Info

with eSN = ^sn^o and eo = 105ierg (typical SN energy) and ew = nwEw with Ew = 1049erg (the typical energy injected by a 20M0 star, taken as representative). nw and nSN are two free parameters and indicate the efficiency of energy transfer from stellar winds and SNe into the ISM, respectively, quantities that are still largely unknown. The total mass of the galaxy is expressed as Mtot(t) = M„(t) + Mgas(t) + Mdark (t) with ML(t) = M*(t) + Mgas(t) and the binding energy of gas is

with

which is the potential well due to the luminous matter and with wld(t) = -Gwld Mgas(t)Mdark, (6.35)

which represents the potential well due to the interaction between dark and luminous matter, where wLD ~ S(1 + 1.37S)/(2^), with S = rL/rD being the ratio between the galaxy's effective radius and the radius of the dark-matter core. The typical model for a BCG has a luminous mass of (108-109) M0, a dark-matter halo ten times larger than the luminous mass, and various values for the parameter S. The galactic wind in these galaxies develops easily but it carries out mainly metals, so the total mass lost in the wind is small.

6.3.3 Results on DIGs and BCGs from purely chemical models

Purely chemical models (Bradamante et al. 1998; Marconi et al. 1994) for DIGs and BCGs have been computed in the last few years by varying the number of bursts, the time of occurrence of bursts tburst, the SF efficiency, the type of galactic wind (differential or normal), the IMF, and the nucleosynthesis prescriptions. The best model of Bradamante et al. (1998) suggests that the number of bursts should be Wbursts < 10, so the SF efficiency should vary from 0.1 to 0.7 Gyr-i for either a Salpeter or a Scalo (1986) IMF (the Salpeter IMF is favored). Metal-enriched winds are favored. The results of these models also suggest that SNe of Type II dominate the chemical evolution and energetics of these galaxies, whereas stellar winds are negligible. The predicted [O/Fe] ratios tend to correspond to overabundance relative to the Solar ratios, owing to the predominance of Type II SNe during the bursts, in agreement with observational data (see the upper panel of Figure 6.15 later). Models with strong differential winds and Nbursts = 10-15 can, however, give rise to negative [O/Fe] ratios. The main difference between DIGs and BCGs, in these models, is that the BCGs suffer a present-day burst, whereas the DIGs are in a quiescent phase.

Figure 6.13. Upper panel: predicted log(N/O) versus 12 + log(O/H) for a model with three bursts of SF separated by quiescent periods and different SF efficiencies here indicated by r. Lower panel: predicted log(C/O) versus 12 + log(O/H). The data in both panels are from Kob-ulnicky and Skillman (1996). The models assume a dark-matter halo ten times larger than the luminous mass and S = 0.3 (Bradamante et al. 1998), see the text.

Figure 6.13. Upper panel: predicted log(N/O) versus 12 + log(O/H) for a model with three bursts of SF separated by quiescent periods and different SF efficiencies here indicated by r. Lower panel: predicted log(C/O) versus 12 + log(O/H). The data in both panels are from Kob-ulnicky and Skillman (1996). The models assume a dark-matter halo ten times larger than the luminous mass and S = 0.3 (Bradamante et al. 1998), see the text.

In Figure 6.13 we show some of the results of Bradamante et al. (1998) compared with data on BCGs: it is evident that the spread in the chemical properties can be simply reproduced by different SF efficiencies, which translate into different wind efficiencies.

In Figure 6.14 we show the results of the chemical-evolution models of Henry et al. (2000). These models take into account exponential infall but not outflow. They suggested that the SF efficiency in extragalactic H ii regions must have been low and that this effect coupled with the primary N production from intermediate mass stars can explain the plateau in log(N/O) observed at low 12 + log(O/H). Henry et al. (2000) also concluded that 12 C is produced mainly in massive stars (yields published by Maeder (1992)) whereas 14 N is produced mainly in intermediate-mass stars (yields published by HG97). This conclusion, however, should be tested also on the abundances of stars in the Milky Way,

Figure 6.14. A comparison between numerical models and data for extragalactic H II regions and stars (filled circles, filled boxes, and filled diamonds); M and S mark the positions of the Galactic H II regions and the Sun, respectively. Their best model is model B with an efficiency of SF of v = 0.03. From Henry et al. (2000).

where the flat behavior of [C/Fe] versus [Fe/H] from [Fe/H] = —2.2 up to [Fe/H] = 0 suggests a similar origin for the two elements, namely partly from massive stars and mainly from low- and intermediate-mass ones (Chiappini et al. 2003b).

Concerning the [O/Fe] ratios, we show results from Thuan et al. (1995) in Figure 6.15, where it is evident that generally BCGs have overabundant [O/Fe] ratios.

Very recently, an extensive study from the SDSS of chemical abundances from emission lines in a sample of 310 metal-poor emission-line galaxies appeared (Izotov et al. 2006). The global metallicity in these galaxies ranges from >7.1(Z0/30) to >8.5(0.7Z0). The SDSS sample is merged with 109 BCGs containing objects of extremely low metallicity. These data, shown in the lower panel of Figure 6.15, substantially confirm previous ones showing that abundances of a-elements do not depend on the O abundance, suggesting a common origin for these elements in stars with M > 10M0, except for a slight increase of Ne/O with metallicity, which is interpreted as due to a moderate dust depletion of O in metal-rich galaxies. An important finding is that all the galaxies studied are found to have log(N/O) > —1.6, which indicates that none of these galaxies is a truly young object, unlike the DLA systems at high redshift which have a log(N/O) > —2.3.

6.3.4 Results from chemo-dynamical models: IZw18 IZw18 is the metal-poorest local galaxy, thus resembling a primordial object. Probably it experienced no more than two bursts of star formation including the present one. The age of the oldest stars in this galaxy is still unknown, although recently Tosi et al. (2006) suggested an age possibly >2 Gyr. The oxygen abundance in IZW18 is 12 + log(O/H) = 7.17-7.26, >15-20 times lower than the Solar abundance of oxygen (12 + log(O/H) = 8.39 (Asplund et al. 2005) and log(N/O = —1.54) to —1.60 (Garnett et al. 1997).

Recently, FUSE provided abundances also for H I in IZw18: the evidence is that the abundances in H I are lower than those in H II (Aloisi et al. 2003; Lecavelier des Etangs

Was this article helpful?

0 0

Post a comment