Euclid: A UV-faint quasar in a highly luminous star-forming host galaxy at z ≈ 7.7

Constraining the co-evolution of supermassive black holes and their host galaxies in the first billion years after the Big Bang is essential for understanding the formation of the earliest cosmic structures. Here, we present IRAM /NOrthern Extended Millimeter Array (NOEMA) observations of the z approximate to 7.7 quasar EUCL J125308.55+705432.3, recently discovered in the first data release of the EuclidWide Survey. We report the most distant detections of [C II] 158 mu m and cold dust emission in a quasar host to date. The [C II] emission line sets the systemic redshift at z = 7.6980 +/- 0.0004. The source exhibits luminosities of L-FIR = 3.6 x 10(12) L-circle dot and L-[C II] = 2 x 10(9) L-circle dot, respectively, a dust mass of 1.4 x 10(8) M-circle dot, and a dynamical mass in the range 0.33-1.3 x 10(10) M-circle dot. Remarkably, despite being nearly two magnitudes fainter in the rest-frame UV (M-1450 = -24.06) than previously known z approximate to 7.5 quasars (< M-1450 > similar to -26.5), EUCL J125308.55 +705432.3 exhibits the brightest [C II] emission among them. This indicates that the host galaxy is actively star-forming, with a star-formation rate >250 M-circle dot yr(-1), consistent with recent findings that UV-faint quasars at z > 6 preferentially reside in [C II]-luminous galaxies. The UV-faintness likely reflects dust obscuration or sub-Eddington accretion, rather than lower host mass, suggesting these systems are at a di fferent stage in their evolution compared to UV-bright quasars. These IRAM /NOEMA observations highlight the power of combining Euclid's wide-area quasar discovery potential with submillimetre follow-up observations to characterise the host galaxies of early supermassive black holes across a broader redshift and luminosity range than previously accessible.