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      ALFALFA Discovery of the Most Metal-Poor Gas-Rich Galaxy Known: AGC 198691

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          Abstract

          We present spectroscopic observations of the nearby dwarf galaxy AGC 198691. This object is part of the Survey of HI in Extremely Low-Mass Dwarfs (SHIELD) project, which is a multi-wavelength study of galaxies with HI masses in the range of 10\(^{6}\)-10\(^{7.2}\)~M\(_{\odot}\) discovered by the ALFALFA survey. We have obtained spectra of the lone HII region in AGC 198691 with the new high-throughput KPNO Ohio State Multi-Object Spectrograph (KOSMOS) on the Mayall 4-m as well as with the Blue Channel spectrograph on the MMT 6.5-m telescope. These observations enable the measurement of the temperature-sensitive [OIII]\(\lambda\)4363 line and hence the determination of a "direct" oxygen abundance for AGC 198691. We find this system to be an extremely metal-deficient (XMD) system with an oxygen abundance of 12+log(O/H) = 7.02 \(\pm\) 0.03, making AGC 198691 the lowest-abundance star-forming galaxy known in the local universe. Two of the five lowest-abundance galaxies known have been discovered by the ALFALFA blind HI survey; this high yield of XMD galaxies represents a paradigm shift in the search for extremely metal-poor galaxies.

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          The chemical composition of the Sun

          The solar chemical composition is an important ingredient in our understanding of the formation, structure and evolution of both the Sun and our solar system. Furthermore, it is an essential reference standard against which the elemental contents of other astronomical objects are compared. In this review we evaluate the current understanding of the solar photospheric composition. In particular, we present a re-determination of the abundances of nearly all available elements, using a realistic new 3-dimensional (3D), time-dependent hydrodynamical model of the solar atmosphere. We have carefully considered the atomic input data and selection of spectral lines, and accounted for departures from LTE whenever possible. The end result is a comprehensive and homogeneous compilation of the solar elemental abundances. Particularly noteworthy findings are significantly lower abundances of carbon, nitrogen, oxygen and neon compared with the widely-used values of a decade ago. The new solar chemical composition is supported by a high degree of internal consistency between available abundance indicators, and by agreement with values obtained in the solar neighborhood and from the most pristine meteorites. There is, however, a stark conflict with standard models of the solar interior according to helioseismology, a discrepancy that has yet to find a satisfactory resolution.
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            Spectrophotometric standards

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              Oxygen abundances in nearby dwarf irregular galaxies

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                Author and article information

                Journal
                2016-03-11
                Article
                10.3847/0004-637X/822/2/108
                1603.03798
                f531c367-e6e2-46aa-bc76-f7056d2306aa

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

                History
                Custom metadata
                14 pages, 7 figures, 4 tables, accepted for publication in ApJ
                astro-ph.GA

                Galaxy astrophysics
                Galaxy astrophysics

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