8. OTHER SPIRALS

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:

• the SFR mainly from H emission and this suggests a correlation between the SFR and the total surface gas density, as discussed previously.

• Abundance gradients are also found in disks of local spirals (see Garnett et al. 1997): in particular, it is found that abundance gradients, expressed in dex/Kpc, are steeper in smaller disks but the correlation disappears if they are expressed in dex/R_d (where R_d is the disk scalelength). It is interesting to note that a universal gradient slope per unit scale length may be explained by viscous disk models (e.g. Lin & Pringle, 1987; Sommer-Larsen & Yoshii, 1989). Further information on gradients is that they are flatter in disks with bars: probably the bar induces radial flows which can wash-out the abundance gradients if their velocity is high enough (see Tinsley, 1980).

• The gas distribution: one finds differences in the gas distribution along the disk betwen field and cluster galaxies, these latter being subject to ram pressure stripping.

• Integrated colors of galaxy disks give information on the distribution of stellar populations along the disks. Several authors (Josey & Arimoto, 1992; Jimenez et al. 1998; Prantzos & Boissier, 2000) have suggested that color gradients, as well as metallicity gradients, can be reproduced by assuming an inside-out formation for disks, as has been suggested for the disk of the Milky Way.

• In Figure 61 we show the results of a paper by Boissier et al. (2001). They conclude that more massive disks are redder, more metal rich and more gas-poor than smaller ones. On the other hand, their estimated SF efficiency (defined as the SFR per unit mass of gas) seems to be similar among different spirals: this leads them to conclude that more massive disks are older than less massive ones. The various quantities in Figure 61 are plotted as functions of the disk circular velocity which is a measure of the dark matter halo of each galaxy. The various curves are obtained by varying the spin parameter , expressed as a / _MW, where _MW refers to the Milky Way. In fact, in the framework of semi-analytical models of galaxy formation the evolution of galactic disks can be described by means of scaling laws calibrated on the Milky Way with V_c and as parameters (e.g. Mo et al. 1998).

  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.

Figure 62. Predicted and observed abundance gradients in M31. Figure and models are from Mollá et al. (1996).

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:

• The disks of spirals have all formed inside-out and the more massive disks have formed faster than the less massive one.

• This translates into a faster gas accretion rate and consequently into a faster SFR.

• In other words, the most massive disks are also the oldest, a conclusion which is not in line with classical predictions from CDM models.