Search

We report the first detection of the metal-bearing molecules sodium sulfide (NaS) and magnesium sulfide (MgS) and the tentative detection of calcium monoxide (CaO) in the interstellar medium (ISM) towards the Galactic Center molecular cloud G+0.693-0.027. The derived column densities are (5.0+-1.1) x 10$^{10}$ cm$^{-2}$, (6.0+-0.6) x $^{10}$ cm$^{-2}$, and (2.0+-0.5) x $^{10}$ cm$^{-2}$, respectively. This translates into fractional abundances with respect to H$_2$ of (3.7+-1.0) x $10^{-13}$, (4.4+-0.8) x $10^{-13}$, and (1.5+-0.4) x $10^{-13}$, respectively. We have also searched for other Na-, Mg- and Ca-bearing species towards this source but none of them have been detected and thus we provide upper limits for their abundances. We discuss the possible chemical routes involved in the formation of these molecules containing metals under interstellar conditions. Finally, we compare the ratio between sulfur-bearing and oxygen-bearing molecules with and without metals, finding that metal-bearing sulfur molecules are much more abundant than metal-bearing oxygen ones, in contrast with the general trend found in the ratios between other non metal- oxygen- and sulfur-bearing molecules. This further strengthen the idea that sulfur may be little depleted in G+0.693-0.027 as a result of the low velocity shocks present in this source sputtering large amounts of material from dust grains., Comment: 18 pages, 7 tables and 7 figures, final version accepted by ApJ

We report the first detection of SiC$_2$ in the interstellar medium. The molecule was identified through six rotational transitions toward G\,+0.693$-$0.027, a molecular cloud located in the Galactic center. The detection is based on a line survey carried out with the GBT, the Yebes 40m, and the IRAM 30m telescopes covering a range of frequencies from 12 to 276 GHz. We fit the observed spectra assuming local thermodynamic equilibrium and derive a column density of ($1.02\pm0.04)\times10^{13}$ cm$^{-2}$, which gives a fractional abundance of $7.5\times10^{-11}$ with respect to H$_2$, and an excitation temperature of $5.9\pm0.2$ K. We conclude that SiC$_2$ can be formed in the shocked gas by a reaction between the sputtered atomic silicon and C$_2$H$_2$, or it can be released directly from the dust grains due to disruption. We also search for other Si-bearing molecules and detect eight rotational transitions of SiS and four transitions of Si$^{18}$O. The derived fractional abundances are $3.9\times10^{-10}$ and $2.1\times10^{-11}$, respectively. All Si-bearing species toward G\,+0.693$-$0.027 show fractional abundances well below what is typically found in late-type evolved stars., Comment: 8 pages, 5 figures

Interstellar amides have attracted significant attentions as they are potential precursors for a wide variety of organics essential to life. However, our current understanding of their formation in space is heavily based on observations in star-forming regions and hence the chemical networks lack the constraints on their early origin. In this work, unbiased sensitive spectral surveys with IRAM 30m and Yebes 40m telescopes are used to systematically study a number of amides towards a quiescent Galactic Centre molecular cloud, G+0.693-0.027. We report the first detection of acetamide (CH3C(O)NH2) and trans-N-methylformamide (CH3NHCHO) towards this cloud. In addition, with the wider frequency coverage of the survey, we revisited the detection of formamide (NH2CHO) and urea (carbamide; NH2C(O)NH2), which had been reported previously towards G+0.693-0.027. Our results are compared with those present in the literature including recent laboratory experiments and chemical models. We find constant abundance ratios independently of the evolutionary stages, suggesting that amides related chemistry is triggered in early evolutionary stages of molecular cloud and remain unaffected by the warm-up phase during the star formation process. Although a correlation between more complex amides and NH2CHO have been suggested, alternative formation routes involving other precursors such as acetaldehyde (CH3CHO), methyl isocyanate (CH3NCO) and methylamine (CH3NH2) may also contribute to the production of amides. Observations of amides together with these species towards a larger sample of sources can help to constrain the amide chemistry in the interstellar medium., Comment: 16 pages, 8 figures, 3 tables, accepted for publication in Monthly Notices of the Royal Astronomical Society

The Central Molecular Zone (CMZ) contains most of the mass of our Galaxy but its star formation rate is one order of magnitude lower than in the Galactic disc. This is likely related to the fact that the bulk of the gas in the CMZ is in a warm ($>$100 K) and turbulent phase with little material in the pre-stellar phase. We present in this Letter observations of deuterium fractionation (D/H ratios) of HCN, HNC, HCO$^{+}$, and N$_{2}$H$^{+}$ towards the CMZ molecular cloud G+0.693-0.027. These observations clearly show, for the first time, the presence of a colder, denser, and less turbulent narrow component, with a line width of $\sim$9 km s$^{-1}$, in addition to the warm, less dense and turbulent broad component with a line width of $\sim$20 km s$^{-1}$. The very low D/H ratio $\le$6$\times$10$^{-5}$ for HCO$^{+}$ and N$_{2}$H$^{+}$, close to the cosmic value ($\sim$2.5$\times$10$^{-5}$), and the high D/H ratios $>$4$\times$10$^{-4}$ for HCN and HNC derived for the broad component, confirm the presence of high-temperatures deuteration routes for nitriles. For the narrow component we have derived D/H ratios $>$10$^{-4}$ and excitation temperatures of $7$ K for all molecules, suggesting kinetic temperatures $\le$30 K and H$_2$ densities $\ge$5$\times$10$^{4}$ cm$^{-3}$, at least one order of magnitude larger than for the broad component. The method presented in this Letter allows to identify clouds on the verge of star formation, i.e. under pre-stellar conditions, towards the CMZ. This method can also be used for the identification of such clouds in external galaxies., Comment: 11 pages, 4 figures, 2 tables, 2 appendix - Accepted for publication in ApJ Letters

Little is known about the chemistry of isocyanates (compounds with the functional group R-N=C=O) in the interstellar medium, as only four of them have been detected so far: isocyanate radical (NCO), isocyanic acid (HNCO), N-protonated isocyanic acid (H$_2$NCO$^+$) and methyl isocyanate (CH$_3$NCO). The molecular cloud G+0.693-0.027, located in the Galactic Centre, represents an excellent candidate to search for new isocyanates since it exhibits high abundances of the simplest ones, HNCO and CH$_3$NCO. After CH$_3$NCO, the next complex isocyanates are ethyl isocyanate (C$_2$H$_5$NCO) and vinyl isocyanate (C$_2$H$_3$NCO). Their detection in the ISM would enhance our understanding of the formation of these compounds in space. We have detected C$_2$H$_5$NCO and H$_2$NCO$^+$ towards G+0.693-0.027 (the former for the first time in the interstellar medium) with molecular abundances of (4.7$-$7.3)$\times$10$^{-11}$ and (1.0$-$1.5)$\times$10$^{-11}$, respectively. A ratio CH$_3$NCO / C$_2$H$_5$NCO = 8$\pm$1 is obtained; therefore the relative abundance determined for HNCO:CH$_3$NCO:C$_2$H$_5$NCO is 1:1/55:1/447, which implies a decrease by more than one order of magnitude going progressively from HNCO to CH$_3$NCO and to C$_2$H$_5$NCO. This is similar to what has been found for e.g. alcohols and thiols and suggests that C$_2$H$_5$NCO is likely formed on the surface of dust grains. In addition, we have obtained column density ratios of HNCO / NCO > 269, HNCO / H$_2$NCO$^+$ $\sim$ 2100 and C$_2$H$_3$NCO / C$_2$H$_5$NCO~<~4. A comparison of the Methyl~/~Ethyl ratios for isocyanates (-NCO), alcohols (-OH), formiates (HCOO-), nitriles (-CN) and thiols (-SH) is performed and shows that ethyl-derivatives may be formed more efficiently for the N-bearing molecules than for the O- and S-bearing molecules., Comment: 5 pages, 5 figures; accepted in A&A

