In this section we will briefly describe the properties of other spirals of the Local Group for which we have enough chemical information.
In external spirals we can measure:
Figure 61. Distribution of the (B-H) color, the 12+log(O/H), the gas fraction and the SFR efficiency (the SFR per unit mass of gas) versus the circular velocity of disks of local spirals.The different lines refer to different values of the spin parameter . Figure from Boissier et al. (2001) where the references to the data can be found.
8.1. Chemical models for external spirals
Several models of chemical evolution of Local Group spirals have been developed in the past years (e.g. Diaz & Tosi, 1984; Mollá et al. 1996; Chiappini et al. 2003). Diaz & Tosi (1984) first modeled the chemical evolution of M31, M33, M83 and M101. Mollá et al. (1996) modeled several spirals of the Local Group (M31, NGC300, M33, NGC628, NGC3198, NGC6946). In Figure 62 we show the predictions, compared to observations, of the Mollá et al. model for M31.
As an another example of abundance gradients and gas distribution in a local spiral galaxy we show in Figure 63 the observed and predicted gas distribution and abundance gradients for the disk of M101. In this case the gas distribution and the abundance gradients are reproduced with systematically smaller timescales for the disk formation relative to the MW (M101 formed faster), and the difference between the timescales of formation of the internal and external regions is smaller (M101 = 0.75 r(Kpc) - 0.5 Gyr, Chiappini et al. 2003) compared to eq. (34).
Figure 63. Upper panel: predicted and observed gas distribution along the disk of M101. The observed HI, H2 and total gas are indicated in the Figure. The large open circles indicate the models: in particular, the open circles connected by a continuous line refer to a model with central surface mass density of 1000 M pc-2, while the dotted line refers to a model with 800 M pc-2 and the dashed line to a model with 600 M pc-2. Lower panel: predicted and observed abundance gradients of C,N,O elements along the disk of M101. The models are the lines and differ for a different threshold density for SF, being larger in the dashed model. All the models are by Chiappini et al. (2003).
Therefore, in the Chiappini et al. (2003) paper it is suggested that the fact that more luminous spirals ( e.g. M101) tend to have shallower abundance gradients than less luminous ones can be interpreted as due to a faster formation (down-sizing) of large spirals, as also hinted at by the results of Boissier et al. (2001).
In summary, from the available studies of spirals in the Local Group we can suggest the following: