As a graduate student in the 1980s, I was warned by senior colleagues to stay clear of the issue of high metallicity. That subject, it was said, had been controversial, and careers had foundered on claims of metallicity greater than Solar. My thesis work was on the Galactic bulge, and, in the long run, it would help to return the subject of super-metal-rich stars to respectability. Still, it is surprising that this is the first meeting on the metal-rich Universe. Spheroidal populations, which have elevated metallicity, account for some 50%-70% of the stellar mass in the local Universe (Fukugita et al. 1998), and are also known as the hosts of black holes (Tremaine et al. 2002). The era of metal production keeps getting pushed back to earlier and earlier epochs; metal lines are found in the most distant quasars, with Feii clearly appearing at z = 6.4 (Barth et al. 2003), less than 1 Gyr after the Big Bang. In the same quasar, ~1010MQ of molecular gas (CO-line emission) is observed (Walter
The Metal-rich Universe, eds. G. Israelian and G. Meynet. Published by Cambridge University Press. © Cambridge University Press 2008.
et al. 2004). The association of quasars with galaxy bulges in formation marks them as evidently prodigious sources of metals in the early Universe.
By 2.4 Gyr after the Big Bang, at z = 2.77, in an exquisite study of a lensed star-forming (so-called Lyman-break) galaxy, MS 1512-cB58, Pettini et al. (2000) find a total metallicity of 0.25 Solar - quite respectable by the standards of the present-day Universe. Galaxies clearly built up their metals early and rapidly. Early spectroscopy of this distant starburst galaxy was of such high signal-to-noise ratio (SNR) that it was necessary to obtain better UV spectra of nearby starbursts in order to have a local comparison sample. The agents of metal buildup are massive stars, and this galaxy is a snapshot of chemical evolution in action. Since the pioneering work of Steidel et al. (1996) it has been found that the Lyman-break galaxy population is surprisingly metal-rich, with evidence of metal-enriched outflows. The work of Erb et al. (2006) uses the classical strong optical lines, shifted to the infrared, to derive abundances in a population of z ~ 2 galaxies. They find supersolar effective yields in their galaxy population and a gas outflow rate of approximately four times the star-formation rate. Although the data are less secure, the abundances derived from the broad lines of quasars have also been claimed to be high (e.g. Hamann et al. 2002).
The detection of high metallicity in this high-redshift galaxy should not come as a surprise to those who have followed the study of the Galactic bulge in recent years. Zoccali et al. (2003) show that the turnoff age for the bulge is comparable to that of a metal-rich halo globular cluster and estimate an age for the bulge in excess of 10 Gyr, a secure demonstration of the great age hinted at in prior studies (Ortolani et al. 1995, Kuijken & Rich 2002). The observation of nearly Solar metallicity at high redshift should come as an expectation, not a surprise.
Yet another means of quantitative measurement of metals in the high-redshift Universe is offered by damped Lyman-alpha systems in quasars. These are gas clouds of sufficient H i column that the associated Lyman-alpha lines have damping wings; associated with these clouds are also metal lines, and it is possible to derive a surprisingly accurate metal abundance for these systems. Dessauges-Zavadsky et al. (2006) analyze systems over a redshift range of 1.8 < z < 2.5, ranging from 1/55 to 1/5 the Solar iron abundance.
Metals at redshift 6 are not confined to quasars or distinct bodies like Lyman-limit or damped Lyman-alpha systems. Metals are distributed widely; Sargent, Simcoe, and collaborators (e.g. Becker et al. 2006) have used statistical methods to find C, O, and Si in the intergalactic medium at z ~ 6, presumably placed there by wind outflow from star-forming galaxies. There are metals in the Universe as far as the eye can see.
Returning closer to home, metal-rich populations are found in surprising venues. The well-studied open cluster NGC 6791 is found to have [Fe/H] = +0.4 even with modern abundance determinations (Gratton et al. 2006; Origlia et al. 2006). The
Sagittarius dwarf spheroidal galaxy has abundances up to Solar (McWilliam et al. 2003; Sbordone et al. 2006). Even the stellar halos of luminous galaxies have metallicities approaching Solar (Mouhcine et al. 2005b), as does the outer disk of M31 some 30kpc from the nucleus (Brown et al. 2006). Regions and stars of high metallicity provide insight into the star-formation process and nucleosynthesis, and it is now clear that regions of high metallicity must be considered to be of great importance in the formation of galaxies and that they are widespread, not only confined to the nuclear regions.
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