Small-Scale Structure in the Interstellar Medium 

 
Introduction	Northwestern Group	Software	Publications	Links
DensePak Na I D absorption maps of the vLSR=3 km s-1 LISM (left) and vLSR=68 km s-1 IVC (right) components toward M15.

Introduction

Basic Premise: Over the past 25 years, the existence of small clumps of dense neutral gas in the interstellar medium (ISM) has been confirmed repeatedly, with a myriad of observational techniques. These small-scale structures ("small" usually meaning less than a parsec in radius) now present many interesting questions for ISM researchers. This page is designed as a home base for this research, as well as to present new results from the team studying this phenomenon at Northwestern University. The next few sections provide a brief overview of the subject. For more detail, refer to the Publications section.

Radio Observations: Using the technique of Very Long Baseline Interferometry (VLBI), astronomers have been able to measure opacity and spectral variations on tiny scales toward multi-component extragalactic radio sources. Additionally, other groups have used pulsars as background sources to detect the structure in the ISM down to the smallest scales. These observations have repeatedly shown small-scale variations on the order of 10 - 100 AU, although the ubiquity of such structure has recently been called into question.

Optical and UV Observations: In the optical (and, to a lesser extent, the ultraviolet), studies have focused on spectral variations, primarily using the Na I D lines. One early technique measured Na I column density variations toward several red giant stars in globular clusters separated by only a few to 10 arcminutes. Another method uses the characteristics of several different interstellar spectral lines measured toward binary star systems to detect small-scale ISM structure down to scales of approximately 10 - 1000 AU. Among other efforts, a new method has been developed to use the DensePak fiber bundle (integral field spectroscopy unit) to map Na I D column density variations toward globular clusters and galaxies on scales down to about 1000 AU.


Na I D2 spectra of the ISM toward the individual stars in the binary mu Cru.

Theory: Because of the large overpressures these small-scale structures would exert on the ambient medium, their ubiquity is not accounted for in the otherwise successful ISM model of McKee and Ostriker (1977). Many theoretical explanations have been proposed, from geometric solutions to fractal models driven by turbulence. More observations will continue to help pin down the causes of these structures.

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Northwestern Group

The group working on small-scale ISM structure at Northwestern University currently consists of:
  • David Meyer, Professor, Department of Physics & Astronomy and Director of the Dearborn Observatory
  • Jim Lauroesch, Research Assistant Professor, Department of Physics & Astronomy

Past members of the group working at Northwestern University include:
  • Sean Andrews, Graduate Student, Institute for Astronomy, University of Hawaii
  • Stefan Cartledge, Postdoctoral Researcher, Department of Physics & Astronomy, Louisiana State University & Agricultural & Medical College
  • Sean Points, Cerro Tololo Inter-American Observatory
Additionally, Michael Faison has worked on this topic using radio data from the Very Large Array (VLA). Other collaborators include (in no particular order) J. Chris Blades (STScI), J. K. Watson, Scott Friedman (STScI), and Kathy Roth (Gemini Observatory).

The group has pioneered the techniques of small-scale structure observations toward binary stars, and mapping column density variations toward globular clusters and galaxies with integral field spectroscopy. These two projects continue in full swing with new FUSE, HST/STIS, and Coude Feed data, as well further proposed integral field spectroscopic observing runs.

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Software

The DPmap code is available for use. This IDL code package contains a program which will create maps of DensePak data similar to those used by the Northwestern Group in their publications regarding integral field spectroscopic mapping of column density variations toward globular clusters. Just read the brief instructions...its simple! We ask that if you publish material using this code, please properly acknowledge Sean Andrews and Jim Lauroesch. DOWNLOAD (gzipped tar file).

Publications


Meetings of Note:,

The Small Ionized and Neutral Structures in the Diffuse Interstellar Medium meeting in June 2006 will provide an opportunity for the radio and optical/UV communities to jointly meet and discuss the various phenomena associated with small scale structures. Professor Meyer is a member of the Scientific Organizing Committee for this meeting, and Professor Lauroesch is an invited speaker.

Refereed:


Conference Proceedings:

  • "The High-Velocity NaI Cloud Complex Toward M81" Lauroesch, Meyer, & Roth 2005, AAS Meeting 207 in Washington D.C.

  • "Probing the Diffuse Interstellar Medium at AU Scales" Lauroesch, J. T., & Meyer, D. M. 2002, AAS Meeting 201 in Seattle, WA

  • "The Complex Interstellar Structure Toward h and Chi Persei" Points, S. D., Meyer, D. M., Lauroesch, J. T., Nguyen, D. T., & Friedman, S. D. 2002, AAS Meeting 201 in Seattle, WA

  • "The Andromeda Project: Mapping the Small-Scale ISM" Lauroesch, J. T., Meyer, D. M., Andrews, S. M., & Roth, K. C. 2001, AAS Meeting 199 in Washington, DC
    A mini-pdf version of the poster can also be downloaded.

  • "The Physical Character of Interstellar Small Scale Structure" Lauroesch, J. T. 2001, in "Gaseous Matter in Galaxies and Intergalactic Space", ed. Roger Ferlet, Matin Lemoine, Jean-Michel Desert, & Brigitte Raban (17th IAP Astrophysics Colloquim), 59

  • "The Small-Scale Structure of High-Velocity Na I Absorption Toward M81" Roth, K. C., Meyer, D. M., & Lauroesch, J. T. 2001, in "Gaseous Matter in Galaxies and Intergalactic Space", ed. Roger Ferlet, Matin Lemoine, Jean-Michel Desert, & Brigitte Raban (17th IAP Astrophysics Colloquim), 211

  • "The Small-Scale Structure of High-Velocity Na I Absorption Toward M81" Roth, K. C., Meyer, D. M., & Lauroesch, J. T. 2001, AAS Meeting 197 in San Diego, CA

  • "Small-Scale Interstellar Na I Structure Toward M92" Andrews, S. M., Meyer, D. M., & Lauroesch, J. T. 2001, AAS Meeting 197 in San Diego, CA

  • "The Small-Scale Structure of Diffuse Interstellar Clouds" Meyer, D. M. 2000, AAS Meeting 196 in Rochester, NY

  • "Small-Scale Interstellar Structure towards HD 32039/40 -- Temporal Na I Variations and HST/STIS Observations" Lauroesch, J. T., Meyer, D. M., & Blades, J. C. 2000, AAS Meeting 195 in Atlanta, GA

  • "A Na I Absorption Map of the Small-Scale Structure in the Interstellar Gas Toward M15" Meyer, D. M., & Lauroesch, J. T. 1999, AAS Meeting 194 in Chicago, IL

  • "AAT and HST Spectroscopy of Small-Scale Interstellar Structure toward the Binary mu Cru" Lauroesch, J. T., Meyer, D. M., Watson, J. K., & Blades, J. C. 1997, AAS Meeting 191 in Washington, DC

  • "Small Scale Structure in the Diffuse ISM" Watson, J. K., & Meyer, D. M. 1996, AAS Meeting 188 in Madison, WI

  • "Small-Scale ISM Structure: The Remarkable Sightline Toward mu Cru" Meyer, D. M., & Blades, J. C. 1995, AAS Meeting 187 in San Antonio, TX
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Links

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Mail any comments to andrews@ifa.hawaii.edu and/or jtl@elvis.astro.northwestern.edu