Download Deep HST Imaging of M33: Reliability and Recovery of the Star

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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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
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
• 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
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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
Observed
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
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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