Astronomy 5500 Galactic Astronomy

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Astronomy 5500Galactic Astronomy

• Goals to develop a knowledge of the tools and
  techniques used for studying the Milky Way, and
  to gain practical experience with them as applied
  to problems arising in Galactic astronomy. The
  dynamics of Galactic rotation and the motion of
  stars and gas about the Galactic centre are
  treated in detail in most textbooks in the field,
  while practical techniques are often covered
  incorrectly or in dated fashion in the same
  sources, as well as in the literature.
• Emphasis is placed on the development of critical
  judgment to separate observational information
  from proposed physical models.

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1. Historical Landmarks The proper study of the
Milky Way Galaxy probably begins in 1610, when
Galileo first discovered that the Milky Way
consists of innumerable faint stars. In 1718
Halley discovered the proper motions of Arcturus,
Sirius, and Aldebaran, and by 1760 Mayer had
published proper motions for some 80 stars based
upon comparisons of their recorded positions. His
results established that the Sun and stars are
not at rest relative to one another in the
Galaxy. The obvious problem with trying to
map our Galaxy from within is that the Sun is but
one of many billions of stars that populate it,
and our vantage point in the disk 8-9 kpc from
the Galactic centre makes it difficult to detect
objects in regions obscured by interstellar dust.
But attempts have been made frequently.
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In 1785 William Herschel derived the first
schematic picture of the Galaxy from optical
star gauging in 700 separate regions of the
sky. He did it by making star counts to the
visual limit of his 20 foot (72-inch diameter)
telescope. He assumed that r N1/3 (i.e. N
r3), and obtained relative thicknesses for the
Galactic disk in the various directions sampled.
No absolute dimensions were established. By 1817
Hershel had adopted a new picture of the Galaxy
as a flattened disk of nearly infinite extension
(similar to the modern picture).
4
In 1837 Argelander, of the Bonn Observatory and
orginator of the BD catalogue, was able to derive
an apex for the solar motion from studying
stellar proper motions. His result is very
similar to that recognized today. Also in 1837,
Frederick Struve found evidence for interstellar
extinction in star count data, which was
considered necessary at that time to resolve
Herschels infinite universe with Olbers
paradox (which had been published in 1823). By
the turn of the century many astronomers felt
that a concerted, detailed effort should be made
to establish reliable dimensions for the Milky
Way. The task was initiated by Kapteyn in 1905
with his plan to study in a systematic fashion
206 special areas, each 1 square, covering most
of the sky the well known Selected Areas for
Galactic research. By then, separately-pursued
research programs into the nature of the Milky
Way system often produced distinctly different
results.
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In 1918, for example, Shapley noted the
asymmetric location of the centre of the globular
cluster system with respect to the Sun, and
suggested that it coincided with the centre of
the Galaxy. But the distance to the Galactic
centre found in such fashion was initially overly
large because of distance scale
problems. Longitude distribution of
globular clusters.
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Kapteyn and van Rhijn published initial results
from star counts in 1920, namely a Galaxy model
with a radius of 4.5 kpc along its major plane
and a radius of 0.8 kpc at the poles. Kapteyn
published an alternate model in 1922 in with the
Sun displaced from the centre, yet by less than
the distance of 15 kpc to the centre of the
globular cluster system established by Shapley.
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The issue reached a turning point in 1920 with
the well known Shapley-Curtis debate on the
extent of the Galactic system. The merits of the
arguments presented on both sides of this debate
have been the subject of considerable study over
the years, but it was years later before the true
extragalactic nature of the spiral nebulae was
recognized. Although Shapley was considered the
winner of the debate, it was Curtis who argued
the correct points. A big step was Hubbles 1924
derivation of the distance to the Andromeda
Nebula using Cepheid variables. Somewhat less
well-known is Lindblads 1926 development of a
mathematical model for Galactic rotation.
Lindblads model was developed further in 1927-28
by Oort, who demonstrated its applicability to
the radial velocity data for stars. Finally, in
1930 Trumpler provided solid evidence for the
existence of interstellar extinction from an
extremely detailed study of the distances and
diameters of open star clusters.
