Serendipitous Discovery of a Young Cluster of Galaxies at <i>z</i> ∼ 0.5 Projected Next to the Nearby Tadpole Galaxy KUG 1138 + 327

Abstract Using a 90 ks Chandra ACIS-S observation in the 0.3–7 keV band, along with complementary Low-Frequency Array and Karl G. Jansky Very Large Array data in the 120–168 MHz and 1–2 GHz ranges, we study the diffuse emission around the nearby dwarf galaxy KUG 1138+327. Our analysis reveals a diffuse X-ray feature on the southern side, disconnected from the galactic disk. This feature exhibits a hard X-ray spectrum, which is highly unusual for an outflow from a dwarf galaxy. We interpret the irregularly shaped feature as hot plasma in a young galaxy cluster at redshift 0.5, supported by X-ray spectral fitting and consistent with the optical redshift of the central elliptical galaxy of a known cluster identified by a red sequence. Additionally, we detect a radio lobe east of the X-ray feature, likely produced by an active galactic nucleus (AGN) offset from the cluster center and confined primarily by ram pressure from the ambient medium. The lobe shows a steep nonthermal radio spectrum, suggesting a cosmic-ray age of ≳5 × 10 7 yr. Assuming energy equipartition between cosmic rays and magnetic fields, we estimate the lobe’s total energy to be ∼9 × 10 56 erg, comparable to the thermal energy in the same volume. This study thus identifies a background young cluster projected next to KUG 1138+327 and highlights the potentially significant role of off-center AGN feedback in shaping the intracluster medium.