Deep hst imaging of m33 the star formation history
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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.

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Deep hst imaging of m33 the star formation history

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
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
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

M33 as a prototypical disk
M33 as a prototypical disk

  • Almost a pure exponential

  • M33 is a low luminosity spiral

Ferguson et al 2006

Corbelli & Salucci 2000

Sfhs from resolved stellar populations
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
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 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
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 history
M33 star formation history

Observed Best fit model Residuals (-3 to 3)

Example from outermost (DISK4) field

Derived reddening distributions
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 history1
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
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
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
M33 metallicities

  • Little inferred metallicity gradient

  • Only mild metallicity evolution?

Integrated sfh
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


  • 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
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
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


Even less related projects
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