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.