Table Of ContentTHE CENTER, BULGE, AND DISK OF THE MILKY WAY
ASTROPHYSICS AND
SPACE SCIENCE LIBRARY
A SERIES OF BOOKS ON THE RECENT DEVELOPMENTS
OF SPACE SCIENCE AND OF GENERAL GEOPHYSICS AND ASTROPHYSICS
PUBLISHED IN CONNECTION WITH THE JOURNAL
SPACE SCIENCE REVIEWS
Editorial Board
R. L. F. BOYD, University College, London, England
W. B. BURTON, Sterrewacht, Leiden, The Netherlands
C. DE JAGER, University of Utrecht, The Netherlands
J. KLECZEK, Czechoslovak Academy of Sciences, Ondfejov, Czechoslovakia
Z. KOPAL, University of Manchester, England
R. LUST, Max-Planck-Institut für Meteorologie, Hamburg, Germany
L. I. SEDOV, Academy of Sciences of the U.S.S.R., Moscow, US.S.R.
Z. SVESTKA, Laboratory for Space Research, Utrecht, The Netherlands
VOLUME 180
THE CENTER, BULGE,
AND DISK OF
THE MILKY WAY
Edited by
LEO BLITZ
Laboratory for Millimeter-wave Astronomy,
University of Maryland, College Park, U.S.A.
SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.
Library of Congress Cataloging-in-Publication Data
The Center, bulge, and disk af the Milky Way / edited by Lea Blitz.
p. cm. -- (Astraphysics and space science 1 ibrary ; v. 180)
Includes bibllagraphlcal references and lndex.
ISBN 978-94-010-5250-4 ISBN 978-94-011-2813-1 (eBook)
DOI 10.1007/978-94-011-2813-1
1. Milky Way. 2. Astraphysics. 1. Blitz, Lea. II. Series.
QB857.7.C39 1992
523.1' 13--dc20 92-24067
Printed on acid-free paper
AII Rights Reserved
© 1992 Springer Science+Business Media Dordrecht
Originally published by Kluwer Academic Publishers in 1992
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic or mechanical,
inc\uding photocopying, recording or by any information storage and
retrieval system, without written permission from the copyright owner.
TABLE OF CONTENTS
Preface Vll
List of Addresses IX
A Coherent Picture of the Innermost Parsec of the Galaxy
Farhad fuse/-Zadeh & Mark Wardle 1
Mid-Infrared Emission From the Galactic Center
Dan Gezari 23
The Evolution of the Galactic Bulge
R. M.Rich 47
The Shape of the Galaxy
David N. Sperge/ 77
The Shape of the Bulge From IRAS MIRAS
Patricia Whitelock & Robill Catchpole 103
HI in the Inner Galaxy
Harvey S. Liszt 111
Molecular Clouds and Young Massive Stars in the Galactic Disk
Leonardo Bran/man 131
The Distribution of Stars in the Disk and Halo of the Galaxy
David W. Latham 155
Index 165
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Frontispiece: NGC .5102 has the morphology of a normal SO galaxy, but has a b
nucleus with the spectrum of an AO star (see Figure 17). The entire bulge of this gal.
has Balmer lines, so we may conclude that it has been involved in the same starbu
that formed the nucleus ~ 1 Gyr ago. We are reminded that bulges can form late in
life of a galaxy. Plate (l03aO+GG1:3) obtained with du Pont 2,.5-m telescope at the 1
Campanas Observatory; North at top, East to the right.
PREFACE
This little book is the outgrowth of three sessions of invited papers given
at the lAD Genaral Assembly in Buenos Aires in 1991. Normally, the papers
given at these sessions are not written up, but what we found was that the
authors were moving the conceptions of the Galaxy in some new directions
and we all agreed that it would be a good thing to write the results up and
gather them together in a single volume. The articles are of uniformly high
quality, and much of what is presented here is new, not just review. Gezari's
stunning high resolution images of the Galactic center made wit h his 10 pm
infrared array camera are presented here for the first time.
The volume is organized in the conventional way, starting from the center
and working out in order of increasing distance. The articles by Yusef-Zadeh
and Wardle, and by Gezari on the inner parsec of the Galaxy present in
the first instance a synthetic view of the diverse phenomena at the center
and in the second, outstanding images from a newly opened array window.
