[Paper Review] Stellar dynamics observations of a double nucleus in M 83

N. Thatte, Matthias Tecza+1 more|arXiv (Cornell University)|Sep 25, 2000

Stellar, planetary, and galactic studies3 citations

TL;DR

This study presents the first direct evidence of a double nucleus in the spiral galaxy M 83 using near-infrared spectroscopy from the VLT ISAAC instrument. Observations reveal two velocity dispersion peaks separated by 2.7 arcseconds, each indicating a dynamical mass of 1.3 × 10⁷ M☉ within 5.4 pc, suggesting either massive star clusters or supermassive dark objects, with the secondary peak likely hidden by >10 mag of extinction at the galaxy's dynamical center.

ABSTRACT

We report on the discovery of a double nucleus in M 83, based on measurements of the line of sight velocity distribution of stars observed at near infrared wavelengths with the VLT ISAAC spectrograph. We observe two peaks separated by 2.7" in the velocity dispersion profile of light from late-type stars measured along a slit 0.6" wide, centered on the peak of K band emission and with P.A. 51.7 degrees. The first peak coincides with the peak of the K band light distribution, widely assumed to be the galaxy nucleus. The second peak, of almost equal strength, almost coincides with the center of symmetry of the outer isophotes of the galaxy. The secondary peak location has little K band emission, and appears to be significantly extincted, even at near infrared wavelengths. It also lies along a mid-infrared bar, previously identified by Gallais et al. (1991) and shows strong hydrogen recombination emission at 1.875 microns. If we interpret the observed stellar velocity dispersion as coming from a virialized system, the two nuclei would each contain an enclosed mass of 13.2 x 10^6 M_sun within a radius of 5.4pc. These could either be massive star clusters, or supermassive dark objects.

Motivation & Objective

• To investigate the stellar kinematics in the nuclear region of M 83 using high-resolution near-infrared spectroscopy.
• To determine whether the observed double peak in velocity dispersion indicates a binary nuclear system or a single nucleus with complex dynamics.
• To estimate the enclosed mass within 5.4 pc around each nucleus using stellar dynamics and Jeans equation modeling.
• To assess the nature of the secondary nucleus, particularly its obscured luminosity and potential role as the dynamical center of the galaxy.
• To explore the connection between the double nucleus, mid-infrared emission, and bar structures in M 83.

Proposed method

• Medium-resolution long-slit spectroscopy was performed with the VLT ISAAC instrument at near-infrared wavelengths (2.238–2.361 µm), covering the major and minor axes of M 83.
• Stellar velocity dispersion profiles were derived from deep CO bandhead features longward of 2.29 µm, using observed spectra and stellar templates from K1III to M5III giants.
• The Jeans equation was applied to estimate the enclosed dynamical mass within 5.4 pc, assuming a virialized system.
• Photometric data from SOFI on the NTT were used to identify the K band photometric peak and the center of symmetry of outer isophotes.
• Extinction estimates were inferred from the lack of K band emission at the secondary nucleus location, despite strong mid-infrared and Hα emission.
• The dynamical center was cross-validated by comparing the secondary nucleus position with the center of symmetry of outer isophotes and mid-infrared bar morphology.

Experimental results

Research questions

• RQ1 Does the velocity dispersion profile in the nuclear region of M 83 reveal evidence of a double nucleus?
• RQ2 What is the enclosed dynamical mass within 5.4 pc around each of the two velocity dispersion peaks?
• RQ3 Why is the secondary nucleus so faint in the K band despite having a significant mass concentration?
• RQ4 Is the secondary nucleus located at the true dynamical center of M 83, as indicated by isophotal symmetry and bar morphology?
• RQ5 What is the nature of the mid-infrared and Hα emission associated with the secondary nucleus?

Key findings

• Two distinct velocity dispersion peaks were detected, separated by 2.7 arcseconds, with the primary peak coinciding with the K band photometric peak and the secondary peak located 2.7 arcseconds southwest.
• Each nucleus is associated with a dynamical mass of 1.3 × 10⁷ M☉ within a radius of 5.4 pc, derived using the Jeans equation under the assumption of virial equilibrium.
• The primary nucleus hosts a stellar population of 2.5 × 10⁶ M☉, consistent with an instantaneous burst of star formation 25–60 million years ago.
• The secondary nucleus is obscured by more than 10 magnitudes of extinction, as evidenced by the lack of K band emission despite strong mid-infrared and Hα recombination emission at 1.875 µm.
• The secondary nucleus aligns precisely with the center of symmetry of the outer isophotes and the mid-infrared bar, indicating it is the true dynamical center of the galaxy.
• The system may be driven by a bar-induced inflow of gas, with star formation triggered along the bar, consistent with the observed mid-infrared and Hα emission.