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Nearby Star-Forming Dwarf Galaxies through HST Eyes

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. In collaboration with: F. Annibali ( STScI ) L. Angeretti (INAF–OAB) G. Clementini (INAF–OAB)

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Nearby Star-Forming Dwarf Galaxies through HST Eyes

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  1. Nearby Star-Forming Dwarf Galaxies through HST Eyes Alessandra Aloisi (STScI) HotSci@STScI Talk 25 August 2010

  2. Nearby Star-Forming Dwarf Galaxies through HST Eyes • 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) • 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 derMarel (STScI) Alessandra Aloisi (STScI) HotSci@STScI Talk 25 August 2010

  3. 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

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

  5. Mapping Galaxy Formation High-Redshift Studies: looking directly back in time by observing distant galaxies (e.g. HDFs, GOODS, HUDF) 2

  6. 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 2

  7. 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 Aparicio & Gallart (2004)

  8. 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 ?

  9. 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 ?

  10. Low-Metallicity Dwarf Star-Forming Galaxies • 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) SBS 1415+437 I Zw 18 DDO 68 D = 13.6 Mpc Z = 1/20 Z HST/ACS D = 18.2 Mpc Z = 1/50 Z HST/ACS D < 9 Mpc Z = 1/50 Z SAO 6 m

  11. RGB Stars in Low-Metallicity Dwarf SFGs I Zw 18 SBS 1415+437 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)

  12. 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”

  13. 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)

  14. Is I Zw 18 Young or Old ? RGB Aloisi et al. (2007) + Press Release STScI-2007-35 “Hubble Finds ‘Dorian Gray’ Galaxy” Izotov & Thuan (2004) + Press Release STScI-2005-35 “Hubble Uncovers a Baby Galaxy in a Grown-Up Universe” Momany et al. (2005)

  15. Variable Stars in I Zw 18 125 days Lowest metallicity Cepheids ever observed ! Z = 1/50 Zo 130 days 8.6 days 139 or 186 days

  16. Variable Stars in I Zw 18 125 days P = 8.6 days Lowest metallicity Cepheids ever observed ! Z = 1/50 Zo 130 days 8.6 days 139 or 186 days Aloisi et al. (2007) Fiorentino et al. (2010) 12

  17. Distance of I Zw 18 • Cepheids – theoretical reddening-free Wesenheit relation for the 3 confirmed Cepheids yields average distanceD = 19 ± 2 Mpc • TRGB – TRGB filtering technique givesD = 18 ± 2 Mpc Distance larger than previously believed; contributed to difficulty in detecting RGB

  18. ULP Cepheids as New Stellar Distance Indicators up to the Hubble Flow Closer metal-poor BCDs are best 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)

  19. 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 ?

  20. On the SFH of SBS 1415+437 Mass 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 CMD dominated 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 ?

  21. 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 ?

  22. Closest Starbursts in Dwarf Galaxies • 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 NGC 4449 NGC 1569 D = 3.8 Mpc Z = 1/4 Z HST/ACS D = 3.4 Mpc Z = 1/5 Z HST/ACS

  23. Spatial Distribution of Stars in Starbursts NGC 4449 Annibali et al. (2008) Starbursts mainly located in the center of dwarf galaxies !

  24. Stars in the Halo of Dwarf Starburst Galaxies NGC 1569 [Fe/H] = – 1.0 1Gyr3Gyr10Gyr I Grocholski et al. (2008; 2010 in prep.) V – I Outer parts of dwarf starburst galaxies composed by intermediate-age/old stars with Z slightly lower than in HII regions!

  25. Outer Halos in Dwarf Starburst Galaxies NGC 1569 NGC 4449 NGC 1569 NGC 4449 Rys et al. (2010, in prep.) 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)

  26. 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 ?

  27. A New Distance to NGC 1569: Definitive Evidence of Interaction Karachentsev et al. (2003) 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 believed D = 3.2 Mpc Galaxy location is now just in front of the IC 342 group Foreground/background M81 Group IC 342 Group NGC 1569 UGCA 92 Grocholski et al. (2008) Grocholski et al. (2010, in prep.) Maffei 1 Group Galaxy has likely just crossed the IC 342 group ! Foreground/background

  28. Evidence of Interaction in NGC 4449 from the Literature Hunter et al. (2002) NGC 4449 Theis & Kohle (2001) models of the interaction of NGC 4449 with DDO 125 (about 0.5 Gyr ago) DDO 125 Hunter et al. (1998)

  29. Evidence of Interaction in NGC 4449 from the Literature Hunter et al. (2002) NGC 4449 Theis & Kohle (2001) models of the interaction of NGC 4449 with DDO 125 (about 4-6 108 Gyr ago) DDO 125 Hunter et al. (1998) Hunter et al. (1999)

  30. New Evidence of Interaction in NGC 4449

  31. New Evidences of Interaction in NGC 4449

  32. New Evidence of Interaction in NGC 4449 Courtesy of Martinez-Delgado (MPIA) Image taken with a 15cm refractor with a very large FOV NGC 4449 is accreting a Fornax dSph-like satellite

  33. 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

  34. 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

  35. Other Ongoing Work: Spectroscopy in Nearby Star-Forming Galaxies Courtesy of Bethan James (STScI)

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