Nemesio J. Rodriguez-Fernandez

Gas flow models in the Milky Way embedded bars

Context. The gas distribution and dynamics in the inner Galaxy present many unknowns, such as the origin of the asymmetry of the lu-diagram of the Central Molecular Zone (CMZ). On the other hand, there is recent evidence in the stellar component of the presence of a nuclear bar that may be slightly lopsided. Aims. Our goal is to characterize the nuclear bar observed in 2MASS maps and to study the gas dynamics in the inner Milky Way taking into account this secondary bar. Methods. We have derived a realistic mass distribution by fitting the 2MASS star count map with a model including three components (disk, bulge and nuclear bar) and we have simulated the gas dynamics in the deduced gravitational potential using a sticky-particles code. Results. Our simulations of the gas dynamics successfully reproduce the main characteristics of the Milky Way for a bulge orientation of 20°-35° with respect to the Sun-Galactic Center (GC) line and a pattern speed of 30-40 km s -1 kpc -1 . In our models the Galactic Molecular Ring (GMR) is not an actual ring but the inner parts of the spiral arms, while the 3-kpc arm and its far side counterpart are lateral arms that extend around the bar. Our simulations reproduce, for the first time, the parallelogram shape of the lu-diagram of the CMZ as the gas response to the nuclear bar. This bar should be oriented by an angle of ∼60°-75° with respect to the Sun-GC line and its mass amounts to (2-5.5) 10 9 M ⊙ . We show that the observed asymmetry of the CMZ cannot be due to lopsidedness of the nuclear bar as suggested by the 2MASS maps. Conclusions. We do not find clear evidence of lopsidedness in the stellar potential. We propose that the observed asymmetry of the central gas layer can be due to the infalling of gas into the CMZ in the I = 1.3°-complex.

Molecular gas chemistry in AGN - I. The IRAM 30 m survey of NGC 1068

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 NGC 1068, a prototypical Seyfert 2 galaxy. Precedent claims that the chemistry of molecular gas in the nucleus of NGC 1068 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 + ,H 13 CO + 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 NGC 1068. We report on the first detection of SiO emission in the CND of NGC 1068. The estimated large abundance of SiO in the CND, X(SiO) ∼ (5−10) × 10 −9 , cannot be attributed to shocks related to star formation, as there is little evidence of a recent starburst in the nucleus of NGC 1068. 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 + .M ost remarkably, the estimated HCO + /HOC + abundance ratio in the nucleus of NGC 1068, ∼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 NGC 1068 has become a giant X-ray Dominated Region (XDR).

Warm H

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.

_\mathsf{2}

Context. The isocyanic acid (HNCO) has an extended distribution in the centers of the Milky Way and the spiral galaxy IC342. Based on both the morphology of the emission and the HNCO abundance with respect to H2, several authors hypothesized that HNCO could be a good tracer of interstellar shocks. Aims. We test this hypothesis by observing a well-known Galactic source whose chemistry is dominated by shocks. Methods. We observed several transitions of HNCO towards L1157-mm and two positions (B1 and B2) in the blue lobe of the molecular outflow. Results. The HNCO line profiles exhibit the same characteristics as other well-known shock tracers such as CH3OH, H2CO, SO, or SO2. The three molecules HNCO, SO2, and OCS are the only ones detected so far whose emission is more intense in B2 than in B1, making these species valuable probes of chemical differences along the outflow. The HNCO abundance with respect to H2 is (0.4–1.8) × 10 −8 in B1 and (0.3–1) × 10 −7 in B2. These abundances are the highest ever measured, and imply an increment with respect to L1157-mm of up to a factor of 83, demonstrating that this molecule is a good shock tracer. Conclusions. we demonstrate that shocks can produce the HNCO abundance measured in galactic nuclei and even higher values. We propose that the gas phase abundance of HNCO is produced by both grain mantle erosion by the shock waves and by neutral-neutral reactions in gas phase involving CN and O2. The observed anticorrelation between CN and HNCO fluxes supports this scenario. The observed similarities between the HNCO emission and sulfur-bearing molecules may be caused by formation pathways that also involve O2.

in the Galactic center region

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 ((O) 63 and 146 µm, (C) 158 µm, (Si  )3 5µm, (S  )2 5µ ma nd (Fe )2 6µm), which are the main coolants of the gas with kinetic temperatures of several hundred K. We also present the full continuum spectra of the dust between 40 and 190 µm. All the clouds exhibit a cold dust component with a temperature of ∼15 K. A warmer dust component is also required to fit the spectra. The temperature of this dust component changes between 27 and 42 K from source to source. We have compared the gas and the dust emission with the predictions from J-type and C-type shocks and photodissociation region (PDRs) models. We conclude that the dust and the fine structure lines observations are best explained by a PDR with a density of 10 3 cm −3 and an incident far-ultraviolet field 10 3 times higher than the local interstellar radiation field. The fine structure line emission arises in PDRs in the interface between a diffuse ionized gas component and the dense molecular clouds. The (C) 158 µ ma nd (Si) 35 µm lines also have an important contribution from the ionized gas component. PDRs can naturally explain the discrepancy between the gas and the dust temperatures. However, these PDRs can only account for 10-30% of the total H2 column density with a temperature of ∼150 K. We discuss other possible heating mechanisms for the rest the warm molecular gas, such as non-stationary PDRs, X-ray Dominated Regions (XDRs) or the dissipation of supersonic turbulence. The central 600 pc of the Galaxy (hereafter the Galactic center, GC) contain up to 10% of the neutral gas of the Milky Way. In spite of the high gas surface density in the GC (several or- ders of magnitude higher than in the disk of the Galaxy) the star formation rate is 10 times lower than that in the disk and the star formation efficiency is only similar to that in the disk. This is probably due to the particular physical conditions of the � Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA. �� Figures 1-4 are only available in electronic form at

