Keywords

We present Atacama large millimeter/submillimeter array (ALMA) and compact array (ACA) [C i]${}^{3}{P}_{1}{-}^{3}{P}_{0}$ ([C i](1–0)) observations of NGC 6240, which we combine with ALMA CO(2–1) and IRAM Plateau de Bure Interferometer CO(1–0) data to study the physical properties of the massive molecular (H₂) outflow. We discover that the receding and approaching sides of the H₂ outflow, aligned east–west, exceed 10 kpc in their total extent. High resolution ($0\buildrel{\prime\prime}\over{.} 24$) [C i](1–0) line images surprisingly reveal that the outflow emission peaks between the two active galactic nuclei (AGNs), rather than on either of the two, and that it dominates the velocity field in this nuclear region. We combine the [C i](1–0) and CO(1–0) data to constrain the CO-to-H₂ conversion factor (${\alpha }_{\mathrm{CO}}$) in the outflow, which is on average $2.1\pm 1.2\,{M}_{\odot }{({\rm{K}}\mathrm{km}{{\rm{s}}}^{-1}{\mathrm{pc}}^{2})}^{-1}$. We estimate that 60 ± 20% of the total H₂ gas reservoir of NGC 6240 is entrained in the outflow, for a resulting mass-loss rate of ${\dot{M}}_{\mathrm{out}}=2500\pm 1200\,{M}_{\odot }\,{\mathrm{yr}}^{-1}\equiv 50\pm 30$ SFR. These energetics rule out a solely star formation-driven wind, but the puzzling morphology challenges a classic radiative-mode AGN feedback scenario. For the quiescent gas, we compute $\langle {\alpha }_{\mathrm{CO}}\rangle =3.2\pm 1.8\,{M}_{\odot }{({\rm{K}}\mathrm{km}{{\rm{s}}}^{-1}{\mathrm{pc}}^{2})}^{-1}$, which is at least twice the value commonly employed for (ultra) luminous infrared galaxies ((U)LIRGs). We observe a tentative trend of increasing ${r}_{21}\equiv {L}_{\mathrm{CO}(2-1)}^{{\prime} }/{L}_{\mathrm{CO}(1-0)}^{{\prime} }$ ratios with velocity dispersion and measure r₂₁ > 1 in the outflow, whereas r₂₁ ≃ 1 in the quiescent gas. We propose that molecular outflows are the location of the warmer, strongly unbound phase that partially reduces the opacity of the CO lines in (U)LIRGs, hence driving down their global ${\alpha }_{\mathrm{CO}}$ and increasing their r₂₁ values.

We revealed the detailed structure of a vastly extended Hα-emitting nebula ("Hα nebula") surrounding the starburst/merging galaxy NGC 6240 by deep narrow-band imaging observations with the Subaru Suprime-Cam. The extent of the nebula is ∼90 kpc in diameter and the total Hα luminosity amounts to L_Hα ≈ 1.6 × 10⁴² erg s⁻¹. The volume filling factor and the mass of the warm ionized gas are ∼10⁻⁴–10⁻⁵ and ∼5 × 10⁸ M_⊙, respectively. The nebula has a complicated structure, which includes numerous filaments, loops, bubbles, and knots. We found that there is a tight spatial correlation between the Hα nebula and the extended soft-X-ray-emitting gas, both in large and small scales. The overall morphology of the nebula is dominated by filamentary structures radially extending from the center of the galaxy. A large-scale bipolar bubble extends along the minor axis of the main stellar disk. The morphology strongly suggests that the nebula was formed by intense outflows—superwinds—driven by starbursts. We also found three bright knots embedded in a looped filament of ionized gas that show head-tail morphologies in both emission-line and continuum, suggesting close interactions between the outflows and star-forming regions. Based on the morphology and surface brightness distribution of the Hα nebula, we propose the scenario that three major episodes of starburst/superwind activities, which were initiated ∼10² Myr ago, formed the extended ionized gas nebula of NGC 6240.

