The Star Clusters of the Magellanic Clouds

Transcription

1 The Dance of Stars MODEST-14 The Star Clusters of the Magellanic Clouds Eva K. Grebel Astronomisches Rechen-Institut Zentrum für Astronomie der Universität Heidelberg Star Clusters in the Magellanic Clouds! All Local Group galaxies with M V < 13 contain star clusters, regardless of morphological galaxy type. (Grebel 2002, IAU Symp. 207 review) What makes the Magellanic Clouds special? Proximity - deep resolved CMDs and spectroscopy of individual stars possible. Richness of their cluster systems. Age range Existence of clusters of all ages, including large numbers of rich, massive young and intermediate- age clusters. Populous star clusters(hodge 1961) Grebel: Star Clusters of the Magellanic Clouds 1

2 Census of Star Clusters in the Magellanic Clouds! Numerous searches for star clusters in the Magellanic Clouds were conducted over the years. Earlier: photographic plates. Now: CCD imaging surveys such as the Magellanic Clouds Photometric Survey (MCPS, Zaritsky et al. 2002, 2004) or the time-domain Optical Gravitational Lensing Experiment (OGLE, e.g., Udalski et al. 2008, 2009). Most recent, most comprehensive catalog of LMC, SMC, and Bridge clusters: Bica et al. 2008, MNRAS 389, 678 A general catalog of extended objects in the Magellanic System LMC: 3103 clusters, 2944 associations. Bridge: 91 clusters, 148 associations. SMC: 546 clusters, 234 associations Grebel: Star Clusters of the Magellanic Clouds 2 Distribution of Star Clusters in the Magellanic Clouds! All Local Group galaxies with M V < 13 contain star clusters, regardless of morphological galaxy type. [Beware: observational incompleteness in eastern part of Bridge] Bica et al. 2008, MNRAS 389, 678 Here old means age > 4 Gyr Grebel: Star Clusters of the Magellanic Clouds 3

3 Distribution of Star Clusters in the Magellanic Clouds! Old LMC clusters trace bar-like structure, slightly rotated against younger star cluster distribution. SMC disk and Bridge better traced by associations. LMC associations also trace leading edge, but its outer clusters form loose ring around entire LMC (not seen in associations). Old LMC clusters also in outer parts (not only in bar). Old SMC clusters preferentially found in outer parts. Associations Clusters Bica et al. 2008, MNRAS 389, Grebel: Star Clusters of the Magellanic Clouds 4 Recent Star Formation History of the LMC! Ages of 1193 populous LMC star clusters from resolved CMDs of the MCPS (Glatt, Grebel, & Koch 2010, A&A 517, 50). Age < 20 Myr 20 Myr age < 50 Myr Ellipses show approximate location of supergiant shells. Glatt et al. 2010, A&A 517, Grebel: Star Clusters of the Magellanic Clouds 5

4 Recent Star Formation History of the LMC! Ages of 1193 populous LMC star clusters from resolved CMDs of the MCPS (Glatt, Grebel, & Koch 2010, A&A 517, 50). 50 Myr age < 100 Myr 100 Myr age < 250 Myr Glatt et al. 2010, A&A 517, Grebel: Star Clusters of the Magellanic Clouds 6 Recent Star Formation History of the LMC! Ages of 1193 populous LMC star clusters from resolved CMDs of the MCPS (Glatt, Grebel, & Koch 2010, A&A 517, 50). 250 Myr age < 500 Myr age > 500 Myr Glatt et al. 2010, A&A 517, Grebel: Star Clusters of the Magellanic Clouds 7

5 Recent Star Formation History of the SMC! Ages of 324 populous SMC star clusters from resolved CMDs of the MCPS (Glatt, Grebel, & Koch 2010, A&A 517, 50). 250 Myr age < 500 Myr age > 500 Myr Black open circles: No resolved CMDs. Glatt et al. 2010, A&A 517, Grebel: Star Clusters of the Magellanic Clouds 8 Diameters and Ellipticities as Function of Distance! Apparent diameter [] Ellipticity Larger D app with increasing galactocentric distance More circular with increasing galactocentric distance Bica et al. 2008, MNRAS 389, Grebel: Star Clusters of the Magellanic Clouds 9

6 Bica et al. 2008, MNRAS 389, 678 [Beware: D app typically 4x smaller than tidal radii R t.] Cluster Size Distributions! Numbers of MC clusters fall off towards large diameters faster than associations. Likely due to dynamical evolution & disruption. In Bridge slopes are similar, possibly due to weaker tidal field. MW GCs: drop off both towards small & large radii drop-off at small radii in MCs may be real, not incomplete- ness. MW GCs Grebel: Star Clusters of the Magellanic Clouds Peak ~ 4 6 pc. SMC associations on average larger than LMC associations. D app [arcmin] Peak tidal radii: ~16 ~30 pc. R t [pc] Associations can have larger D than clusters. 10 Magellanic Cloud Cluster Core Radii vs. Ages! Mackey et al. 2008, MNRAS 386, 65 Glatt et al. 2009, AJ, 138, 138 Spread in cluster core radii (r c ) increases with age. Young massive (> 10 4 M ) clusters: r c ~ 1 2 pc; oldest clusters: up to 8 pc. GCs in Fornax & Sgr dsphs also fit this trend, as do MW GCs (esp. young halo GCs). r c [pc] log (age) [yr] Grebel: Star Clusters of the Magellanic Clouds 11

