X-ray Dominated Regions Research Articles

Modelling molecular clouds and CO excitation in AGN-host galaxies

ABSTRACT We present a new physically motivated model for estimating the molecular line emission in active galaxies. The model takes into account (i) the internal density structure of giant molecular clouds (GMCs), (ii) the heating associated with both stars and the active galactic nuclei (AGNs), respectively, producing photodissociation regions (PDRs) and X-ray-dominated regions (XDRs) within the GMCs, and (iii) the mass distribution of GMCs within the galaxy volume. The model needs, as input parameters, the radial profiles of molecular mass, far-UV flux and X-ray flux for a given galaxy, and it has two free parameters: the CO-to-H2 conversion factor αCO, and the X-ray attenuation column density NH. We test this model on a sample of 24 local (z ≤ 0.06) AGN-host galaxies, simulating their carbon monoxide spectral-line energy distribution (CO SLED). We compare the results with the available observations and calculate, for each galaxy, the best (αCO, NH) with a Markov chain Monte Carlo algorithm, finding values consistent with those present in the literature. We find a median αCO = 4.8 M⊙ (K km s−1 pc2)−1 for our sample. In all the modelled galaxies, we find the XDR component of the CO SLED to dominate the CO luminosity from Jupp ≥ 4. We conclude that, once a detailed distribution of molecular gas density is taken into account, PDR emission at mid-/high-J becomes negligible with respect to XDR.

An X-ray spectroscopy study of structural stability of superhydrogenated pyrene derivatives

ABSTRACT The stability of polycyclic aromatic hydrocarbons (PAHs) upon soft X-ray absorption is of crucial relevance for PAH survival in X-ray dominated regions. PAH stability depends on molecular size but also on the degree of hydrogenation that is related to H2 formation in the interstellar medium. In this project, we intend to reveal the changes of electronic structure caused by hydrogenation and the impact of hydrogenation on the stability of the carbon backbone for cationic pyrene and its hydrogenated derivatives by analysis of near C K-edge soft X-ray photoions. In our experiments, the PAH cations were trapped in a cryogenic radiofrequency linear ion trap and exposed to monochromatic X-rays with energies from 279 to 300 eV. The photoproducts were mass-analysed by means of time-of-flight spectroscopy. Partial ion yields were then studied as a function of photon energy. X-ray absorption spectra computed by time-dependent density functional theory (TD-DFT) aided the interpretation of the experimental results. A very good agreement between experimental data and TD-DFT with short-range corrected functionals for all PAH ions was reached. The near-edge X-ray absorption mass spectra exhibit clear peaks due to C 1s transitions to singly occupied molecular orbitals and to low-lying unoccupied molecular orbitals. In contrast to coronene cations, where hydrogen attachment drastically increases photostability of coronene, the influence of hydrogenation on photostability is substantially weaker for pyrene cations. Here, hydrogen attachment even destabilizes the molecular structure. An astrophysical model describes the half-life of PAH ions in interstellar environments.

Photodissociation and X-Ray-Dominated Regions

The radiation from stars and active galactic nuclei (AGNs) creates photodissociation regions (PDRs) and X-ray-dominated regions (XDRs), where the chemistry or heating is dominated by far-ultraviolet (FUV) radiation or X-ray radiation, respectively. PDRs include a wide range of environments, from the diffuse interstellar medium (ISM) to dense star-forming regions. XDRs are found in the center of galaxies hosting AGNs, in protostellar disks, and in the vicinity of X-ray binaries. In this review, we describe the dominant thermal, chemical, and radiation transfer processes in PDRs and XDRs, as well as give a brief description of models and their use for analyzing observations. We then present recent results from Milky Way, nearby extragalactic, and high-redshift observations. Several important results include the following: ▪ Velocity-resolved PDR lines reveal the kinematics of the neutral atomic gas and provide constraints on the stellar feedback process. Their interpretation is, however, in dispute, as observations suggest a prominent role for stellar winds, whereas they are much less important in theoretical models. ▪ A significant fraction of molecular mass resides in CO-dark gas especially in low-metallicity and/or highly irradiated environments. ▪ The CO ladder and [Ci]/[Cii] ratios can determine if FUV or X rays dominate the ISM heating of extragalactic sources. ▪ With Atacama Large Millimeter/submillimeter Array, PDR and XDR tracers are now routinely detected on galactic scales over cosmic time. This makes it possible to link the star-formation history of the Universe to the evolution of the physical and chemical properties of the gas.

