Bulges of Spiral Galaxies: Stellar

Populations, Structure, and Kinematics

Bhasker Moorthy

Jon Holtzman

Anatoly Klypin

New Mexico State University

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?

Early-typed bulges: Similar

to ellipticals 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- 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)

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)

Late-types: Similar to

disks

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

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

The Galaxy Sample

38 nearby (vres<7000 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; Sil’chenko et al. 2003; Pinkney et al. 2003;

Kormendy & Kennicutt 2004)

Rotational Vs.

Pressure

SupportBulge Ellipticity

(Vm

ax/

bulg

e

(Vm

ax/

bul

ge

Bulge Ellipticity

Based on Binney (1978) and Kormendy & Kennicutt (2004)

Central Line

Strengths

SSP Models from Thomas, Maraston, & Bender (2003);

Crosses from Trager et al. (1998)

[MgFe]’

H

[MgFe]’ [MgFe]’

H

Central Line

Strengths Vs.

Kinematics and

Dynamics

[MgF

e]’

Mgb

/<F

e>

Vmax

Metallicity Gradients

Radius (kpc)

[Mg

Fe

]’

Radius (kpc)

[MgF

e]’

Sa

S0

S0

Sb

Sa

S0

Sb

Sc

Gradients in /Fe

Radius (kpc)M

gb

<F

e>

Radius (kpc)

Mgb

/<F

e>

Sa

Sa

S0

S0 Sb S0

SbSc

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 /Fe

Blue bulges exhibit larger scatter and appear similar to low-luminosity ellipticals in their central stellar populations

Uniformly solar /Fe Metal-poor class: Sb-Sc, emission lines Young metal-rich class: all Hubble types

Central metallicity and /Fe are sensitive to and Vmax Barred galaxies add scatter to Metallicity-Vmax relation but not /Fe–Vmax 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

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?