The Formation History of the Ultra-Faint Dwarf Galaxies

The Formation History of the

Ultra-Faint Dwarf Galaxies

Tom Brown

Space Telescope Science Institute

Jason Tumlinson (STScI), Marla Geha (Yale), Evan N. Kirby (UC Irvine), Don A. VandenBerg (U of Victoria), Ricardo R. Munoz (U de Chile),

Jason S. Kalirai (STScI), Josh D. Simon (Carnegie),

Rorberto J. Avila (STScI), Puragra Guhathakurta (UCO/Lick), Alvio Renzini (INAF), Henry C. Ferguson (STScI),

Luis Vargas (Yale), Mario Gennaro (STScI)

The Formation History of the

Ultra-Faint Dwarf Galaxies

Overview

•

Background: ultra-faint dwarf galaxies

•

Hubble program overview

•

Ages in ultra-faint dwarfs

•

IMF in ultra-faint dwarfs

•

^Summary

Tumlinson (2010)

Dark Matter Distribution

350 kpc

Tumlinson (2010)

Subhalos with star formation continuing past reionization

Tumlinson (2010)

Fossil subhalos - star formation truncated by reionization

Tumlinson (2010)

Most subhalos never form stars at all

SDSS Field of Streams

Belokurov et al. (2007)

Luminosity vs Size

1 10 100 1000

r_h (pc) -2

-4 -6 -8 -10 -12 -14

M V (mag)

Globular Clusters

classical dSphs

ultra-faint dwarfs

Harris (1996) Mateo (1998) Martin et al. (2008)

Luminosity vs Size

Harris (1996) Mateo (1998) Martin et al. (2008)

1 10 100 1000

r_h (pc) -2

-4 -6 -8 -10 -12 -14

M V (mag)

Globular Clusters

classical dSphs

ultra-faint dwarfs

Ground observations imply old populations in UFDs - but how old?

Leo IV (Sand et al. 2010)

MMT

Coma Berenices (Munoz et al. 2010)

CFHT

16

18

20

22

24

-0.5 0 0.5 1 1.5 2

I0

(V-I)₀ UMa IUrsa Major I (Okamoto et al. 2008)

Subaru Bootes I

(Okamoto 2010) Subaru

Hercules (Sand et al. 2009)

LBT

Canes Venatici II (Greco et al. 2008)

WHT

0.0 0.5 1.0 1.5 B-V

Hubble Program Overview

•

Relative ages to <1 Gyr (SNR~100 at main sequence turnoff)

-

Relative to each other (synchronized star formation histories?)

-

Relative to ancient populations (e.g., globular clusters)

•

IMF in old dynamically un-evolved populations

•

Advanced Camera for Surveys (ACS) prime - centered on galaxy

•

Wide Field Camera 3 (WFC3) parallel - in galaxy outskirts

•

113 orbits - F606W (broad V) and F814W (I)

•

Comparison to ACS programs in the Local Group with same bands

-

Dozens of globular clusters (empirical ancient templates)

-

Many pencil-beam surveys of Local Group galaxies

•

Victoria-Regina Isochrones updated to latest physics

•

Leverage Keck spectroscopy to fix metallicities while fitting ages

Ursa Major I

ACS

WFC3

202’’ dist = 97 kpc

MV = -5.5 rh = 11.3’

rh = 318 pc 27 orbits

N E

Leo IV

ACS

WFC3

202’’

dist = 154 kpc MV = -5.8

rh = 4.6’

rh = 206 pc 16 orbits

N E

Leo IV 16 orbits faint limit

V~28.5

Leo IV 16 orbits faint limit

V~28.5

MSTO star V=24.8

MSTO star V=24.9

Leo IV 16 orbits faint limit

V~28.5

M92

(NGC 6341)

[Fe/H]=-2.3 (m-M)o=14.62

E(B-V)=0.023

M92

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 -0.0 0.2

m₆₀₆ - m₈₁₄ (STMAG) 24

22 20 18 16 14 12

m 814 (STMAG)

M92

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 -0.0 0.2

m₆₀₆ - m₈₁₄ (STMAG) 24

22 20 18 16 14 12

m 814 (STMAG)

"Empirical" Isochrone (Ridge Line)

Brown et al. (2005)

M92

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 -0.0 0.2

m₆₀₆ - m₈₁₄ (STMAG) 24

22 20 18 16 14 12

m 814 (STMAG)

Victoria-Regina Theoretical Isochrone Age = 13.0 Gyr [Fe/H]=-2.3

VandenBerg et al. (2012)

Notes:

[O/Fe] higher than assumed previously T_eff-color relation empirically calibrated

Ages

-0.8 -0.6 -0.4 -0.2 0.0 m₆₀₆-m₈₁₄ (STMAG) 28

26 24 22 20 18

m 814 (STMAG)

Hercules

-0.8 -0.6 -0.4 -0.2 0.0 m₆₀₆-m₈₁₄ (STMAG) Leo IV

-0.8 -0.6 -0.4 -0.2 0.0 m₆₀₆-m₈₁₄ (STMAG) UMa I

28 26 24 22 20 18

m 814 (STMAG)

Boo I CVn II ComBer

Brown et al.

