Star Clusters As Witnesses of the Evolutionary History of the Small Magellanic Cloud

JENAM, Symposium 5: Star Clusters – Witnesses of Cosmic History Star Clusters as Witnesses of the Evolutionary History of the Small Magellanic Cloud

Eva K. Grebel Astronomisches Rechen-Institut

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My collaborators:

PhD student Katharina Glatt PhD student Andrea Kayser (both University of Basel & University of Heidelberg) Andreas Koch (U Basel & UCLA / OCIW) Jay Gallagher, D. Harbeck (U Wisc) Elena Sabbi (U Heidelberg & STScI) Antonella Nota, Marco Sirianni (STScI) Monica Tosi, Gisella Clementini (U Bologna) Andrew Cole (U Tasmania) Gary Da Costa (ANU)

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 1 NGC 416 (OGLE) Tracers of the Age-Metallicity Relation:

Star clusters: Easily identifiable. Chronometers of intense star formation events. Single-age, single-metallicity fossils of local conditions.

Star clusters in the SMC: Clusters formed (and survived) for most of its lifetime Closely spaced set of age tracers! Unique property of the SMC. Milky Way: No comparable set of intermediate-age, populous clusters. LMC: Age gap at intermediate ages. 11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 2

Cluster-based Age-Metallicity Relation:

Photometry Spectroscopy

After Da Costa 2002 An inhomogeneous sample: Photometric and spectroscopic metallicities from different techniques Photometric ages from ground-/space-based data of differing depth 11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 3 Getting Homogeneous Ages and Metallicities: PhD thesis Katharina Glatt HST / ACS program to obtain deep CMDs (PI: Gallagher) GO 10396, 29 orbits, executed 2005 – 2006. 6 populous intermediate-age clusters, 1 globular cluster:

NGC 419, Lindsay 38, NGC 416, 339, Kron 3, Lindsay 1, NGC 121.

In common w. Da Costa & Hatzidimitriou: Ln 1, Kr 3, NGC 339, 121. Several strategically chosen field regions. F555W, F814W; WFC (200” x 200”; 0.05”) & HRC (29” x 26”; 0.025”).

Complements HST / ACS program for young SMC clusters

PI: Nota, GO 10248; 10542.

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 4

Getting Homogeneous Ages and Metallicities: PhD thesis Andrea Kayser ESO / VLT program for metallicities (PI: Grebel) 35 hours, executed 2004 – 2006. 12 populous intermediate-age clusters: Ln 11, Ln 116, Ln 32, Ln 38, Kr 28, Kr 44, NGC 152, NGC 339, NGC 361, NGC 411, NGC 416, NGC 419. In common with Da Costa & Hatzidimitriou: Ln 11, NGC 339. In common with HST / ACS: Ln 38, NGC 416, 419, 339. Adjacent field regions. FORS2 MXU (mass exchange unit), grism 1028x+29, OG590+32. R ~ 3,400, 773 – 948 nm (includes NIR Ca II triplet); S/N ~ 70.

Complements ESO/VLT project for younger SMC clusters See paper by Parisi et al. 2008 (arXiv:0808.0018).

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 5 Target Selection:

Photometry Spectroscopy Our sample for spectroscopy Our sample for photometry L116

Da Costa et al. (2002)

Spectroscopic metallicities for as many clusters with only photometry Deep CMDs for as many of the older clusters as possible As much of an age and metallicity coverage as possible As wide an area coverage as possible 11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 6

Target selection: Spatial distribution:

Our spectroscopy targets

Spectroscopic targets of Da Costa & Hatzidimitriou Some of the calibrating clusters Our photometry targets

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 7 The Oldest Cluster in the SMC: NGC 121

Known to be the oldest star cluster in the SMC. Known to be the only globular cluster in the SMC. Suggested to be a few Gyr younger than the oldest GCs in the LMC, For, Sgr, Milky Way.

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 8

The Oldest Cluster in the SMC: NGC 121

Padova & Teramo isochrones provide good fit of main sequence, subgiant branch, lower red giant branch. Do not reproduce slope of upper RGB correctly. Both yield relatively “young” ages; 11 – 12 Gyr.

Glatt et al. 2008, AJ, 135, 1106. [Fe/H]spec = –1.46 dex

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 9 The Oldest Cluster in the SMC: NGC 121

Dartmouth isochrones (Dotter et al. 2007) provide excellent fit of the entire CMD. Yields again a relatively young age. NGC 121 consistently younger by 2–3 Gyr than oldest known GCs.

[Fe/H] = –1.46 dex Glatt et al. 2008, spec AJ, 135, 1106. 11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 10

Glatt et al. 2008, AJ, in press (astroph) Intermediate-Age Clusters in the SMC:

Lindsay 1 Kron 3 NGC 339

NGC 416 Lindsay 38 NGC 419

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 11 Glatt et al. 2008, AJ, in press (astroph) Intermediate-Age Clusters in the SMC:

7.5 Gyr 6.5 Gyr 6 Gyr

~ 1.3 Gyr

6 Gyr 6.5 Gyr

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 12

Glatt et al. 2008, AJ, in press (arXiv:0807.3744) The special case of NGC 419:

Padova isochrones approximate CMD best. No well-defined main-sequence turn-off. Possibly multiple turn-offs; age (range) uncertain.

Glatt et al. 2008, AJ, in press (arXiv:0807.3744), and Sabbi et al., in prep.

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 13 The Age-Metallicity Relation of the SMC

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 14

Membership Criteria

Radial Velocity Only stars within 2 from the mean radial velocity

Position in the CMD Only stars on upper RGB

Distance from the cluster center

Stars within R < 6 · Rcore

Metallicity Outliers discarded

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 15 The Cluster Age-Metallicity Relation of the SMC VLT spectroscopy & ACS photometry of SMC clusters: Metallicity spread at a given age; SMC not well mixed!

Parisi et al. 08 NGC 330 t al. 2008 e latt G t al. 2008, e Kayser

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 16

The Cluster Age-Metallicity Relation of the SMC The AMR in comparison to models: Closed box; continuous SF, no infall or outﬂow of gas Bursting model w. three phases of different SFE (Pagel & Tautvaisiene 1998) Early burst (~ 13 Gyr), quiescent phase, recent burst (4 Gyr). n prep. i

t al. 2008, e Kayser

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 17 The Cluster Age-Metallicity Relation of the SMC

“Inner” SMC clusters; “outer” SMC clusters: Little trend with age and metallicity.

Kayser et al. 2008, in prep.

11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 18

The Cluster Age-Metallicity Relation of the SMC:

No smooth age-metallicity relation. Metallicity spread at a given age; inhomogeneous enrichm. Mild radial trend. NGC 121 confirmed Parisi et al. 08 to be 2–3 Gyr younger NGC 330 than oldest GCs. Delayed GC formation? Dartmouth isochrones work very well for populations > 2 Gyr. Support for depth extent of SMC (at least 17 kpc from cluster distances) Multiple MSTO in NGC 419?

Stay tuned for results on cluster MFs, structure, and field results. 11.09.2008 Grebel, JENAM Symp. 5: SMC Star Clusters 19