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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

The CII 158 microns line is the most important coolant of the interstellar medium in galaxies but substantial variations are seen from object to object. The main source of the emission at a galactic scale is still poorly understood. Previous studies of the CII emission in galaxies have a resolution of several kpc or more so the observed emission is an average of different ISM components. The aim of this work is to study, for the first time, the CII emission at the scale of a spiral arm. We want to investigate the origin of this line and its use as a tracer of star formation. We present CII and OI observations of a segment of a spiral arm of M~31 using the Infrared Space Observatory. The CII emission is compared with tracers of neutral gas (CO, HI) and star formation (H\alpha, Spitzer 24 mu.) The similarity of the CII emission with the Ha and 24 mu images is striking when smoothed to the same resolution, whereas the correlation with the neutral gas is much weaker. The CII cooling rate per H atom increases dramatically from ~2.7e-26 ergs/s/atom in the border of the map to ~ 1.4e-25 ergs/s/atom in the regions of star formation. The CII/FIR(42-122) ratio is almost constant at 2%, a factor 3 higher than typically quoted. However, we do not believe that M~31 is unusual. Rather, the whole-galaxy fluxes used for the comparisons include the central regions where the CII/FIR ratio is known to be lower and the resolved observations neither isolate a spiral arm nor include data as far out in the galactic disk as the observations presented here. A fit to published PDR models yields a plausible average solution of G_0~100 and n~3000 for the PDR emission in the regions of star formation in the arm of M31., Comment: 8 pages, 5 figures. To be published by A&A. Low quality figures. High quality version in http://www.obs.u-bordeaux1.fr/Radio/NRodriguez/out/m31.pdf

An overview is given of ISO results on regions of high excitation ISM and gas, i.e. HII regions, the Galactic Centre and Supernovae Remnants. IR emission due to fine-structure lines, molecular hydrogen, silicates, polycyclic aromatic hydrocarbons and dust are summarised, their diagnostic capabilities illustrated and their implications highlighted., Comment: To be publised in the review book : "ISO science legacy - a compact review of ISO major achievements"

(Abridged) We present fine structure and recombination lines observations of the ionized gas toward a sample of 18 sources located within 300 pc of the center of the Galaxy (GC). The fine structure lines from [NII] and [SIII] have been detected in 16 sources. In 10 sources we have even detected the [OIII] 88 $\mu$m line. The measured N, S and Ne abundances are similar to those measured in the HII regions in the 5-kpc ring and in the nuclei of starburst galaxies. The fine structure lines ratios measured for all the sources can be explained by photo-ionization with an effective temperature of 32000-37000 K and an ionization parameter, $U$, of $-1>\log U > -3$. The highest excitation is found in the Radio Arc region but there must be more ionizing sources distributed over the Galactic center than the known clusters of massive stars. Most of the clouds are located far from the prominent continuum complexes (Sgr C, B ...). However, it is possible that the clouds are ionized by escaped photons from those complexes. The excitation ratios, effective temperature and ionization parameter measured in the GC are similar to those found in some low excitation starburst galaxies. The [NeIII]/[NeII] line ratios measured in the GC sources are consistent with the results of the Thornely et al. (2000) model for a short burst of massive star formation less than 8 Myr ago. We have also found that the [NeII] 13 $\mu$m ~ to far-infrared continuum ratio measured for the GC sources is similar to that of external galaxies, supporting the idea by Sturm et al. (2002) that the far-infrared continuum in Active Galaxies is dominated by dust heated by stellar radiation rather than by the AGN., Comment: Accepted by A&A

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 and the full continuum spectra of the dust between 40 and 190 microns. A warm dust component with a temperature between 27 and 42 K is needed to fit the spectra. 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 10$^3$ cm^-3 and an incident far-ultraviolet field 10$^3$ times higher than the local interstellar radiation field. 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 (short version)., Comment: Accepted for publication by A&A

