High-velocity Clouds

by Woerden, Hugo van; Wakker, Bart P.; Schwarz, Ulrich J.; De Boer, Klaas S.

• ISBN13: 9781402025785
• ISBN10: 1402025785
• Format: Hardcover
• Copyright: 2005-01-31
• Publisher: Kluwer Academic Pub

Summary

This book contains 17 chapters reviewing our knowledge of the high-velocity clouds (HVCs) as of 2004, bringing this together in one place for the first time. Each of the many different aspects of HVC research is addressed by one of the experts in that subfield. These include a historical overview of HVC research and analyses of the structure and kinematics of HVCs. Separate chapters address the intermediate-velocity clouds, the Magellanic Stream, and neutral hydrogen HVCs discovered in external galaxies. Reviews are presented of the Ha emission and of optical and UV absorption-line studies, followed by discussions of the hot Galactic Halo and of the interactions between HVCs and their surroundings. Four chapters summarize the ideas about the origin of the high-velocity gas, with detailed discussions of connections between HVCs and the Galactic Fountain, tidally-stripped material, and remnants of the Milky Way's formation. A chapter outlining what we do not know completes the book. The book comes at a time when knowledge and understanding of the HVCs are making rapid progress, owing to technological, observational, and theoretical developments.

Table of Contents

Preface	ix
History of HVC Research - An Overview	1	(24)
Bart P. Wakker
Klaas S. de Boer
Hugo van Woerden
Discovery; research until 1963	1	(3)
Early radio research: 1963-1968	4	(3)
Developments 1968-1980	7	(5)
Developments 1981-1991	12	(3)
Developments 1992-1999	15	(4)
New Developments since 1999	19	(6)
HVC/IVC Maps and HVC Distribution Functions	25	(30)
Bart P. Wakker
Introduction	26	(1)
Basic HVC and IVC data	27	(11)
Features of the H I sky	38	(2)
Column density distribution	40	(5)
Volume density distribution	45	(2)
Distributions of cloud parameters	47	(4)
Conclusions and outlook	51	(4)
Kinematics of HV and IV Gas	55	(18)
Ulrich J. Schwarz
Klaas S. de Boer
General considerations	55	(1)
Defining the HV and IV gas sample	56	(3)
Systematic motions of high-velocity (HV) gas	59	(4)
Systematic motions of intermediate-velocity (IV) gas	63	(2)
Systematic motions from optical and UV observations	65	(2)
Simple kinematical considerations	67	(2)
N (H) distribution and infall velocity	69	(2)
Some results from further statistical analyses	71	(1)
Conclusions	72	(1)
Intermediate-Velocity Clouds	73	(28)
C. Elise Albert
Laura Danly
Introduction: what are intermediate-velocity clouds?	73	(3)
The distribution of IVCs in space and in velocity	76	(3)
Distances of intermediate-velocity clouds	79	(5)
IVCs and their relationship to low- and high-velocity gas	84	(2)
Abundances and physical conditions	86	(8)
Speculations on the origin and significance of IVCs	94	(3)
Directions for future work	97	(4)
HVCs Related to the Magellanic System	101	(24)
Mary E. Putman
Introduction	101	(1)
The Magellanic Bridge	102	(3)
The Magellanic Stream	105	(6)
The Leading Arm	111	(3)
Theoretical origin models	114	(7)
Relationships to other HVCs?	121	(4)
High-Velocity H I Gas in External Galaxies	125	(20)
Tom Oosterloo
Introduction	125	(1)
H I halos of spiral galaxies	126	(8)
Interactions and accretions	134	(8)
Concluding remarks	142	(3)
The Large- and Small-Scale Structure of HVCs	145	(22)
Ulrich J. Schwarz
Bart P. Wakker
Introduction	145	(2)
