INTRODUCTION TO THE CONCEPT OF STELLAR POPULATIONS
A BRIEF HISTORY OF STELLAR POPULATION STUDIES
formation and evolution of galaxies through
investigation of properties of constituent, luminous parts.
ASSUME: Galaxies can be
broken down into POPULATIONS -
groups of stars, clusters, gas, with shared, or definable
distributions of, properties.
A basic unit of stellar population studies is the "SIMPLE STELLAR POPULATION"
Chemically homogeneous (at least at birth)
One example of an SSP is the typical open star cluster.
where here POPULATIONS refers to what I will call a PRINCIPAL COMPONENT
The GALAXY here is a composite population, but so too may be
the PRINCIPAL COMPONENT POPULATIONS,
which ultimately are likely a superposition of SIMPLE STELLAR POPULATIONS:
Some examples of composite, principal component populations in our Milky Way might be:
Each of the above represent complex groupings of stars and matter, but with distinct
global properties/distributions of chemistry/age/kinematics from one another.
These differences presumably relate to differing mixtures of SSPs.
The smaller the basis set (n, mn), the easier is
the composite population (and, ultimately, the Galactic history) to unravel
Identifying individual SSPs may be hard in complex
galaxy, but, perhaps SSPs are strung together in somewhat simple
(e.g. a star forming disk with steadily enriching series of SSP
"bursts" of increasing rotational velocity about center)
.... that make up a principal component population of a
This is one hope of stellar population studies.
We hope to simplify what may be a quite complex problem to one of finding the patterns in
Principal Component Populations.
CHEMISTRY, e.g. mean
[Fe/H], chemical abundance patterns ([O/Fe], [Ca/Fe],
AGES, reflected, e.g., in
the types of stars seen (their evolutionary state)
TO IDENTIFY AND DEFINE : Principal
component populations that will allow us...
TO RECONSTRUCT: A
complete, physical, chemodynamical evolutionary model
of the Milky Way (or other galactic systems)
Note that these observable trends and attributes are the end result of physical
proceses affecting star formation in a variety of ways.
These physical processes are fundamental to forming chemodynamical models of galaxy evolution, but that,
in themselves, are generally not directly observable, and have to be inferred and constrained by the trends we can
The Ultimate Chemodynamical Model for the Evolution of a Galaxy
might include as descriptors/variables (all time variable):
the evolution of the phase space distribution of stars (and
gas, dark matter)
the evolution of atomic species Xi as the
interstellar gas out of which stars form enriches
the STAR FORMATION RATE
the instantaneous INITIAL MASS FUNCTION, how the
new stars are distributed by mass (which determines how
populations evolve chemically and what kinds of remnants are produced)
BRIEF HISTORY OF MILKY WAY STELLAR POPULATION
The history of understanding the stellar populations of the Milky Way
provides an illustration of the above process.
In the history provided here, the symbol
= "A correlation of properties"
At the beginning of the 20th century, Milky Way studies was
essentially "cosmology", since at the time it was not appreciated
that there were galaxies outside our own.
"Sidereal Universe" = "Kapteyn Universe" = Milky Way.
Until 1920: Curtis-Shapley Debate.
William Herschel's 1785 map of the universe.
Kapteyn's more mathematical description of the Herschel universe as oblate ellipsoids of stellar density.
Early 20th century stellar population studies concerned with taxonomy:
e.g., groups of stars based on:
velocities (proper motions, radial velocities)
stellar types: e.g., colors, brightness,
variability, spectral type
location: e.g., in spiral arms,
in clusters/associations, in bulge
Early work by Lindblad (1936, MNRAS, 97, 15)
on the understanding of stars of different types
in different orbits, characterized by differences
in conserved quantities ("integrals of motion"),
energy (I1) and angular momentum (I2).
Stellar evolution theoretical advances in 1950's -
understanding of chemistry and ages of stars
Allows age dating of clusters,
CMD's (Sandage & Schwarzschild
ages <--> CMD types
Of course, earlier in the 20th century, the Hertzsprung-Russell
Diagram (HR Diagram) was invented; showed that stars do not
occupy random distributions of luminosity and
spectral type/temperature/color, as shown below.)
