The Rotation Curve and Velocity Dispersions of Dwarf Galaxies

1
The Rotation Curve and Velocity Dispersions of
Dwarf Galaxies

• Lei Hao
• Apr, 25th, 2007
• Astro 620 Advanced Radio Astronomy

2
Introduction

• Probe the mass distribution
• Dark Matter content
• Dynamics, evolution and formation of galaxies

3
Kinematics -- Observations

• Using optical spectroscopy (star and gas)
• Traditional long-slit spectra
• Fabry-Perot spectrographs and IFU velocity
  information for the whole field, but time
  consuming
• Emission lines H?, NII, SII
• Distant planetary nebulae and satellite galaxies
  (large distances)
• Individual HII regions (e.g. Rubin Ford, 1970,
  83)

4
Kinematics -- Observations

• HI line
• Advantage Extends further out than the visible
  disk
• Disadvantage not good in the central region
  poor spatial resolution of the beam
• CO line
• Good alternative to H? and HI in the inner disk,
  less extinction
• Maser Lines SiO, OH, H2O
• Super-massive BH in the center of NGC4258.

5
Kinematics of Dwarfs

• Optical (H? and star) and HI
• Pressure-supported systems velocity dispersion
• Rotationally-supported systems rotation velocity
• Methods to measure rotation curve tilted-ring
  model (Begeman 1989)
• The galaxy can be described by a set of
  concentric rings, each ring being characterized
  by a fixed value of the HI surface density and
  circular velocity, and the inclination angle and
  position angle of the major axis.
• Fit with the observed velocity along the major
  axis.

6
Kinematics -- Observations

• Rotation Curve in typical disk galaxies
• Rise rapidly in the central regions and often
  reach 200-300km/s within a few hundred parsecs
  of the center.
• HI disk stay flat until the last point measured ?
  dark matter (exceptions exists)
• The shape of the RC correlates with the optical
  luminosities
• Low-luminosity slowly rising and low amplitude
• High-luminosity fast rising and high amplitude

7
Kinematics -- Observations
8
Kinematics of Dwarfs

• Dwarf galaxies are dark matter dominated. (Mateo
  1989)
• For a given M/L, the central velocity dispersion
  of a self-gravitating system in equilibrium,
  scales as (RcS0)1/2
• Globular clusters have central velocity
  dispersions of 2-15km/s
• Pressure supported dwarf galaxies have Rc 10
  times larger, and S0 103times smaller ? vlt2 km/s
• All low-luminosity dwarfs have velocity
  dispersions of gt7km/s

9
Rotation Curves of Dwarfs

• Dwarf spiral galaxies provide excellent probes of
  the internal structure of dark haloes.
• Dark matter dominate on scales larger than a
  kiloparsec
• Easy to take out the baryonic mass which is
  mostly in the form of HI within the disk.
• Early results show slow rotation, with rotation
  curves that rise monotonically to the last
  measure points.

10
Rotation Curves of Dwarfs

• Dwarf spiral galaxies provide excellent probes of
  the internal structure of dark haloes.
• Dark matter dominate on scales larger than a
  kiloparsec
• Easy to take the baryonic mass which is mostly in
  the form of HI within the disk.

Moore 1994
11
Rotation Curves of Dwarfs

• Core or cusp
• Simulation predict the inner part slope of the
  mass density ? -1 (Navarro et al. 1997, Power at
  al. 2003, Colin et al. 2004) ? -1.2-1.5 (Moore
  et al. 1999, Ghigna et al. 2000 klypin et al.
  2001 Reed et al. 2003 Tasitsiomi et al. 2004)
• Observations ?-0.2 (de Blok et al. 2001a)

12
Rotation Curves of Dwarfs-- Uncertainties

• De Blok, Mcgaugh et al. ? HI RC of dwarf galaxies
  are in contradiction with the CDM predictions.
• Swaters, van den Bosch et al. ? RC of dwarf
  galaxies are consistent with cosmological
  predictions
• Beam smearing, uncertain mass-to-light ratio,
  limited spatial sampling of halos density
  distribution
• Ask for higher resolution, and independent M/L
  ratio

13
Rotation Curves of Dwarfs-- Uncertainties

• De Blok, Mcgaugh et al. ? RC of dwarf galaxies
  are in contradiction with the CDM predictions.
• Swaters, van den Bosch et al. ? RC of dwarf
  galaxies are consistent with cosmological
  predictions
• Beam smearing, uncertain mass-to-light ratio,
  limited spatial sampling of halos density
  distribution
• Ask for higher resolution, and independent M/L
  ratio

Sofue Rubin 2001
14
Rotation Curves of Dwarfs-- Uncertainties

• De Blok, Mcgaugh et al. ? RC of dwarf galaxies
  are in contradiction with the CDM predictions.
• Swaters, van den Bosch et al. ? RC of dwarf
  galaxies are consistent with cosmological
  predictions
• Beam smearing, uncertain mass-to-light ratio,
  limited spatial sampling of halos density
  distribution
• Ask for higher resolution, and independent M/L
  ratio

Swaters et al. 2000
15
Rotation Curves of Dwarfs-- Uncertainties

• De Blok, Mcgaugh et al. ? RC of dwarf galaxies
  are in contradiction with the CDM predictions.
• Swaters, van den Bosch et al. ? RC of dwarf
  galaxies are consistent with cosmological
  predictions
• Beam smearing, uncertain mass-to-light ratio,
  limited spatial sampling of halos density
  distribution
• Ask for higher resolution, and independent M/L
  ratio

