1. Star Formation Histories of Nearby Galaxies from Resolved Stellar Populations: What have we learned? Collaborators: J. Dalcanton, D. Weisz, B. Williams, A. Sarajedini, D. Garnett, E. Skillman, K. McQuinn, ANGST collaboration Jon Holtzman (NMSU)

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 and also can sample a different portion of the galaxy population

3. 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 atmospheres tell us how effective temperature, composition, and surface gravity (from mass and luminosity) are related to spectrum/colors Results embodied in stellar isochrones

4. Recovering star formation histories In principle, distribution of stars in a CMD allow recovery of SFH, e.g. via maximum likelihood Issues –more degeneracies at older ages: time resolution is worse at older ages –IMF hard to constrain: assume constant IMF –Some stars are unresolved binaries –Isochrones aren’t perfect –May be differential reddening in systems with dus t –Errors are challenging to estimate: systematic vs random Lots of time spent on these issues! Dolphin, Tolstoy & Saha, Aparicio & Gallart, Valls- Gabaud & Hernandez, Tosi & Aloissi, Harris & Zaritsky, Holtzman

5. Star formation histories from resolved stellar populations Most work done in Local Group dwarf galaxies: closer and less crowded Can reach oldest main sequence turnoffs Holtzman et al 2006

6. SFH in LG dwarfs Dolphin et al 2005

7. Are LG dwarfs typical? ANGST: nearby galaxy survey (<4 Mpc) Shallower data, but consistent with LG SFHs Weisz et al 2011

8. Implications: dwarf cosmic SFH Does local SFH match that derived from higher redshift observations? Not dramatically different, although SF may be a bit delayed compared to cosmic SFH: evidence for downsizing in dwarfs? Weisz et al 2011

9. Implications: reionization Monelli et al 2010 (LCID collaboration) Cetus dSph Is SF in dwarfs quenched by reionization? Reionization complete by z~6, i.e. > 12.5 Gyr ago Deep data suggests oldest populations extend to less than this

10. Implications: burstiness LG SFHs NOT especially “bursty”/episodic (on Gyr timescales) Dwarf starburst galaxies –Immediate feedback does NOT appear to quench SF (although it certainly could regulate it) McQuinn et al 2010a,b

11. Implications: Origin of dwarf morphologies Weisz et al, 2011 Nature of different types of dwarfs: irregulars, transition, spheroidals Are irregulars transformed into spheroidals? Early SFH looks comparable Dynamical evolution possible (Mayer et al) Chemical issues?

12. Beyond dwarfs SF in dwarfs doesn’t represent a large fraction of SF in galaxies! Star formation histories in disk galaxies –Clues from unresolved observations: Exponentially declining star formation rates? Stellar population gradients: bluer at larger radii Dust, metallicity, and age all contribute –Resolved populations Milky Way challenging because of range of distances, extinction Andromeda challenging: internal extinction, higher surface brightness/crowding (but note PHAT multicycle treasury program!)

13. Almost a pure exponential (but note break) M33 is a low luminosity spiral: lower SB M33 as a prototypical disk Ferguson et al 2006 Corbelli & Salucci 2000

14. 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 Holtzman et al, submitted/in prep

15. 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

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

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

18. M33 Star formation history Clear radial age gradient: –“inside-out?”… –disk growth vs. variation of SF efficiency with radius? Result is robust to isochrone changes, binning, reddening, etc.

19. 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/likelihood of radial migration Williams et al 2009

20. 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

21. M33 metallicities Metallicity gradient evident, but only when separating by age Oldest stars in outermost field inferred very metal poor (c.f. RR Lyraes) … a halo?

22. 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, with peak 4-5 Gyr ago

23. Other (more distant) disks NGC 300 NGC 2976 NGC 404

24. Summary SFHs of dwarfs: –Comparable to global SFH –LG dwarfs appear typical –Not strongly bursty –Morphology driven by environment? –Not globally strongly regulated by background radiation or supernova feedback Disks –Can do SFH in disks, even from shallower data M33: –Not exponentially declining SFR –Radial age gradient –Mild 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., 2010) shows that galaxy has more of a declining SFR!