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The modern era orginated in 1944 when Baade
published his ideas on different stellar
populations. In 1940 during World War II, Grote
Reber had discovered the radio radiation from the
Galactic centre, but it was not until after the
end of the war that the discovery was pursued by
research groups in The Netherlands (Müller and
Oort), the U.S. (Ewen and Purcell), and Australia
(Christiansen), often making use of radio dishes
left behind by German occupation forces. The
prediction and confirmation of the 21-cm
transition of neutral hydrogen in the Galactic
disk initiated the new specialty of radio
astronomy, and led to a boom era in the study of
our Galaxy. Although less popular now than it
was 30 years ago, Galactic astronomy is still an
important area of study.
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Perhaps the best picture of the Galaxy is that
sketched by Sergei Gaposhkin from Australia, as
published in Vistas in Astronomy, 3, 289, 1957.
The lower view is Sergeis attempt to step
outwards by 1 kpc from the Sun.
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Sergei Gaposhkins drawing is crucial for the
insights it provides into the size and nature of
the Galactic bulge, that spheroidal (or
bar-shaped?) distribution of stars surrounding
the Galactic centre. Keep in mind that all such
attempts rely heavily upon the ability of the
human eye (and brain) to distinguish a grand
design from the confusing picture posed by the
interaction of dark dust clouds, bright gaseous
nebulae, and rich star fields along the length of
the Milky Way (see below).
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2. Current Model of the Galaxy The present
picture of the Galaxy has the Sun lying 20 pc
above the centreline of a flattened disk, 80.5
kpc from the Galactic centre. The spheroidal halo
is well established, but the existence of a
sizable central bar and the nature of the spiral
arms are more controversial.
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Another schematic representing the present view
of the Galaxy.
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An alternate picture of the Galaxy from the
instructor in recent years has the Sun lying 20
pc above the centreline of a flattened disk, 81
kpc from the Galactic centre. The spheroidal halo
is well established, and there is an obvious
warping of the Galactic disk in the direction of
the Magellanic Clouds that is best seen in the
fourth Galactic quadrant.
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Best current estimates for the distance of the
Sun from the Galactic centre tend to cluster
around 8 1 kpc R0, although that is not
well-established. Estimates as low as 6.5 kpc
and as high as 10.5 kpc have been
published. The main components of the Galaxy are
the bulge, the disk (which contains the spiral
arms), and the halo, with some debate about the
exact number of subgroups of them. The existence
of a bar at the Galactic nucleus is accepted from
indirect evidence only. There is considerable
evidence for a metallicity gradient in the disk
with stars of higher metallicity lying towards
the Galactic centre. The metal enrichment of the
disk is attributed to evolutionary processes in
stars, which end their lives by adding a rich
supply of heavy elements to the interstellar
medium.
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When the mean metallicity of disk stars is
studied as a function of the age of the stars,
there appears to be a net metallicity growth with
age amounting to ?ltFe/Hgt ? 0.50.7/1010
years, i.e. an increase of Fe/H by 4 1 every
1010 years. The relationship is not zeroed to
the Sun, since solar metallicity is calculated to
have been reached at an age of 2.5 ? 109 years.
Fe/H log(Fe/H)/(F/H)?, i.e. 2? the solar
metallicity is equivalent to Fe/H 0.30. Of
the main components of the Galaxy, there are at
least two components of the halo currently
recognized, as well as some argument about the
number of disk components that can be identified
(thin disk, thick disk, etc.). The components of
spiral arms appear to differ only slightly in
age, and many astronomers would identify them as
a single young Population I component.
16
Table 24.1 from Carroll and Ostlie. Approximate
Values for Parameters Associated with Components
of the Milky Way.
17
Well-recognized characteristics of the
Galaxy 1. Goulds Belt, consisting of nearby
young stars (spectral types O and B) defining a
plane that is inclined to the Galactic plane by
15? to 20?. Its origin is uncertain. The
implication is that the local disk is bent or
warped relative to the overall plane of the
Galaxy. This is not to be confused with the
warping of the outer edges of the Galaxy.
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2. An abundance gradient exists in the Galactic
disk and halo, consistent with the most active
pollution by heavy elements occurring in the
densest regions of these parts of the Galaxy. See
results below from Andrievsky et al. AA, 413,
159, 2004 obtained from stellar atmosphere
analyses of Cepheid variables.
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The abundance gradient is also seen in the halo
according to the distribution of globular
clusters of different metallicity relative to the
Galactic centre (below).