Because the giant and dwarf sequences were
not clearly connected in early HR diagrams, it
was still not clear in the early 20th century which way stars evolved
in the HR diagram, as evidenced by the two images below:
(left) Henry Norris Russell thought stars must contract down the giant
branch and down the main sequence, as illustrated in this 1927
figure (reproduced in Sandage's "History
of Mt. Wilson"). (right) A 1944 letter from George Gamow to Walter Adams
discovered in the Huntington Library Archives shows that Gamow had
notions closer to those we know to be corrrect. You can learn
more about this story from
Sandage, Beaton & Majewski (2016).
The images below show how deeper photometry of globular clusters
by Sandage and Arp in the 1950's made more clear the connection,
and how the evolution could conform to new models of stellar evolution.
(left) Sandage's 1953 color-magnitude diagram of the globular cluster M3.
(right) Sandage & Schwarzschild's (1952, ApJ, 116, 463)
development of an understanding of the positions of
stars in a cluster color-magnitude
diagram on the basis of stellar evolution. Earlier in the
20th century, it was still not clear which way stars evolved
in the HR diagram, because the giant and dwarf sequences were
not clearly connected, as evidenced by the two images below:
Understanding nucleosynthesis in stars
(e.g., Burbidge, Burbidge, Fowler, Hoyle 1957) --
notion of chemical enrichment with time
Connections between structure, kinematics, age, chemistry,
stellar (CMD) types
Chamberlain & Aller (1951) - enrichment
levels "written" in spectra
chemistry <--> gives relative age
Note concept of an
Age-Metallicity Relation (AMR)
Ultraviolet excess as simple way
to get relative metallicities of
stars through photometry, rather than
spectroscopy - metal lines are crowded toward UV
part of spectrum. (Sandage, Eggen: 1950s, 1960s)
Sandage & Walker (1955) - weak-lined
Milky Way field stars look like globular cluster
stars (via Ultraviolet Excess)
cluster stars <-->
calibrate field star age-dating
chemistry (e.g., UV excess) <--> absolute ages
--> Develop a general AMR for Milky Way field stars.
Baade (1944), Oort (1926)
First sweeping collectivization of "stellar populations"
CMD types <-->
Baade's definition of populations based on CMD type.
POP I = Disks = "slow-moving" (OB stars, open
POP II = Bulges, Halos = "fast-moving"
(globular clusters, "cluster variables")
Aside: Note potentially confusing, historical nomenclature that "slow-moving" and
"fast-moving" (and "low" and "high velocity") here are stated with respect to the
Sun, which is itself rotating quickly
about the Galaxy. Thus, the true rotational velocities of the stars are actually
pretty much the opposite of these labels...
extragalactic systems <--> analogs to
Milky Way populations
Nancy Roman (1954), in a spectroscopic study of high and low
velocity stars, makes final ties
metal-weak -- high velocity
metal-rich -- low velocity
groups <--> structure-kinematical groups
1957 Vatican Conference on Stellar Populations:
"Meeting of the world's great astronomical minds" to piece together
and organize understanding of Galactic populations.
Subdivide/refine Baade's broad groupings:
Summary tables from the 1957 Vatican Conference
proceedings. This book makes great reading, because all
of the conversations of participants have been preserved
and recorded in the proceedings. Note that the ages listed
in the table are based on well outdated stellar evolution models,
and are too small by about a factor of two.
modern view of the primary Galactic stellar populations
and their spatial (density law), chemical, and kinematical
Though it should be kept in mind that this conventional picture is
Another view of the Milky Way and its populations. From Buser (2000,
Science, 287, 5450, 69). His caption: Schematic view of the major
components that make up the Galaxy's overall structure, shown in a
cross section perpendicular to the plane of rotation and going
through the sun and the Galactic center. From the observer's vantage
point at the sun's position, the directions to the North (NGP) and
South (SGP) Galactic Poles are particularly suitable for studying the
layered structure and other properties of the stellar disks and halo,
whereas the concentration of gas and dust in the extreme disk severely
obstructs observations of the distant bulge at visual-optical wavelengths.
