星族，则BST 1047将被标记为正在瓦解的潮汐矮星系——这是已知首个此类天体，为潮汐相互作用与矮星系形成及瓦解之间提供了新的联系。我们还将把BST 1047置于正常恒星形成星系已确立的恒星质量-金属丰度关系中，为其起源提供另一项重要检验。最后，我们将利用BST 1047的星族，探测该异常天体中近期与过去恒星形成的空间分布。