REVIEW
The Interstellar Medium of IRAS 08572+3915 NW: H3+ and Warm High Velocity CO
Astrophysics · Submitted 2 Mar 2006 · Published version · 41 references
Pith's verdict: The paper convincingly confirms extragalactic H3+ and reports genuinely new velocity-resolved warm CO absorption, but the CO kinematic story and transient-AGN interpretation are more speculative than the data can firmly support.
T0 · AI review
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Referee report
The paper
The paper claims
Infrared absorption lines reveal a 350 km/s warm CO outflow around a buried active nucleus in IRAS 08572+3915 NW.
The reading
What carries the argument
The two diagnostic probes are absorption lines of H$_3^+$ in the 3.7--3.9 $\mu$m region and the CO fundamental band near 4.9 $\mu$m. H$_3^+$, the ion on which interstellar gas-phase chemistry is built, has the property that its column density depends on the cosmic-ray ionization rate and path length but not on gas density in the steady state, so its detection gives a direct handle on the diffuse gas toward the nucleus. The CO absorption, resolved into individual rotational lines, acts as a velocity-resolved thermometer: the presence of lines through $J=12$ at roughly equal strength shows the warm blueshifted gas is in LTE near 200 K, and the strong asymmetry between blueshifted and redshifted absorption is the key discriminator. The authors favor a model in which the absorbing CO lies on the outer surface of an expanding, optically thick continuum source, giving projected velocities from near zero at the edges to $-350$ km s$^{-1}$ at the center; this geometry, unlike a torus, naturally produces strongly asymmetric absorption.
Extended reading notes
Core claim
Toward the northwest nucleus of the interacting galaxy IRAS 08572+3915, the lowest para and ortho levels of H$_3^+$ give column densities of $1.8\pm0.6\times10^{15}$ cm$^{-2}$ and $2.6\pm0.6\times10^{15}$ cm$^{-2}$, respectively, a total of roughly $4.4\times10^{15}$ cm$^{-2}$, the first robust H$_3^+$ detection in an external galaxy. The CO fundamental band shows a cold component centered near the systemic velocity, a warm blueshifted component that extends from about 0 to at least $-350$ km s$^{-1}$ and is seen in rotational levels up to $J=12$, and a weaker warm redshifted component near $+100$ km s$^{-1}$. The high-$J$ CO population implies a kinetic temperature near 200 K and a density at least $3\times10^6$ cm$^{-3}$, and the column length of the warm blueshifted gas is under 0.01 pc, placing the absorbing gas within roughly 10 pc of the $2\times10^{12}$ $L_\odot$ source. The authors conclude that the line-forming gas sits just outside the dust continuum surface of a buried active nucleus and is being expelled at about 350 km s$^{-1}$ in a transient event that began some 30,000 years ago.
Load-bearing premise
The argument stands or falls on the adopted systemic redshift of the galaxy ($z=0.0583$, from CO(1-0) emission) and on clean removal of telluric absorption by division by standard-star spectra; if the redshift is wrong by more than about 30 km s$^{-1}$, or if residual telluric features mimic the broad blueshifted CO, the decomposition into cold, warm-blueshifted, and warm-redshifted components, and the claimed 350 km s$^{-1}$ extension, does not hold.
Editorial extensions
If this is right
- The same H$_3^+$ and CO absorption technique can be applied to other heavily obscured ULIRGs; the presence or absence of a broad, velocity-asymmetric warm CO component would reveal whether similar transient nuclear events are common.
- A 350 km s$^{-1}$ ejection on a 30,000-year timescale means IRAS 08572+3915 NW is currently in a short active phase, so multi-epoch infrared and X-ray monitoring could catch changes in the absorption profile.
- The H$_3^+$ column density, combined with the elevated cosmic-ray ionization rate seen in Galactic diffuse clouds, implies a column length of less than 100 pc for the diffuse gas, showing that the bulk of the H$_3^+$ is close to the nucleus.
