Denija Crnojević & Eva K. Grebel Texas Tech University &

Dr. Denija Crnojević (Texas Tech University) kindly agreed to present Dr. Eva Grebel’s talk slides during the Division H meeting when she had to cancel her participation in the IAU GA in Hawaii.

A review paper focusing on globular clusters (not clusters in general) will appear in the proceedings of IAU Symposium 312 (eds. R. Spurzem et al.): Grebel, E.K., “Globular Clusters in the Local Group”, to appear in IAU Symp. 312 on “Star Clusters and Black Holes in Across Cosmic Time” held in Beijing 2014, Cambridge Univ. Press, in press.

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 1 M31 and the

M31: Richest GC system in the LG. > 700 GCs! (MW: ~ 160 GCs). Also numerous young clusters.

Huxor et al. 2014, MNRAS, 442, 2165 Halo GCs often still associated with tidal tails of disrupted dwarf

Mackey et al. 2013, galaxies in which MNRAS, 429, 281 they formed. 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 2

Globular Clusters in M31 and the Milky Way

M31 and MW GC metallicity Galleti et al. 2009, distribution functions (here: A&A, 508, 1285; from Lick indices for M31): 245 GCs.

〈[Fe/H]〉M31 ≈ −0.9 dex.

〈[Fe/H]〉MW ≈ −1.3 dex. ➙ M31 GCs overall more metal-rich. 25% of M31 GCs > −0.5 dex. 7% of MW GCs > −0.5 dex. M31: No obvious bimodality like in MW, but 2-component fit with modes at −1.54 and −0.64 dex preferred (in MW: −1.60 and −0.59); also three- component fit possible.

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 3 GCs in M31 Metal-rich GCs: Strongly centered, bulge. Follow HI rotation curve. A few deviant metal-rich GCs in outskirts. “Intermediate”: More widely distributed. “Metal-poor”: widest distribution. Larger velocity dispersion, but still clear rotation signature (contrary to MW GCs!). Galleti et al. 2009, 60’ ∝ 13.8 kpc A&A, 508, 1285 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 4

Cluster age-metallicity relation Clusters in M33 Beasley et al. 2015, MNRAS, 508, 1285 ! M33 disk clusters: Clear ! age-metallicity relation ! (but note large scatter). ! No radial age gradient ! in disk clusters (≤ 6 kpc). ! Clusters younger than 4 ! Gyr: rotate with disk of ! M33 (reduced rotation ! amplitude and increased velocity dispersion ! with higher cluster age; similar to MW). ! Mean metallicity of inner M33 GCs: high ! ( 〈[m/H]〉 = −1.12 ± 0.09 dex). Also kinem- Beasley et al. 2015, ! atics support association with thick disk. MNRAS, 508, 1285 ! At least six outer halo GCs with projected ! galactocentric radii of 10 – 50 kpc. ! Low number: M33 halo stripped in past ! encounters with M31? Age – velocity-dispersion relation Beasley et al. 2015, MNRAS, 508, 1285 Cockcroft et al. 2011, ApJ, 730, 112 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 5 (Globular) Clusters in M31 and the Milky Way

Wang et al. 2013, AJ, 146, 20 Huxor et al. 2014, MNRAS, 442, 2165

", ✕: M31 GCs #: MW GCs : M31 extended GCs Δ: M31 young clusters

Trends: Tidal radii rt increase with (projected) galactocentric distance.

On average larger half-light radii at Rgc > 30 kpc. Extended GCs may fill the gap between (ultrafaint) dSphs and GCs. 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 6

Globular Clusters in the Milky Way , 504 355 rc depends on initial structural conditions of cluster. Little effect of tidal history, while rh quickly adjusts to new potential (Miholics et al. 2014).

Similarity of structural parameters in

GCs in dwarf satellites and MW halo: MackeyGilmore& 2004, MNRAS, Indication of possible accretion origin of “young” halo GCs.

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 7 Globular cluster accretion from dwarf galaxies

Assume: GC metallicity traces host metallicity at time of formation. ❏ Offset in MW GC age-metallicity relation: 0.6 dex (∝ stellar mass ❏ difference of ~ 2 dex). Leaman et al. 2013, ❏ Halo GCs on metal- MNRAS, 436, 122 ❏ poor branch: Well ❏ fitted by age-metal- ❏ licity relation of LG dIrrs. Much of MW halo GC system may have come from ~ 6 – 7 WLM- to LMC-sized dIrrs. ❏ Metal-rich branch: ❏ formed in situ in MW ❏ disk/bulge. See also Forbes & Bridges 2010, MNRAS, 404, 1203

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 8

GC disruption Additionally, GCs may get disrupted by disk and bulge crossings or internal relaxation. A few cases of ongoing dissolution were found in MW halo. (e.g., Odenkirchen et al. 2001, ApJ, 548, L165; Odenkirchen & Jordi & Grebel 2010, A&A, 522, 71). Grebel 2001 Quantifying contrib- ution to halo field by searching for field stars with light element abun- dance anomalies: Possible 2nd genera- tion stars from dis- Martell & Grebel 2010, A&A, 519, 14; solved GCs. Martell et al. 2011, A&A, 534, 136 ➙ ~ 17% of halo field stars originally from GCs. 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 9 Specific GC frequency: S = N · 100.4 (Mv+15) GCs contributed by the Sgr dSph N GC

❏ 5 high-, 4 moderate, 2 low- Law & Majewski 2010, AJ, 718, 1128 ❏ confidence GC members. ❏ ~ 8 ± 2 genuine GCs.

