ner_tags: O O O O O O O O O O B-Model I-Model O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O B-Model I-Model I-Model B-Model O O B-Software I-Software B-Software O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model O O O O O O O O O B-Formula I-Formula I-Formula I-Formula I-Formula B-Formula I-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model I-Model O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O B-Model I-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
tokens: Fig. 12 shows a time frame of each of the gasoline2 simulations compared to the gas surface density data of Corbelli et al. ( 2014 ) in the bottom right. Best fits were found via a combination of by eye and an automated routine using a structural similarity index (SSIM; built into python ’s scipy module) of images of the gas surface density. The simulated disc is first inclined at the orientation of M33, then rotated in increments to assess the suitability of different phase angles. These images are smoothed over a scale of 2 kpc to leave only the strongest features. The SSIM is then calculated for each time-stamp and phase angle (Δ t = 10 My, Δ θ = 10° using the time domain of 0.5–1.0 Gyr, resulting in 1800 images for each calculation). The best fitting of these low-resolution comparisons are then shown in Fig. 12 , now at their natural resolution. Figure 12. Best-fitting gasoline2 models compared to M33 gas distribution (lower right). In Fig. 12 we show one of the better-fitting snapshots of each simulation, orientated in the same manner as the observed M33. Certain features like the arms shown in Fig. 8 are seen in some of these models, with accompanying inter-arm voids. The lowest feedback model displays some very similar features; the upper-right arm structure, the irregularly shaped arm north-west of the centre, and the straight-arm segment in the edge of disc to the south-east. The nature of dynamic spiral features such as these allows for spiral arms that are less regular than the assumed fixed log-spiral features used by density wave-like potentials (Grand, Kawata Cropper 2012 ; Baba, Saitoh Wada 2013 ). Cavities in the gas seen in M33 are a simple result of inter-arm voids. However, the gas does not display the same smaller scale features, like the irregular patchy features seen in the inter-arms of M33. The 5 per cent feedback model shows a more disrupted spiral structure. Similar small arm features are seen as previously, though now the increased feedback stunts their growth considerably. This appears one of the poorer matches to the observational data, with both the feedback cavities and inter-arm regions too small to provide a good match for M33. The remaining higher feedback models provide a good match to the general structure of M33, though neither is a precise match. The 20 per cent model is very effective at creating large ISM cavities, much like the sphNG models, though this comes at the expense of creating very few strong arm features. The cavities in the 20 per cent model may even be too large and numerous, with the disc perhaps showing too great a degree of fragmentation compared to M33. The 10 per cent feedback model is a good middle ground, creating a handful of large cavities but also managing to maintain some elongated arm features. We found that many of the snapshots not shown here produced certain features very well, while missing others entirely. Enough realizations could feasibly eventually produce every feature