Multi-band constraints on the nature of emission line galaxies

Our aim is to explore the nature of emission line galaxies by combining high-resolution observations obtained in different bands to understand which objects are powered by an Active Galactic Nucleus(AGN). From the spectroscopic Palomar survey of nearby bright galaxies, we selected a sample of 18 objects observed with HST, Chandra, and VLA. No connection is found between X-ray and emission line luminosities from ground-based data, unlike what is found for brighter AGN. Conversely, a strong correlation emerges when using the HST spectroscopic data, which are extracted on a much smaller aperture. This suggests that the HST data better isolate the AGN component when one is present, while ground-based line measurements are affected by diffuse emission from the host galaxies. The sample separates into two populations. The 11 objects belonging to the first class have an equivalent width of the [OIII] emission line measured from HST data EW([OIII])>~2 A and are associated with an X-ray nuclear source; in the second group we find seven galaxies with EW([OIII])<~1 A that generally do not show any emission related to an active nucleus (emission lines, X-ray, or radio sources). This latter group includes about half of the Low Ionization Nuclear Emission-line region (LINERs) or transition galaxies of the sample, all of which are objects of low [OIII] line luminosity (<~1E38 erg s-1) and low equivalent width (<~1 A) in ground-based observations. These results strengthen the suggestion that the EW([OIII]) value is a robust predictor of the nature of an emission line galaxy.