AGN impact on the molecular gas in galactic centres as probed by CO lines

ABSTRACT We present a detailed analysis of the X-ray, infrared, and carbon monoxide (CO) emission for a sample of 35 local (z ≤ 0.15), active (LX ≥ 1042 erg s−1) galaxies. Our goal is to infer the contribution of far-ultraviolet (FUV) radiation from star formation (SF), and X-ray radiation from the active galactic nuclei (AGNs), respectively, producing photodissociation regions (PDRs) and X-ray-dominated regions (XDRs), to the molecular gas heating. To this aim, we exploit the CO spectral line energy distribution (CO SLED) as traced by Herschel, complemented with data from single-dish telescopes for the low-J lines, and high-resolution ALMA images of the mid-J CO emitting region. By comparing our results to the Schmidt–Kennicutt relation, we find no evidence for AGN influence on the cold and low-density gas on kpc-scales. On nuclear (r = 250 pc) scales, we find weak correlations between the CO line ratios and either the FUV or X-ray fluxes: this may indicate that neither SF nor AGN radiation dominates the gas excitation, at least at r = 250 pc. From a comparison of the CO line ratios with PDR and XDR models, we find that PDRs can reproduce observations only in presence of extremely high gas densities (n > 105 cm−3). In the XDR case, instead, the models suggest moderate densities (n ≈ 102−4 cm−3). We conclude that a mix of the two mechanisms (PDR for the mid-J, XDR, or possibly shocks for the high-J) is necessary to explain the observed CO excitation in active galaxies.

Dense-gas tracers and carbon isotopes in five 2.5 < z < 4 lensed dusty star-forming galaxies from the SPT SMG sample

The origin of the high star formation rates (SFR) observed in high-redshift dusty star-forming galaxies is still unknown. Large fractions of dense molecular gas might provide part of the explanation, but there are few observational constraints on the amount of dense gas in high-redshift systems dominated by star formation. In this paper, we present the results of our Atacama large millimeter array (ALMA) program targeting dense-gas tracers (HCN(5-4), HCO+(5-4), and HNC(5-4)) in five strongly lensed galaxies from the South Pole Telescope (SPT) submillimeter galaxy sample. We detected two of these lines (S/N > 5) in SPT-125-47 at z = 2.51 and tentatively detected all three (S/N ∼ 3) in SPT0551-50 at z = 3.16. Since a significant fraction of our target lines is not detected, we developed a statistical method to derive unbiased mean properties of our sample taking into account both detections and non-detections. On average, the HCN(5-4) and HCO+(5-4) luminosities of our sources are a factor of ∼1.7 fainter than expected, based on the local L′HCN(5-4) − LIR relation, but this offset corresponds to only ∼2σ if we consider sample variance. We find that both the HCO+/HCN and HNC/HCN flux ratios are compatible with unity. The first ratio is expected for photo-dominated regions (PDRs) while the second is consistent with PDRs or X-ray dominated regions (XDRs) and/or mid-infrared (IR) pumping of HNC. Our sources are at the high end of the local relation between the star formation efficiency, determined using the LIR/[CI] and LIR/CO ratios, and the dense-gas fraction, estimated using the HCN/[CI] and HCN/CO ratios. Finally, in SPT0125-47, which has the highest signal-to-noise ratio, we found that the velocity profiles of the lines tracing dense (HCN, HCO+) and lower-density (CO, [CI]) molecular gas are similar. In addition to these lines, we obtained one robust and one tentative detection of 13CO(4-3) and found an average I 12CO(4-3)/I13CO(4-3) flux ratio of 26.1−3.5+4.5, indicating a young but not pristine interstellar medium. We argue that the combination of large and slightly enriched gas reservoirs and high dense-gas fractions could explain the prodigious star formation in these systems.

