Nancy G. Roman Discovery of the Thick Disk (1950) Adric Riedel 20061101 NGC 4013 (Hubble WFPC-2)

Nancy G. RomanDiscovery of the Thick

Disk(1950)

Adric Riedel20061101

NGC 4013 (Hubble WFPC-2)http://hubblesite.org/newscenter/archive/releases/2001/07/image/a/

•PERIGALACTICA•PERIGALACTIC•PERIGALACTICON•INNERMOST

ORBITAL RADIUS

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Before we begin

http://www.sarjaopas.com/kuvat/Battlestar_Galactica_thumbnail.jpghttp://www.astro-physics.com/gallery/astroph/wsp/ngc2903.htm (cropped)

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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 existencehttp://solarsystem.nasa.gov/people/profile.cfm?Code=RomanN

Dr. RomanPopulations?

>1950Roman 1950

Subsequent Work 1950

ELS 1960Gilmore et al. 1980

Wyse & Gilmore 2000

Current Status


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

NASA Years (1959-~1979)• NASA’s head of Astronomy,

Relativity and Geodesy• NASA responsibilities:

– 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 rocket programs

http://solarsystem.nasa.gov/people/profile.cfm?Code=RomanN

Geodesy: The study of the shape of the earth especially with regard

to gravity

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)


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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.


>1950Roman 1950


ELS 1960Gilmore et al.

1980Wyse & Gilmore

2000Current Status

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”


>1950Roman 1950



1980Wyse & Gilmore

2000Current Status

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”


>1950Roman 1950



1980Wyse & Gilmore

2000Current Status

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


>1950Roman 1950



1980Wyse & Gilmore

2000Current Status

Roman’s 1950• Note the different shapes of the

distributions• Believed to be a real kinematic effect


>1950Roman 1950



1980Wyse & Gilmore

2000Current Status

FLAGRANTLY INACCURATE

Roman’s 1950Dr. Roman

Populations? >1950

Roman 1950Subsequent Work

1950ELS 1960

Gilmore et al. 1980Wyse & Gilmore

2000Current Status

Where do we go from here?Selected further studies of the Thick

Disk


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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”


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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”


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status


• Noted Roman’s 1955 catalogue of stars (as well as their own data) showed a linear correlation between ‘ultraviolet excess’ and eccentricity


>1950Roman 1950



Wyse & Gilmore 2000

Current Status


• 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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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).


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

Theory #4

DiskBulge

¡Hola!

Thick DiskOld Disk

The Current Model

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.)


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

Does the thick disk exist?• 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)


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

In Summary• 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


>1950Roman 1950



Wyse & Gilmore 2000

Current Status

Works Cited:• 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

Nancy G. Roman Discovery of the Thick Disk (1950) Adric Riedel 20061101 NGC 4013 (Hubble WFPC-2)

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