1 / 27

NGC 4696

More Clues to Galaxy Formation: Massive Globular Clusters, Stochastic Self-Enrichment, and Mass/Metallicity Correlations. NGC 4696. HST/ACS. Harris && 2006. Pregalactic dwarf. Proto-GCs. Young massive star clusters (YMCs) forming at ~10 5 M 0 in starburst dwarfs today.

carrie
Download Presentation

NGC 4696

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. More Clues to Galaxy Formation: Massive Globular Clusters, Stochastic Self-Enrichment, and Mass/Metallicity Correlations NGC 4696 HST/ACS Harris && 2006

  2. Pregalactic dwarf Proto-GCs Young massive star clusters (YMCs) forming at ~105 M0 in starburst dwarfs today Starburst dwarf NGC 5253 (ESO/HST)

  3. Bimodal or not? Harris && 2008 Harris && 2006

  4. Disagreements ahead -- Bimodal or not? Harris && 2008 Harris && 2006

  5. Serious questions persist! Is this effect caused by --- (1) A gradual shift of the blue sequence to redder color at higher luminosity? (Mass/Metallicity relation) (2) The disappearance of bimodality altogether at the highest masses? (Threshold enrichment effect) (3) An artifact of photometric measurement procedures? (i.e. not real) If it’s a true, physical MMR then Z ~ M1/2 at high mass, and it may smoothly connect upward to the UCD regime. Does it continue to low mass? Why no red-sequence MMR? Is it present in all galaxies? What is its astrophysical origin?

  6. w Cen (Villanova && 2007) - Multiple populations within a single GC - Different scaling of size vs. mass Evstigneeva et al. 2008 The systematic properties of globular clusters begin to change for M > 2 x 106 M0 … - Appearance of the MMR

  7. Can be helped (partially) by constructing composite samples; e.g grouping Virgo Cluster Survey galaxies into 4 luminosity groups (Mieske && 2006) or combining several supergiants (Harris && 2006) But if amplitude of MMR differs from one galaxy to another, net effect will be diluted in composite samples The basic feature of bimodality is a first-order and (probably) universal effect. The MMR is a second-order effect and harder to trace. Though new, much confusion already exists: Category 1:MMR is present and measurable M87, NGC 1399, several other BCGs and gE’s Category 2:MMR is not present M49; any others? Category 3:presence of MMR not decidable; GC sample too small or does not extend to high enough luminosity Milky Way; M31; dwarf galaxies; most spirals; GC-poor E’s

  8. Most galaxies do not have clusters in the 106 – 107 M0 range 1: strong MMR 2: no MMR 3: Not decidable Milky Way GCs MV [Fe/H]

  9. First, let’s get the measurements straightened out. NGC 5128: d=3.8 Mpc Globular clusters are easily resolved at <1’’ seeing Photometry must account for individually different scale sizes GC profile as seen on image = PSF Intrinsic GC profile rh ~ 1 – 5 parsecs; averages 3 pc  0.3” width

  10. NGC 3311/3309 (A1060) d = 50 Mpc 2 rh ~ 6 pc  0.025” fwhm(PSF) = 0.5”  starlike! psf-fitting photometry is fine Several regimes determined by distance; no single photometric method is suitable for all regimes Gemini-S + GMOS, Wehner & Harris

  11. Aperture photometry r(ap) adjusted for D PSF-fitting photometry HST/ACS imaging of GCs around 6 central supergiants in Abell-type clusters (Harris et al. 2006, 2008) (B,I) bandpasses  metallicity-sensitive Thousands of GCs per galaxy, thus good statistical samples and big luminosity (mass) range 4 distinguishable regimes: compare fwhm of stellar PSF with intrinsic cluster size D (= 2 rh),half-light diameter Well resolved: D >> fwhm(PSF) Partially resolved: D ~ fwhm Marginally resolved: D ~ 0.1 – 0.3 fwhm Unresolved (starlike): D < 0.1 fwhm All this is subject to S/N considerations …