What Lurks in ULIRGs?—Probing the Chemistry and Excitation of Molecular Gas in the Nuclei of Arp 220 and NGC 6240

Abstract We have imaged the dense star-forming regions of Arp 220 and NGC 6240 in the 3 mm band transitions of CO, HCN, HCO+, HNC, and CS at 0.″5–0.″8 resolution using CARMA. Our data set images all these lines at similar resolutions and high sensitivity, and can be used to derive line ratios of faint high excitation lines. In both the nuclei of Arp 220, the HCN/HNC ratios suggest chemistry of X-ray Dominated Regions (XDRs)—a likely signature of an active galactic nucleus. In NGC 6240, there is no evidence of XDR type chemistry, but there the bulk of the molecular gas is concentrated between the nuclei rather than on them. We calculated molecular H2 densities from excitation analysis of each of the molecular species. It appears that the abundances of HNC and HCO+ in Ultra Luminous Infrared Galaxies may be significantly different from those in galactic molecular clouds. The derived H2 volume densities are ∼5 × 104 cm−3 in the Arp 220 nuclei and ∼104 cm−3 in NGC 6240.

A DEEPHERSCHEL/PACS OBSERVATION OF CO(40-39) IN NGC 1068: A SEARCH FOR THE MOLECULAR TORUS

Emission from high-J CO lines in galaxies has long been proposed as a tracer of X-ray dominated regions (XDRs) produced by AGN. Of particular interest is the question of whether the obscuring torus, which is required by AGN unification models, can be observed via high-J CO cooling lines. Here we report on the analysis of a deep Herschel-PACS observation of an extremely high J CO transition (40-39) in the Seyfert 2 galaxy NGC 1068. The line was not detected, with a derived 3$\sigma$ upper limit of $2 \times 10^{-17}\,\text{W}\,\text{m}^{-2}$. We apply an XDR model in order to investigate whether the upper limit constrains the properties of a molecular torus in NGC 1068. The XDR model predicts the CO Spectral Line Energy Distributions for various gas densities and illuminating X-ray fluxes. In our model, the CO(40-39) upper limit is matched by gas with densities $\sim 10^{6}-10^{7}\,\text{cm}^{-3}$, located at $1.6-5\,\text{pc}$ from the AGN, with column densities of at least $10^{25}\,\text{cm}^{-2}$. At such high column densities, however, dust absorbs most of the CO(40-39) line emission at $\lambda = 65.69\, \mu$m. Therefore, even if NGC 1068 has a molecular torus which radiates in the CO(40-39) line, the dust can attenuate the line emission to below the PACS detection limit. The upper limit is thus consistent with the existence of a molecular torus in NGC 1068. In general, we expect that the CO(40-39) is observable in only a few AGN nuclei (if at all), because of the required high gas column density, and absorption by dust.

Intensity ratios for XDR/PDR identification

AbstractThe intensity ratios of HCO+/HCN and HNC/HCN (1-0) reveal the relative influence of star formation and active galactic nuclei (AGN) or black holes on the circum-nuclear gas of a galaxy, allowing the identification of X-ray dominated regions (XDRs) and Photon-dominated regions (PDRs). It is not always clear in the literature how this intensity ratio calculation has been, or should be performed. This paper discusses ratio calculation methods for interferometric data.

Unbiased line surveys of molecular clouds in the Galactic center region

AbstractUsing the IRAM 30 m telescope, we perform a molecular line survey of the 3 and 2 mm wavelength ranges towards 5 selected positions in the Galactic center region, sampling shocked regions, ultraviolet (UV) and X-ray pervaded regions, and positions with rich organic chemistry. These surveys have the potential to be used as chemical templates for different types of activity, such as photodissociated regions (PDRs), shocks and X-ray dominated regions (XDRs). Complementary, molecular surveys done towards extragalactic nuclei, that are also dominated by these physical activities, were carried by our group.

