Quantifying the AGN-driven outflows in ULIRGs (QUADROS) II: evidence for compact outflow regions from HST [OIII] imaging observations

TL;DR

This study uses HST [OIII] imaging to measure the sizes of AGN-driven outflows in ULIRGs, finding most are compact and not galaxy-wide, challenging some feedback models.

Contribution

First detailed HST imaging analysis of outflow sizes in a complete ULIRG sample, revealing predominantly compact outflows.

Findings

Most outflows are compact, with radii less than 1.2 kpc.

Only one galaxy shows a galaxy-wide outflow extending up to 5 kpc.

Results suggest AGN outflows may not be as galaxy-wide as some models predict.

Abstract

The true importance of the warm, AGN-driven outflows for the evolution of galaxies remains uncertain. Measurements of the radial extents of the outflows are key for quantifying their masses and kinetic powers, and also establishing whether the AGN outflows are galaxy-wide. Therefore, as part of a larger project to investigate the significance of warm, AGN-driven outflows in the most rapidly evolving galaxies in the local universe, here we present deep Hubble Space Telescope ( HST) narrow-band [OIII]$\lambda$5007 observations of a complete sample of 8 nearby ULIRGs with optical AGN nuclei. Combined with the complementary information provided by our ground-based spectroscopy, the HST images show that the warm gas outflows are relatively compact for most of the objects in the sample: in three objects the outflow regions are barely resolved at the resolution of HST ($0.065 < R_{[OIII]} < 0.12$ kpc); in a further four cases the outflows are spatially resolved but with flux weighted mean radii in the range $0.65 < R_{[OIII]} < 1.2$ kpc; and in only one object (Mrk273) is there clear evidence for a more extended outflow, with a maximum extent of $R_{[OIII]}\sim5$ kpc. Overall, our observations show little evidence for the galaxy-wide outflows predicted by some models of AGN feedback.