The Galactic Interstellar Medium

409 Pages·1992·8.82 MB·English

by  Professor W. B. Burton

Saas-Fee Advanced Course 21
Lecture Notes 1991
W. B. Burton B. G. Elmegreen
R. Genzel
The Galactic
Interstellar Medium
Saas-Fee Advanced Course 21
Lecture Notes 1991
Swiss Society for Astrophysics and Astronomy
Edited by D. Pfenniger and P. Bartholdi
With 118 Figures
Springer-Verlag
Berlin Heidelberg New York
London Paris Tokyo
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Budapest
Professor W. B. Burton
Sterrewacht Leiden, PO Box 9513, NL-2300 RA Leiden, The Netherlands
Professor B. G. Elmegreen
IBM, Thomas J. Watson Research Center, PO Box 218, Yorktown Heights, NY 10598, USA
Professor R. Genzel
Max-Planck-Institut fiir Physik und Astrophysik, Karl-Schwarzschild-Strasse 1,
W-8046 Garching, Fed. Rep. of Germany
Volume Editors:
Dr. D. Pfenniger
Dr. P. Bartholdi
Observatoire de Geneve, ch. des Maillettes 51, CH-1290 Sauverny, Switzerland
This series is edited by Professor T. J.-L. Courvoisier on behalf of the
Swiss Society for Astrophysics and Astronomy:
Societe Suisse d'Astrophysique et d'Astronomic
Observatoire de Geneve, ch. des Maillettes 51, CH-1290 Sauverny, Switzerland
ISBN 3-540-55805-5 Springer-Verlag Berlin Heidelberg New York
ISBN 0-387-55805-5 Springer-Verlag New York Berlin Heidelberg
Library of Congress Cataloging-in-Publication Data. Burton, W. B. (William Butler), 1940- . The galactic
interstellar medium / W. B. Burton, B. G. Elmegreen, R. Genzel; edited by P. Bartholdi and D. Pfenniger. p. cm.
- (Saas-Fee advanced course 21 lecture notes ; 1991). Includes bibliographical references and index. ISBN 3-540-
55805-5 (Berlin). - ISBN 0-387-55805-5 (New York). 1. Milky Way. 2. Interstellar matter. 3. Molecular clouds.
4. Astrophysics. I. Elmegreen, Bruce G. II. Genzel, R. III. Bartholdi, P. IV. Pfenniger, D. V. Title. VI. Series: Saas-
Fee advanced course ... lecture notes ; 1991. QB857.7.B87 1992 523.1'125-dc20 92-30271
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Foreword
The previous Saas-Fee Advanced Course dedicated to the interstellar medium
took place in 1972. The tremendous scientific advances that have occurred in
this field since then, in particular owing to the availabihty of receivers working
at completely unexplored wavelength bands, fuUy justified a new set of lectures.
As a consequence, the members of the Swiss Society for Astrophysics and As
tronomy voted that "The Galactic Interstellar Medium" should be the subject
of the 1991 course. The 21st Saas Fee Advanced Course took place in Les
Diablerets from 18 to 23 March 1991, gathering together about 80 participants
from all over the world, but mostly from Europe.
According to a rule that has proved to lead to success, but also to chal
lenge the lecturers' energy, the format of a Saas-Fee Advanced Course consists
traditionally of 28 lectures of 45 minutes which take place in the morning and
late afternoon, leaving ample time for discussions, self-study, hiking or skiing.
Despite the inordinate work load imposed, this year's lecturers felt that the
subject was sufficiently dense to increase the lecture time by 1/3! This proved
judicious and left more time for questions and discussions during the lectures.
This volume contains the written form of the dehvered lectures, which fas
cinated many in the audience not only because of the width and depth of the
subject, but because of the high quality of the transmitted information. The
course topics were divided up as foUows: Butler Burton spoke about "Distribu
tion and Observational Properties of the ISM", Bruce Elmegreen about "Large
Scale Dynamics of the ISM", and Reinhard Genzel about "Physics and Chem
istry of Molecular Clouds". Although a complete coverage of the Galactic ISM
is nowadays quite impossible in a one-week course, these written lectures give
probably one of the most up-to-date presentations of the essential facts about
the interstellar medium. Over the coming years this volume should remain
invaluable to students and active or new researchers in the field.
The Saas-Fee Courses are financed in large part by the Swiss Academy of
Natural Sciences, to which the Swiss Society for Astrophysics and Astronomy
belongs. Thanks to continuous support this series of successful lectures has
continued uninterrupted for more than 20 years.
The practical organisation of this year's course was greatly faciHtated
by our secretary, Mrs Irene Scheffre, and our assistants, Daniel FriedH and
Stephane Udry. We thank them for their generous efforts.
