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The Formation Histories and Mass Profiles of Some Elliptical Galaxies

The Formation Histories and Mass Profiles of Some Elliptical Galaxies. Steve Zepf Michigan State University. Overview – Three Questions. 1. Metal-poor globular clusters and early structure formation (the biasing of the metal-poor GC population)

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The Formation Histories and Mass Profiles of Some Elliptical Galaxies

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  1. The Formation Histories and Mass Profiles of Some Elliptical Galaxies Steve Zepf Michigan State University

  2. Overview – Three Questions 1. Metal-poor globular clusters and early structure formation (the biasing of the metal-poor GC population) 2. When were the major formation epoch(s) of elliptical galaxies? (ages of their metal-rich GCs) 3. What are the properties of the dark matter halos around elliptical galaxies? (radial velocities of the globular clusters) topics left out – GC formation, dynamical evolution, low-mass X-ray binary GC conncetion

  3. Why Globular Cluster Systems? • GCs have ~106 stars within a few pc - remarkable densities - readily observable • some GCs are very old – hope of constraining very early structure formation. • GCs are true simple stellar populations (to first order), so determining their age and metallicity is much simpler than for the composite populations in integrated light. • GCs are observed to form in all major star formation events in galaxies. • GCs are found out to very large radii around galaxies – useful dynamical probes of galaxy halos.

  4. one example – NGC 3256 Zepf et al 1999

  5. Another example – the Antennae, Whitmore et al.

  6. Why Globular Cluster Systems? • GCs have ~106 stars within a few pc - remarkable densities - readily observable • some GCs are very old – hope of constraining very early structure formation. • GCs are true simple stellar populations (to first order), so determining their age and metallicity is much simpler than for the composite populations in integrated light. • GCs are observed to form in all major star formation events in galaxies. • GCs are found out to very large radii around galaxies – useful dynamical probes of galaxy halos.

  7. • Distribution because individual GC colors/metallicities can be determined. • Early results from Zepf & Ashman 93 and subsequent work suggest bimodality in optical colors of GC systems of elliptical galaxies. • Large samples establish bimodality as the norm (>60%) for ellipticals, S0s consistent but less certain. e.g. Kundu & Whitmore 01 ~30 Ellipticals and ~30 S0s Bimodality in GC Systems of Elliptical Galaxies

  8. • For mostly old systems, color traces metallicity, use Milky Way and models to do this (age-met degeneracy still present). • Can’t get the two peaks in the observed colors from a single-peaked metallicity distribution. GCs come in two metallicity groups . • Metal-rich GCs - closely tied to bulk of host galaxy - trace galaxy formation/assembly. • Metal-poor GCs - Low metals, extended spatial distribution, and old ages in Milky Way halo - constraints on early structure formation? Note this division in abundance also seen in Galaxy GC system Color and Metallicity Distributions

  9. Formation Sites of Metal-Poor GCs Metal-poor GCs include the oldest surviving fossil structures. When and where they formed constrains early star formation in the universe. Formation redshift can be addressed by determining the “biasing” in GC numbers vs galaxy mass (Rhode & Zepf 2004, Rhode, Zepf, & Santos 2005). Basic idea (Santos 2003)… • More massive galaxies have a greater fraction of their mass collapsed at a given redshift than lower mass galaxies on average. • If metal-poor globular cluster formation has a preferred early formation epoch, then more massive galaxies will have more metal-poor GCs per stellar mass. The earlier the formation epoch, the greater the “bias” in the metal-poor GC population today. • Need observational determination of total number of metal-poor GCs in galaxies with a range of masses  Rhode & Zepf 01,03,04

  10. Observational Requirements • Both wide-field and deep imaging (many extant data only cover ~10% of the GC system). • Need colors to separate blue and red GCs to isolate metal-poor GCs for very early formation. • Katherine Rhode’s PhD thesis at Yale. • Utilize large-area CCDs, multiple colors, new analysis techniques to make both these advances for sample of 4 nearby ellipticals and 9 spirals (ellipticals in Rhode & Zepf 2004, 2001, spirals in Rhode & Zepf 2003 and in prep).

