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Register a new userALMA Investigation of Vibrationally Excited HCN/HCO+/HNC Emission Lines in the AGN-Hosting Ultraluminous Infrared Galaxy IRAS 20551-4250
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Masatoshi Imanishi n Subaru/NAOJ
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We present the results of ALMA Cycle 2 observations of the ultraluminous infrared galaxy, IRAS 20551-4250, at HCN/HCO+/HNC J=3-2 lines at both vibrational-ground (v=0) and vibrationally excited (v2=1) levels. This galaxy contains a luminous buried active galactic nucleus (AGN), in addition to starburst activity, and our ALMA Cycle 0 data revealed a tentatively detected vibrationally excited HCN v2=1f J=4-3 emission line. In our ALMA Cycle 2 data, the HCN/HCO+/HNC J=3-2 emission lines at v=0 are clearly detected. The HCN and HNC v2=1f J=3-2 emission lines are also detected, but the HCO+ v2=1f J=3-2 emission line is not. Given the high-energy level of v2=1 and the resulting difficulty of collisional excitation, we compared these results with those of the calculation of infrared radiative pumping, using the available infrared 5-35 micron spectrum. We found that all of the observational results were reproduced, if the HCN abundance was significantly higher than that of HCO+ and HNC. The flux ratio and excitation temperature between v2=1f and v=0, after correction for possible line opacity, suggests that infrared radiative pumping affects rotational (J-level) excitation at v=0 at least for HCN and HNC. The HCN-to-HCO+ v=0 flux ratio is higher than those of starburst-dominated regions, and will increase even more when thederived high HCN opacity is corrected. The enhanced HCN-to-HCO+ flux ratio in this AGN-hosting galaxy can be explained by the high HCN-to-HCO+ abundance ratio and sufficient HCN excitation at up to J=4, rather than the significantly higher efficiency of infrared radiative pumping for HCN than HCO+.
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We present our ALMA multi-transition molecular line observational results for the ultraluminous infrared galaxy, IRAS 20551-4250, which is known to contain a luminous buried AGN and shows detectable vibrationally excited (v2=1f) HCN and HNC emission lines. The rotational J=1-0, 4-3, and 8-7 of HCN, HCO+, and HNC emission lines were clearly detected at a vibrational ground level (v=0). Vibrationally excited (v2=1f) J=4-3 emission lines were detected for HCN and HNC, but not for HCO+. Their observed flux ratios further support our previously obtained suggestion, based on J=3-2 data, that (1) infrared radiative pumping plays a role in rotational excitation at v=0, at least for HCN and HNC, and (2) HCN abundance is higher than HCO+ and HNC. The flux measurements of the isotopologue H13CN, H13CO+, and HN13C J=3-2 emission lines support the higher HCN abundance scenario. Based on modeling with collisional excitation, we constrain the physical properties of these line-emitting molecular gas, but find that higher HNC rotational excitation than HCN and HCO+ is difficult to explain, due to the higher effective critical density of HNC. We consider the effects of infrared radiative pumping using the available 5-30 micron infrared spectrum and find that our observational results are well explained if the radiation source is located at 30-100 pc from the molecular gas. The simultaneously covered very bright CO J=3-2 emission line displays a broad emission wing, which we interpret as being due to molecular outflow activity with the estimated rate of ~150 Msun/yr.
Aims. The comparative study of several molecular species at the origin of the gas phase chemistry in the diffuse interstellar medium (ISM) is a key input in unraveling the coupled chemical and dynamical evolution of the ISM. Methods. The lowest rotational lines of HCO+, HCN, HNC, and CN were observed at the IRAM-30m telescope in absorption against the lambda 3 mm and lambda 1.3 mm continuum emission of massive star-forming regions in the Galactic plane. The absorption lines probe the gas over kiloparsecs along these lines of sight. The excitation temperatures of HCO+ are inferred from the comparison of the absorptions in the two lowest transitions. The spectra of all molecular species on the same line of sight are decomposed into Gaussian velocity components. Most appear in all the spectra of a given line of sight. For each component, we derived the central opacity, the velocity dispersion, and computed the molecular column density. We compared our results to the predictions of UV-dominated chemical models of photodissociation regions (PDR models) and to those of non-equilibrium models in which the chemistry is driven by the dissipation of turbulent energy (TDR models). Results. The molecular column densities of all the velocity components span up to two orders of magnitude. Those of CN, HCN, and HNC are linearly correlated with each other with mean ratios N(HCN)/N(HNC) = 4.8 $pm$ 1.3 and N(CN)/N(HNC) = 34 $pm$ 12, and more loosely correlated with those of HCO+, N(HNC)/N(HCO+) = 0.5 $pm$ 0.3, N(HCN)/N(HCO+) = 1.9 $pm$ 0.9, and N(CN)/N(HCO+) = 18 $pm$ 9. These ratios are similar to those inferred from observations of high Galactic latitude lines of sight, suggesting that the gas sampled by absorption lines in the Galactic plane has the same chemical properties as that in the Solar neighbourhood. The FWHM of the Gaussian velocity components span the range 0.3 to 3 km s-1 and those of the HCO+ lines are found to be 30% broader than those of CN-bearing molecules. The PDR models fail to reproduce simultaneously the observed abundances of the CN-bearing species and HCO+, even for high-density material (100 cm-3 < nH < 104 cm-3). The TDR models, in turn, are able to reproduce the observed abundances and abundance ratios of all the analysed molecules for the moderate gas densities (30 cm-3 < nH < 200 cm-3) and the turbulent energy observed in the diffuse interstellar medium. Conclusions. Intermittent turbulent dissipation appears to be a promising driver of the gas phase chemistry of the diffuse and translucent gas throughout the Galaxy. The details of the dissipation mechanisms still need to be investigated.
