One of the controversial issues in the study of the chemical evolution of irregular galaxies is the low effective yield of oxygen derived from observations, one solution to this problem is to assume the presence of O-rich galactic outflows.
Several lines of reasoning indicate that O-rich outflows produced by gas rich irregular galaxies are unlikely. Larsen, Sommer-Larsen, & Pagel (2001) from chemical evolution models find that the N/O versus O/H relationship indicates that that O-rich outflows have not played an important role in nearby irregular galaxies (redshifts ~ 0). A similar result has been obtained by Carigi et al. (1995) and Carigi, Colín, & Peimbert (1999) based on the C/O versus O/H values for nearby irregular galaxies. Tenorio-Tagle (2001) from the mixing of metals argues that it is difficult to expel gas in dwarf irregulars with a low rate of star formation, moreover the mass lost would be of well mixed material; in this context Silich et al. (2001) analyze VII Zw 403, a metal poor irregular galaxy, and conclude that the heavy elements produced during the present starburst will not be ejected into the interstellar medium. One way to reduce the difference between the observed effective yield for oxygen and the yield predicted by models is the presence of dark matter (e.g. Carigi et al. 1999).
The N/O versus O/H relation has been studied by many authors (e.g. Garnett 1990, 2001; Pagel et al. 1992; Shields 2001; Skillman 2001; Larsen et al. 2001) there are some aspects of this relation that need further consideration.
The N/O ratio depends on the temperature adopted, specially for objects with
low electron temperature, therefore the errors in the N/O determinations
might be larger for objects with lower temperatures. To determine the N/O
ratio often the temperature derived from the
4363/5007 ratio,
T(O III), is used as representative of the
O+ and N+ zones; from
photoionization models it is found that for objects with T(O
III) >
12360K, the temperature of the O+ + region is higher than the
temperature
of the O+ region, the opposite is found for objects with
T(O III) < 12360K (e.g.
Stasinska 1990);
if this effect is not taken into account the
N/O value for metal rich H II regions (those with
T(O III) <
12360K) will be underestimated while for metal poor H II
regions (those
with T(O III) > 12360K) N/O will be overestimated.
Often it is assumed that the N/O ratio is equal to the N+/O+ ratio (assuming that the O+ zone coincides with the N+ one), while some photoionization models indicate that this is the case, others indicate that it is at best a fair approximation Relaño et al. 2001.
Apparently there are environmental effects present in the N/O versus O/H relation, while Peimbert & Torres-Peimbert (1992) find an underabundance of the N/O ratio for a given O/H ratio in the Bootes Void galaxies, Vílchez & Iglesias-Páramo (2001) find that there is an overabundance of N/O for a given O/H in the dwarf galaxies of the Virgo cluster.