  11. NGC 4472 – 34’ x 34’ field. Ground-based Mosaic image and GC identification Rhode & Zepf 2001, 2004; WFPC2 images in blue, ACS in red

  12. Results: Early-type Sample (Rhode & Zepf 2004, AJ, 127, 302) NGC 4594: 1748 GC candidates selected in BVR NGC 4406: 1400 GC candidates NGC 3379: 321 GC candidates Mosaic image of NGC 4594 FOV = 38’ x 37’, FWHM ~ 1”

  13. Results: Early-type Sample (Rhode & Zepf 2004) Cluster Es Field E/S0s

  14. Results: Early-type Sample (Rhode & Zepf 2004) • Color Distributions • Two (or more) Gaussians fit better than one at >99.9% confidence • NGC 4472, NGC 4406, and NGC 4594 have 60% blue, 40% red GCs • NGC 3379 has 70% blue, 30% red •  Disagrees with collapse+accretion and multi-phase SF models, which predict larger proportions of blue GCs in more luminous galaxies

  15. T  NGC/109MSun Determined observationally for metal poor GCs in galaxies with a range of galaxy masses. Some biasing observed Rhode, Zepf, & Santos 2005 ApJL Observational Results

  16. Hierarchical model with Greg Bryan Most systematics tend to flatten the trend in the data. most likely not extremely early formation zform < ~11 Formation Redshift

  17. Ages of Formation Epoch(s) of Elliptical Galaxies – Why “Metal-Rich” GC Systems? • Metal-rich GC systems trace elliptical galaxy populations spatially and in color. • GC formation observed in all starbursts in local universe – trace major formation episodes. • Each individual GC is a simple stellar population, most basic determination of ages and abundances. GC systems reveal distribution in age and metallicity of major formation events of ellipticals.

  18. Ages of the Formation Epoch(s) How to get ages of GC populations? Optical colors good for finding metallicity peaks, but bad for ages because of age-metallicity degeneracy –older age and higher metal content have same effect on optical colors (colors can be red because of old age or high metal content). Ideas… • luminosity function (assumes mass function) • Balmer absorption lines • optical to near-infrared colors  efficient, effective

  19. I-H color primarily sensitive to metallicity, g-I color more sensitive to age. True of any set of optical to near-infrared color vs optical color – consequence of basic stellar properties. Difference between 3 Gyr (int. age) and 13 Gyr (old age) is about 0.3 mag. Breaking Age-Metallicity Degeneracy with Near-IR to Optical Photometry

  20. First try with this technique – Puzia, Zepf, Kissler-Patig, Hilker, & Minniti 2002 NGC 3115 – primarily consistent with typically old GCs NGC 4365 – surprise! – many GCs with VIK colors only explained by intermediate ages. Important result indicating some elliptical galaxies had substantial formation activity at t <~ 5 Gyr. NGC 3115, NGC 4365 in VIK

  21. Implications Not without controversy • Most galaxy light studies suggest older ages for integrated stellar light of NGC 4365. • Spectroscopy of GCs in NGC 4365 confused – initial confirmation of int. age by Larsen et al. 2003, similar data from same instrument, different result, Brodie et al. 2005. •Only case of possible inconsistency between optical to near-ir colors and optical spectroscopy for GC systems (agree for NGC 1316, NGC 1399, NGC 3115, NGC 4472) • Get better data! Three deep NIC3 pointings for NGC 4365, very high S/N

  22. Independent, deep data – data deep enough to separate optical and near-ir colors. Same result as before – substantial population of NGC 4365 GCs with optical/near-ir colors only accounted for with int. age. Comparison with other archival NIC3 data for NGC 1399 GCs. Mostly old GCs in NGC 1399, with a few int. ages, same as seen in previous spectroscopy. New NICMOS data(Kundu, Zepf, Hempel et al 2005 ApJ, 634, L41 )

  23. Simple test, adopts two populations, one at 13 Gyr, gets best fitting age and fraction of 2nd population. NGC 1399 result of ~10% int. age population same as seen in small spectroscopy sample. NGC 4365 substantial int. age result confirmed Simple Fraction of Int. Age GCs

  24. Is it age? • Test colors by getting much better, independent data. Deep NICMOS H-band imaging of NGC 4365. Much reduced photometric uncertainties.  Colors confirmed, intermediate ages show up in independent, deep data

  25. Is it age? • Test colors by getting much better, independent data. Deep NICMOS H-band imaging of NGC 4365. Much reduced photometric uncertainties.  Colors confirmed, intermediate ages show up in independent, deep data • What about the stellar populations models?

  26. Kundu, Zepf & Hempel 2006 New NIC3 data in center, archival NIC data in outer field with new ACS data. Models lines appear to be mostly correct. Dominant old age in agreement with optical color bimodality and spectroscopic results for M87 M87 – deep NIC3 GO and AR data

  27. Is it age? • Test colors by getting much better, independent data. Deep NICMOS H-band imaging of NGC 4365. Much reduced photometric uncertainties.  Colors confirmed, intermediate show up in independent, deep data • What about the stellar populations models?  Other galaxies match old model prediction lines very well. Can’t shift model lines.

