UV characteristics of UCDs and GCs: When is a star cluster really a galaxy?

WIP Seminar talk at the Institute of Geophysics and Planetary Physics (IGPP) , Lawrence Livermore National Laboratory (LLNL), June 26, 2008

Transcript

1. UV characteristics of UCDs and GCs When is a star cluster really a galaxy? IGPP Seminar talk, June 26, 2008 Work in progress: ESO Conference, Munich, July 21-25, 2008 “Chemical Evolution of Dwarf Galaxies and Stellar Clusters”
2. The oldest star clusters: Globular Clusters Spitzer IR image of the core of the MW
3. Milky Way Globular Clusters derived from the Latin word globulus - a small sphere -  roughly 160 globulars (GCs) in the Milky Way -  90% lie within 100,000 light-years (0.5r stellar halo) -  single age (old !13 Gyr), single (low) metallicity populations (all stars form at the same time) -  Very little or no HI gas and dust (stars + stellar winds drive out gas) -  M/L ratios ≈ 1 (unlike galaxies - NO DARK MATTER) -  typical effective radii of r e = 3-5 pc Back seat bandits: Omega Centauri (ωCen) - old (12 Gyr), massive (5 x 106 M¤) -  metallicity spread (several generations of stars) core of a tidally stripped dwarf galaxy? - contains a 4 x 105 M¤ BH (Gemini observations) M15 : massive and most dense GC (core collapse), probably contains a central black hole.
4. M31 (Andromeda) Globular Clusters M31: - 1012 M¤ roughly 80% mass of the MW -  roughly 460 GCs Back seat bandits: Mayall/G1 : - massive (10 x 106 M¤) x2 mass of ωCen -  metallicity spread (several generations of stars) core of a tidally stripped dwarf galaxy? - large ellipticity Mayall II is named after Nicholas U. Mayall and O.J. Eggen in 1953. Sidney van den Bergh named it G1 in 1977.
5. Globular Clusters in Massive Elliptical Galaxies -  LF predicts 3000-8000 GCs -  single age (old !13 Gyr) -  bimodal metallicity populations red “metal rich” blue “metal poor” GCs are accreted? GCs form with galaxy?
6. -  SE region (30’ across) of the Fornax Cluster. (at d=20 Mpc, 30’~183 kpc) - NGC1399: v =1425 kms-1 (top right) is the central galaxy. -  The first compact object to be discovered is near NGC1404 (v=1947 kms-1) and has v =1591±36 kms-1. Intracluster GCs? The first hints… 1996: (Minniti et al. 1996). Small survey of NGC1399 globular clusters Object near NGC1404s identified as a very compact dwarf galaxy: V=18.5, (V- I) = I.48 1999: (Hilker et al. 1999) Images a large number of Fornax galaxies. Two compact objects are spectroscopically confirmed to be cluster members. “..can be explained by a a very bright GC or a compact elliptical like M32. Another explanation may be that these represent the nuclei of dwarf elliptical galaxies.” Image: CTIO 4m MOSAIC (g’,r’,i’), Karick, Gregg & Drinkwater
7. Fornax Galactic stars V=1491 kms-1
   
