Star Formation Rates in Galaxies

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Star Formation Rates in Galaxies

• Investigations into Color Dependence
• Ben Willett
• U.W.-Madison

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Project Goals

• Find the star forming regions in the galaxies
  from our sample via location of HII regions.
• From these HII regions, gauge the amount of star
  formation.
• Determine any relationships between the amount of
  star formation and the color of the galaxy.

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Preliminaries

• The Interstellar Medium
• HII Regions

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The ISM

• Composed mostly of gas and dust with 100 times
  more gas than dust. lt?gt 1atom/cm3
• Most of the gas composed of HI.
• HI is not uniformly distributed.
• Self gravitation and shocks can cause density
  gradients.

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HII Regions

• HII is ionized hydrogen.
• HII can recombine to a high energy level and then
  decay to the ground state emitting photons at ?
  656.3nm.
• O stars emit light with E gt 13.6ev. HI
  surrounding the O star is ionized.
• HII Regions occur when the number of ionizations
  number of recombinations.
• HII regions fluoresce in the H?.

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HII Regions Star Formation

• How are recently formed stars located?
• ?(O star m.s.) lt 108 yr. ?collapse 106 yr.
• O stars must have formed from the gas indicated
  by the HII region.
• Initial mass functions predict a certain number
  of each kind of star.
• HII regions indicate star formation.

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Procedure

• CCD images taken at CTIO and WIYN.
• IRAF
• On - Off images. H? images are subtracted from
  R-band images to eliminate continuum sources.

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UGCA 270 R-band
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UGCA 270 H?
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UGCA 270 On-Off
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Star Formation Rates

• SFR ? (ltMOBgt NOB / ?OB ) (IMF correction)
• Due to less than perfect observing conditions,
  LOB is not known, consequently NOB is not known
  and exact SFRs cannot be measured.
• Qualitative approach. Provides a measure of the
  instantaneous SFR.

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SFRs and the Colors of Galaxies

• (B - V) color gives an indication of the number
  of youthful stars and thus a measure of the
  average SFR over a period of 1-5Gyr.
• S (ltSFRogt/ltSFR(B-V)gt) provides a measure of the
  deviation from the mean at a given time.

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Implications

• S 1 suggests that SFRs over time are
  deterministic.
• S lt 1 or S gt 1 suggests that SFRs over time are
  chaotic.