  28. Comparison of same NIC data for NGC 4365 and M87 reveals clear difference – only known way to account for this is intermediate age for substantial fraction of NGC 4365 GCs. More NICMOS Results (Kundu, Zepf, Hempel 2006 in prep)

  29. Is it age? • Test colors by getting much better, independent data. Deep NICMOS H-band imaging of NGC 4365. Much reduced photometric uncertainties.  Colors confirmed, intermediate show up in independent, deep data • What about the stellar populations models?  Other galaxies match old model prediction lines very well. Can’t shift model lines. only extant explanation that works for NGC 4365 optical to near-ir is intermediate age. • Optical to near-ir color technique is efficient and effective.What about a galaxy sample?

  30. Optical to near-ir to date (Hempel, Kundu, Puzia, K-P, Geisler, Zepf)

  31. Sample in words NGC 4472 – old NGC 3115, NGC 7192, M87 – old with possible “frosting” NGC 1399 – old with definite ~10% intermediate age NGC 5846 – probable substantial int. age component NGC 4365 – clear intermediate age component Data in hand – Cen A, NGC 3379, NGC 4594 Notes: old for colors and spectra just means t > ~8-10 Gyr, z >~1 Tests higher metallicity GCs, not lower metallicity ones. Expect higher fraction of younger ones at brighter limits. Stochastic effects from finite number of stars come into play. Hempel, Kundu, Puzia, Kissler-Patig, Geisler, Zepf, et al.

  32. Overview of Ages • Current data indicate a modest but significant fraction of Es have substantial intermediate-age GC population in metal-rich population (major formation even at z < 1). Majority have major formation events at z > 1. Of this majority many have small population of intermediate-age GCs (e.g. NGC 1399). • Overall seems consistent with hierarchical merging – some merging today, increasing significantly to the past. • Large optical, near-ir GCS survey enable much more detailed tests (NIC3, SOAR,...). Combine with targeted optical spectroscopy.

  33. Kinematics of Globular Cluster Systems Address two questions 1. Dark matter around elliptical galaxies – how much, spatial distribution? Globular clusters one of best probes way out into the halos. 2. Angular momentum in elliptical galaxies. Why don’t they have any (in their inner regions anyway). Search for angular momentum transported outwards predicted by models.

  34. Globular Clusters provide observable individual objects at very large radii for which the sky dominates the integrated light. e.g. NGC4472 giant Virgo E Photometry - KPNO Mosaic Camera (Rhode & Zepf 2001, 2004) Obviously useful as dynamical probes of halo mass distribution The Utility of GCs for Halo Kinematics

  35. Do Lower Luminosity Ellipticals have Expected Dark Matter Halos? •A few recent results suggest maybe lower luminosity ellipticals are lacking dark matter halos or have halos with either low concentration or modest M/L (e.g. Romanowsky et al). •Test this with VLT multi-fiber observations of globular clusters over a very wide field around best case L* elliptical galaxy, NGC 3379. 1. Confirm or reject previous result. 2. Extend kinematical information to larger radii.

  36. Bergond, Zepf, Romanowsky, Rhode, & Sharples 2006, A&A, 448, 155 • Red points and error bars GC dispersions with radius. • Models:CDM-like halos with standard concentrations in blue hatched region. Blue dashed lines region covered by halo occupation models. Purple line best fit from outer HI ring and CDM halo with most likely concentration. Bottom dashed line is mass traces light.  GC velocity dispersions are in agreement with CDM-like halos. Results for NGC 3379

  37. Angular Momentum in Galaxies • General idea – protogalaxies torqued up by tidal interactions with other protogalaxies, have “spin parameter”   0.05 ( = J E^1/2 / G M^1/2), goes back to Hoyle, confirmed in modern cosmological simulations, mostly independent of mass and environment. • Halos to galaxies – Baryons that make up the parts of the galaxies we see cool, collapse, and spin up. •Similar densities of ellipticals and spirals suggest ellipticals and spirals collapsed by roughly the same amount. But ellipticals have an order of magnitude or more less angular momentum. What happened to ellipticals? •Need to transfer angular momentum. This has long been realized, as has the utility of hierarchical structure formation and merging to transfer ang momentum. •Can we find the angular momentum at large radii around ellipticals?

  38. Kinematics of GC Systems NGC 4472 • Our new VLT Flames data for NGC 4472, up to ~350 velocities out to ~ 75 kpc, builds on 144 GCs (Zepf et al 2000, Sharples et al 1998) to ~ 40 kpc, and another 100 over same area from Cote et al. (2003). Extant results (Z2000 et al) • not much evidence for rotation anywhere (to ~50 kpc). • Metal-poor GC population may have modest rotation. Higher  than more spatially concentrated metal-rich GCs. • No evidence for rotation in metal-rich GC population about any axis -- Upper limit on v/ < 0.34 (99%) Metal-rich GC pop not rotationally supported. Significant angular momentum transport required. Implicates mergers, But where did it go? • M87 consistent with 4472, evidence for ang mom in outer regions, M104 may have counter-rotating outer GCs.

