Extremely metal-poor galaxies: The H i content

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机构：

[1] Filho, M.E.

[2] Winkel, B.

[3] 3,Sánchez Almeida, J.

[4] 3,Aguerri, J.A.

[5] 5,Amorín, R.

[6] Ascasibar, Y.

[7] Elmegreen, B.G.

[8] Elmegreen, D.M.

[9] Gomes, J.M.

[10] Humphrey, A.

[11] Lagos, P.

[12] 3,Morales-Luis, A.B.

[13] 3,Muñoz-Tuñón, C.

[14] Papaderos, P.

[15] Vílchez, J.M.

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| 1600年 / EDP Sciences卷 / 558期

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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. © ESO; 2013;

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