Center for Astrophysics | Harvard & Smithsonian
Lewis McCallum
Astrophysicist working on the nearby Milky Way
I work on three-dimensional maps of the gas, radiation, and magnetic field within 1.25 kpc of the Sun, and simulations that follow how ionized gas evolves through time.
Electron density | 3D reconstruction
Electron density around the Sun
This reconstruction uses the Edenhofer 3D dust map as its gas-density structure and a census of nearby O stars as its ionizing sources. It estimates where free electrons are distributed within 1.25 kpc of the Sun.
Ionized gas | Multiwavelength prediction
Halpha, [N II], and [O III] emission
The same model predicts the emission from the ionized gas. Halpha marks recombining hydrogen, while [N II] and [O III] respond differently to temperature and ionization. Showing them together makes it easier to compare where each line is produced.
Atomic gas | Information Field Theory
The density of local HI
HI4PI tells us where atomic hydrogen appears on the sky and how quickly it is moving. I combine that information with the Edenhofer 3D dust map to work out where the gas sits in physical space. The atomic gas is smoother and more extended than the dust.
Molecular transition | H2 posterior
Where atomic gas becomes molecular
Molecular hydrogen gathers in denser cloud complexes, where the gas becomes shielded from ultraviolet light. These concentrations highlight parts of the local ISM where gas can cool, collapse, and ultimately form stars.
Kinematics | Velocity-resolved HI
How local atomic gas moves
Here colour and moving traces show whether the atomic gas is approaching or receding. Density and velocity are inferred together, which helps separate nearby gas from unrelated emission farther across the Galaxy. The inferred motions can also be compared with maser and young-cluster measurements.
Magnetized ISM | Radial field component
The local magnetic field in three dimensions
This map combines my electron-density reconstruction with the Hutschenreuter et al. Faraday-rotation sky to infer the part of the magnetic field pointing along our line of sight. The moving traces distinguish field directed toward us from field directed away. I only show the reconstruction where the electron-density map provides enough support; the uncertain strip close to the Galactic midplane remains masked.
First-author publications
Papers
These papers cover the 3D mapping and simulation work shown on this site.
About
About my work
I work on models that turn all-sky observations into a physical picture of the Milky Way around us. That means combining radiative transfer, photoionization, radiation hydrodynamics, and Bayesian reconstruction, then checking that the pieces tell a consistent story about the interstellar medium.