  12. HST/ACS Imaging program for BCGs NGC 1407 Eridanus d=23 Mpc MV = -22.35 NGC 3258 Antlia 41 Mpc -21.87 NGC 3268 Antlia 41 Mpc -21.96 NGC 3348 CfA69 41 Mpc -22.13 NGC 4696 Centaurus 42 Mpc -23.31 NGC 7626 Pegasus I 49 Mpc -22.58 (M87 Virgo 16 Mpc -22.4) (Partial list – biggest GCSs out of 12 studied) D = 6 pc at d ~ 40 Mpc  half-light profile width ~ 0.03” compare PSF fwhm = 0.1”  marginally resolved

  13. Photometric technique: • Uniform catalog of detected objects with DAOPHOT • Construct PSF from average of many bright starlike objects • For each individual source, convolve PSF with “King30” model GC profile and vary D(model) to obtain best match (ISHAPE; Larsen 1999) • finally, use fixed-aperture photometry corrected for profile width to obtain final magnitude in each band ISHAPE sample fits 1 px = 0.05” HST/ACS S/N=441 fwhm a=1.3 px b/a = 0.91 S/N=24 fwhm a=0.82 px b/a = 0.50 S/N=108 starlike

  14. Simulations show that the systematically correct intrinsic D (FWHM of GC profile) is returned for D > 0.1 (PSF) (transition boundary from unresolved to marginally resolved) Our regime Growth curves for simulated GC profiles convolved with PSF ISHAPE  solve for best-fit D Measure magnitude through 2.5-px aperture, corrected back to the growth curve for a starlike profile

  15. S/N > 50 !! More tests … Measured size a not affected by modestly elliptical shape b/a, q returned correctly for a > 0.1 PSF

  16. Full, profile-corrected aperture photometry for 6 supergiant ellipticals Previous PSF-fitting data (Harris && 2006) • Trend lines: • blue and red? • linear slope? or top end only? • how steep? N=12000 brighter than MI = -8. Largest sample in existence!

  17. Red sequence: no trend Blue sequence: gradual changeover to MMR toward higher mass Z ~ M0.3+-0.1 RMIX fits of bimodal gaussians within selected magnitude intervals: forces two modes into the solution, but (a) less affected by field contamination, (b) avoids the strong assumption imposed by a ‘linear fit’

  18. The top end: uni- or bi-modal?

  19. A detour: the measured cluster sizes

  20. Trends (?) versus galactocentric distance and metallicity: projection effects, or intrinsic? Low-metallicity GCs average larger size at any galactocentric zone

  21. The MMR is not due to an unaccounted-for size-mass relation.

  22. Input assumptions to self-enrichment model: SNe from >8 M0 stars enrich lower-mass stars while still in formation Salpeter IMF 0.3  100 M0 and SF efficiency f* ~ 0.3 Woosley/Weaver SN yields, and fraction fZ of heavy elements retained in GMC and thus What is responsible for the metallicity distribution function (MDF)? Bailin & Harris 2008 Is a proto-GC - PRE-enriched from the surrounding GMC gas? - internally SELF-enriched by its own SNe within the first few Myr? - stochastic? (can self-enrichment be responsible for the internal dispersion of the MDF?)

  23. Pre-enrichment level for fZ = 0.08 Internal dispersion of MDF due to statistical variation in NSN NSN ~ 1 per 100 M0 Stochastic self-enrichment fails to explain the MDF dispersion at any cluster mass higher than 104 M0

  24. Proto-GC = truncated isothermal sphere  logarithmic potential F(R). All SNe go off while PGC is still highly gaseous; all ejected energy absorbed and thermalized. Gas will leave if outside an “escape radius” defined by total energy > potential energy at edge of cloud. Ejecta become efficiently retained at a characteristic mass (after star formation) Two additional, major factors to add: - reff ~ M1/2 at high mass - fZ is a strong function of M(init) and thus reff as well

  25. Match to BCG data for 6 galaxies • pre-enrichment of each “mode” (blue, red) tuned to match mean color • self-enrichment drives shape of mean MDF at high mass Combined effects of pre-enrichment, self-enrichment, and mass/radius relation

  26. Wehner && 2008 • Basic features of the model: • No MMR for cluster masses < ~106 M0 (i.e., sequences vertical) • Very metal-poor, very massive GCs should be rare (anywhere) • blue and red sequence converge at high-mass end • Similar red-sequence MMR should exist at top end, but smaller amplitude • Internal dispersion and mean metallicity of each mode driven by pre-enrichment

More Related