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Register a new userThe Structure of Dark Molecular Gas in the Galaxy - I: A Pilot Survey for 18-cm OH Emission Towards $l approx 105^{deg}, b approx +1^{deg}$
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We report the first results from a survey for 1665, 1667, and 1720 MHz OH emission over a small region of the Outer Galaxy centered at $l approx 105.0deg , b approx +1.0deg$ . This sparse, high-sensitivity survey ($Delta Ta approx Delta Tmb approx 3.0 - 3.5$ mK rms in 0.55 km/s channels), was carried out as a pilot project with the Green Bank Telescope (GBT, FWHM $approx 7.6$) on a 3 X 9 grid at $0.5deg$ spacing. The pointings chosen correspond with those of the existing $^{12}$CO(1-0) CfA survey of the Galaxy (FWHM $approx 8.4$). With 2-hr integrations, 1667 MHz OH emission was detected with the GBT at $gtrsim 21$ of the 27 survey positions ($geq 78%$ ), confirming the ubiquity of molecular gas in the ISM as traced by this spectral line. With few exceptions, the main OH lines at 1665 and 1667 MHz appear in the ratio of 5:9 characteristic of LTE at our sensitivity levels. No OH absorption features are recorded in the area of the present survey, in agreement with the low levels of continuum background emission in this direction. At each pointing the OH emission appears in several components extending over a range of radial velocity and coinciding with well-known features of Galactic structure such as the Local Arm and the Perseus Arm. In contrast, little CO emission is seen in the survey area; less than half of the $gtrsim 50$ identified OH components show detectable CO at the CfA sensitivity levels, and these are generally faint. There are no CO profiles without OH emission. With few exceptions, peaks in the OH profiles coincide with peaks in the GBT HI spectra (obtained concurrently, FWHM $8.9$), although the converse is not true. We conclude that main-line OH emission is a promising tracer for the dark molecular gas in the Galaxy discovered earlier in Far-IR and gamma-ray emission. Further work is needed to establish the quantitative details of this connection.
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We have mapped faint 1667 OH line emission (TA approx 20 - 40 mK in our approx 30 beam) along many lines of sight in the Galaxy covering an area of approx 4circ times 4circ in the general direction of l approx 108circ, b approx 5circ. The OH emission is widespread, similar in extent to the local HI (r </= 2 kpc) both in space and in velocity. The OH profile amplitudes show a good general correlation with those of HI in spectral channels of approx 1 km/s; this relation is described by TA(OH) approx 1.50 times 10^{-4} TB(HI) for values of TB(HI) </approx 60 - 70 K. Beyond this the HI line appears to saturate, and few values are recorded above approx 90 K. However, the OH brightness continues to rise, by a further factor approx 3. The OH velocity profiles show multiple features with widths typically 2 - 3 km/s, but less than 10% of these features are associated with CO(1-0) emission in existing surveys of the area smoothed to comparable resolution.
We present the Forgotten Quadrant Survey (FQS), an ESO large project that used the 12m antenna of the Arizona Radio Observatory to map the Galactic Plane in the range 220deg$<l<$240deg and -2.5deg$<b<$0deg, both in $^{12}$CO(1-0) and $^{13}$CO(1-0), at a spectral resolution of 0.65 km s$^{-1}$ and 0.26 km s$^{-1}$. Our dataset allows us to easily identify how the molecular dense gas is organised at different spatial scales: from the giant clouds with their denser filamentary networks, down to the clumps and cores that host the newborn stars and to obtain reliable estimates of their key physical parameters. We present the first release of the FQS data and discuss their quality. Spectra with 0.65 km s$^{-1}$ velocity channels have a noise ranging from 0.8 K to 1.3 K for $^{12}$CO(1-0) and from 0.3 K to 0.6 K for $^{13}$CO(1-0). In this paper, we used the $^{12}$CO(1-0) spectral cubes to produce a catalogue of 263 molecular clouds. This is the first selfconsistent, statistical catalogue of molecular clouds of the outer Galaxy, obtained with a subarcminute spatial resolution and therefore able to detect not only the classical giant molecular clouds, but also the small clouds and to resolve the cloud structure at the subparsec scale up to a distance of a few kpc. We found two classes of objects: structures with size above a few parsecs that are typical molecular clouds and may be self-gravitating, and subparsec structures that cannot be in gravitational equilibrium and are likely transient or confined by external pressure. We used the ratio between the Herschel H$_2$ column density and the integrated intensity of the CO lines to calculate the CO conversion factor and we found mean values of (3.3$pm$1.4)$times 10^{20}$ cm$^{-2}$(K km s$^{-1})^{-1}$ and (1.2$pm$0.4)$times 10^{21}$ cm$^{-2}$(K km s$^{-1})^{-1}$, for $^{12}$CO(1-0) and $^{13}$CO(1-0), respectively.
