Spitzer Observations of Transient, Extended Dust in Two Elliptical Galaxies: New Evidence of Recent Feedback Energy Release in Galactic Cores

Spitzer observations of extended dust in two optically normal elliptical galaxies provide a new confirmation of buoyant feedback outflow in the hot gas atmospheres around these galaxies. AGN feedback energy is required to prevent wholesale cooling and star formation in these group-centered galaxies. In NGC 5044 we observe interstellar (presumably PAH) emission at 8 m out to about 5 kpc. Both NGC 5044 and NGC 4636 have extended 70 m emission from cold dust exceeding that expected from stellar mass loss. The sputtering lifetime of this extended dust in the 1 keVinterstellar gas, 107 yr, establishes the time when the dust first entered the hot gas. Evidently the extended dust originated in dusty disks or clouds, commonly observed in elliptical galaxy cores, that were disrupted, heated, and buoyantly transported outward. The surviving central dust in NGC 5044 and NGC 4636 has been disrupted into many small filaments. It is remarkable that the asymmetrically extended 8 m emission in NGC 5044 is spatially coincident with H+[N ii] emission from warm gas. A calculation shows that dust-assisted cooling in buoyant hot gas moving out from the galactic core can cool within a few kiloparsecs in 107 yr, explaining the optical line emission observed. The X-ray images of both galaxies are disturbed. All timescales for transient activity—restoration of equilibrium and buoyant transport in the hot gas, dynamics of surviving dust fragments, and dust sputtering—are consistent with a central release of feedback energy in both galaxies about 107 years ago.