AstroSat UV Deep Field IV. An Extended UV disk around a massive spiral galaxy at z=0.67

Abstract

Extended ultraviolet (XUV) emission in nearby disk galaxies supports the inside-out growth scenario through low-efficiency star formation in their outer regions. However, such detections have largely been limited to the local Universe ($z \sim 0$) due to the need for deep, high-resolution UV imaging. We report the detection of a clumpy XUV disk in a massive, isolated spiral galaxy ($\log(M_*/M_\odot) \approx 11.04$) at $z=0.67$, observed with AstroSat/UVIT. The intrinsic rest frame FUV surface brightness profile, corrected for the instrument PSF, shows a more extended disk than its optical and IR counterparts. The XUV disk reaches nearly twice the optical radius and includes a large UV-bright low surface brightness (LSB) region ($S_{LSB}/S_{K80}\approx15,\ \mu_{FUV}-\mu_K\approx0.8$), consistent with the Type II XUV definition. Additionally, the detection of UV clumps without optical counterparts supports a Type I classification, suggesting gravitational instabilities and recent star formation. These features point to recent cold gas accretion onto the outer disk. From the asymmetric light profile, we estimate a gas accretion rate of $\sim11\ M_\odot$ yr$^{-1}$, providing evidence of active disk growth at intermediate redshift.