The central parts of the Galaxy are better accessible through longer
wavelength infrared and radio observations.
Note the difference between the luminous stellar halo, and the
dark matter halo postulated to exist and in which the luminous
matter is embedded.
Cartoon (left) and modeled (right) illustration of the Galactic
dark matter halo. In right figure the plot is only of the density
of dark matter in a simulated Milky Way halo, with light on a logarithmic scale
and 600,000 light years on a side.
From groupings of populations by properties to the first physical
evolutionary model of the evolution of Milky Way populations
Eggen, Lynden-Bell & Sandage (1962)
Collected data on two main groups of stars
(nearby disk stars, and high velocity stars) under
the assumption that these were representative of
range of types in Galaxy (preconceived picture?)
Found trends between population
characteristics -- i.e., stellar pops not just disconnected
groups but exhibit correlated, continua of
properties, evoking an evolutionary sequence
of events (where the chronometer adopted is the UV-excess,
an indicator of the degree of metal enrichment).
ELS formulated a physical model to account for
continuum of trends.
We shall discuss at greater length later in the semester,
but brief outline of model:
Galaxy starts as ball of unenriched gas.
Self-gravity, spherical collapse begins.
Stars form during collapse, chemical enrichment
of gas proceeds as populations of stars form.
Stars retain chemical
and kinematical character of gas at time of
Conservation of angular momentum creates
"spin-up" of gas as shrinks.
Collapse proceeds 25X vertically, 10X radially
into a disk.
Collapse postulated to be rapid,
freefall timescale (107-108 years or so).
Early-formed stars are metal-poor stars of outer
halo on radial, plunging orbits.
Late formed stars are metal-rich stars of disk
and on circular orbits.
Some later developments (briefly for now and highly selective):
Searle & Zinn (1978): Discovery of likely
age spread in the halo globular clusters -- longer
than "free-fall" timescale.
Postulate late infall of material
from independent "fragments".
Isobe (1974), Saio & Yoshii (1979),
Mihalas & Binney (1980): Realization of
problems with ELS methodology -- selection effects.
Model too simplistic to account for some kinds of
stellar populations found (e.g., retrograde stars).
Need slower formation process for halo.
Early technical understanding that and how galaxies can interact dynamically.
Figure from classic paper by Toomre & Toomre (1972, ApJ, 178, 623) showing early N-body model
of the gravitational interaction of two galaxies (one only modeled as a point particle
mass, due to limits in computing power). These simulations showed how such
interactions create "bridges" and "tails" due to tidal forces.
First large area radio surveys, discovery of
Magellanic Stream and other radio evidence for gaseous filigree in the sky.
Evidence for an earlier Milky Way interaction with another galaxy?
The Magellanic Stream and other 21-cm features as presented by
Mathewson et al. (1974, ApJ, 190, 291).
Codification of computer structure
models (i.e., not really physical evolution models),
ostensibly to aid predictions of star counts
in HST observations (Bahcall &
Soneira 1980, Robin & Creze 1986).
Influence of BS80 model, with TWO
populations, as well as influence of ELS model,
which discussed two main populations creates
a trend to simpler, Baade-like description
of Galaxy (i.e., halo and disk).
Yoshii (1982) and Gilmore & Reid (1983) starcount studies of
structure of Galaxy and (re-)discovery of Vatican
Intermediate Population II ("thick disk")
component of Galaxy.
Gilmore & Reid (1983) two-disk model to fit densities of
stars at the South Galactic Pole.
Name "thick disk" after discovery of such structures in deep photometry of edge-on disks
by Burstein (1979), Tsikoudi (1980), van der Kruit& Searle (1981).
Figures from van der Kruit & Searle (1981, A&A, 95, 105) showing isophotes from their deep photometry
of the edge-on disk NGC 4565, both before and after subtraction of a best fitting "thin disk".
Extensive debate over existence of thick disk
Continuing to today, debate over origin of thick disk. For example:
Created during "Top-down", ELS-like, collapse?
