Deep HST Imaging of M33: the Star Formation History

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