NGC 104: d = 4500 pc, D = 31‘, [Fe/H] = -0.76 dex, t = 12 Gyr, A classic Galactic Globular cluster Introduction • What we are dealing with? – Faint and normally red stars – Very crowded fields – Especially the cores are very hard to resolve – Large telescopes are needed for the core regions – Good spatial resolution is needed • Excellent overview article about GCLs: https://ui.adsabs.harvard.edu/abs/2020arXiv200304093B/abstract • A Galactic Globular Clusters Database http://gclusters.altervista.org/ How many GCLs are there? • Harris GCL catalog (1996, AJ, 112, 1487) provides 157 objects: http://physwww.mcmaster.ca/~harris/mwgc.dat • But still new ones are discovered: New GCLs Camargo, 2018, ApJL, 860, L27 Very difficult to detect New GCLs Ryu & Lee, 2018, ApJL, 863, L38 Only a few stars resolved Remember: more GCLs in the Galactic Bulge Faintness - Crowding 15’ Definition - Radii • Core Radius: Distance at which the apparent surface luminosity has dropped by half • Half-Light Radius: Distance from the core within which half the total luminosity from the cluster is received • Half-Mass Radius: The radius from the core that contains half the total mass • Tidal Radius: Distance from the center at which the external gravitation of the galaxy has more influence over the stars in the cluster than does the cluster itself Density – Profile (King Profile) • Heuristic description of the density law of star clusters (open and globular) by Ivan King (1962, AJ, 67, 471): • General formula: f = f1[(1/r – 1/rt)2 ] f … Stars per square unit or surface density; f1 … Constant; rt … Radius f(r) = 0 k … Constant; rc … core radius Density – Profile (King Profile) • Typical Globular Cluster: 1. rt/rc ~ 30 2. Unit for k is V = 10 mag per square arc minute • The parameters rt and rc can be treated within numerical simulations and can be converted into an „astrophysical quantity“, for example: R … Distance from the Galactic center; M … Mass of the Globular Cluster; Mg … Mass of the Milky Way Calibration of the parameters King, 1962, AJ, 67, 471 King et al., 1968, AJ, 73, 456 King profiles – Gaia DR2 de Boer et al., 2019, MNRAS, 485, 4906 Core not resolved, but still very good coverage 82 GLCs analyzed King profiles – Gaia DR2 Gaia will never give any results for the cores => Additional observations needed de Boer et al., 2019, MNRAS, 485, 4906 King profiles – Gaia DR2 de Boer et al., 2019, MNRAS, 485, 4906 Examples of different shapes Also works for open clusters Sánchez & Alfaro, 2005, ApJ, 696, 2086 IC 2381 (5 Myr) NGC 2194 (300 Myr) Motions of star clusters • Position in Galactic coordinates • Position in the Milky Way [XYZ], distance from the Sun needed • Radial velocity • Proper Motion • Model for the gravitional potential of the Milky Way • Includes: spherical bulge, disk, and spherical dark-matter halo • Bajkova et al., 2020, ApJ, 895, 69 Motion of Globular clusters Sun at [XYZ] = [8, 0, 0] Bajkova & Bobylev, 2020, arXiv:2008.13624 Motion of Globular clusters Sun at [XYZ] = [8, 0, 0] Bajkova & Bobylev, 2020, arXiv:2008.13624 Motion of Open clusters Sun at [XYZ] = [0, 0, 0] and r = 8 kpc Wu et al., 2009, MNRAS, 399, 2146 Motion of Open clusters Sun at [XYZ] = [0, 0, 0] and r = 8 kpc Wu et al., 2009, MNRAS, 399, 2146 Ellipticity Goodwin, 1997, MNRAS, 286, L39 e = 1 - b/a a, b are the semimajor and semiminor axes of the ellipse Ellipticity Chen & Chen, 2010, ApJ, 721, 1790 Globulars in the Galactic Bulge are misaligned due to the gravity of the Galactic center (direction of the white arrows) Ellipticity Chen & Chen, 2010, ApJ, 721, 1790 No obvious correlation of the ellipticity with the age or absolute magnitude Formation of Globular Clusters • Globular Clusters also formed from one GMC • But how are GCLs formed in Galaxies? Taniguchi et al., 1999, ApJ, 526, L13 Formation of Globular Clusters Possible interpretation: the results are inconclusive Two „external Populations“ • Halo Population: – Spherical around the center of the Milky Way – Very extended (Halo) – -2.5 < [Fe/H] < -1 dex – 10 < Age < 15 Gyr • Disk Population (Bulge): – More concentrated around the center of the Milky Way – -0.7 < [Fe/H] < +0.5 dex – Age about 10 Gyr • Continuous transition! Bica et al., 2006, A&A, 450, 105 153 Globulars Two Populations Reddening Although the large distance, no reddening, Halo New Globulars with large reddening and large distance detected Multiple „ internal Populations“ • Multiple Main, AGB and HB Sequences within one Globular were found • Not for all Globulars although same observational quality • No clear morphology detected yet • Also indications for the oldest OCLs Multiple „ internal Populations“ • The ACS Globular Cluster Survey: https://archive.stsci.edu/prepds/acsggct/ • The Gaia-ESO survey https://www.gaia-eso.eu/ • Project SUMO: http://www.iac.es/proyecto/sumo/index.html Piotto et al., 2007, ApJ, 661, L53 NGC 2808 Different He content (Y) can explain the multiple MS Open questions • How can you produce such He abundances? • Different populations (age)? • Intrinsic of the star cluster which means are they formed within the cluster? • Merging processes? • Only in Globular Clusters? • Depending on metallicity? Milone et al., 2009, A&A, 503, 755 Double sub-giant branch but no double Main Sequence Milone et al., 2009, A&A, 503, 755 No correlation with the position within the cluster R … radius from the center Lee et al., 2005, ApJ, 621, L57  Centauri Observations fit different isochrones very well Notice different Z and Y values  Centauri Milone et al., 2017, MNRAS, 469, 800 Different colors SUMO (SUrvey of Multiple pOpulations in Globular Clusters) Monelli et al., 2013, MNRAS, 431, 2126 (first paper) Monelli et al., 2013, MNRAS, 431, 2126 Reddening determination also works for these indices, not only for (U – B) versus (B – V) Monelli et al., 2013, MNRAS, 431, 2126 Red Giant Branch C U,B,I = (U – B) – (B – I) Monelli et al., 2013, MNRAS, 431, 2126 Individual populations Very different characteristics Niederhofer et al., 2017, MNRAS, 465, 4159 Results for old star clusters in the Small Magellanic Cloud Niederhofer et al., 2017, MNRAS, 465, 4159 Results in the Small Magellanic Cloud Niederhofer et al., 2017, MNRAS, 465, 4159 Results in the Small Magellanic Cloud IMBH – Globular Clusters • Intermediate Black Holes (IMBH) as seeds for massive Black Holes • Mass: 100 – 100 000 M⨀ • Important for formation and evolution of Galaxies • Detection via kinematics of central Globular Clusters stars or X-ray emission from the center due to accretion of gas IMBH – Globular Clusters • Zocchi et al., 2017, MNRAS, 468, 4429:  Cen, no identication • Baumgardt et al., 2019, MNRAS, 488, 5340:  Cen and NGC 6624, no identication • Wu & Zhao, 2021, ApJ, 908, 224: 35 Globulars investigated, 4 “weak candidates” IMBH – Globular Clusters Stars close to the IMBH should accelerate IMBH – Globular Clusters • What is needed? 1. Total mass 2. Mass/Luminosity ratio 3. Distance 4. Model for the kinematics after many Gyrs • And then look for anisotropy • Kinematics from HST (Gaia) IMBH – Globular Clusters Retention fraction ... fraction of Black Holes that are inside a certain radius Baumgardt et al., 2019, MNRAS, 488, 5340 IMBH – Globular Clusters Baumgardt et al., 2019, MNRAS, 488, 5340 NGC 6624 IMBH – Globular Clusters • Zocchi et al., 2017, MNRAS, 468, 4429 Result: no evidence for an IMBH in  Cen Notice the differences of the listed cluster parameters from the literature The rotation of GCLs • Rotation as dissolving mechanism for GCLs • Sollima et al., 2019, MNRAS, 485, 1460 – 15 of 62 investigated GCLs are rotating – Used radial velocities and proper motions • Cordoni et al., 2020, ApJ, 880, 18 – 2 of 6 investigated GCLs are rotating – Used radial velocities and proper motions – Analysis of two different internal populations Rotating GCLs Both populations (1G and 2G) seem to rotate differently, only on 2.5s Cordoni et al., 2020, ApJ, 880, 18 Sollima et al., 2019, MNRAS, 485, 1460 Different parameter spaces Rotating GCLs Different references find rotation or not (X) for the same GCL GCL Code of different references Sollima et al., 2019, MNRAS, 485, 1460