The Legacy of the HST for Studies of Stellar Populations in the Local Group

Jon Holtzman, New Mexico State University

• Contributions of HST to stellar populations
  • HST resolves faint stars in Local Group galaxies
    • Local Group contains a few dozen galaxies: two large ones, several ``medium-sized'' ones, and lots of little ones

  • In nearby Milky Way companions, HST provided first accurate CMDs fainter than the oldest main sequence turnoff.
    • One can probe fainter than the oldest main sequence turnoff in Milky Way companions only: Fornax, Carina, Ursa Minor, Draco, Sculptor, Leo I, Leo II, Phoenix. HST pioneered studies of these; however, superb data can now be obtained from the ground, with a much wider field coverage.

    • lower main sequence still accessible only with HST

    • HST allows photometry of individual stars in dense star clusters, e.g. LMC, SMC, Fornax (and MW!)

  • For M31 companions and ``isolated'' galaxies of the LG, one can reach the horizontal branch with HST, but not quite down to the oldest main sequence turnoff with HST/WFPC2; should be able to reach the oldest MS turnoff with ACS!

  • Numerous LG galaxies have been observed by HST. Most of these have been in programs designed to study single objects. In addition, proprietary time limit of HST encourages rapid analysis and publication of individual fields

  • To attempt to address more global questions, and to provide a useful legacy of HST results, we (Holtzman, Afonso (NMSU), Dolphin (NOAO)) have an archival project to make a compilation of stellar photometry from all LG observations made with HST/WFPC2.
    • HST LOCAL GROUP STELLAR POPULATIONS ARCHIVE ((photometry in progress)

    • Scientific goals:
      • IMF in Milky Way companions

      • Photometric metallicities and metallicity spread in LG galaxies

      • Cluster/ISM interactions

      • LBV identification

      • Consistent star formation histories of LG galaxies
  
  
• Review: some notable results from HST
  • Star clusters
    • MW globular clusters: LF and IMF below 0.75 solar masses (e.g. Paresce et al.): similar between all clusters and with turnover around 0.3 solar masses

    • LMC star clusters: IMF in R136 still debated(e.g. Sirianni et al., Zinnecker et al., Hunter et al.); age of the oldest LMC clusters similar to oldest MW clusters (e.g. Olsen et al., Johnson et al.); LMC age gap still exists (e.g. Rich et al.)

    • M31 globulars: structural parameters (e.g. Barmby et al., Grillmair et al.); horizontal branch brightness dependence on metallicity (e.g. Ajhar et al, Fusi Pecci et al., Rich et al.)

    • M33 globulars (e.g. Sarajedini et al.) - some may be younger than MW GCs!

    • Structural parameters of clusters

  • Field: star formation histories (many: Tolstoy et al.; Gallart, Aparicio et al.; Bertelli; Dolphin; Ng; Hurley-Keller, Mateo et al.; Han; Hernandez, Valls-Gabaud & Gilmore; Tosi et al.; Olsen; Harris; Cole, Smecker-Hane; Holtzman et al.).

  
  

• Star formation histories: motivation
  • Comparison with high redshift observations, e.g. SF history of the Universe, faint blue galaxies (note age-redshift relation)

  • Understand star/galaxy formation by looking at similarities/differences between SF history in different locations within a galaxy, proximity to other galaxies, etc.

  • Constrain numerical models of SF, as modellers begin to be able to create ``mock'' Local Groups

• Specific questions:
  • Is the SF history of galaxy correlated with any global parameters (e.g., morphology, mass, location, etc.)?

  • Is the IMF universal?

  • Is star formation bursty? What triggers star formation?

  • Is there more than one mode of star formation? IMF, cluster/field, etc.

  • How does SF history vary with location within galaxies, and why?

  • How old are the oldest stars in galaxies? Is this age different for different galaxies?

  • How important is gas inflow/outflow? Is it a function of galaxy type/ environment?

  
  

• What comprises a star formation history of a galaxy? Time history of:
  • Star formation rate

  • Metallicity distribution

  • IMF

  • Dynamical mixing and merger history

• Star formation histories: techniques
  • Coarse/qualitative SF history can be determined from presence/absence of key types of stars: horizontal branch stars, RR Lyraes, red clump stars, AGB stars, carbon stars, upper main sequence stars, supergiants, distinct main sequence turnoffs, etc. (e.g., Phoenix)

  • In principle, can derive full SF histories based on locations of stars in CMDs. Basic principle: what combination of simple stellar pops provide best match to observed data? Need to define best match. Also important to consider whether best match is in fact a good match.
    Simple stellar populations
    Composite stellar populations

  • Sensitivity to age decreases as age increases. Age-metallicity relation makes these problems more severe for older ages.

