Tentative detection of the circumgalactic medium of the isolated low-mass dwarf galaxy WLM

Abstract

We report a tentative detection of the circumgalactic medium (CGM) of Wolf–Lundmark–Melotte (WLM), an isolated, low-mass (logM*/M⊙ ≈ 7.6), dwarf irregular galaxy in the Local Group (LG). We analyse an HST/COS archival spectrum of a quasar sightline (PHL2525), which is 45 kpc (0.5 virial radius) from WLM and close to the Magellanic Stream (MS). Along this sightline, two ion absorbers are detected in Si II, Si III, Si IV, C II, and C IV at velocities of ∼−220 km s⁻¹ (Component v-220) and ∼−150 km s⁻¹ (Component v-150). To identify their origins, we study the position–velocity alignment of the components with WLM and the nearby MS. Near the magellanic longitude of PHL2525, the MS-related neutral and ionized gas moves at ≲−190 km s⁻¹, suggesting an MS origin for Component v-220, but not for Component v-150. Because PHL2525 passes near WLM and Component v-150 is close to WLM’s systemic velocity (∼−132 km s⁻¹), it is likely that Component v-150 arises from the galaxy’s CGM. This results in a total Si mass in WLM’s CGM of M^(CGM)_(Si)∼(0.2−1.0)×10⁵ M⊙ using assumption from other COS dwarf studies. Comparing M^(CGM)_(Si) to the total Si mass synthesized in WLM over its lifetime (∼1.3 × 10⁵ M⊙), we find ∼3 per cent is locked in stars, ∼6 per cent in the ISM, ∼15–77 per cent in the CGM, and the rest (∼14–76 per cent) is likely lost beyond the virial radius. Our finding resonates with other COS dwarf galaxy studies and theoretical predictions that low-mass galaxies can easily lose metals into their CGM due to stellar feedback and shallow gravitational potential.