There is observational evidence that nuclear winds and X-rays can heavily influence the physical conditions and chemical abundances of molecular gas in the circumnuclear disks (CND) of Active Galactic Nuclei (AGN). In this paper we probe the chemical status of molecular gas in the CND of NGC1068, a prototypical Seyfert 2 galaxy. Precedent claims that the chemistry of molecular gas in the nucleus of NGC1068 is abnormal by galactic standards were based on the high HCN/CO luminosity ratio measured in the CND. Results from new observations obtained in this survey have served to derive abundances of molecular species such as SiO, CN, HCO+, HOC+, H13CO+ and HCO. These estimates are complemented by a re-evaluation of molecular abundances for HCN, CS and CO, based on previously published single-dish and interferometer observations of NGC1068. We report on the first detection of SiO emission in the CND of NGC1068. The estimated large abundance of SiO in the CND, X(SiO)~5-10x10^{-9}, cannot be attributed to shocks related to star formation, as there is little evidence of a recent starburst in the nucleus of NGC1068. Alternatively, we propose that silicon chemistry is driven by intense X-ray processing of molecular gas. We also report on the first extragalactic detection of the reactive ion HOC+. Most remarkably, the estimated HCO+/HOC+ abundance ratio in the nucleus of NGC1068, ~30-80, is the smallest ever measured in molecular gas. The abundances derived for all molecules that have been the subject of this survey are compared with the predictions of models invoking either oxygen-depletion or X-ray chemistry in molecular gas. Our conclusions favour an overall scenario where the CND of NGC1068 has become a giant X-ray Dominated Region (XDR)., Comment: 16 pages (including 9 figures and 6 tables); accepted by A&A

This paper deals with the heating and the ionization of the interstellar medium (ISM) in the 500 central pc of the Milky Way (hereafter Galactic center, GC). We review the results of the Infrared Space Observatory (ISO) observations of a sample of GC molecular clouds located far from thermal radiocontinuum or far-infrared sources. For the first time, we have been able to study in detail the dust continuum spectra from 40 to 190 microns founding a warm (30-40 K) dust component in addition to the well known 15-20 K component. Fine-structure lines observations have revealed the presence of diffuse ionized gas associated with the molecular clouds. The effective temperature of the ionizing radiation is higher than 33000 K. ISO has also allow us to measure the fraction of warm (~ 150 K) H2 in the GC clouds, which is on average of 30 %. The observations of the warm (a few 100 K) neutral gas are compatible with a Photon Dominated Region (PDR) scenario., Comment: 4 pages, 3 figures. To appear in the Proceedings of the 4th Cologne-Bonn-Zermatt-Symposium "The Dense Interstellar Medium in Galaxies" eds. S. Pfalzner, C. Kramer, C. Straubmeier, & A. Heithausen (Springer: Berlin)

We present large scale 9'x 27'(25 pc x 70 pc) far-IR observations around Sgr B2 using the Long-wavelength spectrometer (LWS) on board the Infrared Space Observatory (ISO). The spectra are dominated by the strong continuum emission of dust, the widespread molecular absorption of light hydrides (OH, CH and H2O) and the fine structure lines of [NII], [NIII], [OIII], [CII] and [OI]. The molecular richness in the outer layers of Sgr B2 is probed by the ISO-LWS Fabry-Perot (35 km s^-1) detections towards Sgr B2(M), where more that 70 lines from 15 molecular and atomic species are observed at high signal to noise ratio., Comment: 46 pages, 10 figures, 5 tables, accepted in ApJ part I. (Figs. 1, 2, 3, 9 and 10 have been bitmapped to low resolution)

