Resolved Stellar Populations outside the Local Group

Alessandra Aloisi (STScI/ESA)

Science with the New HST after SM4Bologna – 30 January 2008

Collaborators

F. Annibali, A. Grocholski, C. Leitherer, J. Mack, M. Sirianni, & R. van der Marel (STScI)

L. Angeretti, G. Clementini, R. Contreras, G. Fiorentino, M. Maio, D. Romano, & M. Tosi (INAF-OAB)

M. Marconi & I. Musella (INAF-OAC)E. Held & L. Greggio (INAF-OAP)

A. Saha (NOAO)

Hierarchical Galaxy Formation

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• dwarf galaxies first to form stars

• bigger galaxies form by merging of these building blocks

High-mass galaxies’ oldest pop must be as old as low-mass galaxies’ pop or younger

Mapping Galaxy Formation

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Mapping Galaxy Formation

1. High-Redshift Studies looking directly back in time by observing distant galaxies (e.g. HDFs, GOODS, HUDF)

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Mapping Galaxy Formation

1. High-Redshift Studies looking directly back in time by observing distant galaxies (e.g. HDFs, GOODS, HUDF)

Mapping Galaxy Formation

1. High-Redshift Studies looking directly back in time by observing distant galaxies (e.g. HDFs, GOODS, HUDF)

2. Stellar Archaeology studying nearby galaxies by resolving their present-day stellar populations

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Mapping Galaxy Formation

1. High-Redshift Studies looking directly back in time by observing distant galaxies (e.g. HDFs, GOODS, HUDF)

2. Stellar Archaeology studying nearby galaxies by resolving their present-day stellar populations

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Mapping Galaxy Formation

1. High-Redshift Studies looking directly back in time by observing distant galaxies (e.g. HDFs, GOODS, HUDF)

2. Stellar Archaeology studying nearby galaxies by resolving their present-day stellar populations

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Courtesy Elena Sabbi (STScI)

Resolving Galaxies with HST Imaging

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Resolving Galaxies with HST Imaging

• images in multiple bands BVI (optical) & JH (NIR)

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Resolving Galaxies with HST Imaging

Sextans ACTIO• images in multiple bands

BVI (optical) & JH (NIR)

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Hunter (1997)

Resolving Galaxies with HST Imaging

Sextans ACTIO

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Hunter (1997)

Sextans AHST/WFPC2

Dohm-Palmer et al. (2002)

• images in multiple bands BVI (optical) & JH (NIR)

Resolving Galaxies with HST Imaging

Sextans A

Dolphin et al. (2003)

Sextans A

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• images in multiple bands BVI (optical) & JH (NIR)

• CMD of resolved stars

Resolving Galaxies with HST Imaging

Dolphin et al. (2003)

Sextans A

RGBT

TP-AGB (C stars)

Cepheids• images in multiple bands BVI (optical) & JH (NIR)

• CMD of resolved stars

• distance RGBT, TP-AGB, Cepheids

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Resolving Galaxies with HST Imaging

Sextans A

RGBT

TP-AGB (C stars)

Cepheids• images in multiple bands BVI (optical) & JH (NIR)

• CMD of resolved stars

• distance RGBT, TP-AGB, Cepheids

• star formation history

Aparicio & Gallart (2004)

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TO

Resolving Galaxies with HST Imaging

Sextans A

RGBT

TP-AGB (C stars)

Cepheids• images in multiple bands BVI (optical) & JH (NIR)

• CMD of resolved stars

• distance RGBT, TP-AGB, Cepheids

• star formation history

Aparicio & Gallart (2004)

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TO

All galaxies studied in sufficient detail so far contain ancient populations

What does it really mean to go outside the Local Group?

Grebel (1999)

• distance > 1 Mpc

• different types of galaxies accessible:

Giant Ellipticals

Active Galaxies (starbursts & BCDs)

• only filters F606W & F814W really feasible !

dSphsdEsdSph/dIrrsdIrrs

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What is beyond the Local Group?

Courtesy Tom Brown (STScI)

closest giant EllipticalNGC 5128 (Centaurus group)

D = 3.8 Mpc

closest Starburst NGC 1569 (IC 342 group ? )

D = 3.2 Mpc

closest metal-poor BCDUGC 4483 (M81 group)

D = 3.4 Mpc

and more …

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The Closest Giant Elliptical: NGC 5128

NGC 5128 WEH

• some fields (r < 30 kpc) observed with WFPC2 down to the RGBT

• deepest field (r ~ 37 kpc) observed with ACS/WFC down to the RC

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The Closest Giant Elliptical: NGC 5128

NGC 5128 WEH

• some fields (r < 30 kpc) observed with WFPC2 down to the RGBT

• deepest field (r ~ 37 kpc) observed with ACS/WFC down to the RC

6Rejkuba et al. 2005

The Closest Giant Elliptical: NGC 5128

NGC 5128 WEH

• some fields (r < 30 kpc) observed with WFPC2 down to the RGBT

• deepest field (r ~ 37 kpc) observed with ACS/WFC down to the RC

6Rejkuba et al. 2005

Metal-rich all the way out !

