Using Globular Clusters to Omega Centauri Study Elliptical Galaxies Terry Bridges Australian Gemini Office 10,000 1,000,000 stars up to 1000 stars/pc3 typical sizes ~10 parsec Mike Beasley (IAC, Tenerife) Favio Faifer, Juan Carlos Forte (Argentina) Duncan Forbes (Swinburne) Karl Gebhardt (Univ. Texas) + Gemini staff!!!! Dave Hanes (Queen's) Mark Norris, Ray Sharples (Durham) Steve Zepf (Michigan State Univ.) The View Isn t Bad... M13 Inside a Globular Cluster... very bright & massive: 104-106 x Sun very old: 10-15 billion years, roughly co-eval generally metal-poor, and chemically homogeneous Found in all types of galaxies ~150 in MW 500-1000 in M31 Y > 10,000 in some ellipticals Young globular cluster in the LMC Massive globular cluster (?) in M31 (G1)
Young globular clusters in merging galaxies-- The Antennae Why are Globular Clusters Useful? Stellar Laboratories -- lots of stars & dense: many interactions -- perfect testbeds of stellar structure and evolution Galaxy Structure, Dynamics, and Dark Matter -- found throughout galaxies (disk, halo, bulge) -- used to determine size and shape of Milky Way -- useful probes for studying galactic dark matter Star Formation and Galaxy Evolution -- record of galactic star formation and how galaxies evolve Bimodal GC Color Distributions GC/Galaxy Formation Models Two populations of GCs: a blue (metal-poor) pop, and a red (metal-rich) pop Blue Red 1. Formation of ellipticals/gcs in mergers (Schweizer 87, Ashman & Zepf 92) N Implies multiple epochs and/or mechanisms of GC formation z 2. In situ/multi-phase collapse (Forbes, Brodie & Grillmair 97) 3. Accretion/stripping (Cote et al. 98) However, both age and metallicity affect colours! GC Colour 4. Hierarchical merging (Beasley et al. 02) 2 & 4 require (temporary) truncation of GC formation at high redshift Spectroscopy Allows... 1) GC metallicities, ages, and abundance ratios via line indices and population synthesis models 2) GC kinematics (rotation, velocity dispersion) 3) dark matter content of host galaxies via GC radial velocities and stellar kinematics Our Ultimately learn more about the structure, formation, and dynamics of elliptical galaxies Gemini/GMOS Program
NGC 3379 N524 (Norris et al. 2008) N3379 (Pierce et al. 2006) Pierce et al. 2006, MNRAS, 366, 1253 NGC 3379: nearby (11 Mpc), E0, MV = -21.1, in Leo Group Previous planetary nebula velocities (Romanowsky et al. 2003) showing no evidence for dark matter! GMOS: ~0.4 nm spectral resolution, 3800-6660 nm 10 hours on one field N4649 (Bridges et al. 2006) N3923 (Norris et al. 2008) Deriving GC Ages and Metallicities NGC 3379/3384 GCs Norris et al. 2008 22 GCs from GMOS Plot indices sensitive (2 non-gcs) (circles) to age (e.g. Hβ) and 14 GCs from the VLT (Puzia et al. 04) (crosses) 1 Gyr Age metallicity (e.g. Mg,Fe) N3384 Overlay stellar N3379 4 GCs which could belong to either N3379 or N3384-2.25 population synthesis 12 Gyr [Fe/H] models to get ages (open circles) +0.7 and metallicities NGC 3379: GC Ages/Metallicities NGC 3379: GC Abundance Ratios [Z/H] GCs cover a wide range of metallicity α-element abundance [E/Fe] ratios seem to decrease with increasing metallicity All GCs older than 10 Gyr No evidence for young GCs Consistent with old ages for NGC 3379 stars Age (Gyr) [Fe/H]
Deriving Mass Profiles NGC 3379: Dark Matter Halo Velocity Dispersion I(R) = projected surface brightness profile Derive NGC 3379 mass profile using spherical Jeans equation, with stellar & GC ν(r) = 3D luminosity profile σp2(r) = projected velocity velocities, assuming isotropic orbits dispersion profile vr2(r) = 3D velocity dispersion GC & PNe velocity dispersions disagree (2-3 ) profile Mass-to-Light Ratio -- do they have different orbits? Mass-to-light increases with radius, implying existence of a dark-matter halo in N3379 NGC 4649 10 kpc Radius (Arcsec) NGC 4649: GC Ages/Metallicities Bridges et al. 2006, MNRAS, 373, 157 Pierce et al. 2006, MNRAS, 368, 325 NGC 4649: abundances less than solar bright E2 (MV = -22.4) in Virgo subcluster, rich GC system [Z/H] 3-4 apparently young GCs (2-3 Gyr) with supersolar metallicities X-ray data show extended dark matter halo No evidence for recent star formation: perhaps 8 hours with GMOS, 38 (of the young GCs were captured from other galaxies? 39) confirmed GCs No detected rotation in GC system: v/σ < 0.6 Age (Gyr) NGC 4649: GC Abundance Ratios As in N3379, alpha Most GCs are old with [E/Fe] NGC 4649: Dark Matter Halo element abundance ratio Spherical, isotropic models using stellar, GC data decreases with GC velocity dispersion inconsistent with model with no dark matter increasing metallicity Excellent agreement between GC and X-ray mass profiles Mass-to-light increases with radius: DM halo in N4649 [Fe/H] Radius (Arcsec) 20 kpc