❏ SN ~ 5 – 9 for initial Sgr ❏ luminosity of MV = −15. ❏ M54: GC at core of Sgr ❏ (formed elsewhere and sank to ❏ center via dynamical friction). ❏ In the (HB-type, [Fe/H] plane), ❏ Sgr is contributing “young halo” ❏ GCs with predominantly red HBs ❏ (Arp 2, NGC 4147) and “old halo” GCs (!) with mainly blue HBs. ❏ When fully disrupted, Sgr will (probably) have contributed up to 3 – 4 ❏ metal-rich, young objects to the Galactic halo, which have no counter- ❏ counterparts even among the “young halo” GCs.

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 10

5 Globular Clusters in Fornax dSph GCs #1 – 3, #5 in Fornax: −2.5 dex −2.1 dex ~ indistinguishable in age; as old as oldest MW GCs. GC #4: ~ 3 Gyr younger, −1.9 dex. Buonanno et al. 1999, AJ, 118, 1671

−2.4 dex −2.2 dex

Buonanno et al. 1998, ApJ, 501, L33 Note 2nd parameter effect in ~ coeval, metal- poor GCs! Smith et al. 1996, AJ, 111, 1596 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 11 NGC 185 Globular Clusters in M31’s Dwarf Ellipticals ! NGC 147: 8 – 10 GCs. Mainly old stellar populations, gas/dust free. ! NGC 185: ~ 7 GCs (incl. “young” and “old halo” GCs). Strong intermediate-age ! populations, some intermed.-age clusters (a few Gyr), nuclear cluster, gas, dust. ! NGC 205: ~ 6 GCs (incl. “young” and “old halo” GCs). 7 – 11 Gyr, −1.1 – −2 dex. ! Many intermed.-age clusters esp. ~ 1 – 2 Gyr; [m/H] > −0.8 dex. Nuclear cluster.

Ellipse: B = 25 mag/arcsec2 isophote SN of dEs in Virgo & Fornax clusters

consistent with those of M31’s dEs.

Veljanoski et al. 2013, MNRAS, 435, 3654

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 12

Distribution of Star Clusters in the Magellanic Clouds

❏ “Old” LMC clusters (blue dots: > 4 Gyr) 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.

LMC Magellanic Bridge SMC

Associations Bica et al. 2008, MNRAS 389, 678 Clusters

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 13 Recent History of the LMC ❏ Ages of 1193 populous LMC star clusters from resolved CMDs taken from the ❏ Magellanic Clouds Photometric Survey (Glatt, Grebel, & Koch 2010, A&A 517, 50). ❏ Note how star formation migrates along the bar and how different disk regions ❏ become active at different times. Age < 20 Myr 20 Myr ≤ age < 50 Myr

30 Dor

Ellipses show approximate location of supergiant shells. 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 14

Recent Star Formation History of the LMC

❏ Ages of 1193 populous LMC star clusters from resolved CMDs taken from the ❏ Magellanic Clouds Photometric Survey (Glatt, Grebel, & Koch 2010, A&A 517, 50).

50 Myr ≤ age < 100 Myr 100 Myr ≤ age < 250 Myr

250 Myr: approx. one rotation period of the LMC. 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 15 Recent Star Formation History of LMC and SMC

❏ Ages of populous LMC and SMC star clusters from resolved CMDs from the ❏ Magellanic Clouds Photometric Survey (Glatt, Grebel, & Koch 2010, A&A 517, 50). 250 Myr ≤ age < 500 Myr LMC clusters

Age histo- grams of populous LMC and SMC star SMC clusters clusters from re- solved CMDs:

Similar peaks in the age distribution may Star clusters formed during the have been triggered by close encounters previous rotation period of the LMC. between the Clouds (and the MW). 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 16

Magellanic Cloud Cluster Core Radii vs. Ages

❏ Spread in cluster

❏ core radii (rc) ❏ increases with age. , 65 ❏ Young massive 386 4 ❏ (> 10 M!) clusters: , 138 ❏ rc ~ 1 – 2 pc; 138

❏ oldest clusters: [pc] c ❏ up to 8 pc. r ❏ GCs in Fornax & ❏ Sgr dSphs also et al. 2009, AJ, AJ, al.2009,et ❏ fit this trend, as

Mackeyal.2008,et MNRAS Glatt ❏ do MW GCs (esp. ❏ “young” halo GCs).

log (age) [yr] 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 17 LMC Star Cluster Survivability Cluster frequency

divided by field SFH.