Observations	147	(2)
Physical characteristics	149	(6)
Characterization of small-scale structure	155	(10)
Discussion and conclusions	165	(2)
Ionized Gas Associated with HVCs	167	(16)
Stephen L. Tufte
Introduction	168	(1)
The observations: high-resolution Fabry-Perot spectroscopy of emission lines	169	(1)
A brief summary of recent emission-line observations	170	(4)
Are HVCs extragalactic? The evidence from Hα	174	(3)
Measuring metal abundances in HVCs -- the ionization correction	177	(2)
The disk/halo connection	179	(2)
Conclusions and future directions	181	(2)
The Coldest Phase in Halo High-Velocity Gas: Dust and Molecules	183	(12)
Philipp Richter
Klaas S. de Boer
Measurements of dust and molecules in the Milky Way Halo	183	(1)
Presence of dust in halo gas	184	(4)
Measurements of molecules in halo gas	188	(4)
Molecular ``globules'' in the outer halo?	192	(1)
Concluding remarks	193	(2)
Distances and Metallicities of HVCs	195	(32)
Hugo van Woerden
Bart P. Wakker
Introduction	195	(2)
Methods of distance determination	197	(3)
Distance determination from absorption lines	200	(5)
Distances of high-velocity clouds	205	(6)
Metallicity determination - requirements and problems	211	(4)
Ion abundances and metallicities of HVCs	215	(7)
Origins	222	(1)
Needs and prospects	223	(4)
The Hot Halo	227	(24)
Klaas S. de Boer
Toward the concept of a hot halo	227	(1)
Absorption by highly-ionized species in the halo	228	(2)
Large-scale distribution of high ions in the halo	230	(6)
Emission by high ions	236	(1)
Diffuse X-ray emission	237	(2)
Special lines of sight	239	(4)
Models for the presence of high ions	243	(2)
FUSE and the hot halo	245	(1)
Association of O VI with HVCs	246	(1)
Concluding remarks	247	(4)
HVCs Interacting with Their Environment	251	(22)
Christian Bruns
Ulrich Mebold
Introduction	251	(1)
The influence of ram pressure	252	(2)
Evidence for interaction with the ambient halo medium	254	(11)
Evidence from UV, X-ray and γ-ray observations	265	(2)
Numerical simulations	267	(2)
Summary and conclusions	269	(4)
Warps, Polar Rings and High-Velocity Clouds	273	(24)
Linda S. Sparke
Introduction	273	(1)
Warps in the outer disks of galaxies	274	(7)
Galaxies with polar rings	281	(16)
High-Velocity Clouds: The Missing Link?	297	(16)
Leo Blitz
David N. Spergel
Peter J. Teuben
Dap Hartmann
The missing galaxy Problem	297	(2)
Evidence for a Local Group origin	299	(6)
Implications and speculations	305	(3)
Predictions and comparison with other observations	308	(2)
Other possible origins	310	(3)
Compact, Isolated High-Velocity Clouds	313	(28)
W. Butler Burton
Robert Braun
Vincent de Heij
Introduction	314	(2)
An all-sky CHVC catalog extracted from the LDS and HIPASS	316	(2)
Principal morphological observables of the ensemble of CHVCs	318	(7)
The CHVC ensemble modeled as a Local Group population	325	(7)
Properties of individual CHVCs imaged at high resolution	332	(5)
Concluding remarks	337	(4)
The Origin of the High-Velocity Clouds	341	(30)
Joel N. Bregman
Introduction	342	(1)
Galactic models for the HVCs	343	(14)
Gas streams from dwarf galaxies	357	(3)
The Local Group debris model	360	(1)
The impact of HVCs onto the disk	361	(3)
Discussion and summary	364	(7)
Unsolved Mysteries of High-Velocity Clouds	371	(20)
Robert A. Benjamin
Introduction	371	(1)
What surrounds HVCs?	372	(8)
HVCs and H I: is that all there is?	380	(3)
Are high-velocity clouds important	383	(3)
Concluding thoughts	386	(5)
Index	391