Galaxy Classification in the 21st Century: The Story Behind The de Vaucouleurs Atlas of Galaxies

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Galaxy Classification in the 21st Century The
Story Behind The de Vaucouleurs Atlas of Galaxies

• Ron Buta
• University of Alabama

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Published by Cambridge University Press, February
2007
Gerard de Vaucouleursborn April 25, 1918 Paris,
Francedied October 7, 1995 Austin, Texas
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Who was Gerard de Vaucouleurs?

• He was an extragalactic astronomer at the
  University of Texas at Austin who made many
  important contributions to astronomy
• He was also my PhD supervisor from 1978-1983

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What is the de Vaucouleurs Atlas of Galaxies?
- a comprehensive illustration of a modified
version of the de Vaucouleurs (1959) revised
Hubble-Sandage classification system using modern
digital images - illustrations of 523 galaxies
are accompanied by a review of research
describing how different morphologies are
interpreted.
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The old Hubble classification system (1936)
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De Vaucouleurs Classification Volume 1959
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Cross section at stage Sb
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How does this classification differ from Hubbles?

• Treated galaxy morphology as continuous
• recognized an intermediate bar strength category
  SAB
• extended sequence to later types Sd, Sm, Im
• carefully treated ring phenomena recognized
  outer pseudorings R
• recognized family and variety among S0s in same
  manner as for spirals

