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The Galactic Bulge

The Galactic Bulge. R. Michael Rich (UCLA) , Christian Howard, David Reitzel, (UCLA), Andreas Koch (Leicester), HongSheng Zhao (St. Andrews), Roberto de Propris (CTIO), Andy McWilliam (Carnegie), Jon Fulbright(JHU), Livia Origlia (Bologna) Annie Robin (Besancon). Support: NSF AST-0709479.

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The Galactic Bulge

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  1. The Galactic Bulge R. Michael Rich (UCLA), Christian Howard, David Reitzel, (UCLA), Andreas Koch (Leicester), HongSheng Zhao (St. Andrews), Roberto de Propris (CTIO), Andy McWilliam (Carnegie), Jon Fulbright(JHU), Livia Origlia (Bologna) Annie Robin (Besancon) Princeton 2009 Support: NSF AST-0709479

  2. Motivation • “Spheroids” 50-70% of stellar mass in local Universe. (Fukugita et al. 1998) • Lyman-break (z > 3) galaxies metal-rich star formation and evidence for metal-rich winds. Chemical evolution. • Epoch, population of bulge formation?? EROs? BzK, LBG? Other? • Unique formation history, stellar population (link to extragalactic spheroids, high Mg2 index (Worthey et al. 1993) • Bulges host supermassive black holes. Formation process still unknown. Related to bulge formation? (Gebhardt et al., Ferrarese et al., Ghez et al.). • Metal rich halos are widespread: M31 (Durrell 1994, Rich 1996) other galaxies (Mouhcine et al. 2005). • ~15 hot Jupiter transit planets discovered using HST imaging of Bulge field (Sahu et al. 2006). Princeton 2009

  3. Composition is a window into galaxy formation and chemical/dynamical evolution • Type II supernovae are produced by massive stars (>8 M) on very short (107 to 108 yr) timescales. • Type I supernovae produce Fe-peak elements on longer ~109 yr timescales. • Produce large amounts of the elements through the Fe-group (Z < 31) and probably some r-process. • Known for high α-element (O, Mg, Ne, Si, S, Ar, Ca, Ti) to Fe yields. • Detailed production dependent on progenitor mass: • O, Mg primarily produced by M > 25 M progenitors. • Si, S produced in the 15-25 Mrange. • Ca, Ti through Zn produced over nearly all masses. Princeton 2009

  4. Bulges appear be either spheroidal (classical) or barlike (pseudobulge) Canonical formation picture is that spheroidal forms via early mergers, while pseudobulges/bars evolve from a buckling instability over longer timescales. Milky Way has dynamics characteristic of pseudobulges, yet age/chemistry consistent with rapid formation. Princeton 2009

  5. High [O/Fe] reflects early burst of star formation Matteuci et al. 1999 Princeton 2009

  6. Imelli et al. 2004; Elmegreen et al. (2008) - major merger origin Clumps dissipate rapidly into bulge or Classical early merger.. Multiple star forming clumps might produce kinematic groups with distinct chemical fingerprints. Abadi 2003 Imelli et al. 2004 Princeton 2009

  7. UV-Luminous Galaxies in the Local Universe (Lyman Break Galaxy Analogs) Overzier et al. 2007 Princeton 2009

  8. N-body bar models attractive for representing the bulge However, extended formation models favored; bar survival? Bar dissolves due to central mass (Norman et al. 1996) Combes 09- bar resurrection via gas inflow Vertical thickening of the bar into a bulge would leave no abundance gradient in the z-direction. Princeton 2009

  9. Do some bulges populate halos?Do bulge debris populate halos? Mouhcine, Ferguson, Rich, Brown, Smith 2005 ? M31 halo Koch et al. 2007 Princeton 2009

  10. “bulge” appears to be distinct structure 2MASS Oph SFR (foreground) Baade's Window Sgr dSph (28 kpc) Wyse et al. (1997) Princeton 2009

