Download The Space Density of Spiral Galaxies as f(magnitude, size and

The Stellar Populations,
Mass-to-Light Ratios
and
Dark Matter
in Spiral Galaxies
Roelof S. de Jong Steward Observatory
Eric Bell
Rob Kennicutt
Rob Swaters
Rob Olling
Don McCarthy
Cedric Lacey
Overview
• Introduction
• Ages and metallicities of stellar populations
26 February 2001
Rijks Universiteit Groningen
– description of method
– scaling laws with structural parameters
• Galaxy evolution modeling
• Mass-to-light ratios of stellar populations
– correlation with population colors
– constraints from rotation curves
– application to Tully-Fisher relation
• Future work
Galaxy Formation and Evolution
• Huge progress, both observational and theoretical:
26 February 2001
Rijks Universiteit Groningen
– observational: e.g. the star formation history of the
Universe and of local group galaxies
– theoretical: hierarchical galaxy formation models in
CDM-like universes
• Something is missing:
We do not not where, when and especially why stars
are forming in particular galaxies
Galaxy Evolution and Structural Parameters
26 February 2001
Rijks Universiteit Groningen
• What
drives parameters:
the Star Formation
History
and
the
Structural
luminosity,
scale
size,
Chemical
Evolution within
galaxies?
surface brightness,
mass,disk
velocity
distribution
– current star formation in disks semi-regular, related to
morphology
andstudies:
structural
parameters
Statistical
scaling
relations
– are spirals determined by initial conditions or are infall
and outflow important?
– how is galaxy evolution related to the luminous and dark
matter distribution and galaxy dynamics?
• What is the distribution of dark and luminous matter?
– can we explain the Tully-Fisher relation?
– does dark matter really follow NFW profile distributions?
– do we need alternative gravity (e.g. MOND)?
Stellar populations Color-Color diagrams
26 February 2001
Rijks Universiteit Groningen
Gyr
Bruzual & Charlot models
Gyr
Data & Samples
• Face-on disk galaxies with
– data in at least 3 passbands (of which one IR)
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– good colors over at least 2 disk scale lengths
• Samples:
– de Jong & van der Kruit 1994
◊ BVRIHK of 64 face-on field spirals
– Verheijen et al. 1998
◊ BVRK of 34 Ursa Major Cluster spirals
– Bell et al. 1999
◊ BVRIK of 23 Low Surface Brightness galaxies
Total sample of 121 galaxies
26 February 2001
R-K
Rijks Universiteit Groningen
Radial Color-Color Observations
BR
BR
Maximum Likelihood Fitting
• Make model grid of e-t/τ Star Formation History and
metallicity
Rijks Universiteit Groningen
– parameterize SFH by average age <A>
• Determine minimum Χ2 between models and data
– use all available passbands
– take calibration, flatfield and sky errors into account
• Repeat for all radii
26 February 2001
• Use Monte Carlo simulations to determine uncertainties
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Rijks Universiteit Groningen
Local Age & Local Metallicity versus
Local Surface Brightness
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Age vs Surface Brightness & Luminosity
26 February 2001
Rijks Universiteit Groningen
Metals vs Surface Brightness & Luminosity
26 February 2001
Rijks Universiteit Groningen
What determines SFH and Metals?
Surface Brightness or Luminosity?
Remember
luminosity
and surface
brightness
are correlated!
The Galaxy Space Density
26 February 2001
Rijks Universiteit Groningen
Surface Brightness & Magnitude
Space density of
spiral galaxies
corrected for
selection effects
(de Jong & Lacey 2000)
26 February 2001
Rijks Universiteit Groningen
Are Ages mainly determined by
Surface Brightness or Luminosity?
26 February 2001
Rijks Universiteit Groningen
Is metallicity mainly determined by
Surface Brightness or Luminosity?
Summary observations
26 February 2001
Rijks Universiteit Groningen
• Ages are mainly determined by surface brightness,
suggesting inside-out disk formation
• Metallicity is determined by surface brightness and
total luminosity
• The observed scatter is larger than observational errors
So what are the caveats?
