Faculty: Professors Howes, Lang, Spangler
Students: Thomas Zimmerman
At the University of Iowa, the study of the interstellar medium and the Galactic center relies on a combination of ground and space-based observations, complemented by theoretical efforts. In particular, Iowa faculty and students are frequent usersof the Very Large Array and Very Long Baseline Array radio telescopes, and work on data obtained by the NASA Great Observatories (Chandra, Spitzer and Hubble Space Telescopes).
The Galactic Center Environment
The center of our Milky Way galaxy harbors a 4-million solar mass black hole in addition to dense concentrations of gas, strong magnetic fields and powerful young stellar clusters. The interplay of these components gives rise to episodic and energetic activity that arises from our Galactic nucleus. At only 25,000 light years distant, the Galactic center provides an opportunity to study these processes in detail. Professor Lang has recently completed several surveys of the central environment:
(1) the first comprehensive high resolution survey of neutral hydrogen in this region using the Very Large Array. This is an important study which provides fundamental insights concerning the kinematics of the central region (including the massive black hole at its center), as well as the dynamic of star formation and ionization,
(2) the first Hubble Space Telescope survey of the ionized gas as traced by the Paschen alpha line (near-infrared) reveals unprecedented detail in the interstellar medium and the interaction between the stars and gas, and
(3) the first radio polarimetric survey carried out by the VLA. This survey will help to clarify the configuration and organization of the magnetic field in this region of the Galaxy, which is thought to be much stronger and more well-ordered than in the Galactic disk. A recent "Great Observatories" panorama of the Galactic Center is shown at right.
Caption for image: View of the central 50 pc (~150 light years) of the
Galactic center showing ionized gas (traced by Paschen alpha emission observed by Hubble Space Telescope), hot plasma (traced in the X-ray by the Chandra X-ray Observatory) and warm dust (traced by mid-infrared radiation by the Spitzer Space Telescope).
The Interstellar Medium: The Impact of Massive Stellar Clusters
Professor Lang also is interested in determining how many such young, massive clusters (similar to the ones found in the Galactic center) exist throughout the Galactic disk. To do this, she is carrying out a multi-wavelength effort (with collaborators at Rochester Institute of Technology)
to image the interstellar medium (ionized gas and diffuse emission from warm gas traced in the radio and infrared) surrounding a large number of massive cluster
candidates. The Milky Way is thought to have as many as several hundred powerful young clusters.
The Interstellar Medium: The Soft X-ray Background
Professor McEntaffer studies the origin and variability of the 1/4 keV
soft X-ray background. The ROSAT observatory discovered that the
local ISM is dominated by million degree diffuse gas. However, this
emission is variable and tied to the variability of the solar wind
leading to charge exchange of the solar wind with interstellar
neutrals as a significant emission mechanism for the background. The
time variability of important charge exchange lines will be studied by
a suite of suborbital sounding rocket flights occurring between
The Interstellar Medium: Plasma Aspects
The interstellar medium is not a quiescent environment but rather a turbulent one, and this interstellar turbulence significantly affects the evolution of the galaxy, in particular the galactic magnetic field and the spatial distribution of star formation. Although the driving mechanisms supporting the pervasive turbulence in the interstellar medium have not been clearly identified, leading candidates are supernovae, strong stellar winds and outflows from massive stars, and the magnetorotational instability. This turbulence is believed to play an important role in the generation of the kiloparsec-scale galactic magnetic field through dynamo action. In the denser molecular clouds, turbulent motions are believed to play a dominant role in the regulation of the rate of star formation. Improving observations of this turbulence and advancing our theoretical understanding of its driving mechanisms and the resulting effect of galactic processes is an important frontier of astrophysical research.
Professor Spangler is a leading expert in the use of interstellar scintillation, sensitive to the electron density fluctuations in the turbulent interstellar medium, to probe the characteristics of the interstellar plasma turbulence. His work has identified one of the "Great Power Laws in the Sky," a spectrum of electron density fluctuations over 12 orders of magnitude in scale. The spectral index appears to be consistent with a value of -5/3, the prediction from turbulence theory. Professor Howes employs analytical models of turbulence and high-performance direct numerical simulations to study the dynamics and dissipation of turbulence in weakly collisional plasma conditions relevant to the interstellar medium, with an aim to connect numerical predictions of the plasma turbulence to the observations from interstellar scintillation.
Chandran, B. D. G., Quataert, E., Howes, G. G., Xia, Q., and Pongkitiwanichakul, P., 2009,
Lang, C.C., Drout, M.R 2008, The magnetic environment in the central region of nearby galaxies, 2008, Journal of Physics: Conference Series, 131, 012032
Lang, C.C. 2008, Non-thermal Emission in Sagittarius B?, GCNEWS – The Galactic Center Newsletter,, January 2008.
Lang, C.C., Kaaret, P., Corbel, S. & Mercer 2007, A. A Radio Nebula Surrounding the Ultraluminous X-Ray Source in NGC 5408, ApJ. 666, 79
Wang, Q.D., Dong, H. and Lang, C.C. 2006, “The interplay between star formation and the nuclear environment of our Galaxy: deep X-ray observations of the Galactic centre Arches and Quintuplet clusters”, MNRAS, 371, 38
Lang, C.C., Johnson, K.E., Goss, W.M. & Rodriguez, L.F. 2005 “Stellar Winds and Embedded Star Formation in the Galactic Center Arches & Quintuplet Clusters: Multifrequency Radio Observations, AJ, 130, 2185
McEntaffer, R. L., & Cash, W., “Soft X-ray Spectroscopy of the Cygnus Loop Supernova Remnant”, ApJ, 680, 328-335, 2008.
Spangler, S. 2009, Plasma Turbulence in the Local Bubble, Sp. Sci. Rev.
Whiting, C., Spangler, S., Ingeby, L. Haffner, M. 2009, Confirmation of a Faraday Rotation Measure Anomaly in Cygnus, Ap.J.