Hubble Space Telescope Observations of NGC 6240: a Case Study of an Ultra-Luminous Infrared Galaxy with Obscured Activity

Abstract

We present results from an HST study of the morphology and kinematics of NGC 6240. This merging galaxy with a double nucleus is one of the nearest and best-studied ultraluminous infrared galaxies. HST resolves both nuclei into seperate components. The distance between the northern and southern optical/near-infrared components is greater than that observed in radio and X-ray studies, arguing that even in K-band we may not be seeing all the way through the dust to the true nuclei. The ionized gas does not display rotation around either of the nuclei, or equilibrium motion in general. There is a strong velocity gradient between the nuclei, similar to what is seen in CO data. There is no such gradient in our stellar kinematics. The velocity dispersion of the gas is larger than expected for a cold disk. We also map and model the emission-line velocity field at an off-nuclear position where a steep velocity gradient was previously detected in ground-based data. Overall, the data indicate that line-of-sight projection effects, dust absorption, non-equilibrium merger dynamics, and the possible influence of a wind may be playing an important role in the observed kinematics. Chandra observations of hard X-rays have shown that both of the nuclei contain an Active Galactic Nucleus (AGN). The HST data show no clear sign of the two AGNs: neither continuum nor narrow-band imaging shows evidence for unresolved components in the nuclei, and there are no increased emission line widths or rapid rotation near the nuclei. This underscores the importance of X-ray data for identifying AGNs in highly dust-enshrouded environments.

arXiv commentLaTeX, 32 pages, 9 figures, 2 tables, accepted for publication in The Astronomical Journal (Jan 2004). Paper with high-resolution (non-compressed) color figures in gzipped postscript format available at http://www.stsci.edu/~marel/psgzdir/ngc6240v11.ps.gz

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Cited by in corpus (38)

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• Outflows in Infrared-Luminous Starbursts at z < 0.5. II. Analysis and Discussion
• Fueling Low-Level AGN Activity Through the Stochastic Accretion of Cold Gas
• The Relation Between Quasar and Merging Galaxy Luminosity Functions and the Merger-Induced Star Formation Rate of the Universe
• Constraints on the Star Formation Rate in Active Galaxies
• High Resolution Imaging of Warm and Dense Molecular Gas in the Nuclear Region of the Luminous Infrared Galaxy NGC6240
• ALMA [CI]${}^3{P}_1{-}^3{P}_0$ observations of NGC6240: a puzzling molecular outflow, and the role of outflows in the global ${\alpha }_{\mathrm{CO}}$ factor of (U)LIRGs
• NGC6240: Merger-Induced Star Formation & Gas Dynamics
• The Core of NGC 6240 from Keck Adaptive Optics and HST NICMOS Observations
• NGC6240: extended CO structures and their association with shocked gas
• High resolution mapping of CO(1-0) in NGC6240
• HIFI Spectroscopy of ${\mathrm{H}}_2\mathrm{O}$ submm Lines in Nuclei of Actively Star Forming Galaxies
• The Molecular Gas in the NGC 6240 Merging Galaxy System at the Highest Spatial Resolution
• Mass of the Southern Black Hole in NGC 6240 from Laser Guide Star Adaptive Optics
• The Exceptional Soft X-ray Halo of the Galaxy Merger NGC 6240
• Optical Properties of Host Galaxies of Extragalactic Nuclear Water Masers
• Dense and Warm Molecular Gas between Double Nuclei of the Luminous Infrared Galaxy NGC 6240
• Giant H$\alpha$ nebula surrounding the starburst merger NGC 6240
• AGN impact on the molecular gas in galactic centers as probed by CO lines
• Imaging the Molecular Outflows of the Prototypical ULIRG NGC 6240 with ALMA
• Examining the Seyfert - Starburst Connection with Arcsecond Resolution Radio Continuum Observations
• The Toomre Sequence Revisited with HST/NICMOS: Nuclear Brightness Profiles and Colors of Interacting and Merging Galaxies
• Sub-Arcsecond Mid-Infrared Observations of NGC 6240: Limitations of AGN-Starburst Power Diagnostics
• Nuclear absorption and emission in the AGN merger NGC 6240: the hard X-ray view
• HI and OH Absorption toward NGC6240
• Circumnuclear Star Clusters in the Galaxy Merger NGC 6240, Observed with Keck Adaptive Optics and HST
• Dark Matter Halo Mergers I: Dependence on Environment & Redshift Evolution
• Optical spectral classification of southern ultraluminous infrared galaxies
• The Structure of Active Merger Remnant NGC 6240 from IRAC Observations
• NGC 6240 Supermassive Black Hole Binary dynamical evolution based on Chandra data
• The JWST/NIRSpec view of the nuclear region in the prototypical merging galaxy NGC 6240
• Dissecting the Extended X-ray Emission in the Merging Pair NGC 6240
• Molecular Gas in the Nuclear Region of NGC 6240
• Three Spectacular HII-buried-AGN Galaxies from SDSS
• Dynamical evolution timescales for the triple supermassive black hole system in NGC 6240
• Two Epochs of Very Large Array Observations of Water Maser Emission in the active galaxy NGC 6240
• Outflowing shocked gas dominates the NIR H${}_2$ emission from the dual AGN NGC6240