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.
• Determine any relationships between the amount of star formation and the color of the galaxy.
Preliminaries
• The Interstellar Medium
• HII Regions
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.
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.
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.
Procedure
• CCD images taken at CTIO and WIYN.
• IRAF
• “On - Off” images. H images are subtracted from R-band images to eliminate continuum sources.
UGCA 270 R-band
UGCA 270 H
UGCA 270 “On-Off”
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.
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.
Implications
• S ~ 1 suggests that SFR’s over time are deterministic.
• S < 1 or S > 1 suggests that SFR’s over time are chaotic.