A&A 558, A18 (2013) DOI: 10.1051/0004-6361/201322098 c  ESO 2013 Astronomy & Astrophysics Extremely metal-poor galaxies: The H  content ⋆,⋆⋆ M. E. Filho 1 , B. Winkel 2 , J. Sánchez Almeida 3,4 , J. A. Aguerri 3,4 , R. Amorín 5,6 , Y. Ascasibar 7 , B. G. Elmegreen 8 , D. M. Elmegreen 9 , J. M. Gomes 1 , A. Humphrey 1 , P. Lagos 1 , A. B. Morales-Luis 3,4 , C. Muñoz-Tuñón 3,4 , P. Papaderos 1 , and J. M. Vílchez 5 1 Centro de Astrofísica da Universidade do Porto, 4150-762 Porto, Portugal e-mail: mfilho@astro.up.pt 2 Max-Planck Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany 3 Instituto de Astrofísica de Canarias, 38200 La Laguna (Tenerife), Spain 4 Departamento de Astrofísica, Universidad La Laguna, 38206 La Laguna (Tenerife), Spain 5 Instituto de Astrofísica de Andalucía, 18008 Granada, Spain 6 INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Rome, Italy 7 Universidad Autónoma de Madrid, 28049 Madrid, Spain 8 IBM, T. J. Watson Research Center, Yorktown Heights, NY 10598, USA 9 Vassar College, Department of Physics and Astronomy, Poughkeepsie, NY 12604, USA Received 18 June 2013 / Accepted 12 July 2013 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 H  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 in- formation was complemented with H  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 H  integrated ﬂux densities are between 1 and 15 Jy km s -1 , while line widths are between 20 and 120 km s -1 . H  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 H  line proﬁle is system- atically accompanied by an asymmetry in the optical morphology. Typically, the g-band stellar mass-to-light ratios are ∼0.1, whereas the H  gas mass-to-light ratios may be up to two orders of magnitude larger. Moreover, H  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 H  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 H  gas into stars. The dynamical masses inferred from the H  line widths imply that the stellar mass does not exceed 5% of the dynamical mass, while the H  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 H  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 H  gas and the stellar/nebular component, implying that, in these sources, the H  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 H  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 H  component suggests kine- matical disruption and hints at a primordial composition. Key words. galaxies: fundamental parameters – radio lines: galaxies – techniques: spectroscopic 1. Introduction According to the hierarchical paradigm of structure formation, massive galaxies assemble through mergers and cannibalism of ⋆ Full Fig. 1, Tables 3–5 are available in electronic form at http://www.aanda.org ⋆⋆ Reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/558/A18 smaller systems. Interactions between galaxies and secular pro- cesses induce episodes of star-formation. Stellar winds and the death of stars chemically enrich both the interstellar medium and the subsequent stellar generations. In this scenario, extremely metal-poor dwarf galaxies (XMPs) should be common in the early Universe, whereas they should be very rare at low red- shift (York et al. 2000; Pustilnik et al. 2005; Guseva et al. 2007; Izotov et al. 2012; Mamon et al. 2012). Indeed, one of the most recent searches in the Sloan Digital Sky Survey (SDSS) Data Article published by EDP Sciences A18, page 1 of 30