[Paper Review] Multiwavelength campaign on Mrk 509 VII. Relative abundances of the warm absorber

K. C. Steenbrugge, J. S. Kaastra+15 more|arXiv (Cornell University)|Aug 11, 2011

Galaxies: Formation, Evolution, Phenomena61 references14 citations

TL;DR

This study determines precise relative abundances of C, N, Ne, Mg, Si, Ca, and Fe relative to oxygen in the warm absorber of the Seyfert 1 galaxy Mrk 509 using high-resolution XMM-Newton RGS and Chandra LETGS spectra from a multiwavelength campaign. It finds that most elements exhibit proto-solar abundance ratios, with sulfur slightly underabundant, providing robust constraints on nuclear enrichment processes in AGN environments.

ABSTRACT

Context. The study of abundances in the nucleus of active galaxies allows us to investigate the evolution of abundance by comparing local and higher redshift galaxies. However, the methods used so far have substantial drawbacks or rather large uncertainties. Some of the measurements are at odds with the initial mass function derived from the older stellar population of local elliptical galaxies. Aims. We determine accurate and reliable abundances of C, N, Ne, and Fe relative to O from the narrow absorption lines observed in the X-ray spectra of Mrk 509. Methods. We use the stacked 600 ks XMM-Newton RGS and 180 ks Chandra LETGS spectra. Thanks to simultaneous observations with INTEGRAL and the optical monitor on-board XMM-Newton for the RGS observations and HST-COS and Swift for the LETGS observations, we have an individual spectral energy distribution for each dataset. Owing to the excellent quality of the RGS spectrum, the ionisation structure of the absorbing gas is well constrained, allowing for a reliable abundance determination using ions over the whole observed range of ionisation parameters. Results. We find that the relative abundances are consistent with the proto-solar abundance ratios: C/O = 1.19$\pm$0.08, N/O = 0.98$\pm$0.08, Ne/O = 1.11$\pm$0.10, Mg/O = 0.68$\pm$0.16, Si/O = 1.3$\pm$0.6, Ca/O = 0.89$\pm$0.25, and Fe/O = 0.85$\pm$0.06, with the exception of S, which is slightly under-abundant, S/O = 0.57$\pm$0.14. Our results, and their implications, are discussed and compared to the results obtained using other techniques to derive abundances in galaxies.

Motivation & Objective

• To determine accurate and reliable relative abundances of key elements (C, N, Ne, Fe, etc.) in the warm absorber of Mrk 509 using high-resolution X-ray spectroscopy.
• To resolve discrepancies in abundance measurements from previous methods, which suffer from large uncertainties or systematic biases.
• To compare the derived abundances with proto-solar ratios and with abundances in other astrophysical environments to infer enrichment history and nuclear processes.
• To validate the results using absorption measure distribution modeling and cross-checks with independent abundance measurements in similar AGN.
• To assess the reliability of broad emission-line-based abundance determinations in high-redshift quasars, which may be biased by resonance scattering.

Proposed method

• Stacked 600 ks XMM-Newton RGS and 180 ks Chandra LETGS spectra were used to analyze narrow X-ray absorption lines from the warm absorber.
• Simultaneous multiwavelength observations from INTEGRAL, XMM-Newton Optical Monitor, HST-COS, and Swift provided individual spectral energy distributions for each dataset.
• The ionisation structure of the absorbing gas was well constrained by the high spectral resolution of the RGS, enabling reliable abundance determination across a wide range of ionisation parameters.
• Abundance ratios were derived by fitting observed absorption lines from multiple ionisation states of C, N, Ne, Mg, Si, S, Ca, and Fe relative to oxygen.
• Absorption measure distribution (AMD) modeling was applied to verify the abundance results, particularly for less precisely constrained elements like Si and Ca.
• Results were cross-validated against absolute abundance measurements from UV spectra of Mrk 279 and compared with abundances in local elliptical galaxies, cluster cores, and quasar broad emission lines.

Experimental results

Research questions

• RQ1 What are the relative abundances of C, N, Ne, Mg, Si, Ca, and Fe relative to oxygen in the warm absorber of Mrk 509?
• RQ2 Are the derived abundances consistent with proto-solar ratios, and if not, what does the deviation imply about nucleosynthetic processes?
• RQ3 How do the abundance ratios in Mrk 509 compare to those in other AGN, local galaxies, and intra-cluster media?
• RQ4 To what extent are broad emission-line-based abundance determinations in high-redshift quasars biased by resonance scattering effects?
• RQ5 What do the abundance patterns reveal about the origin and evolution of the absorbing gas—particularly whether it originates from the accretion disk or torus?

Key findings

• The relative abundance ratios in Mrk 509's warm absorber are consistent with proto-solar values: C/O = 1.19 ± 0.08, N/O = 0.98 ± 0.08, Ne/O = 1.11 ± 0.10, Mg/O = 0.68 ± 0.16, Si/O = 1.3 ± 0.6, Ca/O = 0.89 ± 0.25, and Fe/O = 0.85 ± 0.06.
• Sulphur is slightly underabundant relative to proto-solar ratios, with S/O = 0.57 ± 0.14, indicating a potential deficiency in S-rich nucleosynthetic contributions.
• The abundance ratios for O/Fe, Ne/Fe, Si/Fe, S/Fe, and Ca/Fe are higher than in the cores of galaxy clusters, suggesting a dominant role for core-collapse supernovae and stellar winds in enriching the interstellar medium of Mrk 509.
• The Mg/Fe and Si/Fe ratios in Mrk 509 are consistent with those observed in the hot halo ISM of local elliptical galaxies, though O/Fe is higher than in the hot ISM.
• The abundance ratios derived from X-ray absorption lines are consistent with absolute abundances measured in Mrk 279 using UV absorption lines, supporting the reliability of the method.
• Broad emission-line-based abundance determinations in high-redshift quasars are found to be unreliable due to resonance scattering of Lyα photons, which distorts the N v line profile and biases abundance estimates.