We report SOFIA-upGREAT spectroscopic imaging of the [C II] 158um spectral line, as well as a number of [O I] 63um spectra, across a 67x45 pc field toward the Sgr B region in our Galactic center. The fully-sampled and velocity-resolved [C II] images have 0.55 pc spatial and 1 km/s velocity resolutions. We find that Sgr B extends as a coherent structure spanning some 34 pc along the Galactic plane. Bright [C II] emission encompasses Sgr B1 (G0.5-0.0), the G0.6-0.0 HII region, and passes behind and beyond the luminous star forming cores toward Sgr B2 (G0.7-0.0). Sgr B is a major contributor to the entire Galactic center's [C II] luminosity, with surface brightness comparable to [C II] from the Arches region. [C II], 70um, and 20cm emission share nearly identical spatial distributions. Combined with the lack of [C II] self-absorption, this indicates that these probes trace UV on the near surfaces of more extended clouds visible in CO isotopologues and 160um continuum. Stars from regions of local star formation likely dominate the UV field. Photodissociation regions and HII regions contribute similar amounts of [C II] flux. The extreme star formation cores of Sgr B2 contribute negligible amounts to the total [C II] intensity from the Sgr B region. Velocity fields and association with a narrow dust lane indicate that they may have been produced in a local cloud-cloud collision. The cores are likely local analogs of the intense star formation regions where ideas to explain the "C+ deficit" in ultra-luminous galaxies can be tested., Comment: 22 pages, 13 figures. Accepted for publication in the Astrophysical Journal

We report here the first detection in the interstellar medium of the cyanomidyl radical (HNCN). Using the Yebes 40m and the IRAM 30m telescopes, we have targeted the doublets of the $N$=2$-$1, 4$-$3, 5$-$4, 6$-$5, and 7$-$6 transitions of HNCN toward the molecular cloud G+0.693-0.027. We have detected three unblended lines of HNCN, these are the $N$=6$-$5 doublet and one line of the $N$=4$-$3 transition. Additionally we present one line of the $N$=5$-$4 transition partially blended with emission from other species. The Local Thermodynamic Equilibrium best fit to the data gives a molecular abundance of (0.91$\pm$0.05)$\times$10$^{-10}$ with respect to H$_2$. The relatively low abundance of this species in G+0.693-0.027, and its high reactivity, suggest that HNCN is possibly produced by gas-phase chemistry. Our work shows that this highly reactive molecule is present in interstellar space, and thus it represents a plausible precursor of larger prebiotic molecules with the NCN backbone such as cyanamide (NH$_2$CN), carbodiimide (HNCNH) and formamidine (NH$_2$CHNH)., Comment: Accepted in Monthly Notices of the Royal Astronomical Society Letters

The lines of HOC$^+$, HCO and CO$^+$ are considered good tracers of photon-dominated regions (PDRs) and X-ray dominated regions. We study these tracers towards regions of the Sgr B2 cloud selected to be affected by different heating mechanisms. We find the lowest values of the column density ratios of HCO$^+$ versus HOC$^+$, HCO and CO$^+$ in dense HII gas, where UV photons dominate the heating and chemistry of gas. HOC$^+$, HCO and CO$^+$ abundances and the above ratios are compared with those of chemical modeling, finding that high temperature chemistry, a cosmic-ray ionization rate of 10$^{-16}$ s$^{-1}$ and timescales $>$10$^{5.0}$ years explain well the HOC$^+$ abundances in quiescent Sgr B2 regions, while shocks are also needed to explain the highest HCO abundances derived for these regions. CO$^+$ is mainly formed in PDRs since the highest CO$^+$ abundances of $\sim$(6-10)$\times$10$^{-10}$ are found in HII regions with electron densities $>$540 cm$^{-3}$ and that CO$^+$ emission is undetected in quiescent gas. Between the ratios, the HCO$^+$/HCO ratio is sensitive to the electron density as it shows different values in dense and diffuse HII regions. We compare SiO J=2-1 emission maps of Sgr B2 with X-ray maps from 2004 and 2012. One known spot shown on the 2012 X-ray map is likely associated with molecular gas at velocities of 15-25 km s$^{-1}$. We also derive the X-ray ionization rate of $\sim$10$^{-19}$ s$^{-1}$ for Sgr B2 regions pervaded by X-rays in 2004, which is quite low to affect the chemistry of the molecular gas.

(abridged) The ambiguous origin of [CII] 158um in the interstellar medium complicates its use for diagnostics concerning the star-formation rate and physical conditions in photodissociation regions (PDRs). We observed the giant HII region N11 in the Large Magellanic Cloud with SOFIA/GREAT in order to investigate the origin of [CII] to obtain the total H2 gas content, the fraction of CO-dark H2 gas, and the influence of environmental effects such as stellar feedback. We present an innovative spectral decomposition method that allows statistical trends to be derived. The [CII] line is resolved in velocity and compared to HI and CO, using a Bayesian approach to decompose the profiles. A simple model accounting for collisions in the neutral atomic and molecular gas was used in order to derive the H2 column density traced by C+. The profile of [CII] most closely resembles that of CO, but the integrated [CII] line width lies between that of CO and that of HI. Using various methods, we find that [CII] mostly originates from the neutral gas. We show that [CII] mostly traces the CO-dark H2 gas but there is evidence of a weak contribution from neutral atomic gas preferentially in the faintest components. Most of the molecular gas is CO-dark. The fraction of CO-dark H2 gas decreases with increasing CO column density, with a slope that seems to depend on the impinging radiation field from nearby massive stars. Finally we extend previous measurements of the photoelectric-effect heating efficiency, which we find is constant across regions probed with Herschel, with [CII] and [OI] being the main coolants in faint and diffuse, and bright and compact regions, respectively, and with PAH emission tracing the CO-dark H2 gas heating where [CII] and [OI] emit. Our study highlights the importance of velocity-resolved PDR diagnostics and higher spatial resolution for HI observations., Comment: Accepted for publication in A&A

Aims: We study the spatial distribution and kinematics of water emission in a ~64 pc$^2$ region of the Galactic Center (GC) around Sgr A*. We also analyze the water excitation to derive the physical conditions and water abundances in the CND and the `quiescent clouds'. Methods: We presented the integrated intensity maps of the ortho 1$_{10}-1_{01}$, and para 2$_{02}-1_{11}$ and 1$_{11}-0_{00}$ water transitions observed with the HIFI instrument on board Herschel. To study the water excitation we used ground state ortho and para H$_2^{18}$O transitions. In our study, we also used SPIRE continuum measurements of the CND. Using a non-LTE radiative transfer code, the water line profiles and dust continuum were modeled. We also used a rotating ring model to reproduce the CND kinematics represented by the PV diagram. Results: We identify the water emission arising from the CND, the Western Streamer, and the 20 and 50 km s$^{-1}$ clouds. The ortho water maps show absorption structures in the range of [-220,10] km s$^{-1}$. The PV diagram shows that the 2$_{02}-1_{11}$ H$_2$O emission traces the CND. We derive high X$_{H_2O}$ of $\sim$(0.1-1.3)$\times$10$^{-5}$, V$_t$ of 14-23 km s$^{-1}$ and T$_d$ of 15-45 K for the CND, and the lower X$_{\rm H_2O}$ of 4$\times$10$^{-8}$ and V$_t$ of 9 km s$^{-1}$ for the 20 km s$^{-1}$ cloud. Collisional excitation and dust effects are responsible for the water excitation in the southwest lobe of the CND and the 20 km s$^{-1}$ cloud, whereas only collisions can account for the water excitation in the northeast lobe of the CND. We propose that the water vapor in the CND is caused by grain sputtering by shocks of 10-20 km s$^{-1}$, with some contribution of high temperature and cosmic-ray chemistries plus a PDR chemistry. The low X$_{\rm H_2O}$ derived for the 20 km s$^{-1}$ cloud could be partially a consequence of the water freeze-out on grains., Comment: 15 pages, 11 figures

