Chemical evolution of dwarf irregular and blue compact galaxies
Key Laboratory for Research in Galaxies and Cosmology, Shanghai
Astronomical Observatory, CAS, 80
Nandan Road, 200030, Shanghai,
2 Department of Physics, Astronomy Division, Trieste University, via G.B. Tiepolo 11, 34131 Trieste, Italy
3 Osservatorio Astronomico di Trieste, INAF, via G.B. Tiepolo 11, 34131 Trieste, Italy
Accepted: 20 February 2011
Aims. Dwarf irregular and blue compact galaxies are very interesting objects since they are relatively simple and unevolved. We aim at deriving the formation and chemical evolution history of late-type dwarf galaxies and at comparing it with DLA systems.
Methods. We present new models for the chemical evolution of these galaxies by assuming different regimes of star formation (bursting and continuous) and different kinds of galactic winds (normal and metal-enhanced). The dark-to-baryonic mass ratio is assumed to be ten in these models. The chemical evolution model follows the evolution of He, C, N, O, S, Si, and Fe in detail. We have collected the most recent data on these galaxies and compared them with our model’s results. We also collected data for damped-Lyman α-systems.
Results. Our results show that in order to reproduce all the properties of these galaxies, including the spread in the chemical abundances, the star formation should have proceeded in bursts and the number of bursts should be not more than ten in each galaxy, and that metal-enhanced galactic winds are required. The presence of metal-enhanced galactic winds can by itself reproduce the observed mass-metallicity relation, although an increasing efficiency of star formation and/or number and/or duration of bursts can also reproduce such a relation equally well.
Conclusions. Metal-enhanced winds, together with an increasing amount of star formation with galactic mass, are required to explain most of the properties of these galaxies. Normal galactic winds, where all the gas is lost at the same rate, do not reproduce the features of these galaxies. On the other hand, a global increase in the amount of star formation (increasing efficiency, number of bursts, or burst duration) with galactic mass is able by itself to reproduce the mass-metallicity relation even without winds, but without metal-enhanced winds is not able to explain many other constraints. We suggest that these galaxies should have suffered a different number of bursts varying from two to ten and that the efficiency of metal-enhanced winds should not have been too high (λmw ~ 1). We predict for these galaxies present-time type Ia SN rates from 0.00084 and 0.0023 per century. Finally, by comparing the abundance patterns of damped Lyman-α objects with our models, we conclude that they are very likely the progenitors of the current dwarf irregulars.
Key words: galaxies: evolution / galaxies: abundances / galaxies: dwarf / galaxies: irregular
© ESO, 2011