Lectures
Lecture note may be found here.
"The Interstellar Environment at the Galactic Center"
Mark Morris (UCLA)
LECTURE 1: The Central Molecular Zone: the distribution and dynamics
of molecular clouds in the central 300 pc of the Galaxy
This lecture will focus on the characteristics of the central
reservoir of interstellar molecular gas, and the possible reasons why the
temperature, density and local velocity dispersion are unusually high
compared to other locations in our Galaxy. The orbital dynamics of clouds in
the bar potential of our Galaxy will also be described, and the importance of
the strong tidal fields of the Center will be discussed.
LECTURE 2: I. The Galactic Center Radiation Field, and its Effect on Molecular Clouds
II. The Chemistry of GC Clouds
The internal structure of clouds will be examined in this lecture,
including the HII regions and photodissociation regions at cloud surfaces.
The observation that Galactic center clouds have both warm and hot components will be discussed, as will the evidence for X-ray dominated
regions (XDRs). The evidence will be presented for why the
chemistry of Galactic center clouds seems to differ in some respects from that
seen in molecular clouds in the Galactic disk. In particular, the Galactic
center is unusually rich in complex organic molecules; grain surface
chemistry has been invoked to account for this.
LECTURE 3: The Galactic Center Magnetic Field
I will review the morphological evidence for the magnetic field
geometry and strength in the Galactic center, from both radio and infrared
observations. The large-scale field in the Galactic center appears to have a dipolar geometry, but there is some question about
whether the implied strong
field is pervasive throughout the inner few hundred parsecs of the
Galaxy, or whether the magnetically organized features are local, or perhaps
organized cylindrically. This debate will be reviewed. The field orientations
inside and outside of molecular clouds are orthogonal to each other.
This can be ascribed to the shear of GC clouds, and it leads to
very interesting possibilities for what might happen at cloud surfaces.
Finally, ideas for the origin of the GC magnetic field will be discussed.
LECTURES 4: Star Formation in the Central Molecular Zone
Because the physical conditions of GC clouds, as well as their strong magnetic fields, are so different from those in the Galactic disk
clouds, the initial conditions for star formation are quite different. In the
GC, this favors high-mass stars, and the initial mass function there is
expected to be relatively flat. Observations of a few massive clusters are consistent with this expectation. In this lecture, the current
observations of star formation in the GC are reviewed, and the
question of whether formation in clusters is the dominant mode of
star formation will be addressed. The final point will be a discussion
of the mass budget of interstellar gas in the GC: what are the sources
and sinks for interstellar gas in the GC?
"Dust grains and magnetic fields in the Galactic center"
Giles Novak (Northwestern University)
LECTURE 1: Dust emission and extinction in the Galactic center region
I will cover features seen on large scales, such as the dense ridge,
as well as dust heating and temperature. I will also discuss the
hot dust in and within the circumnuclear disk, and the distribution of
PAH emission.
LECTURE 2: Theory of grain alignment
I will explain why the older ideas on grain alignment via paramagnetic
dissipation are being replaced by the newer idea that grain alignment
proceeds via the radiative torque mechanism. I will review the
observational evidence from near-IR polarimetry of nearby Galactic disk
clouds which supports the idea that grains in highly obscured regions
are less well aligned, a prediction of the radiative torque mechanism.
LECTURE 3: Mapping magnetic fields in the Galactic center via dust emission
polarimetry
I will review far-IR/submm polarimetry of the CND, the 20 km/s and 50
km/s clouds, the arched filaments region, the sickle, Sgr B2, as well as
the large scale map produced by SPARO at South Pole. I will discuss the
interpretation of these maps, including the magnetic models for the CND
developed in the early 1990s.
LECTURE 4:Synthesis with other techniques for mapping magnetic fields in the
Galactic center
I will summarize Zeeman detections and upper limits, recent near-IR
results, and Faraday rotation measurements. I will also present some
very speculative attempts to synthesize these results with evidence for
poloidal fields in the non-thermal gas.
"Non-Thermal Signals from the Galactic Centre"
Roland Crocker (Monash University)
LECTURE 1: "High-energy Photon Signals from the Galactic Centre"
As revealed in observations by a plethora of space- and
ground-based instruments, the Galactic centre (GC) displays both
point-like and diffuse emission from soft X-ray (~keV) all the way up to
hard gamma-ray (>100 GeV) energies. Emission at these various
wavelengths is characteristic of particular processes either associated
with discrete astrophysical sources in the region (including the
super-massive black hole located at the Galactic dynamical centre) or
occurring diffusively in the GC interstellar medium. I consider the
implications of these various high-energy photon signals.
LECTURE 2: "Large-scale Radio Continuum Emission from the Galactic
At ~GHz frequencies the region around the Galactic centre (GC) presents
a very rich phenomenology. Of particular interest a distinct but diffuse
non-thermal radio source ("DNS") covering a large solid angle (6 degrees
x 2 degrees) around the GC has been detected between 74 MHz and 330 MHz
by LaRosa et al. Application of the "equipartition" principle to this
source suggests a large-scale magnetic field throughout the region of
~10 micro gauss. On the other hand, observations by Morris and
co-workers of the so-called non-thermal filaments found throughout the
inner few hundred pcs -- long and thin flux tubes illuminated by
synchrotron emission from high-energy electrons -- suggest a pervasive
milli gauss field through the GC region. I describe our recent analysis
of archival radio data which extends the spectrum of the DNS up to 10
GHz and, moreover, uncovers a downbreak in this spectrum. Such a
downbreak is very likely due to synchrotron cooling of the the emitting
electron population. Our analsis of this break points to a field
amplitude of at least 100 micro gauss averaged over the DNS region.
LECTURE 3: "The Galactic Centre ISM"
polarimetry
I consider the further implications of the recent uncovering by us of a
downbreak in the radio spectrum of the diffuse, non-thermal source
detected on large scales around the Galactic centre (introduced in
lecture 2). In particular I will discuss similarities between the
conditions identified by us as pertaining in the GC environment and
those found by Thompson et al. (2006) to pertain within the dense,
central regions of starburst galaxies. These similarities -- which
suggest that the GC may be regarded as akin to a starburst in miniature
-- include: i) throughout the GC the cosmic ray energy density seems to
be sub-equipartition with respect to the energy density in the other ISM
phases (including the very hot (8 keV) plasma, the magnetic field and
the dense and turbulent molecular matter) but these phases would seem to
be close to equipartition with each other; ii) the GC environment seems
to be in the "calorimeter limit" (Voelk 1989) suggesting that it should
fall close to the far-infrared--radio-continuum correlation as is,
indeed, the case; and iii) recent observations of diffuse, TeV emission
from the GC region by the HESS instrument suggest that the observed
radio emission may have a significant contribution from (and may even be
dominated by) synchrotron emission from _secondary_ electrons created in
collisions between primary hadronic rays and ambient molecular hydrogen.
LECTURE 4: "PET Scanning the Galactic Bulge"
It has been known for thirty years that around 10^43 positrons
annihilate per second in the Galactic bulge but, to date, no compelling
candidate source or mechnanism for the origin of these positrons has
been identified. I will review the evidence around this important
astrophysical mystery.
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