Stellar Dynamics and the Implications on the Merger Evolution in NGC 6240

Abstract

We report near-infrared integral field spectroscopy of the luminous merging galaxy NGC 6240. Stellar velocities show that the two K-band peaks separated by 1. ′′ 6 are the central parts of inclined, rotating disk galaxies with equal mass bulges. The dynamical masses of the nuclei are much larger than the stellar mass derived from the K-band light, implying that the progenitor galaxies were galaxies with massive bulges. The K-band light is dominated by red supergiants formed in the two nuclei in starbursts, triggered ≈ 2× 10 7 years ago, possibly by the most recent perigalactic approach. Strong feedback effects of a superwind and supernovae are responsible for a short duration burst (≈ 5 × 10 6 years) which is already decaying. The two galaxies form a prograderetrograde rotating system and from the stellar velocity field it seems that one of the two interacting galaxies is subject to a prograde encounter. Between the stellar nuclei is a prominent peak of molecular gas (H 2 , CO). The stellar velocity dispersion peaks there indicating that the gas has formed a local, self-gravitating concentration decoupled from the stellar gravitational potential. NGC 6240 has previously been reported to fit the paradigm of an elliptical galaxy formed through the merger of two galaxies. This was based on the near-infrared light distribution which follows a r 1/4 -law. Our data cast strong doubt on this conclusion: the system is by far not relaxed, rotation plays an important role, as does self-gravitating gas, and the near-infrared light is dominated by young stars.NGC 6240 was observed with the MPE near-infrared imaging spectrograph 3D (Weitzel et al. 1996) in conjunction with the tip-tilt correction adaptive optics system ROGUE (Thatte et al. 1995) in two observing runs. 3D is an integral field spectrograph that simultaneously obtains spectra for each of 256 spatial pixels covering a square field of view with over 95% fill factor. In both observing runs the spectral resolving power (R ≡ λ/∆λ) was 2000 and Nyquist sampled using two settings of a piezo-driven flat mirror.The first observing run took place in April 1996 at the ESO 2.2 m telescope on La Silla, Chile. The pixel scale was 0. ′′ 3 per pixel, and a wavelength range from 2.18µm to 2.45µm was covered. The total on-source integration time was 4600 s with individual frame integration times of 300 s or 400 s. The same amount of time was spent off-source 1 ′ E and W of the nuclear region of NGC 6240 for sky background subtraction. The seeing during the observations was better than 0. ′′ 8.

N‐Band Imaging of Seyfert Nuclei and the Mid‐Infrared–X‐Ray Correlation

“…More speciÐcally, it is overluminous in hard X-rays by about an order of magnitude. Possibly, the dust extinction toward the nucleus of NGC 6240 is much higher than assumed based on NIR colors (Tecza et al 2000 ;Alexander et al 2000). Indeed, NGC 6240 is the most distant object of our sample, and the observations sample larger spatial scales (approximately kiloparsecs).…”

NGC 6240: merger-induced star formation and gas dynamics

“…A template spectrum, that of the M3 III star HD 176617, was prepared in the same way. Although both Sugai et al (1997) and Tecza et al (2000) used K Ib stars, they also both noted that M III stars provided almost equally good fits to the K-band features (see Fig. 5 in Tecza et al 2000).…”

“…Tacconi et al (1999) find the cold gas, as traced by CO(2-1) emission, to be concentrated between the two nuclei. The 1-0 S(1) H 2 line emission is one of the most powerful found in any galaxy to date (Joseph et al 1984), probably excited in shocks (van der Werf et al 1993;Sugai et al 1997;Tecza et al 2000;Lutz et al 2003). The stellar kinematics (Tecza et al 2000) display rotation centred on each of the two nuclei, and extraordinarily large stellar velocity dispersions (∼350 km s −1 ) between them.…”

“…The 1-0 S(1) H 2 line emission is one of the most powerful found in any galaxy to date (Joseph et al 1984), probably excited in shocks (van der Werf et al 1993;Sugai et al 1997;Tecza et al 2000;Lutz et al 2003). The stellar kinematics (Tecza et al 2000) display rotation centred on each of the two nuclei, and extraordinarily large stellar velocity dispersions (∼350 km s −1 ) between them. Tecza et al (2000) find evidence for a recent starburst in the central kpc (the region encompassing the two nuclei), and Pollack et al (2007) resolve a few dozen clusters in the nuclear region that are also consistent with a recent starburst.…”

“…In this paper we build on the qualitative picture put forward by Tecza et al (2000) that NGC 6240 is a merger already past its first close encounter, where the luminosity arises from a very recent burst of star formation, and therefore in which the mass must be due to an older stellar population. We use new adaptive optics integral field K-band spectra, interferometric mm CO(2-1) data, and merger star formation histories from numerical simulations, to perform a quantitative analysis of the mass, luminosity, age, and origin of the young and old stellar populations.…”