NGC 3923 NGC 4649: GC Orbits Norris et al. 2008, MNRAS, 385, 40 Axisymmetric, orbit-based models. Gravitational potential: with DM (top) or without DM (bottom) GC orbits are isotropic out to ~100, become tangential beyond that radius With GC and X-ray data, we confirm a DM halo with >95% significance NGC 3923: Galaxy Stellar Light New technique to get host galaxy stellar light spectra from same MOS slits as GC targets -- Allows study of galaxy light out to large radius (2 4 Re) Rotation of 31 ± 13 km/s for NGC 3923 stars (no rotation found in GC system) NGC 3923: Metallicities and Ages All GCs appear older than 10 Gyr: no young GCs NGC 3923: luminous (MV = -21.9) E4, ~21 Mpc, shell galaxy, in average-size group 3 GMOS fields (central, SW, NE), 8 hours on each field, ~80 confirmed GCs total NGC 3923: Metallicities and Ages NGC 3923: GCs and Dark Matter Spherical, isotropic models using stellar and GC data Good agreement in velocity GCs span a wide range in metallicity All GCs consistent with [α/fe] = 0.3 Stellar light consistent in age, metallicity, and [α/fe] with metal rich GCs dispersion for GCs and stars Also good agreement in mass-to-light profile for GCs and X-ray data Increase in mass-to-light with radius supports DM halo
NGC 524 Norris et al. 2008, in preparation NGC 524: S0 in a small group, ~33 Mpc, MV = -22.6 NGC 524: Stellar Velocity Field Excellent agreement between stellar velocity field from SAURON and GMOS: Vrot = 130 km/s at 315 SAURON (1 Re) 103 ± 17 km/s at 304 GMOS (3 Re) SAURON integral-field spectroscopy (Ensellem et al. 2004) 2 GMOS fields, ~50 confirmed GCs, 8 hours/field Only stellar light extraction so far, GC velocities in progress Conclusions Most GCs in elliptical galaxies appear to be old, but span a wide range in metallicity. Some young GCs found Dark matter halos seem to be common around early-type galaxies-- supports X-ray data In galaxies with X-ray data (e.g. N4649), we can study both the galaxy DM halos and the GC orbits 150-200 GC velocities may remove need for X-ray data New technique for measuring stellar light kinematics using GC slits-- can study stellar light to large radius Future: larger GC samples for more galaxies combine with multi-fiber data at larger radius Galaxy Spectrum Extraction Advantages over galaxy starlight GCs are simple stellar populations single age and metallicity GCs can be studied out to several Reff probe DM halos/galaxy cluster potential Galaxy starlight usually only sampled in center 1 Reff difficult to disentangle different stellar populations recent but unimportant (in mass) star formation episode can dominate
Bimodal Color Distributions Bimodality: Implications NGC 4472 (M49) Puzia et al. (1999) AJ Bimodal color distributions globular cluster sub-populations V N Color differences are due to age differences and /or metallicity differences V-I Multiple epochs and/or mechanisms of formation [Fe/H] V-I = 0.95-1.5 1.15-0.5 The Importance of Spectroscopy ddd Globular Clusters as Dynamical Probes Globular Clusters (GCs) are found in large numbers in early-type galaxies, out to large radius ( 5-10 R eff ) GC velocities can be used to study the GC kinematics and galactic dark matter content out to large radius -- GCs complement stellar light, PNe, and X-ray data By combining observations and simulations, study the formation and dynamics of early-type galaxies and their GCs e.g. merger model predicts outer-halo rotation Still a very data-poor field: < 10 early-type galaxies with > 50 GC velocities Astronomical Spectroscopy