Robust Detection of Hot Intragroup Medium in Optically Selected, Poor Galaxy Groups by eROSITA

Abstract

Over the last several decades, extensive research has been conducted on the baryon cycles within cosmic structures, encompassing a broad mass range from dwarf galaxies to galaxy clusters. However, a notable gap in understanding the cosmic baryon cycle is the poor galaxy groups with halo masses around 10 ^13 M _⊙ . Poor galaxy groups, like our own Local Group, are prevalent throughout the Universe, yet robust detection of their hot, X-ray-emitting intragroup medium (IGrM) has remained elusive. The presence of this hot IGrM is crucial for addressing the long-standing “missing baryons” problem. Previous ROSAT-based studies were limited by a small number of X-ray bright samples, thus restricting the scope of their findings. Here, we show a robust detection of this hot IGrM in a large, optically selected poor groups sample, based on the stacked X-ray images from the eROSITA Final Equatorial Depth Survey. These groups are identified in DESI Legacy Survey with a mass range of log( M _halo / h ^−1 M _⊙ ) = 11.5–13.5 and a redshift range of z = 0.1–0.5. Additionally, our results indicate that despite its presence in virtually groups at all sizes, this gas component is still not sufficient to recover the universal baryon fraction, and hence the “missing baryons” problem still persists in poor galaxy groups.

Keywords

• Galaxy groups
• X-ray astronomy
• Missing mass
• Hot intergalactic medium