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Star Formation Rates in Galaxies

Star Formation Rates in Galaxies. Investigations into Color Dependence Ben Willett U.W.-Madison. 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.

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Star Formation Rates in Galaxies

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  1. Star Formation Rates in Galaxies Investigations into Color Dependence Ben Willett U.W.-Madison

  2. 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.

  3. Preliminaries • The Interstellar Medium • HII Regions

  4. The ISM • Composed mostly of gas and dust with 100 times more gas than dust. <> = 1atom/cm3 • Most of the gas composed of HI. • HI is not uniformly distributed. • Self gravitation and shocks can cause density gradients.

  5. 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 > 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.

  6. HII Regions & Star Formation • How are recently formed stars located? • (O star m.s.) < 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.

  7. Procedure • CCD images taken at CTIO and WIYN. • IRAF • “On - Off” images. H images are subtracted from R-band images to eliminate continuum sources.

  8. UGCA 270 R-band

  9. UGCA 270 H

  10. UGCA 270 “On-Off”

  11. Star Formation Rates • SFR  (<MOB>NOB / OB ) *(IMF correction) • Due to less than perfect observing conditions, LOB is not known, consequently NOB is not known and exact SFR’s cannot be measured. • Qualitative approach. Provides a measure of the “instantaneous” SFR.

  12. SFR’s 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 = (<SFRo>/<SFR(B-V)>) provides a measure of the deviation from the mean at a given time.

  13. Implications • S ~ 1 suggests that SFR’s over time are deterministic. • S < 1 or S > 1 suggests that SFR’s over time are chaotic.

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