  11. The Inner Bulge < 200 pc: A complex picture Outside of 200 pc the bulge is dominated by old stars; bar angle ~20o (Gebhardt et al. 2002); bar has 3 kpc length The Galactic Center region has star formation, supermassive black hole Serabyn & Morris (1996) argue that the nuclear region is dominated by the r^-2 cusp. Launhardt et al. 2002 map it. Nuclear region has continuous star formation but has some old stars. Launhardt et al. 2003 Princeton 2009

  12. Bar structure from red clump distance; Babusiaux & Gilmore 2005 Princeton 2009

  13. Stellar Evolution in the Bulge AGB 10^7 yr (Mira, gM OH/IR PNe Red clump Red Giant Branch (RGB) ~ 5x10^8 yr TiO bands can cause RGB tip to be faint Horizontal Branch (HB) RR Lyr EHB ?? Far-UV Main sequence ~10^10 yr H-burning 1 Lsun Princeton 2009

  14. Simple model of chemical evolution known since Rich (1990) Simple model fit (Rich et al. 07)to Zoccali et al. 2008 data at -6o Princeton 2009

  15. BRAVA First proposal 2003 Rich et al. 2007 ApJ 658, L29 Rich+ 2007 astro-ph/0710.5162 Howard et al. 2007 astro-ph 0807.3967; ApJ in press. Strategy: Use M giants brighter than clump that can be observed even in high extinction fields Select M giants from 2MASS survey (excellent, uniform, astrometry and photometry; ease of developing links to spectra for a public database Clear red giant branch easily seen in 2MASS data Cross correlation from 7000 - 9000A (include Ca IR triplet) Abundances from either future IR studies or from modeling of optical spectra 3x10 min exposures with Hydra fiber spectrograph at CTIO Blanco 4m; ~100 stars/field R~4000 8,000 stars to date Princeton 2009

  16. All stars pass through K/M giant phase; avoid region of CMD with overlap from disk and nearby red clump [Fe/H]=-1.3 and +0.5 RGB Adjust selection for reddening Princeton 2009

  17. Full sample, dereddened Princeton 2009

  18. Sample BRAVA spectra Princeton 2009

  19. Princeton 2009

  20. Earlier state of art PNe Beaulieau et al. 2000 Vrot sigma Princeton 2009

  21. Zhao (1996) self-consistent Rotating Bar model Schwarzschild method Surf brightness, dynamics constrain orbit families Can populate N-body bar with particles launched according to the model prescription Model can be modified to respond to new data Princeton 2009

  22. Solid body rotation not present at -4o; Zhao model needs more retrograde orbits Data Zhao (1996) Princeton 2009

  23. Rebinned Beaulieau data (light open crosses) show Pne agree well with rotation curve but dispersion curve low. Pne data courtesy S. Beaulieau Princeton 2009

  24. Rotation curve turns over from solid body at ~ 0.5 kpc Clarkson et al. 2008 (PM) Howard et al. 2008 (BRAVA RV) Princeton 2009

  25. Kinematics independent of color, luminosity Samples bifurcated by Red/blue or bright/faint are identical. This was not the case in Sharples et al. 1990; we have followed their exclusion of bright likely foreground disk stars. Princeton 2009

  26. -4o velocity distributions Princeton 2009

  27. Larger samples have not confirmed 2 stream candidates; all candidates will be followed up. Reitzel et al. (2007) simulations suggest ~1 “real” stream Stream followup important. Possible origins from disrupted globular clusters or dwarf galaxies, groups of stars in unusual orbit families; all candidates presently assumed to be Poisson statistics caused. Princeton 2009

  28. Cold streams? Disk l=-30; sigma 46 km/s No coincidence with Sgr dwarf at +140 k/s Princeton 2009

  29. Summed and shifted data show no departure from Gaussian: No evidence of cold (disky) or hot (halo-like) subcomponents Minor axis, N=822 Minor + b=-4o major, N=3022 Princeton 2009

  30. l-v plot also shows no cold (disk) or hot (spheroid) components Princeton 2009

  31. New Results at -8o (Howard et al. 2008 Princeton 2009

  32. Disk -30,-4 The -8o field lacks hot or cold subocomponents population, even at l=+/- 8 Summed and velocity shifted Princeton 2009