Made by secular "Bottom-up", heating scenario?
Made by a dramatic "disk stirring event", for example, a merger into the disk?
The remains of some other galaxy absorbed by Milky Way?
Migrated stars from the innerg Galaxy?
Refurbished HST and other
large glass leads to revolution in resolution of extragalactic
detailed study of deep CMDs of ever more distant galactic
and star cluster systems, deeper study
of MW clusters, etc..
Lower panels show unprecedentedly
deep CMD of the globular cluster NGC6397
by King et al. (1997), showing stars all of the way down
the main sequence to the hydrogen burning limit and including
the white dwarf sequence. Top panels show HST proper motions
used to clean the cluster of contaminating field stars.
HST color-magnitude diagram of the Leo I dwarf spheroidal galaxy
at 270 kpc distance, by Gallart et al. (1999, AJ, 118, 2245).
HST CMDs, velocity distributions and derived age-metallicity distributions
of stellar populations in fields of M31 via stellar population synthesis
modeling. From Brown et al. (2007, ApJ, 658, L95).
Big scopes create explosion of high resolution
spectroscopic study of multi-element chemical
abundance patterns in nearby galaxies.
--> Provides detailed understanding of star formation
and evolution in galaxies at low redshifts.
An early plot showing the distribution of [O/Fe] vs. [Fe/H] in Galactic stars.
The plot gives information on the relative contributions of Type Ia and
Type II supernovae at different times in the Galaxy's history. From
Matteucci & Recchi 2001, ApJ, 558, 351.
--> More recently, with access (last decade)
to stars in more distant MW satellites,
provides some puzzles w.r.t. the prevailing models
of hierarchical galaxy formation.
(left) Plot showing the distribution of [O/Fe] vs. [Fe/H] in Galactic stars (small symbols)
compared to a Milky way satellite (the Sculptor dSph, pink symbols).
The plot shows that the putative subcomponents used to build large galaxies
like the Milky Way have different abundance patterns.
From Geisler et al. 2005, 129, 1428. (right) Similar diagram for other
"alpha-elements" (and their sum) for Milky Way stars of different
stellar populations (cyan=halo, green=thick disk, red=think disk)
and for Milky Way dwarf spheroidal
satelites (squares). From Venn (2005).
--> APOGEE is now greatly expanding the reach of such work, allowing chmistry to be explored over
all parts of the MIlky Way, and allowing the ability to study the variations in chemistry,
enabling reconstruction of the chemical history of different stellar populations.
Remarkably clear evidence in APOGEE and other surveys that there are DISTINCT populations with
separate chemical evolutions, in the nearby parts of the Milky Way.
The "low alpha" sequence seen below is dominated by what we regard as "thin disk" stars, and
varies in mean metallicity as one goes from the center of the disk outward.
The "high alpha" sequence (traced by the line in the figure) seem to be the same distribution everywhere (the
density of stars in the plot is a sampling variation, but the relative metallicity distribution in that
sequence is apparently unvarying with position in the Galaxy). It looks to be representative of
the "thick disk"/Intermediate Populaiton II.
APOGEE results for
the [α/Fe] vs. [M/H] diagram for 70,000 red giant stars across the Galactic disk, broken up in
zones of radius and distance from the Galactic plane. From Hayden et al. (2015).
Even the distribution functions for individual elements (here, iron) for different parts
of the Galaxy hold important clues to post-star formation mixing of stars from place to place.
In this figure showing the Metallicity Distribution Function across the Galaxy, we see
the variation in the distribution shape, which Hayden et al. (2015) interpret as having to do
with radial migration of stars to different radii in the disk.
--> Detailed patterns of more "obscure" elements and the notion
that stellar enrichment across all chemical elements is
somewhat stochastic (e.g., a function of the very specific stars
that went supernova to create the enrichment) leads to hope
of "chemical fingerprinting" stars to their birth sites.
Summary of chemical abundances PREDICTED for a variety of elements in galaxies of the
Local Group. APOGEE and other surveys of Galactic chemistry, like GALAH and the Gaia ESO Survey
are now delivering such multielement data.
