The Story of DDO 154: An Isolated Dwarf Irregular Galaxy with a Huge HI Extent

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The Story of DDO 154 An Isolated Dwarf Irregular
Galaxy with a Huge HI Extent

• Sabrina Stierwalt
• Astro 620
• April 25, 2007

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Serendipitious Detection 1984

• dI w/ a dozen HII regions
• HI content at least 7 times the optical extent
• rising rot curve
• out to 15 kpc
• resolution too poor to make conclusions on
  kinematics

Krumm Burstein 1984
Distance Uncertainty DDO 154 is projected
against the Coma cluster and could be a part of
Coma I (10 Mpc) or of Canes Venaticorum (4 Mpc).
Tully-Fisher relation suggests larger distance
but brightest stars can be resolved. Shorter
distance also reflected by low heliocentric
velocity.
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Further Mapping Imaging 1988

• VLA mapping and Kitt Peak imaging
• Found optical diameter of 3.5 kpc (MB-13.81)
  and HI extending to 18.5 kpc (5 Rho) at 1019
  cm-2 level gt MH/LB5.4 8.0
• HI stars oriented similarly out to extent of
  optical disk beyond that HI warped
• No molecular gas found

Carignan Freeman 1988
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Carignan Freeman Rotation Curve
Carignan Freeman 1988
Dark Matter Halo Model Isothermal
Sphere Rcore 3kpc ? 28km/s ?c 0.015
Msun/pc3
At last point of rotation curve (r7.5
kpc) Mstars 5 x 107 Msun MHI 2.7 x 108 Msun
Mtot 3.8 x 109 Msun (M/LB)total 80
Mdark/Mlum 12 90 mass is dark (curve dominated
by dark matter for r gt1kpc)
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HI vs Dark Matter Distributions
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Comparison w/ Massive Galaxies
Compare w/ NGC 3198 Mdark/Mlum4,
Mtot/LB18 Dwarf galaxies may have less-massive
dark matter haloes with higher central densities
and smaller radii
Does Tully-Fisher Relation break down for this
galaxy (and other low-mass galaxies)? OR is TFR
more of a relation between velocity and luminous
mass rather than luminosity?
Carignan Beaulieu 1989
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DDO 154 A Test for MOND
Milgrom Braun compared a MOND-derived rotation
curve with Carignan Freemans derived rotation
curve Conclusion If you choose the right value
for (M/L) you can find MOND curves that fit well.
MOND
Newtonian dynamics
Milgrom Braun 1988
Solid (M/L)0.1 Dashed (M/L)0.5 x CF
1988 data Dotted (HI only)
CF 1988 chose (M/L) 1.0
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Recovering the Missing Flux
Carignan Freeman 1988 Carignan
Purton 1998
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Carignan Purton 1998
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Carignan Purton 1998
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Carignan Purton 1998
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Is this the End?
Carignan Purton claim to have found the total
mass (3 x 109 Msun) and the total extent (8 kpc)
of DDO 154.
Carignan Purton 1998
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Truncation of the HI Disk

• Hoffman et al 2001 traced HI out to 20 (18.5
  kpc)
• determined the HI disk was abruptly truncated at
  15 (roughly 14 kpc) at a column density of 1019
  cm-2
• Corbelli Salpeter (1993) predict such
  truncation of outer disks due to photoionization
  by extragalactic UV radiation

Hoffman et al 2001
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Possible Decline
Hoffman et al 2001
6.2 kpc
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Fine Structure of DDO 154

• Gas beyond 6 (5.6 kpc) very smooth, no
  structures gt 300 pc, HI far from stars
• Gas within 6 (5.6 kpc) structures of a few
  hundred pc found (but still well outside optical
  reg)
• Velocity dispersions everywhere fit well with
  single Gaussian (7-8 km/s WNM)
• 2 prominent cavities East West

Hoffman et al 2001
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Fine Structure of DDO 154 (cont)
Unexplained Cavity of the East 40 arcseconds
(600 kpc) Expansion velocity of 10.3
km/s Matches faint smudge in DSS2 blue
West
Unexplained Cavity of the West 60 arcseconds
(900 kpc) Expansion velocity of 16.8 km/s No
evidence for assoc starlight
East

• What is causing these holes?
• Multiple SN comparable size but no stars nearby
  (perhaps UV data would help) no expansion
  evidence
• Collision with a HVC no HVCs around
• Gravitational instabilities during spiral
  structure formation no evidence for spiral arms

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HII Regions in DDO 154
Kennicutt Skillman 2001
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DDO 154 A Quiescent dI

• Applied relation w/ Ha luminosity to get
  integrated SFR of .0015 Msun/yr (low on average
  but not atypical of dIs of this mass)
• Estimated avg past SFR (.006 Msun/yr) in two
  ways
• 1)compared Ha EW w/ evolution synthesis models
• 2) Used total blue L, assumed M/LB1.2, and
  assumed age of 10 Gyr
• Determined current SFR 2-4 times lower than in
  past
• Combined amount of neutral gas w/ current SFR to
  get gas consumption time of 220 Gyr

Kennicutt Skillman 2001
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Metallicity-Luminosity Relation
Kennicutt Skillman 2001
Oxygen abundance 6 solar (on the metal-poor
side for dwarf irregulars but nowhere near IZw18)
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Nitrogen Abundance for DDO 154
Kennicutt Skillman 2001
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References

• Krumm Burstein 1984 The Extended Hydrogen
  Envelope around the Dwarf Galaxy DDO 154
• Carignan Freeman 1988 DDO 154 A Dark
  Galaxy?
• Milgrom Braun 1988 The Rotation Curve of DDO
  154 A Particularly Acute Test of the Modified
  Dynamics
• Carignan Beaulieu 1989 Optical and HI Studies
  of the Gas-Rich Dwarf Irregular Galaxy DDO 154
• Carignan Purton 1998 The Total Mass of DDO
  154
• Hoffman et al 2001 The Fine Structure and
  Outskirts of DDO 154
• Skillman et al 2001 HII Regions and Abundances
  in the Dark Galaxy DDO 154 and the Chemical
  Evolution of Dwarf Irregular Galaxies