REVIEW
Imaging Molecular Gas in the Luminous Merger NGC 3256 : Detection of High-Velocity Gas and Twin Gas Peaks in the Double Nucleus
Astrophysics · Submitted 3 Mar 2006 · 95 references
Pith's verdict: Solid first interferometric view of molecular gas in NGC 3256 with a real detection of high-velocity outflow gas, but the quantitative outflow rate and depletion time depend on an unverified CO-to-H2 conversion factor in the outflowing gas.
Why Pith says this
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Referee report
The paper
The paper claims
The starburst wind in the merging galaxy NGC 3256 is expelling molecular gas at roughly ten solar masses per year, comparable to the rate at which the starburst consumes gas.
The reading
What carries the argument
The load-bearing observational tool is the Submillimeter Array's imaging of $^{12}$CO(2–1), $^{13}$CO(2–1), and C$^{18}$O(2–1) at roughly $1''\times2''$ (170 × 340 pc) resolution, which lets the authors construct position–velocity diagrams and a central $5''$ spectrum. The outflow interpretation is carried by the coincidence of the extreme-velocity CO wings with the previously known blueshifted Na I D absorption near 300 km s$^{-1}$, by the absence of a K-band mass concentration or stellar velocity gradient between the nuclei that rotation would require, and by a comparison of the outflow's mechanical luminosity with the mechanical energy available from a starburst model. The outflow mass, rate, and energy are computed from the CO(2–1) flux in the wings using a CO-to-H$_2$ conversion factor, and the depletion time is obtained from the flux fraction and the crossing time.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is that the center of the luminous merger NGC 3256 contains a substantial amount of molecular gas moving at extreme velocities: up to 420 km s$^{-1}$ on the redshifted side and 240 km s$^{-1}$ on the blueshifted side relative to the systemic velocity, located between the two nuclei rather than on either one. The paper models this high-velocity gas as a molecular outflow from the main gas disk, entrained by the superwind produced by the central starburst; the mass in the fast component is about $1.4\times10^7\,M_\odot$ (red) and $0.3\times10^7\,M_\odot$ (blue), giving an outflow rate of roughly $10\,M_\odot\,\mathrm{yr}^{-1}$, a momentum flux of about $10^{29}$ N, and a kinetic luminosity of about $2\times10^{34}$ W. From the flux fraction (3% of the central $5''$ CO flux) and a crossing time of about 2 Myr, the authors derive a molecular-gas depletion time of about 70 Myr, making the outflow competitive with star formation in removing gas. A second claimed discovery is that each of the two nuclei has a compact CO peak and a steep east–west velocity gradient, interpreted as a small rotating gas disk with dynamical mass of order $10^9\,M_\odot$ within 170 pc.
Load-bearing premise
The claim's quantitative force rests on assuming that the CO emission of the fast-moving gas has the same luminosity per unit molecular gas mass as the quiescent disk gas, even though the paper states that the outflow's CO emissivity is unknown and probably different.
Editorial extensions
If this is right
- If the outflow interpretation holds, starburst-driven winds in luminous mergers can remove molecular gas at roughly the same rate that star formation consumes it, about 10 solar masses per year.
- The central molecular gas would be dispersed or depleted on a timescale of about 70 million years, short enough to shape the lifetime of the starburst.
- The twin mini-disks would make NGC 3256 a sibling of Arp 220, supporting the idea that massive rotating gas disks around individual merger nuclei are a common late-stage feature.
- The high $^{12}$CO/$^{13}$CO ratios and their radial gradients imply that merger gas is largely warm, turbulent, moderate-opacity gas, with subthermal excitation in the outer disk.
- Because only the fast wing of the outflow is detected, the total molecular outflow—including slower gas—is likely larger than the measured $10\,M_\odot\,\mathrm{yr}^{-1}$.
Reading between the lines
- The 70 Myr depletion time is less sensitive than the absolute rate to the unknown CO emissivity of the outflow, because it uses only the flux fraction and the crossing time; if the outflow is warmer and more diffuse, the absolute rate would fall but the depletion estimate would survive better.
- The non-detection of the high-velocity gas in $^{13}$CO and C$^{18}$O is consistent with optically thin outflow gas; deeper observations of these isotopologues could directly measure the outflow's column density and test the mass estimate.
- The blueshifted CO and Na I absorption likely trace the same wind; joint modeling of the atomic and molecular phases could yield the entrainment efficiency and the fraction of the wind that is molecular.
- The mini-disk interpretation predicts that higher-resolution observations will keep the steep velocity gradients centered on the radio/X-ray nuclei; a resolved rotation curve would also test whether the gradient is rotation rather than a compact outflow.