(2012, 2013 in prep)

-0.8 -0.6 -0.4 -0.2 0.0 m₆₀₆-m₈₁₄ (STMAG) 28

26 24 22 20 18

m 814 (STMAG)

Hercules M92

-0.8 -0.6 -0.4 -0.2 0.0 m₆₀₆-m₈₁₄ (STMAG)

Leo IV M92

-0.8 -0.6 -0.4 -0.2 0.0 m₆₀₆-m₈₁₄ (STMAG)

UMa I M92

28 26 24 22 20 18

m 814 (STMAG)

Boo I M92 CVn II M92 ComBer M92

Brown et al.

(2012, 2013 in prep)

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 m₆₀₆-m₈₁₄ (STMAG)

29 28 27 26 25 24

m 814 (STMAG)

Hercules

CMDs of the UFDs, all look very similar

Composite UFD CMD looks like a single-age

population (to first order)

Brown et al. (2012, 2013 in prep)

CMDs of the UFDs, all look very similar

Composite UFD CMD looks like a single-age

population (to first order)

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 m₆₀₆-m₈₁₄ (STMAG)

29 28 27 26 25 24

m 814 (STMAG)

Hercules Leo IV

Brown et al. (2012, 2013 in prep)

CMDs of the UFDs, all look very similar

Composite UFD CMD looks like a single-age

population (to first order)

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 m₆₀₆-m₈₁₄ (STMAG)

29 28 27 26 25 24

m 814 (STMAG)

Hercules Leo IV CVn II

Brown et al. (2012, 2013 in prep)

CMDs of the UFDs, all look very similar

Composite UFD CMD looks like a single-age

population (to first order)

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 m₆₀₆-m₈₁₄ (STMAG)

29 28 27 26 25 24

m 814 (STMAG)

Hercules Leo IV CVn II UMa I

Brown et al. (2012, 2013 in prep)

CMDs of the UFDs, all look very similar

Composite UFD CMD looks like a single-age

population (to first order)

Brown et al. (2012, 2013 in prep)

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 m₆₀₆-m₈₁₄ (STMAG)

29 28 27 26 25 24

m 814 (STMAG)

Hercules Leo IV CVn II UMa I Boo I

Brown et al. (2012, 2013 in prep)

CMDs of the UFDs, all look very similar

Composite UFD CMD looks like a single-age

population (to first order)

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 m₆₀₆-m₈₁₄ (STMAG)

29 28 27 26 25 24

m 814 (STMAG)

Hercules Leo IV CVn II UMa I Boo I Com Ber

-0.6 -0.5 -0.4 -0.3 m₆₀₆-m₈₁₄ (STMAG)

25.5 25.0 24.5 24.0

m 814 (STMAG)

Hercules

12.5 Gyr isochrone

-0.6 -0.5 -0.4 -0.3

m₆₀₆-m₈₁₄ (STMAG)

11.5 Gyr Model

12.5 Gyr isochrone

Brown et al.

(2012, 2013 in prep)

-0.6 -0.5 -0.4 -0.3 m₆₀₆-m₈₁₄ (STMAG)

25.5 25.0 24.5 24.0

m 814 (STMAG)

Hercules

12.5 Gyr isochrone

-0.6 -0.5 -0.4 -0.3

m₆₀₆-m₈₁₄ (STMAG)

12.5 Gyr Model

12.5 Gyr isochrone

Brown et al.

(2012, 2013 in prep)

-0.6 -0.5 -0.4 -0.3 m₆₀₆-m₈₁₄ (STMAG)

25.5 25.0 24.5 24.0

m 814 (STMAG)

Hercules

12.5 Gyr isochrone

-0.6 -0.5 -0.4 -0.3

m₆₀₆-m₈₁₄ (STMAG)

13.5 Gyr Model

12.5 Gyr isochrone

Brown et al.

(2012, 2013 in prep)

-0.6 -0.5 -0.4 -0.3 m₆₀₆-m₈₁₄ (STMAG)

25.5 25.0 24.5 24.0

m 814 (STMAG)

Hercules

12.5 Gyr isochrone

-0.6 -0.5 -0.4 -0.3

m₆₀₆-m₈₁₄ (STMAG)

12.5 Gyr Model

12.5 Gyr isochrone

Brown et al.