Rich's beautifully argued paper presents a strong case for a bulge which
formed subsequent to the globular cluster population. The Spergel paper is
an attempt to unify work he and I have done arguing for the pn'sence of a
bar and also for a larger triaxial spheroid that controls the kinematics of the
disk. The article by Whitelock is a new analysis of the IRAS sources at the
Galactic Center arguing that the Milky Way has a barred spiral structure.
Liszt's paper presents a new look at an old subject: the distribution of III in
the disk of the Galaxy, and Bronfman's article is an outstanding synthesis of
molecular line surveys of the Galactic plane, but also contains new results
from high density CS surveys. Latham's article presents the most recent
information from population studies about trying to identify whether the
thick and the thin disks are distinct stellar populations.
All in all, those who attended the sessions were rewarded with an un
conventional program with outstanding new results about the large scale
structure of the Milky Way. This volume is an attempt to share those re
sults with a larger audience.
Leo Blitz
College Park
May 1992
VII
LIST OF ADDRESSES
Leo Blitz,
Laboratory for Millimeter-wave Astronomy, Depanment of Astronomy,
University of Maryland, College Park. MD 20742. USA
Leonardo Bronfman,
Universidad de Chile, Observatorio Astronomico Nacional.
Casilla 36-D. Santiago. Chile
Robin Catchpole,
Royal Greenwich Observatory, Madingley Road,
Cambridge CB3 OEZ. United Kingdom
Dan Gezari,
NASA Goddard Space Flight Center. Laboratory for Astronomy/Solar Physics.
Code 685. Greenbelt. MD 20771. USA
David Latham,
Center jor Astrophysics, 60 Garden Street, Cambridge. MA 02138. USA
Harvey Liszt,
National Radio Astronomy Observatory. Edgemont Road.
Charlottesville. VA 22903. USA
Michael Rich,
Depanment Q!'Astronomy, Columbia University. Box 52 Pupin Hall.
538 West 120th Street. New York, NY 10027. USA
David Spergel,
Princeton University Observatory. Peyton Hall. Princeton. NJ 08544. USA
Mark Wardle,
Nonhwestem University, Depanment of Physics and Astronomy.
Evanston. IL 60201. USA
Patricia Whitelock,
South African Astronomical Observatory, PO Box 9.
Observatory. Cape 7935. South Africa
Farhad Yusef-Zadeh,
Nonhwestem University, Depanment Qf Physics and Astronomy.
Evanston. IL 60201, USA
ix
A COHERENT PICTURE OF THE INNERMOST PARSEC
OF THE GALAXY
FARHAD YUSEF-ZADEH and MARK WARDLE
Department of Physics and Astronomy, Northwestern University
Abstract. This review attempts to provide a framework within which recent radio and
infrared results related to the inner parsec of the Galaxy can be interpreted. The unifying
theme is the outflow from the Galactic center. This wind arises predominant.l~' from the IRS
16 complex which lies within 1/1 of a massive black hole candidate. We present compelling
morphological and, in some cases, kinematic evidence for the interaction of this outflow
i) with a mass-losing supergiant, known as IRS 7, located within the innPf light year of
the dynamical cent.er of t.he Galaxy, ii) with the continuous flow of ionizpc\ gas associated
with Sgr A West which is orbiting the Galact.ic cent.er, iii) with the massiv(' black hole
candidate which is thought to be coincident with a nonthermal compact radio source,
known as Sgr A·, and iv) wit.h t.he circuIIInuclear IIIolecular disk which engulfs the inner
few parsecs of the Galactic center.
1. Introduction
The Galactic Center is obscured at visi ble wavelengths by more than 28
magnitudes of intervening dust and gas. Most of our understanding of this
region is, therefore, based on observations in the infrared or at longer wave
lengths (see reviews by Brown and Liszt 1984 and Genzel and Townes 1987).
This strong radio and infrared source is often the first object ('xamined with
new instrumentation or with new techniques. Indeed, it was present at the
birth of radio astronomy when Karl Jansky (1932) first report ed the detec
tion of radio emission from the Galactic center. More r('centiy. ad vances in
radio and infrared imaging techniques have revealed all ('1I0rnIOUS morpho
logical com plexity in this region. and numerous com POI}('I1ts wi 1 h difr<'rpnt
scale sizes have been recognized. Because of this cOlllplexity. a consistent
understanding of the relationship between different fE'aturp:-- is lacking. In
this review we shall concentrate on the activity in the innermost parsec. and
try and present a coherent picture of what is happening then'. \\7(' begin by
outlining the morphology of the region.
