1. 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

2. 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

3. 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

4. M33 as a prototypical disk
Almost a pure exponential
M33 is a low luminosity spiral
Ferguson et al 2006
Corbelli & Salucci 2000

5. 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

6. 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!

7. 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

8. 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

9. M33 star formation history
Observed Best fit model Residuals (-3 to 3)
Example from outermost (DISK4) field

10. Derived reddening distributions
Inner fields have more reddening
Inner fields have broader reddening distribution
In all fields, reddening is larger for younger stars

11. M33 Star formation history
Clear radial age gradient
Only innermost field has declining SFR
Result is robust to isochrone changes, binning, reddening, etc.

12. 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

13. 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

14. M33 metallicities Little inferred metallicity gradient
Only mild metallicity evolution?

15. 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

16. Implications 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

17. 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, …

18. 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
SDSSIII- APOGEE

19. 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
SDSSIII - MARVELS