We report the first detection in the interstellar medium of the Z-isomer of cyanomethanimine (HNCHCN), an HCN dimer proposed as precursor of adenine. We identified six transitions of Z-cyanomethanimine, along with five transitions of E-cyanomethanimine, using IRAM 30m observations towards the Galactic Center quiescent molecular cloud G+0.693. The Z-isomer has a column density of (2.0$\pm$0.6)$\times$10$^{14}$ cm$^{-2}$ and an abundance of 1.5$\times$10$^{-9}$. The relative abundance ratio between the isomers is [Z/E]$\sim$6. This value cannot be explained by the two chemical formation routes previously proposed (gas-phase and grain surface), which predicts abundances ratios between 0.9 and 1.5. The observed [Z/E] ratio is in good agreement with thermodynamic equilibrium at the gas kinetic temperature (130$-$210 K). Since isomerization is not possible in the ISM, the two species may be formed at high temperature. New chemical models, including surface chemistry on dust grains and gas-phase reactions, should be explored to explain our findings. Whatever the formation mechanism, the high abundance of Z-HNCHCN shows that precursors of adenine are efficiently formed in the ISM., Comment: Accepted in Monthly Notices of the Royal Astronomical Society Letters

We present an analysis of gas densities in the central R=300 parsecs of the Milky Way, focusing on three clouds: GCM-0.02-0.07 (the 50 km/s cloud), GCM-0.13-0.08 (the 20 km/s cloud), and GCM0.25+0.01 (the "Brick"). Densities are determined using observations of the J=(3-2), (4-3), (5-4), (10-9), (18-17), (19-18), (21-20), and (24-23) transitions of the molecule HC3N. We find evidence of at least two excitation regimes for HC3N and constrain the low-excitation component to have a density less than 10^4 cm^-3 and the high-excitation component to have a density between 10^5 and 10^6 cm^-3. This is much less than densities of 10^7 cm^-3 that are found in Sgr B2, the most actively star-forming cloud in the Galactic center. This is consistent with the requirement of a higher density threshold for star formation in the Galactic center than is typical in the Galactic disk. We are also able to constrain the column density of each component in order to determine the mass fraction of 'dense' (n>10^5 cm^-3) gas for these clouds. We find that this is ~15% for all three clouds. Applying the results of our models to ratios of the (10-9) and (3-2) line across the entire central R=300 pc, we find that the fraction of gas with n>10^4 cm^-3 increases inward of a radius of ~140 pc, consistent with the predictions of recent models for the gas dynamics in this region. Our observations show that HC3N is an excellent molecule for probing the density structure of clouds in the Galactic center., Comment: 20 pages, 8 figures, 3 tables. ApJ accepted

We present 1"-resolution ALMA observations of the circumnuclear disk (CND) and the environment around SgrA*. The images unveil the presence of small spatial scale CO (J=3-2) molecular "cloudlets" within the central pc of the Milky Way, moving at high speeds, up to 300 km/s along the line-of-sight. The CO-emitting structures show intricate morphologies: extended and filamentary at high negative-velocities (v_LSR < -150 km/s), more localized and clumpy at extreme positive-velocities (v_LSR > +200 km/s). Based on the pencil-beam CO absorption spectrum toward SgrA* synchrotron emission, we also present evidence for a diffuse gas component producing absorption features at more extreme negative-velocities (v_LSR < -200 km/s). The CND shows a clumpy spatial distribution. Its motion requires a bundle of non-uniformly rotating streams of slightly different inclinations. The inferred gas density peaks are lower than the local Roche limit. This supports that CND molecular cores are transient. We apply the two standard orbit models, spirals vs. ellipses, invoked to explain the kinematics of the ionized gas streamers around SgrA*. The location and velocities of the CO cloudlets are inconsistent with the spiral model, and only two of them are consistent with the Keplerian ellipse model. Most cloudlets, however, show similar velocities that are incompatible with the motions of the ionized streamers or with gas bounded to the central gravity. We speculate that they are leftovers of more massive, tidally disrupted, clouds that fall into the cavity, or that they originate from instabilities in the inner rim of the CND and infall from there. Molecular cloudlets, all together with a mass of several 10 M_Sun, exist around SgrA*. Most of them must be short-lived: photoevaporated by the intense stellar radiation field, blown away by winds from massive stars, or disrupted by strong gravitational shears., Comment: Accepted for publication in A&A. 21 pages, 19 figures (includes corrections by language editor)

We present an unbiased spectral line survey toward the Galactic Centre (GC) quiescent giant molecular cloud (QGMC), G+0.693 using the GBT and IRAM 30$\,$ telescopes. Our study highlights an extremely rich organic inventory of abundant amounts of nitrogen (N)-bearing species in a source without signatures of star formation. We report the detection of 17 N-bearing species in this source, of which 8 are complex organic molecules (COMs). A comparison of the derived abundances relative to H$_2$ is made across various galactic and extragalactic environments. We conclude that the unique chemistry in this source is likely to be dominated by low-velocity shocks with X-rays/cosmic rays also playing an important role in the chemistry. Like previous findings obtained for O-bearing molecules, our results for N-bearing species suggest a more efficient hydrogenation of these species on dust grains in G+0.693 than in hot cores in the Galactic disk, as a consequence of the low dust temperatures coupled with energetic processing by X-ray/cosmic ray radiation in the GC., Comment: 24 pages, 23 figures, 7 tables, accepted for publication in MNRAS

We present a large set of spectral lines detected in the $40"$ central region of the starburst galaxy NGC 253. Observations were obtained with the three instruments SPIRE, PACS and HIFI on board the Herschel Space Observatory, upGREAT on board of the SOFIA airborne observatory, and the ground based APEX telescope. Combining the spectral and photometry products of SPIRE and PACS we model the dust continuum Spectral Energy Distribution (SED) and the most complete $^{12}$CO Line SED reported so far toward the nuclear region of NGC 253. Properties and excitation of the molecular gas were derived from a three-component non-LTE radiative transfer model, using the SPIRE $^{13}$CO lines and ground based observations of the lower-$J$ $^{13}$CO and HCN lines, to constrain the model parameters. Three dust temperatures were identified from the continuum emission, and three components are needed to fit the full CO LSED. Only the third CO component (fitting mostly the HCN and PACS $^{12}$CO lines) is consistent with a shock/mechanical heating scenario. A hot core chemistry is also argued as a plausible scenario to explain the high-$J$ $^{12}$CO lines detected with PACS. The effect of enhanced cosmic ray ionization rates, however, cannot be ruled out, and is expected to play a significant role in the diffuse and dense gas chemistry. This is supported by the detection of ionic species like OH$^+$ and H$_2$O$^+$, as well as the enhanced fluxes of the OH lines with respect to those of H$_2$O lines detected in both PACS and SPIRE spectrum., Comment: 36 pages, 16 figures, submitted, reviewed and accepted for publication in the Astrophysical Journal on April 12th 2018