We present the first observations of H¹³CN $(1-0)$, H¹³CO⁺$(1-0)$, and SiO $(2-1)$in NGC 6240, obtained with the IRAM Plateau de Bure Interferometer. Combining a Markov Chain Monte Carlo code with Large Velocity Gradient (LVG) modeling, and with additional data from the literature, we simultaneously fit three gas phases and six molecular species to constrain the physical condition of the molecular gas, including mass−luminosity conversion factors. We find $\sim {10}^{10}{M}_{\odot }$ of dense molecular gas in cold, dense clouds (${T}_{{\rm{k}}}\sim 10$ K, ${n}_{{{\rm{H}}}_{2}}\sim {10}^{6}$ cm⁻³) with a volume filling factor $\lt 0.002,$ embedded in a shock heated molecular medium (${T}_{{\rm{k}}}\sim 2000$ K, ${n}_{{{\rm{H}}}_{2}}\sim {10}^{3.6}$ cm⁻³), both surrounded by an extended diffuse phase (${T}_{{\rm{k}}}\sim 200$ K, ${n}_{{{\rm{H}}}_{2}}\sim {10}^{2.5}$ cm⁻³). We derive a global ${\alpha }_{{\rm{CO}}}={1.5}_{1.1}^{7.1}$ with gas masses ${\mathrm{log}}_{10}\left(M/[{M}_{\odot }]\right)={10.1}_{10.0}^{10.8},$ dominated by the dense gas. We also find ${\alpha }_{{\rm{HCN}}}={32}_{13}^{89},$ which traces the cold, dense gas. The [¹²C]/[¹³C] ratio is only slightly elevated (${98}_{65}^{230}$), contrary to the very high [CO]/[¹³CO] ratio (300–500) reported in the literature. However, we find very high [HCN]/[H¹³CN] and [HCO⁺]/[H¹³CO⁺] abundance ratios $({300}_{200}^{500})$ which we attribute to isotope fractionation in the cold, dense clouds.

The European Very Long Baseline Interferometry (VLBI) Network was used at two epochs in 2003 and 2009 to obtain multi-frequency high-resolution images of the merger galaxy NGC 6240 in order to study the radio properties of all compact high-brightness components in the galaxy. Our observations at milliarcsecond resolution detected the northern and southern nuclei and two radio components, which we interpret as long-lived luminous supernovae associated with circumnuclear starburst activity in the southern nucleus. The new VLBI data support the presence of an active galactic nucleus (AGN) together with starburst activity in the southern nucleus and provide some evidence for an AGN in the northern nucleus. Both nuclei display an inverted spectrum at lower GHz frequencies. The spectrum of the southern nucleus indicates thermal free–free absorption on parsec scales, consistent with the presence of an AGN.

Studies of 22 GHz H₂O maser emission from the merging galaxy NGC 6240 with double nuclei are presented. Two epochs of Very Large Array (VLA) observations in the A-configuration in spectral-line mode were carried out at 0.1 arcsec resolution by covering the redshifted velocity range of ∼300 km s⁻¹ with respect to the systemic velocity of the galaxy. The purpose of these new observations is twofold: to detect an H₂O maser that an earlier VLA observation pinpointed in the southern nucleus in the northern nucleus as well to clarify the kinematics of the double nuclei, and to understand the origin of the maser in the galaxy. In the second epoch, one velocity feature peaking at V_LSR = 7491.1 km s⁻¹, redshifted by ∼200 km s⁻¹ relative to the systemic velocity, was detected only toward the southern nucleus. The detection of an H₂O maser feature at or near this velocity had never been reported in earlier observations. However, including the known velocity features at redshifted velocities, no other velocity features were observed toward either nuclei throughout these epochs. The maser remains unresolved at an angular resolution of ∼01, corresponding to a linear size of less than about 45 pc. The two epochs of VLA observations show that the maser intensity is variable on timescales of at least three months, while the correlation between the maser intensity and the radio continuum intensity is not certain from our data. It is plausible that the maser in NGC 6240 is associated with the activity of an active galactic nucleus in the southern nucleus. Alternatively, the maser can be explained by star-forming activity at the site of massive star formation in the galaxy.