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3. The orbital speed of the Sun about the
Galactic centre is about 250 km s1, as
determined from the measured velocities of local
group galaxies, as well as from a gap in the
local velocity distribution of stars
corresponding to plunging disk stars. This fact
is actually NOT well recognized by most
astronomers.
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4. The Galactic bulge is spheroidal, although
some researchers believe it displays a boxy
structure at infrared wavelengths suggestive of a
central bar viewed nearly edge on. A
mapping (right) of Milky Way planetary nebulae in
Galactic co-ordinates (Majaess et al.
MNRAS, 398, 263, 2009) suggests a more spheroidal
structure typical of galaxies like NGC 4565
(top). The nature of the Galactic bulge is still
unclear. The surface brightness follows a de
Vaucouleurs law.
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5. The Galaxy is a spiral galaxy. But does it
have 2 arms or 4, and can it be matched by a
logarithmic spiral? A grand design spiral
pattern is not obvious in the plot of the
projected distribution of Cepheids (points) and
young open clusters (circled points) below
(Majaess et al. 2009).
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A schematic representation of what are considered
to be major spiral features. How would you
connect the points? Most recent studies
consider the Cygnus feature to be a spur or minor
arm, and the Perseus feature is considered to be
a major arm! There is an Outer Perseus Arm in
many deep surveys. It lies gt4 kpc from the Sun In
the direction of the Galactic anticentre.
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6. The Galactic disk is warped, presumably from a
gravitational interaction with the Magellanic
Clouds. The warp is evident in 21cm maps of
neutral hydrogen restricted (by radial velocity)
to lie at large distances from the Galactic
centre (below).
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7. The Galaxy has a magnetic field that appears
to be coincident with its spiral arms (or
features), with the likely geometry of the
magnetic field lines running along the arms. Weak
fields of tens of mGauss are typically measured.
The evidence for the presence of a magnetic field
comes from the detection of interstellar
polarization in the direction of distant stars
(see below).
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8. Note features in Carroll and Ostlie that are
NOT included in the list spiral structure the
Milky Ways central bar 3-kpc expanding
arm dark matter halo evidence of dark
matter Can you understand why?
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3. Stellar Reference Frames and Proper
Motions If we define the equatorial coordinates
of a star to be a1 and d1 at epoch T1, and a2 and
d2 at epoch T2, then a2 a1 (m n sin a
tan d µa)(T2 T1) and d2 d1 (n cos a
µd)(T2 T1), where m and n are the terms for
general precession. As determined by Newcomb with
respect to observations of planets and asteroids,
with known (or estimated) masses of solar system
objects used to establish a dynamical rest frame,
the constant of luni-solar precession is given
by p 50".2910 0".0222 T per
year, where T is the number of elapsed centuries
since J2000.0, i.e. p 50".2688 per year for
the year J1900.0. Thus, for example p(1985.0)
50".2910 0".0222 (85.0/100) per year
50".27213 per year.
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General precession consists of two terms p1
luni-solar precession, and ? planetary
precession (a function of a only). Thus, m
p1 cos e ? ? 3s.07496 0s.00186T /year,
and n p1 sin e? 1s.33621 0s.00057T
/year 20".0431 0".0085T /year, where e is the
obliquity of the ecliptic. The parameters µa and
µd are the proper motions in right ascension and
declination, respectively. In other
words and the net proper motion of an object
is given by µ (µa cos d)2 µd2½. Accurate
proper motions for stars therefore require small
internal errors of observation as well as a
detailed knowledge of the inertial reference
frame and the resulting precession constants
(which are not well determined).
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The steps usually taken to determine reliable
proper motions for stars are (i) Meridian
telescopes and accurate clocks are used to
establish reliable position measurements for
bright stars, with stellar observations also
being used (if possible) to establish the
location of the celestial pole for the epoch of
the observations. (ii) The current right
ascensions and declinations for all program stars
are obtained from repeated measurements of each
stars meridian crossing times as well as its
culmination points measured on the telescopes
large altitude circle. (iii) Published
precession and nutation constants are used to
reduce the observations to a common nearby epoch
in time, and the results are published as a
Catalogue of Stellar Positions.
