Metallicities from absorption lines

As in other areas of astrophysics, a cross-check of the high metallicities deduced from the broad emission lines in AGN with an independent abundance diagnostic is highly desirable. The narrow absorption lines seen in QSO spectra at redshifts close to the emission redshift are generally considered to be different from those of most 'intervening' QSO absorbers (in the sense that they arise in the QSO environment) and there are claims in the literature that the metallicity of these 'associated' absorbers is indeed supersolar (e.g. Petitjean et al. 1994). The difficulty

1180 1200 1220 1240 1260 Wavelength (A)

Figure 20.2. Left panel: a portion of the spectrum of Q2343-BX415 obtained with the Echelle Spectrograph and Imager (ESI) on the Keck II telescope, showing the region encompassing the Lyman-alpha (Lya) line. A DLA with N(H i) = 1 x 1021 cm-2 absorbs away most of the QSO Lya emission line since both are at redshift z = 2.57393. Right panel: the chemical-abundance pattern determined by Rix etal. (2007) for the proximate DLA in Q2343-BX415. Metallicities are shown relative to Solar (long-dashed line at [X/H] = 0.0); the height of each box reflects the uncertainty in the abundance of the corresponding element.

1180 1200 1220 1240 1260 Wavelength (A)

Figure 20.2. Left panel: a portion of the spectrum of Q2343-BX415 obtained with the Echelle Spectrograph and Imager (ESI) on the Keck II telescope, showing the region encompassing the Lyman-alpha (Lya) line. A DLA with N(H i) = 1 x 1021 cm-2 absorbs away most of the QSO Lya emission line since both are at redshift z = 2.57393. Right panel: the chemical-abundance pattern determined by Rix etal. (2007) for the proximate DLA in Q2343-BX415. Metallicities are shown relative to Solar (long-dashed line at [X/H] = 0.0); the height of each box reflects the uncertainty in the abundance of the corresponding element.

here is that these abundance determinations require photoionisation models to take into account unobserved ion stages, given that most proximate absorption systems exhibit a high degree of ionisation.

Among the various classes of QSO absorbers, those with the most secure and extensive abundance measures are the DLAs (Wolfe et al. 2005) - when the neutral-hydrogen column density exceeds N(H i) = 2 x 1020 cm-2, the gas is predominantly neutral and one observes directly the dominant ion stages of a relatively large variety of elements in the rest-frame ultraviolet (UV) absorption spectrum. Proximate DLAs are rare, but Rix et al. (2007, in preparation) have recently studied in detail the chemical composition of one such system in Q2343-BX415. This faint (R = 20.2) QSO was discovered in the course of the survey for UV-selected (or 'BX') galaxies at z ~ 2 by Steidel et al. (2004).

As can be seen from Figure 20.2 (left panel), the QSO spectrum reveals a strong (N(H i) = 1 x 1021 cm-2) DLA at a redshift that coincides with that of the QSO emission lines (zabs = zem = 2.573 93). In their study, Rix et al. determined the abundances (or limits) for 13 elements, from C to Zn (illustrated in the right panel of Figure 20.2), and concluded that the properties of this DLA are broadly similar to those of more conventional damped systems at zabs < zem. In particular, its abundance pattern is not unusual, with a metallicity of Z ~ 5Z© (which evidently applies to a-capture elements and to Zn) and a mild depletion of refractory species similar to that seen in diffuse interstellar clouds in the Milky Way. The elements N and Mg are somewhat anomalous in exhibiting lower abundances than expected on the basis of the other eleven elements considered. We do not know the location of this DLA relative to BX415. The low degree of ionisation and excitation of the gas

(the latter measured via the fine-structure line of C ii at 1335 À - see Wolfe et al. (2005)) make it unlikely that it is in the immediate vicinity of the AGN. However, the DLA may be formed in the interstellar medium (ISM) of the QSO host or of a nearby galaxy that is part of the same large-scale structure. In any case, these results show that absorption-line systems at the same redshifts as the background QSOs against which they are being viewed are not always metal-rich. In the case of BX415 there may in fact be little difference between the metallicity of the DLA and that of the QSO itself, which appears to be sub-solar given the weakness of its N emission lines (Rix et al. 2007 in preparation).

Was this article helpful?

0 0

Post a comment