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

M15 Mv=-9 M=1e6

NGC6388Mv=-10M=1.5e6

Fornax Mv=-13 M=68e6 Sculptor Mv=-11 M=6e6

Carina Mv=-9 M=13e6 Sextans Mv=-9 M=19e6

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 ResultsLanfranchi & 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

Belokurov et al. (2007)

The Discovery of Ultra Faint dwarfs

The Ultra Faint Dwarfs

Faint GC

M 71

Mv=-5.6

Pal 13

Mv=-3.5

Old OC

NGC 7789

Mv=-4.7

NGC 188

Mv=-3.5GC and OC have sizes < 10 pc

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: Rperi = 11 kpc Dinescu et al. 2005Carina: Rperi = 20 kpc Piatek et al. 2003Fornax: Rperi = 138 kpc Dinescu et al. 2004Sculptor: Rperi = 120 kpc Dinescu et al. 2004Ursa Minor: Rperi = 40 kpc 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.