Clues to Nuclear Star Cluster Formation from Edge-on Galaxies

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Clues to Nuclear Star Cluster Formation from
Edge-on Galaxies

• Anil Seth (U. of Washington --gt CfA)
• Collaborators
• Julianne Dalcanton
• Paul Hodge
• Victor Debattista

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Outline

• Edge-on galaxy sample (Seth et al. 2005a,b --gt
  stellar pop. of thick disks)
• Nuclear Cluster Background
• Nuclear Cluster morphology
• multiple components
• Stellar Populations Mass
• Formation Mechanisms

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Edge-on Galaxy Sample

• HST/ACS snapshot data
• 2x350 sec exposures in F606W F814W (V I).
• 14 galaxies
• Distance 2-20 Mpc
• Late-type (Sbc)
• Low-Mass (low Av)
• Edge-on (i gt 80?)

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NGC 4244
2MASS K band image
ACS 2-color image
5x5 closeup of nuclear cluster
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What are nuclear star clusters?

• Common in dE (dE,N), and spiral galaxies.
• Late-type spirals (face-on) 75 have
  nuclear clusters (Boker et al. 2002)
• Same size as Milky Way globulars (reff 3 pc),
  but brighter (MI -12) and more massive (3x106
  Msol) (Boker et al. 2004, Walcher et al. 2005).
• Multiple Epochs of Star Formation (Long et al.
  2002, Sarzi et al. 2005)

Globular Clusters
Nuclear Clusters
N
MI
MI histogram from Boker, 2004
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Why do we care about nuclear clusters?

• Visible record of nuclear star formation /
  accretion
• Linked to supermassive black hole formation?
• Progenitors of ?Cen-like globular clusters, UCDs,
  blue-tilt?

Bulge Luminosity vs. Nuclear Cluster Mass for
spiral galaxies. Courtesy Joern Rossa (in prep)
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How do nuclear clusters form?
IC 342

• Two Scenarios
• Dynamical friction makes globular clusters sink
  to center.
• Formed in situ from gas transported to the
  center.

CO emission color-coded by velocity Schinnerer
et al. 2003
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Nuclear Clusters in our Sample

• Of 14 galaxies in our sample
• 9 have cluster candidates
• 3 nuclear clusters have multiple components
• Morphology
• Color
• Aligned with galaxy disk

HST/ACS F814W images Each image is 2x2, black
bar 10pc Sorted by distance
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Edge-on nuclear clusters same size and magnitude
as face-on nuclear clusters. (Boker et al.
2004)

• Faintest 2 clusters similar to MW GCs.
• Most are ?Cen like objects
• Brightest object (NGC 4206) similar to
  ultra-compact dwarfs

Black symbols F606W F814W fits for reff Other
Data Boker et al. 2004, Drinkwater et al. 2003,
Martini Ho 2004, Harris 1996, Huxor et al.
2005, De Propris et al. 2005
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Color maps reveal multiple components

• Bluer disks/rings and redder spheroidal
  components.
• Spheroid vs. Disk color difference 0.3-0.6 mags,
  implies disk age lt 1 Gyr

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Stellar populations in the NGC 4244 nuclear
cluster
1 Gyr
70 Myr

• 45 minute long-slit spectrum from APO 3.5m
• Photometry from the Sloan and 2MASS surveys
• Single-age fits Two ?2 minima at 70 Myr
  and 1 Gyr

• Combination of two ages significantly improves
  fits
• Best spectral fit AV0.5, M71Myr1.7x105 Msol
  M1Gyr3.3x106 Msol
• Additional 10 Gyr component or continuous SFR
  fits equally well.

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A Dynamical Mass

• Spectrum shows emission lines offset from
  absorption
• Emission associated with compact blue HII region,
  w/ projected distance of 0.8819 pc
• Assuming circular orbit ?v 24 km/sec --gt
• Mass 2.51.7-1.2 x 106 Msol
• Similar to ?Cen other nuclear clusters (Walcher
  et al. 2005)

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Distribution of Stellar Populations (NGC 4244)
Data

• Mass from two-age fits consistent with dynamical
  mass.
• Mass of 70 Myr component matches luminosity of
  disk from morphological fits.

Model (Disk Spheroid
Residual
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Summary of Results

• 3 of 14 galaxies have nuclear clusters with both
  a red spheroidal and blue disk component.
• The disk component is aligned with the galaxy
  disk.
• The stars in the disk components are lt1 Gyr in
  age.
• In NGC 4244, multiple stellar populations of 70
  Myr and 1 Gyr, older ages also possible.
• NGC 4244 nuclear cluster mass 3x106 Msol

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Formation Mechanism

• results support in situ formation model!
• Assuming stars form in a disk
• must happen multiple times (?t 0.5 Gyr)
• must lose angular momentum and heat vertically.
• What mechanisms?
• Evaporation? No
• Interaction with galaxy potential?

Possible! Gas can be transported to
center. Milosavljevic, 2004
Evaporation high angular momentum stars
preferentially lost. Causes reduction in
ellipticity ?? gt 40 ? trelax
trelax 109 years Fall Frenk 1985, Shapiro
Marchant, 1976
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Interactions with galaxy potential can heat
disks.

• Cluster disks are not perfectly aligned with
  galaxy disks (?? lt 10?)
• Clusters may not be stationary wrt galaxy
  potential
• Other Possibilities hybrid, disks at different
  orientations.

Initial misalignment 15 degrees Initial offset
10 pc Time units 3x105 years Assumes nuclear
disk (hr 3 pc, M 105 Msol) and galaxy
potential like NGC 4244
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The End
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Edge-on Galaxy Sample
Galaxy Type Vmax km/sec D Mpc
IC5052 SBcd 79 6.0
IRAS09312-3248 Sc 142 13
NGC3501 Sc 136 16
NGC4144 SBc 67 7.5
NGC4183 Sc 103 19
NGC4206 Sbc 124 18
NGC4244 Sc 93 4.4
NGC4517 Sc 137 7.2
NGC5023 Sc 77 6.6

• HST/ACS snapshot data 700 sec exposures in
  F606W F814W (V I).
• 14 galaxies, 9 have candidate nuclear star
  clusters.
• Late-type (Sbc)
• Low-Mass
• Edge-on (i gt 80?)

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Morphological Fitting

• Fit clusters using ishape (Larsen, 1999) and IDL
  programs.
• Best single component fits were King and Moffat
  profiles
• Multiple component clusters aligned with galaxy
  disk!

Axial ratios vs. Position Angle relative to
galaxy disk for elliptical King profile fits. Two
connected data points indicate F606W and F814W
model fits. Blue points multiple component
clusters.
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Multiple ComponentsAddition of disk component
leads to qualititative and quantitative
improvements in the fits.
ORIG. IMAGE KING RESID. DISKKING RESID
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Stellar Populations in the NGC 4244 nuclear
cluster

• Obtained 45m long-slit spectrum with APO 3.5m
• Detected as a point source in Sloan and 2MASS
  surveys
• Fit photometry and spectroscopy to single-stellar
  population models.
• Two ?2 minima at 70 Myr and 1 Gyr

Blue lines joint ?2 minima
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• Best fit AV0.5, M71Myr1.7x105 Msol
  M1Gyr3.3x106 Msol
• Addition of 10 Gyr component significantly
  improves photometry
• Sanity check Disk component from morphological
  fit would be 1.2x105 Msol assuming an age of 71
  Myr.

Top single-age fits, Middle 2-age fit,
Bottom Residuals
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Simulations show quick timescale for heating!