Constraining Nuclear Molecular Gas Content with High-resolution CO Imaging of GOALS Galaxies

TL;DR

The study addresses how nuclear molecular gas content affects black hole mass estimates in gas-rich mergers. By combining high-resolution ALMA CO(2-1) imaging and continuum data with independent kinematic modeling (3DBarolo), the authors quantify the gas mass within tens of parsecs of the nuclei in III Zw 035 and IRAS F01364-1042 and derive independent M_enc values from cold gas. They find CO-based M_enc values are significantly lower than previous warm-gas-based measurements, which can move the inferred BH masses onto SMBH scaling relations, while continuum-based gas masses depend strongly on the uncertain dust temperature. The results imply that nuclear gas content and its angular momentum dynamics can influence SMBH growth and challenge simple Bondi-like accretion prescriptions, though a larger sample is needed to generalize these conclusions.

Abstract

We present measurements of the cool molecular gas mass around the nuclei of two gas-rich mergers, III Zw 035 and IRAS F01364-1042, whose enclosed masses (M$_\mathrm{enc}$) within the central 40-80 pc would be overmassive if attributed entirely to the supermassive black hole mass (SMBH) and compared to SMBH-galaxy scaling relations. Our gas mass measurements are derived from Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 long-baseline observations of CO(J=2-1) and 230 GHz continuum emission at 14-20 pc resolution, which probes below the resolving limit of the previous black hole mass measurements. Subtracting molecular gas mass from these enclosed masses is not enough to reconcile with BH-galaxy relationships, but independently measuring M$_\mathrm{enc}$ using the cold CO(2-1) gas does shift the black holes down to their expected values. Still, these ALMA data reveal respective molecular gas masses of $\sim$3$\times$10$^7$ to $\sim$6$\times$10$^8$ M$_\odot$ within 70 pc of these black holes, which could challenge some black hole accretion models that assume nuclear gas like this has no angular momentum.

Figures (8)

• Figure 1: I-Band Hubble ACS images (GO-10592; PI: Evans) of III Zw 035 (left) and IRAS F01364$-$1042 (right) with CO(2-1) moment 0 contours overlaid in black. Moment 0 contours are from 10$^{-0.4}$ to 10$^{0.5}$ Jy beam$^{-1}$ km s$^{-1}$ for III Zw 035 and 10$^{-0.8}$ to 10$^{0.5}$ Jy beam$^{-1}$ km s$^{-1}$ for IRAS F01364$-$1042 in 7 logarithmically-spaced intervals. III Zw 035's companion galaxy still retains much of its large-scale structure, while in IRAS F01364$-$1042 the two are in a later stage of the merger process.
• Figure 2: Top: CO(2-1) moment 0 (integrated intensity) maps for III Zw 035 (left) and IRAS F01364$-$1042 (right) with Band 6 continuum contours at 230 GHz for III Zw 035 and 228 GHz for IRAS F01364$-$1042 with levels at 3, 6, 12, 24, 48 $\times$ (rms) Jy beam$^{-1}$ (rms values are 32.7 $\times$ and 29.4 $\times$$\mu$Jy beam$^{-1}$ for III Zw 035 and IRAS F01364$-$1042 respectively). White ellipses indicate the beam sizes of the CO images (33 $\times$ 28 for III Zw 035 and 43 $\times$ 40 mas for IRAS F01364$-$1042. Bottom; Middle: CO(2-1) moments 1 (velocity) and 2 (velocity dispersion) maps. Black contours are the same as in the top two panels for continuum, while magenta contours are moment 0 contours from 10$^{-0.4}$ to 10$^{0.5}$ Jy beam$^{-1}$ km s$^{-1}$ for III Zw 035 and 10$^{-0.8}$ to 10$^{0.5}$ Jy beam$^{-1}$ km s$^{-1}$ for IRAS F01364$-$1042 in 7 logarithmically-spaced intervals. Velocity maps for both galaxies are clipped to a 3$\sigma$ CO(2-1) detection. Continuum center, where we expect the black hole to be located, is indicated by a black cross. Red and blue arrows indicate the directionality (red and blue-shifts relative to the galaxy's systemic velocity) of the molecular outflows.
• Figure 3: Integrated measurements and calculated masses within the boxed aperture for IRAS F01364$-$1042 and III Zw 035. First row: enclosed CO(2-1) flux from images described in Section \ref{['sec:MeasurementsandImaging']}. Second row: M$_{\mathrm{enc}}$ calculated from CO(2-1) fluxes shown in above panels using method described in Section \ref{['sec:CO21gasmass']}. Third row: integrated continuum flux densities at 230 GHz (III Zw 035) and 228 GHz (IRAS F01364$-$1042) extracted from images described in Section \ref{['sec:MeasurementsandImaging']}. Bottom row: M$_{\mathrm{enc}}$ calculated from continuum fluxes shown in above panels using method described in Section \ref{['sec:contgasmass']}. Grey regions are Medling2015 M$_{\mathrm{enc}}$ ranges. III Zw 035 requires a T$_D$$\gtrsim$ 175 K to have a dust-derived gas mass lower than the previous M$_{\mathrm{enc}}$ while IRAS F01364$-$1042 requires a T$_D$$\gtrsim$ 19 K.
• Figure 4: Spectral index (top) and uncertainty maps (bottom) for III Zw 035 (left) and IRAS F01364$-$1042 (right). The beam sizes of the continuum images used to create the spectral index maps (0.075$^{\prime\prime}$$\times$ 0.052$^{\prime\prime}$ for III Zw 035 and 0.134$^{\prime\prime}$$\times$ 0.110$^{\prime\prime}$ for IRAS F01364$-$1042) are shown in the lower left corners in magenta. The black contours correspond to the Band 6 combined line-free continuum emission at 230 GHz for III Zw 035 and 228 GHz for IRAS F01364$-$1042 with levels at 3, 6, 12, 24, 48 $\times$ (rms) Jy beam$^{-1}$ (rms values are 32.9 and 38.3 $\mu$Jy for III Zw 035 and IRAS F01364$-$1042 respectively). Their beam sizes are shown in the lower left corners in black (31 $\times$ 25 mas for III Zw 035 and 29 $\times$ 28 mas for IRAS F01364$-$1042).
• Figure 5: Position-velocity (PV) diagrams of the CO emission modeled across the major axis in III Zw 035 (left) & IRAS F01364$-$1042 (right). Top: PV models from $^{3\mathrm{D}}$Barolo outputs. Black contours correspond to 3, 6, 9, and 12 $\times$ rms of high resolution PV diagrams made in CARTA at the same central position with the same high resolution input cubes described in Section \ref{['sec:MeasurementsandImaging']}. The white, horizontal lines represent velocities of zero with respect to systematic. Bottom: Inclination-corrected M$_{\mathrm{enc}}$ calculated using the methods described in Section \ref{['sec:PVdiagrams']}. M$_{\mathrm{enc}}$ was calculated on tilted rings that were separated by single beam widths. Both M$_{\mathrm{enc}}$ values (measured at radii of a single beam FWHM indicated by the black circles) are about an order of magnitude lower than the measurements made in Medling2015.