The Multipurpose InfraRed Imaging System (MIRIS) performed the MIRIS Pa{alpha} Galactic Plane Survey (MIPAPS), which covers the entire Galactic plane within the latitude range of -3{deg} < b < +3{deg} at Pa{alpha} (1.87 um). We present the first result of the MIPAPS data extracted from the longitude range of l = 96.5{deg}-116.3{deg}, and demonstrate the data quality and scientific potential of the data by comparing them with H{alpha} maps obtained from the INT Photometric H{alpha} Survey (IPHAS) data. We newly identify 90 H II region candidates in the WISE H II region catalog as definite H II regions by detecting the Pa{alpha} and/or H{alpha} recombination lines, out of which 53 H II regions are detected at Pa{alpha}. We also report the detection of additional 29 extended and 18 point-like sources at Pa{alpha}. We estimate the E(B-V) color excesses and the total Lyman continuum luminosities for H II regions by combining the MIPAPS Pa{alpha} and IPHAS H{alpha} fluxes. The E(B-V) values are found to be systematically lower than those estimated from point stars associated with H II regions. Utilizing the MIPAPS Pa{alpha} and IPHAS H{alpha} images, we obtain an E(B-V) map for the entire region of the H II region Sh2-131 with an angular size of ~2.5{deg}. The E(B-V) map shows not only numerous high-extinction filamentary features but also negative E(B-V) regions, indicating H{alpha} excess. The H{alpha} excess and the systematic underestimation of E(B-V) are attributed to light scattered by dust.
Synchrotron emission pervades the Galactic plane at low radio frequencies, originating from cosmic ray electrons interacting with the Galactic magnetic field. Using a low-frequency radio telescope, the Murchison Widefield Array (MWA), we measure the free-free absorption of this Galactic synchrotron emission by intervening HII regions along the line of sight. These absorption measurements allow us to calculate the Galactic cosmic-ray electron emissivity behind and in front of 47 detected HII regions in the region $250^circ < l < 355^circ$, $|b| < 2^circ$. We find that all average emissivities between the HII regions and the Galactic edge along the line of sight ($epsilon_b$) are in the range of 0.24$,,sim,,$0.70$,,$K$,,$pc$^{-1}$ with a mean of 0.40$,,$K$,,$pc$^{-1}$ and a variance of 0.10$,,$K$,,$pc$^{-1}$ at 88$,,$MHz. Our best model, the Two-circle model, divides the Galactic disk into three regions using two circles centring on the Galactic centre. It shows a high emissivity region near the Galactic centre, a low emissivity region near the Galactic edge, and a medium emissivity region between these two regions, contrary to the trend found by previous studies.
We present excitation temperatures $T_{ex}$ for the OH 18-cm main lines at 1665 and 1667 MHz measured directly in front of the W5 star-forming region, using observations from the Green Bank Telescope and the Very Large Array. We find unequivocally that $T_{ex}$ at 1665 MHz is greater than $T_{ex}$ at 1667 MHz. Our method exploits variations in the continuum emission from W5, and the fact that the continuum brightness temperatures $T_C$ in this nebula are close to the excitation temperatures of the OH lines in the foreground gas. The result is that an OH line can appear in emission in one location and in absorption in a neighboring location, and the value of $T_C$ where the profiles switch from emission to absorption indicates $T_{ex}$. Absolute measurements of $T_{ex}$ for the main lines were subject to greater uncertainty because of unknown effects of geometry of the OH features. We also employed the traditional expected profile method for comparison with our continuum background method, and found that the continuum background method provided more precise results, and was the one to definitively show the $T_{ex}$ difference. Our best estimate values are: $T^{65}_{ex} = 6.0 pm 0.5$ K, $T^{67}_{ex} = 5.1 pm 0.2$ K, and $T^{65}_{ex} - T^{67}_{ex} = 0.9 pm 0.5$ K. The $T_{ex}$ values we have measured for the ISM in front of W5 are similar to those found in the quiescent ISM, indicating that proximity to massive star-forming regions does not generally result in widespread anomalous excitation of OH emission.
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