Star Formation Histories of Nearby Galaxies from Resolved

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. Mc. Quinn, ANGST collaboration

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

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

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 vs random • Lots of time spent on these issues! • Dolphin, Tolstoy & Saha, Aparicio & Gallart, Valls. Gabaud & Hernandez, Tosi & Aloissi, Harris & Zaritsky, Holtzman

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

SFH in LG dwarfs Dolphin et al 2005

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

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

Implications: reionization Cetus d. Sph 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)

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) Mc. Quinn et al 2010 a, b

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

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!)

M 33 as a prototypical disk • Almost a pure exponential (but note break) • M 33 is a low luminosity spiral: lower SB Ferguson et al 2006 Corbelli & Salucci 2000

HST data on M 33 HST/ACS: 4 radial fields, 3 deep, F 475 W/F 606 W/F 814 W HST/WFPC 2: 4 radial fields, F 300 W, 4 deep parallel fields HST/NICMOS: 4 radial fields, short Holtzman et al, submitted/in prep HST/ACS: 8 parallel fields

M 33 photometry • • F 475/F 814 W top; F 606 W/F 814 W bottom Depth increases with radius (crowding) Clear differential reddening in inner fields Clear age range in all fields

M 33 star formation history Observed Best fit model Residuals (-3 to 3 ) Example from outermost (DISK 4) field

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

M 33 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.

M 33 surface mass density evolution Williams et al 2009 • 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

M 33: stellar M/L ratios • SFH variations lead to stellar M/L variations of almost factor of two • Shallower fields give consistent results with deeper

M 33 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?

Integrated SFH • Assuming Ferguson et al (2006) profile and crude assigment of observed SFHs to radial bins, can calculate integrated SFH for M 33 • Integrated SFH is not exponentially declining, SFR has been roughly constant, with peak 4 -5 Gyr ago

Other (more distant) disks • NGC 300 • NGC 2976 • NGC 404

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 • M 33: – – 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 • M 33 manages to have continued star formation to present despite the proximity of M 31 – Note comparable study of more isolated, but otherwise comparable, NGC 300 (Gogarten et al. , 2010) shows that galaxy has more of a declining SFR!