How do black holes form? Do they form first, and trigger the birth of galaxies around them, or do galaxies form first and stimulate the formation of black holes? How do black holes grow? Do they and grow in a merger tree as galaxies collide? Or do they accumulate their mass by hydrodynamic accretion from surrounding gas and stars in a single galaxy? TPF will image the circum-nuclear disks of systems such as M106 (NGC 4258). The H₂O maser disk in that galaxy requires significant IR pumping near 10 microns - thus such disk will be very bright. Darwin/TPF-I will compliment ELT imaging. High contrast between the AGN and the disk required high-fidelity imaging, search for the dominant mode of AGN feeding, probe the launch and collimation regions of AGN jets.

The Galactic center contains the nearest massive black hole (3.6 x 10⁶ M_o), a uniquely dense star cluster containing up more that 10 million stars per cubic parsec, and a remarkable group of high-mass stars with Wolf-Rayet-like properties. Darwin/TPF-I will be able to trace the distribution of lower mass stars, and probe the distribution of dust and plasma in the immediate vicinity of the central black hole.

Galaxies contain exotic systems in which one or more stars orbits an exotic star or a collapsed object such as a white dwarf, neutron star, or black hole. Mass transfer can result is mass ejections in the form of excretion disks such as those seen around contact binaries and symbiotic systems. In others, mass transfer produces accretion disks which drive powerful winds or jets. In accreting neutron star or black-hole systems, mass transfer can produce relativistic jets which often mimic the behavior of quasars (hence the term micro-quasar). However, in these systems, phenomena occur on time-scales orders of magnitudes shorter than in AGN. Exotic and symbiotic systems include massive stars that have undergone recent eruptions such as eta Carinae, Roche-lobe overflow systems that have shed circumstellar disks such as WeBo1, and the micro-quasars such as SS 433. Infrared emission can be produced by warm dust in circumstellar tori, by molecules, highly ionized species such as neon and argon, or continuum processes such as synchrotron radiation or the inverse Compton effect. Darwin/TPF-I will revolutionize the investigation of these systems by probing the inner AU-scale regions where these flows are energized.

Supermassive black holes are found in the centers of many galaxies. When these objects are fed by strong accretion flows, they eject relativistic jets, powerful winds, and can power intense luminosity. Darwin/TPF-I will enable the diagnosis of physical and chemical properties of active galactic nuclei (AGN) at all redshifts. Its 2 to 10 mas angular resolution will produce resolutions ranging from under 1 pc for the nearest AGN to 10s of pc for the most distant. Darwin/TPF-I will provide a look at the stellar and interstellar environments of these million to 10 billion Solar mass black holes in unprecedented detail. Emission lines such as Br α and [Ne II], and Argon will trace the ionized and shock excited components of the circum-nuclear environment. Are mini-spirals such as that seen in our own Milky Way common? Continuum interferometry will map the circum-nuclear distributions of stars. Synchrotron emission produced by relativistic particles gyrating in nuclear magnetic fields will trace non-thermal continua. The distributions of silicate dust, ices, and PAHs can be used to trace the warm gas and dust distributions in the circum-nuclear environments of AGN with unprecedented resolution.

Interferometric measurements of lensed objects may provide the highest resolution studies of stars and interstellar media in the distant Universe. By combining the milli-arcsecond resolution of Darwin/TPF-I with the natural magnification of a gravitational lens, linear resolutions of less than 1 parsec in the high-redshift Universe can be achieved.