Abstract
Aims. We wish to improve the accuracy of oscillator strength values for several Ni II UV transitions and measure for the first time the f-value of a few other weak transitions for which no laboratory nor astronomical measurement is presently available. Methods. Four quasars displaying five damped Lyman α systems with relatively strong Ni II lines were selected. From the analysis of the excellent high resolution spectra available, we determined the relative f-value of Ni II transitions by comparing the strength of the corresponding absorption profiles. To quantify the latter, we used the apparent optical depth method for resolved features, equivalent width measurements for optically thin lines and line fitting with VPFIT. Absolute f-values were then derived by relating our determinations to the available laboratory measurements. Results. Thanks to the good signal-to-noise ratio of the spectra and to the suitable properties of the absorption systems investigated, we are able to significantly improve the determination of the f-value for 13 Ni II transitions falling in the 1317–1804 Å interval. Our results are found to be consistent with other earlier determinations for ten of these transitions; our median relative accuracy for these f-values is 6.5%. For three weak transitions near 1502, 1773, and 1804 Å, which have not been detected previously in astronomical spectra, we can get a first measurement of their f-value. Conclusions. Our work illustrates that, thanks to the redshift and the absence of variations of physical constants on cosmological scales, the analysis of absorption lines induced by remote gas in quasar spectra can nowadays provide valuable constraints on atomic data in the UV range.
Highlights
Absorption line studies of interstellar or intergalactic gas seen in the spectra of bright sources such as stars, quasars, or gammaray bursts provide critical information on the physical conditions prevailing in the intervening material lying along these lines of sight
There are three available spectra for this quasar: two UVES (2000 and 2011–2012; the latest has a FWHM of 6.0 km s−1 and is of higher S/N) and one of lower S/N obtained with HIRES in 1994
As compared to hot stars, distant quasars with their high redshift absorption systems, offer the great advantage of providing opacity measurements for UV transitions using groundbased 8–10 m class telescopes. Another advantage of quasar absorption systems over interstellar lines is that the absorption profile induced by ions associated with damped Lyman α (DLA) often extends over an interval of 100 km s−1 or more, leading to a direct, unbiased estimate of the opacity
Summary
Absorption line studies of interstellar or intergalactic gas seen in the spectra of bright sources such as stars, quasars, or gammaray bursts provide critical information on the physical conditions prevailing in the intervening material lying along these lines of sight. For the strong 1317 and 1370 Å transitions, a rough estimate was provided by Ellison et al (2001a) from the analysis of Ni ii lines in a quasar damped Lyman α (DLA) system These transitions were still poorly characterised, which led Jenkins & Tripp (2006) to use GHRS/HST spectra for a sample of reddened stars, resulting in a significant improvement of the determination of the relative 1317, 1370, 1454, and 1741 Å f -values. Dessauges-Zavadsky et al (2006) obtained an independent measurement of the 1317 Å f -value (consistent with the one given by Jenkins & Tripp) from the analysis of five DLA systems Despite all these studies, the accuracy of f -values for some major Ni ii transitions (such as those at 1317 and 1370 Å) is still no better than about 10%. We conclude with some prospects concerning the use of distant quasar spectra to constrain f -values and summarise our main results (Sect. 5)
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