ISM, Star Formation Thresholds

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ISM, Star Formation Thresholds Very Extended HI
Disks in Dwarf Galaxies and GALEX

• March 14, 2007
• April 11, 2007
• Astro 620
• Sabrina Stierwalt

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Why study the ISM in dwarfs?

• As MB decreases, MHIM Analysis of HI line
  profiles can tell us the temperature and density
  structure of the ISM.
• ISM in nearby dwarfs (where metallicites are low)
  may mimic conditions of the early universe.
• Is there any evidence for dark matter provided by
  the ISM?
• Are dwarfs supported by rotation? If yes, can we
  get info on the outskirts of the galaxy from a
  rotation curve? If no, what is going on instead?
  
• We see fluctuations in the star formation rate in
  dwarf galaxies - what is happening in the ISM to
  cause these changes?

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What Makes up the ISM?

• Cold, neutral gas HI
• Cold, molecular gas H2, CO
• Ionized gas HII regions, diffuse comp
• Hot x-ray gas
• Dust

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Neutral HI in Galaxies
UGC7178 van Zee et al 1997
NGC4013 Bottema 1995
M81
Ho II Stewart et al 1999
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NGC5903 System Appleton et al 1990
Peculiar HI disks around ellipticals could mean
gas was accreted.
Cluster spirals get stripped of gas via ram
pressure stripping from ICM (stars
unaffected) Being stripped NGC 4522 Kenney et al 2004
IC 3392 Crow et al 2005
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Leo Triplet Haynes, Giovanelli Roberts 1979
Whirlpool Rots et al 1990
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The Baryonic Tully-Fisher Relation
Squares vcW20/2 Circles vcvflat Bothum et al
1995 Verheijen et al 1997 Eder Schombert
2000 McGaugh de Blok 1998 Matthews et al 1998
Md? vc4
McGaugh et al 2000
The correction was only for gas content (HI He)
so other baryonic components unlikely to play a
large role (i.e. cold molecular gas in the disk,
hot ionized gas in the halo, or baryonic MACHOs).
McGaughs plug for MOND Relationship btwn
baryonic mass and rotational velocity fits well
with the idea that baryonic mass is total mass.
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NGC 3741 A Really Extended HI Disk
Begum et al 2005

• MH/LB5.8, HI gas disk 8.3 RHo
• Mdyn3 x 109 Msun Mdyn/LB107 (dark!!)
• Is this a 13th mag dwarf that somehow acquired a
  lot of gas OR is this a galaxy that should have
  been much brighter but failed to convert gas into
  stars? (i.e. do the baryons match the halo?)

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NGC 3741 A Really Extended HI Disk

• Even though dwarfs tend to have higher Mdyn/L
  ratios, they have Mdyn/Mbary similar to those of
  L galaxies
• BUT galaxy formation simulations tell us that
  dwarfs should have lower fractions of baryons
  (reionization inhibits baryon capture in small
  haloes and good fraction of those baryons that
  are captured are then lost due to energy input
  from SF)

Begum et al 2005
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NGC 3741 A Really Extended HI Disk

• Baryonic fraction (w/in extent of the gas disk)
  0.18 comparable to other galaxies with less
  extended HI disks
• Even though Mdyn/Mbary high for extended disk
  galaxies, still lies within range of rest of
  sample
• Perhaps location plays an important role not so
  isolated that accreting material unlikely and not
  so crowded that perturbation likely

Begum et al 2005
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Looking for Signs of Rotation in Virgo
Van Zee et al 1997 sample found all to be
dominated by rotation
UGC 7178
UGC 5764
UGC 5716
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Looking for Signs of Rotation in the Local Group
Young Lo 1996 Leo A
Young Lo 1997 Sag DIG
Young Lo 1997 LGS 3
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Star Formation Thresholds
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Star Formation Thresholds in Bright Galaxies
Schmidt-Kennicutt Power Law
?SFR ? ?ga a1.4
Toomre Instability Criterion
Rot sheer Thermal gas pressure
Gravity

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Star Formation Thresholds in LSBGs

• Clear threshold has not been found for low
  surface brightness galaxies and star formation
  found in gas densities below thresholds
  calculated by Kennicutt (1989)
• Possible reasons for inhibited star formation in
  gas-rich, blue, low-L systems
• System may be young and undergoing its first
  episode of star formation
• Massive star formation may never have been
  triggered by external sources (collisions or
  tides)
• Gas surface density may be too low to allow star
  formation to percolate throughout disk

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Comparing LSBDGs and Normal Dwarfs
van Zee Haynes et al 1997
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Comparing LSBDGs and Normal Dwarfs
van Zee et al 1997
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van Zee Haynes et al 1997
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Global Star Formation Thresholds

• Gas densities tend to be much lower than
  stability criterion (and lower than threshold for
  spirals at 0.67)
• Larger systems have higher ratios but no
  difference for those w/ current SF vs those w/out
• Perhaps our emphasis should be local rather than
  global

normals
LSBDGs
1 UGCA 20 6 UGC 11820 2 UGC 2684 7 UGC 191 3 UGC
3174 8 UGC 634 4 UGC 5716 9 UGC 891 5 UGC 7178 10
UGC 5764
van Zee Haynes et al 1997
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Local Star Formation Thresholds
van Zee Haynes et al 1997

