Ionized gas at the edge of the Central Molecular Zone

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 figure

Disentangling the excitation conditions of the dense gas in M17 SW

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 accepte

Characterizing the transition from diffuse atomic to dense molecular clouds in the Magellanic clouds with [CII], [CI], and CO

We present and analyze deep Herschel/HIFI observations of the [CII] 158um, [CI] 609um, and [CI] 370um lines towards 54 lines-of-sight (LOS) in the Large and Small Magellanic clouds. These observations are used to determine the physical conditions of the line--emitting gas, which we use to study the transition from atomic to molecular gas and from C^+ to C^0 to CO in their low metallicity environments. We trace gas with molecular fractions in the range 0.1<f(H2)<1, between those in the diffuse H2 gas detected by UV absorption (f(H2)<0.2) and well shielded regions in which hydrogen is essentially completely molecular. The C^0 and CO column densities are only measurable in regions with molecular fractions f(H2)>0.45 in both the LMC and SMC. Ionized carbon is the dominant gas-phase form of this element that is associated with molecular gas, with C^0 and CO representing a small fraction, implying that most (89% in the LMC and 77% in the SMC) of the molecular gas in our sample is CO-dark H2. The mean X_CO conversion factors in our LMC and SMC sample are larger than the value typically found in the Milky Way. When applying a correction based on the filling factor of the CO emission, we find that the values of X_CO in the LMC and SMC are closer to that in the Milky Way. The observed [CII] intensity in our sample represents about 1% of the total far-infrared intensity from the LOSs observed in both Magellanic Clouds.Comment: 32 pages, 21 figures, Accepted to Ap

The 492 GHz emission of Sgr A* constrained by ALMA

We report linearly polarized continuum emission properties of Sgr A* at $\sim$492 GHz, based on the Atacama Large Millimeter Array (ALMA) observations. We used the observations of the likely unpolarized continuum emission of Titan, and the observations of C\textsc{i} line emission, to gauge the degree of spurious polarization. The Stokes I flux of 3.6$\pm$0.72 Jy during our run is consistent with extrapolations from the previous, lower frequency observations. We found that the continuum emission of Sgr A* at $\sim$492 GHz shows large amplitude differences between the XX and the YY correlations. The observed intensity ratio between the XX and YY correlations as a function of parallactic angle may be explained by a constant polarization position angle of $\sim$158$^{\circ}$$\pm$3$^{\circ}$. The fitted polarization percentage of Sgr A* during our observational period is 14\%$\pm$1.2\%. The calibrator quasar J1744-3116 we observed at the same night can be fitted to Stokes I = 252 mJy, with 7.9\%$\pm$0.9\% polarization in position angle P.A. = 4.1$^{\circ}$$\pm$4.2$^{\circ}$. The observed polarization percentage and polarization position angle in the present work appear consistent with those expected from longer wavelength observations in the period of 1999-2005. In particular, the polarization position angle at 492 GHz, expected from the previously fitted 167$^{\circ}$$\pm$7$^{\circ}$ intrinsic polarization position angle and (-5.6$\pm$0.7)$\times$10$^{5}$ rotation measure, is 155$^{+9}_{-8}$, which is consistent with our new measurement of polarization position angle within 1$\sigma$. The polarization percentage and the polarization position angle may be varying over the period of our ALMA 12m Array observations, which demands further investigation with future polarization observations.Comment: 10 pages, 6 figures, 1st referee report received and revise

HIFI spectroscopy of low-level water transitions in M82

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&

GREAT [CII] and CO observations of the BD+40{\deg}4124 region

The BD+40\degree4124 region was observed with high angular and spectral resolution with the German heterodyne instrument GREAT in CO J = 13 \rightarrow 12 and [CII] on SOFIA. These observations show that the [CII] emission is very strong in the reflection nebula surrounding the young Herbig Ae/Be star BD+40\degree4124. A strip map over the nebula shows that the [CII] emission approximately coincides with the optical nebulosity. The strongest [CII] emission is centered on the B2 star and a deep spectrum shows that it has faint wings, which suggests that the ionized gas is expanding. We also see faint CO J = 13 \rightarrow 12 at the position of BD+40\degree4124, which suggests that the star may still be surrounded by an accretion disk.We also detected [CII] emission and strong CO J = 13 \rightarrow 12 toward V1318 Cyg. Here the [CII] emission is fainter than in BD+40\degree4124 and appears to come from the outflow, since it shows red and blue wings with very little emission at the systemic velocity, where the CO emission is quite strong. It therefore appears that in the broad ISO beam the [CII] emission was dominated by the reflection nebula surrounding BD+40\degree4124, while the high J CO lines originated from the adjacent younger and more deeply embedded binary system V1318 Cyg

A thorough view of the nuclear region of NGC 253 - Combined Herschel, SOFIA and APEX dataset

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 201

A complex storm system in Saturn’s north polar atmosphere in 2018

Producción CientíficaSaturn’s convective storms usually fall in two categories. One consists of mid-sized storms ∼2,000 km wide, appearing as irregular bright cloud systems that evolve rapidly, on scales of a few days. The other includes the Great White Spots, planetary-scale giant storms ten times larger than the mid-sized ones, which disturb a full latitude band, enduring several months, and have been observed only seven times since 1876. Here we report a new intermediate type, observed in 2018 in the north polar region. Four large storms with east–west lengths ∼4,000–8,000 km (the first one lasting longer than 200 days) formed sequentially in close latitudes, experiencing mutual encounters and leading to zonal disturbances affecting a full latitude band ∼8,000 km wide, during at least eight months. Dynamical simulations indicate that each storm required energies around ten times larger than mid-sized storms but ∼100 times smaller than those necessary for a Great White Spot. This event occurred at about the same latitude and season as the Great White Spot in 1960, in close correspondence with the cycle of approximately 60 years hypothesized for equatorial Great White Spots.Ministerio de Economía, Industria y Competitividad - Fondo Europeo de Desarrollo Regional (project AYA2015-65041-P)Gobierno Vasco (project IT-366-19

High-resolution absorption spectroscopy of the OH 2Pi 3/2 ground state line

The chemical composition of the interstellar medium is determined by gas phase chemistry, assisted by grain surface reactions, and by shock chemistry. The aim of this study is to measure the abundance of the hydroxyl radical (OH) in diffuse spiral arm clouds as a contribution to our understanding of the underlying network of chemical reactions. Owing to their high critical density, the ground states of light hydrides provide a tool to directly estimate column densities by means of absorption spectroscopy against bright background sources. We observed onboard the SOFIA observatory the 2Pi3/2, J = 5/2 3/2 2.5 THz line of ground-state OH in the diffuse clouds of the Carina-Sagittarius spiral arm. OH column densities in the spiral arm clouds along the sightlines to W49N, W51 and G34.26+0.15 were found to be of the order of 10^14 cm^-2, which corresponds to a fractional abundance of 10^-7 to 10^-8, which is comparable to that of H_2O. The absorption spectra of both species have similar velocity components, and the ratio of the derived H_2O to OH column densities ranges from 0.3 to 1.0. In W49N we also detected the corresponding line of ^18OH