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Construction and Evolution of the Galaxy
Where do the dwarf galaxies fit in?
Matthew Shetrone
February 26, 2009
Why we care about dwarf galaxies
<- CDM
vs.
HDM ->
CDM wins and suggests
that smallest scales form
first and build larger
galaxies.
What are these smallest
scales that form first?
Lots of dwarf satellites!
Why the focus on dSph galaxies?
Biggest GC
Fornax Mv=-13 M=68e6
Sculptor Mv=-11 M=6e6
M15
Mv=-9
M=1e6
NGC6388
Mv=-10
M=1.5e6
Carina Mv=-9 M=13e6
Sextans Mv=-9 M=19e6
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•
•
•
•
Where are abundances done on these
dwarf galaxies
Keck
– HIRES (35k) published (V=18 for S/N 30 in 4 hours).
– DEIMOS (6k) published (V=19 For S/N 40 in 1 hour).
Subaru
– HDS (45k) published (V=17 for S/N 55 in 2.5 hours).
VLT
– UVES published (45k) limited to V=19 for S/N 30 in 5 hours.
– FLAMES-UVES (45k) limited to V=18 for S/N 30 in 5 hours.
– FLAMES-Giraffe published (20k) limited to V=19 for S/N 30 in 7 hours.
Magellan Landon Clay
– Mike (20k) published (V=17 for S/N 40 in 1.5 hours).
HET
– HRS (15k) limited to V=18 for S/N 30 in 3 hours.
Gemini
– Phoenix (50k) published (K=13 for S/N 50 in 2 hours).
The slower evolution and low alphas
From Tolstoy, Hill & Tosi ARAA 2009
Modeling the Results
Lanfranchi & Matteucci 2004
Standard (but sophisticated)
chemical evolution models
including metal-rich winds,
super novae and AGB yields,
and SFH from HST
photometry.
When tuned to match the
Milky Way these models can
match the dSph galaxies
including the alpha, iron peak
and rare earth elements.
They find slower chemical evolution and high wind efficiency.
Danger of a small sample
An Nbody/Tree
SPH simulation
of Sextans
dwarf galaxy.
(Revaz et al.
2009)
The slower evolution and low alphas
Tolstoy, Hill & Tosi ARAA 2009 Sag Fnx Scl Car MW
The Discovery of Ultra Faint dwarfs
Belokurov et al. (2007)
The Ultra Faint Dwarfs
GC and OC have sizes < 10 pc
Faint GC
M 71
Mv=-5.6
Pal 13
Mv=-3.5
Old OC
NGC 7789
Mv=-4.7
NGC 188
Mv=-3.5
Ultra Faint dwarfs
Simon & Geha 2007, ApJ, 670, 313
Preliminary results from UFD
From Tolstoy, Hill & Tosi ARAA 2009
Why these are so hard
From Simon & Geha 2007
Observing large samples at lower resolution
For many years people have been measuring the CaT lines
and determining a metallicity analog.
Shetrone et al. 2009 have been able to analyze the weaker
Fe, Ti and Mg lines in these spectra to get individual
abundances for the Leo II dSph: errors on individual stars
are large but the mean trends are reliable.
Evan Kirby has been developing a technique for determining
alpha and Fe for DEIMOS-Keck spectra in a large number
of dSph galaxies. This technique is growing in
sophistication and will soon measure individual
abundances.
Preliminary and new results
dSph and the MW Halo
We have already begun the comparison with the Milky Way by
comparing the chemical abundance patterns.
The result?
The halo could/might look like the most metal-poor dSph stars but
definitely does not look like the more metal-rich (relatively) dSph
stars.
Who cares? The halo clearly formed fairly quickly before such stars
would have formed and thus everything is consistent, right?
Metallicity Distribution Function
Helmi et al. 2006 find a difference between halo and dSph,
while Schoerck et al. 2008 do not.
Interpretation seems to depend upon where you normalize
the distribution function.
What is the goal?
A question of where to normalize
An Nbody/Tree
SPH simulation
of Sextans
dwarf galaxy.
(Revaz et al.
2009)
Metallicity Distribution Function
Interpretation seems to depend upon where you normalize
the distribution function.
What is the goal? To model the early fast evolution of
metallicity (ie. SN II from first and second generations).
Should avoid stars with Type Ia contributions for [Fe/H] plot
OR plot again [alpha/H].
dSph and the MW Halo
Very few dSph have full space motions, but those that do don’t come
within 10 kpc to the Galactic center or the solar neighborhood:
Canis Major:
Carina:
Fornax:
Sculptor:
Ursa Minor:
R
R
R
R
R
peri
peri
peri
peri
peri
= 11 kpc
= 20 kpc
= 138 kpc
= 120 kpc
= 40 kpc
Dinescu et al. 2005
Piatek et al. 2003
Dinescu et al. 2004
Dinescu et al. 2004
Piatek et al. 2005
Leo II has little to no tidal interaction with MW (Siegel et al. 2008)
Even LMC and SMC MAY be on first approach (Belsa et al. 2007)
Sagittarius is the exception.
The Local Inner vs. Outer Halo
The Local
Inner vs. Outer
Abundance
Results
There is a larger
dispersion for
outer halo
abundances.
Roederer 2009
Some Science Results
• Results discussed on the dSph
– Slower evolution for dwarf galaxies where winds important
– Sub-solar [alpha/Fe] in the most metal-rich stars.
– Origin of the most metal-poor galactic halo stars may be the least luminous
“galaxies”.
• Results not discussed on the dSph
– Constraints on the origins for Mn, Cu, Al and Na.
– Evidence for different origin and production rates of the light and heavy alpha
elements, e.g. Ca up while Mg down.
– Multiple populations within the dwarfs (younger/metal-rich centrally
concentrated).
• What Surveys may hold for dSph and the Milky Way
– Distribution of Ultra Faint Dwarfs over the sky
– In situ samples for outer halo
– A small sample of confirmed escapee metal-rich dSph star in the halo.