  • Deriving SF history complicated by possible uncertainties in stellar models, photometric calibration, reddening, distance, binarity. LF more robust, but non-unique.

  • Various groups have been working on these techniques (Tolstoy et al.; Gallart, Aparicio et al.; Bertelli; Dolphin; Ng; Hurley-Keller, Mateo et al.; Han; Hernandez, Valls-Gabaud & Gilmore; Tosi et al.; Olsen; Harris; Cole, Smecker-Hane; Holtzman et al.). Detailed approaches differ.

  • Healthy skepticism required. Comparison of results from different techniques limited so far, but not amazing consistent - Coimbra experiment

  • Systematic and correlated errors may be present between different techniques

  • Are the results robust? MAYBE, MAYBE NOT!

  • Is there potential to make them robust? YES
    • Differences in SF history between different locations may be easier.
    • Incorporate more external information

  
  

• Results to date and implications
  • Star formation histories are complex for most of these galaxies. In fact, derived SF histories are frustratingly heterogeneous! (HST compilation, Grebel compilation)
    • Range of ages present at all morphogical types

    • Burst/no-burst differences

    • Some correlations with mass, but note exceptions

    • Some correlation with proximity to large galaxy, but note exceptions (LMC/SMC)

    • With somewhat limited data, IMF appears to be consistent from location to location
      • lower main sequence (HST) constrains IMF without assumptions about SF history.

      • IMF has now been measured in systems covering 2 dex in metallicity in range of environments with no obvious variations: Draco (Grillmair et al.), Ursa Minor (Wyse et al.), SMC (Holtzman et al.), LMC (Holtzman et al.), R136 (Hunter et al., Zinnecker et al.), Galactic Bulge (Holtzman et al., Zoccali et al. 2000)

      • Caveats: Limited mass range, mild variations certainly not ruled out

  • Encouragingly, age-metallicity relations appear to come out of the color-magnitude diagram analysis! However, metallicity dispersion must be included in derivation of star formation history (also in chemical evolution models!)
    • Example: LMC

    • Example: Phoenix

  • Current results make crude predictions for metallicity distributions. Note difference between overall metallicity distribution and metallicity distribution of giants. Independent metallicity determinations will significantly help to confirm validity of derived SF histories and constrain them

  • Does field SF trace cluster SF? PROBABLY NOT
    • LMC
    • IC10
    Are there different modes of SF? MAYBE

  • To what extent is SF bursty? MAYBE NOT SO MUCH

  • Is there a single initial epoch of star formation in the Local Group? MAYBE, BUT HARD TO GET SUFFICIENT ACCURACY

  • Is there any preferred epoch for the start or cessation of star formation? NOT OBVIOUSLY

  • Do the SF histories of the two Magellanic Clouds differ significantly? MAYBE NOT. Is star formation in the Clouds triggered by mutual interactions or interaction with the Milky Way?

  • Is the IMF variable? MAYBE NOT!

  • Are Local Group dwarfs candidates for galaxies observed at higher redshift? PROBABLY NOT! Are they any good for anything? MAYBE!

  
  

• Directions
  • Uniform compilation /analysis of data sets. To what extent is SF bursty? Patterns of SF history, metallicity, metallicity spread, etc. Need high precision, wide field observations. HST archival project plus ground-based key projects.

  • Independent metallicity determinations: photometric and/or spectroscopic. Metallicity distributions can constrain chemical evolution.

  • Couple chemical evolution models with CMD information
    • Determine SF history that can simultaneously match CMD as well as abundance distribution/abundances of individual stars.

    • Using the SF history plus nucleosynthesis calculations, look for self-consistency, correct relative abundances of heavy elements.

    • Adjust chemical evolution models, e.g. for inflow/outflow/yields/IMF in an attempt to achieve self consistency.

  • Dynamical information may be crucial for understanding star formation triggering, and may help to reveal patterns where none currently appear to exist.

  • To connect Local Group studies with higher redshift observations, need to get past dwarf galaxies: Milky Way( fit), M31, M32, M33. ACS should allow probing of oldest populations, TAC-willing
    • Echelle spectroscopy of Hipparcos stars

    • GAIA, SIM critical for MW star formation history

  • More distant objects: gEs. Need to exploit information content of the red clump

  • SF histories in Local Group likely provides best place to tune star formation parameterizations in hydrodynamical cosmological simulations.

• The legacy of HST for Local Group stellar populations studies is still yet to be determined!