Herschel Spectroscopy of Early-type Galaxies

Abstract

We present Herschel spectroscopy of atomic lines arising in photodissociation regions as well as ionization regions of nearby early-type galaxies (ETGs), focusing on the volume-limited Atlas3D sample. Our data include the [C ii ], [O i ], and [N ii ] 122 and 205 μ m lines, along with ancillary data including CO and H i maps. We find that ETGs have [C ii ]/FIR ratios slightly lower than spiral galaxies in the KINGFISH sample, and several ETGs have unusually large [N ii ] 122/[C ii ] ratios. The [N ii ] 122/[C ii ] ratio is correlated with UV colors and there is a strong anti-correlation of [C ii ]/FIR with NUV-K seen in both spirals and ETGs, likely due to a softer radiation field with fewer photons available to ionize carbon and heat the gas. The correlation thus makes a [C ii ] deficit in galaxies with redder stellar populations. The high [N ii ] 122/[C ii ] (and low [C ii ]/FIR) line ratios could also be affected by the removal of much of the diffuse, low-density gas, which is consistent with the low H i /H 2 ratios. [C ii ] is now being used as a star-formation indicator, and we find that it is just as good for ETGs as in spirals. The [C ii ]/CO ratios found are also similar to those found in spiral galaxies. Through the use of the [N ii ] 205 μ m line, estimates of the percentage of [C ii ] emission arising from ionized gas indicate that a significant portion could arise in ionized regions.