The most metal-poor galaxies

Metallicity is a key parameter that controls many aspects in the formation and evolution of stars and galaxies. In this review we focus on the metal deficient galaxies, in particular the most metal-poor ones, because they play a crucial role in the cosmic scenery. We first set the stage by discussing the difficult problem of defining a global metallicity and how this quantity can be measured for a given galaxy. The mechanisms that control the metallicity in a galaxy are reviewed in detail and involve many aspects of modern astrophysics: galaxy formation and evolution, massive star formation, stellar winds, chemical yields, outflows and inflows etc. Because metallicity roughly scales as the galactic mass, it is among the dwarfs that the most metal-poor galaxies are found. The core of our paper reviews the considerable progress made in our understanding of the properties and the physical processes that are at work in these objects. The question on how they are related and may evolve from one class of objects to another is discussed. While discussing metal-poor galaxies in general, we present a more detailed discussion of a few very metal-poor blue compact dwarf galaxies like IZw18. Although most of what is known relates to our local universe, we show that it pertains to our quest for primeval galaxies and is connected to the question of the origin of structure in the universe. We discuss what QSO absorption lines and known distant galaxies tell us already? We illustrate the importance of star-forming metal-poor galaxies for the determination of the primordial helium abundance, their use as distance indicator and discuss the possibility to detect nearly metal-free galaxies at high redshift from Ly$\alpha$ emission.Comment: 96 pages, 12 figures. To appear in the A&A Review. Version including proof correction

Massive (?) starburst hosts of blue compact galaxies (BCGs)

We present optical spectroscopy and deep optical/near-IR photometry of 4 luminous metal-poor blue compact galaxies (BCGs) and two of their companions. With the aid of spectral evolutionary models (SEMs) and structural parameters derived from the surface photometry we discuss the properties of the central starbursts and the halo populations of the galaxies. Special attention is paid to the effects of dust, chemical inhomogeneities and contamination of nebular emission to the halo light. The optical/near-IR colour index profiles show a sharp distinction between the starburst and the host. The hosts have luminosity profiles characteristic of massive ellipticals and remarkably red colours, typical of a relatively {\it metal-rich} stellar population of {\it old age}. These properties are in conflict with the relatively low luminosities. The situation can best be explained if the hosts have an unusually large amount of dark matter that can hinder the outflow of metals from the system. The indicated difference in metallicity between the halo and the young starburst disproves the recurrent burst scenario and supports different origins of the two populations. We conclude that these BCGs are undergoing mergers between early type galaxies/thick disks and gas-rich galaxies or intergalactic HI clouds, in many respects reminiscent of a retarded formation of massive ellipticals.Comment: 30 pages, 21 figures, accepted for publication in Astronomy and Astrophysic

Extended Tidal Structure In Two Lyman Alpha-Emitting Starburst Galaxies

We present new VLA C-configuration HI imaging of the Lyman Alpha-emitting starburst galaxies Tol 1924-416 and IRAS 08339+6517. The effective resolution probes neutral gas structures larger than 4.7 kpc in Tol 1924-416, and larger than 8.1 kpc in IRAS 08339+6517. Both systems are revealed to be tidally interacting: Tol 1924-416 with ESO 338-IG04B (6.6 arcminutes = 72 kpc minimum separation), and IRAS 08339+6517 with 2MASX J08380769+6508579 (2.4 arcminutes = 56 kpc minimum separation). The HI emission is extended in these systems, with tidal tails and debris between the target galaxies and their companions. Since Lyman Alpha emission has been detected from both of these primary systems, these observations suggest that the geometry of the ISM is one of the factors affecting the escape fraction of Lyman Alpha emission from starburst environments. Furthermore, these observations argue for the importance of interactions in triggering massive star formation events.Comment: ApJ, in press; 11 pages, 2 color figure

The dark matter halos of the bluest low surface brightness galaxies

We present BVI photometry and long-slit Halpha rotation curve data obtained with ESO VLT/FORS2 for six low surface brightness galaxies with extremely blue colours and very faint central regions. We find no evidence for a steep central density cusp of the type predicted by many N-body simulations of cold dark matter (CDM) halos. Our observations are instead consistent with dark matter halos characterized by cores of roughly constant density, in agreement with previous investigations. While unremarkable in terms of the central density slope, these galaxies appear very challenging for existing CDM halo models in terms of average central halo density, as measured by the Delta_(V/2) parameter. Since most of our target galaxies are bulgeless disks, our observations also disfavour a recently suggested mechanism for lowering the central mass concentration of the halo by means of a fast collapse phase, as this scenario predicts that the original CDM profile should still be detectable in bulgeless galaxies. Other potential ways of reconciling the CDM predictions with these observations are discussed.Comment: 14 pages, 9 figures, accepted for publication in Astronomy & Astrophysic