Presentation on theme: "Deep HST Imaging of M33: the Star Formation History"— Presentation transcript:
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 keyStar 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 theoreticallyObservations of galaxies at high redshift provide an indication of when stars were formed, so long as integrated star formation rate indicators are validNearby 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 crowdedProblem: not clear that SF in dwarfs represents a large fraction of SF in galaxies!Star formation histories in disk galaxiesMilky Way actually challenging because of range of distances, extinctionClues from unresolved observations:Exponentially declining star formation rates?Stellar population gradientsProblems: dust
4 M33 as a prototypical disk Almost a pure exponentialM33 is a low luminosity spiralFerguson et al 2006Corbelli & 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 compositionStellar atmosperes tell us how effective temperature, composition, and surface gravity (from mass and luminosity) are related to spectrum/colorsResults 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 perfectIn practice, assume constant IMFIn reality, isochrones aren’t perfect. Also, many stars are unresolved binaries.In disks, differential reddening is presentErrors are challenging to estimateLots 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 fieldsHST/NICMOS: 4 radial fields, shortHST/ACS: 8 parallel fields
8 M33 photometry F475/F814W top; F606W/F814W bottom Depth increases with radius (crowding)Clear differential reddening in inner fieldsClear 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 reddeningInner fields have broader reddening distributionIn all fields, reddening is larger for younger stars
11 M33 Star formation history Clear radial age gradientOnly innermost field has declining SFRResult 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 evolutionRadial age gradient implies evolution of disk scale lengthNote possibility/likelikhood of radial migration
13 M33: stellar M/L ratiosSFH variations lead to stellar M/L variations of almost factor of twoShallower fields give consistent results with deeper
14 M33 metallicities Little inferred metallicity gradient Only mild metallicity evolution?
15 Integrated SFHAssuming Ferguson et al (2006) profile and crude assigment of observed SFHs to radial bins, can calculate integrated SFH for M33Integrated 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 SFRRadial age gradientNarrow 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 disksM33 manages to have continued star formation to present despite the proximity of M31Note 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 starsHST/WFC3calibration of photometric metallicity indicators (funded!)Bulge Treasury programNew proposal(s): nearby dwarfs metallicity distribution functions, …
18 Less related projectsHST/WFC3 program on: Star Formation in Nearby Galaxies (funded!)Echelle spectroscopy of Hipparcos subgiantsSolar neighborhood age-metallicity relationSolar neighborhood abundance ratio patternsSolar neighborhood star formation historySDSSIII- APOGEE
19 Even less related projects Velocity function in VirgoSDSS-II SN surveyPublication of full set of survey photometryPhotometric identification of type Ia SNVariable star studies in stripe 82?Orphan optical bursts (MJ & Bernie)Asteroseismology projects with the 1m / 3.5mIdeas for higher accuracy photometryVelocity precision with 3.5m echelle?Feeding the echelle with the 1mSDSSIII - MARVELS