Bulges of Spiral Galaxies: Stellar Populations, Structure, and Kinematics

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Bulges of Spiral Galaxies Stellar Populations,
Structure, and Kinematics

• Bhasker Moorthy
• Jon Holtzman
• Anatoly Klypin
• New Mexico State University

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Motivation Why Bulges?

• Key to understanding origin of Hubble sequence
• Bridge together properties of disks and
  ellipticals
• Provide insight into a wide range of mechanisms
  involved in the formation and evolution of
  galaxies
• Merging and accretion
• Star formation
• Feedback from SN II and SN 1a
• Galactic winds
• Secular processes - bar formation, vertical and
  radial transport, disk heating, new star
  formation, bar destruction?

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Early-typed bulges Similar to ellipticals
Late-types Similar to disks

• Correlation in scale lengths (eg MacArthur et
  al. 2003)
• Small Sersic indices (eg Balcells et al. 2003)
• Smaller SSP ages than ellipticals (Proctor
  Sansom 2002)
• Similar colors (eg MacArthur et al. 2004)
• Emission lines (Prugniel et al. 2001)
• B/P bulges bars (eg Chung Bureau
  2004 Athanassoula 2005)
• Rotational support (eg Kormendy Illingworth
  1982)

• Same or similar fundamental plane relation
  (Falcón-Barroso et al. 2002)
• Similar light profiles (eg Baggett et al. 1998
  Carollo et al. 1997)
• Similar luminosity-weighted (SSP) ages and Mg-s
  relations (eg Idiart et al. 1996)
• Color and line strength gradients (Balcells
  Peletier 1994 Fisher et al. 1996)
• Milky Way bulge stars are predominantly old with
  larger Mg/Fe ratios than disk stars (eg Feltzing
  Gilmore 2000 Fulbright et al. 2004)

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Stellar Populations and Formation Mechanisms

• Luminosity-weighted ages and abundance ratios
  (eg. Mg/Fe) constrain epochs and duration of star
  formation
• Similarities between bulge and disk populations
  suggestive of secular evolution
• Dissipationless Secular Evolution
• Decrease in scale length might amplify
  metallicity gradients while increase in velocity
  dispersion might wash them out
• Disk-driven evolution with gas funneling
• Could trigger new star formation, producing a
  negative metallicity gradient (Friedli et al.
  1994)
• Not necessarily secular

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Our Project

• Study stellar populations, structure, and
  kinematics of bulges as a means of constraining
  their formation mechanisms
• Particularly interested in seeing
• whether or not stellar populations
• show evidence for secular evolution
• Long-slit spectroscopy with
• ARC 3.5m/DIS at APO
• Wavelength coverage 4000-8000 Å at
• 6-8 Å resolution
• Absorption (Lick indices) and emission
• lines
• Rotation curves and velocity dispersion profiles
• Imaging with ARC 3.5m/SPIcam
• Bulge-to-disk decomposition to determine disk
  contamination and obtain structural properties

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The Galaxy Sample

• 38 nearby (vreslt7000 km/s) mostly isolated
  galaxies spanning a wide range in Hubble type
  (S0-Sc)
• Selected galaxies whose bulges are very similar
  in color to their disks and a control sample with
  redder bulges (Balcells Peletier 1994 de Jong
  van der Kruit 1994)
• Included 3 other galaxies previously identified
  as having disk-like bulges based on their
  structure and kinematics (Erwin et al. 2003 de
  Zeeuw et al. 2002 Silchenko et al. 2003
  Pinkney et al. 2003 Kormendy Kennicutt 2004)

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Rotational Vs. Pressure Support
(Vmax/s)bulge
(Vmax/s)bulge
Bulge Ellipticity
Based on Binney (1978) and Kormendy Kennicutt
(2004)
Bulge Ellipticity
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Central Line Strengths
Hb
Hb
MgFe
SSP Models from Thomas, Maraston, Bender
(2003) Crosses from Trager et al. (1998)
MgFe
MgFe
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Central Line Strengths Vs. Kinematics and
Dynamics
MgFe
Mgb/ltFegt
s
Vmax
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Metallicity Gradients
Sa
S0
S0
MgFe
S0
MgFe
Sb
Sc
Sa
Sb
Radius (kpc)
Radius (kpc)
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Gradients in a/Fe
Sa
S0
S0
Sb
S0
Mgb/ltFegt
MgbltFegt
Sc
Sb
Sa
Radius (kpc)
Radius (kpc)
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Main Results

• Red bulges are similar to luminous ellipticals in
  their central stellar populations
• Hubble types S0-Sb
• Intermediate-large SSP age
• Super-solar Z/H
• Super-solar a/Fe
• Blue bulges exhibit larger scatter and appear
  similar to low-luminosity ellipticals in their
  central stellar populations
• Uniformly solar a/Fe
• Metal-poor class Sb-Sc, emission lines
• Young metal-rich class all Hubble types
• Central metallicity and a/Fe are sensitive to s
  and Vmax
• Barred galaxies add scatter to Metallicity-Vmax
  relation but not a/FeVmax relation
• Gradients support disk-driven evolution picture
  for many galaxies
• Bulges of barred galaxies, boxy/peanut bulges,
  and bulges with disk-like kinematics are more
  often similar to their disks in their stellar
  populations

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Additional Hints

• Galaxies with largest central metallicities are
  barred or have b/p bulges
• Extra enrichment from bar-driven gas inflow?
• NGC 2599 and late-typed blue bulges Unbarred
  but bulge stellar populations nearly identical to
  those of disk
• Secular evolution with bar destruction?