We present a study of diffuse extended ionised gas toward three clouds located in the Galactic Centre (GC). One line of sight (LOS) is toward the 20 km s$^{-1}$ cloud (LOS$-$0.11) in the Sgr A region, another LOS is toward the 50 km s$^{-1}$ cloud (LOS$-$0.02), also in Sgr A, while the third is toward the Sgr B2 cloud (LOS+0.693). The emission from the ionised gas is detected from H$n\alpha$ and H$m\beta$ radio recombination lines (RRLs). He$n\alpha$ and He$m\beta$ RRL emission is detected with the same $n$ and $m$ as those from the hydrogen RRLs only toward LOS+0.693. RRLs probe gas with positive and negative velocities toward the two Sgr A sources. The H$m\beta$ to H$n\alpha$ ratios reveal that the ionised gas is emitted under local thermodynamic equilibrium conditions in these regions. We find a He to H mass fraction of 0.29$\pm$0.01 consistent with the typical GC value, supporting the idea that massive stars have increased the He abundance compared to its primordial value. Physical properties are derived for the studied sources. We propose that the negative velocity component of both Sgr A sources is part of gas streams considered previously to model the GC cloud kinematics. Associated massive stars with what are presumably the closest HII regions to LOS$-$0.11 (positive velocity gas), LOS$-$0.02 and LOS+0.693 could be the main sources of UV photons ionising the gas. The negative velocity components of both Sgr A sources might be ionised by the same massive stars, but only if they are in the same gas stream.

Phosphorus (P) is one of the essential elements for life due to its central role in biochemical processes. Recent searches have shown that P-bearing molecules (in particular PN and PO) are present in star-forming regions, although their formation routes remain poorly understood. In this Letter, we report observations of PN and PO towards seven molecular clouds located in the Galactic Center, which are characterized by different types of chemistry. PN is detected in five out of seven sources, whose chemistry is thought to be shock-dominated. The two sources with PN non-detections correspond to clouds exposed to intense UV/X-rays/cosmic-ray radiation. PO is detected only towards the cloud G+0.693$-$0.03, with a PO/PN abundance ratio of $\sim$1.5. We conclude that P-bearing molecules likely form in shocked gas as a result of dust grain sputtering, while are destroyed by intense UV/X-ray/cosmic ray radiation., Comment: 5 pages, 3 figures, 2 tables, accepted for publication in MNRAS Letters

We present a systematic survey of multiple velocity-resolved H$_2$O spectra using Herschel/HIFI towards nine nearby actively star forming galaxies. The ground-state and low-excitation lines (E$_{\rm up}\,\le 130\,{\rm K}$) show profiles with emission and absorption blended together, while absorption-free medium-excitation lines ($130\,{\rm K}\, \le\, E_{\rm up}\,\le\,350\,{\rm K}$) typically display line shapes similar to CO. We analyze the HIFI observation together with archival SPIRE/PACS H$_2$O data using a state-of-the-art 3D radiative transfer code which includes the interaction between continuum and line emission. The water excitation models are combined with information on the dust- and CO spectral line energy distribution to determine the physical structure of the interstellar medium (ISM). We identify two ISM components that are common to all galaxies: A warm ($T_{\rm dust}\,\sim\,40-70\,{\rm K}$), dense ($n({\rm H})\,\sim\,10^5-10^6\,{\rm cm^{-3}}$) phase which dominates the emission of medium-excitation H$_2$O lines. This gas phase also dominates the FIR emission and the CO intensities for $J_{\rm up} > 8$. In addition a cold ($T_{\rm dust}\,\sim\,20-30\,{\rm K}$), dense ($n({\rm H})\sim\,10^4- 10^5\,{\rm cm^{-3}}$) more extended phase is present. It outputs the emission in the low-excitation H$_2$O lines and typically also produces the prominent line absorption features. For the two ULIRGs in our sample (Arp 220 and Mrk 231) an even hotter and more compact (R$_s\,\le\,100$ pc) region is present which is possibly linked to AGN activity. We find that collisions dominate the water excitation in the cold gas and for lines with $E_{\rm up}\le300\,{\rm K}$ and $E_{\rm up}\le800\,{\rm K}$ in the warm and hot component, respectively. Higher energy levels are mainly excited by IR pumping., Comment: Accepted by ApJ, in press

The physical state of the gas in the central 500 pc of NGC~5128 (the radio galaxy Centaurus A - Cen A), was investigated using the far-infrared fine-structure lines of carbon, oxygen, and nitrogen, as well as the CO(4-3) molecular line. The circumnuclear disk (CND) is traced by emission from dust and the neutral gas ([CI] and CO). A gas outflow with a line-of-sight velocity of 60 km/s is evident in both species. The center of the CND is bright in [OI], [OIII], and [CII]; [OI]63mu emission dominates that of [CII] even though it is absorbed with optical depths of 1.0-1.5. The outflow is well-traced by the [NII] and [NIII] lines and also seen in the [CII] and [OIII] lines that peak in the center. Ionized gas densities are moderate in the CND and low everywhere else. Neutral gas densities range from 4000 per cm3 (outflow, extended thin disk ETD) to 20 000 per cm3 (CND). The CND radiation field is weak compared to the ETD starburst field. The outflow has a much stronger radiation field. The total mass of all the CND gas is 9 x 10^(7) M(o) and the mass of the outflowing gas is only 15%-30% of that. The outflow most likely originates from the shock-dominated CND cavity surrounding the central black hole. With a factor of three uncertainty, the mass outflow rate is about 2 M(o)/yr, a thousand times higher than the accretion rate of the black hole. Without replenishment, the CND will be depleted in 15-120 million years. However, the outflow velocity is well below the escape velocity., Comment: 14 pages, 9 figures. Accepted by Astronomy & Astrophysics

We present large-scale submm observations towards the Sgr A Complex covering ~300 arcmin2. These data were obtained in the frame of the HEXGAL Program with the Herschel-HIFI satellite and are complemented with submm observations obtained with the NANTEN2/SMART telescope as part of the NANTEN2/SMART Central Nuclear Zone Survey. The observed species are CO(4-3) observed with the NANTEN2/SMART telescope, and [CI](1-0), [CI](2-1), [NII](1-0), and [CII](3/2-1/2) observed with the Herschel-HIFI satellite. The observations are presented in a 1 km/s spectral resolution and a spatial resolution ranging from 46" to 28". The spectral coverage of the three lower frequency lines is +-200 km/s, while in the two high frequency lines, the upper LSR velocity limit is +94 km/s and +145 km/s for the [NII] and [CII] lines, respectively. The spatial distribution of the emission in all lines is very widespread. The bulk of the CO emission is found towards Galactic latitudes below the Galactic plane, and all the known molecular clouds are identified. Both neutral atomic carbon lines have their brightest emission associated with the +50 km/s cloud. Their spatial distribution at this LSR velocity describes a crescent-shape structure, which is probably the result of interaction with the energetic event (one or several supernovae explosions) that gave origin to the non-thermal Sgr A-East source. The [CII] and [NII] emissions have most of their flux associated with the thermal Arched-Filaments and the H Region and bright spots in [CII] emission towards the Central Nuclear Disk (CND) are detected. Warm Gas at very high (|Vlsr| > 100 km/s) LSR velocities is also detected towards the line of sight to the Sgr A Complex, and it is most probably located outside the region, in the X1 orbits.