HNCO enhancement by shocks in the L1157 molecular outflow

The physical conditions of the Galactic center (GC) clouds moving with non-circular velocities are not well-known. We have studied the physical conditions of these clouds with the aim of better understanding the origin of the outstanding physical conditions of the GC molecular gas and the possible effect of the large scale dynamics on these physical conditions.Using published CO(1-0) data, we have selected a set of clouds belonging to all the kinematical components seen in the longitude-velocity diagram of the GC. We have done a survey of dense gas in all the components using the J=2-1 lines of CS and SiO as tracers of high density gas and shock chemistry. We have detected CS and SiO emission in all the kinematical components. The gas density and the SiO abundance of the clouds in non-circular orbits are similar those in the nuclear ring (GCR). Therefore, in all the kinematical components there are dense clouds that can withstand the tidal shear. However, there is no evidence of star formation outside the GCR. The high relative velocity and shear expected in the dust-lanes along the bar major axis could inhibit the star formation process, as observed in other galaxies. The high SiO abundances derived in the non-circular velocity clouds are likely due to the large-scale shocks that created the dust lanes

ISO observations of the Galactic center interstellar medium. Neutral gas and dust

We present observations of C2H5OH toward molecular clouds in Sgr A and Sgr B2 and associated with thermal and nonthermal features in the Galactic center (GC). 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-1. While most of the clouds associated with the thermal features do not show C2H5OH emission, that associated with the nonthermal radio arc shows emission. The fractional abundance of C2H5OH in most of the clouds with radial velocities between 0 and 70 km s-1 in Sgr A and Sgr B2 is relatively high, of a few times 10-8. The C2H5OH abundance decreases by more than a factor of 10 (10-9) 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 ~105 yr. The implications of this finding in the origin of the shocks in the GC is briefly discussed.

Coupling the dynamics and the molecular chemistry in the galactic center

Context. The [Cii] 158 µm 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 and candidates range from photodissociation regions (PDRs) to the cold neutral or diffuse warm ionized medium. 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. Aims. 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. Methods. We present [Cii ]a nd [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 (Hα, Spitzer 24 µm). Results. The similarity of the [Cii] emission with the Hα and 24 µm 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 ∼2.7 × 10 −26 erg s −1 atom −1 in the border of the map to ∼1.4 × 10 −25 erg s −1 atom −1 in the regions of star formation. The [Cii]/FIR42−122 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 wholegalaxy 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 G0 ∼ 100 and n ∼ 3000 for the PDR emission in the regions of star formation in the arm of M 31.

Large-Scale Grain Mantle Disruption in the Galactic Center

Context. After several generations of interferometers in radioastronomy, wide-field imaging at high angular resolution is today a major goal for trying to match optical wide-field performances. Aims. All the radio-interferometric, wide-field imaging methods currently belong to the mosaicking family. Based on a 30 years old, original idea from Ekers & Rots, we aim at proposing an alternate formalism. Methods. Starting from their ideal case, we successively evaluate the impact of the standard ingredients of interferometric imaging, i.e. the sampling function, the visibility gridding, the data weighting, and the processing of the short spacings either from single-dish antennas or from heterogeneous arrays. After a comparison with standard nonlinear mosaicking, we assess the compatibility of the proposed processing with 1) a method of dealing with the effect of celestial projection and 2) the elongation of the primary beam along the scanning direction when using the on-the-fly observing mode. Results. The dirty image resulting from the proposed scheme can be expressed as a convolution of the sky brightness distribution with a set of wide-field dirty beams varying with the sky coordinates. The wide-field dirty beams are locally shift-invariant as they do not depend strongly on position on the sky: their shapes vary on angular scales typically larger or equal to the primary beamwidth. A comparison with standard nonlinear mosaicking shows that both processing schemes are not mathematically equivalent, though they both recover the sky brightness. In particular, the weighting scheme is very different in both methods. Moreover, the proposed scheme naturally processes the short spacings from both single-dish antennas and heterogeneous arrays. Finally, the sky gridding of the measured visibilities, required by the proposed scheme, may potentially save large amounts of hard-disk space and cpu processing power over mosaicking when handling data sets acquired with the on-the-fly observing mode. Conclusions. We propose to call this promising family of imaging methods wide-field synthesis because it explicitly synthesizes visibilities at a much finer spatial frequency resolution than the one set by the diameter of the interferometer antennas.

[CII] emission and star formation in the spiral arms of M 31

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 (