Are There Dark Galaxies in the Local Group

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Are There Dark Galaxiesin the Local Group?

• Josh Simon
• Caltech

Tim Robishaw (Berkeley) Leo Blitz
(Berkeley) Andrew Cole (Minnesota) Martin
Weinberg (UMass) Martin Cohen (Berkeley)
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Outline

• Missing satellites and HVCs
• Known or suspected dark galaxies
• LGS 3 HVC
• Complex H
• High-resolution maps of HVCs
• Are all HVCs dark galaxies?

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CDM and the Missing Satellites
CDM predicts large numbers of subhalos (300-1000
for a Milky Way-sized galaxy)
Milky Way only has 15 known satellites
What happened to the rest of them?
Springel et al. 2001
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CDM and the Missing Satellites
CDM predicts large numbers of subhalos (300-1000
for a Milky Way-sized galaxy)
Milky Way only has 15 known satellites
What happened to the rest of them?
Springel et al. 2001
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Are HVCs The Missing Dwarfs?

• There are thousands of HVCs in the Local Group,
  and hundreds of them are compact and isolated
• HVCs have the right masses if they are located at
  distances of several hundred kpc

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Problems with HVCs

• Unknown distances and masses
• Difficult to study at high resolution

Key question How far away are typical HVCs?
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Problems with HVCs

• Unknown distances and masses
• Difficult to study at high resolution

Key question How far away are typical HVCs?
Do HVCs have dark matter halos?
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• Are there dark galaxies in the Local Group?

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Dark Galaxies

• Yes!
• But how many?

1. HVC 127-41-330
2. (?) Complex H
3 - 300.
???
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LGS 3 and HVC 127-41-330
Blitz Robishaw 2000
Robishaw, Simon, Blitz 2002
Leiden-Dwingeloo Survey (beam 36)
Arecibo (beam 3.3)
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LGS 3 and HVC 127-41-330
LGS 3
HVC
Tidal tails
Robishaw, Simon, Blitz 2002
Leiden-Dwingeloo Survey (beam 36)
Arecibo (beam 3.3)
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HVC Rotation Curve
At d700 kpc, Mdyn 6MHI At d100 kpc, Mdyn
39MHI
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What is HVC 127-41-330?

• Cloud of gas in Local Group
• Likely at 700 kpc
• MHI 5.5 x 106 M?
• Dark matter-dominated
• No stars

Robishaw, Simon, Blitz 2002
The first dark galaxy!
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Complex H
Location in the Galactic plane makes it unique
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Complex H Kinematics
Lockman 2003

• For b
• Therefore dVLSR/db Vz
• Inclined, retrograde, circular orbit at
• d 27 kpc fits the data MHI 2 x 107 M?

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Is Complex H a dwarf galaxy?

• In the IR, many
• sources are visible
• towards the core of
• Complex H

• CO observations show that they are all in the
  Milky Way

SFR
Simon et al. 2006
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Is Complex H a dwarf galaxy?

• What about an old population?

Milky Way main sequence and red giant tracks
for d 2 kpc
2MASS CMD of Complex H
Dwarf galaxy tracks for d 18, 27, 36 kpc
M
Simon et al. 2006
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What is Complex H?

• Cloud of gas near Milky Way
• Likely at 27 kpc
• MHI 2.0 x 107 M?
• No star formation MHI/M 20

If dark matter
If no dark matter
The first cold accretion flow?
Dark galaxy 2
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• Are many or most HVCs dark galaxies?

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HVC Models
Extra/circumgalactic (Oort 1966, Blitz et al.
1999) - d few hundred kpc - m 105 -
107 M? - dark matter halos -
gravitationally bound
Hot halo (Maller Bullock 2004) - d 150
kpc - m 105 - 107 M? - no dark matter
- pressure-confined
Tidal debris (no formal model) - d 10 - 50
kpc - m 103 - 105 M? - no dark matter
- not bound

• Questions about the bulk of the HVC population
• Are they gravitationally bound?
• Do they have dark matter halos?

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Are HVCs bound?

• If not, calculate expansion timescale
• 5.3 x 107 yr
• Not stable for very long
• What about dynamical masses?
• Assuming virialization
• 300 (d/100 kpc)-1 kpc)-1
• Must have dark matter to be bound!

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Audience Quiz
Which of these HVCs is not rotating?
HVC 072-22
HVC 01847
HVC 19129
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Audience Quiz
HVC 072-22
HVC 01847
HVC 19129
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HVC Image Gallery
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Three More Rotating HVCs?
HVC 072-22
HVC 01847
HVC 050-27
Vrot 14 km s-1 Mdyn/MHI 78 (d/100 kpc)-1
Vrot 6 km s-1 Mdyn/MHI 22 (d/100 kpc)-1
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Three More Rotating HVCs?
HVC 072-22
HVC 01847
Vrot 14 km s-1 Mdyn/MHI 78 (d/100 kpc)-1
Vrot 6 km s-1 Mdyn/MHI 22 (d/100 kpc)-1
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Summary

• HVC 127-41
• Cloud kinematics require dark matter, no stars -
  dark galaxy
• Complex H
• Large HI mass, no star formation, no stars -
  dark galaxy or cold accretion stream?
• Other HVCs
• Several also show signs of rotation
• If the rotation is real, they are DM-dominated

HVCs appear compatible with the dark galaxy
hypothesis