tokens: 3. Enclosed gas mass profiles as a function of radius for the known potential Galactic YMCs precursors. The transition point from solid to lower opacity indicates the beam resolution of the data. The triangle markers correspond to the initial conditions used by Banerjee Kroupa ( 2013 ) to simulate the monolithic formation of the R136 YMC. The circular markers correspond to the initial conditions used by Banerjee Kroupa ( 2014 ) to simulate the monolithic formation of the NGC 3603 YMC. The star markers correspond to the initial conditions given in the fifth row of table 1 in Assmann et al. ( 2011 ) to simulate a monolithically forming star cluster. As these simulations are 3D models, and the data that we discuss in this paper are 2D projections, we must take care to ensure that we are making a fair comparison to these models. In Banerjee Kroupa ( 2013 , 2014 ), they present their initial conditions as initial cluster mass, initial gas mass and half-mass radius. Using their assumed ∼33 per cent SFE, we combine these masses to obtain the total gas mass prior to star formation. We also use the following relation to convert the half-mass radius to the Plummer radius (Heggie Hut 2003 ) – Assmann et al. ( 2011 ) already provide the Plummer radius. We then take the total 3D gas mass for all of the models and convert this to a projected mass (Heggie Hut 2003 ) – Overall Fig. 3 shows that the initial projected masses within a given radius for these various models generally too high, particularly at smaller radii. We note that the Banerjee Kroupa ( 2014 ) model for NGC 3603 appears consistent with observations at radii > 0.5 pc. However, it is overdense on smaller spatial scales. Whilst the models generally predict mass surface densities that are greater than the observed clouds, this is not unambiguously the case. The highly star-forming clouds such as Sagittarius B2, W49 and W51 all lie very close to these models. Though we reiterate that the mass estimates, particularly in the highly star-forming clouds, are upper limits. Accounting for temperature effects would create an even larger disparity between the observations and simulations. We also note that the simulated models assume that both the gas and stars are initially distributed according to a Plummer distribution, which is not true for the gas in the clouds in our sample (see Section 4 ). This is a critical difference between the true conditions in the ISM and those assumed in these models. The gravitational potential of the gas is dominant until a significant number of stars have formed within the gas, and it is clear that the initial potential does not arise from a Plummer-like gravitational potential. 4 DISCUSSION In Walker et al. ( 2015 ), we used a sample of YMCs and possible YMC precursor clouds at the Galactic Centre to attempt to distinguish between two commonly argued modes of YMC formation – popping or conveyor-belt modes. We did this simply by comparing their mass surface density
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