HST FOC spectroscopy of the NLR of NGC 4151. I. Gas kinematics

TL;DR

This study uses high-resolution HST spectroscopy to analyze gas kinematics in NGC 4151's narrow-line region, revealing a transition from dark halo influence to a dominant central mass, with detailed insights into gas motions and mass distribution.

Contribution

First detailed kinematic analysis of NGC 4151's NLR at sub-arcsecond resolution, isolating individual cloud motions and revealing the mass distribution transition from dark halo to central concentration.

Findings

Gas in the extended region shows normal rotation in the galactic plane.

Inner gas kinematics are dominated by a central mass concentration with near-Keplerian fall-off.

Estimated mass within 60 pc is approximately 10^9 solar masses.

Abstract

We present the results from a detailed kinematic analysis of both ground-based, and Hubble Space Telescope/Faint Object Camera long-slit spectroscopy at sub-arcsec spatial resolution, of the narrow-line region of NGC 4151. In agreement with previous work, the extended emission gas (R > 4") is found to be in normal rotation in the galactic plane, a behaviour that we were able to trace even across the nuclear region, where the gas is strongly disturbed by the interaction with the radio jet, and connects smoothly with the large scale rotation defined by the neutral gas emission. The HST data, at 0.029" spatial resolution, allow us for the first time to truly isolate the kinematic behaviour of the individual clouds in the inner narrow-line region. We find that, underlying the perturbations introduced by the radio ejecta, the general velocity field can still be well represented by planar rotation down to a radius of ~ 0.5" (30 pc), distance at which the rotation curve has its turnover. The most striking result that emerges from our analysis is that the galaxy potential derived fitting the rotation curve changes from a "dark halo" at the ENLR distances to dominated by the central mass concentration in the NLR, with an almost Keplerian fall-off in the 1"< R < 4" interval. The observed velocity of the gas at 0.5" implies a mass of M ~ 10E9 M(sol) within the inner 60 pc. The presence of a turnover in the rotation curve indicates that this central mass concentration is extended. The first measured velocity point (outside the region saturated by the nucleus) would imply an enclosed mass of ~ 5E7 M(sol) within R ~ 0.15" (10 pc) which represents an upper limit to any nuclear point mass.