The HI and CO components of the interstellar medium (ISM) are usually used to derive the dynamical mass M_dyn of nearby galaxies. Both components become too faint to be used as a tracer in observations of high-redshift galaxies. In those cases, the 158 $\mu$m line of atomic carbon [CII] may be the only way to derive M_dyn. As the distribution and kinematics of the ISM tracer affects the determination of M_dyn, it is important to quantify the relative distributions of HI, CO and [CII]. HI and CO are well-characterised observationally, however, for [CII] only very few measurements exist. Here we compare observations of CO, HI, and [CII] emission of a sample of nearby galaxies, drawn from the HERACLES, THINGS and KINGFISH surveys. We find that within R_25, the average [CII] exponential radial profile is slightly shallower than that of the CO, but much steeper than the HI distribution. This is also reflected in the integrated spectrum ("global profile"), where the [CII] spectrum looks more like that of the CO than that of the HI. For one galaxy, a spectrally resolved comparison of integrated spectra was possible; other comparisons were limited by the intrinsic line-widths of the galaxies and the coarse velocity resolution of the [CII] data. Using high-spectral-resolution SOFIA [CII] data of a number of star forming regions in two nearby galaxies, we find that their [CII] linewidths agree better with those of the CO than the HI. As the radial extent of a given ISM tracer is a key input in deriving M_dyn from spatially unresolved data, we conclude that the relevant length-scale to use in determining M_dyn based on [CII] data, is that of the well-characterised CO distribution. This length scale is similar to that of the optical disk., Comment: Accepted for publication in the Astronomical Journal

This paper presents [CII], [CI] and CO emission line maps of the star-forming regions N66, N25+N26, and N88 in the metal-poor Local Group dwarf galaxy SMC. The spatial and velocity structure of the large HII region N66 reveals an expanding ring of shocked molecular gas centered on the exciting star cluster NGC346, whereas a more distant dense molecular cloud is being eroded by UV radiation from the same cluster. In the N25+N26 and N88 maps, diffuse [CII] emission at a relatively low surface brightness extends well beyond the compact boundaries of the bright emission associated with the [CII] regions. In all regions, the distribution of this bright [CII] emission and the less prominent [CI] emission closely follows the outline of the CO complexes, but the intensity of the [CII] and [CI] emission is generally anticorrelated, which can be understood by the action of photodissociation and photoionization processes. Notwithstanding the overall similarity of CO and [CII] maps, the intensity ratio of these lines varies significantly, mostly due to changes in CO brightness. [CII] emission line profiles are up to 50% wider in velocity than corresponding CO profiles. A radiative transfer analysis shows that the [CII] line is the dominant tracer of (CO-dark) molecular hydrogen in the SMC. CO emission traces only a minor fraction of the total amount of gas. The similarity of the spatial distribution and line profile shape, and the dominance of molecular gas associated with [[CII] rather than CO emission imply that in the low-metallicity environment of the SMC the small amount of dense molecular gas traced by CO is embedded in the much more extended molecular gas traced only by [CII] emission. The contribution from neutral atomic and ionized hydrogen zones is negligible in the star-forming regions observed., Comment: 17 pages, 16 figures, Accepted in A&A

The circumnuclear disk (CND) of the Galactic Center is exposed to many energetic phenomena coming from the supermassive black hole Sgr A* and stellar activities. These energetic activities can affect the chemical composition in the CND by the interaction with UV-photons, cosmic-rays, X-rays, and shock waves. We aim to constrain the physical conditions present in the CND by chemical modeling of observed molecular species detected towards it. We analyzed a selected set of molecular line data taken toward a position in the southwest lobe of the CND with the IRAM 30m and APEX 12-meter telescopes and derived the column density of each molecule using a large velocity gradient (LVG) analysis. The determined chemical composition is compared with a time-dependent gas-grain chemical model based on the UCL\_CHEM code that includes the effects of shock waves with varying physical parameters. Molecules such as CO, HCN, HCO$^+$, HNC, CS, SO, SiO, NO, CN, H$_2$CO, HC$_3$N, N$_2$H$^+$ and H$_3$O$^+$ are detected and their column densities are obtained. Total hydrogen densities obtained from LVG analysis range between $2 \times 10^4$ and $1 \times 10^6\,$cm$^{-3}$ and most species indicate values around several $\times 10^5\,$cm$^{-3}$, which are lower than values corresponding to the Roche limit, which shows that the CND is tidally unstable. The chemical models show good agreement with the observations in cases where the density is $\sim10^4\,$cm$^{-3}$, the cosmic-ray ionization rate is high, $>10^{-15} \,$s$^{-1}$, or shocks with velocities $> 40\,$km s$^{-1}$ have occurred. Comparison of models and observations favors a scenario where the cosmic-ray ionization rate in the CND is high, but precise effects of other factors such as shocks, density structures, UV-photons and X-rays from the Sgr A* must be examined with higher spatial resolution data., Comment: 17 Pages, 13 figures, accepted for publication in A&A

OH is a key molecule in H2O chemistry, a valuable tool for probing physical conditions, and an important contributor to the cooling of shock regions. OH participates in the re-distribution of energy from the protostar towards the surrounding ISM. Our aim is to assess the origin of the OH emission from the Cepheus A massive star-forming region and to constrain the physical conditions prevailing in the emitting gas. We thus want to probe the processes at work during the formation of massive stars. We present spectrally resolved observations of OH towards the outflows of Cepheus A with the GREAT spectrometer onboard the SOFIA telescope. Three triplets were observed at 1834.7 GHz, 1837.8 GHz, and 2514.3 GHz (163.4, 163.1, and 119.2 microns), at angular resolutions of 16.3", 16.3", and 11.9", respectively. We present the CO (16-15) spectrum at the same position. We compared the integrated intensities in the redshifted wings to shock models. The two triplets near 163 microns are detected in emission with blending hyperfine structure unresolved. Their profiles and that of CO can be fitted by a combination of 2 or 3 Gaussians. The observed 119.2 microns triplet is seen in absorption, since its blending hyperfine structure is unresolved, but with three line-of-sight components and a blueshifted emission wing consistent with that of the other lines. The OH line wings are similar to those of CO, suggesting that they emanate from the same shocked structure. Under this common origin assumption, the observations fall within the model predictions and within the range of use of our model only if we consider that four shock structures are caught in our beam. Our comparisons suggest that the observations might be consistently fitted by a J-type model with nH > 1e5 cm-3, v > 20 km/s, and with a filling factor of ~1. Such a high density is generally found in shocks associated to high-mass protostars., Comment: 10 pages, 7 figures, 5 tables

We probe the chemical and energetic conditions in dense gas created by radiative feedback through observations of multiple CO, HCN and HCO$^+$ transitions toward the dense core of M17 SW. We used the dual band receiver GREAT on board the SOFIA airborne telescope to obtain maps of the $J=16-15$, $J=12-11$, and $J=11-10$ transitions of $^{12}$CO. We compare these maps with corresponding APEX and IRAM 30m telescope data for low- and mid-$J$ CO, HCN and HCO$^+$ emission lines, including maps of the HCN $J=8-7$ and HCO$^+$ $J=9-8$ transitions. The excitation conditions of $^{12}$CO, HCO$^+$ and HCN are estimated with a two-phase non-LTE radiative transfer model of the line spectral energy distributions (LSEDs) at four selected positions. The energy balance at these positions is also studied. We obtained extensive LSEDs for the CO, HCN and HCO$^+$ molecules toward M17 SW. The LSED shape, particularly the high-$J$ tail of the CO lines observed with SOFIA/GREAT, is distinctive for the underlying excitation conditions. The critical magnetic field criterion implies that the cold cloudlets at two positions are partially controlled by processes that create and dissipate internal motions. Supersonic but sub-Alfv\'enic velocities in the cold component at most selected positions indicates that internal motions are likely MHD waves. Magnetic pressure dominates thermal pressure in both gas components at all selected positions, assuming random orientation of the magnetic field. The magnetic pressure of a constant magnetic field throughout all the gas phases can support the total internal pressure of the cold components, but it cannot support the internal pressure of the warm components. If the magnetic field scales as $B \propto n^{2/3}$, then the evolution of the cold cloudlets at two selected positions, and the warm cloudlets at all selected positions, will be determined by ambipolar diffusion., Comment: 26 pages, 13 figures, A&A accepted