Physical conditions and chemistry of molecular gas in galactic centers

AbstractStudying the molecular phase of the interstellar medium in galaxy nuclei is fundamental for the understanding of the onset and evolution of star formation and the growth of supermassive black holes. We can use molecules as observational tools exploiting them as tracers of chemical, physical and dynamical conditions. The molecular physical conditions in galaxy centers show large variety among galaxies, but in general the average gas densities (traced by e.g. HCN) and temperatures (probed by e.g. H2CO, NH3) are greater than in their disks. Molecular gas and dust is being funneled to the centers of galaxies by spiral arms, bars, and interactions - and one example of this is the minor merger NGC1614. Gas surface densities are also greater in galaxy nuclei and in extreme cases they become orders of magnitudes larger than what we find in the center of our own Milky Way. We can use IR excited molecular emission to probe the very inner regions of galaxies with deeply obscured nuclei where N(H2)>1024 cm−2 - for example the luminous infrared galaxy (LIRG) NGC4418. Abundances of key molecules such as HCN, HCO+, HNC, HC3N, CN, H3O+ are important tools in identifying the nature of buried activity and its evolution. Standard astrochemical scenarios (including X-ray Dominated regions (XDRs) and Photon Dominated Regions (PDRs)) are briefly discussed in this review and how we can use molecules to distinguish between them. High resolution studies are often necessary to separate effects of excitation and radiative transfer from those of chemistry - one example is absorption and effects of stimulated emission in the ULIRG Arp220. The nuclear activity in luminous galaxies often drives outflows and winds and in some cases molecular gas is being entrained in the outflows. Sometimes the molecular gas is carrying the bulk of the momentum. We can study the structure and physical conditions of the molecular gas to constrain the mass outflow rates and the evolution and nature of the driving source and two examples are discussed here: NGC1377 and Mrk231.

ROVIBRATIONALLY RESOLVED DIRECT PHOTODISSOCIATION THROUGH THE LYMAN AND WERNER TRANSITIONS OF H2FOR FUV/X-RAY-IRRADIATED ENVIRONMENTS

Using ab initio potential curves and dipole transition moments, cross-section calculations were performed for the direct continuum photodissociation of H{sub 2} through the B{sup 1}{Sigma}{sup +}{sub u} <- X{sup 1}{Sigma}{sup +}{sub g} (Lyman) and C{sup 1}{Pi}{sub u} <- X{sup 1}{Sigma}{sup +}{sub g} (Werner) transitions. Partial cross-sections were obtained for wavelengths from 100 A to the dissociation threshold between the upper electronic state and each of the 301 bound rovibrational levels v''J'' within the ground electronic state. The resulting cross-sections are incorporated into three representative classes of interstellar gas models: diffuse clouds, photon-dominated regions, and X-ray-dominated regions (XDRs). The models, which used the CLOUDY plasma/molecular spectra simulation code, demonstrate that direct photodissociation is comparable to fluorescent dissociation (or spontaneous radiative dissociation, the Solomon process) as an H{sub 2} destruction mechanism in intense far-ultraviolet or X-ray-irradiated gas. In particular, changes in H{sub 2} rotational column densities are found to be as large as 20% in the XDR model with the inclusion of direct photodissociation. The photodestruction rate from some high-lying rovibrational levels can be enhanced by pumping from H Ly{beta} due to a wavelength coincidence with cross-section resonances resulting from quasi-bound levels of the upper electronic states. Given the relatively large sizemore » of the photodissociation data set, a strategy is described to create truncated, but reliable, cross-section data consistent with the wavelength resolving power of typical observations.« less

The Effects of Doubly Ionized Chemistry on SH + and S +2 Abundances in X-Ray-dominated Regions

Recent laboratory measurements for the S^+2 + H2 reaction find a total rate coefficient significantly larger than previously used in theoretical models of X-ray dominated regions (XDRs). While the branching ratio of the products is unknown, one energetically possible route leads to the SH+ molecule, a known XDR diagnostic. In this work, we study the effects of S^+2 on the formation of SH+ and the destruction of S^+2 in XDRs. We find the predicted SH+ column density for molecular gas surrounding an Active Galactic Nucleus (AGN) increases by as much as 2 dex. As long as the branching ratio for S^+2 + H2 -> SH+ + H+ exceeds a few percent, doubly ionized chemistry will be the dominant pathway to SH+, which then initiates the formation of other sulfur-bearing molecules. We also find that the high rate of S^+2 + H2 efficiently destroys S^+2 once H2 forms, while the S^+2 abundance remains high in the atomic hydrogen region. We discuss the possible consequences of S^+2 in the atomic hydrogen region on mid-infrared diagnostics. The enhanced SH+ abundance has important implications in the study of XDRs, while our conclusions for S^+2 could potentially impact the interpretation of Spitzer and SOFIA observations.

