Alex S. Hill: Research

The Interstellar Medium

The interstellar medium of our Galaxy is an important part of the star formation cycle. Cold (∼100 K) clouds of atomic and molecular gas and dust collapse under their own gravity to form stars and planets. Late in the life of a star, nuclear fusion builds hydrogen and helium into heavier elements, which were not present in significant amounts immediately after the Big Bang, such as carbon, nitrogen, oxygen, and iron. When the star exhausts its nuclear fuel and dies, it either ejects a planetary nebula (Sun-like stars) or explodes in a supernova (stars more massive than about 5 solar masses). These deaths return some of the gas in the star — still mostly hydrogen and helium, but now enriched with heavy elements by the fusion processes — to the interstellar medium. The enriched gas then mixes with the surrounding interstellar medium, out of which subsequent generations of molecular clouds and stars form. Thus, all the heavy elements present on the Earth were formed in now-dead stars and cycled to us through the interstellar medium.

The interstellar medium is composed of three thermally stable phases: “cold” (∼100 K), primarily neutral gas, “warm” (∼8000 K), neutral and ionized gas, and “hot” (∼10⁶ K), ionized gas. The three phases are thought to be in rough pressure equilibrium.

Warm Ionized Medium

I am primarily interested in the warm, ionized component (WIM) of the interstellar medium. The WIM consists of a diffuse plasma layer which occupies about 20% of the volume of a 2000 pc (∼6000 light years) thick layer centered on the midplane of the Galaxy; for comparison, the thin stellar disk, which includes relatively young stars such as our Sun, is a few hundred parsecs thick, and the neutral hydrogen disk is thinner than that of the stars. The WIM is pervasive and isothermal with a temperature of 8000 K and a space-averaged, mid-plane electron number density of 0.03 particles cm^-3.

The WIM is heated and ionized by a combination of the radiation from hot stars and supernova shock fronts. However, because the hot, massive stars are mostly within a few hundred parsecs of the midplane, the mechanism by which the radiation and shocks reach 1000 pc above the midplane is unknown, so the pervasive nature of the WIM is surprising.

The WIM is primarily observed in two ways. It was first discovered as a consequence of the 1968 discovery of pulsars. Radio signals from pulsars propagate through the WIM en route to a telescope on the Earth; the free electrons both disperse (slow down low frequencies more than high frequencies) and scatter the passing radio waves. Also, as free electrons recombine with ions, transitions within the newly recombined atoms result in emission lines which can be detected with optical telescopes. The dominant ion in the WIM is ionized hydrogen (H⁺, or a proton); the strongest optical transition within the hydrogen atom is from the third to the second level. This transition, called Balmer-Alpha or Hα, has a wavelength of 656 nm (red light).

Wisconsin H-Alpha Mapper

The Wisconsin H-Alpha Mapper, affectionately known as WHAM, is a spectroscopic telescope specially designed to study the faint, optical emission lines from the WIM. It was used to complete the WHAM Northern Sky Survey, the first absolutely calibrated, kinematically resolved sky survey of the Hα emission line. The survey detected Hα emission in all directions.

We are still mining the original survey data to better understand the WIM. With Prof. Bob Benjamin, Prof. Ron Reynolds, and Dr. Matt Haffner, I have recently found that the emission from the WIM is beautifully described by a lognormal distribution. A lognormal distribution of density is characteristic of a density distribution established by isothermal turbulence, so we are applying simulations of isothermal turbulence developed by Dr. Grzegorz Kowal and Prof. Alex Lazarian to the WIM. These models promise to improve our understanding of the role turbulence plays in heating the WIM.

Pulsar Scintillation

As an undergraduate at Oberlin College, I studied the WIM using pulsar scintillation with Prof. Dan Stinebring. Pulsars, although fascinating in their own right, are primarily interesting to me because they are bright sources of coherent radiation which serve as excellent probes of the WIM. Turbulence in the WIM causes density variations, and the radio waves from a pulsar scatter off of those density variations. This causes multi-path propagation from the pulsar to a radio telescope on the Earth, so initially coherent waves arrive out of phase.

By observing the variations in intensity of the pulsar with time and observing frequency (right), we can reconstruct information about the density inhomogeneities in the WIM. We find that the scattering of radio waves by the pulsar occurs primarily in a discrete scattering region. The scattering region occupies only a few percent of the distance from the pulsar, and the location of the scattering region is constant for a given pulsar over a twenty year period. In one remarkable instance, we found evidence for a discrete, physical structure that refracted pulsar signals through angles of 10 mas. The nature of these physical structures is unknown, but they are probably less than an AU (the distance from the Earth to the Sun) in size and are highly overpressured with respect to the surrounding warm ionized medium.

See my publications for more details on my work both with WHAM and with pulsar scintillation.