Supermassive black holes occupy the centers of many, possibly most galaxies. Their immediate surroundings, however, and much of their host galaxies, are filled with a relatively tenuous gas. When black holes are actively accreting (active galactic nuclei or AGN), they can light up their surroundings, and when this gas is lit we call it the Narrow Line Region or NLR.
It's called the Narrow Line Region because we see it in unresolved AGN spectra the form of narrow emission lines such as doubly ionized oxygen ([O III] at 5007 Å) or neutral hydrogen (Hα at 6563 Å).
These narrow lines tell us that the gas is slow, low-density, and relatively distant from the black hole, from a few light years to many thousands. These properties make the NLR very responsive to slow processes which affect much or all of a whole galaxy, like AGN feedback and AGN shutdown, which are topics that I study.
These lines are particularly powerful when we can spatially resolve them, because then we can derive physical parameters such as gas temperature and density in different regions of a galaxy. We can learn how intensely AGN emission is ionizing its host galaxy, or where gas outflows from an AGN are pushing through interstellar gas at locally supersonic speeds (shocks!).Hubble has the angular resolution to derive this information on physically interesting scales. By using filters to block light outside of the most interesting wavelengths, we get pictures of the intensity of a single emission line!
In NGC 3393, my collaborators and I compared high-resolution line ratio images of [O III], [S II] and Hα and saw that while material inside the ionization cones was very intensely ionized (like a Seyfert galaxy), the immediate outside was characteristic of what we see in Low Ionization Nuclear Emission Regions (LINERs). This suggests that the AGN disk wind could be surrounded by higher-density shocked gas, or possibly that we can observe a well-defined region outside of which the AGN emission is filtered through its dusty torus.
Thanks to a new Hubble program (#15350, 19 orbits), I am now leading my collaborators in a much broader investigation of these phenomena. [O III] and Hα data are common, but with new [S II] and Hβ data, we will get a more comprehensive picture of how these AGN interact with their host galaxies.
Integrated Field Units or IFUs (such as GMOS on Gemini) on ground-based telescopes might not have Hubble's spatial resolution, but they excel in sensitivity, and give us the whole spectrum at the same time! This allows us to construct a very complicated physical picture which includes gas velocities and simultaneous imaging of many different diagnostic lines.
IFUs on the James Webb Space Telescope will allow us to study very sensitive images of high-resolution emission line diagnostics in the infrared (such as [Ne V]), which also penetrate the dust and gas which commonly surround supermassive black holes. This will allow us to study how AGN interact with their environments.
ACIS on the Chandra X-ray Observatory is also an IFU, since it can measure the energy of each photon as it hits the detector! Chandra allows us to create images of powerful diagnostic lines such as O VII, O VIII, and Ne IX, which lets us see where AGN outflows are shocking gas in the host galaxy, and where feedback is dominated by photoionizing radiation from the accretion disk.
Chandra's spatial resolution makes it powerful, but to take full advantage of its capabilities you need far more observing time than is available to astronomers. Lynx is therefore a necessary and natural step. Its energy resolution will be much better, separating emission lines and extracting velocity information as cleanly as optical and infrared IFUs.
The collecting area of Lynx is also much larger. This is extremely important! Gathering a few photons over the course of a whole day allows educated guesses about physics with powerful statistical techniques. But full spectral fitting often requires insanely long observations. Lynx will open up X-ray astronomy wide for the whole community, making its powerful diagnostics accessible to all astronomers even with modest observing times.