A radially broad collisional cascade in the debris disk of $\gamma$ Ophiuchi observed by JWST

The A1V star $\gamma$ Oph, at a distance of 29.7 pc, is known from Spitzer imaging to host a debris disk with a large radial extent and from its spectral energy distribution to host inner warm dust. We imaged $\gamma$ Oph with JWST/MIRI at 15 and 25.5 microns, which reveal smooth and radially broad emission that extends to a radius of at least 250 au at 25.5 microns. In contrast to JWST findings of an inner small-grain component with distinct ringed substructures in Fomalhaut and Vega, the mid-infrared radial profile combined with prior ALMA imaging suggests a radially broad steady-state collisional cascade with the same grain size distribution throughout the disk. This further suggests that the system is populated by a radially broad planetesimal belt from tens of au or less to well over 200 au, rather than a narrow planetesimal belt from which the observed dust is displaced to appear broad. The disk is also found to be asymmetric, which could be modelled by a stellocentric offset corresponding to a small eccentricity of $\sim$0.02. Such a disk eccentricity could be induced by a mildly eccentric $<$$10\,M_\mathrm{Jup}$ giant planet outside 10 au, or a more eccentric companion up to stellar mass at a few au, without producing a resolvable radial gap in the disk.