(2012, 2013 in prep)

Issues still being worked out

•

Distance & reddening to each UFD

-

Can affect age by ~0.5 Gyr

•

Field contamination in HST CMDs

-

Can mask trace sub-populations

•

Field contamination in Keck spectroscopy

-

Relatively metal-rich contaminants can drive a minority population that is ~1 to 2 Gyr younger in fit

•

[O/Fe] to assume in models at low [Fe/H]

-

Can affect age by ~0.5 Gyr

Initial Mass Function

Limited test in ideal population

•

Mass range is not large:

~0.5 - 0.8 MSun in 4: Hercules, Leo IV, UMa I, CVn II

~0.35 - 0.8 MSun in 2: Boo I & Com Ber

•

Old but dynamically un-evolved (unlike globular clusters)

•

IMF is a free parameter in many high-redshift studies

•

Test theoretical predictions IMF evolution

(e.g., Tumlinson 2010, Munoz et al. 2009, Bovill et al.

2009, Koposov et al. 2009, Li et al. 2010)

-

Larger characteristic mass at high z?

(due to higher CMB temperature)

24 25 26 27 28 m₈₁₄ (STMAG)

10 100

number of stars

0.764 0.747 0.698 0.617 0.521 mass (MO .)

Hercules

25 26 27 28

m₈₁₄ (STMAG) 10

100

0.766 0.755 0.716 0.643 0.548 mass (MO .)

Leo IV

Geha et al. (2013)

24 25 26 27 28 m₈₁₄ (STMAG)

10 100

number of stars

0.764 0.747 0.698 0.617 0.521 mass (MO .)

Hercules

Best-fit power law (_=1.16)

25 26 27 28

m₈₁₄ (STMAG) 10

100

0.766 0.755 0.716 0.643 0.548 mass (MO .)

Leo IV

Best-fit power law (_=1.31)

Geha et al. (2013)

24 25 26 27 28 m₈₁₄ (STMAG)

10 100

number of stars

0.764 0.747 0.698 0.617 0.521 mass (MO .)

Hercules

Best-fit power law (_=1.16) Salpeter (1955; _=2.35)

25 26 27 28

m₈₁₄ (STMAG) 10

100

0.766 0.755 0.716 0.643 0.548 mass (MO .)

Leo IV

Best-fit power law (_=1.31) Salpeter (1955; _=2.35)

Geha et al. (2013)

24 25 26 27 28 m₈₁₄ (STMAG)

10 100

number of stars

0.764 0.747 0.698 0.617 0.521 mass (MO .)

Hercules

Best-fit power law (_=1.16) Salpeter (1955; _=2.35)

Bottom-light power law (_=0.5)

25 26 27 28

m₈₁₄ (STMAG) 10

100

0.766 0.755 0.716 0.643 0.548 mass (MO .)

Leo IV

Best-fit power law (_=1.31) Salpeter (1955; _=2.35)

Bottom-light power law (_=0.5)

Geha et al. (2013)

Summary

•

Ultra-faint dwarf galaxies formed bulk (all?) of their stars 12 to 13 Gyr ago (z ~ 4 to 8)

•

No intermediate-age (< 10 Gyr) stars

-

Unlike most classical dwarf galaxies

•

Within current uncertainties, star formation is synchronized

-

Unclear if it predates reionization

•

IMF is flatter than Salpeter & other populations where directly measured (SMC, U Minor, MW)

Backup Slides

-4 -3 -2 -1 [Fe/H]

-1 0 1 2

[O/Fe]

Frebel (2009)

[O/Fe]

significantly

affects the age of the isochrone one would fit to a

color magnitude diagram

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2

m₆₀₆-m₈₁₄ (STMAG) 8

6 4 2 0

m 814 (STMAG)

[O/Fe]

significantly

affects the age of the isochrone one would fit to a

color magnitude diagram

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2

m₆₀₆-m₈₁₄ (STMAG) 8

6 4 2 0

m 814 (STMAG)

[Fe/H]=-2.35 [O/H]=-1.95 age=13.40 Gyr

[O/Fe]

significantly

affects the age of the isochrone one would fit to a

color magnitude diagram

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2

m₆₀₆-m₈₁₄ (STMAG) 8

6 4 2 0

m 814 (STMAG)

[Fe/H]=-2.35 [O/H]=-1.95 age=13.40 Gyr

[Fe/H]=-2.35 [O/H]=-1.71 age=12.75 Gyr