We report ~2 resolution Atacama Large Millimeter/submillimeter Array observations of the HCN(1-0), HCO+(1-0), CO(1-0), CO(2-1), and CO(3-2) lines towards the nearby merging double-nucleus galaxy NGC 3256. We find that the high density gas outflow traced in HCN(1-0) and HCO+(1-0) emission is co-located with the diffuse molecular outflow emanating from the southern nucleus, where a low-luminosity active galactic nucleus (AGN) is believed to be the dominant source of the far-infrared luminosity. On the other hand, the same lines were undetected in the outflow region associated with the northern nucleus, whose primary heating source is likely related to starburst activity without obvious signs of AGN. Both HCO+(1-0)/CO(1-0) line ratio (i.e. dense gas fraction) and the CO(3-2)/CO(1-0) line ratio are larger in the southern outflow (0.20$pm$0.04 and 1.3$pm$0.2, respectively) than in the southern nucleus (0.08$pm$0.01, 0.7$pm$0.1, respectively). By investigating these line ratios for each velocity component in the southern outflow, we find that the dense gas fraction increases and the CO(3-2)/CO(1-0) line ratio decreases towards the largest velocity offset. This suggests the existence of a two-phase (diffuse and clumpy) outflow. One possible scenario to produce such a two-phase outflow is an interaction between the jet and the interstellar medium, which possibly triggers shocks and/or star formation associated with the outflow.
We present high resolution (0.4) IRAM PdBI and ALMA mm and submm observations of the (ultra) luminous infrared galaxies ((U)LIRGs) IRAS17208-0014, Arp220, IC860 and Zw049.057 that reveal intense line emission from vibrationally excited ($ u_2$=1) J=3-2 and 4-3 HCN. The emission is emerging from buried, compact (r<17-70 pc) nuclei that have very high implied mid-infrared surface brightness $>$$5times 10^{13}$ L$_{odot}$ kpc$^{-2}$. These nuclei are likely powered by accreting supermassive black holes (SMBHs) and/or hot (>200 K) extreme starbursts. Vibrational, $ u_2$=1, lines of HCN are excited by intense 14 micron mid-infrared emission and are excellent probes of the dynamics, masses, and physical conditions of (U)LIRG nuclei when H$_2$ column densities exceed $10^{24}$ cm$^{-2}$. It is clear that these lines open up a new interesting avenue to gain access to the most obscured AGNs and starbursts. Vibrationally excited HCN acts as a proxy for the absorbed mid-infrared emission from the embedded nuclei, which allows for reconstruction of the intrinsic, hotter dust SED. In contrast, we show strong evidence that the ground vibrational state ($ u$=0), J=3-2 and 4-3 rotational lines of HCN and HCO$^+$ fail to probe the highly enshrouded, compact nuclear regions owing to strong self- and continuum absorption. The HCN and HCO$^+$ line profiles are double-peaked because of the absorption and show evidence of non-circular motions - possibly in the form of in- or outflows. Detections of vibrationally excited HCN in external galaxies are so far limited to ULIRGs and early-type spiral LIRGs, and we discuss possible causes for this. We tentatively suggest that the peak of vibrationally excited HCN emission is connected to a rapid stage of nuclear growth, before the phase of strong feedback.
We obtained high resolution (0.25 to 0.90) observations of HCN and HCO+ J=3-2 of the ultraluminous QSO galaxy Mrk231 with the IRAM Plateau de Bure Interferometer. We find luminous HCN and HCO+ 3-2 emission in the main disk and we detect compact (r<90 pc) vibrationally excited HCN 3-2, v2=1f emission centred on the nucleus. The velocity field of the vibrationally excited HCN is strongly inclined (PA=155 deg.) compared to the east-west rotation of the main disk. The nuclear molecular mass is estimated to 8e8 Msun with an average N(H2)of 1.2e24 cm-2. Prominent, spatially extended (>350 pc) line wings are found for HCN 3-2 with velocities +-750 km/s. Line ratios indicate that the emission is emerging in dense gas n=1e4 - 5e5 cm-3 of elevated HCN abundance X(HCN)=1e-8 to 1e-6. High X(HCN) also allows for the emission to originate in gas of more moderate density. We tentatively detect nuclear emission from the reactive ion HOC+ with HCO+/HOC+=10-20. The HCN v2=1f line emission is consistent with the notion of a hot, dusty, warped inner disk of Mrk231 where the v2=1f line is excited by bright mid-IR 14 micron continuum. We estimate the vibrational temperature T_vib to 200-400 K. We propose that 50% of the main HCN emission may have its excitation affected by the radiation field through IR pumping of the vibrational ground state. The HCN emission in the line wings, however, is more extended and thus likely not strongly affected by IR pumping. Our results reveal that dense clouds survive (and/or are formed) in the AGN outflow on scales of at least several hundred pc before evaporating or collapsing. The elevated HCN abundance in the outflow is consistent with warm chemistry possibly related to shocks and/or X-ray irradiated gas. An upper limit to the mass and momentum flux is 4e8 Msun and 12L_AGN/c, respectively, and we discuss possible driving mechanisms for the dense outflow.
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