Figure from Gibson 2007, IAU Symp. 241: Stellar Populations as Building Blocks of Galaxies,
eds. A Vazdekis & R. Peletier.
HST and big scopes bring new insights into galaxies
evolution at high redshifts.
--> Can image morphology, obtain photometry of presumed
high redshift analogues to Local Group galaxies.
--> QSO absorption line studies bring information on
chemical and dynamical state of gas in galaxies at
Damped Lyman Alpha Cloud at z=2 seen in absorption of a higher redshift
QSO (note emission line). The amount of absorption needed to make a
trough this deep is consistent with the mass thought to exist in
spiral disks. Could these be young versions of the Milky Way? From
The so-called "Madau plot", which shows the evolution of the
star formation rate with time, derived for Hubble Deep Field galaxies
by Madau (1998). If the plot were made as a function of time instead
of redshift, the peak would be more symmetric and centered on 60%
of the lookback time from the present age of the universe.
Growth of ever more realistic N-body codes, to match
an improved understanding of the input physics.
--> Better models of Galaxy formation and interactions.
Early and relatively simplistic
N-body model of the collapse of a Milky Way-like galaxy (a la Eggen, Lynden-Bell & Sandage), with
gas (yellow), stars (red), and dark matter (purple), by Peter Williams and Alistair Nelson
(The Edinburgh Parallel Computing Center).
A 2008 movie showing the formation of a galaxy like our own with only the stars and gas. It assumes a Cold Dark Matter Universe. The movie shows the distribution of gas and stars from after the Big bang to the present time. Every second corresponds to 70 million years and the frame is 500.000 light years across. Note the periodic formation of strong central bars.
A 2010 movie showing separately the evolution of gas (red to yellow, left) and cold dark matter
(purple to white, right) densities during the formation of a galaxy similar to the Milky Way.
Volume rendered by Rob Crain and Jim Geach, from a 3D hydrodynamical cosmological simulation
by Rob Crain and the GIMIC/OWLS consortia. From http://www.youtube.com/watch?v=6nupGMmaXEI.
A 2011 movie claiming to be "the world's first realistic simulation of the formation of our
home galaxy". From groups at the University of Zurixh and UC-Santa Cruz. "The new results
were partly calculated on the computer of the Swiss National Supercomputing Center (CSCS)
and show, for instance, that there has to be stars on the outer edge of the Milky Way."
--> Models of the growth of structure
in the universe promote notion of the importance of Cold Dark
Matter in the universe, and the possibility of a universal
mass profile (e.g., the "Navarro-Frenk-White" or NFW profile)
for hierarchical structure growth.
where a is a scale length and ρ0 is a normalizing density.
N-body model of the growth of structure showing gas (left) and dark
matter (right) by Navarro, Frenk & White (1995, MNRAS,275,720).
Growing recognition of importance of dark matter in galaxies,
local dark matter in the Galaxy, and searches for
its source, distribution.
That the fractional dark matter content of dwarf galaxies increases with decreasing
The dependence of metallicity ([Fe/H]), mass-to-light ratio and mass (within 300 pc) as a function of luminosity for MW dSph galaxies. Though their luminosities span almost
5 orders of magnitude, for MW dSphs the mass within 300 pc remains nearly constant at
about 107 soalr masses. The derived M/L rises dramatically with decreasing
luminosity. Figure from Geha et al. (2009).
At the same time, a recognition that the standard theory of Cold Dark Matter,
so successful on large scales, seems to have problems on galaxy scales -- see below...
Growing recognition of possible importance of
late infall, accretion of small satellites, in
formation of Galaxy (as predicted by Cold Dark
Matter theories for growth of structure in the
Discovery of a potentially disrupting Milky Way
dwarf galaxy, the Sagittarius dSph.
Image from Ibata et al. (1995, MNRAS, 277, 781).
First quality large-scale, multiwavelength views of Milky Way. E.g.,
Image of the Milky Way from the COBE satellite.
Image of the Milky Way from the IRAS satellite.
Neutral hydrogen, 21-cm map of the Milky Way from Dickey & Lockman (1990).