– Changes in the IMF
– Other Stellar Population Synthesis models
– The effect of dust reddening
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Rijks Universiteit Groningen
IMF uncertainty
Salpeter IMF
Scalo IMF
Spectral synthesis model uncertainty
Bruzual & Charlot
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Rijks Universiteit Groningen
Kodama & Arimoto
The effect of Dust Extinction
• Extinction will mainly effect metallicity determinations
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Rijks Universiteit Groningen
i.e. reddening vector runs parallel to metallicity color gradients
• Reddening not the main cause of the observed trends because:
– we are using face-on galaxies
– of the limits set by overlapping and edge-on galaxy
The effect of Dust Extinction
• Extinction will mainly effect metallicity determinations
26 February 2001
Rijks Universiteit Groningen
i.e. reddening vector runs parallel to metallicity color gradients
• Reddening not the main cause of the observed trends because:
– we are using face-on galaxies
– of the limits set by overlapping and edge-on galaxy
– we see no dependence on galaxy inclination
– colors are mainly determined by least obscured stars
– patchy dust structure reduces reddening effect
– reddening is caused by absorption only, not by scattering
26 February 2001
Rijks Universiteit Groningen
Dust modeling with scattering
• Scattering preferably to
face on direction
• Reddening follows
absorption curve, not
extinction curve
• For low optical depth
reddening insignificant
Conclusion Age & Metallicity Caveats
• Only very unusual IMFs can mimic our results
26 February 2001
Rijks Universiteit Groningen
• Other Spectral Synthesis Models will only change the
absolute age and metallicity values
• Dust will at most effect metallicities a bit
The relative rankings of
Ages & Metallicities
are Robust
Simple Galaxy Evolution Models
• Simple closed box models:
– Start with exponential gas disk
26 February 2001
Rijks Universiteit Groningen
– Form stars according to Schmidt law: (surface density)n
– Instantaneous recycling of metals
– Maximum likelihood fitting on resulting integrated colors
• Additional bells and whistles:
– Mass dependent metal free gas infall
– Mass dependent enriched gas blowout
– Mass dependent epoch of formation
– Fluctuations due to small starbursts
26 February 2001
Rijks Universiteit Groningen
Galaxy evolution models
Mass dependent
formation epoch model with star burst
Closed
box model
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Rijks Universiteit Groningen
Modeling conclusions
• Simple closed box models with a star formation rate
dependent on local gas density explains the basic
observed trends between stellar ages & metallicities
and galaxy surface brightness parameters
• Enriched gas blowout or mass dependent formation
epoch models are needed to explain the metallicity
dependence on total luminosity of the galaxy
• Small burst of star formation explains the scatter on
the observed relations
• What about masses instead of luminosities?
Why stellar M/Ls?
26 February 2001
Rijks Universiteit Groningen
• Stellar M/Ls needed to do dynamics in situations
where we have more matter than just stars, e.g.
– (baryonic) Tully-Fisher and other scaling relations
– rotation curve decomposition
• Dynamics is needed to model star formation and
galaxy evolution
• How? Many approaches possible:
– Milky Way kinematics
– galaxy kinematics
◊ streaming motions induced by bars or spiral arms
◊ vertical velocity dispersion in stellar disks
– stellar population synthesis
26 February 2001
Rijks Universiteit Groningen
Galaxy evolution models
Closed
box model
Mass dependent
formation epoch model with star bursts
The optical color of a stellar
population is a good M/L indicator
Even in K mass-to-light ratio varies by factor of 2
Color-ML for hierarchical galaxy model
26 February 2001
Rijks Universiteit Groningen
B
I
Even a hierarchical
galaxy formation
model shows strong
correlation between
color and M/L
K
Cole et al. (2000) models
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Rijks Universiteit Groningen
Different population synthesis models
• The slope of the
color-M/L relation
is independent of
stellar population
synthesis models
used
26 February 2001
Rijks Universiteit Groningen
Different Initial Mass Functions
• The slope of the
color-M/L relation
is independent of
models and IMFs
used
• The normalization of
the relation depends
on the IMF used, i.e.
the amount of low
mass stars
26 February 2001
Rijks Universiteit Groningen
Rotation curve M/L constraint
Maximum disk constraints
• A Salpeter IMF is too
massive
• Distribution suggests
IMF similar in most
galaxies and close to
maximum disk for a
fraction of the galaxies
bad data point due to beam smearing
data Verheijen (1997)
26 February 2001
Rijks Universiteit Groningen
• The color-M/L relation
must be normalized
below all maximum
disk values
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Rijks Universiteit Groningen
Stellar Mass Tully-Fisher relation
• Raw
Tully Tully-Fisher
Stellar
etmasses
al. (1998)
derived
relation has
from different
different
extinction
slopes
corrections
passbands
and offsets
using
in
makes
different
the
thecolor-M/L
slopes
passbands
steeper,
relation
but
do not
agree
bring
to them
within
10%agreement
into
rms
• The Tully-Fisher relations
derived from different
passbands are identical to
within the errors
• The slope is very steep
Vrot ~ M*4.5
Baryonic Tully-Fisher relation
26 February 2001
Rijks Universiteit Groningen
• Add in the HI gas mass
to calculate the baryonic
Tully-Fisher relation
• The slope is less steep
than stars only and less
than
Vrot ~ Mbar3.5
• Slope problematic for
MOND, but consistent
wit hierarchical CDM
galaxy formation models
with some fine-tuning
26 February 2001
Rijks Universiteit Groningen
Future work: Stellar Velocity Dispersions
An isothermal disk yields:
26 February 2001
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Future work: Rotation Curves
Future work: stellar populations
26 February 2001
Rijks Universiteit Groningen
Ages and metallicities of
resolved stellar populations
in nearby disk galaxies
Ages of young star clusters
in merging galaxies
Conclusions
• Local star formation history in disks mainly set by local
surface density, resulting in inside-out disk formation
26 February 2001
Rijks Universiteit Groningen
• Metallicity regulated by both surface density and mass
• Realistic galaxy evolution models predict a strong
correlation between population color and M/L
• Maximum disk constraints support this observationally
and suggest that a Salpeter IMF is too massive
• The stellar mass Tully-Fisher relation is independent of
originating passband
• The baryonic Tully-Fisher relation has a maximal slope
of about 3.5 +/- 0.2