(Abridged) Cold and warm absorbers have beeen detected in all types of active
galaxies (AGN) from low to high redshift. This gas, located in the black hole
region of AGN, is thought to play an important role in AGN unification
scenarios, in explaining the X-ray background, in black hole growth and AGN
evolution.
We provide a review of the observations of dusty and dust-free warm and cold
absorbers at low and high redshift, including most recent results and exciting
questions still open. Emphasis is on the science issues that we will be able to
address with XEUS for the first time, particularly at high redshift, including:
(i) determination of metal abundances of X-ray (cold) absorbers by detection of
metal absorption edges, (ii) analysis of the composition of dust mixed with
cold and ionized gas (K-edges of metals in cold dust and cold gas will be
resolvable from each other for the first time), (iii) measurement of the
velocity field of the gas, (iv) utilization of these results to investigate the
evolution of gas and dust in AGN from high to low redshift: the evolution of
abundances, dust content, ionization state, amount and velocity of gas, and its
role in feeding the black hole.
We emphasize the importance of iron absorption measurements with XEUS at high
redshift for two key issues of cosmology: the early star formation history of
the universe, and the measurement of cosmological parameters. As an example, we
discuss recent XMM-Newton observations of the high-redshift BAL quasar APM
08279+5255.Comment: 8 pages, to appear in the proc. of the workshop "XEUS - studying the
evolution of the universe", G. Hasinger et al. (eds), MPE Report, in press.
Related papers on the high-z BAL quasar APM 08279+5255 (and MPG press
release), and on ionized absorbers are available at
http://www.xray.mpe.mpg.de/~skomossa
