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Context. Carbon monoxide (CO) is a poor tracer of H2in the diffuse interstellar medium (ISM), where most of the carbon is not incorporated into CO molecules, unlike the situation at higher extinctions. Aims. We present a novel, indirect method for constraining H2column densities (NH2) without employing CO observations. We show that previously recognized nonlinearities in the relation between the extinction,AV(H2), derived from dust emission and the H Icolumn density (NH I) are due to the presence of molecular gas. Methods. We employed archival (NH2) data, obtained from the UV spectra of stars, and calculatedAV(H2) toward these sight lines using 3D extinction maps. The following relation fits the data: logNH2= 1.38742 (logAV(H2))3− 0.05359 (logAV(H2))2+ 0.25722 logAV(H2) + 20.67191. This relation is useful for constrainingNH2in the diffuse ISM as it requires onlyNH Iand dust extinction data, which are both easily accessible. In 95% of the cases, the estimates produced by the fitted equation have deviations of less than a factor of 3.5. We constructed aNH2map of our Galaxy and compared it to the CO integrated intensity (WCO) distribution. Results. We find that the average ratio (XCO) betweenNH2andWCOis approximately equal to 2 × 1020cm−2(K km s−1)−1, consistent with previous estimates. However, we find that theXCOfactor varies by orders of magnitude on arcminute scales between the outer and the central portions of molecular clouds. For regions withNH2≳ 1020cm−2, we estimate that the average H2fractional abundance,fH2= 2NH2/(2NH2+NH I), is 0.25. Multiple (distinct) largely atomic clouds are likely found along high-extinction sightlines (AV≥ 1 mag), hence limitingfH2in these directions. Conclusions. More than 50% of the lines of sight withNH2≥ 1020cm−2are untraceable by CO with aJ= 1−0 sensitivity limitWCO= 1 K km s−1.
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This content will become publicly available on September 1, 2025
H 3 + absorption and emission in local (U)LIRGs with JWST/NIRSpec: Evidence for high H 2 ionization rates
We study the 3.4 − 4.4 μm fundamental rovibrational band of H3+, a key tracer of the ionization of the molecular interstellar medium (ISM), in a sample of 12 local (d < 400 Mpc) (ultra)luminous infrared galaxies ((U)LIRGs) observed with JWST/NIRSpec. TheP,Q, andRbranches of the band are detected in 13 out of 20 analyzed regions within these (U)LIRGs, which increases the number of extragalactic H3+detections by a factor of 6. For the first time in the ISM, the H3+band is observed in emission; we detect this emission in three regions. In the remaining ten regions, the band is seen in absorption. The absorptions are produced toward the 3.4 − 4.4 μm hot dust continuum rather than toward the stellar continuum, indicating that they likely originate in clouds associated with the dust continuum source. The H3+band is undetected in Seyfert-like (U)LIRGs where the mildly obscured X-ray radiation from the active galactic nuclei might limit the abundance of this molecule. For the detections, the H3+abundances,N(H3+)/NH = (0.5 − 5.5)×10−7, imply relatively high ionization rates,ζH2, of between 3 × 10−16and > 4 × 10−15s−1, which are likely associated with high-energy cosmic rays. In half of the targets, the absorptions are blueshifted by 50–180 km s−1, which is lower than the molecular outflow velocities measured using other tracers such as OH 119 μm or rotational CO lines. This suggests that H3+traces gas close to the outflow-launching sites before it has been fully accelerated. We used nonlocal thermodynamic equilibrium models to investigate the physical conditions of these clouds. In seven out of ten objects, the H3+excitation is consistent with inelastic collisions with H2in warm translucent molecular clouds (Tkin ∼ 250–500 K andn(H2) ∼102 − 3cm−3). In three objects, dominant infrared pumping excitation is required to explain the absorptions from the (3,0) and (2,1) levels of H3+detected for the first time in the ISM.
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- Award ID(s):
- 2142300
- PAR ID:
- 10569721
- Publisher / Repository:
- Astronomy and Astrophysics
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 689
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- L12
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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