The RESOLVE and ECO Gas in Galaxy Groups Initiative: The Group Finder and the Group H i–Halo Mass Relation

We present a four-step group-finding algorithm for the Gas in Galaxy Groups (G3) initiative, a spin-off of the z ∼ 0 REsolved Spectroscopy Of a Local VolumE (RESOLVE) and Environmental COntext (ECO) surveys. In preparation for future comparisons to intermediate redshift (e.g., the LADUMA survey), we design the group finder to adapt to incomplete, shallow, or nonuniform data. We use mock catalogs to optimize the group finder’s performance. Compared to friends-of-friends (with false-pair splitting), the G3 algorithm offers improved completeness and halo-mass recovery with minimal loss of purity. Combining it with the volume-limited H i census data for RESOLVE and ECO, we examine the H i content of galaxy groups as a function of group halo mass. Group-integrated H i mass M H I,grp rises monotonically over halo masses M halo ∼ 1011–1014.5 M ⊙, pivoting in slope at M halo ∼ 1011.4 M ⊙, the gas-richness threshold scale. We present the first measurement of the scatter in this relation, which has a median of ∼0.3 dex and is asymmetric toward lower M H I,grp. We discuss interesting tensions with theoretical predictions and prior measurements of the M H I,grp–M halo relation. In an appendix, we release RESOLVE DR4 and ECO DR3, including updates to survey redshifts, photometry, and group catalogs, as well as a major expansion of the ECO H i inventory with value-added data products.

我们为星系群中的气体（G3）项目提出了一种四步的星系群寻找算法，该项目是本地体积的约0红移解析光谱（RESOLVE）和环境背景（ECO）调查的一个分支。为了将来与中等红移（例如，LADUMA调查）进行比较做准备，我们设计的星系群寻找器能够适应不完整、较浅或不均匀的数据。我们使用模拟星表来优化星系群寻找器的性能。与朋友的朋友算法（带有错误对拆分）相比，G3算法在纯度损失最小的情况下提高了完整性和晕质量恢复能力。将其与RESOLVE和ECO的体积限制氢Ⅰ普查数据相结合，我们研究了星系群的氢Ⅰ含量随星系群晕质量的变化。星系群积分氢Ⅰ质量$M_{HI,grp}$在晕质量$M_{halo}\sim10^{11}-10^{14.5}M_{\odot}$范围内单调上升，在晕质量$M_{halo}\sim10^{11.4}M_{\odot}$（富气阈值尺度）处斜率发生转折。我们首次测量了这种关系中的弥散度，其中值约为0.3 dex，并且在较低的$M_{HI,grp}$方向上不对称。我们讨论了与理论预测以及先前对$M_{HI,grp}-M_{halo}$关系的测量之间有趣的矛盾。在附录中，我们发布了RESOLVE DR4和ECO DR3，包括对调查红移、光度测量和星系群星表的更新，以及ECO氢Ⅰ清单的大幅扩展以及增值数据产品。