Mean [M/H] = – 0.64

Mean Age = 8.5 Gyrs

Similarities with M31 Halo in the LG

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M31

Similarities with M31 Halo in the LG

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M31 haloACS/WFC

Brown et al. (2003)

Similarities with M31 Halo in the LG

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M31 haloACS/WFC

Brown et al. (2003)

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

The Closest Starburst: NGC 1569

• deep field observed with ACS/WFC down to the RC

• distance is 1 Mpc larger than previously

believed D = 3.2 Mpc

• RC/HB at the detection limit

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NGC 1569ACS/WFC

The Closest Starburst: NGC 1569

• deep field observed with ACS/WFC down to the RC

• distance is 1 Mpc larger than previously

believed D = 3.2 Mpc

• RC/HB at the detection limit

Grocholski, Aloisi et al. (in prep.)

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NGC 1569ACS/WFC

V – I

I

Grocholski, Aloisi et al. (in prep.)

The Closest Starburst: NGC 1569

• deep field observed with ACS/WFC down to the RC

• distance is 1 Mpc larger than previously

believed D = 3.2 Mpc

• RC/HB at the detection limit

Grocholski, Aloisi et al. (in prep.)

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NGC 1569ACS/WFC

V – I

I

Grocholski, Aloisi et al. (in prep.)Grocholski, Aloisi et al. (in prep.)

I

V – I

[Fe/H] = – 1.0 NGC 1569 Halo1Gyr 3Gyr 10Gyr

The Closest Starburst: NGC 1569

• deep field observed with ACS/WFC down to the RC

• distance is 1 Mpc larger than previously

believed D = 3.2 Mpc

• RC/HB at the detection limit

Morphology of RGB, presence of RC and lack (?) of HB suggest metal-rich and intermediate-age stars in the halo once again !

Grocholski, Aloisi et al. (in prep.)

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NGC 1569ACS/WFC

V – I

I

Grocholski, Aloisi et al. (in prep.)Grocholski, Aloisi et al. (in prep.)

I

V – I

[Fe/H] = – 1.0 NGC 1569 Halo1Gyr 3Gyr 10Gyr

The Closest Metal-Poor BCD: UGC 4483• deep field observed with WFPC2 down to the RGB

Izotov & Thuan (2002)

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UGC 4483ACS/WFC

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V – I

The Most Metal-Poor BCD at the borders of the Local Volume: I Zw 18

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RGB Stars in I Zw 18

11Aloisi et al. 2007

Variable Stars in I Zw 18

Lowest metallicity Cepheids

ever observed !

Z = 1/50 Zo

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125 days

8.6 days

130 days

139 or 186 days

Variable Stars in I Zw 18

Lowest metallicity Cepheids

ever observed !

Z = 1/50 Zo

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125 days

8.6 days

130 days

139 or 186 days

Aloisi et al. 2007

P = 8.6 days

Distance of I Zw 18

• Cepheids – theoretical reddening-free Wesenheit relation for the 3 confirmed Cepheids yields average distance D = 19 ± 2 Mpc

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Distance of I Zw 18

• Cepheids – theoretical reddening-free Wesenheit relation for the 3 confirmed Cepheids yields average distance D = 19 ± 2 Mpc

• TRGB – TRGB filtering technique gives D = 18 ± 2 Mpc

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Distance of I Zw 18

• Cepheids – theoretical reddening-free Wesenheit relation for the 3 confirmed Cepheids yields average distance D = 19 ± 2 Mpc

• TRGB – TRGB filtering technique gives D = 18 ± 2 Mpc

Distance larger than previously believed; contributed to difficulty in detecting RGB

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PL Relation vs. Metallicity

Fiorentino et al. 2007 (to be submitted)

Closer metal-poor BCDs need to be additionally investigated in order to better constrain PL relation at low metallicity

Several BCDs available within the Local Volume !14

HST UV Spectroscopy after SM4

Aloisi et al. 2003

COS & STIS will allow studies of the neutral ISM in star-forming systems (e.g., FUSE study of I Zw 18)

In particular, COS will be crucial in the FUV

to characterize the realO abundances from the 1300-

1350 Å region

Confirmation of the metallicity offset between neutral and ionized gas ?

Constraints to chemical evolution models

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