Probing Black Hole/Galaxy Co-Evolution at Cosmic Dawn in High Definition with a Gravitationally Lensed Quasar

Gravitational lensing brightens the images of quasars by a significant amount and provides a powerful probe of the physical properties of quasar population and their host galaxies. At z=6.51, recently discovered quasar J0439 is the most distant gravitationally lensed quasar known and the first such object detected at the epoch of reionization. Shallow HST/ACS observations reveal a compact lensing system, with Einstein ring radius of 0.2" and a high magnification factor of 51 for the central point source. J0439 is the brightest quasar observed at both optical and far-infrared wavelengths, indicating strong black hole (BH) accretion activities accompanied by a powerful starburst with an intrinsic star formation rate SFR >1000 solar mass per year. A shallow, low resolution ALMA image has resolved the lensed quasar host into a 1"-long arc. We propose to carry out deep HST/WFC3 F125W and F160W imaging to study the quasar host galaxy. It will detect and spatially resolved the rest-frame UV radiation from the host galaxy, allowing measurements of its unobscured star formation rate and size. The combination of the new HST observations with approved deep ALMA observations will measure the host galaxy SED, and, further aided by the extra resolution gain from lensing magnification, probe star formation and dust obscuration in its nuclear region. With additional multi- wavelength observations from X-ray through radio, J0439 will provide a unique laboratory to study BH/galaxy co-evolution at cosmic dawn in unprecedented detail. In particular, future JWST observations would probe the ISM kinematics in this starburst/quasar system within the BH sphere of influence