Molecular gas in Tidal Dwarf Galaxies: Exploring the conditions for star formation

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Molecular gas in Tidal Dwarf Galaxies Exploring
the conditions for star formation
Ute Lisenfeld Universidad de Granada, Spain

• In collaboration with
• Jonathan Braine
• Pierre-Alain Duc
• Stephane Leon
• Vassilis Charmandaris
• Elias Brinks
• C. Mundell
• P. Appleton
• E. Schinnerer

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• Motivation and Goals
• TDGs are similar to classical dwarf galaxies
  except for their high metallicty.
• ? CO is a good tracer of the molecular gas
• TDGs are ideal objects to study the relation
  between molecular gas and SF in dwarf galaxies
• Goal Test validity of SF laws in TDGs. Test
  conditions for onset of SF
• Outline of the talk
• Describe observations of molecular gas in TDGs
  (and similar objects)
• Test laws of SF
• Main conclusions
• Molecular gas is (almost) everywhere where HI is
• Prescene of molecular gas is not enough for the
  onset of SF, kinematics plays an important role
  as well
• SF laws seems to proceed in a similar way as in
  spirals/classical dwarfs

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What are TDG?

• A self-gravitating entity formed out of the
  debris in tidal interactions
• Self-gravitating, i.e not just a agglomerations
  of stars and gas (hard to decide observationally)
  
• Can contain both stars and gas from debris
• Future development unclear, but potential to
  become a dwarf galaxy

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What do we know about TDGs?

• Produced in galaxy interactions in tidal tails
• Gas segregation HI at end of tails, most CO in
  parent galaxies
• Most TDGs have two main stellar components
• young stars recently formed
• older stars (1 Gyr) from parent galaxies

NGC 7252
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What do we know about TDGs?

• Gaseous and stellar properties similar to
  classical dwarf irregular and blue compact dwarfs
  ..
• .except their high metallicity
• typical of outer regions of spiral galaxies
• higher than in classical dwarf systems of
  comparable size
• do not follow luminosity-metallicity relation

(Duc et al. 2000)
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Single-dish CO observations in a sample of TDGs
12 galaxies observed, 9 detected
Arp 105
NGC 2992/93
Braine et al. 2000 Braine et al. 2001
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Some more examples
NGC 7252
NGC 4676
NGC 5291
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CO spectra

• CO lines coincides spatially and kinematically
  with HI
• spatial coincidence with Ha
• Interpretation HI ? H2 ? star formation
• BUT this is not the entire picture

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Molecular gas fraction and SF efficieny similar
to spirals
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Interferometric observations of TDGs NGC 2992/3
Brinks, Duc Walter (2004)
CO (thick contours) and Ha (thin contours) over
HI (grey)
HI (contours) over Rband

• CO is found where SF takes place (? Ha)
• Interferometric observations yield only 25 of
  flux from single-dish measurements ? smoothly
  distributed component present

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Spatially resolved observations of an old TDG
VCC 2062 (Duc et al. 2007)
True color image (BVR) of VCC 2062. Superimposed
HI (white contours), Ha (red contours)
True color image (BVR) of NGC 4694 and VCC
2062 Superposed HI (blue) and Ha (red)
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Spatially resolved observations of VCC 2062
CO follows HI very well in distribution and
kinematics
Integrated CO (red) and CO (blue) line
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Kinematical properties of the gas
HI position-velocity diagram along cloud -
Narrow lines in SF region - Wide lines in
southern part
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A special case The Hickson Compact Group
Stephans Quintet
Intergalactic starburst SQ A
Potential TDG SQ B
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Atomic hydrogen
All the HI outside of galaxies!
From Williams et al. (2002)
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Abundant molecular gas!!
SQ B MH2 7x108 Mo MH2/MHI 0.6
SQ A MH2 3.1x109 Mo MH2/MHI 1.1
Lisenfeld et al. 2002
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Different line shapes in both regionsGood
kinematical agreement with HI
SQ A wide, two velocities (FWHM 140 km/s)
SQ B narrower (FWHM 50km/s) Maximum of CO
coincides with Ha
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Observations of SQ B with Plateau de Bure
Interferometer

• Two concentrations of CO, coinciding with SF
  regions
• Comparison with IRAM 30m observations 50 of
  molecular gas in diffuse component

Lisenfeld et al. 2004
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The interacting system Arp 94

• It consists of
• NGC 3226
• Hubble type E2 pec
• NGC 3227
• Hubble type SAB pec
• Seyfert nucleus (one of the original Seyfert
  galaxies of Seyfert)

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HI observations with the VLA revealed (Mundell
et al. 1995)- northern and southern tidal
tail- dwarf companion close to NGC
3227J10231952HI diameter 8.9 kpcHI mass
3.8x108 Mo
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Deep images show star formation in J10231953
Blue image (Mundell et al. 2004) from the Isaac
Newton Telescope
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• IRAM 30m observations of CO of J1023.
• Expected detect molecular gas in the SF region
• But
• (Lisenfeld et al. 2008)

