The contributions of old star populations to the radiation of nearby elliptical and lenticular galaxies have been underestimated, according to Alison Crocker, a postdoctoral research associate at the University of Massachusetts Amherst. Crocker’s discovery affects the estimates of star formation rates within galaxies. The degree to which regions of star formation are responsible for measured radiation now appears to be overestimated.

The lecture, entitled “It’s Not All Star Formation: Old Star Influences in Local Galaxies,” was sponsored by the Department of Physics and Astronomy. Crocker is a member of the Dartmouth College Class of 2006.

Elliptical and lenticular galaxies, or “early-type galaxies,” have a low ratio of ongoing star formation to total galactic mass, or low specific star formation rates (sSFR).

Current estimates of star formation rates in a galaxy are based on the measured flux of various electromagnetic wavelengths. This assumes that the radiation from younger, hotter stars – indicative of recent star formation – is responsible for the detected ionized gas and radiation from heated dust grains within that galaxy.

This method works for spiral galaxies, which have high sSFR, where the luminosity of the hottest young stars dominates. This occurs even though such stars have short lives and the lowest abundances.

However, Crocker stated that approximately 75 percent of early-type galaxies have ionized gas, which is well above the 25 percent that show signs of star formation through the presence of cold molecular gas, the material that forms stars. Even when other sources of emission, such as active galactic nuclei, are considered there is an excess of galaxies with ionized gas.

For some of the early-type galaxies with molecular gas, the locations of ionized gas correspond, suggesting star formation as the cause of radiation. For some of them, however, they do not.

According to Crocker, infrared emission is commonly used to trace star formation rates because it originates from heated dust surrounding the hottest types of young stars. However, her research has found that the total radiation output in the infrared range from the early-type galaxies that show signs of star formation has too large of a contribution from old stars.

This discovery of significant radiative contributions by old stars in early-type galaxies will affect estimates concerning low-sSFR regions.

According to Crocker, her investigation of early-type galaxies has produced no evidence yet for the commonly-held assumption that star formation in early-type galaxies differs significantly from the laws suggested for formation in spiral-type galaxies.

Crocker also discussed her research of the mid-infrared 8-micron line in spiral galaxies. This emission originates from photons captured by molecules known as polycyclic aromatic hydrocarbons (PAH), one of the types of dust grains common within galaxies.

A component of the diffuse PAH emission from a galaxy can be identified as originating from regions of star formation. This is done by locating where it is coupled with the H-alpha emission line, which would only originate from such regions, according to Crocker.

An analysis of the spiral galaxy NGC 628 revealed that around 25 percent of the PAH emission was not related to star formation. Crocker stated that 25 percent may be a common figure for the contribution to dust heating by old stars.