Geneva, June 1992 D. Pfenniger and P. Bartholdi
Contents
Distribution and Observational Properties of the ISM
By W.B. Burton (With 72 figures)
Chapters 5 and 6 are co-authored, respectively, with E.R. Deul & H.S. Liszt
1. Introduction 1
1.1 Some Qualifying Remarks 1
1.2 Some Suggested General References on Various Aspects of Galactic
Structure 2
References 4
2. Observations of HI and Other Tracers of the Low-Latitude Morphology
of the Interstellar Medium in Our Galaxy 5
2.1 Observations of Pervasive, Heavily Blended HI Emission from the
Low-Latitude Galactic Gas Layer 5
2.2 Observations of CO and Other Tracers of the Gas Layer Morphology 15
2.3 Global Optical-Depth Effects and the Interpretation of Measured HI
Intensities 20
2.4 Interpretation of the HI Integrated Emission and the Majdmum
Brightness Observed: Is the Low-|6| HI Gas Layer Globally Trans
parent? 22
2.5 Small-Scale Structure in Heavily-Blended Low-|6| HI Emission Spec
tra 29
References 34
3. Galactic Rotation and the Constants Specifying the Kinematic and Lin
ear Scales of the Milky Way; Line-Broadening Mechanisms Determining
Profile Shapes at Low Latitudes 36
3.1 Introduction; Galactic Rotation 36
3.2 The Distance of the Sun from the Galactic Center, RQ 38
3.3 The Galactic Circular-Velocity Rotation Curve Evaluated at R<Ro]
the Value of e{R) at JRo, 6>o 39
3.4 The Galactic Circular-Velocity Rotation Curve Evaluated at R>Ro 42
3.5 Line-Broadening Mechanisms Other than Simple Circular Galactic
Rotation 43
3.6 Considerations Regarding the Velocity-to-Distance Transformation 52
References 57
VII
4. Global Structural Distribution of Low-Latitude Tracers of the Inner
Galaxy 59
4.1 Mapping Limitations Imposed by Our Embedded Perspective ... 59
4.2 Longitudinal Distributions of Low-|6| Tracers 61
4.3 Radial Distributions Characterizing the Population I Layer 66
4.4 Vertical Thickness and Degree of Flatness of the Inner-Galaxy Gas
and Dust Layer 72
References 77
5. Comparative Global Properties of Interstellar Dust and Gas in the
Galaxy 79
5.1 Introduction 79
5.2 Correlation Between HI and 100-/Am Intensities 81
5.3 Radial Unfolding 85
5.4 Morphological Properties 87
5.5 Concluding Remarks 96
References 98
6. Kinematics and Distribution of Neutral Interstellar Gas in the Galactic
Bulge Region 100
6.1 Use of the Terminal-Velocity Measure to Determine the Inner-
Galaxy Rotation Curve 100
6.2 The Role of Absorption in Determining the Paucity of HI Emission
in the Z > 0°, v < Okms"^ Quadrant at 6 c^ O"* 105
6.3 The Tilted Nature of the Inner-Galaxy Gas Distribution Ill
6.4 Modelling Some Aspects of the Kinematics and Distribution of the
Gas Layer in the Bulge 114
6.5 Some Brief Remarks on the Form of the Potential Dominating the
Motions in the Bulge Region, and on the Situation in M31 122
References 124
7. The Warped and Flaring Layer of Atomic and Molecular Gas in the
Outer Galaxy 126
7.1 Introduction; the Shape of the Outer Galactic Disk 126
7.2 The Shape of the Warped and Flaring HI Layer Extending to the
Far Outer Galaxy 128
7.3 Comments Regarding the Apparent Lopsidedness of the Outer
Galaxy 140
7.4 The Shape of the Molecidar-Cloud Ensemble in the Outer Galaxy 142
7.5 Warps in the Two Nearby Galaxies M31 and M33 149
References 153
VIII
Large Scale Dynamics of the Interstellar Medium
By Bruce G. Elmegreen (With 12 figures)
1. Introduction 157
References 158
2. Basic State of the Interstellar Gas 159
2.1 Giant Clouds 159
2.2 Giant Holes 164
2.3 Hierarchical Structure 167
2.4 Gas Motions 170
2.5 Magnetism 172
References 173
3. Large Scale Heating and CooHng 178
3.1 Energy Sources 178
