Nearby Star-Forming Dwarf Galaxies through HST Eyes

Alessandra Aloisi (STScI)

HotSci@STScI Talk 25 August 2010

Nearby Star-Forming Dwarf Galaxies through HST Eyes

Alessandra Aloisi (STScI)

HotSci@STScI Talk 25 August 2010

In collaboration with:

F. Annibali (STScI)L. Angeretti (INAF–OAB)G. Clementini (INAF–OAB)R. Contreras (INAF–OAB)G. Fiorentino (INAF–OAB)L. Greggio (INAF-OAP)A. Grocholski (STScI)E. Held (INAF-OAP)C. Leitherer (STScI)M. Marconi (INAF–OAN)I. Musella (INAF-OAN)J. Mack (STScI)D. Romano (INAF-OAB)A. Rys (IAC)A. Saha (NOAO)M. Sirianni (STScI/ESA)M. Tosi (INAF–OAB)R. van der Marel (STScI)

Hierarchical Galaxy Formation

• Dwarf galaxies are first systems to collapse and start forming stars

• Massive galaxies form by merging/accretion of these building blocks

• Starbursts usually associated with merging/accretion phenomena

Mapping Galaxy Formation

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

2

Mapping Galaxy Formation

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

2

Mapping Galaxy Formation

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

Stellar Archaeology:studying nearby galaxies by resolving their present-day stellar populations and reconstructing their star-formation history

Mapping Galaxy Formation

Aparicio & Gallart (2004)

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

Stellar Archaeology:studying nearby galaxies by resolving their present-day stellar populations and reconstructing their star-formation history

Key Questions to be addressed in the Nearby Universe

• Do primordial (< 1 Gyr old) galaxies exist in the local universe?

• Are the oldest stars in dwarf galaxies as old as the oldest stars in massive galaxies?

• Do present-day dwarf galaxies resemble the high-redshift building blocks of massive galaxies or are there systematic differences in the properties of “progenitors” and “survivors”?

• What is the observational evidence for the hierarchical build-up of dwarf galaxies ?

• Are starbursts in the local Universe always associated with merging/accretion phenomena ?

Key Questions to be addressed in the Nearby Universe

• Do primordial (< 1 Gyr old) galaxies exist in the local universe?

• Are the oldest stars in dwarf galaxies as old as the oldest stars in massive galaxies?

• Do present-day dwarf galaxies resemble the high-redshift building blocks of massive galaxies or are there systematic differences in the properties of “progenitors” and “survivors”?

• What is the observational evidence for the hierarchical build-up of dwarf galaxies ?

• Are starbursts in the local Universe always associated with merging/accretion phenomena ?

• Z < 1/20 Z (as inferred from Oxygen in HII regions)• chemically similar to primordial galaxies in the early

Universe• PI of 3 HST/ACS programs in Cycle 11, 14, and 17• Targets chosen to be the most metal-poor BCDs in low-

density environments (e.g., voids)

Low-Metallicity Dwarf Star-Forming Galaxies

D = 13.6 MpcZ = 1/20 ZHST/ACS

SBS 1415+437 I Zw 18 DDO 68

D = 18.2 MpcZ = 1/50 ZHST/ACS

D < 9 MpcZ = 1/50 ZSAO 6 m

RGB Stars in Low-Metallicity Dwarf SFGs

Aloisi et al. (2007) Aloisi et al. (2005)

RGB feature: oldest stars at least 1-2 Gyr old (z ~ 1)

Bulk (~ 80%) of the galaxy mass in this old stellar population (as inferred by LF)

SBS 1415+437 I Zw 18

Is I Zw 18 Young or Old ?

Izotov & Thuan (2004)+

Press Release STScI-2005-35

“Hubble Uncovers a Baby Galaxy in a

Grown-Up Universe”

Is I Zw 18 Young or Old ?

Izotov & Thuan (2004)+

Press Release STScI-2005-35

“Hubble Uncovers a Baby Galaxy in a

Grown-Up Universe”

Momany et al. (2005)

Is I Zw 18 Young or Old ?

Izotov & Thuan (2004)+

Press Release STScI-2005-35

“Hubble Uncovers a Baby Galaxy in a

Grown-Up Universe”

Momany et al. (2005)

Aloisi et al. (2007)+

Press Release STScI-2007-35“Hubble Finds ‘Dorian Gray’

Galaxy”

RGB

Variable Stars in I Zw 18

Lowest metallicity Cepheids

ever observed !

Z = 1/50 Zo

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

12

125 days

8.6 days

130 days

139 or 186 days

Aloisi et al. (2007)Fiorentino et al. (2010)

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

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

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

ULP Cepheids as New Stellar Distance Indicators up to the Hubble Flow

Closer metal-poor BCDs arebest places were to look for ULP Cepheids due to low metallicity and recent SFH

Several BCDs available within the Local Volume !

A couple of ground-based proposals (Gemini, TNG) already approved in 2010 to better characterize ULP Cepheids in NGC 1705 & IZw18 (PIs: Saha & Marconi)

IZw18

Bird et al. (2009)

Key Questions to be addressed in the Nearby Universe

• Do primordial (< 1 Gyr old) galaxies exist in the local universe?

• Are the oldest stars in dwarf galaxies as old as the oldest stars in massive galaxies?

• Do present-day dwarf galaxies resemble the high-redshift building blocks of massive galaxies or are there systematic differences in the properties of “progenitors” and “survivors”?

• What is the observational evidence for the hierarchical build-up of dwarf galaxies ?

• Are starbursts in the local Universe always associated with merging/accretion phenomena ?

