Detection of Interstellar H[TINF]2[/TINF]D[TSUP]+[/TSUP] Emission (original) (raw)
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Herschel/PACS spectroscopy of NGC 4418 and Arp 220: H2O, H2^{18} O, OH,^{18} OH, O^ 0, HCN and NH3
Arxiv preprint arXiv: …, 2011
Full range Herschel/PACS spectroscopy of the (ultra)luminous infrared galaxies NGC 4418 and Arp 220, observed as part of the SHINING key programme, reveals high excitation in H 2 O, OH, HCN, and NH 3 . In NGC 4418, absorption lines were detected with E lower >800 K (H 2 O), 600 K (OH), 1075 K (HCN), and 600 K (NH 3 ), while in Arp 220 the excitation is somewhat lower. While outflow signatures in moderate excitation lines are seen in Arp 220 as have been seen in previous studies, in NGC 4418 the lines tracing its outer regions are redshifted relative to the nucleus, suggesting an inflow withṀ 12 M ⊙ yr −1 . Both galaxies have compact and warm (T dust 100 K) nuclear continuum components, together with a more extended and colder component that is much more prominent and massive in Arp 220. A chemical dichotomy is found in both sources: on the one hand, the nuclear regions have high H 2 O abundances, ∼10 −5 , and high HCN/H 2 O and HCN/NH 3 column density ratios of 0.1−0.4 and 2−5, respectively, indicating a chemistry typical of evolved hot cores where grain mantle evaporation has occurred. On the other hand, the high OH abundance, with OH/H 2 O ratios of ∼0.5, indicates the effects of X-rays and/or cosmic rays. The nuclear media have high surface brightnesses ( 10 13 L ⊙ /kpc 2 ) and are estimated to be very thick (N H 10 25 cm −2 ). While NGC 4418 shows weak absorption in H 18 2 O and 18 OH, with a 16 O-to-18 O ratio of 250−500, the relatively strong absorption of the rare isotopologues in Arp 220 indicates 18 O enhancement, with 16 O-to-18 O of 70−130. Further away from the nuclear regions, the H 2 O abundance decreases to 10 −7 and the OH/H 2 O ratio is reversed relative to the nuclear region to 2.5−10. Despite the different scales and morphologies of NGC 4418, Arp 220, and Mrk 231, preliminary evidence is found for an evolutionary sequence from infall, hot-core like chemistry, and solar oxygen isotope ratio to high velocity outflow, disruption of the hot core chemistry and cumulative high mass stellar processing of 18 O.
The Astrophysical Journal, 2015
We modelled recent observations of UV absorption of HD and H 2 in the Milky Way and toward damped/sub-damped Lyman alpha systems at z=0.18 and z > 1.7. N(HD)/N(H 2) ratios reflect the separate self-shieldings of HD and H 2 and the coupling introduced by deuteration chemistry. Locally, observations are explained by diffuse molecular gas with 16 cm −3 n(H) 128 cm −3 if the cosmic-ray ionization rate per H-nucleus ζ H = 2×10 −16 s −1 as inferred from H 3 + and OH +. The dominant influence on N(HD)/N(H 2) is the cosmic-ray ionization rate with a much weaker downward dependence on n(H) at Solar metallicity, but dust-extinction can drive N(HD) higher as with N(H 2). At z > 1.7, N(HD) is comparable to the Galaxy but with 10x smaller N(H 2) and somewhat smaller N(H 2)/N(H I). Comparison of our Galaxy and the Magellanic Clouds shows that smaller H 2 /H is expected at sub-Solar metallicity and we show by modelling that HD/H 2 increases with density at low metallicity, opposite to the Milky Way. Observations of HD would be explained with higher n(H) at low metallicity but high-z systems have high HD/H 2 at metallicity 0.04 Z 2 Solar. In parallel we trace dust-extinction and self-shielding effects. The abrupt H 2 transition to H 2 /H ≈ 1-10% occurs mostly from self-shielding although it is assisted by extinction for n(H) 16 cm −3. Interior H 2 fractions are substantially increased by dust extinction below 32 cm −3. At smaller n(H), ζ H , small increases in H 2 triggered by dust extinction can trigger abrupt increases in N(HD).
