## 4. Initial conditionsThe radial distribution of molecular gas in the Milky Way, as
traced by CO, is essentially confined inside the solar circle, with a
marked hole between 1 and 3 kpc and a strong central
concentration associated to the nuclear ring/disc, whereas the HI
presents a rather constant surface density extending far beyond
and an abrupt decline towards the
centre below 3 kpc. The total gas mass within
is estimated to
,
comprising
of H with kpc, and a total mass , resulting in within the solar circle. Both radial and vertical profiles are Gaussian, and the disc is linearly flaring with radius, as observed in HI from a few kpc of the centre out to at least (Merrifield 1992), achieving a thickness of 136 pc at . The fast radial decline is aimed to spare SPH particles in the outer regions and hence increase the spatial resolution near the centre at fixed number of particles. The observed gas deficit between 1 and 3 kpc needs not to be reproduced since shocks will naturally deplete this region. The gas particles initially have pure circular velocities derived from the axisymmetric part of the total potential at . The stellar and dark mass is divided into the same three components as in Paper I: a stellar nucleus-spheroid (NS), a stellar disc and a dark halo (DH), with their initial axisymmetric mass distribution described by the same analytical formulae, except for the disc. In Paper I we indeed noted that the adopted double exponential discs evolve into discs with too less mass outside the bar region according to the COBE/DIRBE near-IR and bulge microlensing data, and possibly an excess of mass in the central region when compared to the HI terminal velocity constraints. Instead of increasing the disc scale length, we choose here to soften the initial central mass density taking: where is the total disc mass, the scale length, the scale height and the normalised vertical profile. The surface density remains exponential in the external disc, but continuously and differentiably joins an inner Gaussian distribution at , with a central value reduced by 40% relative to the purely exponential case. To compensate for the enhanced spatial density near the plane caused by the additional gas component, we also replace the exponential vertical profile of Paper I by van der Kruit's (1988) profile: with , between the exponential () and isothermal () cases. The choice of the parameters are based on the simulations performed
in Paper I, giving a strong weight to the
orbits versus HI terminal velocities
test. The best models regarding this test are m06t4600 and m04t3000,
whose initial axisymmetric models share the following interdependent
properties: (i) a ratio of disc over NS mass of 0.8 within the
spheroidal volume kpc (where
and
), (ii) a total NS+disc mass of
in the same volume and (iii) a circular velocity of
190 km s © European Southern Observatory (ESO) 1999 Online publication: April 28, 1999 |