We present large scale 9' x 27' (25 pc x 70 pc) far-IR observations of the Sgr B2 complex using the spectrometers on board the Infrared Space Observatory (ISO). The far-IR spectra are dominated by the strong continuum emission of dust and by the fine structure lines of high excitation potential ions (NII, NIII and OIII) and those of neutral or weakly ionized atoms (OI and CII). The line emission has revealed a very extended component of ionized gas. The study of the NIII 57 microns/NII 122 microns and OIII 52/88 microns line intensity ratios show that the ionized gas has a density of n_e~10^{2-3} cm^-3 while the ionizing radiation can be characterized by a diluted but hard continuum, with effective temperatures of ~35000 K. Photoionization models show that the total number of Lyman photons needed to explain such an extended component is approximately equal to that of the HII regions in Sgr B2(N) and (M) condensations. We propose that the inhomogeneous and clumpy structure of the cloud allows the radiation to reach large distances through the envelope. Therefore, photodissociation regions (PDRs) can be numerous at the interface of the ionized and the neutral gas. The analysis of the OI (63 and 145 microns) and CII (158 microns) lines indicates an incident far-UV field (G_0, in units of the local interstellar radiation field) of 10^{3-4} and a H density of 10^{3-4} cm^{-3} in such PDRs. We conclude that extended photoionization and photodissociation are also taking place in Sgr B2 in addition to more established phenomena such as widespread low--velocity shocks., Comment: 5 pages, to be published in: Astron. Nachr., Vol. 324, No. S1 (2003), Special Supplement "The central 300 parsecs of the Milky Way", Eds. A. Cotera, H. Falcke, T. R. Geballe, S. Markoff

We present infrared and millimeter observations of molecular gas, dust and ionized gas towards a sample of clouds distributed along the 500 central pc of the Galaxy. The clouds were selected to investigate the physical state, in particular the high gas temperatures, of the Galactic center region (GCr) clouds located far from far-infrared of thermal radio continuum sources. We have found that there is ionized gas associated with the molecular gas. The ionizing radiation is hard (~35000 K) but diluted due to the inhomogeneity of the medium. We estimate that ~30 % of the warm molecular gas observed in the GCr clouds is heated by ultra-violet radiation in photo-dissociation regions., Comment: 5 pages, to be published in: Astron. Nachr., Vol. 324, No. S1 (2003), Special Supplement "The central 300 parsecs of the Milky Way", Eds. A. Cotera, H. Falcke, T. R. Geballe, S. Markoff

We present some recent studies on the physical conditions and the chemistry of the interstellar medium (ISM) in the Galactic center region (GCR). Large scale shocks in the context of a bar potential can play a role in explaining the observed properties, at least in some particular regions. Alternatively, we propose that the observed properties (inhomogeneity, high temperatures of the molecular gas, rich chemistry,...) of the GCR ISM can be the consequence of the recent past ($10^6-10^7$ years ago) of massive star formation in the GCR., Comment: Slightly enlarged version of the Contribution to the conference "The evolution of galaxies II: basic building blocks", Eds. Sauvage, Vigroux, Schaerer, Madden. 2 pages

We present Infrared Space Observatory (ISO) observations of fine structure lines toward the Radio Arc region, close to Sgr A, in the Galactic center region (GCR). The ionization at large scale is dominated by the clusters of massive stars in this region: the Quintuplet and the Arches cluster. Any other possible mechanism as interaction with the magnetic fields revealed by the Non-Thermal Filaments (NTFs) must play a minor role. The radiation in this area is relatively hard ($T_\mathrm{eff}\sim 33-35$ kK) but diluted. It reaches large distances due to the inhomogeneity of the medium., Comment: 2 pages, 2 figures. Contribution to the conference "The evolution of galaxies II: basic building blocks", Eds. Sauvage, Vigroux, Schaerer, Madden, Use kluwer.cls

(Short version) We present an analysis of selected fine structure lines observed ISO toward the Radio Arc in the Galactic center region (GCR). We study the density of the ionized gas and the large scale ionization structure in a region of ~30x30 pc^2 by means of the NeIII 15.6 mum/NeII 12.8 mum and NIII 57 mum/NII 122 mum line ratios. The electron densities (n_e) derived from the OIII lines ratio indicate the presence of diffuse ionized material with n_e of \~10^{1.8-2.6} cm^{-3}. The NIII/NII ratio show two clear gradients, one pointing toward the Quintuplet cluster and the other pointing toward the Arches cluster. The model used to explain the ionization structure shows that the large scale ionization of the whole region can be accounted for by the UV radiation produced by the Quintuplet and the Arches cluster. Any other ionization mechanism should play a minor role. We also investigate the influence of the clusters on the bubble of warm dust seen in the MSX infrared images. The warm dust is well correlated with the ionized gas indicating that the dust is also heated by the radiation from both clusters. Furthermore, the elliptical rather than circular symmetry of some structures like the Thermal Filaments can also be explained when one considers the combined effects of both the Arches and the Quintuplet clusters. The bubble is filled with continuum emission of hard X rays and with emision from the 6.4 keV line of neutral or low ionized Fe., Comment: 13 pages, 5 figures included in the paper and two independent PNG figures, to be published in A&A