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The first essential of a classification scheme
is to be well-illustrated. Ivan R. King,
conference summary
Origins of the Project Meeting at Osservatorio
Astronomico di Capodimonte, SantAgata, Italy 1990
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The Hubble Classification System
E. Hubbles revision to his classification system
was published as a large atlas of images by Allan
Sandage in 1961 This was followed much later by
the Carnegie Atlas of Galaxies in 1994 These
atlases outline Hubbles and Sandages ideas very
well The de Vaucouleurs viewpoint had never been
illustrated in atlas form with large-scale images
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Colleague Harold G. Corwin, Jr., who also worked
on RC2 and RC3
-American Astronomical Society meeting, summer
2001 - Harold and I talk to Simon Mitton of
Cambridge University Press about the idea of a de
Vaucouleurs galaxy atlas - Simon suggests writing
a proposal - web proposal prepared in fall 2001 -
project approved and contract signed January
2002 - proposed delivery date Sept. 1, 2003 -
actual delivery date Nov. 12, 2005!
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Motivations behind the Atlas
- in 1990, classical galaxy classification and
morphology were viewed with some disdain - the
subject was almost purely descriptive physical
interpretations were lacking - nevertheless,
people were beginning to realize the significance
of features such as bulges, disks, bars, rings,
and spiral structure to galaxy evolution - as
technology advanced, it became possible to study
morphology in wavebands outside the optical
realm - with the Hubble Space Telescope,
morphological studies of high redshift galaxies
became feasible for the first time - this
rejuvenated interest in classical morphology
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Why classical galaxy morphology and
classification is still relevant
- no physical approach has yet provided a
description of galaxies that can fully supplant
classical morphological analysis - people still
choose samples for study based on catalogued
galaxy types - classification is an efficient
process for nearby galaxies it can train more
quantitative approaches for studies of more
distant galaxies - while it is tempting to
relegate the details of galaxy morphology to
unimportance, these details may nevertheless
carry information on galaxy evolution - dark
matter domination is not sufficient justification
to ignore the details
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Why prepare an atlas to illustrate de
Vaucouleurss viewpoint?
- de Vaucouleurss revision provides a better
description of galaxies without being too
unwieldy - most astronomers use de Vaucouleurss
classifications from the Reference catalogues -
de Vaucouleurs paid more attention to certain
details that have a bearing on the way galaxies
evolve
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Goals of the Atlas
- to make the de Vaucouleurs revised
Hubble-Sandage (VRHS) system more accessible and
understandable to astronomy students,
researchers, and amateurs - to summarize how much
people have learned about the physical basis of
morphology since the time the classification
system was published. The subject is no longer
purely descriptive!
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Beginning of the Project
- fall 2002 sabbatical - I begin soliciting
images for the Atlas - Steve Odewahn sends the
first batch of Palomar 1.5m images - Brent Tully
sends a tape with many of his best images - I
also solicit images from the Ohio State
University Bright Galaxy Survey and other
sources - as images are received, I quickly begin
to fill up my disk storage space - MIRACLE
Seagate 80 Gbyte hard drives suddenly make the
Atlas feasible!
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Highlights from the Atlas
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Atlas Units
The digital images in The de Vaucouleurs Atlas of
Galaxies are displayed in the common unit of
magnitudes per square arcsecond
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Factors Influencing Galaxy Classification and
Morphology
- no galaxy can be brought into a lab to be
viewed from any direction or distance nearby
galaxies always better resolved than distant
ones - wavelength dependence -interstellar
extinction - expanding Universe - evolution -
impact of total mass and environment
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Effects of tilt on morphology
NGC 1433
NGC 5905
NGC 5792
NGC 2713
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Wavelength-dependence of galaxy morphology
NGC 6951 - blue light
NGC 6951 - near-infrared
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The ( r) and (s) varieties
NGC 2523 SB(r)
NGC 1300 SB(s)
NGC 5364 SA(r)
NGC 5247 SA(s)
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SA, SAB, and SB families at ( r) variety
NGC 5364 - SA(r)
NGC 309 - SAB(r)
NGC 3660 - SB(r)
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Varieties among SB0 galaxies
NGC 4340 - SB( r)0
NGC 4665 - SB( s)0
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Bulge inconsistency in Sa galaxies
NGC 3081 - a small bulge Sa
NGC 4378 - a large bulge Sa
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Recent Classification Developments
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Sb galaxy luminosity classes
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Sc galaxy luminosity classes
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Elmegreen Arm Classes
Flocculent vs density wave spirals
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Recognizing edge-on Barred Galaxies
NGC 128
X-shape reveals 3D bar structures in these
unsharp-masked images
NGC 1381
NGC 2788A
NGC 3203
NGC 4710
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NGC 7020
Non-edge-on galaxies showing similar X-shapes
NGC 1527
IC 5240
NGC 4429
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Counterwinding Spiral Galaxies
m0-6
m0
m1
m2
NGC 4622 - V-band
NGC 4622 - Fourier component images
This galaxy has oppositely winding sets of spiral
arms.
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Counterwinding Spiral Galaxies
ESO 297-27
This galaxy has an inner single arm and 2-3
oppositely winding outer arms.
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New morphologies the outer Lindblad resonance
(OLR) subclasses
Formation of an R1 pseudoring
Formation of an R2 pseudoring
M. P. Schwarz 1984 - test-particle simulations of
a barred galaxy R. Buta 1985 1995 - Catalogue of
Southern Ringed Galaxies
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OLR Subclasses of Outer Pseudorings
R1
R1 R2
Orbit families behind these features
R2
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Nuclear Rings - small rings of star formation
located in centers of barred galaxies
ESO 565-11
NGC 1326
U-band
V-band
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Secondary or Nuclear Bars
Secondary bars can have any angle relative a
primary bar, implying they are independent
features
NGC 1291- leading
NGC 6782 - trailing
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Color index maps show the distribution of dust
and star formation
NGC 1097 B-I
NGC 1097 - B-band
Red, dusty regions are light Blue, star-forming
regions are dark
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Kormendy and Bender revised the classification of
E galaxies to include boxy and disky shapes
Disky elliptical galaxy
Boxy elliptical galaxy
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The effect of Galactic extinction on Galaxy
Morphology
Maffei 1
Maffei 2
Near-IR light - extinction much reduced
Blue light- extinction strong, galaxies almost
invisible
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The effects of environment on galaxy morphology
NGC 4580 - in Virgo Cluster. Star formation only
in inner regions
NGC 4245 - in Coma I group. Should have an outer
ring but doesnt!
These galaxies have truncated star forming disks
due to interactions with a cluster or group
environment
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The quantification of bar strength in terms of
forces
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Quantify bar strength in terms of forces inferred
from near-IR images
Tangential to radial force ratio map of NGC 6951,
showing islands of maximum tangential forcing
NGC 6951 - 2.2 micron (near-IR) image
Use this kind of map to quantify bar strength
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If a galaxy has both a bar and spiral arms, it is
possible to separate the two features using
Fourier techniques. This improves bar strength
estimates.
NGC 6951 - separated images
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A dust-penetrated classification scheme based
on near-IR images (Block, Puerari, Buta)
Dominant two-armed spirals
Pitch angle class
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Intrinsic shapes of inner rings and outer
pseudorings, a way of linking these features to
resonances in barred galaxies
Inner rings oval in shape and aligned parallel
to bars
Outer pseudorings two types in the OLR subclasses
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Rings and HII region distributions
Inner rings are zones of active star formation.
H-alpha images reveal locations of HII regions.
The way these HII regions are distributed around
inner rings is sensitive to intrinsic ring shape
(numbers in right cols.) but not to bar strength
(numbers in left cols).
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Large low surface brightness (LSB) galaxies
important contributors to mass density in
Universe but often missed in catalogues
Companion to NGC 7531 - large LSB
Malin 1 - giant LSB
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Examples of Atlas Pages
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Thank You!