NASA's most recent
multiwavelength Milky Way poster (from http://mwmw.gsfc.nasa.gov/mmw_product.html.)
Magnification of apparent brightness by microlensing.
Discovery of brown dwarfs, extrasolar planetary
systems as extension of stellar evolution theory
and possible dark matter source (but seems less likely now,
based on microlensing experiments).
Spectrum of the brown dwarf Gl229B compared to the very similar spectrum of Titan,
the gaseous satellite of Saturn. From Gordon Walker, 1998 C.S. Beals Lecture,
First large area, high resolution, digital sky surveys
(SDSS, 2MASS) allow first
accurate, large-scale, systematic probes of the Galaxy.
2MASS image of the Milky Way.
First clear signature of a dwarf galaxy stellar tidal stream:
Map of the distribution of M giant stars seen in the 2MASS
survey, in two apparent magnitude bins. The Sagittarius stream
is evident. From Majewski et al. (2003, ApJ, 599, 1082).
The distribution of M giant stars seen in the 2MASS
survey seen in projection onto a plane perpendicular to the
Galactic plane. The Sagittarius stream is seen
wrapping around the Milky Way in a nearly polar orbit.
From Majewski et al. (2003, ApJ, 599, 1082).
The substructure of the outer Galaxy becomes clear:
The "field of streams" from the SDSS survey.
From Belokurov et al. (2006, ApJ, 642, L137).
Further/better CDM modeling on Milky Way-scale (birth
of "near-field cosmology"):
Model of the formation of a Milky Way-sized galaxy in
a CDM model by Ben Moore's Zurich group.
But recognition of the failure of the
otherwise largely successful concordance
cosmology with dark matter on Galactic scales:
Missing satellites problem.
Current CDM models predict that the Milky Way should have
hundreds of satellites. Model from Ben Moore's Zurich group.
Angular momentum problem: CDM
can't make large enough galactic disks.
Core versus cusps problem: CDM
predicts galaxies to follow NFW to
a cusped center, but this is contrary
to what is observed in real galaxies.
Predicted dark matter density profiles for a galaxy resembling our own is shown but the green and magenta lines. Two empirical parameterizations
of what we actually see are given by the black and cyan lines. From
Expectation of luminous halo substructure from late infall of
CDM models mated with tidal disruption models make predictions
of halos filled with streams.
Models from Kathryn Johnston and James Bullock.
Discovery and mapping of actual stellar streams around the Milky Way
and Andromeda, and convergence of observation and theory of halo
(Left) Known Milky Way streams as mapped by Majewski & Law.
(Right) Model of Milky Way from Bullock & Johnston model.
In past few years (!) a substantial increase in the number of known
Local Group dwarf galaxies (mostly satellites of Milky Way and Andromeda).
About a dozen new Milky Way dwarf galaxy satellites have been found in the SDSS data
(doubling known number).
Strong evidence that the Milky Way's center is
(left) Evidence from the distances to measured
red clump star distances in Milky Way bulge fields
point to an x-shape for the center. From Zoccali &
McWilliam (2010, ApJ, 724, 1491). (right)
An x-shaped bulge seen in an edge-on galaxy,
ESO597-G036, as studied by Bureau et al. (2006, MNRAS,
Extensive observations of globular clusters, both
with HST imaging and high resolution imaging, suggest
` that some, perhaps ALL, are not just simple stellar
(left) Using new filtering ("hk"), Lee et al. (2009, Nature)
shows that some clusters show multiple RGBs.
(right) Most clusters are showing spreads of chemical
abundances in member stars, like this Na-O anticorrelation,
which is thought to be caused by variable pollution by
a first population of stars onto a second generation in
the same cluster. From Caretta et al. 2010, A&A, 516).
New explorations of the inner Milky Way, enabled by
work in the infrared (APOGEE!).
(left) APOGEE observing the Galactic bulge during
its first week of operations, May 2011.
(right) First light observations by APOGEE of a field
of candidate open clusters in the constellation Cygnus.
Pieces of APOGEE spectra show how chemical and velocity
information can be derived for member stars in each
candidate cluster (large circles). Stars in each cluster
targeted with an APOGEE fiber are shown by small circles.
