Denija Crnojević & Eva K. Grebel Texas Tech University ... · Denija Crnojević & Eva K. Grebel...

Denija Crnojević & Eva K. Grebel Texas Tech University & Heidelberg University

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

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 Local Group globular clusters (not star 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 Galaxies Across Cosmic Time” held in Beijing 2014, Cambridge Univ. Press, in press.


M31 and the Milky Way

Halo GCs often still associated with tidal tails of disrupted dwarf galaxies in which they formed.

Mackey et al. 2013, MNRAS, 429, 281

Huxor et al. 2014, MNRAS, 442, 2165

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


M31 and MW GC metallicity distribution functions (here: from Lick indices for M31): 〈[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.

Globular Clusters in M31 and the Milky Way

Galleti et al. 2009, A&A, 508, 1285;

245 GCs.


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!).

GCs in M31

Galleti et al. 2009, A&A, 508, 1285 60’ ∝ 13.8 kpc


Clusters in M33 Cluster age-metallicity relation

Age – velocity-dispersion relation

Beasley et al. 2015, MNRAS, 508, 1285

Beasley et al. 2015, MNRAS, 508, 1285

Beasley et al. 2015, MNRAS, 508, 1285 Cockcroft et al. 2011, ApJ, 730, 112

!  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- !  atics support association with thick disk. !  At least six outer halo GCs with projected !  galactocentric radii of 10 – 50 kpc. !  Low number: M33 halo stripped in past !  encounters with M31?


Trends: Tidal radii rt increase with (projected) galactocentric distance. On average larger half-light radii at Rgc > 30 kpc.

(Globular) Clusters in M31 and the Milky Way

Huxor et al. 2014, MNRAS, 442, 2165

Wang et al. 2013, AJ, 146, 20

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

Extended GCs may fill the gap between (ultrafaint) dSphs and GCs.


Globular Clusters in the Milky Way

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Similarity of structural parameters in GCs in dwarf satellites and MW halo: Indication of possible accretion origin

of “young” halo GCs.

rc depends on initial structural conditions of cluster. Little effect of

tidal history, while rh quickly adjusts to new potential (Miholics et al. 2014).


Assume: GC metallicity traces host galaxy metallicity at time of formation.

❏  Offset in MW GC age-metallicity relation: 0.6 dex (∝ stellar mass ❏  difference of ~ 2 dex). ❏  Halo GCs on metal- ❏  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.

Leaman et al. 2013, MNRAS, 436, 122

Globular cluster accretion from dwarf galaxies

See also Forbes & Bridges 2010, MNRAS, 404, 1203


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; Jordi & Grebel 2010, A&A, 522, 71).

Quantifying contrib- ution to halo field stars by searching for field stars with light element abundance anomalies: Possible 2nd generation stars from dis- solved GCs. ➙ ~ 17% of halo field stars originally from GCs.

GC disruption

Martell & Grebel 2010, A&A, 519, 14; Martell et al. 2011, A&A, 534, 136

Odenkirchen & Grebel 2001


GCs contributed by the Sgr dSph ❏  5 high-, 4 moderate, 2 low- ❏  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.

Law & Majewski 2010, AJ, 718, 1128

Specific GC frequency: SN = NGC · 100.4 (Mv+15)


5 Globular Clusters in Fornax dSph GCs #1 – 3, #5 in Fornax: ~ indistinguishable in age; as old as oldest MW GCs. GC #4: ~ 3 Gyr younger, −1.9 dex.

Buonanno et al. 1998, ApJ, 501, L33

−2.5 dex −2.1 dex

−2.4 dex −2.2 dex

Buonanno et al. 1999, AJ, 118, 1671

Note 2nd parameter effect in ~ coeval, metal-poor GCs! Smith et al. 1996, AJ, 111, 1596


!  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.

Globular Clusters in M31’s Dwarf Ellipticals

Veljanoski et al. 2013, MNRAS, 435, 3654

SN of dEs in Virgo & Fornax clusters

consistent with those of M31’s dEs.

Ellipse: B = 25 mag/arcsec2 isophote

NGC 185


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.

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

LMC

SMC Magellanic

Bridge


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). ❏  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

Ellipses show approximate location of supergiant shells.

30 Dor


Recent Star Formation History of the LMC

250 Myr: approx. one rotation period of the LMC.

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

❏  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).


Recent Star Formation History of LMC and SMC

250 Myr ≤ age < 500 Myr

❏  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).

Star clusters formed during the previous rotation period of the LMC.

LMC clusters

SMC clusters

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

Similar peaks in the age distribution may have been triggered by close encounters between the Clouds (and the MW).


Magellanic Cloud Cluster Core Radii vs. Ages

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❏  Spread in cluster ❏  core radii (rc) ❏  increases with age.

❏  Young massive ❏  (> 104 M!) clusters: ❏  rc ~ 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 [p

c]

log (age) [yr]


LMC Star Cluster Survivability

LMC

➙  Characteristic lifetime of a ≥ 104 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).

Cluster frequency divided by field SFH.

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No. of clusters formed per unit stellar mass ~ constant up to 200 Myr. Then sharp drop − onset of dissolution.


SMC Field and Cluster Age-Metallicity Relation

❏  Field star AMR ❏  overall traces ❏  cluster AMR. ❏  At any ❏  given age: ❏  range of ❏  [Fe/H]; ❏  SMC ❏  clusters ❏  did not form ❏  from well-mixed ❏  material. ❏  Clusters ❏  formed and ❏  survived even ❏  while field SFR ❏  was low.

Cignoni et al. 2013, ApJ 775, 83

Large symbols: Clusters with spectroscopic [Fe/H]. Small symbols: Clusters with photometric [Fe/H].

Thick lines: Field AMR from HST CMDs.


LMC GCs: Light element abundance variations

❏  Old, metal-poor LMC ❏  GCs NGC 1786, ❏  NGC 2210, and NGC ❏  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 ❏  clusters do not. ➙ Higher metallicity ❏  and/or lower mass ❏  to blame? ❏ 

Mucciarelli 2012, MemSAI, 19, 179

MW GCs


Globular clusters in isolated dIrrs: NGC 6822

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 central 2 kpc. Only metal-poor in outer halo (≥ 2.6 kpc). Weak net rotation with underlying spheroid, not with HI disk.

Hwang et al. 2014, ApJ, 783, 49

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


Clusters in isolated dIrrs: IC10

R25

Lim & Lee 2015, ApJ, 804, 123

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

quality ages

!  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).

Lim & Lee 2015, ApJ, 804, 123


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.


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.).

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