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Molecular gas extended over whole galaxy! In
general good kinematical agreement with
HI. Towards west Line blending with CO from NGC
3227? do a fitting of 2 Gaussian lines to
disentangle.
CO green HI red
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Look at SF properties of 3 regions
2. High HI surface density
3. Low HI surface density
1. SF region
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• Lines at region 3
• Wider lines (total widths of 100 km/s)
• Fit by a single gaussian
• 2N(H2)/N(H) 0.7-1.2
• 2N(H2)N(H) 1.2-1.6 1021 cm-2

• Lines at region 1
• Thin lines (total widths of 40 km/s)
• Fit by a single gaussian
• 2N(H2)/N(H) 0.5
• 2N(H2)N(H) 1.8 1021 cm-2

• Lines at region 2
• Wider lines (total widths of 100 km/s)
• Fit by two gaussian
• 2N(H2)/N(H) 1.1
• 2N(H2)N(H) 3.1 1021 cm-2

3
2
1
Green Data Red Gaussian Fit Black Residual
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Summary of observational results

• Unresolved observations Good spatial (and
  kinematical) agreement between HI, CO and H?
• Spatially resolved single-dish observations
  Close relation between CO and HI, CO (almost)
  everywhere where HI is, including regions without
  SF
• In particular, there are places with high gas
  column density, but no SF. (Similar finding in
  tidal tail by Maybhate et al. 2007)
• Interferometric observations
• Close relation between CO and H?.
• Considerable (50-75) of missing flux indicating
  presence of smoothly distributed CO component.
• Kinematics of CO and HI (VCC 2062, J10231953)
• Narrow lines width in SF regions
• Wide lines outside SF regions

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Theories of star formation

• When SF has started SFR is well described by a
  Schmidt law relating SFR/area and gas column
  density
• SF thresholds gas column density has to be
  higher than a certain minimal/critical value
• Minimal gas density enabling the formation of
  cool gas necessary for SF.
• Value about 6 Msol/pc2 (Elmegreen 2002), but
  dependent on metallicity and radiation field
• Critical gas column density (large scale effects)
• Toomre critical density gravitation vs. gas
  pressure and coriolis force
• Shear (Elmegreen 87, 91, 93,) gravitation vs.
  Gas pressure and shear --gt better description for
  dwarf galaxies (Hunter, Elemegreen Baker 98)

Large part of this summary is based on Elmegreen
(2002)
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Theories of star formation

• Small scale effects Turbulence
• Can trigger SF by compression of preexisting gas
  clouds
• Can also inhibit SF if the motions continously
  force the gas to break up to pieces that are
  samller than a thermal Jeans mass.
• Role of trigger Elmegreen (2002) concludes that
  triggers (turbulence, pressure from other stars)
  play an important role to trigger SF if a cool
  phase of gas is available.
• SF by gravitational instability alone is also
  possible.

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Apply this to TDGs/intergalactic SF Schmidt law

• Schmidt law (Kennicutt 1998) followed by TDGs
  (within uncertainty, because generally low
  spatial resolution)
• --gt Origin of this law cannot be cloud-cloud
  collision due to spiral arm (Silk 1997)

x
x
X VCC2062 and J1023
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Apply this to TDGs/intergalactic SF

• Minimal gas density
• J10231953 gas column density 10-20 Msol/pc2
• above typcial values for minimal column density
• Presence of molecular gas also indicative that
  cool gas can exist
• Threshold found by Maybhate for tidal tails 3
  Msol/pc2 -gt close to the minimal threshold. Could
  explain their finding that in some cases at this
  column density no SF takes place
• VCC 2062 gas column density about 10 Msol/pc2

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Large scale kinematics

• Toomre criterion Needs a rotating object
• J10231953 not clear that this is the case, but
  try.
• Derive mcrit sk/pG between 4 and 5 Msol/pc2 --gt
  little variation over the cloud, trend to have
  lower values in the north (no SF)
• Shear ?crit 2.5?A/?G
• (A is Oort constant)
• J10231953 Low values
• (?crit lt 1 Msol/pc2) due to close
• to linear velocity field

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Small scale kinematics

• Lines profile does not allow us to draw
  conclusions about properties of GMC, they are
  presumably the same everywhere.
• Wide lines --gt Typically found where no SF takes
  place
• High velocity dispersion among clouds --gt large
  scale turbulence.
• We could see turbulence inhibiting SF due to
  break up of clouds.
• High turbulence lower volume density
• Streaming motions along tidal tails (should be
  visible in large scale velocity map as well)
• Narrow lines
• --gt typically found where SF takes place
• Low velocity dispersion (J1023)
• Gravitationally bound (Vcc2062)
• Indication of large scale
• compression/trigger (??)

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Conclusions

• Molecular gas fraction, SF efficiency and Schmidt
  law in TDGs similar as in spirals
• Gas column density is not enough to understand if
  SF takes place
• The reason for the lack of SF is (in the few
  cases discussed here)
• Not minimal density (molecular gas is everywhere)
• Not due to galaxy-wide dynamical process
  (Toomre/Shear)
• Related to the line width of the gas SF happens
  in dynamically cool gas. This could indicate
• Lack of large-scale turbulence
• Lack of streaming motions
• Indication of gravitationally bound object
• Indication of compression? Trigger?