• Sites of SF slightly offset from peaks in HI
  column density (young stars could be ionizing
  immediate environment)
• Peak gas densities hover around 1021cm-2 whether
  SF is current or not

• If threshold breached locally SF begins, gas
  depleted until density falls back below the limit
  (self-regulating feedback) normal dwarfs
  started w/ higher gas surface density but once
  extra gas turned into stars, density too low to
  efficiently percolate SF throughout the rest of
  the disk
• Skillman (1987) suggested that this empirical
  threshold may come from critical column density
  of dust needed to shield molecular gas from UV
  radiation limit may be function of molecular
  abundance

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Local Star Formation Thresholds
A study of faint dwarf galaxies using the GMRT
looked for the existence of a threshold NHI and
found the values to differ by a factor of 4 for
different dwarfs.
Begum et al 2005
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Leo A

• Faint, dI w/ very recent SF (
• SFR higher in past creating significant red
  stellar population but metallicity still only 2
  solar and SF relatively low now

Sag DIG

• Gas-rich (9x106 solar masses), dI currently
  forming massive stars
• Similar to LGS 3 in optical luminosity but 20
  times more gas
• HI in shape of ring w/ central depression

LGS 3

• Red dSph dominated by old stellar population (no
  evidence for HII regions)
• .008 solar metallicity
• HI in shape of ring w/ central depression

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Young Lo 1996 Leo A
All 3 galaxies have diffuse HI haloes that cover
3 times their optical extent.
Young Lo 1997 Sag DIG
Young Lo 1997 LGS 3
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H? and HI Comparisons

• Peaks in HI slightly offset from HII regions in
  both Sag DIG and Leo A
• No evidence of HII regions in LGS 3
• Young stars could be ionizing immediate
  environment
• Young massive stars creating small-scale
  structure in the ISM

Young Lo 1997 Sag DIG
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Velocity Dispersions Leo A Sag DIG
Young Lo 1996 Leo A
High Dispersion Component Warm Neutral Medium
(WNM) Global 80 of galaxy
Low Dispersion Component Cold Neutral Medium
(CNM) Associated w/ Recent SF
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Velocity Dispersions LGS 3

• All profiles fit very well with a single Gaussian
• No evidence for the multi-phase medium found in
  Leo A and Sag DIG
• Shows only broader (possibly warm) phase

Young Lo 1997 LGS 3
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Link between Star Formation Asymmetry Small
Scales
Young et al 2003
GR 8
Asymmetric profiles correlate with SF sites just
as well as CNM
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Link between Star Formation Asymmetry Large
Scales
Asymmetry vs Halpha
CNM vs Halpha
(Young et al 2003)
Trends could be product of SF Young massive
stars inject KE into ISM and resulting
acceleration of gas not isotropic. Added energy
may also act as negative feedback and destroy the
CNM.
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Galaxy Evolution Explorer

• Launched April 2003
• 1350 to 2800 A
• 5 photometric surveys (ranging from all-sky w/
  limit mAB20-21 to 4 sq deg ultra-deep w/ limit
  mAB26)
• 3 spectroscopic surveys (wide-field, medium deep,
  and deep)

Image Credit Caltech

• Compare UV properties with SF rate, extinction,
  starburst history, IMF, and metallicity for
  nearby galaxies
• Will combine UV images spectra with data from
  surveys like SDSS, 2dF, 2MASS, IRAS, and ALFALFA
• Use this local calibration to study more
  distant galaxies (up to z2) and history of star
  formation

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Extended UV Disks
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GALEX Dwarf Galaxies IC1613
UV
Optical
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Summary

• We dont get the full story without looking
  beyond the stars.
• Some dwarfs seem to be dominated by rotation
  while others clearly are not.
• A clear threshold column density required for SF
  does not seem to exist for dwarfs as it does for
  brighter galaxies
• Narrow (low velocity dispersion) gas component
  required but not sufficient for SF
• Ism not quiescent even in galaxies with low SF
  like Leo A
• There is a lot we dont know about dwarf
  galaxies!!

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References

• Lo et al 1993 The HI Structure of Nine
  Intrinsically Faint Dwarf Galaxies
• Young Lo 1996 The Neutral Interstellar Medium
  in Nearby Dwarf Galaxies. I. Leo A
• van Zee et al 1997 A Comparative Study of Star
  Formation Thresholds in Gas-Rich Low Surface
  Brightness Dwarf Galaxies
• Young Lo 1997 The Neutral Interstellar Medium
  in Nearby Dwarf Galaxies. III. Sagittarius DIG,
  LGS 3, and Phoenix
• McGaugh et al 2005 The Baryonic Tully-Fisher
  Relation
• Young et al 2003 Star Formation and the
  Interstellar Medium in Four Dwarf Irregular
  Galaxies
• Begum et al 2005 A Dwarf Galaxy w/ a Giant HI
  Disk
• Begum et al 2005 Gas Distribution, Kinematics
  and Star Formation in Faint Dwarf Galaxies
• Thilker et al 2005 Recent Star Formation in the
  Extreme Outer Disk of M83

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Connection to BCDs

• Either BCDs are young (starbursts periodically
• If latter idea is true, we should see quiescent
  versions of BCDs in between bursts these would
  look like the gas-rich dwarf irregulars we observe