The Star-Formation and the Chemical Enrichment History Of

Star Formation Histories in Andromeda

Thomas M. Brown Space Telescope Science Institute Collaborators

H. Ferguson, E. Smith, J. Kalirai, P. Guhathakurta, K. Gilbert, R.M. Rich, D. Reitzel, A. Koch, A. Sweigart, R. Kimble, A. Renzini, M. Geha, D. VandenBerg, E. Kirby, R. Munoz, J. Simon, R. Avila

Courtesy of Jason Ware: http://galaxyphoto.com Stellar archaeology in nearby galaxies

• The most direct age diagnostic comes from resolving both the dwarf and giant stars, including the “main sequence turnoff” • In the 1950s, this technique was applied to star clusters in our own Galaxy • In the 1990s, such studies were expanded to the satellite galaxies of the Milky Way • With the launch of the Advanced Camera for Surveys (ACS) on Hubble, it became feasible to apply this technique in populations 1 Mpc away (Andromeda) 1

horizontal branch 2 RGB

3 subgiant branch (STMAG) 6 Gyr 4 10 Gyr F814W turnoff 14 Gyr m 5

main sequence 6 -0.6 -0.5 -0.4 mF606W-mF814W (STMAG) 1

2 [Fe/H]=-1.31 8 Gyr 3 10 Gyr 12 Gyr (STMAG) 4

F814W [Fe/H]=-0.71 m 5 8 Gyr 10 Gyr 12 Gyr 6 -0.6 -0.5 -0.4 mF606W-mF814W (STMAG) WFPC2

ACS First M31 deep ﬁeld

51 arcmin (11 kpc) from nucleus M31 halo (11 kpc)

210 x 210 arcsec

800 x 800 pc

25.0

25.2

25.4

25.6 magnitude

25.8 24 26.0 0.00 0.75 1.50 25 RR Lyrae phase 26

I 27

30 0.5 1.0 1.5 V-I Solar analog Brown et al. 2004, AJ, 127, 2738; Jeffery et al. 2011, AJ, 141, 171

RRab P=0.464d RRc P=0.275d 25.0 25.0

25.5 25.5

STMAG 26.0 26.0

26.5 26.5 0.5 1.0 0.5 1.0 Phase Phase 21 26.0 Eclipsing binary P=3.037d Mira 22 26.5 23 27.0 24 STMAG 27.5 25 28.0 26 0.5 1.0 0 10 20 30 40 Phase Days 0.263 0.267 0.274 0.275 0.280 0.283 0.287 0.289 0.301 0.301 0.306 25.0 mF606W 25.5 mF814W 26.0

0.313 0.313 0.327 0.329 0.330 0.331 0.338 0.339 0.339 0.351 0.353 25.0 mF606W 25.5 mF814W 26.0

0.361 0.366 0.366 0.382 0.442 0.464 0.496 0.506 0.529 0.532 0.533 25.0 mF606W 25.5 mF814W 26.0

0.534 0.552 0.554 0.554 0.559 0.572 0.573 0.580 0.583 0.589 0.611 25.0 mF606W 25.5 mF814W 26.0

0.621 0.627 0.627 0.631 0.634 0.678 0.687 0.714 0.726 0.735 0.774 25.0 mF606W 25.5 mF814W 26.0

0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0 Phase

26 (STMAG) F814W m 28

-0.8 -0.4 0.0 0.4 mF606W-mF814W 22 M31 halo

26 (STMAG) 28 F814W m 80% 30

50% 32 -1.0 -0.5 0.0 0.5 -1.0 -0.5 0.0 0.5 Brown et al. 2003, ApJ, 592, L17 mF606W-mF814W (STMAG) 22 M31 halo 47 Tuc [Fe/H]=-0.7 24

26 (STMAG) 28 F814W m 80% 30

50% 32 -1.0 -0.5 0.0 0.5 -1.0 -0.5 0.0 0.5 Brown et al. 2003, ApJ, 592, L17 mF606W-mF814W (STMAG) 22 M31 halo

(STMAG) 28 F814W m 30

32 Fitting region -1.0 -0.5 0.0 0.5 Starﬁsh code mF606W-mF814W (STMAG) (Harris & Zaritsky 2001) M31 halo age = 13 Gyr 27 -2.3 <_ [Fe/H] <_ 0.5

(STMAG) 29 F814W m 30

-0.7 -0.5 -0.3 -0.7 -0.5 -0.3 Brown et al. 2003, ApJ, 592, L17 mF606W-mF814W (STMAG) M31 halo 11 <_ age _< 14 Gyr 27 -2.3 <_ [Fe/H] <_ 0.5

(STMAG) 29 F814W m 30

-0.7 -0.5 -0.3 -0.7 -0.5 -0.3 Brown et al. 2003, ApJ, 592, L17 mF606W-mF814W (STMAG) M31 halo 2 <_ age _< 14 Gyr 27 -2.3 <_ [Fe/H] <_ 0.5