Dense gas in normal and active galaxies

Dense molecular medium plays essential roles in galaxies. As demonstrated by the tight and linear correlation between HCN(1–0) and FIR luminosities among star-forming galaxies, from very nearby to high-z ones, the observation of a dense molecular component is indispensable to understand the star formation laws in galaxies. In order to obtain a general picture of the global distributions of dense molecular medium in normal star-forming galaxies, we have conducted an extragalactic CO(3–2) imaging survey of nearby spiral galaxies using the Atacama Submillimeter Telescope Experiment (ASTE). From the survey (ADIoS; ASTE Dense gas Imaging of Star-forming galaxies), CO(3–2) images of M 83 and NGC 986 are presented. Emphasis is placed on the correlation between the CO(3–2)/CO(1–0) ratio and the star formation efficiency in galaxies. In the central regions of some active galaxies, on the other hand, we often find enhanced or overluminous HCN(1–0) emission. The HCN(1–0)/CO(1–0) and HCN(1–0)/HCO+(1–0) intensities are often enhanced up to ∼0.2–0.3 and ∼2–3, respectively. Such elevated ratios have never been observed in the nuclear starburst regions. One possible explanation for these high HCN(1–0)/CO(1–0) and HCN(1–0)/HCO+(1–0) ratios is X-ray induced chemistry in X-ray dominated regions (XDRs), i.e., the overabundance of the HCN molecule in the X-ray irradiated dense molecular tori. If this view is true, the known tight correlation between HCN(1–0) and the star-formation rate breaks in the vicinity of active nuclei. Although the interpretation of these ratios is still an open question, these ratios have a great potential for a new diagnostic tool for the energy sources of dusty galaxies in the ALMA era because these molecular lines are free from dust extinction.

Chemistry in luminous AGN and starburst galaxies

Molecular line emission is a useful tool for probing the highly obscured inner kpc of starburst galaxies and buried AGNs. Molecular line ratios serve as diagnostic tools of the physical conditions of the gas—but also of its chemical properties. Both provide important clues to the type and evolutionary stage of the nuclear activity. While CO emission remains the main tracer for molecular distribution and dynamics, molecules such as HCN, HNC, HCO+, CN and HC3N are useful for probing the properties of the denser (n≳104 cm−3), star-forming gas. Here I discuss current views on how line emission from these species can be interpreted in luminous galaxies. HNC, HCO+ and CN are all species that can be associated both with photon dominated regions (PDRs) in starbursts—as well as X-ray dominated regions (XDRs) associated with AGN activity. HC3N line emission may identify galaxies where the starburst is in the early stage of its evolution.

Luminous HC3N line and the HCN/HCO+ ratio in NGC 4418

Aims. We investigate the properties of the nuclear molecular gas and address the nature of the deeply buried source driving the IR emission of NGC 4418. Methods. We present IRAM 30 m observations and basic non-LTE, single component radiative transport modelling of HNC, HCN, HCO + , CN, HC3N, and H2CO Results. We find that NGC 4418 has a rich molecular chemistry – including unusually luminous HC3N J = 10–9, 16–15, and 25–24 line emission – compared to the other high density tracers. We furthermore detect: ortho-H2CO 2–1, 3–2; CN 1–0, 2–1; HCO + , 1–0. 3–2, HCN 3–2, HNC 1–0, 3–2, and tentatively OCS 12–11. The HCN, HCO + ,H 2CO, and CN line emission can be fitted to densities of n = 5× 10 4 −10 5 cm −3 and gas temperatures Tk = 80–150 K. Both HNC and HC3N are, however, significantly more excited than the other species, which requires higher gas densities or radiative excitation through mid-IR pumping. The HCN line intensity is fainter than that of HCO + and HNC for the 3–2 transition, in contrast to previous findings for the 1–0 lines, where the HCN emission is the most luminous. Assuming all line emission is emerging from the same gas, abundances of the observed species are estimated to be similar to each other within factors of 2–5. The most noteworthy of these is the high abundance of HC3N and a small-to-moderate abundance ratio between HCN and HCO + . Conclusions. We tentatively suggest that the observed molecular line emission is consistent with a young starburst, where the emission can be understood as emerging from dense, warm gas with an additional PDR component. We find that X-ray chemistry is not required to explain the observed mm-line emission, including the HCN/HCO + 1–0 and 3–2 line ratios. The luminous HC3N line emission is an expected signature of dense, starforming gas. A deeply buried AGN cannot be excluded, but its impact on the surrounding molecular medium is then suggested to be limited. However, detailed modelling of HC3N abundances in X-ray dominated regions (XDRs) should be carried out. The possibility of radiative excitation should also be investigated further.