The size of the moon is shown for scale.
Discovery of a stellar nuclear disk as seen in the radial velocities of APOGEE
data, by Schonrich et al. (2015). Figure shows a map of stars observed by APOGEE
in the direction of the Galactc center, color-coded by radial velocity. The thin rotating
disk of stars is obvious.
Perhaps most exciting -- New abilities to measure the ages of individual stars through the
methods of asteroseismology, gyrochronology, and C/N abundance ratios. Age is perhaps the most important
parameter for any investigation of evolutionary processes, but until recently we could generally
only age date clusters of stars reliably. These new abilities to measure ages for individual
field stars across the Galaxy is radically advancing our ability to explore this history of our
Galaxy's stellar populations.
Video showing the distribution of stellar ages measured from C/N abundance ratios (calibrated by
stars having ages from asteroseimology) in giant stars as
measured by the APOGEE survey. Colored dots show the 70,000 APOGEE RGB stars by age. As the video
shows, the oldest stars are found in the Galactic center, while younger stars are seen farther out in the
disk. This suggests that the Galaxy grows from the inside-out. Data and video from Ness et al. (2016).
SOME CHALLENGES OF STELLAR POPULATION STUDIES
Some information on the past can be erased and be unrecoverable. For
--> Stochastic "scrambling" of stellar motions by dynamical
relaxation, scattering off of Giant Molecular Clouds, the bar, spiral arms,
"Churning and blurring".
--> Tidal/evaporative processes act to winnow small star systems
(e.g., dwarf galaxies,
star clusters) down; change their appearance or remove them completely from sample.
--> Convection in stars alters chemical abundance patterns
present in atmospheres -- dredges up/introduces new elements, destroys
--> Astration: Stars pick up material from ISM (small effect).
Problems of perspective, confounded by extinction by dust. For example:
Some aspects of MW difficult to study.
- No clear view of bulge and inner Galaxy.
2MASS image of the point source catalog but with directions of the sky
having more than 1 magnitude of visual extinction by dust shaded grey. One can
see how difficult it is to explore the main part of the Milky Way, where most of the
- However, Skrutskie/2MASS
working in the infrared (less extinction): enhancement of stellar density
as seen in the Galactic plane by bar becomes obvious.
2MASS image showing relative densities of 30,000 carbon stars in the disk
of the Milky Way. Our position marked with green cross. The carbon stars
appear to show the Galactic Center to have a bar shape, tilted to the line of
- Compare to external views of other galaxies, e.g., M31,
for which a central bar has just been discovered by
Beaton, Majewski, Skrutskie et al. (2006):
Image from Beaton et al. (200x).
...and for which the halo has now been explored over a large area of the
Image showing the minimum size of the M31 halo as determined by the
SPLASH collaboration (e.g., Guhathakurta et al. 2005, Kalirai et al. 2009).
Image showing streams and newly discovered dSph satellites around the Andromeda
Galaxy from Geraint Lewis and the PANDAS collaboration.
Deep imaging of the spiral NGC 5907 shows similar tidal
streams as we know exist around the Milky Way. Image by Jay
The halo substructure remaining around the soiral galaxy
NGC 3521 is evident in this deep image by Jay Gabany.
- We know rotation curves of external galaxies
much better than that of MW. Cannot do outside
of solar circle very well.
Summary of measures of the Galactic rotation curve. From
Heather Morrison's web page: http://smaug.astr.cwru.edu/heather/222/mar1.html.
- What is on other side of Galaxy or beyond disk?
(e.g., recent Sloan "ring"; Sgr dwarf galaxy)
Schematic representation of the Sagittarius dwarf behind the Galactic bulge
(R. Wyse, JHU.).
Connecting Milky Way to extragalactic context
without similar, external view.
- Until Morgan (1950's) recognized nearby spiral
arms in MW, still uncertain whether we were a
Discovery of the Galactic spiral arms by Morgan, Whitford & Code's (1953,
ApJ, 118, 318) observations of blue supergiant stars.
