Globular Clusters and Galaxy Building Blocks. Young-Wook Lee Yonsei University, Seoul, Korea. Where are the relics of building blocks that formed stellar component of the Galaxy? Globular clusters as galaxy building blocks?
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Yonsei University, Seoul, Korea
Globular clusters as galaxy building blocks?
Peebles & Dicke 1968: “Originated as gas clouds before the galaxies formed”
Freeman 1993: Remaining nuclei of nucleated dwarf ellipticals?
Not all, but some might be…
Do we have evidence?
Early hint from spectroscopy (Norris+96)Not just a spread , but discrete RGBs in optical CMD! Multiple pops having different metal (heavier elements) abundances Direct evidence for SNe enrichment Remaining nucleus of a disrupted dwarf galaxy!
Super-He-rich Subpopulations in w CentauriEvidence from MS & New Y2 Isochrones
Model: Lee, Joo+2005
Observation: Bedin, Piotto+2004
Norris 04; Piotto+05;
Joo & Lee 10, in prep.
Super-He-rich Subpopulations in w CentauriEvidence from Extended HB (EHB)
Observation: Ferraro et al. 2004
Joo & Lee 2010
(See also Lee+05)
Super-He-rich Subpopulation in M54+SgrEvidence from SGB & EHB (Joo & Lee 10, in prep.)
For EHB & SGB split
Narrow-band Ca &Stromgren b, y filters
hk = (Ca-b) – (b-y)
“a measure of Ca abundance” (Anthony-Twarog+91)
Obs data : Lee, J.-W. +09
Da Costa+09 : [Fe/H] = -1.89 & -1.63
Marino+09 : [Fe/H] = -1.82 & -1.68
Both Ca (Fe) & He are enriched in 2nd population!
Small DZ + DY + Dt
Only weakly extended HB
(Han+09; CTIO 4m)
U is more sensitive to metal lines!
Confirmed by Ca-by photometry
(Red: Ca-rich, Blue:Ca-poor: J.-W. Lee+09)
Enhancements of (1) “lighter elements”
(N, Al, Na; red dotted-line), (2) heavy elements
(Ca, Fe…), & (3) He are required.
Star formation & chemical enrichment history in GCs with multiple RGBs
Most, if not all, EHB GCs show multiple populations (RGBs)…
Therefore, we use EHB as a proxy for multiple populations (RGBs)…
GCs with extended HB (EHB GCs) = GCs with multiple populations (Lee+07)
GCs with multiple pops (EHB GCs) are distinct from “normal” GCs!
Evidence 1:Presence of SNe enrichment! system was much more massive, was able to withstand SNe winds! M > 107 - 108Msun (i.e., dwarf galaxy)
Lee+2007, ApJ, 661, L49
Database: Harris 2003
EHB GCs are kinematically decoupled from normal GCs!
Lee+2007, ApJ, 661, L49
Orbital Kinematics based on Radial Velocity
Database: Harris 2003
(Lee+2007, ApJ, 661, L49)
EHB GCs: Memory of chaotic
Normal GCs: Evidence for
Occurrence of this by random selection < 1/105 (0.001%) !
EHB GCs are more enhanced in Helium (on average)!
Helium abundance from “R-method”:Data from Salaris+2004
EHB : 0.272±0.008
OH+D/B : 0.240±0.006
YH : 0.235±0.009
Difference is more than 4s!
EHB GCs are metal-poor!
MDF is peaked at [Fe/H] = -1.6
R ≤ 8 Kpc
1. SNe enrichment (Multiple RGBs)
3. Orbital kinematics
4. Helium abundance
5. Metallicity distribution function
& Absence of DM is not a serious problem (Saitoh+06)
NGC 1399 (CTIO 4m, Kim+09)
Subaru ~10hrs Exp. (S. Kim+10, in prep.)
Bright GCs (V < 22.5) in Virgo M87
GMT will provide much better data!
(1) Passive Evolution or Residual Star Formation?
(2) E galaxies prevailed by He enhanced population?
Model with He-enhanced pop (zform > 5)
Chung, Lee, & Yoon, in prep
Schiavon et al. 2006
A possible scenario?
1. Formation of metal-poor (bluer RGB) stars Normal He, metal-poor, no light-elements enhanced (or depleted)2. Pollution by fast rotating massive stars Enhance He, and enhance/deplete “lighter elements” Formation of Na-rich O-poor stars (+Mixing )? 3. Most massive (M > 8M⊙) metal-poor stars explode as SNe II Metal enrichment + He enrichment (system was much more massive, was able to withstand SNe winds!) Quenching of SF for a while?4. Pollution by intermediate-mass (3-7M⊙ ) AGB stars Add more He, and simultaneously enhance/deplete “lighter elements” 5. Formation of metal-rich (redder RGB) stars from the gas now enriched in overall metallicity, He, and “lighter elements”
Present-day Galactic GCs are ensemble of heterogeneous objects originated from three distinct phases of the Milky Way formation!
(1) EHB GCs: remaining cores or relics of primordial Galaxy building blocks expected in the LCDM hierarchical merging paradigm
(2) Normal GCs in the Inner Halo: genuine GCs formed in the dissipational collapse of a transient gas-rich inner halo system that eventually formed the Galactic disk (ELS 1962)
(3) Normal GCs in the Outer Halo: genuine GCs formed in the outskirts of outlying building blocks that later accreted to the outer halo of the Milky Way (Searle & Zinn 1978)
Two pops defined from Na-O anticorrelation are not identical to two pops defined from Ca-by photometry (Han & Lee, in prep.)
Two Pops defined from photometry:
No clear separation in Na-O plane
(Data from Marino+09,J.-W. Lee+09)
Two Pops defined from Na-O plane:
No clear separation in hk CMD
For the spectroscopic confirmation of heavy elements difference claimed from Ca-by photometry, stars in two populations defined from photometry should be observed in spectroscopy! (cf. Carretta+10)
This critical test has not been done with enough stars (cf. J.-W. Lee+09), but Teff & g should be very well determined in spectroscopy since expected D[Fe/H] is comparable to measurement error (0.15 dex)!
Photometry of bright RGB stars in globular clusters & halo fields in nearby galaxies
If diffraction limited, reliable photometry might be possible to 1-3 mags below RGB tip at Fornax/Virgo distances (Tolstoy 2006; GMT Science Case Nov. 2006).
(1) Measurement of global metallicity from NIR RGB color, such as J-K.
(2) Discovery of multiple RGBs, if any (w Cen-like)?
(3) distance, etc…