Overluminous HNC line emission in Arp 220, NGC 4418 and Mrk 231. Global IR pumping or XDRs?

Context. In recent studies of 3 mm J = 1 - 0 HNC emission from galaxies it is found that the emission is often bright which is unexpected in warm, star forming clouds. We propose that the main cause for the luminous HNC line emission is the extreme radiative and kinematical environment in starburst and active nuclei. Aims. To determine the underlying excitational and chemical causes behind the luminous HNC emission in active galaxies and to establish how HNC emission may serve to identify important properties of the nuclear source. Methods. We present mm and submm JCMT, IRAM 30 m and CSO observations of the J = 3 - 2 line of HNC and its isomer HCN in three luminous galaxies and J = 4 - 3 HNC observations of one galaxy. The observations are discussed in terms of physical conditions and excitation as well as in the context of X-ray influenced chemistry. Results. The ultraluminous mergers Arp 220 and Mrk 231 and the luminous IR galaxy NGC 4418 show the HNC J 3 - 2 emission being brighter than the HCN 3 - 2 emission by factors of 1.5 to 2. We furthermore report the detection of HNC J = 4 - 3 in Mrk 231. Overluminous HNC emission is unexpected in warm molecular gas in ultraluminous galaxies since I( HNC) greater than or similar to I( HCN) is usually taken as a signature of cold (10 - 20 K) dark clouds. Since the molecular gas of the studied galaxies is warm (T(k) greater than or similar to 40 K), we present two alternative explanations to the overluminous HNC: a) HNC excitation is affected by pumping of the rotational levels through the mid-infrared continuum and b) XDRs (X-ray Dominated Regions) influence the abundances of HNC. HNC may become pumped at 21.5 mu m brightness temperatures of T(B) greater than or similar to 50 K, suggesting that HNC-pumping could be common in warm, ultraluminous galaxies with compact IR- nuclei. This means that the HNC emission is no longer dominated by collisions and its luminosity may not be used to deduce information on gas density. On the other hand, all three galaxies are either suspected of having buried AGN - or the presence of AGN is clear (Mrk 231) - indicating that X-rays may affect the ISM chemistry. Conclusions. We conclude that both the pumping and XDR alternatives imply molecular cloud ensembles distinctly different from those of typical starforming regions in the Galaxy, or the ISM of less extreme starburst galaxies. The HNC molecule shows the potential of becoming an additional important tracer of extreme nuclear environments.

ISO observations of the Galactic center interstellar medium

The 500 central pc of the Galaxy (hereafter GC) exhibit a widespread gas component with a kinetic temperature of 100–200 K. The bulk of this gas is not associated to the well-known thermal radio continuum or far infrared sources like Sgr A or Sgr B. How this gas is heated has been a longstanding problem. With the aim of studying the thermal balance of the neutral gas and dust in the GC, we have observed 18 molecular clouds located at projected distances far from thermal continuum sources with the Infrared Space Observatory (ISO). In this paper we present observations of several fine structure lines ([Oi] 63 and 146 μm, [Cii] 158 μm, [Si ii] 35 μm, [S i] 25 μm and [Fe ii] 26 μm), which are the main coolants of the gas with kinetic temperatures of several hundred K. We also present the full continuum spectra of the dust between 40 and 190 μm. All the clouds exhibit a cold dust component with a temperature of ~15 K. A warmer dust component is also required to fit the spectra. The temperature of this dust component changes between 27 and 42 K from source to source. We have compared the gas and the dust emission with the predictions from J-type and C-type shocks and photodissociation region (PDRs) models. We conclude that the dust and the fine structure lines observations are best explained by a PDR with a density of 103 cm-3 and an incident far-ultraviolet field 103 times higher than the local interstellar radiation field. The fine structure line emission arises in PDRs in the interface between a diffuse ionized gas component and the dense molecular clouds. The [Cii] 158 μm and [Si ii] 35 μm lines also have an important contribution from the ionized gas component. PDRs can naturally explain the discrepancy between the gas and the dust temperatures. However, these PDRs can only account for 10–30% of the total H2 column density with a temperature of ~150 K. We discuss other possible heating mechanisms for the rest the warm molecular gas, such as non-stationary PDRs, X-ray Dominated Regions (XDRs) or the dissipation of supersonic turbulence.

