Water Maser Disk and a Supermassive Black Hole at the Nucleus of the Active Galaxy NGC 7738

TL;DR

This paper uses very long baseline interferometry to map H2O megamasers in the nucleus of NGC 7738, revealing an edge-on, sub-parsec disk whose rotation is significantly influenced by disk self-gravity rather than a pure central point mass. A two-component Mestel-disk model is applied to the rotation data, yielding a central black hole mass of about $1.2\times10^6\,M_\odot$ and a disk mass of several $\times10^6\,M_\odot$ within ~0.22 pc, with a rotation profile $V_ ext{rot}(r) \propto r^{-0.14}$ indicating non-Keplerian dynamics. The mean central density is extraordinarily high, supporting the SMBH interpretation, while X-ray measurements imply a very low Eddington ratio and a modest mass-accretion rate, suggesting a radiatively inefficient inner flow. The findings highlight the importance of disk self-gravity in parsec-scale maser disks and set the stage for higher-sensitivity VLBI follow-ups to refine BH and disk mass estimates.

Abstract

We present the results of very long baseline interferometry (VLBI) observations of water vapor masers in the nucleus of the LINER galaxy NGC 7738. The red- and blue-shifted and newly detected systemic maser features show an almost edge-on disk located at a distance of ${0.031}\mbox{-}{0.222}$ pc from the galactic center and rotating with a velocity of $324\mbox{-}454$ km s$^{-1}$ . The velocity field of the disk indicates sub-Keplerian rotation, suggesting a non-negligible disk mass. The Mestel disk model reveals the central and disk masses to be $(1.2 \pm 0.4) \times 10^6$ $M_{\odot}$ and $(4.7 \pm 1.5) \times10^6$ $M_{\odot}$, respectively. The mean volume density within the inner radius of the disk [$(1.2 \pm 0.5) \times 10^{10}$ $M_{\odot}$ $\mathrm{pc^{-3}}$] strongly suggests the existence of a supermassive black hole at the center.

Figures (8)

• Figure 1: Optical image of NGC 7738 adopted from Digitized Sky Surveys.The scale is 3'.0 $\times$3'.0 in R.A. and Decl., and north is top and east is left.
• Figure 2: Cross-power spectral amplitudes of the visibilities with four antennas (FD, LA, PT, and VLA) toward NGC 7738; we excluded longer-baseline visibilities for this spectrum to avoid coherence loss. No continuum $(2.7\>\mathrm{mJy})$ subtraction is applied. The arrow denotes the systemic velocity of the galaxy, $V_\mathrm{LSR}={6598}$>$\mathrm{km\>s^{-1}}$. The newly detected maser features are observed at $V_\mathrm{LSR}=6559$--$6613$>$\mathrm{km\>s^{-1}}$ (systemic features).
• Figure 3: (a) Distribution of the maser spots in the nuclear region of NGC 7738. (b) Same as (a) but without error bars. The origin of the coordinates, ($\Delta \mathrm{RA}, \Delta \mathrm{Decl})=(0\>\mathrm{mas}, 0\>\mathrm{mas})$, is the peak position of the strongest maser feature of the systemic features at $V_\mathrm{LSR}=6581$>$\mathrm{km\>s^{-1}}$. The colored circles indicate the peak positions of the maser features detected at the $\geq {6}\>\sigma$ level. The color indicates the LSR velocity of the maser spots in >$\mathrm{km\>s^{-1}}$. Dotted lines with a position angle of $\mathrm{PA}={\timeform{8.6D}}$ are the result of a least-squares fitting of the circles with errors in (a), being not through $(\Delta\mathrm{RA}, \Delta \mathrm{Decl})=(0\>\mathrm{mas},0\>\mathrm{mas})$.
• Figure 4: (a) Distribution of maser spots in the frame of the coordinates of $X$ along $\mathrm{PA}={\timeform{8.6D}}$. (b) Same as (a) but without error bars. The figures are a rewrite of figure \ref{['fig:spots']} with the dotted line ($\mathrm{PA}={\timeform{8.6D}}$) in figure \ref{['fig:spots']} as the $X$ line. The origin of the coordinates, ($\Delta{X}, \Delta Y)=(0\>\mathrm{mas}, 0\>\mathrm{mas})$, is equal to ($\Delta \mathrm{RA}, \Delta \mathrm{Decl})=(0.00\>\mathrm{mas}, 0.13\>\mathrm{mas})$ in figure \ref{['fig:spots']}.
• Figure 5: (a) Position-velocity diagram along the $X$ axis in figure \ref{['fig:pa']}. (b) Same as (a) but without error bars. The size of the circles represents the signal-to-noise ratio. Vertical dotted lines indicate the dynamical center of the maser disk ($X_0={0.13}\>\mathrm{mas}={0.06}\>\mathrm{pc}$) and horizontal dotted lines the system velocity ($V_\mathrm{sys}={6598}$>$\mathrm{km\>s^{-1}}$). Dotted curves are the result of least-squares fitting of the high-velocity maser spots to equation (\ref{['eq:vrot']}) in the text.