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ASTR 5610, Majewski [SPRING 2016]. Lecture Notes

ASTR 5610 (Majewski) Lecture Notes


INTRODUCTION TO THE CONCEPT OF STELLAR POPULATIONS
and
A BRIEF HISTORY OF STELLAR POPULATION STUDIES

GOAL: Understand 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" (SSP):

One example of an SSP is the typical star cluster (either open or globular).


IN PRINCIPLE:

where here POPULATIONS refers to what I will call a PRINCIPAL COMPONENT POPULATION.

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:

"Basis Vectors"

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.


MORE SPECIFICALLY:


The Ultimate Chemodynamical Model for the Evolution of a Galaxy might include as descriptors/variables (all time variable):


BRIEF HISTORY OF MILKY WAY STELLAR POPULATION STUDIES

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"

  1. 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" = Milky Way.

    Until 1920: Curtis-Shapley Debate.

  2. Early 20th century stellar population studies concerned with taxonomy:
  3. Stellar evolution theoretical advances in 1950's - understanding of chemistry and ages of stars
  4. Connections between structure, kinematics, age, chemistry, stellar (CMD) types

    1. Chamberlain & Aller (1951) - enrichment levels "written" in spectra
    2. 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)

    3. Sandage & Walker (1955) - weak-lined Milky Way field stars look like globular cluster stars (via Ultraviolet Excess)
    4. cluster stars <--> calibrate field star age-dating


      chemistry (e.g., UV excess) <--> absolute ages

      --> Develop a general AMR for Milky Way field stars.


    5. Baade (1944), Oort (1926)


      First sweeping collectivization of "stellar populations"
    6. CMD types <--> structural components

      Baade's definition of populations based on CMD type.

      Click here for Baade discussion figures.

      structural components <--> kinematical groups

      • POP I = Disks = "slow-moving" (OB stars, open clusters)
      • 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...

      Note also:

      extragalactic systems <--> analogs to Milky Way populations


    7. Nancy Roman (1954), in a spectroscopic study of high and low velocity stars, makes final ties
    8. metal-weak -- high velocity

      metal-rich -- low velocity

      age-metallicity groups <--> structure-kinematical groups


    9. 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.


    10. A "conventional", modern view of the primary Galactic stellar populations and their spatial (density law), chemical, and kinematical properties.

      Though it should be kept in mind that this conventional picture is still debated.

      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 http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/RotationsReckon.html and http://www.mpa-garching.mpg.de/mpa/research/current_research/hl2003-12/hl2003-12-en.html.
  5. 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 their formation.
      • 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).
      • Today:


        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.


  6. Some later developments (briefly for now and highly selective):


    • 1970s:


      • 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.
      • First large area radio surveys, discovery of Magellanic Stream and other radio evidence for gaseous filigree in the sky.

        The Magellanic Stream and other 21-cm features as presented by Mathewson et al. (1974, ApJ, 190, 291).

    • 1980s:


      • 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 Conference Intermediate Population II ("thick disk") component of Galaxy.
      • Gilmore & Reid (1983) two-disk model to fit densities of stars at the South Galactic Pole.
      • Extensive debate over existence of thick disk ensues.
      • Continuing to today, debate over origin of thick disk.


    • 1990s:


      • Refurbished HST and other large glass leads to revolution in resolution of extragalactic systems and 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.
    • 2000s:

      --> 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.

      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 high redshift.

      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 zebu.uoregon.edu/~imamura/ 209/may10/baryon.html.

      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.

      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. From http://www.youtube.com/watch?v=n0jRObc7_xo.



      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." From http://www.youtube.com/watch?v=MncUDWhPB_E.
      --> 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 local dark matter in the Galaxy. Searches for its source, distribution.
    • 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 universe).
    • 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.,

      • infrared...

        Image of the Milky Way from the COBE satellite.
        Image of the Milky Way from the IRAS satellite.
      • radio...

        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.)
    • Microlensing surveys:

      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, www.astro.ubc.ca/E-Cass/1998-JS/ thetalk/thetalk.html.
    • 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. From http://cfcpwork.uchicago.edu/seminars/talks/040206/slideshow/3.html.
          From http://cfcpwork.uchicago.edu/seminars/talks/040206/slideshow/4.html
        • 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 http://ned.ipac.caltech.edu/level5/Sept05/Gondolo/Gondolo3.html.
      • Expectation of luminous halo substructure from late infall of satellites.

      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 substructure.

      (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 x-shaped:

      '
      (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, 370, 733).

    • Extensive observations of globular clusters, both with HST imaging and high resolution imaging, suggest ` that some, perhaps ALL, are not just simple stellar populations.

      ' '
      (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

Physical Limitations
Technical Limitations


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.

We may also represent the chemical and star formation history of a galaxy by way of the HODGE POPULATION BOX (Hodge 1989).

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:

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

Unfortunately, hard to construct this information for external galaxies:

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).

READING ASSIGNMENTS:


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All material copyright © 2003,2006,2008,2010,2012,2014,2016 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 551 and Astronomy 5610 at the University of Virginia.