The ATLAS3D project - X. On the origin of the molecular and ionized gas in early-type galaxies

Abstract

We make use of interferometric CO and H i observations, and optical integral‐field spectroscopy from the ATLAS3D survey, to probe the origin of the molecular and ionized interstellar medium (ISM) in local early‐type galaxies. We find that 36 ± 5 per cent of our sample of fast‐rotating early‐type galaxies have their ionized gas kinematically misaligned with respect to the stars, setting a strong lower limit on the importance of externally acquired gas (e.g. from mergers and cold accretion). Slow rotators have a flat distribution of misalignments, indicating that the dominant source of gas is external. The molecular, ionized and atomic gas in all the detected galaxies are always kinematically aligned, even when they are misaligned from the stars, suggesting that all these three phases of the ISM share a common origin. In addition, we find that the origin of the cold and warm gas in fast‐rotating early‐type galaxies is strongly affected by environment, despite the molecular gas detection rate and mass fractions being fairly independent of group/cluster membership. Galaxies in dense groups and the Virgo cluster nearly always have their molecular gas kinematically aligned with the stellar kinematics, consistent with a purely internal origin (presumably stellar mass loss). In the field, however, kinematic misalignments between the stellar and gaseous components indicate that at least 42 ± 5 per cent of local fast‐rotating early‐type galaxies have their gas supplied from external sources. When one also considers evidence of accretion present in the galaxies’ atomic gas distributions, ≳46 per cent of fast‐rotating field ETGs are likely to have acquired a detectable amount of ISM from accretion and mergers. We discuss several scenarios which could explain the environmental dichotomy, including preprocessing in galaxy groups/cluster outskirts and the morphological transformation of spiral galaxies, but we find it difficult to simultaneously explain the kinematic misalignment difference and the constant detection rate. Furthermore, our results suggest that galaxy mass may be an important independent factor associated with the origin of the gas, with the most massive fast‐rotating galaxies in our sample (MK≲−24 mag; stellar mass of ≈8 × 1010 M⊙) always having kinematically aligned gas. This mass dependence appears to be independent of environment, suggesting it is caused by a separate physical mechanism.

TheHerschelVirgo Cluster Survey

“…This strengthens our suggestion that the ETG most likely to contain dust and molecular gas are slow rotators in the Virgo cluster and fast rotators in the field. Davis et al (2011) find that the gas has a purely internal origin (and a smaller amount) in fast rotators residing in dense environments, while it has an external origin, possibly leading to larger amounts, in about half of the fast rotators in the field. They also find that the dominant source of gas is external for slow rotators, which are mainly found in dense enviroments.…”

Tracing the extreme interplay between radio jets and the ISM in IC 5063

“…to the H I emission profile. In early-type galaxies, molecular and atomic neutral gas tends to show similar kinematics (Davis et al 2011). Thus, the new observations can also serve as a comparison for this puzzling difference.…”

A star formation study of the ATLAS3Dearly-type galaxies with the AKARI all-sky survey

“…Galaxy mergers are therefore frequently invoked as a possible mechanism to supply cold gas and dust to ETGs. For example, Davis et al (2011) studied the kinematics of stars, ionised gas and molecular gas in ETGs and, due to the pervasiveness of kinematic misalignments between stars and gas, suggested that the majority of them may obtain cold gas from external sources. Internal sources are however also proposed by several authors.…”