We have mapped the NGC 2023 reflection nebula in [CII] and CO(11--10) with the heterodyne receiver GREAT on SOFIA and obtained slightly smaller maps in 13CO(3--2), CO(3--2), CO(4--3), CO(6--5), and CO(7--6) with APEX in Chile. We use these data to probe the morphology, kinematics, and physical conditions of the C II region, which is ionized by FUV radiation from the B2 star HD37903. The [CII] emission traces an ellipsoidal shell-like region at a position angle of ~ -50 deg, and is surrounded by a hot molecular shell. In the southeast, where the C II region expands into a dense, clumpy molecular cloud ridge, we see narrow and strong line emission from high-J CO lines, which comes from a thin, hot molecular shell surrounding the [CII] emission. The [CII] lines are broader and show photo evaporating gas flowing into the C II region. Based on the strength of the [13CII] F=2--1 line, the [CII] line appears to be somewhat optically thick over most of the nebula with an optical depth of a few. We model the physical conditions of the surrounding molecular cloud and the PDR emission using both RADEX and simple PDR models. The temperature of the CO emitting PDR shell is ~ 90 -- 120 K, with densities of 10^5 -- 10^6 cm^-3, as deduced from RADEX modeling. Our PDR modeling indicates that the PDR layer where [CII] emission dominates has somewhat lower densities, 10^4 to a few times 10^5 cm^-3, Comment: Accepted by A&A

To determine the properties of the ionized gas at the edge of the CMZ near Sgr E we observed a small portion of the edge of the CMZ near Sgr E with spectrally resolved [C II] 158 micron and [N II] 205 micron fine structure lines at six positions with the GREAT instrument on SOFIA and in [C II] using Herschel HIFI on-the-fly strip maps. We use the [N II] spectra along with a radiative transfer model to calculate the electron density of the gas and the [C II] maps to illuminate the morphology of the ionized gas and model the column density of CO-dark H2. We detect two [C II] and [N II] velocity components, one along the line of sight to a CO molecular cloud at -207 km/s associated with Sgr E and the other at -174 km/s outside the edge of another CO cloud. From the [N II] emission we find that the average electron density is in the range of about 5 to 25 cm{-3} for these features. This electron density is much higher than that of the warm ionized medium in the disk. The column density of the CO-dark H$_2$ layer in the -207 km/s cloud is about 1-2X10{21} cm{-2} in agreement with theoretical models. The CMZ extends further out in Galactic radius by 7 to 14 pc in ionized gas than it does in molecular gas traced by CO. The edge of the CMZ likely contains dense hot ionized gas surrounding the neutral molecular material. The high fractional abundance of N+ and high electron density require an intense EUV field with a photon flux of order 1e6 to 1e7 photons cm{-2} s{-1}, and/or efficient proton charge exchange with nitrogen, at temperatures of order 1e4 K, and/or a large flux of X-rays. Sgr E is a region of massive star formation which are a potential sources of the EUV radiation that can ionize the gas. In addition X-ray sources and the diffuse X-ray emission in the CMZ are candidates for ionizing nitrogen., Comment: 12 pages, 9 figures

We present the results of two Mopra 3-mm spectral line surveys of the Lines of Sight (LOS) toward the Galactic Center (GC) molecular complexes Sgr B2 (LOS+0.693) and Sgr A (LOS-0.11). The spectra covered the frequency ranges of ~77-93 GHz and ~105-113 GHz. We have detected 38 molecular species and 25 isotopologues. The isotopic ratios derived from column density ratios are consistent with the canonical values, indicating that chemical isotopic fractionation and/or selective photodissociation can be considered negligible (<10%) for the GC physical conditions. The derived abundance and rotational temperatures are very similar for both LOSs, indicating very similar chemical and excitation conditions for the molecular gas in the GC. The excitation conditions are also very similar to those found for the nucleus of the starburst galaxy NGC 253. We report for the first time the detection of HCO and HOC+ emission in LOS+0.693. Our comparison of the abundance ratios between CS, HCO, HOC+ and HCO+ found in the two LOSs with those in typical Galactic photodissociation regions (PDRs) and starbursts galaxies does not show any clear trend to distinguish between UV and X-ray induced chemistries. We propose that the CS/HOC+ ratio could be used as a tracer of the PDR components in the molecular clouds in the nuclei of galaxies.

We present new CO and C^o line measurements of the compact circumnuclear disk in the center of NGC 128 (Centaurus~A) obtained with the Herschel Space Observatory, as well as SEST, JCMT, and APEX. The Cen A center CO ladder is quite different from those of either star-burst galaxies or AGNs. In addition, the relative intensity of the central Cen A [CI] emission lines is much greate than that in any other galaxy. The CO surface brightness of the compact circumnuclear disk (CND) is significantly higher than that of the much more extended thin disk (ETD) in the same line of sight. Our LVG and PDR/XDR models suggest that much of the CND gas is relatively cool (25 - 80 K) and not very dense (~ 300 cm^{-3}) if the heating is by UV photons, although there is some gas in both the CND and the ETD with a much higher density of ~30 000 cm^{-3}. Finally, there is also high-excitation, high-density phase in the CND (but not in the ETD), either in the form of an extreme PDR but more likely in the form of an XDR. The total gas mass of the Cen A CND is 8.4 x 10^{7} M(sun), uncertain by a factor of two. The CO-H2 conversion factor is 4 x 10^{20} K km/s, also within a factor of two., Comment: 14 pages, 5 figures; A&A in press

Starburst galaxies are undergoing intense periods of star formation. Understanding the heating and cooling mechanisms in these galaxies can give us insight to the driving mechanisms that fuel the starburst. Molecular emission lines play a crucial role in the cooling of the excited gas. With SPIRE on the Herschel Space Observatory we have observed the rich molecular spectrum towards the central region of NGC 253. CO transitions from J=4-3 to 13-12 are observed and together with low-J line fluxes from ground based observations, these lines trace the excitation of CO. By studying the CO excitation ladder and comparing the intensities to models, we investigate whether the gas is excited by UV radiation, X-rays, cosmic rays, or turbulent heating. Comparing the $^{12}$CO and $^{13}$CO observations to large velocity gradient models and PDR models we find three main ISM phases. We estimate the density, temperature,and masses of these ISM phases. By adding $^{13}$CO, HCN, and HNC line intensities, we are able to constrain these degeneracies and determine the heating sources. The first ISM phase responsible for the low-J CO lines is excited by PDRs, but the second and third phases, responsible for the mid to high-J CO transitions, require an additional heating source. We find three possible combinations of models that can reproduce our observed molecular emission. Although we cannot determine which of these are preferable, we can conclude that mechanical heating is necessary to reproduce the observed molecular emission and cosmic ray heating is a negligible heating source. We then estimate the mass of each ISM phase; $6\times 10^7$ M$_\odot$ for phase 1 (low-J CO lines), $3\times 10^7$ M$_\odot$ for phase 2 (mid-J CO lines), and $9\times 10^6$ M$_\odot$ for phase 3 (high-J CO lines) for a total system mass of $1\times10^{8}$ M$_\odot$.