(STMAG) 29 F814W m 30

-0.7 -0.5 -0.3 -0.7 -0.5 -0.3 Brown et al. 2003, ApJ, 592, L17 mF606W-mF814W (STMAG) Halo Distribution in Age and Metallicity 0.5

0.0

-0.5

-1.0 [Fe/H]

-1.5

-2.0

2 4 6 8 10 12 14 Brown et al. 2006, ApJ, 652, 323 Age (Gyr) Mergers

• Best-ﬁt model has 40% of the stellar mass younger than 10 Gyr and metal-rich • Some of the stars belong to the textbook “old metal-poor population” • At the time, we suggested the intermediate-age stars in the halo originated in a major merger or series of smaller mergers • We now know what merger that was... 3

In 2004, we disk obtained 1 deep ﬁelds NGC205 in the tidal 0 M31 (degrees) stream and d halo outer disk -1

stream 30 kpc -2 in disk plane

-3 (star count map from 3 2 1 0 -1 -2 -3 Ferguson et al. 2002) j (degrees) M31 stream (20 kpc)

210 x 210 arcsec

800 x 800 pc -1 vhel (km s ) -600 -500 -400 -300 -200 -100 -600 -500 -400 -300 -200 -100 Distribution of 12 8

velocities in stream stars 4 and halo are distinct 0 22 stream halo (matched)

24 Distribution of ages and metallicities in 26 stream and halo are (STMAG) F814W m nearly identical 28

-1.0 -0.5 0.0 0.5 -1.0 -0.5 0.0 0.5

Brown et al. 2006, ApJ, 636, L89 mF606W-mF814W (STMAG) Stream Distribution in Age and Metallicity 0.5

0.0

-0.5

-1.0 [Fe/H]

-1.5

-2.0

2 4 6 8 10 12 14 Brown et al. 2006, ApJ, 652, 323 Age (Gyr) Halo Distribution in Age and Metallicity 0.5

0.0

-0.5

-1.0 [Fe/H]

-1.5

-2.0

2 4 6 8 10 12 14 Brown et al. 2006, ApJ, 652, 323 Age (Gyr) Star formation history of the stream

• Most likely explanation: the inner spheroid of Andromeda is polluted with debris from the stream’s progenitor (or objects like it) • This explanation is supported by simulations and kinematic data (Fardal et al. 2007; Gilbert et al. 2007) • The stream is the merger event that explains the population in our original halo ﬁeld 3 In 2006, we obtained 2 new images in the 1 extended halo. NGC205 21 kpc ﬁeld 0 M31

(degrees) halo and two d 11 kpc 35 kpc ﬁelds -1 21 kpc completed 35 kpc before ACS 30 kpc -2 in disk plane failure.

-3 (star count map from 3 2 1 0 -1 -2 -3 Ferguson et al. 2002) j (degrees) M31 halo (21 kpc)

210 x 210 arcsec

800 x 800 pc M31 halo (35 kpc)

210 x 210 arcsec

800 x 800 pc M31 halo (35 kpc)

210 x 210 arcsec

800 x 800 pc -1 vhel (km s ) -600 -500 -400 -300 -200 -100 -600 -500 -400 -300 -200 -100 Distribution of 12 velocities in each ﬁeld 8 is broad and centered stars 4 0 on M31 systemic 22 halo - 21 kpc halo - 11 kpc (matched)

21 kpc ﬁeld is older 26

and somewhat more (STMAG) F814W

metal-poor m 28

-1.0 -0.5 0.0 0.5 -1.0 -0.5 0.0 0.5

Brown et al. 2007, ApJ, 658, L95 mF606W-mF814W (STMAG) -1 vhel (km s ) -600 -500 -400 -300 -200 -100 -600 -500 -400 -300 -200 -100 Distribution of 12 velocities in each ﬁeld 8 is broad and centered stars 4 0 on M31 systemic 22 halo - 35 kpc halo - 11 kpc (matched)

35 kpc ﬁeld is older 26

and somewhat more (STMAG) F814W

metal-poor m 28

-1.0 -0.5 0.0 0.5 -1.0 -0.5 0.0 0.5

Brown et al. 2008, ApJ, 658, L121 mF606W-mF814W (STMAG) Age & Metallicity Distribution in 11 kpc Field 0.5

0.0

-0.5

-1.0 [Fe/H]

-1.5

-2.0

2 4 6 8 10 12 14 Brown et al. 2006, ApJ, 652, 323 Age (Gyr) Age & Metallicity Distribution in 21 kpc Field 0.5

0.0

-0.5

-1.0 [Fe/H]

-1.5

-2.0

2 4 6 8 10 12 14 Brown et al. 2007, ApJ, 658, L95 Age (Gyr) Age & Metallicity Distribution in 35 kpc Field 0.5

0.0

-0.5

-1.0 [Fe/H]

-1.5

-2.0

2 4 6 8 10 12 14 Brown et al. 2008, ApJ, 658, L121 Age (Gyr) Star Formation Histories at 21 kpc and 35 kpc