Sgr A * is the brightest radio source within the imler few degrees of the
Galactic center region, and is locatpd very close to Ih(, dYliamical center
of the Galaxy. An unusual bright, early-type infrared cOlllplex, IRS IG, lies
within'" I" of Sgr A*, as drawn schematically in FigllJ'(' I. IHS 16 consists
of at least 1.5 components at 21tll1 (Bckart cl al. 1991), and appears to be the
source of a strong wind with velocity of order 700 kill s-I (Hall. Kleilllllalll1,
& Scovilk 1982; GebaJle ct al. 19X4. 1987, 1991; AII(,11 fI (ii, 19!)O). Both
Sgr A * and IRS I G lie withil1 all ionized cavity at the c(,lIter of a ril1g of
neutral material (the circlllllllllrlear "disk", see H'views by (;(,lIz('1 19X9 and
L. Blit:: (ed.). The Center. Bulge, alld Disk o/the Milky Way, 1-22.
© 1992 Kluwer Academic Publishers.
2 F. YUSEF-ZADEH AND MARK WARDLE
Genzel and Townes 1987). Within the cavity, three "arms" of ionized gas
are in orbital motion around Sgr A *f IRS 16 (Ekers et al. 1983). The closest
segment of ionized gas is known as the Bar within which a "mini-cavity"
has also been identified (Yusef-Zadeh, Morris and Ekers 1989, 1990). The
kinematics of the ionized gas suggests a continuous flow along the individual
arms, some aspect of which is consistent with orbital motion of gas around
Sgr A * or IRS 16 (Lacy et al. 1980; Serabyn et al. 1988; van Gorkom et al.
1984 ).
In the next two sections we consider the two objects, Sgr A * and IRS
16, which dominate the energetics of the inner parsec. Sgr A * is the best
candidate for the concentrated mass which drives the dynamics of the inner
parsec, whereas IRS 16 is the source of the ionized outflow from the region.
We then discuss the evidence for the interaction between these sources.
In particular, we suggest that the luminosity of Sgr A* is a result of the
capture of a portion of the outflow from IRS 16. We go on to explore the
dynamical consequences of the outflow in order to explain the morphology
of the orbiting material in Sgr A West.
2. Sgr A *: A Million Solar Mass Black Hole?
Sgr A * is the brightest radio source wi thin the inner few degrees of the
Galactic center region, and is located very close (if not coincident) to the
dynamical center of the Galaxy, embedded within an evolved stellar cluster.
It is compact having a size < 20 AU and based on VLBI measurements it is
elongated at 8.4 GHz along a position angle of 82° with an axial ratio of 0.53.
It is variable, and shows nonthermal characteristics with a spectrum resem
bling the cores of extragalactic radio sources (Lo 1989; Zhao et al. 1989;
Jauncey et al. 1989). These sources are thought to be powered by accretion
onto a massive black hole, thus it is not surprising that Sgr A * itself has
become a black-hole candidate. Indeed, it is suspected that the nuclei of
most, if not all, galaxies contain a massive black hole in a quiescent state.
The behavior of the velocity dispersion of the stars in the nuclei M31, M32,
NGC 4592 and NGC 4594 are consistent with the presence of a black hole
(Dressler and Richstone 1988; Kormendy 1988). Perhaps the best evidence
for massive black hole is seen toward the center of M31 with a mass range
4 - 5 X 107 MG. As such, this estimate is more than an order of magnitude
higher than the mass estimate for Sgr A *. Curiously, if Sgr A * were placed
at the distance of M31, the surface brightness of the compact radio source
associated with M31 would be weaker than Sgr A * by more than an order
of magnitude (P. Crane, private comm.)
A large mass for Sgr A *, of order 3 X 106 MG, is indeed suggested by its
low proper motion (Backer & Sramek 1982), by the velocities of the ionized
gas orbiting the Galactic center (Serabyn et al. 1988; Lacy, Achtermann
