Aaron Dotter (STScI)Ata Sarajedini (Florida)Jay Anderson (STScI)

November 3, 2010

What can globular clusters tell us about the formation of the 

Galactic halo?

2Micron All Sky Survey

The Milky Way

Messier 92

What is a globular cluster?

4

 Why study globular clusters?

   They are bright (historically important)

   They are numerous

   Each one is a homogeneous population

       same chemical composition*

        same age                                                                                                       *(sort of)

5

Background:The Milky Way has about 150 globular clusters

The most distant (AM1) is at 120 kpc 

2/3 inside the solar circle, only 8 outside 30 kpc

Making a Color-MagnitudeDiagram for Globular Cluster

Omega Centauri

Jay Anderson, STScI

1

This is the Early-Release Image of Omega Centauri, taken by WFC3/UVIS on board the

Hubble Space Telescope (HST)

http://hubblesite.org/gallery/wallpaper/pr2009025q/

2

This image was made by combining

separate red, green, and blue images. 4

The red image is from filter F814W, which sees only very red light.

5

The green image is from filter F336W, which sees only blue light.

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The combined image again.

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The colors are so extreme because…

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… the red stars have almost no blue light, and the blue stars have almost no red light.

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Astronomers like to study the colors of stars in a quantitative way.

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They like to sort the stars by color, putting the blue stars on the left and the red stars on the right.

13a

They like to sort the stars by color, putting the blue stars on the left and the red stars on the right.

13b

They like to sort the stars by color, putting the blue stars on the left and the red stars on the right.

13d

They like to sort the stars by color, putting the blue stars on the left and the red stars on the right.

13e

They like to sort the stars by color, putting the blue stars on the left and the red stars on the right.

13f

They like to sort the stars by color, putting the blue stars on the left and the red stars on the right.

13h

They like to sort the stars by color, putting the blue stars on the left and the red stars on the right.

13i

They like to sort the stars by color, putting the blue stars on the left and the red stars on the right.

13j

Note that there are very few extreme stars; most stars are white, meaning

they have a balanced spectrum.14

Astronomers also like to characterize the stars in terms of brightness.

14

They like to sort the stars, putting the bright stars on top,

and the faint stars on the bottom.15a

They like to sort the stars, putting the bright stars on top,

and the faint stars on the bottom.15c

They like to sort the stars, putting the bright stars on top,

and the faint stars on the bottom.15d

They like to sort the stars, putting the bright stars on top,

and the faint stars on the bottom.15e

They like to sort the stars, putting the bright stars on top,

and the faint stars on the bottom.15g

They like to sort the stars, putting the bright stars on top,

and the faint stars on the bottom.15h

They like to sort the stars, putting the bright stars on top,

and the faint stars on the bottom.15i

They like to sort the stars, putting the bright stars on top,

and the faint stars on the bottom.15k

This is called a Color-Magnitude Diagram

(CMD).16

When Astronomers first plotted stars this way, they realized that stars don’t

fall just anywhere in the diagram.17

The unmistakable order in diagrams like this led astronomers to develop theories

to explain stellar evolution.19

The vast majority of stars lie along the Main Sequence (MS).

20a

MS

Stars don’t move along this sequence; rather they sit at the same place for a long

time fusing their hydrogen into helium.20b

MS

The more massive stars consume their hydrogen fuel much faster than the

lower-mass stars.20d

MS

When fuel becomes sparse in the stellar core, stars readjust their internal structure and move red-ward along the Sub-Giant Branch (SGB).

21

MS

SGB

They start to burn the hydrogen in a shell around the core and become big and bloated as they move up the Red Giant Branch (RGB).

22a

MS

SGB

RGB

When the core has enough mass, it is finally able to ignite helium into carbon.

22c

MS

SGB

RGB

The star readjusts its structure again and finds itself on the Horizontal Branch (HB).

23a

MS

SGB

RGBHB

The helium fuel is not as potent as the hydrogen, so it runs out quickly.

23b

MS

SGB

RGBHB

When the helium is gone, the star has no more fuel. With nothing left to burn, it fades away into

blue darkness as a White Dwarf (WD).24

MS

SGB

RGBHB

WD

Since a star’s color and brightness tell us

its evolutionary phase, we can easily

identify stars by phase in the image.

25

MS

SGB

RGBHB

WD

On to business...

● Globular clusters give us insight into the formation of the part of a galaxy where they're found

● Using models, we can measure both chemical composition and age for a globular cluster

● Hubble has observed ~50% of our GCs

47

M13d(VI)~1

47 Tucd(VI)~0.15

The Horizontal Branch (HB) can vary in appearance based on a few different parameters intrinsic to a stellar population, e.g. a globular cluster

48

Sea

rle 

& Z

inn

 (19

78)

47 Tuc

M13

Che

mic

al C

om

po

sitio

n

HB Morphology

49

Background

Outer halo first explored during 1960's:●Sandage & Wallerstein●van den Bergh●Sandage & Wildey

Searle & Zinn 1978:HBs become redder with increasing RGC

Led to suggestion that outer halo did not form in a short period of time

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The most metalpoor GCs are predicted—by standard horizontal branch models—to be young because the HBs move back to the red.

This conflicts with main sequence turnoff age estimates, which are the gold standard for stellar evolution.

Lee, Demarque, & Zinn (1994)

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The Outermost Halo: 50+ kpc

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AM1 (120 kpc) and Pal 14 (70 kpc) are 1.52 Gyr younger than the inner halo.(Dotter, Sarajedini, & Yang, 2008)

53

Harris et al. (1997) found NGC 2419 to be typical of old, metalpoor GCs.  Sandquist & Hess (2008) who found multimodal HB with a faint blue tail.

54

55Dotter et al. (2010)

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Age

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Galaxy Formation: Initial Collapse vs. Subsequent Accretion

1. How many of the Galactic GCs are from initial collapse?

2. How many from later accretions of dwarf galaxies?Dwarf galaxies exhibit different AMR and abundance ratios

Connection between inner halo (#1) and outer halo (#2)?

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How many blue?How many red?

59

The NotSoOuter Halo: 10 – 50 kpc

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How many blue?How many red?

61Muratov & Gnedin (2010)

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Can't do it with one type of accreted galaxy!                   (Mackey & Gilmore 2004)

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*Some GCs depart from the haloor bulge trends in [/Fe] ratios

Boxes are abundances from dSphs

Some GCs clearly follow dSph trend

(Priztl, Venn, & Irwin 2005)

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The “standard” HB models assume constant or Reimers mass loss.  Neither of these choices is appropriate for red giants.

If we assume the main sequence turnoff ages are correct, how should mass loss vary?                                                                         (Dotter 2008)

Assuming mass loss relation from previous slide.

Look at the old GCs.

Age = 13 Gyr

Dots – 1,000 MC             realizations

Large circles  data

Implications:●Stochastic effects●mass loss ~ metallicity

               (Dotter 2008)

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Movies shown in the talk are available from:

http://mesa.sourceforge.nethttp://hubblesite.org/newscenter/archive/releases/2010/28/video/b/http://hubblesite.org/newscenter/archive/releases/2010/28/video/d/

http://mesa.sourceforge.net/http://hubblesite.org/newscenter/archive/releases/2010/28/video/b/http://hubblesite.org/newscenter/archive/releases/2010/28/video/d/