Molecular Hydrogen Excitation around Active Galactic Nuclei
The Astrophysical Journal, 2005
We report R ∼ 3000 Very Large Telescope ISAAC K-band spectroscopy of the nuclei (i.e. central 100-300 pc) of nine galaxies hosting an active galactic nucleus. For five of these we also present spectra of the circumnuclear region out to 1 kpc. We have measured a number of molecular hydrogen lines in the ν = 1 − 0, 2 − 1, and 3 − 2 vibrational transitions, as well as the Brγ and He i recombination lines, and the Na i stellar absorption feature. Although only three of the galaxies are classified as type 1 Seyferts in the literature, broad Brγ (FWHM 1000 km s −1) is seen in seven of the objects. The ν = 1 − 0 emission appears thermalised at temperatures T ∼ 1000 K. However, the ν = 2 − 1 and ν = 3 − 2 emission show evidence of being radiatively excited by far-ultraviolet photons. The photo-dissociation region models that fit the data best are, as for the ultraluminous infrared galaxies in Davies et al. (2003), those for which the H 2 emission arises in dense clouds illuminated by intense FUV radiation. The Na i stellar absorption line is clearly seen in six of the nuclear spectra of these AGN, indicating the presence of a significant population of late type stars. It is possible that these stars are a result of the same episode of star formation that gave rise to the stars heating the PDRs. It seems unlikely that the AGN is the dominant source of excitation for the near infrared H 2 emission: in two of the nuclear spectra H 2 was not detected at all, and in general we find no evidence of suppression of the 2-1 S(3) line, which may occur in X-ray irradiated gas. Our data do not reveal any significant difference between the nuclear and circumnuclear line ratios, suggesting that the physical conditions of the dominant excitation mechanism are similar both near the AGN and in the larger scale environment around it, and that star formation is an important process even in the central 100 pc acround AGN.
H[TINF]2[/TINF] Emission from the Inner 400 Parsecs of the Galaxy
The Astrophysical Journal, 1996
We have mapped the H 2 v = 1 → 0 S(1) (λ = 2.1215 µm) emission line along a 400 pc long strip and in a 50 pc region in the Galactic center. There is H 2 emission throughout the surveyed region. The typical de-reddened (A K = 2.5 mag) H 2 v = 1 → 0 S(1) surface brightness, ∼ 3 × 10 −5 ergs s −1 cm −2 sr −1 , is similar to the surface brightness in large-scale photon-dominated regions in the galactic disk. We investigate two possible excitation mechanisms for the H 2 emission: UV-excitation by photons from OB stars, and shock waves, and conclude that UV-excitation is more likely. The total H 2 v = 1 → 0 S(1) luminosity in the inner 400 pc region of the Galaxy is 8000 L ⊙. The ratio of the H 2 to far-IR luminosity in the inner 400 pc of the Galaxy agrees with that in starburst galaxies and ultraluminous infrared bright galaxies.
Observational Studies on the Near-Infrared Unidentified Emission Bands in Galactic H II Regions
The Astrophysical Journal, 2014
Using a large collection of near-infrared spectra (2.5-5.4 µm) of Galactic H II regions and H II region-like objects, we perform a systematic investigation of the astronomical polycyclic aromatic hydrocarbon (PAH) features. 36 objects were observed by the use of the infrared camera onboard the AKARI satellite as a part of a directer's time program. In addition to the well-known 3.3-3.6 µm features, most spectra show a relatively-weak emission feature at 5.22 µm with sufficient signal-to-noise ratios, which we identify as the PAH 5.25 µm band previously reported. By careful analysis, we find good correlations between the 5.25 µm band and both the aromatic hydrocarbon feature at 3.3 µm and the aliphatic ones at around 3.4-3.6 µm. The present results give us convincing evidence that the astronomical 5.25 µm band is associated with C-H vibrations as suggested by previous studies and show its potential to probe the PAH size distribution. The analysis also shows that the aliphatic to aromatic ratio of I 3.4−3.6 µm /I 3.3 µm decreases against the ratio of the 3.7 µm continuum intensity to the 3.3 µm band, I cont,3.7 µm /I 3.3 µm , which is an indicator of the ionization fraction of PAHs. The mid-infrared color of I 9 µm /I 18 µm also declines steeply against the ratio of the hydrogen recombination line Brα at 4.05 µm to the 3.3 µm band, I Brα /I 3.3 µm. These facts indicate possible dust processing inside or at the boundary of ionized gas.
