Extremely metal-poor galaxies: The H i content^⋆,⋆⋆

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 i 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 H i 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 i integrated flux densities are between 1 and 15 Jy km s^-1, while line widths are between 20 and 120 km s^-1. H i 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 i line profile is systematically accompanied by an asymmetry in the optical morphology. Typically, the g-band stellar mass-to-light ratios are ~0.1, whereas the H i gas mass-to-light ratios may be up to two orders of magnitude larger. Moreover, H i 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 i 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 i gas into stars. The dynamical masses inferred from the H i line widths imply that the stellar mass does not exceed 5% of the dynamical mass, while the H i 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 i 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 i gas and the stellar/nebular component, implying that, in these sources, the H i 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 i 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 i component suggests kinematical disruption and hints at a primordial composition.