Current Challenges of Astrophysical Photoionized Plasmas

Photoioniation and photoexcitation has been known to be important in astrophysical plasmas since the early study of emission line nebulae, and the study of optical and UV emission lines is a classical subject. Recently, this has broadened to include emission in other wavelength bands. In the X-ray band, Chandra and XMM-Newton have revealed rich spectra from active galaxies and compact binaries, and this has motivated new efforts at modeling these plasmas. This has led to insights about a component of the gas in these systems which had been previously unknown, observationally, and which has implications for our understanding of the global mass budget. In this talk I will review the observational data on these plasmas, and discuss some recent results on modeling.