LMC

No. of clusters formed per unit stellar mass ~ constant up to 200 Myr.

Then sharp drop − onset of al.2013,et MNRAS,430, 676 dissolution. Baumgardt

4 ➙ Characteristic lifetime of a ≥ 10 M! cluster: 200 Myr ➙ (1/2 the value for same mass in solar neighborhood; Lamers et al. 2005) ➙ For t > 200 Myr: ~ 90% of clusters destroyed per 1 dex in log t (assuming a constant cluster formation rate). 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 18

SMC Field and Cluster Age-Metallicity Relation

❏ Field star AMR Large symbols: Clusters with spectroscopic [Fe/H]. ❏ overall traces Small symbols: Clusters with photometric [Fe/H]. ❏ cluster AMR. Thick lines: Field AMR from HST CMDs. ❏ At any ❏ given age: Cignoni et al. ❏ range of 2013, ApJ ❏ [Fe/H]; 775, 83 ❏ SMC ❏ clusters ❏ did not form ❏ from well-mixed ❏ material. ❏ Clusters ❏ formed and ❏ survived even ❏ while field SFR ❏ was low.

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 19 LMC GCs: Light element abundance variations

❏ Old, metal-poor LMC ❏ GCs NGC 1786, ❏ NGC 2210, and NGC MW GCs ❏ 2257 ([Fe/H] ~ −2 to ❏ −1.5 dex) show Na – O ❏ anti-correlations ❏ just like old MW ❏ GCs. ➙ 2nd generation SF? ❏ Intermediate-age ❏ populous LMC Mucciarelli 2012, MemSAI, 19, 179 ❏ clusters do not. ➙ Higher metallicity ❏ and/or lower mass ❏ to blame? ❏ 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 20

Globular clusters in isolated dIrrs: NGC 6822

Veljanoski et al. 2015, MNRAS, 452, 320 Huxor et al. 2013, MNRAS, 429, 1039 Ages ≥ 8 Gyr

7 GC candidates (5 extended, 2 very compact, luminous).

SN ~ 7. Ext. GCs old, ≥ 8 Gyr. Both metal-poor (≈ −2 dex) and metal-rich (≈ −0.9 dex) GCs in Hwang et al. 2014, ApJ, 783, 49 central 2 kpc. Only metal-poor in outer halo (≥ 2.6 kpc). Weak net rotation with underlying spheroid, not with HI disk. 08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 21 Lim & Lee 2015, Clusters in isolated ApJ, 804, 123 dIrrs: IC10 ! No known GCs. ! Young clusters (< 10 Myr) mainly in ! off-centered HII region (main body). ! Main body: clusters with a few Myr ! to > 1 Gyr. 3 peaks: 6,100 Myr, 4 Gyr. ! (All age estimates via photom. SEDs.)

! Halo (R ≥ R25): metal-poor, old clusters

! (1 – 10 Gyr; reff from ~2 to ~8 pc). R25 Lim & Lee 2015, ApJ, 804, 123

Lim & Lee 2015, hatched: high- ApJ, 804, 123 quality ages

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 22

Summary

❏ Differences in GCs of M31/MW: Many M31 GCs show bulge kinematics. ❏ Rotating “halo” with higher dispersion and more metal-rich than in MW. ❏ MW: Halo GCs on radial orbits; no preferred rotation direction; fewer GCs. ➙ Globular cluster formation seems to have happened in very ❏ different ways in these two massive spirals. ❏ For both galaxies: Part of the GCs were accreted from dwarf galaxies. ❏ Range of GCs observed; from compact/luminous to diffuse/extended.

❏ Metal-rich GCs and GCs with smaller rh more centrally concentrated.

❏ All massive dwarf galaxies contain GCs, often high specific frequencies, SN. ❏ Intermediate-age and young clusters: More centrally concentrated. ❏ Clear cluster age-metallicity relation and age-velocity dispersion relation. ❏ Young clusters trace spatially resolved recent star formation history, ❏ including possible cluster formation enhancement by interactions. ❏ Chemistry helps to trace modes of star formation (rapid/slow SF, possible ❏ 2nd generation SF in GCs) and gas infall history.

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 23 Outlook / Expected Major Developments

❏ More cluster detections especially in extended galactic outskirts. ➙ Further improvement of star cluster census. ❏ In addition to integrated multi-band photometry (ground-based ~4m-class tel.): ❏ Increasingly spectroscopic characterization (ground-based 8-10 m tel.) ❏ Photometry: Resolved deep CMDs (HST). ➙ Estimates of cluster ages, metallicities, kinematics; masses, lifetimes. ➙ Possibly more information on light element variations; multiple populations.

Beyond the Local Group: ❏ Vastly improved census & characterization of young ❏ clusters in star-forming galaxies via HST surveys like ❏ LEGUS (HST Legacy UV Survey) and ground-based follow-up. ❏ Continued improvements of globular cluster census and characterization ❏ via HST photometry and ground-based spectra (8-10 m tel.).

08/07/2015 Crnojevic & Grebel: Local Group Star Clusters 24