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tokens: in Section 2.3.2 . 3.3. Gravitationally Unstable Disk Figure 3 (right) shows the simulated observations for a gravitationally unstable disk with Σ ∝ R −0.5 and T ∝ R −0.75 . The disk mass is 0.24 M ⊙ , leading to a disk-to-star-mass ratio of 0.49. Even with this relatively massive disk, the combined system mass lies within the limits discussed in Section 2.3.1 . The simulated unsharp masking observation reproduces the large-scale spiral arms, the dark crescents, and the bright inner ellipse, while the deprojected image shows two large-scale spirals analogous to Figure 1 (right). 4. Discussion The combination of our hydrodynamic modeling and simulated observations allows us to put strong constraints on the disk structure from which the submillimeter continuum emission originates. Throughout all of our models, we find that a steeply declining surface mass density beyond ≈300 au is key to producing a close match to the masked image. Figure 4 shows the scaled radial intensity profile of our successful self-gravitating disk and the radial intensity profile of the successful companion simulations compared to the observational data from Pérez et al. ( 2016 ) and the model of Andrews et al. ( 2009 ). The steep decline beyond ≈300 au in our simulation manifests itself in the form of a steep emission profile at large radii that turns over at approximately the right radius, matching the observed profile well. The modest scaling factor for the self-gravitating disk (∼1.5) is well within uncertainties associated with dust-to-gas ratios and/or grain opacities for circumstellar disks. Figure 4. Radial emission profiles for the successful gravitationally unstable disk simulation (red), which has been decreased by a factor of ∼1.5 (see the text) and the successful companion disk simulation (blue). Also shown are data from Pérez et al. ( 2016 ; black points) and the best-fit model from Andrews et al. ( 2009 ; black dashes). The steep decline beyond ≈300 au is key to matching the observed morphology in the simulated images. We note that the resulting Toomre profile dips marginally below 1 for a limited radial range in our successful self-gravitating disk. However, this disk shows no sign of fragmentation even though we evolve it for many dynamical times for this radial range. On the other hand, a companion beyond the spirals (≳300 au) is unlikely to have formed by core accretion. This suggests that, should the companion hypothesis be correct, this would hint that the Elias 2–27 disk may have formed a fragment by gravitational instability in the past. We also stress the importance of applying an unsharp mask to our simulated observations. While such a mask is primarily applied to increase the contrast of the image, subtle changes are introduced to the resulting image that allow us to remove models from consideration. Changes in the surface density profile affect the radial locations at which there is an excess or deficit with respect to a Gaussian mask. This