On the SFH of SBS 1415+437Mass dominated by very old(> 6 Gyr) more metal-poor (Z < 0.0004) stars

SFR dominated by young(< 10 Myr) more metal-rich(Z > 0.001) stars

Number of stars in the CMDdominated by stars with intermediate age (25 Myr – 0.4 Gyr) and intermediate metallicity (0.0004 < Z < 0.001)

SFH not inconsistent with SF started as early as 12.5 Gyr ago (z~7), just around the epoch of re-ionization ?

Key Questions to be addressed in the Nearby Universe

• Do primordial (< 1 Gyr old) galaxies exist in the local universe?

• Are the oldest stars in dwarf galaxies as old as the oldest stars in massive galaxies?

• Do present-day dwarf galaxies resemble the high-redshift building blocks of massive galaxies or are there systematic differences in the properties of “progenitors” and “survivors”?

• What is the observational evidence for the hierarchical build-up of dwarf galaxies ?

• Are starbursts in the local Universe always associated with merging/accretion phenomena ?

• starburst properties similar to LBGs at z ~ 3• place where to characterize processes related to galaxy formation/evolution, i.e., merging and accretion• PI of 2 HST/ACS programs in Cycle 14 and 15• Targets chosen to be isolated starburst galaxies

Closest Starbursts in Dwarf Galaxies

D = 3.8 MpcZ = 1/4 ZHST/ACS

NGC 4449 NGC 1569

D = 3.4 MpcZ = 1/5 ZHST/ACS

Spatial Distribution of Stars in Starbursts

Annibali et al. (2008)

Starbursts mainly located in the center of dwarf galaxies !

NGC 4449

Stars in the Halo of Dwarf Starburst Galaxies

Grocholski et al. (2008; 2010 in prep.)

[Fe/H] = – 1.0 1Gyr 3Gyr 10Gyr

NGC 1569

Outer parts of dwarf starburst galaxies composed by intermediate-age/old stars with Z slightly lower than in HII regions!

V – I

I

Outer Halos in Dwarf Starburst Galaxies

No evidence for stellar population age/metallicity gradients within the halos

Outer stellar envelopes in dwarf starburst galaxies are distinct halos (not extensions of inner disks)

Rys et al. (2010, in prep.)

NGC 1569 NGC 4449 NGC 1569 NGC 4449

Key Questions to be addressed in the Nearby Universe

• Do primordial (< 1 Gyr old) galaxies exist in the local universe?

• Are the oldest stars in dwarf galaxies as old as the oldest stars in massive galaxies?

• Do present-day dwarf galaxies resemble the high-redshift building blocks of massive galaxies or are there systematic differences in the properties of “progenitors” and “survivors”?

• What is the observational evidence for the hierarchical build-up of dwarf galaxies ?

• Are starbursts in the local Universe always associated with merging/accretion phenomena ?

A New Distance to NGC 1569: Definitive Evidence of Interaction

Distance quoted in the literature before our HST/ACS observations:D = 2.2 Mpc

New distance inferred from RGBT in our HST/ACS data is ~ 1 Mpc larger than previously believedD = 3.2 Mpc

Galaxy location is now just in front of the IC 342 group

Grocholski et al. (2008)Grocholski et al. (2010, in prep.)

Karachentsev et al. (2003)

NGC 1569UGCA 92

Foreground/background

Foreground/background

M81 Group

IC 342 Group

Maffei 1 Group

Galaxy has likely just crossed the IC 342 group !

Evidence of Interaction in NGC 4449 from the Literature

Hunter et al. (1998)

DDO 125

NGC 4449

Hunter et al. (2002)

Theis & Kohle (2001) models of the interaction

of NGC 4449 with DDO 125(about 0.5 Gyr ago)

Evidence of Interaction in NGC 4449 from the Literature

Hunter et al. (1998)

DDO 125

NGC 4449

Hunter et al. (2002)

Theis & Kohle (2001) models of the interaction

of NGC 4449 with DDO 125(about 4-6 108 Gyr ago)

Hunter et al. (1999)

New Evidence of Interaction in NGC 4449

New Evidences of Interaction in NGC 4449

New Evidence of Interaction in NGC 4449Courtesy of Martinez-Delgado (MPIA)

Image taken with a 15cm refractor with a very large FOV

NGC 4449 is accreting a Fornax dSph-like satellite

Concluding Remarks• Deep HST/ACS CMDs provide detailed new insights into the SFH and

properties of nearby dwarf star-forming galaxies

• Underlying old (> 1-2 Gyr) populations that dominate the mass are present in even the most metal-poor systems

• Low-metallicity systems may have SF for look-back times comparable to a Hubble time, so in this sense they would be similar to the building blocks at high z

• Low metallicities in very metal-poor systems likely due to very low SF activity prior to the current burst and low-density environment.

• NGC 4449 is first observational evidence of accretion in a dwarf galaxy. Intense SF in NGC 1569 is now explained with the galaxy being part of a group.

• Cepheids with the lowest metallicity and the longest periods ever detected in I Zw 18

Other Ongoing Work: Spectroscopy in Nearby Star-Forming Galaxies

Spectroscopy important to understand metal content of the different galaxy components (i.e., stars and gas) and to provide additional information on the chemical evolution of nearby star-forming systems

• STIS spectra of A-F stars in the starburst dwarf galaxy NGC 1569 to infer abundances in stars younger than ~ 50 Myr

• FUSE (900-1200 Å) spectra of a sample of starburst galaxies to infer metal content in the neutral gas via absorption lines studies

• COS (1150-1700 Å) spectra of a subset of the FUSE sample to disentangle observational biases in the FUSE spectral region (saturation, depletion, contamination by ionized gas) and to study multiple sightlines within the same galaxy

Other Ongoing Work: Spectroscopy in Nearby Star-Forming Galaxies

Courtesy of Bethan James (STScI)