The Infrared Space Observatory (ISO) has shown the complexity of the Galactic center (GC) Interstellar medium (ISM) detecting, not only large column densities of warm molecular gas (H2), but the emission of neutral atoms and ions of low ionization potential (CII, OI, SiII,...) that should arise in shocked or photon-dominated regions (PDRs). In addition, ISO has also detected emission from ions like SIII, NeII, ArII, or NII (in some clouds we have even detected NeIII and OIII) that should arise from HII regions that were previously unsuspected due to the non-detection of Hydrogen recombination lines. Here we review some ISO results on the large scale study of the GC ISM and in particular, on the heating mechanisms of the clouds. Although, shocks should play an important role on the physics and chemistry of the GC ISM, ISO shows that the effect of radiation on the heating of the gas cannot be ruled out with the simple argument that the dust temperature is lower than that of the gas., Comment: 4 pages, 1 figure included in the paper and two independentGIF figures, to be published in the proc. of 'The Promise of FIRST' symposium. ESA SP-460

We present observations of C2H5OH toward molecular clouds in Sgr A, Sgr B2 and associated with thermal and non-thermal features in the Galactic center. C2H5OH emission in Sgr A and Sgr B2 is widespread, but not uniform. C2H5OH emission is much weaker or it is not detected in some molecular clouds in both complexes, in particular those with radial velocities between 70 and 120 km/s. While most of the clouds associated with the thermal features do not show C2H5OH emission, that associated with the Non-Thermal Radio Arc shows emission. The fractional abundance of C2H5OH in most of the clouds with radial velocities between 0 and 70 km/s in Sgr A and Sgr B2 is relatively high, of few 1e-8. The C2H5OH abundance decreases by more than one order of magnitude (aprox. 1e-9) in the clouds associated with the thermal features. The large abundance of C2H5OH in the gas-phase indicates that C2H5OH has formed in grains and released to gas-phase by shocks in the last aprox. 1e5 years., Comment: In press in Astrophysical Journal Letters 7 pages, 1 table, 1 figure

We present ISO observations of several H2 pure-rotational lines (from S(0) to S(5)) towards a sample of 16 molecular clouds distributed along the central ~ 500 pc of the Galaxy. We also present C18O and 13CO J=1->0 and J=2->1 observations of these sources made with the IRAM-30m telescope. With the CO data we derive H2 densities of 10e(3.5-4.0) cm-3 and H2 column densities of a few 10e22 cm-2. We have corrected the H2 data for ~ 30 magnitudes of visual extinction using a self-consistent method. In every source, we find that the H2 emission exhibits a large temperature gradient. The S(0) and S(1) lines trace temperatures (T) of ~150 K while the S(4) and S(5) lines indicate temperatures of ~ 600K. The warm H2 column density is typically ~1-2 x 10e22 cm-2, and is predominantly gas with T=150 K. This is the first direct estimate of the total column density of the warm molecular gas in the Galactic center region. These warm H2 column densities represent a fraction of ~ 30 % of the gas traced by the CO isotopes emission. The cooling by H2 in the warm component is comparable to that by CO. Comparing our H2 and CO data with available ammonia NH3 observations from literature one obtains relatively high NH3 abundances of a few 10e(-7) in both the warm and the cold gas. A single shock or Photo-Dissociation Region (PDR) cannot explain all the observed H2 lines. Alternatives for the heating mechanisms are discussed., Comment: 14 pages including figures, to be published in A&A

In this letter we present a map of the GC in the 1-0 line of SiO covering the same region than the ASCA observations of the Fe 6.4 keV line. The SiO emission is restricted to molecular clouds with radial velocity between 10 and 60 km/s in the Sgr A and Sgr B complexes. We find a correlation between the SiO morphology and the spatial distribution of the Fe 6.4 keV line, on the large scale and also within Sgr A and Sgr B. The SiO abundance increases by a factor of 20 in the regions with strong Fe 6.4 keV line. This indicates that the Fe 6.4 keV line mainly arises from molecular clouds with large gas phase abundance of refractory elements. We discuss the implications of the correlation on the origin of the hard X-rays, and the heating and the chemistry of the molecular clouds in the GC, Comment: 4 pages, 2 figures