- Now some understanding of spiral structure of the Milky Way, though
still incomplete due to obscuration problems.
(Left) Our best guess at the spiral arm structure of the Milky Way. Note that the
one arm should be called "Scutum-Crux" (not "Scuturn-Crux"). From wikimedia.
(Right) Schematic view of our current notion of Milky Way structure. From
http://www.daviddarling.info/images/Milky_Way_schematic.jpg. The right panel is inverted to compare to upper images.
- No rotation curve, can't accurately put MW on Tully-Fisher
- Current best evidence puts Milky Way 2-sigma off the T-F relation.
The Tully-Fisher Relation correlates the mass of a galaxy, as
measured by the rotation curve, typically measured in HI, and the
optical or NIR luminosity of the galaxy. The relation is a useful
rung on the distance scale ladder. From www.astro.columbia.edu/ ~bureau/astronomy.html.
As may be seen by some examples given above, there is hope that problems of
perspective can eventually be solved.
Difficult to study stars in other galaxies to level in MW,
--> HST, CCDs, large telescopes, adaptive optics making/will make
huge advances here possible.
Outer reaches of Milky Way challenging, particularly for
dynamics and parallaxes (distance scale problem).
--> GAIA, HST and radio observations will bring/have brung astrometry
to microarcsecond levels, enabling
motions of stars in outer Galaxy to be measured well.
Accretion means many subsystems (subpopulations) in Galaxy.
Each subsystem is a unique galactic environment to ecplore.
Systematic mapping needed.
--> Huge databases, like Sloan, 2MASS, HIPPARCOS, and faster
computers are allowing great
inroads into mapping Galactic phase space.
REPRESENTATIONS OF COMPLEX POPULATIONS IN MULTIVARIATE SPACE
-- SOME EXAMPLES YOU WILL SEE IN THIS CLASS
Most commonly, will use color-magnitude diagrams.
We will learn how to interpret such diagrams to understand something about the
star formation history of a system.
For example, decomposing the Carina Galaxy into SSPs:
Color-magnitude diagram of the Carina dwarf spheroidal galaxy
by Smecker-Hane et al. (1994, AJ, 108, 507) showing evidence of
the superposition of multiple simple stellar populations.
Evidence for three bursts of star formation but with similar abundance
We may also represent the chemical and star formation history of a galaxy by
way of the HODGE POPULATION BOX (Hodge
- AGE vs. [Fe/H] vs. STAR FORMATION RATE
- Note definition of metallicity [Fe/H]:
Note that the Hodge box variables are derived, not observed.
Hodge Population Box for Carina rather simple:
Hodge Population Box for a more complex (fictional) galaxy:
More or less continuous star formation could be
one "principal population component" made up of
numerous SSPs (e.g. an enriching disk) ... or
multiple pops -- need dynamics
From Grebel (199x).
Current estimation of the Milky Way's Hodge Population Box, as viewed from above:
An estimation of the Hodge Population Box for the Milky Way,
using a variety of available data. See Majewski (1999, in Globular Clusters,
eds. C. Martinez Roger et al., Cambridge Univ. Press).
Chemodynamical models require dynamical constraints/
observations as well
We can construct, analogously to Hodge box, a
DYNAMICAL POPULATION BOX,
with one axis a measure of the ratio of
ORDERED MOTION (Vrotation) to DISORDERED MOTION (σ = velocity dispersion):
e.g. Eggen, Lynden-Bell and Sandage Formation Model:
Unfortunately, hard to construct this information for external galaxies:
Line-of-sight, Doppler velocities hard enough, integrated information,
v sin i problems.
Proper motions? Just now contemplating doing this
for the very nearest galaxies
Milky Way is one "laboratory galaxy" where we can
get detailed info on chemistry, age, r, and kinematics
of stellar populations (we assume M.W. is "typical").
An estimation of the Dynamical Population Box for the Milky Way,
using a variety of available data. See Majewski (1999, in Globular Clusters,
eds. C. Martinez Roger et al., Cambridge Univ. Press).
Baade (1944) articles
Majewski ARAA article, Introduction and Section 1.