Insights into Star Formation Histories of Nearby Galaxies from Resolved Stellar Populations

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

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 dust • Errors are challenging to estimate: systematic vsrandom • 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 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 Monelli et al 2010 (LCID collaboration)

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 • 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? Weisz et al, 2011

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. M33 as a prototypical disk • Almost a pure exponential (but note break) • M33 is a low luminosity spiral: lower SB 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!