Extremely metal-poor galaxies: The H i content

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. 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.

On the properties of the interstellar medium in extremely metal-poor blue compact dwarf galaxies

“…XBCDs are found to fall off the mass/luminosity -metallicity relation compiled from literature data in Filho et al (2013), which is consistent with the presence of chemically unpolluted gas in these systems. It is worth remarking in this context that Sánchez almeida et al (2014) studied, using long-slit spectroscopy, the oxygen abundance along the major axis of seven SF galaxies, including HS 2236+1344.…”

“…A&A 569, A110 (2014) These systems are very scarce in the nearby Universe (Izotov & Thuan 2007;Kniazev et al 2004;Papaderos et al 2008;Guseva et al 2009; see also Skillman 2012) with only about 100 of them currently known (see Morales-Luis et al 2011;Filho et al 2013, for recent compilations of literature data). The most metalpoor ones are SBS 0335-052 W (12 + log(O/H) 6.9 .…”

Star formation sustained by gas accretion

“…These galaxies have a number of properties consistent with disks being assembled by accretion of gas (Elmegreen et al 2012b;Sánchez Almeida et al 2013. In particular, they are enshrouded by large amounts of neutral gas, so that M g /M is typically as large as 20 (Filho et al 2013). The ratio is so large that normal stellar evolution cannot produce enough metals for their HI gas to have the same metallicity as that observed in the HII regions of the galaxies, which are already extremely metal poor because XMP galaxies are selected to have Z < 0.1 Z .…”

“…In a systematic search for XMP galaxies in SDSS-DR7, Morales-Luis et al (2011) find that 75 % of them have either cometary shape or are formed by chained knots. Likewise, from the comprehensive catalog of 140 known XMPs used by Filho et al (2013), 80 % have cometary structure or two or multiple star-forming regions. For reference, only 0.2 % of the star-forming galaxies in the Kiso survey are cometary (Elmegreen et al 2012b).…”

Extreme emission-line galaxies out toz~ 1 in zCOSMOS

“…Although there might be an inevitable overlap between them (e.g., some clumpy/chain or cometary/tadpole EELGs may be also interpreted in terms of interactions) a more detailed description of both the distribution of the star-forming regions and the shape of the underlying diffuse component in broadband images remains interesting. In particular, it may be useful to study the possible mechanisms responsible for the origin of the starburst and chemical enrichment in these galaxies (e.g., Papaderos et al 2008;Filho et al 2013;Sánchez Almeida et al 2013, 2014Amorín et al 2009) and to compare them with galaxies of similar morphologies identified at higher redshifts (e.g., Elmegreen et al 2012Elmegreen et al , 2013Amorín et al 2012b). …”