  33. BRAVA data at -8o (1kpc): Evidence of cylindrical rotation. Also note solid-body like rotation field at -8o Howard et al. 2008 Kormendy & Illingworth (1982) found cylindrical rotation for NGC 4565; Samson et al. (2000) found abund. Gradient. Princeton 2009

  34. -4 -8 Zhao Princeton 2009

  35. Fabry-Perot Surveys: Rangwala et al. 2008 Narrow band image Radial velocities Line profile Princeton 2009

  36. Rangwala 2008 R.V. for all stars with Ca T gives Large numbers in velocity distributions NGC 6522 Princeton 2009

  37. Generally good agreement between FP and BRAVA but FP finding higher dispersions for red clump compared to gM stars at l+/- 5o gM should evolve from RCG so dispersion a mystery Princeton 2009

  38. Contradiction: Abundance gradient in the outer bulge Cylindrical rotation a characteristic of pseudobulges, but should not exhibit abundance gradient, since buckling models are not dissipative. Location on Binney plot similar to NGC 4565. Zoccali et al. 2008 Princeton 2009

  39. No minor axis rotation; more data needed Princeton 2009

  40. Survey Fields 2005: blue 2006: red 2007: green Goal: Grid of fields at 1 deg intervals, covering 10x10 deg box, pushing as close to plane as possible Princeton 2009

  41. The Milky Way shares much in common with NGC 4565 (peanut bulge, abudance gradient) Kormendy & Kennicutt 2004 MW Princeton 2009

  42. Kormendy Illingworth 82 Proctor et al. 00 Princeton 2009

  43. The Age/Pseudobulge Paradox Clarkson et al. 09 Fux N-body bar is best fit Howard et al. 09 ~99% of bulge older than 5Gyr; pure 10+ Gyr likely (Clarkson+ 08, 09 Cylindrical rotation, morphology, consistent with pseudobulge (young?) Abundance gradient of MW, NGC 4565 – but how? If N-body models? Princeton 2009

  44. The Age/Pseudobulge Paradox Fux “disk” Fux N-body bar is best fit Howard et al. 09 Fux spheroid Princeton 2009

  45. Bulge Spectroscopy Before Keck Low resolution spectroscopy; challenging due to high extinction, stellar crowding, high metallicity. High resolution spectroscopy almost impossible (McWilliam & Rich 1994 analyzed CTIO echelle spectra of 11 giants). R=16,000,S/N=30 Galactic globular clusters, disk, and halo composition well understood; bulge remains as last frontier. Princeton 2009

  46. Bulge Spectroscopy After Keck HIRES: R=45,000 and higher, S/N~60: metal rich giants feasible NIRSPEC: R=25,000 cross-dispersed infrared echelle, opens both cool red giants and optically obscured stars Note: NIRSPEC remains as the only cross-dispersed infrared echelle spectrograph with R>20,000 on an 8-10m class telescope. Need for atmospheric dispersion standard star, flat field, arcs, for each wavelength setting makes other IR echelles (Phoenix, CRIRES) far less efficient, especially for faint stars. Princeton 2009

  47. Sample Spectra: Relative Intensity Wavelength (Å) Princeton 2009

  48. Keck Bulge Giants: Baade’s Window (Shifted) Relative Intensity Wavelength (Å) Wavelength (Å) Princeton 2009

  49. A New Approach to Abundance Analysis • Use the thick disk giant Arcturus, not the Sun, as the abundance standard. • Issues like non-plane parallel atmosphere, mass loss, CN molecule opacity, etc. are similar between Arcturus and the bulge giants. • Derive Arcturus-based gf values for weak iron lines that remain on the linear part of the curve of growth even in metal rich stars. Confirm the line list with the local metal rich giant mu Leo. • Result: robust iron abundance scale, and confirmation of stars with [Fe/H]=+0.5 • Method also adopted by Lecureur et al. 2007. Princeton 2009

  50. Fulbright, McW, Rich 2006 definitive bulge iron abundance relative to Arcturus. New linelist is usable over the full abundance range of our sample. Princeton 2009

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