Abstract

Context. Extremely metal-poor (XMP) galaxies are chemically, and possibly dynamically, primordial objects in the local Universe. Aims. Our objective is to characterize the Hi content of the XMP galaxies as a class, using as a reference the list of 140 known local XMPs compiled by Morales-Luis et al. (2011). Methods. We have observed 29 XMPs, which had not been observed before at 21 cm, using the Effelsberg radio telescope. This information was complemented with Hi data published in literature for a further 53 XMPs. In addition, optical data from the literature provided morphologies, stellar masses, star-formation rates and metallicities. Results. Effelsberg Hi integrated flux densities are between 1 and 15 Jy kms-1, while line widths are between 20 and 120 kms-1. Hi integrated flux densities and line widths from literature are in the range 0.1-200 Jy km s-1 and 15-150 km s-1, respectively. Of the 10 new Effelsberg detections, two sources show an asymmetric double-horn profile, while the remaining sources show either asymmetric (seven sources) or symmetric (one source) single-peak 21 cm line profiles. An asymmetry in the Hi line profile is systematically accompanied by an asymmetry in the optical morphology. Typically, the g-band stellar mass-to-light ratios are similar to 0.1, whereas the Hi gas mass-to-light ratios may be up to two orders of magnitude larger. Moreover, Hi gas-to-stellar mass ratios fall typically between 10 and 20, denoting that XMPs are extremely gas-rich. We find an anti-correlation between the Hi gas mass-to-light ratio and the luminosity, whereby fainter XMPs are more gas-rich than brighter XMPs, suggesting that brighter sources have converted a larger fraction of their Hi gas into stars. The dynamical masses inferred from the Hi line widths imply that the stellar mass does not exceed 5% of the dynamical mass, while the Hi mass constitutes between 20 and 60% of the dynamical mass. Furthermore, the dark matter mass fraction spans a wide range, but can account, in some cases, for over 65% of the dynamical mass. XMPs are found to be outliers of the mass -and luminosity -metallicity relation, whereby they lack metals for their estimated dynamical mass and luminosity, suggesting the presence of pristine gas. However, they generally follow the luminosity -and baryonic mass Tully-Fisher relation, indicating that the Hi gas is partly virialized and contains some rotational support. 60% of the XMP sources show a small velocity offset (10-40 km s-1) between the Hi gas and the stellar/nebular component, implying that, in these sources, the Hi gas is not tightly coupled to the stars and ionized gas. The effective yields provided by oxygen are often larger than the standard theoretical yields, suggesting that the observed Hi gas is relatively metal-free. 80% of the XMP sources present asymmetric optical morphology 60 XMPs show cometary structure, 11 show two bright star-forming knots and 18 show multiple star-forming regions. Star-formation rates are found to be similar to those typically found in BCDs. However, specific star-formation rates are high, with timescales to double their stellar mass, at the current rate, of typically less than 1 Gyr. Conclusions. XMP galaxies are among the most gas-rich objects in the local Universe. The observed Hi component suggests kinematical disruption and hints at a primordial composition.