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(iv) Several such catalogues are reduced to a
common epoch to establish the proper motions,
systematic observatory errors, systematic
precession constant errors, etc. for a common set
of stars. The resulting collection of positions
and proper motions is a Compilation
Catalogue. (v) When several such catalogues are
combined with a new set of planetary observations
used to redefine the inertial reference frame for
the precessional corrections, the resulting
compilation of positions and motions tied to the
inertial reference frame is known as a
Fundamental Catalogue, e.g. the FK4 and
FK5. (vi) Proper motion data for stars in
Position Catalogues but not in a Fundamental
Catalogue are obtained by establishing Catalogue
corrections tied to the Fundamental Catalogue
reference frame. Several such non-fundamental
catalogues exist, of which the SAO Catalogue and
AGK3 are two examples.
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A simple way of assessing the problem of deriving
reliable proper motions for stars is to consider
the various sources of error involved The
proper motion of a star in a fundamental
catalogue, µF, is given by µF P S G (µ
sF eF) , where P the effect arising from
an error in precession, S the effect caused by
the solar motion, G the effect resulting from
galactic rotation, µ the residual motion of
the star after removal of P, S and G (i.e. the
stars space motion), sF the systematic error
in the fundamental system (always a
possibility!), and eF the accidental error for
a particular star.
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Usually, catalogued µF values are used for as
many stars as possible, distributed at random in
position and magnitude, to remove µ, sF, and eF
from discussion. Any subsequent analyses of µF
values to derive S and G may therefore contain an
error caused by P. In fact, evidence for a
residual error in precession for the FK4 system
(i.e. P gt 0) was the primary motivation behind
the studies leading to the production of the FK5
Catalogue. A possible alternate route is to use
galaxies as reference objects for the positions
of stars, which is possible for astronomical
imaging. In that case the proper motion of a
star, µG, measured in such fashion relative to
galaxies, is given by µG S G (µ sG
eG) , with the symbols as given
previously. Proper motions obtained in such
fashion do not involve uncertainties in the
precession corrections.
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The resulting differences in proper motion are
µF µG P (sF sG eF eG). Thus, analyses
of large numbers of stars measured by both
techniques can be used to establish P, provided
that the terms in brackets are completely
randomized. The potential advantages of
measuring stars relative to galaxies was realized
in the 50s and 60s, and led to the development
of two observatory programs to test the concept.
A Lick Observatory program (Vasilevskis) used the
0.5-m astrograph with 6 ? 6 photographic
plates, and typically 60 galaxies per field (to
19th magnitude). The galaxies (faint and fuzzy)
were used to establish plate constants, the
correct orientations, etc., with limited success.
A Pulkovo Observatory program (Fatchikin) used
the normal astrograph with 2 ? 2 plates, and
typically only 1 or 2 bright galaxies (to 9th
magnitude) per field. The galaxies were used to
standardize the plates, with stars on the plates
used to establish the reference system, plate
constants, etc.
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The Pulkovo program results were clearly based on
a different standardization from that used in the
Lick program. A Yale-Columbia program was a
southern replication of the Lick program using an
0.5-m astrograph with 6 ? 6 plates, but also
with superior optics to the Lick and Carnegie
astrographs. Hanson (AJ, 80, 379, 1975 IAU
Symp., 85, 71, 1980) used some of the Lick
program plates for the central (and later outer)
regions of the Hyades cluster field for his Ph.D.
thesis study of the Hyades cluster distance based
upon proper motions. The study was heavily
criticized by Luyten (Publ.Univ.Minnesota, XLI,
1975), who argued that the technique suffers from
the non-stellar nature of the galaxy images,
which assures that stars and galaxies in the
fields are measured in completely different ways.
He suggested an alternative method of tying the
measurements to quasar images, although that may
not be practical for fields like the Hyades.
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Uncertainties in proper motion measurements are
typically of order 0".005 /year to 0".010
/year, although the results of the Hipparcos
mission have generated stated precisions closer
to 0".001 /year . Proper motion studies are also
made for open and globular clusters, where they
are used to study membership probabilities for
cluster stars or space motions of the clusters.
In the case of membership testing, membership
discrimination is based upon the analysis of
proper motions relative to some inferred field
star distribution. Recent variants use
each stars position in the cluster, in addition
to proper motion, to specify membership.
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An example of proper motions used to establish
membership probabilities for stars in the open
cluster M11 (McNamara et al. AAS, 27, 117,
1977).