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

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

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

• Almost a pure exponential

• M33 is a low luminosity spiral

M33 as a prototypical disk

Ferguson et al 2006

Corbelli & Salucci 2000

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

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!

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

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

M33 star formation historyObserved Best fit model Residuals (-3 to 3)

Example from outermost (DISK4) field

Derived reddening distributions

• Inner fields have more reddening

• Inner fields have broader reddening distribution

• In all fields, reddening is larger for younger stars

M33 Star formation history

• Clear radial age gradient

• Only innermost field has declining SFR

• Result is robust to isochrone changes, binning, reddening, etc.

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

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

M33 metallicities

• Little inferred metallicity gradient

• Only mild metallicity evolution?

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

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

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, …

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

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