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Deep HST Imaging of M33: the Star Formation History . Jon Holtzman, Roberto Avila (NMSU) Julianne Dalcanton, Ben Williams (UW) Ata Sarajedini (UFl) Don Garnett (Arizona). Williams et al, ApJL 695, L15; Holtzman et al, AJ, submitted. Star Formation Histories.

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deep hst imaging of m33 the star formation history

Deep HST Imaging of M33: the Star Formation History

Jon Holtzman, Roberto Avila (NMSU)

Julianne Dalcanton, Ben Williams (UW)

Ata Sarajedini (UFl)

Don Garnett (Arizona)

Williams et al, ApJL 695, L15; Holtzman et al, AJ, submitted

star formation histories
Star Formation Histories
  • Galaxies are the observable building blocks of the Universe: understanding how and when they are assembled is key
  • Star formation histories record the buildup of stellar mass: include history of star formation rate, history of metallicity distribution, history of stellar mass distribution (IMF)
  • Understanding star formation is key: it’s a critical aspect of galaxy formation that is not currently very well understood theoretically
  • Observations of galaxies at high redshift provide an indication of when stars were formed, so long as integrated star formation rate indicators are valid
  • Nearby galaxies provide a fossil record of star formation
star formation histories from resolved stellar populations
Star formation histories from resolved stellar populations
  • Most work done in Local Group dwarf galaxies: closer and less crowded
  • Problem: not clear that SF in dwarfs represents a large fraction of SF in galaxies!
  • Star formation histories in disk galaxies
    • Milky Way actually challenging because of range of distances, extinction
    • Clues from unresolved observations:
      • Exponentially declining star formation rates?
      • Stellar population gradients
      • Problems: dust
m33 as a prototypical disk
M33 as a prototypical disk
  • Almost a pure exponential
  • M33 is a low luminosity spiral

Ferguson et al 2006

Corbelli & Salucci 2000

sfhs from resolved stellar populations
SFHs from resolved stellar populations
  • Stellar evolution tells us how mass, composition, and age of a star are related to luminosity, effective temperature, and composition
  • Stellar atmosperes tell us how effective temperature, composition, and surface gravity (from mass and luminosity) are related to spectrum/colors
  • Results embodied in stellar isochrones
recovering star formation histories
Recovering star formation histories
  • In principle, distribution of stars in a CMD allow recovery of SFH so long as degeneracies across entire diagram are not present and isochrones are perfect
  • In practice, assume constant IMF
  • In reality, isochrones aren’t perfect. Also, many stars are unresolved binaries.
  • In disks, differential reddening is present
  • Errors are challenging to estimate
  • Lots of time spent on these issues!
hst data on m33
HST data on M33

HST/ACS: 4 radial fields, 3 deep, F475W/F606W/F814W

HST/WFPC2: 4 radial fields, F300W, 4 deep parallel fields

HST/NICMOS: 4 radial fields, short

HST/ACS: 8 parallel fields

m33 photometry
M33 photometry
  • F475/F814W top; F606W/F814W bottom
  • Depth increases with radius (crowding)
  • Clear differential reddening in inner fields
  • Clear age range in all fields
m33 star formation history
M33 star formation history

Observed Best fit model Residuals (-3 to 3)

Example from outermost (DISK4) field

derived reddening distributions
Derived reddening distributions
  • Inner fields have more reddening
  • Inner fields have broader reddening distribution
  • In all fields, reddening is larger for younger stars
m33 star formation history1
M33 Star formation history
  • Clear radial age gradient
  • Only innermost field has declining SFR
  • Result is robust to isochrone changes, binning, reddening, etc.
m33 surface mass density evolution
M33 surface mass density evolution
  • Can use SFH to infer surface stellar mass density and its evolution
  • Radial age gradient implies evolution of disk scale length
  • Note possibility/likelikhood of radial migration
m33 stellar m l ratios
M33: stellar M/L ratios
  • SFH variations lead to stellar M/L variations of almost factor of two
  • Shallower fields give consistent results with deeper
m33 metallicities
M33 metallicities
  • Little inferred metallicity gradient
  • Only mild metallicity evolution?
integrated sfh
Integrated SFH
  • Assuming Ferguson et al (2006) profile and crude assigment of observed SFHs to radial bins, can calculate integrated SFH for M33
  • Integrated SFH is not exponentially declining, SFR has been roughly constant, or even increased in past several Gyr
  • Can do SFH in disks, even from shallower data
  • No dramatic implications from one galaxy! But for M33:
    • Not exponentially declining SFR
    • Radial age gradient
    • Narrow metallicity distribution and limited metallicity evolution --> gas inflow important?
    • Population gradient implies stellar M/L gradient that may need to be taken account of, e.g. in mass modelling of disks
  • M33 manages to have continued star formation to present despite the proximity of M31
    • Note comparable study of more isolated, but otherwise comparable, NGC300 (Gogarten et al., submitted) shows that galaxy has more of a declining SFR!
  • Larger sample, e.g. ANGST and more, might start to become more representative
other related projects
Other related projects
  • Star formation histories of Local Group Dwarfs: do different current morphologies have common progenitors?
  • ANGST survey: star formation histories from more distant galaxies/more luminous stars
  • HST/WFC3
    • calibration of photometric metallicity indicators (funded!)
    • Bulge Treasury program
    • New proposal(s): nearby dwarfs metallicity distribution functions, …
less related projects
Less related projects
  • HST/WFC3 program on: Star Formation in Nearby Galaxies (funded!)
  • Echelle spectroscopy of Hipparcos subgiants
    • Solar neighborhood age-metallicity relation
    • Solar neighborhood abundance ratio patterns
    • Solar neighborhood star formation history
even less related projects
Even less related projects
  • Velocity function in Virgo
  • SDSS-II SN survey
    • Publication of full set of survey photometry
    • Photometric identification of type Ia SN
    • Variable star studies in stripe 82?
    • Orphan optical bursts (MJ & Bernie)
  • Asteroseismology projects with the 1m / 3.5m
    • Ideas for higher accuracy photometry
    • Velocity precision with 3.5m echelle?
    • Feeding the echelle with the 1m