AT2025ulz and S250818k: Leveraging DESI spectroscopy in the hunt for a kilonova associated with a sub-solar mass gravitational wave candidate

Supplementary material to DESI's publication AT2025ulz and S250818k: DESI Host Galaxy Properties to comply with the data management plan. Abstract On August 18th, 2025, the LIGO–Virgo–KAGRA collaboration reported a sub-threshold gravitational wave candidate detection consistent with a sub-solar-mass neutron star merger, denoted S250818k. An optical transient, AT2025ulz, was discovered within the localization region. AT2025ulz initially appeared to meet the expected behavior of kilonova (KN) emission, the telltale signature of a binary neutron star merger. The transient subsequently rebrightened after ~5 days and developed spectral features characteristic of a Type IIb supernova. In this work, we analyze the observations of the host galaxy of AT2025ulz obtained by the Dark Energy Spectroscopic Instrument (DESI). From the DESI spectrum, we obtain a secure redshift of z = 0.084840±0.000006, which places the transient within 2σ of the gravitational wave distance. Our analysis of the host galaxy's spectral energy distribution reveals a star-forming, dusty galaxy with stellar mass ∼10^10 M⊙, broadly consistent with the population of both short gamma-ray bursts and core-collapse supernova host galaxies. We also present our follow-up of DESI-selected candidate host galaxies using the Fraunhofer Telescope at the Wendelstein Observatory, and show the promise of DESI for associating or rejecting candidate electromagnetic counterparts to gravitational wave alerts. These results emphasize the value of DESI’s extensive spectroscopic dataset in rapidly characterizing host galaxies, enabling spectroscopic host subtraction, and guiding targeted follow-up. Data Here we give the data for all figure as well as the spectrum of the host AT2025ulz. We include the host photometry, the various images, the host spectrum of AT2025ulz, the fastspecfit results of other sGRBs. The data used to compare the SFR and masses of GRBs and CC SN.适配数据管理计划的DESI研究补充材料：AT2025ulz与S250818k的DESI宿主星系属性 摘要 2025年8月18日，LIGO-Virgo-KAGRA合作组报告了一次亚阈值引力波候选体探测结果，该信号与亚太阳质量中子星并合事件相符，被命名为S250818k。在该信号的定位区域内，发现了光学暂现源AT2025ulz。该暂现源最初表现出符合千新星（kilonova，KN）辐射的预期特征，而千新星是双中子星并合的标志性电磁信号。后续该暂现源在约5天后再次增亮，并呈现出IIb型超新星的光谱特征。 本研究针对暗能量光谱仪（Dark Energy Spectroscopic Instrument，DESI）获取的AT2025ulz宿主星系观测数据展开分析。通过DESI光谱，我们得到了精度可靠的红移值z=0.084840±0.000006，该红移对应的距离使得暂现源处于引力波距离的2σ置信区间内。对宿主星系的光谱能量分布分析表明，该星系为正在进行恒星形成活动的尘埃星系，恒星质量约为10^10 M⊙，这与短伽马射线暴（short gamma-ray bursts，sGRBs）和核心坍缩型超新星的宿主星系群体特征大体相符。我们还展示了使用文德尔施泰因天文台（Wendelstein Observatory）的夫琅禾费望远镜（Fraunhofer Telescope）对DESI筛选出的候选宿主星系开展的后续观测，并证实了DESI在关联或排除引力波预警对应候选电磁对应体方面的应用潜力。上述结果凸显了DESI大规模光谱数据集的重要价值：其可快速完成宿主星系的特征刻画、实现光谱宿主扣除，并为靶向后续观测提供指导。 数据 本部分提供所有图表所需数据以及AT2025ulz宿主星系的光谱数据。涵盖内容包括宿主星系测光数据、各类观测图像、AT2025ulz的宿主光谱，以及其他短伽马射线暴（short gamma-ray bursts，sGRBs）的fastspecfit拟合结果。本次提供的数据用于对比伽马射线暴与核心坍缩型超新星的恒星形成率（Star Formation Rate，SFR）及质量参数。