Table Of ContentObscuration of Quasars by
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Dust and the Reddening
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a Mechanism in Parkes-Quasars
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Franco John Masci
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v A thesis submitted in total fulfillment of the requirements
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X for the degree of Doctor of Philosophy
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September 1997
School of Physics
The University of Melbourne
I dedicate this thesis to my mother,
whose love and support has made everything possible
Declaration
I hereby declare that my thesis entitled ‘Obscuration of Quasars by Dust and
the Reddening Mechanism in Parkes-Quasars’ is the result of my own work
and contains nothing which is the outcome of work done in collaboration,
with the exception of certain observations. Where work by others has been
used, appropriateacknowledgement andreferences aregiven. This thesisisnot
substantially the same, as a whole or in part, as anything I have submitted
for a degree or diploma or any other qualification at any other University.
This thesis is less than 100,000 words in length, exclusive of figures, tables,
appendices and bibliographies.
Franco J. Masci
Acknowledgements
What apleasure itistowake uponthis beautifulSpring Melbourne daywitha
finished thesis, and the thoughts of all those people who have made it possible.
Firstly, I thank my official supervisor, Rachel Webster and my unofficial
supervisor, Paul Francis for their enthusiasm and guidance throughout the last
four years. I feel I have been very lucky to have had two supervisors both of
whom have been generous with their time and ideas. I owe an enormous debt
of gratitude to Rachel Webster who has taught me the benefits of a careful
and rigorous approach to research. I thank her for shortening my sentences
and dealing with all those administrative issues in the University. I extend my
warmest thanks to her for everything she has done for me.
PaulFrancis’contributiontothisthesishasbeenincalculable. Hedisplayed
enormous patience in teaching me the techniques of observing and data reduc-
tion with IRAF. I also thank him for his seemingly boundless knowledge of
statistics. I wish to say grazie molto.
This work has benefited enormously from many conversations, long and
short, both with numerous visitors to the department and at various confer-
ences and meetings. I wish to extend a special thanks to Michael Drinkwater,
Dick Hunstead, Bev Wills, Anne Kinney, Loredana Bassani, Bill Priedhorsky,
Bruce Peterson, Kurt Liffman and Vic Kowalenko.
I am also indebted to the graduate students of ‘Room 359’ (the ‘Astro
Room’) for discussions and suggestions which only rarely were concerned with
the field of astrophysics. Thanks in particular for all the chess games and all
the help with my computer-related traumas and spelling dilemmas. I wish
you all a prosperous future. A warm thanks to our school librarian Kamala
Lekamge for her patience and gratitude. I specially thank Maurizio Toscano
and Mathew Britton for lifting my spirits with the finest of Melbourne’s cui-
sine.
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A special thanks to various members of the Melbourne University Moun-
taineering Club for rejuvenating my academic life with numerous expeditions,
far and close, and for sharing my appreciation of the natural environment.
I would particularly like to thank my mother for her tremendous love and
support throughout my entire scholastic life. Given the hardships faced fol-
lowing the passing away of my father in 1977, my academic position would
not of been possible without her enormous help. I thank her wholeheartedly
for making my life possible – grazie molto e non ti dimentico mai. I also
thank my brother Kevin and his family for providing constant support and
encouragement.
I gratefully acknowledge the University of Melbourne for its financial sup-
port.
September 25, 1997
Melbourne
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Abstract
A majority of quasar surveys have been based on criteria which assume strong
blue continua or a UV-excess. Any amount of dust along the line-of-sight
is expected to drastically extinguish the optical/UV flux leading to a selec-
tion bias. Radio surveys however should suffer no bias against extinction by
dust. Recently, a large complete sample of radio-selected quasars has become
available(the‘Parkes sample’). Amajorityofthesesources exhibit optical–to–
near-infrared continua that are exceedingly ‘red’, very unlike those of quasars
selected optically. The purpose of this thesis, broadly speaking, is to explore
the problem of incompleteness in optical quasar surveys due to obscuration
by dust, and to interpret the relatively ‘red’ continua observed in the Parkes
quasar sample.
The first part of this thesis explores the observational consequences of an
intervening cosmological dust component. A preliminary study explores the
effects of different foreground dust distributions (on galaxy-cluster scales to
the visible extent of normal galaxies) on obscuration of background sources.
Numerical simulations of dusty-galaxies randomly distributed along the line-
of-sight with simplified assumptions are then performed, and implications on
optical counts of quasars and absorption-line statistics are explored. This
foundation is extended by considering the effects of more complicated models
of foreground obscuration where the dust content evolves with redshift. The
Parkes sample is used to constrain evolutionary andphysical properties of dust
in intervening systems. The contribution of line-of-sight galactic dust to the
reddening observed in this sample is also constrained.
