Nancy G. Roman
Discovery of the Thick Disk
(1950)
Adric Riedel
20061101
NGC 4013 (Hubble WFPC-2)
http://hubblesite.org/newscenter/archive/releases/2001/07/image/a/
Before we begin
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PERIGALACTICA
PERIGALACTIC
PERIGALACTICON
INNERMOST
ORBITAL RADIUS
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http://www.sarjaopas.com/kuvat/Battlestar_Galactica_thumbnail.jpg
Dr. Nancy Grace Roman
• Born 1925
• BA Swarthmore College 1946
• PhD University of Chicago 1949 (with
William Morgan)
• Four other honorary degrees
• Research at Yerkes Observatory and
Naval Research Laboratory
• Has an asteroid (2516 Roman) named
after her
• Joined NASA four months into its
existence
http://solarsystem.nasa.gov/people/profile.cfm?Code=RomanN
NASA Years (1959-~1979)
• NASA’s head of Astronomy,
Relativity and Geodesy
• NASA responsibilities:
Geodesy: The study of the
shape of the earth especially
with regard to gravity
– Experiments on Gemini, Apollo, Skylab,
Spacelab
– IUE
– Hubble Space Telescope (Early stages)
– IRAS
– Had a part in AXAF (Chandra), COBE,
Space Shuttle Payload designs
– Various high-energy balloon and
http://solarsystem.nasa.gov/people/profile.cfm?Code=RomanN
rocket programs
Post-NASA (~1979-Present)
• Consultant for Aerospace companies (Hughes, etc)
especially dealing with the Space Telescope (Hubble)
and AXAF (Chandra)
• Various funding chairs
• Digitizing and cleaning of astronomical catalogs (eg
Vizier)
• Head of the Astronomical Data Center 1995-1996
• Currently retired (though apparently still does Astronomy
education work)
• Last scientific appearance: AAS 207 (2005)
Roman’s 1999
• Correlation of high velocity and ‘late’ spectral type
noticed at least as early as Oort (1926)
• All globular clusters have high velocities
• 75% of RR Lyrae stars have high velocities
• Baade discovered the nuclei of galaxies were red giants
(no bright blue stars) in 1944: Developed Population II
Roman’s 1999
• Roman’s 1950 paper came before Baade’s work was
accepted (the “Vatican Symposium” in 1958)
• Subdwarfs had not been studied yet- thought of as
simply metal-poor stars if at all.
Roman’s 1950
• Studied a number of F5 through G5 stars from the Yerkes
spectral atlas (Morgan et al. 1943) with the intent to
develop a spectral classification-parallax system (Rubin,
1999)
• “…Among the late F- and early G- type stars there
appear to be two groups of stars which occur with
comparable frequency and which can be distinguished
spectroscopically, though they occupy the same region of
the H-R diagram… the weakness of the hydrogen lines
would indicate a later spectral type, while the weakness
of the remaining lines would indicate an earlier type”
Roman’s 1950
• Roman assigned the stars compromise classifications
• The real remarkable point of the paper was that not
only could these populations be distinguished
spectroscopically, they could be distinguished
kinematically.
• Roman (1999) points out this was noticed as early as
1914 by Adams and Kohlschütter. “The existence of high
radial velocities amongst stars having what is generally
considered an early type of spectrum is shown by these
results although there can be no doubt that such cases
are rare”
Roman’s 1950
• Roman’s 1950 paper is the first systematic study to show
this unexpected behavior.
• Large (but overlapping) radial velocity measurements
suggest two different groups
Roman’s 1950
• Note the different shapes of the distributions
• Believed to be a real kinematic effect
Roman’s 1950
Where do we go from here?
• Selected further studies of the thick disk
Roman’s 1999
• Roman’s discovery prompted several people doing similar
research on stars to attempt to characterize subdwarfs
• “Chamberlain and Aller (1951) published a detailed
comparison of the spectra of two of the subdwarfs… They
found that both subdwarfs could be represented by an F-type
model atmosphere with a temperature near 6300K but
abnormally small amounts of calcium and iron compared to
hydrogen”
http://antwrp.gsfc.nasa.gov/apod/image/0102/m55cmd_mochejska_big.jpg
Roman’s 1954
• A summary of Chamberlain’s discovery of subdwarfs (not
A stars, but F stars with low metals)
• Defines metallicity for 17 stars currently being observed
out of a program of 500
• Gives eccentricities and perigalactic distances (assuming
M=+5 given the stars in the sample with known
parallaxes)
• Demonstrated that stars with eccentric orbits could be
selected from generic high-motion stars entirely based on
spectral features
• “In addition, the small perigalactic distances found for
these stars, indicates that we may be dealing with a
sample of the population of the Galactic Bulge”
Theory #1
Roman’s 1955
• Catalog collecting proper motion and radial
measurements of 571 known high velocity stars
• New spectral type measurements (now including the VI
subdwarf type) as well as estimations of the eccentricity
assuming the sun is 8.2 kpc from galactic center, moving
216 km/s
Roman’s 1999
• All high velocity stars were eventually all classified as
Population II (Baade’s bulge/halo), despite Roman’s
evidence of a difference.
• Among other places, Roman’s 1955 paper is cited in
“Galactic Astronomy” by Binney & Merrifield (1998) as
important to Eggen, Lynden-Bell & Sandage’s study of
the Milky Way formation
The Vatican Symposium (1958)
• According to O’Connell (1958) the Vatican Symposium
arrived at the following classifications for Baade’s
populations.
