The HI Content of Extremely Metal-Deficient Blue Compact Dwarf Galaxies
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Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 14 June 2018 (MN LATEX style file v2.2) The H i content of extremely metal-deficient blue compact dwarf galaxies T. X. Thuan1 , K. M. Goehring1, J. E. Hibbard2, Y. I. Izotov3 and L. K. Hunt4 1Astronomy Department, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325, USA; [email protected], [email protected] 2National Radio Astronomy Observatory, Charlottesville, VA 22903, USA; [email protected] 3Main Astronomical Observatory, National Academy of Sciences of Ukraine, 03680 Kyiv, Ukraine; [email protected] 4INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy; [email protected] 19 January 2015 ABSTRACT We have obtained new H i observations with the 100 m Green Bank Telescope (GBT) for a sample of 29 extremely metal-deficient star-forming Blue Compact Dwarf (BCD) galaxies, selected from the Sloan Digital Sky Survey spectral data base to be extremely metal-deficient (12 + log O/H 6 7.6). Neutral hydrogen was detected in 28 galaxies, a 97% detection rate. Combining the H i data with SDSS optical spectra for the BCD sample and adding complementary galaxy samples from the literature to extend the metallicity and mass ranges, we have studied how the H i content of a galaxy varies with various global galaxian properties. There is a clear trend of increasing gas mass fraction with decreasing metallicity, mass and luminosity. We obtain the relation i ∝ −0.3 M(H )/Lg Lg , in agreement with previous studies based on samples with a smaller luminosity range. The median gas mass fraction fgas for the GBT sample is equal to 0.94 while the mean gas mass fraction is 0.90±0.15, with a lower limit of ∼0.65. The H i depletion time is independent of metallicity, with a large scatter around the median value of 3.4 Gyr. The ratio of the baryonic mass to the dynamical mass of the metal-deficient BCDs varies from 0.05 to 0.80, with a median value of ∼0.2. About 65% of the BCDs in our sample have an effective yield larger than the true yield, implying that the neutral gas envelope in BCDs is more metal-deficient by a factor of 1.5–20, as compared to the ionized gas. Key words: galaxies: dwarf – galaxies: fundamental parameters – galaxies: irregular – galaxies: ISM – galaxies: starburst 1 INTRODUCTION previous star formation and metal enrichment. The discov- arXiv:1609.02358v1 [astro-ph.GA] 8 Sep 2016 ery of XMD galaxies at high z may have to wait until the The formation and evolution of the first galaxies in the uni- advent of the JWST and 30 m-class ground-based telescopes. verse remains a key issue in cosmology. It is now thought that large massive star-forming galaxies form in a hierar- We adopt here a different approach. Instead of search- chical manner from the assembly of smaller dwarf systems, ing for high-z metal-deficient objects, we focus our atten- through accretion and merger processes. These galaxy in- tion on XMD star-forming dwarf galaxies in the local uni- teractions trigger the formation of stars which enrich the verse. They are the most promising local proxies of chemi- interstellar gas in metals by stellar winds and supernovae. cally unevolved galaxies in the early universe, and are usu- In this scenario, the number of extremely metal-deficient ally found among a class of dwarf galaxies undergoing in- (XMD) dwarf galaxies should be high in the early universe tense bursts of star formation called Blue Compact Dwarf but considerably smaller at the present epoch (Mamon et al. (BCD) galaxies (Thuan & Martin 1981). The optical spec- 2012). tra of the BCDs are characterized by a blue continuum However, while much progress has been made in finding on which are superimposed strong narrow emission lines. large populations of galaxies at high (z >3) redshifts (e.g. XMD BCDs are very rare (Izotov, Thuan & Guseva 2012). Steidel et al. 2003; Adelberger et al. 2005), truly chemically For more than three decades, one of the first BCD discov- unevolved galaxies remain elusive in the high-z universe. ered, I Zw 18 (Sargent & Searle 1970), held the record as The spectra of distant galaxies generally indicate the pres- the most metal-deficient emission-line galaxy known, with ence of a substantial amount of heavy elements, implying an oxygen abundance [O/H]= 12 + log O/H = 7.17 0.01 in ± c 0000 RAS 2 T. X. Thuan et al. its northwestern component and 7.22 0.02 in its southeast- and a dark energy density parameter ΩΛ = 0.73 (Riess et al. ± ern component (Thuan & Izotov 2005) ( 3% solar, adopt- 2011). ∼ ing the solar abundance 