Molecular gas chemistry in NGC 1068

We have studied how the active nucleus of the Seyfert 2 galaxy NGC 1068 influences the chemistry of its 200 pc circumnuclear disk of molecular gas (CND). Results from new observations made with the IRAM 30m telescope have served for deriving abundances of molecular species such as SiO, CN, HCO, HOC, HCO and HCO. These estimates are complemented with a re-evaluation of abundances of HCN, CS and CO, based on previously published observations. We report on the first detection of significant SiO emission in the CND of NGC 1068. We also report on the first extragalactic detection of the reactive ion HOC. Our conclusions favour an overall scenario where the CND of NGC 1068 has become a giant X-ray Dominated Region (XDR). Active Galactic Nuclei (AGN) are able to inject vast amounts of energy into their host galaxies, carried by nuclear winds and strong X-rays fields. In particular, X-ray dominated regions (XDR) could become the dominant sources of emission for molecular gas in the circumnuclear disks (CND) of AGN. The first evidence of a ‘peculiar’ chemistry in the CND of AGN was found by Sternberg et al. (1994), who measured an abnormally high HCN/CO abundance ratio in the nucleus of NGC 1068 (∼10−3−10−2). Different explanations have been advanced to account for this anomalous HCN chemistry. The selective depletion of gas-phase oxygen in the dense molecular clouds could enhance the HCN-to-CO abundance ratio (Sternberg et al. 1994). Alternatively, an increased X-ray ionization of molecular clouds near the AGN could be at work Lepp & Dalgarno (1996). While the aforementioned scenarios reproduce the measured enhancement of HCN relative to CO in NGC 1068, their predictions about other molecular species differ. In order to constrain observationally the choice of the optimum scenario, we have obtained with the IRAM 30m mm-telescope new spectra of 6 molecular species: SiO, HCO, HCO, HCO, HOC and CN (see Usero et al. 2004). We have also included in our analysis the results from previous observations of CO (Schinnerer et al. 2000), HCN (Tacconi et al. 1994), and CS (Tacconi et al. 1997). This database has served for estimating the abundances of eight molecular species in the CND of NGC 1068, using LVG calculations. While models invoking oxygen depletion in molecular gas successfully fit the HCN/CO ratio measured in the CND, they fail to account for our estimates of the HCN/HCO and CN/HCN abundance ratios. On the contrary, XDR models can satisfactorily fit these values. The detection of SiO and HOC emission in the CND of NGC 1068 (Fig. 1) gives further support to the XDR chemistry scenario. The large overall abundance of SiO estimated in the CND (∼(5-10)×109) cannot be explained by shocks driven by star formation on molecular gas, as there is counterevidence of a recent starburst in the nucleus of NGC 1068. The processing of 10 A dust grains by X-rays, as a mechanism to enhance Si chemistry in the gas phase, would explain the large SiO abundances of the CND. The observation of HOC in the CND

Infrared Response of H2to X‐Rays in Dense Clouds

The excitation by X-rays and cosmic rays of molecular hydrogen in interstellar clouds is analyzed. We carried out detailed calculations of entry efficiencies in rovibrational levels of H2 following impact with fast electrons produced by X-ray ionization of the gas. The competing effect of collisional excitation, and quenching by the ambient gas is examined in detail. Up to date values for H-H2 collisional rate coefficients are adopted, and some derivations of H2-H2 rovibrational rate coefficients from existing literature data are proposed. Several models as a function of temperature, density, and ionization rate are presented. We found that H2 infrared emission in X-ray dominated regions (XDR) is potentially observable for temperatures and ionization rates lower than certain critical values (typically T < 1000 K and zeta/n(sub H) < 10(exp -15) cc/s where zeta is the ionization rate). At higher temperatures, collisional excitation by the ambient gas dominates the population of low vibrational levels, and at higher values of zeta/n(sub H) the abundance of H2 is negligible. If such conditions are satisfied, the resulting infrared emission spectrum can be used as a diagnostic of nearby X-ray sources such as in cooling flows in galaxy clusters, quasars, Seyfert galaxies and supernova remnants. The intensity ratio of the 2-1S(1) and 1-0S(1) lines measured for the Seyfert galaxy NGC 1275 is consistent with X-ray pumping.