We used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observe the J_KaKc=3_03-2_02, 3_22-2_21, and 3_21-2_20 transitions of para-H_2CO at 218 GHz simultaneously to determine kinetic temperatures of the dense gas in the central molecular zone (CMZ) of our Galaxy. The map extends over approximately 40 arcmin x 8 arcmin (~100x20 pc^2) along the Galactic plane with a linear resolution of 1.2 pc. The strongest of the three lines, the H_2CO (3_03-2_02) transition, is found to be widespread, and its emission shows a spatial distribution similar to ammonia. The relative abundance of para-H_2CO is 0.5-1.2 10^{-9}, which is consistent with results from lower frequency H_2CO absorption lines. Derived gas kinetic temperatures for individual molecular clouds range from 50 K to values in excess of 100 K. While a systematic trend toward (decreasing) kinetic temperature versus (increasing) angular distance from the Galactic center (GC) is not found, the clouds with highest temperature (T_kin > 100 K) are all located near the nucleus. For the molecular gas outside the dense clouds, the average kinetic temperature is 65+/-10 K. The high temperatures of molecular clouds on large scales in the GC region may be driven by turbulent energy dissipation and/or cosmic-rays instead of photons. Such a non-photon-driven thermal state of the molecular gas provides an excellent template for the more distant vigorous starbursts found in ultraluminous infrared galaxies (ULIRGs)., Comment: 23 pages, 11 figures, A&A in press

We report SOFIA/GREAT, Herschel/HIFI, and ground-based velocity-resolved spectroscopy of carbon monoxide (CO) rotational transitions from J=2-1 to J=16-15 toward two positions in the circum-nuclear disk (CND) in our Galactic center. Radiative transfer models were used to derive information on the physical state of the gas traced by CO. The excitation of the CO gas cannot be explained by a single physical component, but is clearly the superposition of various warm gas phases. In a two-component approach, our large velocity gradient (LVG) analysis suggests high temperatures of ~200 K with moderate gas densities of only ~10^4.5 cm^-3 for the bulk of the material. A higher excited phase, carrying ~20-30% of the column densities, is warmer (~300-500 K) but only slightly denser ~10^5.3 cm^-3. These densities are too low to self-stabilize the clumps against their high internal turbulence and fall below the Roche density (>10^7 cm^-3) at 1.5 pc galactocentric distance. We conclude that the bulk of the material in the CND is not organized by self-gravity nor stable against tidal disruption, and must be transient., Comment: Accepted in A&A special issue for SOFIA/GREAT

Molecular globules and pillars are spectacular features, found only in the interface region between a molecular cloud and an HII-region. Impacting Far-ultraviolet (FUV) radiation creates photon dominated regions (PDRs) on their surfaces that can be traced by typical cooling lines. With the GREAT receiver onboard SOFIA we mapped and spectrally resolved the [CII] 158 micron atomic fine-structure line and the highly excited 12CO J=11-10 molecular line from three objects in Cygnus X (a pillar, a globule, and a strong IRAS source). We focus here on the globule and compare our data with existing Spitzer data and recent Herschel Open-Time PACS data. Extended [CII] emission and more compact CO-emission was found in the globule. We ascribe this emission mainly to an internal PDR, created by a possibly embedded star-cluster with at least one early B-star. However, external PDR emission caused by the excitation by the Cyg OB2 association cannot be fully excluded. The velocity-resolved [CII] emission traces the emission of PDR surfaces, possible rotation of the globule, and high-velocity outflowing gas. The globule shows a velocity shift of ~2 km/s with respect to the expanding HII-region, which can be understood as the residual turbulence of the molecular cloud from which the globule arose. This scenario is compatible with recent numerical simulations that emphazise the effect of turbulence. It is remarkable that an isolated globule shows these strong dynamical features traced by the [CII]-line, but it demands more observational studies to verify if there is indeed an embedded cluster of B-stars., Comment: Letter accepted by A&A (SOFIA special issue)

The hydroxyl radical (OH) is found in various environments within the interstellar medium (ISM) of the Milky Way and external galaxies, mostly either in diffuse interstellar clouds or in the warm, dense environments of newly formed low-mass and high-mass stars, i.e, in the dense shells of compact and ultracompact HII regions (UCHIIRs). Until today, most studies of interstellar OH involved the molecule's radio wavelength hyperfine structure (hfs) transitions. These lines are generally not in LTE and either masing or over-cooling complicates their interpretation. In the past, observations of transitions between different rotational levels of OH, which are at far-infrared wavelengths, have suffered from limited spectral and angular resolution. Since these lines have critical densities many orders of magnitude higher than the radio wavelength ground state hfs lines and are emitted from levels with more than 100 K above the ground state, when observed in emission, they probe very dense and warm material. We probe the warm and dense molecular material surrounding the UCHIIR/OH maser sources W3(OH), G10.62-0.39 and NGC 7538 IRS1 by studying the $^2\Pi_{{1/2}}, J = {3/2} - {1/2}$ rotational transition of OH in emission and, toward the last source also the molecule's $^2\Pi_{3/2}, J = 5/2 - 3/2$ ground-state transition in absorption. We used the Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe these OH lines, which are near 1.84 THz ($163 \mu$m) and 2.51 THz ($119.3 \mu$m). We clearly detect the OH lines, some of which are blended with each other. Employing non-LTE radiative transfer calculations we predict line intensities using models of a low OH abundance envelope versus a compact, high-abundance source corresponding to the origin of the radio OH lines., Comment: Accepted for publication in A&A (SOFIA/GREAT special issue)

We present and analyze two spectrally resolved high-J CO lines towards the molecular outflow Cep E, driven by an intermediate-mass class 0 protostar. Using the GREAT receiver on board SOFIA, we observed the CO (12--11) and (13--12) transitions (E_u ~ 430 and 500 K, respectively) towards one position in the blue lobe of this outflow, that had been known to display high-velocity molecular emission. We detect the outflow emission in both transitions, up to extremely high velocities (~ 100 km/s with respect to the systemic velocity). We divide the line profiles into three velocity ranges that each have interesting spectral features: standard, intermediate, and extremely high-velocity. One distinct bullet is detected in each of the last two. A large velocity gradient analysis for these three velocity ranges provides constraints on the kinetic temperature and volume density of the emitting gas, >~ 100 K and > ~ 10^4 cm^-3, respectively. These results are in agreement with previous ISO observations and are comparable with results obtained by Herschel for similar objects. We conclude that high-J CO lines are a good tracer of molecular bullets in protostellar outflows. Our analysis suggests that different physical conditions are at work in the intermediate velocity range compared with the standard and extremely high-velocity gas at the observed position., Comment: Accepted for publication in A&A (SOFIA/GREAT special issue)

We have mapped 20 molecular lines in the Central Molecular Zone (CMZ) around the Galactic Centre, emitting from 85.3 to 93.3 GHz. This work used the 22-m Mopra radio telescope in Australia, equipped with the 8-GHz bandwidth UNSW-MOPS digital filter bank, obtaining \sim 2 km/s spectral and \sim 40 arcsec spatial resolution. The lines measured include emission from the c-C3H2, CH3CCH, HOCO+, SO, H13CN, H13CO+, SO, H13NC, C2H, HNCO, HCN, HCO+, HNC, HC3N, 13CS and N2H+ molecules. The area covered is Galactic longitude -0.7 to 1.8 deg. and latitude -0.3 to 0.2 deg., including the bright dust cores around Sgr A, Sgr B2, Sgr C and G1.6-0.025. We present images from this study and conduct a principal component analysis on the integrated emission from the brightest 8 lines. This is dominated by the first component, showing that the large-scale distribution of all molecules are very similar. We examine the line ratios and optical depths in selected apertures around the bright dust cores, as well as for the complete mapped region of the CMZ. We highlight the behaviour of the bright HCN, HNC and HCO+ line emission, together with that from the 13C isotopologues of these species, and compare the behaviour with that found in extra-galactic sources where the emission is unresolved spatially. We also find that the isotopologue line ratios (e.g. HCO+/H13CO+) rise significantly with increasing red-shifted velocity in some locations. Line luminosities are also calculated and compared to that of CO, as well as to line luminosities determined for external galaxies., Comment: 27 pages, 15 figures, 12 tables, accepted by MNRAS