3.2 Energy Dissipation 181
3.3 Magnetic Diffusion 187
References 189
4. Explosions 191
4.1 Nonhnear Waves and Shock Fronts 191
4.2 Discontinuous Fronts 194
4.3 Continuous Magnetic Pressure Fronts 208
4.4 Common Explosions 211
4.5 SupersheUs and Chimneys 217
4.6 Bowshocks and Jets 219
References 220
5. Gas Flow in Spiral Density Waves 224
5.1 An Introduction to SteUar Density Waves 224
5.2 Gas Flow in Stellar Density Waves 231
References 245
6. Gas Instabilities 248
6.1 Thermal InstabiHty 248
6.2 Gravitational Instability 251
6.3 Instabihties in Galactic Disks 252
6.4 Two Fluid Gravitational Instabihties 256
6.5 Instabihties in Galactic Spiral Shocks 257
6.6 Collapse of Elhpsoids 260
References 262
7. Global Star Formation 264
7.1 The Star Formation Rate and Surface Density Threshold 264
7.2 Exponential Disks in Galaxies 267
7.3 A Universal Galaxy Rotation Curve 269
7.4 Star Formation and Spiral Density Waves 269
7.5 Starburst Galaxies 272
References 273
IX
Physics and Chemistry of Molecular Clouds
By Reinhard Genzel (With 34 figures)
Introduction 275
1. Tools of Infrared and Radio Astronomy 277
1.1 Sumniary of Infrared/Radio Spectroscopy 277
1.2 Radiative Transport and Excitation 287
1.3 Estimates of Cloud Column Densities and Masses 308
1.4 Measuring Temperature and Density 313
References 317
2. Heating and Cooling Processes 319
2.1 Photon Domiinated/Photodissociation Regions 320
2.2 Shocks in Dense Interstellar Clouds 325
2.3 Heating by Energetic Particles 331
2.4 Heating by Ambipolar Diffusion 333
References 335
3. Interstellar Chemistry 337
3.1 Chemistry of the Gas 337
3.2 Composition of Dust Grains 343
References 346
4. Structure and Dynamics of Molecular Clouds 349
4.1 The Spatial Structure of Molecular Clouds 349
4.2 Energy Balance of Molecidar Clouds 353
4.3 An Example of a Star Forming Cloud: Orion 356
4.4 The Orion-KL Star Forming Core 361
4.5 Outflows in Star Forming Regions 367
References 367
5. Circumstellar Clouds and Masers 371
5.1 A Close-Up View of BN and IRc2 371
5.2 Circumstellar Disks in Low Mass Star Formation Regions 372
5.3 Interstellar Masers 372
References 377
6. Molecular Clouds in the Galactic Center and External Galaxies 379
6.1 Our Galaxy: The Central 500pc 379
6.2 Molecular Gas in Starburst Galaxies 381
6.3 Neutral Gas in Photon Dominated Regions 386
6.4 Molecular Gas in AGN's 388
References 390
Subject Index 393
List of Previous Saas-Fee Advanced Courses:
1991 THE GALACTIC INTERSTELLAR MEDIUM,
W.B. Burton, E.G. Elmegreen, R. Genzel
1990 ACTIVE GALACTIC NUCLEI,
R. Blandford, H. Netzer, L. Woltjer
1989 THE MILKY WAY AS A GALAXY,
G. Gilmore, I. King, P. van der Kruit
1988 RADIATION IN MOVING GASEOUS MEDIA,
H. Frisch, R.P. Kudritzki, H.W. Yorke
1987 LARGE SCALE STRUCTURES IN THE UNIVERSE,
A.C. Fabian, M. Geller, A. Szalay
1986 NUCLEOSYNTHESIS AND CHEMICAL EVOLUTION,
J. Audouze, C. Chiosi, S.E. Woosley
1985 HIGH RESOLUTION IN ASTRONOMY,
R.S. Booth, J.W. Brault, A. Labeyrie
1984 PLANETS, THEIR ORIGIN, INTERIOR AND ATMOSPHERE,
D. Gautier, W.B. Hubbard, H. Reeves
1983 ASTROPHYSICAL PROCESSES IN UPPER MAIN SEQUENCE STARS,
A.N. Cox, S. Vauclair, J.P. Zahn
* 1982 MORPHOLOGY AND DYNAMICS OF GALAXIES,
J. Binney, J. Kormendy, S.D.M. White
1981 ACTIVITY AND OUTER ATMOSPHERES OF THE SUN AND STARS,
F. Praderie, D.S. Spicer, G.L. Withbroe
* 1980 STAR FORMATION,
J. AppenzeUer, J. Lequeux, J. Silk
* 1979 EXTRAGALACTIO HiGH ENERGY ASTROPHYSICS,
F. Pacini, C. Ryter, P.A. Strittmatter
* 1978 OBSERVATIONAL COSMOLOGY,
J.E. Gunn, M.S. Longair, M.J. Rees
* 1977 ADVANCED STAGES IN STELLAR EVOLUTION,
I. Iben Jr., A. Renzini, D.N. Schramm
* Out of print
XI