WISDOM project -- XVIII. Molecular gas distributions and kinematics of three megamaser galaxies

Abstract

The co-evolution of galaxies and supermassive black holes (SMBHs) underpins our understanding of galaxy evolution, but different methods to measure SMBH masses have only infrequently been cross-checked. We attempt to identify targets to cross-check two of the most accurate methods, megamaser and cold molecular gas dynamics. Three promising galaxies are selected from all those with existing megamaser SMBH mass measurements. We present Atacama Large Millimeter/sub-millimeter Array (ALMA) 12CO(2-1) and 230-GHz continuum observations with angular resolutions of about 0.5". Every galaxy has an extended rotating molecular gas disc and 230-GHz continuum source(s), but all also have irregularities and/or non-axisymmetric features: NGC1194 is highly inclined and has disturbed and lopsided central 12CO(2-1) emission; NGC3393 has a nuclear disc with fairly regular but patchy 12CO(2-1) emission with little gas near the kinematic major axis, faint emission in the very centre and two brighter structures reminiscent of a nuclear ring and/or spiral; NGC5765B has a strong bar and very bright 12CO(2-1) emission concentrated along two bisymmetric offset dust lanes and two bisymmetric nuclear spiral arms. 12CO(2-1) and 12CO(3-2) observations with the James Clerk Maxwell Telescope are compared with the ALMA observations. Because of the disturbed gas kinematics and the impractically long integration times required for higher angular resolution observations, none of the three galaxies is suitable for a future SMBH mass measurement. Nonetheless, increasing the number of molecular gas observations of megamaser galaxies is valuable, and the ubiquitous disturbances suggest a link between large-scale gas properties and the existence of megamasers.

Figures (5)

• Figure 1: Optical images of our three target galaxies. In each panel the ALMA CO(2-1) emission is overlaid as contours (except the upper-right panel) and the ALMA primary beam is shown as a blue dashed circle. The bottom row panels show zoom-in images. Left column: NGC 1194. Upper panel: SDSS $r$-band image of the whole galaxy. Lower panel: HST Wide-field Camera 3 (WFC3) F438W (blue), F814W (green) and F160W (red) composite of the central region. Middle column: NGC 3393. Upper panel: Digitized Sky Survey-2 red-band image of the whole galaxy. Lower panel: HST/WFC3 F336W (blue), F814W (green) and F160W (red) composite of the central region. Right column: NGC 5765B (in the bottom-left corner of each panel) and its companion NGC 5765A. Upper panel: HST/WFC3 F814W image. Lower panel: unsharp-masked HST/WFC3 F814W image.
• Figure 2: Data products of NGC 1194. Top-left: CO(2-1) zeroth-moment (integrated flux) map. The overlaid black solid circle shows the JCMT beam (assuming perfect pointing accuracy) at the same frequency. The magenta dashed line indicates the position angle of the maser disc (extrapolated in spatial extent). Top-centre: CO(2-1) first-moment (intensity-weighted mean velocity) map. The overlaid black dashed line shows the kinematic major axis. Top-right: CO(2-1) second-moment (intensity-weighted velocity dispersion) map. Bottom-left: CO(2-1) integrated spectrum synthesised from the ALMA datacube (orange histogram, with uncertainties indicated as darker shades) and JCMT spectrum (grey histogram). Bottom-centre: CO(2-1) PVD along the kinematic major axis, whose position angle is listed in the top-left corner. Bottom-right:$230$-GHz continuum map. The magenta dashed line again indicates the position angle of the maser disc (extrapolated in spatial extent). The spatial extent shown is smaller than that of the moments maps, as illustrated by the overlaid orange square. The synthesised beam is shown in the bottom-left corner of all the maps.
• Figure 3: As \ref{['fig:NGC1194_maps']} but for NGC 3393. The JCMT beam (assuming perfect pointing accuracy) at the CO(3-2) frequency is also shown in the top-left panel as a black dashed circle, and the corresponding spectrum is shown in the bottom-left panel as a black dashed histogram. The ALMA CO(2-1) primary beam full width at $50\%$ (i.e. the usual primary beam definition) and at $20\%$ (the maximal extent of the datacube) of the maximum are also shown in the top-left panel as blue and red solid circles, respectively. Due to the scarcity of gas along the kinematic major axis, the kinematic position angle of $45\degr$ was estimated by eye rather than by a fit. The CO(2-1) emission reaches the edge of the ALMA FoV.
• Figure 4: As \ref{['fig:NGC3393_maps']} but for NGC 5765B (although the red solid circle is not visible and the range of position angles of the maser disc due to its warp is indicated by two magenta dashed lines in the relevant panels).
• Figure 5: Radio -- millimetre integrated continuum flux densities and power-law fits of the nuclear source (red) and south-west compact source (blue) detected in NGC 3393. The $8.4$- and $4.9$-GHz measurements are from Koss+2015ApJ807.149.