HI in Dwarf Galaxies

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HI in Dwarf Galaxies
Elias Brinks (Univ. Hertfordshire)

• DWARF GALAXIES AS ASTROPHYSICAL AND COSMOLOGICAL
  PROBES Ringberg Castle, 12-17 March 2006

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Why look at HI?

• The majority of the baryons are in the gas phase
• Vital ingredient for SF (temperature, pressure,
  vel. disp., turbulence, etc.)
• Tells about on-going and past interactions
• Gives clues about the 3-D shape of dwarfs
• Indispensable in determining Mdyn and hence DM
  content and its distribution

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Questions

• Can we explain the HI structure?
• Evidence for impact of massive SF holes or
  artefacts?
• HI dispersions why the same in dwarfs and
  spirals?
• Are our estimates on HI scale heights
  correct/reasonable?
• The prospects of the SKA

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Key facts

• HI extends well beyond R25
• Rotation is solid body, often out to Rmax
• dIrr are DM dominated throughout
• DM fraction typically Mdyn ? 10 x Mbaryon
• As MB decreases, MHI M?
• Lower mass limit MHI 5 x 106 - 107 M?

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Key facts (contd)

• Velocity dispersion 6 - 9 km s-1
• Star Formation occurs above
• N(HI) ? 5 x 1020 cm-2
• dIrr frequently have companions ( 50)
• There are no Dark Galaxies

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NGC6822
de Blok Walter 2000
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IC 2574
Walter Brinks 1999
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dIrrs and companions
Taylor et al. 1995
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? You neeeeeever walk?alone?..
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Typical mass distributions
Main dwarfs
companions
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SBS0335-052
Pustilnik et al. 2001
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The HI Nearby Galaxy Survey THINGS

• High resolution (6" x 5 km s-1, or better) HI
  maps of a sample of 40 nearby (
• Targets were selected to include all Hubble types
  and a range SF activity
• Complements the Spitzer SINGS survey
• Fabian Walter, Elias Brinks, Erwin de Blok, Rob
  Kennicutt, Frank Bigiel, et al.

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THINGS dIrr Galaxy Sample
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Fireworks
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Cassiopeia
Typical kinetical energy deposited 1051 erg per
SN
Energy Red 0.3-1.55 keV, Green 1.55-3.34 keV,
Blue 3.34-10 keV
Chandra
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Multiple SNe
Up to several hundred SNe explode within a small
(few tens of pc) volume and within a short (Myr) time, creating few 100pc to few kpc large
cavities.
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Why bother?

• Superbubble blow-out can cause
• Galactic fountain (HVCs?)
• Contamination of the halo (link to DLA and metal
  lines towards QSOs)
• Dispersal of metals across galaxy disks
• Enrichment of IGM (important at high z)
• Investigate self-regulating (propagating or
  stochastic) star formation
• Energetics of the ISM, turbulence, etc.

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LMC

• 1344 pointing mosaic
• 1 (15 pc) resolution
• 23 supergiant shells
• 103 giant shells
• individual shells are
• detected in soft X-rays
• HII regions and OB-
• associations found on
• rims
• findings compatible
• with wind and SN-
• blown structures

Kim et al. 1999
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Results dwarfs versus spirals

• Diameters of shells in dwarf galaxies are larger
  than in spirals
• Expansion velocities in dwarfs are lower than in
  spirals
• Kinetic energies are the same, i.e. independent
  of host galaxy

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The why and how

• The gravitational potential of dwarfs is
  shallower than in spirals, hence for a constant
  velocity dispersion (as is observed!) the HI disk
  is thicker
• The volume density in the plane of dwarf galaxies
  is lower than in spirals
• Solid-body rotation in dwarfs preserves features

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Scale height, h, and ?
Velocity dispersion is self-regulated SNe stir
up (heat) the ISM which needs to cool before
the next generation of SF can commence.
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Alternatives

• Chance superpositions
• HVC infall
• rare, if at all
• Ram pressure
• proposed for Holmberg II to enlarge embryonic
  shells
• Turbulence
• nopemodel data cubes dont resemble observations

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Dark Galaxies
Promoted by the Cardiff group Davies et al.
2004 Minchin et al. 2005
HI data cube
NGC 4254
VIRGOHI21
NGC 4262
http//www.astro.cardiff.ac.uk/groups/galaxies/dar
kgalaxy.html
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NGC 4388 Virgo Cloud
Oosterloo van Gorkom 2005
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HIJASS J10216842
IC 2573
Walter et al. 2005
HIJASS J10216842
Boyce et al. 2001
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The FutureSKA!

• For those not familiar with HI, this is in
  fact a very weak line the great strength of the
  SKA will lie in its vastly increased collecting
  area. In brief, we can routinely expect in HI
• 1? (100 pc _at_ 20 Mpc) to 1020 cm-2 in 12 hr
• dIrr out to z0.5
• and in continuum out to z2-3

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SKA HI sensitivity
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Summary

• HI is Great!!!
• HI in dwarfs extends well beyond R25
• Solid body rotation and standard vel. disp.
• dIrr are scaled down spirals, but
• their HI disks are thick
• SF occurs above empirical 5 x 1020 cm-2 threshold
• Dwarfs can have companions
• There are no Dark Dwarfs

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Info
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