GULP: Galaxy UV Legacy Project

The Galaxy UV Legacy Project (GULP) is a Cycle 28 Treasury program with the Hubble Space Telescope (HST) designed to characterize resolved massive stars, OB associations, and young star clusters (YSCs) in 26 nearby star-forming galaxies. Utilizing the ACS/SBC F150LP and WFC3/UVIS F218W filters, combined with extensive archival observations, GULP provides an unprecedented panchromatic 8-band view from the Far-UV to the I-band. The target galaxies were carefully selected to span a broad range of metallicities, masses, morphological types, and star formation rates, thereby enabling detailed studies of star formation processes across different galactic environments.
This paper introduces the GULP survey, detailing its observational strategy, data processing, and initial scientific results for the irregular barred starburst dwarf galaxy NGC 4449, used as a test case. We derived the physical parameters and ages for thousands of stars using the Binary Populations And Spectral Synthesis (BPASS) models, and found that the younger stars and clusters are predominantly concentrated along the galaxy's central bar, and that over the past <50 Myr star formation progressively migrated from northeast to southwest.
We used the F150LP, F218W, and F275W filters to investigate how the "UV-bump" at lambda 2175 AA correlates with the intensity of the UV radiation. The "UV-bump" is detected in many areas of the galaxy, but is absent in the regions of most intense and recent star formation. This strongly supports the scenario where UV radiation from young, massive stars effectively destroys the small dust grains responsible for the UV-bump''.

星系紫外遗产项目（Galaxy UV Legacy Project, GULP）是哈勃空间望远镜（Hubble Space Telescope, HST）第28周期的金库专项项目，旨在对26个邻近恒星形成星系中的已解析大质量恒星、OB星协和年轻星团（young star clusters, YSCs）进行特性表征。该项目利用ACS/SBC F150LP与WFC3/UVIS F218W滤光片，并结合海量存档观测数据，提供了从远紫外到I波段的前所未有的全色八波段观测视野。

项目目标星系经过精心遴选，覆盖了宽泛的金属丰度、质量、形态类型以及恒星形成率范围，从而能够针对不同星系环境中的恒星形成过程开展精细化研究。

本文详细介绍了GULP巡天项目，阐述了其观测策略、数据处理流程，并以不规则棒状星暴矮星系NGC 4449作为测试案例，展示了初步科学成果。研究团队采用双星种群与光谱合成（Binary Populations And Spectral Synthesis, BPASS）模型，为数千颗恒星推导了物理参数与年龄信息；研究发现，年轻恒星与星团主要集中在该星系的中央棒状结构中，且在过去不足5000万年的时间里，恒星形成活动逐渐从东北方向向西南方向迁移。

研究团队借助F150LP、F218W与F275W滤光片，探究了波长2175埃处的"紫外隆起（UV-bump）"与紫外辐射强度之间的关联。该"紫外隆起"在星系的诸多区域均有被探测到，但在恒星形成活动最为剧烈且新近的区域中却不存在。这一发现有力支持了如下理论：年轻大质量恒星发出的紫外辐射会有效破坏产生该"紫外隆起"的微小尘埃颗粒。