We present observations of the rotational ortho-water ground transition, the two lowest para-water transitions, and the ground transition of ionised ortho-water in the archetypal starburst galaxy M82, performed with the HIFI instrument on the Herschel Space Observatory. These observations are the first detections of the para-H2O(111-000) (1113\,GHz) and ortho-H2O+(111-000) (1115\,GHz) lines in an extragalactic source. All three water lines show different spectral line profiles, underlining the need for high spectral resolution in interpreting line formation processes. Using the line shape of the para-H2O(111-000) and ortho-H2O+(111-000) absorption profile in conjunction with high spatial resolution CO observations, we show that the (ionised) water absorption arises from a ~2000 pc^2 region within the HIFI beam located about ~50 pc east of the dynamical centre of the galaxy. This region does not coincide with any of the known line emission peaks that have been identified in other molecular tracers, with the exception of HCO. Our data suggest that water and ionised water within this region have high (up to 75%) area-covering factors of the underlying continuum. This indicates that water is not associated with small, dense cores within the ISM of M82 but arises from a more widespread diffuse gas component., Comment: 5 pages, 4 figures. Accepted for publication in A&A

We present high resolution HIFI spectroscopy of the nucleus of the archetypical starburst galaxy M82. Six 12CO lines, 2 13CO lines and 4 fine-structure lines are detected. Besides showing the effects of the overall velocity structure of the nuclear region, the line profiles also indicate the presence of multiple components with different optical depths, temperatures and densities in the observing beam. The data have been interpreted using a grid of PDR models. It is found that the majority of the molecular gas is in low density (n=10^3.5 cm^-3) clouds, with column densities of N_H=10^21.5 cm^-2 and a relatively low UV radiation field (GO = 10^2). The remaining gas is predominantly found in clouds with higher densities (n=10^5 cm^-3) and radiation fields (GO = 10^2.75), but somewhat lower column densities (N_H=10^21.2 cm^-2). The highest J CO lines are dominated by a small (1% relative surface filling) component, with an even higher density (n=10^6 cm^-3) and UV field (GO = 10^3.25). These results show the strength of multi-component modeling for the interpretation of the integrated properties of galaxies., Comment: Accepted for publication in A&A Letters

We present a systematic study of the HNCO, C18O, 13CS, and C34S emission towards 13 selected molecular clouds in the Galactic center region. The molecular emission in these positions are used as templates of the different physical and chemical processes claimed to be dominant in the circumnuclear molecular gas of galaxies. The relative abundance of HNCO shows a variation of more than a factor of 20 amo ng the observed sources. The HNCO/13CS abundance ratio is highly contrasted (up to a factor of 30) between the shielded molecular clouds mostly affected by shocks, where HNCO is released to gas-phase from grain mantles, and those pervaded by an intense UV radiation field, where HNCO is photo-dissociated and CS production favored via ion reactions. We propose the relative HNCO to CS abundance ratio as a highly contrasted diagnostic tool to distinguish between the influence of shocks and/or the radiation field in the nuclear regions of galaxies and their relation to the evolutionary state of their nuclear star formation bursts., Comment: 25 pages, 5 figures, Accepted for publication in ApJ

We present the first detection of complex aldehydes and isomers in three typical molecular clouds located within 200pc of the center of our Galaxy. We find very large abundances of these complex organic molecules (COMs) in the central molecular zone (CMZ), which we attribute to the ejection of COMs from grain mantles by shocks. The relative abundances of the different COMs with respect to that of CH3OH are strikingly similar for the three sources, located in very different environments in the CMZ. The similar relative abundances point toward a unique grain mantle composition in the CMZ. Studying the Galactic center clouds and objects in the Galactic disk having large abundances of COMs, we find that more saturated molecules are more abundant than the non-saturated ones. We also find differences between the relative abundance between COMs in the CMZ and the Galactic disk, suggesting different chemical histories of the grain mantles between the two regions in the Galaxy for the complex aldehydes. Different possibilities for the grain chemistry on the icy mantles in the GC clouds are briefly discussed. Cosmic rays can play an important role in the grain chemistry. With these new detections, the molecular clouds in the Galactic center appear to be one of the best laboratories for studying the formation of COMs in the Galaxy., Comment: 20 pages, 4 figures, accepted in ApJ

We present high sensitivity observations of the complex organic molecules (COMs) CH3OH, C2H5OH, HCOOCH3, HCOOH and H2CO and of SiO toward the quiescent dark cloud L183 and the molecular outflow L1448-mm. We have not detected C2H5OH, HCOOCH3 and SiO in L183 and in the quiescent gas of L1448-mm. The abundances of CH3OH, H2CO and SiO are enhanced by factors 4-20 in the shock precursor component, and those of CH3OH and SiO by 3 and 4 orders of magnitude in the shocked gas, without substantial changes (< factor of 2) in the abundances of C2H5OH and HCOOCH3 relative to that of CH3OH. The large enhancements of SiO and CH3OH can be explained by the shock ejection of an important fraction of the grain mantle material into gas phase. Our upper limits to the C2H5OH/CH3OH and HCOOCH3/CH3OH ratios are consistent with the rather constant ratios measured in hot cores and Galactic center clouds. However, the upper limits to the HCOOCH3/CH3OH and the HCOOH/CH3OH ratios are at least one order of magnitude smaller than those found in"hot corinos" surrounding low mass protostars. We speculate that the observed abundances of COMs in different objects are consistent with a sort of "universal" grain mantle composition which is locally changed by the processes of low mass star formation., Comment: 10 pages, 1 figure. Accepted ApJ Letters

We study the origin of large abundances of complex organic molecules in the Galactic center (GC). We carried out a systematic study of the complex organic molecules CH3OH, C2H5OH, (CH3)2O, HCOOCH3, HCOOH, CH3COOH, H2CO, and CS toward 40 GC molecular clouds. Using the LTE approximation, we derived the physical properties of GC molecular clouds and the abundances of the complex molecules.The CH3OH abundance between clouds varies by nearly two orders of magnitude from 2.4x10^{-8} to 1.1x10^{-6}. The abundance of the other complex organic molecules relative to that of CH3OH is basically independent of the CH3OH abundance, with variations of only a factor 4-8. The abundances of complex organic molecules in the GC are compared with those measured in hot cores and hot corinos, in which these complex molecules are also abundant. We find that both the abundance and the abundance ratios of the complex molecules relative to CH3OH in hot cores are similar to those found in the GC clouds. However, hot corinos show different abundance ratios than observed in hot cores and in GC clouds. The rather constant abundance of all the complex molecules relative to CH3OH suggests that all complex molecules are ejected from grain mantles by shocks. Frequent (similar 10^{5}years) shocks with velocities >6km/s are required to explain the high abundances in gas phase of complex organic molecules in the GC molecular clouds. The rather uniform abundance ratios in the GC clouds and in Galactic hot cores indicate a similar average composition of grain mantles in both kinds of regions. The Sickle and the Thermal Radio Arches, affected by UV radiation, show different relative abundances in the complex organic molecules due to the differentially photodissociation of these molecules., Comment: 18 pages, 10 Postscript figures, uses aa.cls, aa.bst, 10pt.rtx, natbib.sty, revsymb.sty revtex4.cls, aps.rtx and aalongtabl.sty. Accepted in A&A 2006. version 2. relocated figures and tables. Language editor suggestions. added references