A Precision Measurement of the Mass of the Black Hole in NGC 3258 from High-Resolution ALMA Observations of its Circumnuclear Disk (1906.06267v1)

Abstract: We present $\sim0.10^{\prime\prime}-$resolution Atacama Large Millimeter/submillimeter Array (ALMA) CO(2$-$1) imaging of the arcsecond-scale ($r \approx 150$ pc) dusty molecular disk in the giant elliptical galaxy NGC 3258. The data provide unprecedented resolution of cold gas disk kinematics within the dynamical sphere of influence of a supermassive black hole, revealing a quasi-Keplerian central increase in projected rotation speed rising from 280 km s$^{-1}$ at the disk's outer edge to $>400$ km s$^{-1}$ near the disk center. We construct dynamical models for the rotating disk and fit beam-smeared model CO line profiles directly to the ALMA data cube. Our models incorporate both flat disks and tilted-ring disks that provide a better fit of the mildly warped structure in NGC 3258. We show that the exceptional angular resolution of the ALMA data makes it possible to infer the host galaxy's mass profile within $r=150$ pc solely from the ALMA CO kinematics, without relying on optical or near-infrared imaging data to determine the stellar mass profile. Our model therefore circumvents any uncertainty in the black hole mass that would result from the substantial dust extinction in the galaxy's central region. The best model fit yields $M_\mathrm{BH} = 2.249\times10^9$ $M_\odot$ with a statistical model-fitting uncertainty of just 0.18\%, and systematic uncertainties of 0.62\% from various aspects of the model construction and 12\% from uncertainty in the distance to NGC 3258. This observation demonstrates the full potential of ALMA for carrying out highly precise measurements of $M_\mathrm{BH}$ in early-type galaxies containing circumnuclear gas disks

• The paper presents a precise SMBH mass measurement of NGC 3258 at 2.249×10⁹ M☉ using ALMA’s CO(2–1) line observations.
• It employs high-resolution dynamical modeling, including flat disk and tilted-ring models, to accurately resolve the black hole's sphere of influence.
• The study demonstrates how ALMA overcomes dust extinction, refining SMBH mass scaling relations for early-type galaxies.

Analysis of Black Hole Mass Measurement in NGC 3258 Using High-Resolution ALMA Observations

The paper presented by Boizelle et al. investigates the supermassive black hole (SMBH) in the elliptical galaxy NGC 3258 using high-resolution CO(2–1) line observations from the Atacama Large Millimeter/submillimeter Array (ALMA). This research contributes to the precise determination of black hole masses in early-type galaxies, which is fundamental for understanding the co-evolution of galaxies and their central black holes.

Methodology and Data

The data were obtained through ALMA, providing $\sim0.1$ arcsecond resolution observations of the circumnuclear disk in NGC 3258. The angular resolution of these observations resolves the kinematics within the black hole's sphere of influence, achieving a precision unattainable with previous techniques. The authors construct dynamical models of the rotating disk, incorporating both flat disk and tilted-ring models. The latter accounts for a mildly warped disk, allowing a more accurate characterization of the structure and kinematics.

The standard approach for estimating the host galaxy's mass profile from imaging is circumvented due to the severe dust extinction in the galaxy's center. Instead, the mass profile is directly inferred from the ALMA CO(2–1) kinematics, avoiding potential uncertainties related to extinction. This methodological choice underlines the potential of molecular gas dynamics for precise SMBH mass measurements, unaffected by obscuration issues.

Results and Robust Claims

The SMBH mass is measured to be $M_{\bullet} = 2.249 \times 10^9 M_{\odot}$, with a statistical model-fitting uncertainty of 0.18% and systematic uncertainties from model construction contributing approximately 0.62%. Systematic uncertainties from distance estimation are notably more significant (around 12%). This precision highlights the exceptional capability of ALMA observations, underlining that resolving the dynamical influence of the black hole with high angular precision minimizes systematic uncertainties traditionally associated with optical and infrared measurements.

Discussion and Implications

The findings indicate that the local $M_{\bullet}$–$\sigma_{\star}$ and $M_{\bullet}$–$L$ relationships do not universally apply across all types of galaxies, particularly for the most luminous early-type galaxies, such as NGC 3258. The discrepancy between the observed black hole mass and that predicted by empirical scaling relationships emphasizes the complexity of SMBH and galaxy co-evolution, particularly for massive early-type galaxies and brightest cluster galaxies (BCGs).

The paper underscores ALMA's role in refining black hole mass determinations, particularly in cases where optical and infrared methodologies are hindered by dust. The results imply that high-resolution molecular gas dynamics can anchor SMBH mass scaling relations at the high-mass end with minimal bias from obscuration. This capability is crucial for statistical studies involving large galaxy samples and precise empirical relationship calibrations, advancing our understanding of galactic evolution.

Future Prospects

The capability to resolve the SMBH's sphere of influence at such precision opens new avenues for observing SMBHs in other galaxies with similar methodologies, particularly for those heavily obscured in optical wavelengths. The development of a comprehensive sample of precise BH masses using ALMA could significantly refine the current understanding of SMBH growth, merging processes, and their influence on host galaxy properties across cosmic times.

In conclusion, this paper demonstrates a pivotal step in high-precision astrophysical measurements that leverage advancements in observational technology, specifically ALMA, to enhance our comprehension of the fundamental processes governing the dynamics of galaxies and their central black holes.