A Masing Molecular Cloud in the Central Parsecs of Mrk 348

Abstract

We report new observations of the H2O megamaser in the Seyfert 2 galaxy Mrk 348. The line is redshifted by ∼130 km s^-1 with respect to the systemic velocity, is extremely broad, with a FWHM of 130 km s^-1, and has no detectable high velocity components within 1500 km s^-1 on either side of the observed line. The unusual line profile led us to suspect that this source, like NGC 1052 [1], might belong to a class of megamaser galaxies in which the amplified emission is the result of an interaction between the radio jet and an encroaching molecular cloud, rather than occurring in a circumnuclear disk. Our initial VLBA observations show that the maser emission emanates entirely from a region ≤0.25 pc in extent, located toward a continuum component thought to be associated with the receding jet [6]. The very high linewidth occurring on such small spatial scales and the rapid variability indicate that the H₂O emission is more likely to arise from a shocked region at the interface between the energetic jet material and the molecular gas in the cloud where the jet is boring through, than simply as the result of amplification by molecular clouds along the line of sight to the continuum jet. The orientation of the radio jets close to the plane of the sky also results in shocks with the preferred orientation for strong masers from our vantage point. Singledish monitoring with the Effelsberg 100m telescope showed that the line and continuum emission “flared” on very similar timescales. The close temporal correlation between this activity in the maser emission and the continuum flare further suggest that the masing region and the continuum hotspots are nearly equidistant from the central engine and may be different manifestations of the same dynamical events. The study of this newly discovered type of H₂O megamaser can provide detailed information about the conditions in the ISM in the central 1-10 pc of active galactic nuclei.