Excited OH+, H2O+, and H3O+in NGC 4418 and Arp 220
Astronomy & Astrophysics, 2013
We report on Herschel/PACS observations of absorption lines of OH + , H 2 O + and H 3 O + in NGC 4418 and Arp 220. Excited lines of OH + and H 2 O + with E lower of at least 285 and ∼200 K, respectively, are detected in both sources, indicating radiative pumping and location in the high radiation density environment of the nuclear regions. Abundance ratios OH + /H 2 O + of 1−2.5 are estimated in the nuclei of both sources. The inferred OH + column and abundance relative to H nuclei are (0.5−1) × 10 16 cm −2 and ∼2 × 10 −8 , respectively. Additionally, in Arp 220, an extended low excitation component around the nuclear region is found to have OH + /H 2 O + ∼ 5−10. H 3 O + is detected in both sources with N(H 3 O +) ∼ (0.5−2) × 10 16 cm −2 , and in Arp 220 the pure inversion, metastable lines indicate a high rotational temperature of ∼500 K, indicative of formation pumping and/or hot gas. Simple chemical models favor an ionization sequence dominated by H + → O + → OH + → H 2 O + → H 3 O + , and we also argue that the H + production is most likely dominated by X-ray/cosmic ray ionization. The full set of observations and models leads us to propose that the molecular ions arise in a relatively low density (10 4 cm −3) interclump medium, in which case the ionization rate per H nucleus (including secondary ionizations) is ζ > 10 −13 s −1 , a lower limit that is several × 10 2 times the highest current rate estimates for Galactic regions. In Arp 220, our lower limit for ζ is compatible with estimates for the cosmic ray energy density inferred previously from the supernova rate and synchrotron radio emission, and also with the expected ionization rate produced by X-rays. In NGC 4418, we argue that X-ray ionization due to an active galactic nucleus is responsible for the molecular ion production.
Looking for Pure Rotational H[TINF]2[/TINF] Emission from Protoplanetary Disks
The Astrophysical Journal, 2002
We report on a limited search for pure-rotational molecular hydrogen emission associated with young, pre-main-sequence stars. We looked for H 2 v = 0 J = 3 → 1 and J = 4 → 2 emission in the mid-infrared using the Texas Echelon-Cross-Echelle Spectrograph (TEXES) at NASA's 3m Infrared Telescope Facility. The high spectral and spatial resolution of our observations lead to more stringent limits on narrow line emission close to the source than previously achieved. One star, AB Aur, shows a possible (2σ) H 2 detection, but further observations are required to make a confident statement. Our non-detections suggest that a significant fraction, perhaps all, of previously reported H 2 emission towards these objects could be extended on scales of 5 ′′ or more.
Monthly Notices of the Royal Astronomical Society, 2013
We discuss physical conditions in Galactic neutral hydrogen based on deep, high velocity resolution interferometric H i 21cm absorption spectroscopy towards 33 compact extra-galactic radio sources. The H i 21cm optical depth spectra have root-mean-square noise values 10 −3 per 1 km s −1 velocity channel, i.e., sufficiently sensitive to detect H i 21cm absorption by the warm neutral medium (WNM). Comparing these spectra with Hi 21cm emission spectra from the Leiden-Argentine-Bonn (LAB) survey, we show that some of the absorption detected on most sightlines must arise in gas with temperatures higher than that in the stable cold neutral medium (CNM). A multi-Gaussian decomposition of 30 of the H i 21cm absorption spectra yielded very few components with line widths in the temperature range of stable WNM, with no such WNM components detected for sixteen of the thirty sightlines. We find that some of the detected H i 21cm absorption along thirteen of these sightlines must arise in gas with spin temperatures larger than the CNM range. For these sightlines, we use very conservative estimates of the CNM spin temperature and the non-thermal broadening to derive strict upper limits to the gas column densities in the CNM and WNM phases. Comparing these upper limits to the total H i column density, we find that typically at least 28% of the gas must have temperatures in the thermally unstable range (200 − 5000 K). Our observations hence robustly indicate that a significant fraction of the gas in the Galactic interstellar medium has temperatures outside the ranges expected for thermally stable gas in two-phase models.
The Excitation of N 2 H + in Interstellar Molecular Clouds. II. Observations
The Astrophysical Journal, 2007
We present observations of the J ¼ 1Y0, 2Y1, and 3Y2 rotational transitions of N 2 H + and N 2 D + toward a sample of prototypical dark clouds. The data have been interpreted using nonlocal radiative transfer models. For all sources previously studied through millimeter-continuum observations, we find a good agreement between the volume density estimated from our N 2 H + data and that estimated from the dust emission. This confirms that N 2 H + depletion is not very efficient in dark clouds for densities as large as 10 6 cm À3 , and also points out that a simultaneous analysis based on millimeter-continuum, N 2 H + and N 2 D + observations should lead to reliable estimates for the temperature and density structure of cold dark clouds. From multiline modeling of N 2 H + and N 2 D + , we derive the deuterium enrichment in the observed clouds. Our estimates are similar or higher than previous ones. The differences can be explained by the assumptions made on the cloud density profile and by the chemical fractionation occurring in the clouds. For two of the observed objects, L183 and TMC 2, multiposition observations have allowed us to derive the variation of the N 2 D þ /N 2 H þ abundance ratio with the radius. We have found that it decreases by an order of magnitude for radii greater than a few 0.01 pc (i.e., outside the central cores). Inside the dense condensations, the fractionation is efficient and, compared to the abundance ratio expected from statistical considerations based on the cosmic D/H ratio, the deuterium enrichment is estimated to be ' 0:1Y 0:5 ð Þ; 10 5 .