Topic: "It's not all star formation: old star influences in local galaxies" (Video)
Abstract: Star formation is responsible for much of the appearance of local galaxies. The copious ultraviolet and optical radiation from young stars demarcates such prominent features as spiral arms, rings, bars and individual star clusters. In fact, the young luminous stars are usually assumed to be the sole cause of gas ionization and dust heating in galaxies. This assumption is the backbone of deriving global galaxy star formation rates at both high and low redshift. However, this direct connection to star formation expectedly breaks down in some cases. Early-type galaxies have the smallest ratios of ongoing star formation to total mass (specific star formation rates) and the old stars in these galaxies not only heat a large fraction of their dust, but surprisingly, can also dominate their gas ionization and UV emission. Using high-resolution infrared data, we can also separate quiescent regions within local spiral galaxies and compute the contribution from old stars. Recent work using the Spitzer Space Telescope measures this old-star related emission from small dust grains called polycyclic aromatic hydrocarbons (PAHs) and future work using new data from the Herschel Space Observatory will additionally constrain this important quantity for larger dust grains. These examples highlight the need to consider the contribution of old stars to solve the galaxy-wide energy balance and to be aware of their contribution whenever star formation rates are measured in lower specific star formation rate locations.