• These fields span the transition between inner “bulge-like” halo and outer “classical” halo • The halo fields at 21 and 35 kpc exhibit lower metallicities and older ages, but are not purely ancient - A population entirely >10 Gyr ruled out at >8 sigma • Intermediate-age stars are not surprising, given the myriad streams criss-crossing the halo in wide-field star count maps (McConnachie et al. 2009, Ibata et al. 2007) • Dwarf galaxies of the Local Group exhibit extended star formation histories (Orban et al. 2008, Weisz et al. 2011) 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

Tumlinson (2010) Belokurov et al. (2007) Luminosity vs Size -14 classical dSphs -12 Globular Clusters -10

-8 (mag) V

M -6

-4 ultra-faint dwarfs -2

Harris (1996) 1 10 100 1000 Mateo (1998) Martin et al. (2008) rh (pc) Luminosity vs Size -14 classical dSphs -12 Globular Clusters -10

-8 (mag) V

M -6

-4 ultra-faint dwarfs -2

Harris (1996) 1 10 100 1000 Mateo (1998) Martin et al. (2008) rh (pc) Ursa Major I N 27 orbits E

WFC3 ACS

202’’ dist = 97 kpc MV = -5.5 rh = 11.3’ rh = 318 pc Hercules Leo IV Ursa Major I 22

(STMAG) 814

m 26 26

-1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 Brown et m606 - m814 (STMAG) m606 - m814 (STMAG) m606 - m814 (STMAG) al. (2012) Hercules Leo IV Ursa Major I 22

(STMAG) 814

m 26 26 M92: 13.7 Gyr [Fe/H]=-2.3 26

-1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 Brown et m606 - m814 (STMAG) m606 - m814 (STMAG) m606 - m814 (STMAG) al. (2012) Hercules Leo IV Ursa Major I 22

(STMAG) 814

m 26 26 M92: 13.7+/-1 Gyr 26

-1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 Brown et m606 - m814 (STMAG) m606 - m814 (STMAG) m606 - m814 (STMAG) al. (2012) Hercules Leo IV Ursa Major I 22

(STMAG) 814

m 26 26 M92: 13.7+/-1 Gyr [Fe/H]=-3.2 26

-1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 Brown et m606 - m814 (STMAG) m606 - m814 (STMAG) m606 - m814 (STMAG) al. (2012) Hercules Leo IV Ursa Major I 22

(STMAG) 814

m 26 26 M92: 13.7+/-1 Gyr [Fe/H]=-3.2 26

-1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 Brown et m606 - m814 (STMAG) m606 - m814 (STMAG) m606 - m814 (STMAG) al. (2012) Hercules Assume spectroscopic 22 • metallicity distribution (Kirby et al. 2008; Simon & Geha 2007) 24 • Allow ages in ﬁne grid of isochrones to ﬂoat (STMAG)

814 Maximum-Likelihood ﬁt to m • 26 main sequence turnoff and M92: 13.7+/-1 Gyr [Fe/H]=-3.2 subgiant branch • Mean age = 13.6 Gyr 28 +/-0.2 Gyr statistical +/-0.6 Gyr systematic ([O/Fe]) -1.0 -0.8 -0.6 -0.4 -0.2 Brown et +/-0.6 Gyr systematic (distance) m606 - m814 (STMAG) al. (2012) Hercules age(Hercules) = age(M92) simulated CMD 22

24 (STMAG) 814 m 26 M92: 13.7+/-1 Gyr [Fe/H]=-3.2

-1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 Brown et m606 - m814 (STMAG) m606 - m814 (STMAG) al. (2012) Hercules age(Hercules) = age(M92) age(Hercules) = age(M92) - 2.0 Gyr simulated CMD simulated CMD 22

24 (STMAG) 814 m 26 M92: 13.7+/-1 Gyr [Fe/H]=-3.2

-1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 Brown et m606 - m814 (STMAG) m606 - m814 (STMAG) m606 - m814 (STMAG) al. (2012) Hercules age(Hercules) = age(M92) age(Hercules) = age(M92) + 2.0 Gyr simulated CMD simulated CMD 22

24 (STMAG) 814 m 26 M92: 13.7+/-1 Gyr [Fe/H]=-3.2

-1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 -1.0 -0.8 -0.6 -0.4 -0.2 Brown et m606 - m814 (STMAG) m606 - m814 (STMAG) m606 - m814 (STMAG) al. (2012) Summary • All sightlines through the Andromeda halo exhibit intermediate-age stars, indicating an active merger history • The Giant Stellar Stream looks remarkably similar to the inner halo, implying the inner halo of Andromeda is polluted by stars stripped from the stream’s progenitor • Most dwarf galaxies exhibit extended star formation histories, and the remnants of past mergers can be seen criss- crossing the Andromeda halo • New observations of the ultra-faint dwarf galaxies demonstrate they that they are purely ancient, suggesting the faintest galaxies were truncated by reionization • All of our Andromeda data (coadded & resampled images, artiﬁcial star tests, photometric catalogs) available at MAST