Swaters et al. 2000
16
Rotation Curves of Dwarfs-- Uncertainties

• De Blok, Mcgaugh et al. ? RC of dwarf galaxies
  are in contradiction with the CDM predictions.
• Swaters, van den Bosch et al. ? RC of dwarf
  galaxies are consistent with cosmological
  predictions
• Beam smearing, uncertain mass-to-light ratio,
  limited spatial sampling of halos density
  distribution
• Ask for higher resolution, and independent M/L
  ratio
• Mcgaugh et al. 2001 with higher resolution H?
  data for inner region ? similar RC as before,
  still in contradiction with the CDM predictions
• Rhee et al. 2004 ? many systematic biases to
  consider. Each bias can be small, but all add up
  to produce significant effects. (Inclination
  bar bulge) titled-ring model produces an
  underestimate of the central rotational
  velocities. (typically 20)

17
Rotation Curves of Dwarfs-- exceptions
Carignan Puche 1990
18
Rotation Curves of Dwarfs-- exceptions
Ostlin et al. 1999 RC for BCDs
19
Velocity Dispersion of Dwarfs

• How low-mass dwarf galaxies supported
• Rotation velocities become comparable to
  disordered motions (intrinsic gas dispersions,
  bulk motions by SN and stellar winds)
• A 107-108M? galaxy with a radius 2kpc has
  circular rotation speed of 5-15km/s, while warm
  diffuse HI gas with T6000K has velocity
  dispersion of 7km/s.
• DsphdIrr LGS3 and Phoenix
• Optically similar to dwarf spheroidals
• Retain a substantial amount of interstellar gas

20
Velocity Dispersion of Dwarfs

• Young Lo 1997
• HI studies of Sag DIG (dIrr), LGS3 (dIrr/dsph)
  and phoenix (dIrr/dsph)
• Sag DIG and LGS3 have extended HI, phoenix
  unsure.
• They are random motion supported. (only within
  local group do we have dIrr that are not rotation
  supported)

21
Velocity Dispersion of Dwarfs

• Young Lo 1997
• HI studies of Sag DIG (dIrr), LGS3 (dIrr/dsph)
  and phoenix (dIrr/dsph)
• Sag DIG and LGS3 have extended HI, phoenix
  unsure.
• They are random motion supported. (only within
  local group do we have dIrr that are not rotation
  supported)

22
Velocity Dispersion of Dwarfs

• Young Lo 1997
• HI studies of Sag DIG (dIrr), LGS3 (dIrr/dsph)
  and phoenix (dIrr/dsph)
• Sag DIG and LGS3 have extended HI, phoenix
  unsure.
• They are random motion supported. (only within
  local group do we have dIrr that are not rotation
  supported)
• Sag DIG is decomposed into broad (?10km/s) and
  narrow (?5km/s) components. Broad component
  distributed throughout the galaxy and the narrow
  component concentrated into a small number of
  prominent clumps.

23
Velocity Dispersion of Dwarfs

• Young Lo 1997
• HI studies of Sag DIG (dIrr), LGS3 (dIrr/dsph)
  and phoenix (dIrr/dsph)
• Sag DIG and LGS3 have extended HI, phoenix
  unsure.
• They are random motion supported. (only within
  local group do we have dIrr that are not rotation
  supported)
• Sag DIG is decomposed into broad (?10km/s) and
  narrow (?5km/s) components. Broad component
  distributed throughout the galaxy and the narrow
  component concentrated into a small number of
  prominent clumps.

24
Velocity Dispersion of Dwarfs

• Young Lo 1997
• HI studies of Sag DIG (dIrr), LGS3 (dIrr/dsph)
  and phoenix (dIrr/dsph)
• Sag DIG and LGS3 have extended HI, phoenix
  unsure.
• They are random motion supported. (only within
  local group do we have dIrr that are not rotation
  supported)
• Sag DIG is decomposed into broad (?10km/s) and
  narrow (?5km/s) components. Broad component
  distributed throughout the galaxy and the narrow
  component concentrated into a small number of
  prominent clumps.
• LGS3 do not have narrow component the lack of
  cold component may be the reason for the lack of
  starformation in LGS3

25
Velocity Dispersion of Dwarfs

• Young Lo 1997
• HI studies of Sag DIG (dIrr), LGS3 (dIrr/dsph)
  and phoenix (dIrr/dsph)
• Sag DIG and LGS3 have extended HI, phoenix
  unsure.
• They are random motion supported. (only within
  local group do we have dIrr that are not rotation
  supported)
• Sag DIG is decomposed into broad (?10km/s) and
  narrow (?5km/s) components. Broad component
  distributed throughout the galaxy and the narrow
  component concentrated into a small number of
  prominent clumps.
• LGS3 do not have narrow component the lack of
  cold component may be the reason for the lack of
  starformation in LGS3

26
References

• Sofue Rubin ARAA, 2001, 39, 137
• Rhee et al. 2004, ApJ, 617, 1059
• Young Lo, 1997, ApJ, 490, 710
• Moore, 1994, Nature, 370, 629
• Noordermeer, et al. 2007, MNRAS,/0701731
• McGaugh et al. 2001, AJ, 122, 2381
• Begeman 1989, AA, 223, 47
• Swaters et al 2000, ApJ, 531, 107
• De Blok McGaugh 1997 MNRAS, 290, 533