Original abstract
Molecular gas in the merging starburst galaxy NGC 3256 has been imaged with the Submillimeter Array at a resolution of 1'' x 2'' (170 x 340 pc at 35 Mpc). This is the first interferometric imaging of molecular gas in the most luminous galaxy within z=0.01. There is a large disk of molecular gas (r > 3 kpc) in the center of the merger with a strong gas concentration toward the double nucleus. The gas disk having a mass of ~3*10^9 Msun in the central 3 kpc rotates around a point between the two nuclei that are 850 pc apart on the sky. The molecular gas is warm and turbulent and shows spatial variation of the intensity ratio between CO isotopomers. High-velocity molecular gas is discovered at the galactic center. Its velocity in our line of sight is up to 420 km/s offset from the systemic velocity of the galaxy; the terminal velocity is twice as large as that due to the rotation of the main gas disk. The high-velocity gas is most likely due to a molecular outflow from the gas disk, entrained by the starburst-driven superwind in the galaxy. The molecular outflow is estimated to have a rate of ~10 Msun/yr and to play a significant role in the dispersal or depletion of molecular gas from the galactic center. A compact gas concentration and steep velocity gradient are also found around each of the twin nuclei. They are suggestive of a small gas disk rotating around each nucleus. If these are indeed mini-disks, their dynamical masses are ~10^9 Msun within a radius of 170 pc.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
Molecular gas in the merging starburst galaxy NGC 3256 has been imaged with the Submillimeter Array at a resolution of 1'' x 2'' (170 x 340 pc at 35 Mpc). This is the first interferometric imaging of molecular gas in the most luminous galaxy within z=0.01. There is a large disk of molecular gas (r > 3 kpc) in the center of the merger with a strong gas concentration toward the double nucleus. The gas disk having a mass of ~3*10^9 Msun in the central 3 kpc rotates around a point between the two nuclei that are 850 pc apart on the sky. The molecular gas is warm and turbulent and shows spatial variation of the intensity ratio between CO isotopomers. High-velocity molecular gas is discovered at the galactic center. Its velocity in our line of sight is up to 420 km/s offset from the systemic velocity of the galaxy; the terminal velocity is twice as large as that due to the rotation of the main gas disk. The high-velocity gas is most likely due to a molecular outflow from the gas disk, entrained by the starburst-driven superwind in the galaxy. The molecular outflow is estimated to have a rate of ~10 Msun/yr and to play a significant role in the dispersal or depletion of molecular gas from the galactic center. A compact gas concentration and steep velocity gradient are also found around each of the twin nuclei. They are suggestive of a small gas disk rotating around each nucleus. If these are indeed mini-disks, their dynamical masses are ~10^9 Msun within a radius of 170 pc.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Molecular gas in the merging starburst galaxy NGC 3256 has been imaged with the Submillimeter Array at a resolution of 1'' x 2'' (170 x 340 pc at 35 Mpc). | method 0.86 | 0.93 | 0.04 | asserted 1.00 |
| s1 | This is the first interferometric imaging of molecular gas in the most luminous galaxy within z=0.01. | contribution 0.82 | 0.62 | 0.05 | asserted 1.00 |
| s2 | There is a large disk of molecular gas (r > 3 kpc) in the center of the merger with a strong gas concentration toward the double nucleus. | result 1.00 | 0.92 | 0.10 | asserted 1.00 |
| s3 | The gas disk having a mass of ~3*10^9 Msun in the central 3 kpc rotates around a point between the two nuclei that are 850 pc apart on the sky. | result 1.00 | 0.92 | 0.10 | asserted 1.00 |
| s4 | The molecular gas is warm and turbulent and shows spatial variation of the intensity ratio between CO isotopomers. | result 1.00 | 0.91 | 0.03 | asserted 1.00 |
| s5 | High-velocity molecular gas is discovered at the galactic center. | result 1.00 | 0.95 | 0.25 | asserted 1.00 |
| s6 | Its velocity in our line of sight is up to 420 km/s offset from the systemic velocity of the galaxy; the terminal velocity is twice as large as that due to the rotation of the main gas disk. | result 1.00 | 0.93 | 0.11 | asserted 1.00 |
| s7 | The high-velocity gas is most likely due to a molecular outflow from the gas disk, entrained by the starburst-driven superwind in the galaxy. | interpretation 1.00 | 0.74 | 0.29 | hedged possibility 1.00 |
| s8 | The molecular outflow is estimated to have a rate of ~10 Msun/yr and to play a significant role in the dispersal or depletion of molecular gas from the galactic center. | result 0.51 | 0.91 | 0.25 | asserted 0.70 |
| s9 | A compact gas concentration and steep velocity gradient are also found around each of the twin nuclei. | result 1.00 | 0.94 | 0.06 | asserted 1.00 |
| s10 | They are suggestive of a small gas disk rotating around each nucleus. | interpretation 1.00 | 0.79 | 0.06 | hedged possibility 1.00 |
| s11 | If these are indeed mini-disks, their dynamical masses are ~10^9 Msun within a radius of 170 pc. | interpretation 0.54 | 0.79 | 0.07 | hedged possibility 0.81 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:astro-ph/0603079 by the authors.
References
Discussion
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