We present ISO observations of the S(0), S(1), S(2), and S(3) rotational lines of molecular hydrogen from two molecular clouds near the Galactic Center (GC). We have also measured continuum dust emission at infrared wavelengths with ISO and the rotational radio lines J=1-0 of 13CO and C18O and J=2-1 of C18O with the IRAM-30m telescope. Using the dust continuum spectra and the CO lines we derive a total visual extinction of 15-20 magnitudes toward these GC clouds. After correcting the H2 data for extinction, the gas temperatures are \~250 K and the column densities of warm gas are ~2e21 cm-2. This is the first direct measure of the H2 column densities of the warm component; with this, we estimate an NH3 abundance in the warm gas of ~ 2e-7. The column density of warm gas is, at least, a factor of 100 larger than the corresponding column densities derived from the warm dust. The observed ortho-to-para ratio (OTPR) is ~ 1, clearly below the local thermodynamical equilibrium (LTE) OTPR for gas at 250 K of ~3. Low velocity shocks (~ 10 km/s) are the most likely explanation for the column densities of warm gas and dust and the non-LTE H2 OTPR., Comment: 12 pages, 5 figures, accepted by A&A

We present SWS and LWS ISO observations towards a strip across the photodissociation region (PDR) in the reflection nebula NGC 7023. SWS02 and LWS01 spectra have been taken towards the star and the brightest infrared filaments located NW and SW from the star (hereafter NW and SW PDRs). In addition, SWS02 spectra have been taken towards two intermediate positions. While all the PDR tracers (the [CII] 157.7$\mic$, [OI] 63.2 and 145.6$\mic$, [HI] 21cm and the H$_2$ rotational lines) present a ring-like morphology with the peaks toward the NW and SW PDRs and a minimum around the star, the SiII emission is filling the hole of this ring with the peak towards the star. This morphology can only be explained if the SiII emission arise in the lowest extinction layers of the PDR (A$_v$ $<$ 2 mag) and the HII region. At least 20% -- 30% of the Si must be in gas phase in these layers. For A$_v$ $\geq$ 2 mag, the Si is mainly in solid form ($\delta$ Si = -1.3). In both, the NW and SW PDRs, the intensities of the H$_2$ rotational lines can only be fitted by assuming an ortho-to-para-H$_2$ ratio lower than 3 in gas with rotation temperatures from 400 to 700 K. Therefore, there is a non-equilibrium OTP ratio in the region. The comparison between the OTP ratio derived from H$_2$ vibrational lines and the pure H$_2$ rotational lines, shows that the OTP ratio increases from $\sim$ 1.5 to 3 across the photodissociation region with larger values in the less shielded gas (A$_v$$<$ 0.7 mag). This behavior is interpreted as a consequence of an advancing photodissociation front., Comment: 14 pages, 6 figures, other comments

We have observed the S(0), S(1), S(2), S(3), S(4) and S(5) rotational lines of molecular hydrogen (H2) towards the peak of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. The observed H2 line ratios show that they arise in warm gas with kinetic temperatures ~300 - 700 K. However, the data cannot be fitted by an ortho- to para- (OTP) ratio of 3. An OTP ratio in the range ~1.5 - 2 is necessary to explain our observations. This is the first detection of a non-equilibrium OTP ratio measured from the H2 pure-rotational lines in a PDR. The existence of a dynamical PDR is discussed as the most likely explanation for this low OTP ratio., Comment: 4 pages, 3 figures

An overview is given of ISO results on regions of high excitation ISM and gas, i.e. HII regions, the Galactic Centre and Supernovae Remnants. IR emission due to fine-structure lines, molecular hydrogen, silicates, polycyclic aromatic hydrocarbons and dust are summarized, their diagnostic capabilities illustrated and their implications highlighted.