Eggen, Lynden-Bell & Sandage 1962
• Standard model of Galaxy formation
• Attempted to model the high-velocity Population II stars
as a result of the formation of the galaxy
– Cylindrical Axisymmetric Geometry
– Galaxy starts as nearly all gas
– Galaxy then collapses radially; at some point radial collapse
stops and only vertical collapse continues
– At the same time, star formation occurs and enriches the
collapsing material, creating a gradation of metallicity
Eggen, Lynden-Bell & Sandage 1962
• Noted Roman’s 1955 catalogue of stars (as well as their
own data) showed a linear correlation between
‘ultraviolet excess’ and eccentricity
Eggen, Lynden-Bell & Sandage 1962
• The extremely eccentric stars could not have formed at
the edges of the cloud, supported against gravity by
the pressure
• If galaxy was in dynamic equilibrium when the stars
formed, large amounts of kinetic energy would be
needed to knock stars THAT askew
• Therefore stars formed before the galaxy settled down
to its present equilibrium.
• Therefore, galaxy formation took less than 200 Myr
(one rotation) to collapse
• Kinematics are the basis of all modern studies and
textbooks
Theory #2
Gilmore & Reid’s 1983
• Where some authors find the ELS timescale for galaxy
collapse too short to reproduce the smooth metallicity
gradient, Gilmore and Reid find that the distribution is
not smooth.
• Made a survey down to absolute magnitude 19 of stars
towards the South Galactic Pole (not magnitude limited)
• Assume the thick disk has similar metallicities to the old
disk
• Using a mass-luminosity relation and an exponential
density law for the galaxy, they find the galaxy is best
fit by TWO exponentials
Gilmore & Reid’s 1983
• Gilmore & Reid’s scale heights for the thin disk and
thick disk are ~300 pc and ~1350 pc (depending on
stellar type).
Gilmore & Reid’s 1983
• They come to the same conclusion as Roman (1954),
where the thick disk is an extension of the spheroid
• They don’t actually cite Roman, just ELS
Other theories
• Norris (1987) finds similarities in metallicity between
Red Giant Branch stars in open clusters and globular
cluster-like Red Giant Branch stars in the thick disk
• The thick disk and the old disk may be the same, or at
least indistinguishable by metallicity alone
• Gilmore, Wyse & Kuijken (1989) review the ELS paper
and question the assumption that stars remain on the
orbits they formed on.
• They find results consistent with old stars being boosted
into the Thick Disk
Theory #3
Gilmore, Wyse & Norris’s 2002
• It’s been known for some time that galaxies accumulate
dwarf spheroidal galaxies
• Perhaps the thick disk is the debris from the merger,
remnants of outlying areas in the spheroidal galaxy.
• Current examples have been shown as kinematically
different from the canonical ‘thick disk’
• Perhaps the thick disk is the sum of many such merger
events
Wyse, Gilmore & Norris’s 2006
• Their models (?) predict azimuthal streaming velocities
around 100 km/sec
• Studied lines of sight toward Carinae and Draco and
found an excess of those velocities
Theory #4
The Current Model
Halo
Bulge
Thick Disk
Old Disk
Disk
A selection of theories:
• 1.) The Thick disk is an extension of the bulge of our
galaxy (Roman, Gilmore & Reid)
• 2.) The Thick disk is the product of star formation
during the collapse of the galaxy (ESL)
• 3.) The Thick disk is old stars perturbed from the plane
(Norris, Gilmore)
• 4.) The Thick disk is fringe stars from galaxies we ate
(Gilmore et al.)
Does the thick disk exist?
• Gilmore & Wyse (1989) note that Bahcall disproved
the existence of the thick disk in 1984, and then reproved it using the same data and different (better?)
constants
• Roman (1957) notes that Population II stars have more
in common with low-velocity stars than with each other
• What can be agreed on:
– Stars at greater scale heights have lower metallicities
– Stars with extreme kinematics have lower metallicities (Roman
1950,1954,1955)
– With accurate numbers for parameters, the thick disk does
exist (Gilmore et al. 1989)
In Summary
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Work is still being done
Dr. Roman’s 1950 discovery appears to be genuine
The galaxy is still not well understood
The thick disk probably exists- confirmation by several
independent researchers
Works Cited:
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Eggen et al. 1962. ApJ, 136, 748E
Gilmore, G, & Reid, N. 1983. MNRAS 202, 1025.
Gilmore et al. 1989. ARA&A 27, 555G
Gilmore et al. 2002. ApJ, 574, L39
Majewsky, S.N. 1993, ARA+A, 31, 575M
Norris, J. 1987. AJ, 93, 616N
O’Connell 1959. JRASC, 53, 45
Roman, N.G. 1950. ApJ, 112, 554R
--------------- 1954. AJ 59, 307R
--------------- 1955. ApJS, 2, 159
--------------- 1957. AJ, 62, 146R
--------------- 1999. Ap&SS, 267, 37-44
--------------- http://www.mc.cc.md.us/Departments/planet/Nancy/Nancy.htm.
October 31, 2006.
Rubin, V.C. 1999. ApJ 525, 401
Wyse et al. 2006. ApJ, 639, L13-L16
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