12+logO/H = 8.76 of Steffen et al. 2015). Only in 2005 has I Zw 18 been superceded in its rank by SBS 0335–052W with a metallicity [O/H] = 7.12 2 H I OBSERVATIONS (Izotov, Thuan & Guseva 2005). Because of the scarcity of XMD emission-line galaxies, 2.1 The GBT sample we stand a much better chance of discovering them in very large spectroscopic surveys such as the Sloan Digital Sky We have constructed our XMD BCD sample by applying Survey (SDSS, York et al. 2000). We have carried out a the following selection criteria to the SDSS DR7 spectral systematic search for such objects with [O/H] in the SDSS database. As described in Izotov et al. (2012), we first select spectroscopic data release 7 (DR 7) (Abazajian et al. 2009). them on the basis of the relative fluxes of particular emission iii 6 Imposing cut-offs in metallicity and redshift results in a total lines as measured on the SDSS spectra: [O ]λ4959/Hβ 1 ii 6 sample of 29 XMD BCDs. Similar searches for XMD galax- and [N ]λ6853/Hβ 0.1. These spectral properties select ies in the SDSS have been carried out by Morales-Luis et al. out uniquely low-metallicity dwarfs since no other type of (2011) and S´anchez Almeida et al. (2016). We found a to- galaxy possesses them. Second, since one of the main sci- i tal of 10 galaxies in common between the present sam- entific objectives here is to study how the H properties ple and that of S´anchez Almeida et al. (2016) so that the of star-forming galaxies vary with metallicity, it is impor- two samples are likely to have similar properties, and the tant to have accurate heavy element abundances for the H i characteristics discussed here probably apply to the XMD BCDs. Thus, we have included in our sample only iii S´anchez Almeida et al. (2016) objects as well. those BCDs that have a well-detected [O ]λ4363 line as The focus of this paper is the study of the neutral hy- this electron temperature-sensitive emission line allows a di- drogen content of these XMD objects. There have been pre- rect and precise abundance determination. Third, we have i set a metallicity cut-off [O/H] 6 7.6 ( 8% solar) to choose vious H studies of this type of extremely metal-poor galax- ∼ ies. Thus, Filho et al. (2013) have carried out a single-dish only XMD galaxies. This metallicity threshold has been sug- H i study with the Effelsberg radio telescope of a subsample gested by Izotov & Thuan (1999) to characterize very young of 29 XMD galaxies selected from the Morales-Luis et al. galaxies, with most of their stellar populations formed not more than 1 Gyr ago. Lastly, we have chosen the BCDs (2011) list. We will use part of the data of those authors ∼ to supplement our own and will compare with our results to be not more distant than 85 Mpc, so we can measure i ∼ with theirs when warranted. A handful of interferometric their H emission with a good signal-to-noise ratio within H i maps of other XMD galaxies have also been obtained by a reasonable integration time with the GBT. This distance upper limit corresponds to a recession velocity of 6200 km the Pune group with the Giant Metrewave radio telescope −1 ∼ (see Ekta, Pustilnik & Chengalur 2009; Ekta & Chengalur s . These selection criteria result in a total sample of 29 2010, and references therein). In Section 2, we define the XMD BCDs. In contrast to many previous BCD samples i XMD BCD sample and describe observations of their neu- that have been observed in H (e.g. Thuan & Martin 1981; tral hydrogen content with the Robert C. Byrd Green Bank Thuan et al. 1999b; Huchtmeier, Krishna & Petrosian 2005; Telescope (GBT) at the National Radio Astronomy Obser- Filho et al. 2013), this BCD sample is unique in that : 1) it 6 6 vatory 1. This sample will be referred to hereafter as the contains only very metal-deficient objects (7.35 [O/H] GBT sample. Section 3 describes the H i data along with 7.60) as compared to previous samples which include many > derived ancillary data such as metallicities, star formation objects with [O/H] 7.7. This allows us to study very chemi- rates, stellar masses needed to study trends of the neutral cally unevolved galaxies; 2) it has O abundances determined gas content with other properties of the BCDs. In Section precisely for each object, using the direct method based on iii 4, we discuss several comparison samples compiled from the the [O ]λ4363 line. This is in contrast to the approximate literature, useful for studying the H i content of star-forming abundances derived for many galaxies in previous samples, galaxies over a wider range of heavy element abundances and using the statistical strong-line method, because of the un- iii stellar masses.