The second part examines the continuum properties of Parkes quasars in
the framework of a number of absorption and emission mechanisms to as-
sess the importance of extinction by dust. Three classes of theories are ex-
plored: ‘intrinsically red’ AGN emission models, dust extinction models, and
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host-galaxy light models. Simple models are developed and tested against
the available data. Several new correlations between spectral properties are
predicted and identified observationally. For the dust model, we explore the
effects of dust on soft X-rays and compare our predictions with ROSAT data.
Possible physical dust properties are constrained. I then consider the possi-
bility that a ‘red’ stellar component from the host galaxies contributes to the
observed reddening. This contribution is quantified using a novel spectral fit-
ting technique. Finally, an observational study of near-infrared polarisation is
presented to distinguish between two models for the reddening: the intrinsic
‘synchrotron emission model’, and the dust model. Combined with spectral
and photometric data, these observations are used to constrain various emis-
sion and dust absorption models.
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Preface
Some ofmy work which has been published, or is due to bepublished, andthat
which is not my own, but is extensively used and of substantial importance in
this thesis is outlined as follows:
Chapter 1: this chapter involves an introduction and review and thus is not
original.
Chapter 2: to be published as Masci & Francis (1997):
Masci,F.J. & Francis,P.J., “Diffuse Dust and Obscuration of the Background
Universe.” Publications of the Astronomical Society of Australia, (1997, sub-
mitted).
TheCluster-QSOtwo-pointangularcorrelationdatausedinsection2.3isfrom
the literature.
Chapter 3: published as Masci & Webster (1995):
Masci,F.J. & Webster,R.L., “Dust Obscuration in the Universe.” Publications
of the Astronomical Society of Australia, 12, p.146 (1995).
The initial stages of the derivation for the probability distribution function
in dust optical depth follows from Wright (1986). Except where specifically
stated in this and all chapters that follow, all photometric and spectroscopic
observations (and their reduction) for the Parkes quasar sample are the result
of the following collaborations: Drinkwater etal. (1997) and Francis etal.
(1997).
Chapter 4: to be published as Masci & Webster (1997):
Masci,F.J. & Webster,R.L., “Constraining the Dust Properties of Galaxies Us-
ing Background Quasars” (1997, in preparation).
The parameterisation assumed for our evolutionary dust model is partially
based on previous indirect studies of the global evolution in heavy metals.
Chapter 5: various aspects of the material presented in section 5.3 was pub-
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lished in Masci & Webster (1996):
Masci,F.J. & Webster,R.L., “Red Blazars”: Evidence Against a Synchrotron
Origin. In Proceedings of IAU Conference 163: ‘Accretion Phenomena and
Related Outflows.’ ASP Conference Series, 121, p.764 (1996).
The mathematical formalism used to model the synchrotron process (sec-
tion 5.3.3), and explore diagnostics for X-ray absorption (section 5.6.1) is
from the literature. The observational tests presented in sections 5.4.1, 5.5.1
and 5.5.3 were devised in collaboration with Dr. P. Francis, and will appear
in Francis etal. (1997). Soft X-ray data for Parkes quasars was provided by
Siebert etal. (1997), and my analysis will appear in Masci etal. (1998; in
preparation).
Chapter 6: this chapter consists of original research and will be published as
Masci & Webster (1998):
Masci,F.J. & Webster,R.L., “Host Galaxy Contribution to the Colours of ‘Red’
Quasars.” Monthly Notices of the Royal Astronomical Society, (1998; submit-
ted).
Chapter 7: Dr. P. Francis assisted generously in the polarimetry observations.
A loose description of the reduction procedure was provided by Dr. J. Bailey
of which no published material exists. Modelling of the polarimetry data is
based on extensions of existing models from a number of independent studies.
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Contents
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . i
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvi
1 Introduction 1
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Plan of this Thesis . . . . . . . . . . . . . . . . . . . . 3
1.2 The Study of Cosmic Dust . . . . . . . . . . . . . . . . . . . . 4
1.3 The Interaction of Radiation with Dust and Observational Di-
agnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3.1 Extinction . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.2 Other Diagnostics . . . . . . . . . . . . . . . . . . . . . 9
1.4 The Cosmological Distribution of Dust . . . . . . . . . . . . . 11
1.4.1 Dust in Local Galaxies . . . . . . . . . . . . . . . . . . 12
1.4.2 High Redshift Dust and Evolution . . . . . . . . . . . . 13
1.5 Dust and Quasars . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.5.1 Obscuration of Quasars by Intervening Dust . . . . . . 14
1.5.2 Dust in the Quasar Environment . . . . . . . . . . . . 16
1.5.3 Optically Dust-Obscured Quasars . . . . . . . . . . . . 18
1.6 Radio-Selected, Optically Reddened Quasars . . . . . . . . . . 21
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