  39. Kinematics of GC Systems NGC 4472 • Our new VLT Flames data for NGC 4472, up to ~350 velocities out to ~ 75 kpc, builds on 144 GCs (Zepf et al 2000, Sharples et al 1998) to ~ 40 kpc, and another 100 over same area from Cote et al. (2003). Extant results (Z2000 et al) • not much evidence for rotation anywhere (to ~50 kpc). • Metal-poor GC population may have modest rotation. Higher  than more spatially concentrated metal-rich GCs. • No evidence for rotation in metal-rich GC population about any axis -- Upper limit on v/ < 0.34 (99%) Metal-rich GC pop not rotationally supported. Significant angular momentum transport required. Implicates mergers, But where did it go? • M87 consistent with 4472, evidence for ang mom in outer regions, M104 may have counter-rotating outer GCs.

  40. SUMMARY • Globular Cluster Systems show the episodic formation history of their host elliptical galaxies. • Not all of these episodes are old (intermediate-age GC populations in normal ellipticals). Broad agreement with hierarchical merging models. • Optical to near-infrared colors of GC systems efficient, straightforward way to constrain ages of major formation episodes. • Metal-poor GCs are “biased” with galaxy mass, potential to constrain their formation sites/epochs. • Dark matter halos ubiquitous around elliptical galaxies. • Angular momentum transport needed for elliptical galaxy formation  mergers. Out to large radii in at least one well-studied case.

  41. Hempel, Zepf, Kundu, & Geisler 2006 Ongoing survey, NGC 4472 results show old ages in one of strongest bimodal cases, as expected. NGC 4472 – new data

  42. Models on previous page Bruzual-Charlot, here Anders & Fritze v. Alvensleben and Maraston. Basically same answers. Optical to near-ir approach is astrophysically straightforward. Insensitivity to Models

  43. Kundu, Zepf et al optical to near-ir colors of objects with spectroscopy. Note optical to near-ir data around re , B05 at large r . NGC 4365 – photo and spect.

  44. Derived from • Spherical Jeans eqn and isotropic orbits • smoothed  distribution • Observed GCS profile Compared to • Mass profile from hot gas observed in X-rays (Irwin & Sarazin 1997) Hydrostatic equilibrium for gas and isotropic orbits for stars give the same answer within the errors. (M/L)B  12 at 34 kpc, increases roughly as sqrt (r) to larger radii. Mass Profile of NGC 4472

  45. V-I colors have more age sensitivity than V-K colors  V-I, V-K can break age-metallicity degeneracy. Plot on the right shows the Milky Way and M31 GCs and various models for 12 Gyr and a range of [Fe/H] Younger ages will have blue V-I for a given V-K. Note good agreement of models with CMD ages. Potential of near-IR photometry

  46. Low-Mass X-ray Binary -Globular Cluster Connection • A LMXB is a compact object accreting from a companion low-mass star. • LMXBs are X-ray bright – typical L ~ 1037 ergs/s Extragalactic obs are more typically 1038 • In Milky Way, only ~13 LMXBs in Galactic GCs. This is 10% of total LMXB population, even though less than 0.1% of the stars are in GCs. 1) Need more numbers for stats and 2) LMXBs are clearly over-represented in GCs – dynamical formation in dense environment.  Great progress in extragalactic studies

  47.  Much of the X-ray emission from early-type galaxies is from LMXBs  Many (~50%) of the LMXBs are in globular clusters NGC 4472 as an example Chandra image in Grayscale HST-WFPC2 dark outlines Chandra sources circled, black circles are matches with GCs found with HST. Kundu, Maccarone, & Zepf 2002, ApJL

  48. What observables determine LMXB presence? Kundu, Maccarone, & Zepf 2002, Sarazin et al. 2003, MKZ03 KMZP03, Jordan et al. 2004, KMZ 2004… • Globular cluster luminosity (mass) – probability of a LMXB scales roughly linearly with GC mass • Globular cluster color (metallicity) – probability of a LMXB roughly 3x larger for red (metal-rich) GCs relative to blue (metal-poor) GCs Statistically marginal correlations with globular cluster size and distance from galaxy center. Metallicity effect unexpected, what’s the evidence?

  49. NGC 4472 – KMZ02 Small points - globular clusters Dark Points – LMXB – GC matches Shows LMXBs preferentially in higher L,redder GCs. Metallicity effect at very high significance. KMZ 2002

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