- If the warm blueshifted gas is in LTE at 200 K with $n(\mathrm{H}_2) \gtrsim 3\times10^6$ cm$^{-3}$, the same gas should be visible in other molecular tracers such as H$_2$ or OH at comparable velocities; future observations can test this.
Reading between the lines
- The expanding-shell model predicts that the continuum source should be partially covered at the highest blueshifted velocities; a measurement of the covering fraction versus velocity would test whether the geometry is a shell or separated clouds.
- Orientation effects may matter: if the outflow is collimated rather than spherical, a small survey of similar ULIRGs should show a mix of blue-dominated and red-dominated CO absorption, and the ratio could be used to infer the average opening angle of the ejection.
- The tentative 100 K excitation temperature of H$_3^+$ could be sharpened by observing the R(2,2)l and R(3,3)l lines; depending on the result, the H$_3^+$ may trace either a warmer diffuse component or a contribution from dense clouds, which would change the derived pathlength.
- Comparing the CO absorption profile with the 10 $\mu$m silicate feature at the same resolution could establish whether the same dust and gas are moving together, and thus whether the outflow is dusty or nearly dust-free.
Original abstract
We confirm the first detection of the molecular ion H3+ in an extragalactic object, the highly obscured ultraluminous galaxy IRAS 08572+3915 NW. We also have detected absorption lines of the fundamental band of CO in this galaxy. The CO absorption consists of a cold component close to the systemic velocity and warm, highly blueshifted and redshifted components. The warm blueshifted component is remarkably strong and broad and extends at least to -350 km/s. Some analogies can be drawn between the H3+ and cold CO in IRAS08572+3915 NW and the same species seen toward the Galactic center. The profiles of the warm CO components are not those expected from a dusty torus of the type thought to obscure active galactic nuclei. They are probably formed close to the dust continuum surface near the buried and active nucleus and are probably associated with an unusual and energetic event there.
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
We confirm the first detection of the molecular ion H3+ in an extragalactic object, the highly obscured ultraluminous galaxy IRAS 08572+3915 NW. We also have detected absorption lines of the fundamental band of CO in this galaxy. The CO absorption consists of a cold component close to the systemic velocity and warm, highly blueshifted and redshifted components. The warm blueshifted component is remarkably strong and broad and extends at least to -350 km/s. Some analogies can be drawn between the H3+ and cold CO in IRAS08572+3915 NW and the same species seen toward the Galactic center. The profiles of the warm CO components are not those expected from a dusty torus of the type thought to obscure active galactic nuclei. They are probably formed close to the dust continuum surface near the buried and active nucleus and are probably associated with an unusual and energetic event there.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We confirm the first detection of the molecular ion H3+ in an extragalactic object, the highly obscured ultraluminous galaxy IRAS 08572+3915 NW. | result 0.94 | 0.94 | 0.69 | asserted 1.00 |
| s1 | We also have detected absorption lines of the fundamental band of CO in this galaxy. | result 0.99 | 0.95 | 0.08 | asserted 1.00 |
| s2 | The CO absorption consists of a cold component close to the systemic velocity and warm, highly blueshifted and redshifted components. | result 1.00 | 0.88 | 0.12 | asserted 1.00 |
| s3 | The warm blueshifted component is remarkably strong and broad and extends at least to -350 km/s. | result 1.00 | 0.93 | 0.12 | asserted 1.00 |
| s4 | Some analogies can be drawn between the H3+ and cold CO in IRAS08572+3915 NW and the same species seen toward the Galactic center. | interpretation 0.88 | 0.49 | 0.07 | hedged possibility 0.87 |
| s5 | The profiles of the warm CO components are not those expected from a dusty torus of the type thought to obscure active galactic nuclei. | interpretation 0.68 | 0.84 | 0.17 | asserted 0.99 |
| s6 | They are probably formed close to the dust continuum surface near the buried and active nucleus and are probably associated with an unusual and energetic event there. | interpretation 1.00 | 0.70 | 0.42 | hedged possibility 0.99 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:astro-ph/0603041 by the authors.
References
How the literature uses it
1 arXiv paper cite this in 1 sentence
Discussion
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