7 Magellanic Cloud Cluster Core Radii vs. Ages! Mackey et al. 2008, MNRAS 386, 65 Galactic young halo GCs mainly at Galactocentric distances > ~ 15 kpc. Progression of cluster structural properties with time? Internal dynamical evolution or Grebel: Star Clusters of the Magellanic Clouds 12 Magellanic Cloud Cluster Core Radii vs. Ages! Hurley & Mackey 2010, MNRAS 408, 2353 Differences in initial cluster size may play important role (Hurley & Mackey 2010). Extended clusters (ECs) also in outer regions of other massive galaxies (M31; Huxor et al. 2005) and other dwarfs. 2 modes of GC formation, one with typical half-light radii of ~ 3 pc, and extended mode with r 1/2 ~ 10 pc? (Da Costa et al. 2009, see also Baumgardt et al. 2010) Diffuse star clusters can form directly in regions where tidal forces are low (Elmegreen 2008). ECs with r 1/2 up to 30 pc (depending on galactic tidal field) can form from standard cluster evolution if clusters are close to filling their tidal radii initially (Hurley & Mackey 2010). ECs such as some Galactic outer halo GCs probably formed in dwarfs (like the MCs) and were later accreted Grebel: Star Clusters of the Magellanic Clouds 13

8 Star Cluster Survivability! Baumgardt et al. 2013, MNRAS, 430, 676 LMC cluster masses vs. age: Note lack of low- mass clusters for ages > 1 Gyr. Due to dissolution or difficulty of detecting older, sparse clusters? Consider age (dissolution) bias. Dotted line: 50% completeness. Dashed line: Present analysis; ages > 10 7 yr; masses > 5000 M, M V > Grebel: Star Clusters of the Magellanic Clouds 14 Star Cluster Survivability! No. of clusters formed per unit stellar mass ~ constant up to 200 Myr. Then sharp drop onset of dissolution. Cluster frequency divided by field SFH. LMC Primordial gas expulsion either has little effect, or is only effective during first 10 Myr, or only affects clusters with M < 5000 M. Characteristic lifetime of a 10 4 M cluster: 200 Myr (1/2 the value for same mass in solar neighborhood; Lamers et al. 05) For t > 200 Myr ~ 90% of clusters destroyed per 1 dex in log t Grebel: Star Clusters of the Magellanic Clouds 15 Baumgardt et al. 2013, MNRAS, 430, 676

9 Clusters with multiple MS TOPs?! LMC, SMC: Multiple main-sequence turn- offs and/or red clumps. Suggested explanations: Extended or multiple SF episodes; age spreads of Myr? Cluster mergers? Interacting binaries? Stellar rotation? Not seen in younger ( Myr) clusters. (e.g., Girardi et al. 2011; Yang et al. 2012, 2013; Bastian & de Mink 2009, Bastian et al. 2013) Grebel: Star Clusters of the Magellanic Clouds Yang et al Glatt et al. 2008, AJ 136, Differences between LMC/SMC Clusters! LMC: at least 15 old globular clusters. Variety of abundances and horizontal branch types. Oldest LMC GCs as old as oldest Galactic GCs. (e.g., Johnson et al. 1999). Belong kinematically to disk; no kinematic halo GCs. LMC: Extended hiatus in star cluster formation at intermediate ages; the famous age gap. SMC: Only one genuine, old GC; NGC121; 2-3 Gyr younger than old Galactic halo clusters. SMC: fairly continuous age sequence of clusters. Difference not understood. Puzzling if Clouds were long-term bound, interacting pair Grebel: Star Clusters of the Magellanic Clouds 17 (e.g., Glatt et al. 2008)

10 LMC: Field and Cluster Age-Metallicity Relation! Field stars seem to trace cluster AMR rather well. Initial rapid enrichment. Cluster age gap appears to coincide with low field SFR; enrichment mainly via SNe Ia. But be aware of caveats in deriving field AMR from either photometry or spectra! Note the gap in the age distribution between ~ 4 and ~ 9 Gyr. Note the large scatter in metallicity at a given age. Harris & Zaritsky 2009, AJ 138, Grebel: Star Clusters of the Magellanic Clouds 18 SMC Field and Cluster Age-Metallicity Relation! Field AMR overall traces cluster AMR. At any given age: range of [Fe/H]; SMC not well mixed. Clusters formed and survived even while field SFR was low. Large symbols: Clusters with spectroscopic [Fe/H]. Small symbols: Clusters with photometric [Fe/H]. Thick lines: Field AMR from HST CMDs. Cignoni et al. 2013, ApJ 775, Grebel: Star Clusters of the Magellanic Clouds 19

11 Summary! Clusters found in LMC, SMC, Bridge. Young clusters/field stars: irregular distribution, different activity centers (incl. bar), loci change with time. Recent age distribution peaks may coincide with times of close encounters between Clouds and/or Milky Way. Cluster-to-field-star ratio ~ constant for Myr. Characteristic cluster life time ~ 200 Myr (LMC). Main dissolution mechanism probably two-body relaxation. SMC cluster age-metallicity relation: Not well mixed in metallicity at any given age! (Apparently also in LMC) Differences LMC/SMC clusters (reasons unclear): LMC age gap (~4 ~9 Gyr) vs. continuous age range. Many old GCs vs. 1 GC 2 3 Gyr younger than old GCs. Multiple stellar populations under debate Grebel: Star Clusters of the Magellanic Clouds 20 Coming soon:! HTTP The Hubble Tarantula Treasury Survey (ACS & WFC3) Grebel: Star Clusters of the Magellanic Clouds 21

12 SMC Clusters with HST! Glatt et al. 2008, AJ 136, 1703 and AJ, 135, 1106 NGC 121 Kron 3 NGC Gyr 11 Gyr 6.5 Gyr Grebel: Star Clusters of the Magellanic Clouds 22