   V=1312 kms-1
   
   V=1591 kms-1
   
   V=1920 kms-1
   
   V=1355 kms-1
   
   Image: ~3x2 degree B,V,I mosaic, taken with the CTIO Curtis Schmidt Telescope (Karick, Gregg & Drinkwater ) +90 2dF follow-up AAOmega VLT/Flames
8. We find similar objects in the Virgo + 16 more from AAOmega spectroscopy “Ultra-compact Dwarf” (UCD) galaxies
9.   Unusually luminous globular clusters. (Hilker et al. 1999; Mieske et al. 2002) Extremely luminous star clusters formed in galaxy interactions. (Fellhauer & Kroupa 2002, Maraston et al. 2004) Remnant nuclei of dE,Ns or low luminosity spirals which have been tidally disrupted or “threshed” during passages close to the central cluster galaxy, NGC1399. (“UCDs”: Bekki et al. 2001, 2003) Extreme cases of known galaxies. (e.g. dE,Ns with very faint halos) Highly compact galaxies formed in the early Universe? (Blanchard et al. 1992, Tegmark et al. 1997) -  is there an upper mass limit for normal GCs? -  are the ICOs single age and metallicity like MW GCs? -  do the ICOs fall on the L ∝ σ 1/4 relation or is there are turnover point? -  smooth radial distribution? -  ICOs should be more concentrated than dEs -  requires low σ, radial orbits and very compact cores ωCen, M22, G1? -  ICOs should be more metal rich? -  mixed ages? W3? Possible explanations for IGCs
10. Follow-up Observations: HST/STIS imaging of UCDs HST/STIS imaging of 5 UCDs and a comparison Fornax Cluster dE,N, FCC303 (Drinkwater et al. 2003)   UCD profiles are well fitted by King and de Vaucouleurs R1/4 law profiles. Effective radii range from, r e = 10-22 pc. UCD3, the most luminous UCD, required a 60 pc scale length exponential halo. FCC303 (dE,N) has an 8 pc core and a 300 pc halo. FCC303 UCD1 UCD2 UCD3 UCD4 UCD5 Galactic GCs have typical effective radii of r e = 3-5 pc. The most compact normal dwarf galaxies in the Virgo Cluster have scale lengths of 160 pc.
11. Follow-up Observations: VLT/UVES & Keck/ESI Spectroscopy VLT/UVES and Keck/ESI Spectroscopy of 5 UCDs and a comparison dE,N, FCC303 (Drinkwater et al. 2003 & Evstigneeva et al. 2007) Velocity dispersions range from, σ = 24 - 37 km s-1 considerably higher than Galactic GCs. UCDs lie well off the GC: L ∝ σ1.7 relation in a previously unoccupied region, but on an extrapolation of the elliptical galaxy: L ∝ σ4 Faber-Jackson relation. Combining the HST data, mass-to-light ratios range from M/L ~ 2 - 4 solar masses. The largest GCs in M31 have M/L ~ 1-2 solar masses. The location of the UCDs and dE,Ns supports the “galaxy threshing” model (Bekki et al. 2001, 2003). A normal dE,N halo, accounting for ~98% of the dE,Ns luminosity is tidally stripped, reducing the total luminosity by a factor of 100 but barely changing the central velocity dispersion. Over time this disruption contributes diffuse stellar material to the ICM. The surviving nuclei are dispersed into intra cluster space or added to the envelopes of brighter galaxies where they can masquerade as stars.
12. Recent Results: Keck/LRIS Spectroscopy – part 1 Keck/ESI Spectroscopy of 5 UCDs and FCC303 (Gregg 2008., in prep)   Observations taken in September 2003 Short exposures but good S/N. 6Å resolution. UCD2 and UCD6 both have stronger Ca II H absorption than Ca II K, indicating a younger, ~1-3 Gyr stellar component. UCD1 has strong Mg, Hb absorption as well as a strong CN feature, similar to NGC1399. This suggests that it may have started out as a larger more metal rich stellar system.
13. What can UV observations tell us? Far UV: 1344 - 1786 Å (GALEX) Near UV: 1771 - 2831 Å (GALEX) -  hand wavy explanation: UV light traces star formation -  the UV light of an integrated population is an effective age indicator and can be used to break the well known “age-metallicity” degeneracy. HOW? -  the far UV is sensitive to the excess UV emission produced by EXTREME (Teff > 15,000K) horizontal branch stars (EBH). This “UV-excess” is observed in many early-type (elliptical) galaxies. Elliptical galaxies and large spiral bulges exhibit this excess for λ < 2000A by stars with T~22,000K WHY? -  EBH population is governed mainly by envelope mass-loss processes on the RGB and by the helium abundance, which in-turn is sensitive to the age and metal abundance of stars.
14. Remember Omega Centauri (ωCen) ? - old (12 Gyr), massive (5 x 106 M¤) -  metallicity spread (several generations of stars) core of a tidally stripped dwarf galaxy? - contains a 4 x 105 M¤ BH (Gemini observations) Double MS - large variation in He abundance -  large enrichment parameter (>90, ~2 for MW stars) -  larger number of “hot” stars - large FUV flux Canonical Old SP models: For a given metallicity, the Universe needs to be ~3-5 Gyr older than WMAP. i.e. normal level of He enrichment Would take longer to output the Observed UV flux.
15. Remember Galactic Globular Clusters? -  roughly 160 globulars (GCs) in the Milky Way -  90% lie within 100,000 light-years (0.5r stellar halo) -  single age (old !13.2), single (low) metallicity populations (all stars form at the same time) -  Very little or no HI gas and dust (stars + stellar winds drive out gas) -  M/L ratios ≈ 1 (NO DARK MATTER) -  typical effective radii of r e = 3-5 pc The rest of the Milky Way do not exhibit the same FUV-excess seen in ωCen. Neither do the M31 Globulars
16. …but some the M87 Globulars in Virgo are UV-luminous - 66 GCs with STIS (NUV+FUV) and optical photometry. -  distinct UV-optical properties compared to MW and M31 GCs. -  the most metal rich are near solar and overlap the local E galaxy sample in terms of their Mg2 line indices.
17. Sohn et al. 2006 Without He enrichment The GCs are too old
18. GALEX FUV + NUV observations of the Fornax Cluster ~ 34ks exposures in FUV (1344 - 1786Å) and NUV (1771 - 2831 Å) -  1.2 deg FOV with 1.5” pixels (~4.5” FWHM) -  UCDs and GCs are unresolved -  HST/ACS (V&I) photometry, CTIO (g,r,i,z) photometry
19. GALEX FUV+NUV, HST (V&I) and CTIO (g,r,i,z) photometry. galaxies stars
20. IMACS spectroscopy of Fornax UCDs + NGC1399 GCs Medium resolution (1.3A), high S/N (~50) spectroscopy of 32 Fornax UCDs, 61 GCs and 2 dE,Ns in order to measure line strength indices, especially Mg2 absorption.
21. Next time… Keck/HIRES Spectroscopy & Keck/LGSAO imaging of bright M31 GCs. - better velocity dispersion measurements - better core sizes and M/L ratios (for the GCs that have larger ellipticities and metallicity spreads)