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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(cropped)
Dr. Nancy Grace Roman
Dr. 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
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
http://solarsystem.nasa.gov/people/profile.cfm?Code=RomanN
NASA Years (1959-~1979)
Dr. Roman
Populations? >1950
• NASA’s head of Astronomy,
Relativity and Geodesy
• NASA responsibilities:
Geodesy: The study of the Roman 1950
Subsequent Work 1950
shape of the earth especially
ELS 1960
with regard to gravity Gilmore et al. 1980
Wyse & Gilmore 2000
– Experiments on Gemini, Apollo, Skylab,
Current Status
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)
Dr. Roman
• 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 19951996
• Currently retired (though apparently still does
Astronomy education work)
• Last scientific appearance: AAS 207 (2005)
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Roman’s 1999
Dr. Roman
Populations? >1950
• 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 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Roman’s 1999
Dr. Roman
Populations? >1950
Roman 1950
Subsequent Work 1950
• 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.
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Roman’s 1950
Dr. Roman
• 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”
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Roman’s 1950
Dr. Roman
• 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”
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Roman’s 1950
Dr. Roman
• Roman’s 1950 paper is the first systematic
study to show this unexpected behavior.
• Large (but overlapping) radial velocity
measurements suggest two different groups
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Roman’s 1950
Dr. Roman
Populations? >1950
• Note the different shapes of the distributions
• Believed to be a real kinematic effect
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
FLAGRANTLY INACCURATE
Roman’s 1950
Dr. Roman
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Dr. Roman
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Where do we go from here?
Selected further studies of the Thick Disk
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Roman’s 1999
Dr. Roman
Populations? >1950
• 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 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
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”
Dr. Roman
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Theory #1
Roman’s 1955
Dr. Roman
Populations? >1950
• 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 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Roman’s 1999
Dr. Roman
Populations? >1950
• 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
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
The Vatican Symposium (1958)
Dr. Roman
Populations? >1950
• According to O’Connell (1958) the Vatican
Symposium arrived at the following
classifications for Baade’s populations.
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Eggen, Lynden-Bell & Sandage 1962
Dr. Roman
• 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
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Eggen, Lynden-Bell & Sandage 1962
Dr. Roman
Populations? >1950
• Noted Roman’s 1955 catalogue of stars (as
well as their own data) showed a linear
correlation between ‘ultraviolet excess’ and
eccentricity
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Eggen, Lynden-Bell & Sandage 1962
Dr. Roman
Populations? >1950
• 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
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Theory #2
Gilmore & Reid’s 1983
Dr. Roman
• 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
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Gilmore & Reid’s 1983
Dr. Roman
Populations? >1950
• Gilmore & Reid’s scale heights for the thin
disk and thick disk are ~300 pc and ~1350
pc (depending on stellar type).
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Gilmore & Reid’s 1983
Dr. Roman
Populations? >1950
• 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
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Other theories
Dr. Roman
Populations? >1950
• 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
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Theory #3
Gilmore, Wyse & Norris’s 2002
Dr. Roman
Populations? >1950
• 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
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Wyse, Gilmore & Norris’s 2006
Dr. Roman
Populations? >1950
• 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
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Theory #4
The Current Model
¡Hola!
Bulge
Thick Disk
Old Disk
Disk
A selection of theories:
Dr. Roman
Populations? >1950
• 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.)
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
Does the thick disk exist?
Dr. Roman
• Gilmore & Wyse (1989) note that Bahcall
disproved the existence of the thick disk in
1984, and then re-proved 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)
Populations? >1950
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
In Summary
Dr. Roman
Populations? >1950
• 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- confirmed by
several independent researchers
Roman 1950
Subsequent Work 1950
ELS 1960
Gilmore et al. 1980
Wyse & Gilmore 2000
Current Status
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
Battlestar Galactica ©2006 Scifi Channel.
Image of NGC 2903 by Trent Kjell ©2003 astro-physics.com.
All other images from NASA