Serendipitous discovery of a young cluster of galaxies at $z \sim 0.5$ projected next to the nearby tadpole galaxy KUG 1138 + 327

Résumé : 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 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 $\gtrsim 5 \times 10^{7}$~yr. Assuming energy equipartition between cosmic rays and magnetic fields, we estimate the lobe's total energy to be $\sim 9 \times 10^{56}~\mathrm{erg}$, 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.