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.