We made CO(1-0) observations of 103 lines of sight in the core and envelope of the high-latitude cloud MBM 40 to determine how the CO-H_2 conversion factor (X_CO) varies throughout the cloud. Calibrating X_CO with CH data at similar resolution (1' for CO, 1.5' for CH) yields values of X_CO ranging from 0.6 10^20 to 3.3 10^20 cm^-2 [K km s^-1]^-1 with an average of 1.3 10^20 cm^-2 [K km s^-1]^-1. Given that the cloud has a peak reddening of 0.24 mag, it should be classed as a diffuse rather than a translucent molecular cloud. The mass obtained from the CO data and our values of X_CO is 9.6 M(solar) for the core, 12 M(solar) for the envelope, and 10 M(solar) for the periphery of the cloud. A third of the molecular mass of the cloud is found in a region with E(B-V) < 0.12 mag. With these mass estimates, we determine that the cloud is not gravitationally bound.
We have conducted B, g, V, and R-band imaging in a 45x40 arcmin^2 field containing part of the high Galactic latitude translucent cloud MBM32, and correlated the intensity of diffuse optical light S_ν(λ) with that of 100 micron emission S_ν(100um). A chi^2 minimum analysis is applied to fit a linear function to the measured correlation and derive the slope parameter b(λ)= ΔS_ν(λ) / ΔS_ν(100um) of the best-fit linear function. Compiling a sample by combining our b(λ) and published ones, we show that the b(λ) strength varies from cloud to cloud by a factor of 4. Finding that b(λ) decreases as S_…
We conducted high-sensitivity, high-velocity resolution CO(1-0) observations in a region containing a portion of the diffuse molecular cloud MBM 53 to determine whether weak CO emission was present. The results of our observations increase the amount of CO-detectable molecular gas in the region by a factor of two. The increased molecular mass for the cloud, if applicable to the molecular clouds in the entire Pegasus-Pisces region, decreases the dark molecular gas content from 58% of the total H2 mass to ~ 30%. If the results for MBM53 are applicable to other diffuse clouds, then the fraction o…
We show that the XCO factor, which converts the CO luminosity into the column density of molecular hydrogen has similar values for dense, fully molecular gas and for diffuse, partially molecular gas. We discuss the reasons of this coincidence and the consequences for the understanding of the interstellar medium.
Aims: We wish to separate and quantify the CO luminosity and CO-H2 conversion factor applicable to diffuse but partially-molecular ISM when H2 and CO are present but C+ is the dominant form of gas-phase carbon. Methods: We discuss galactic lines of sight observed in \HI, HCO+ and CO where CO emission is present but the intervening clouds are diffuse (locally \AV\ $\la 1$ mag) with relatively small CO column densities $\NCO \la 2\times10^{16}\pcc$. We separate the atomic and molecular fractions statistically using \EBV\ as a gauge of the total gas column density and compare NH2 to the observed …
We wish to relate the degree scale structure of galactic diffuse clouds to sub-arcsecond atomic and molecular absorption spectra obtained against extragalactic continuum background sources. To do this, we used the ARO 12m telescope to map J=1-0 CO emission at 1' resolution over 30' fields around the positions of 11 background sources occulted by 20 molecular absorption line components, of which 11 had CO emission counterparts. We compare maps of CO emission to sub-arcsec atomic and molecular absorption spectra and to the large-scale distribution of interstellar reddening. The main results are:…
A new method to determine the CO-to-H$_2$ conversion factor $X_{\rm CO}$ using absorption of diffuse X-ray emission by local molecular clouds was developed. It was applied to the Ophiucus (G353+17) and Corona Australis (G359-18) clouds using CO-line and soft X-ray archival data. We obtained a value $X_{\rm CO} =1.85 \pm 0.45 \times 10^{20} {\rm H_2 cm^{-2}/(K~ km~ s^{-1})}$ as the average of least-$chi^2$ fitting results for R4 (0.7 keV) and R5 (0.8 keV) bands. Full resolution pdf available at http://www.ioa.s.u-tokyo.ac.jp/~sofue/htdocs/2016Xco/
We reanalyzed interstellar Cl I and Cl II spectra acquired with the Copernicus satellite. The directions for this study come from those of Crenny & Federman and sample the transition from atomic to molecular rich clouds where the unique chemistry leading to molecules containing chlorine is initiated. Our profile syntheses relied on up-to-date laboratory oscillator strengths and component structures derived from published high-resolution measurements of K I absorption that were supplemented with Ca II and Na I D results. We obtain self-consistent results for the Cl I lines at 1088, 1097, and 13…
We present polynomial conversions between each of 11 different strong line gas-phase metallicity calibrations, each based on $\sim$ 1.1 million star-forming spaxels in the public Sloan Digital Sky Survey (SDSS) Data Release 15 (DR15) Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. For this sample, which is $\sim$ 20 times larger than previous works, we present 5th order polynomial fits for each of 110 possible calibration conversions, for both Small Magellanic Cloud (SMC)-type and Milky Way (MW)-type dust corrections. The typical $2σ$ scatter around our polynomial fits is 0…
Carbon monoxide (CO) provides crucial information about the molecular gas properties of galaxies. While $^{12}$CO has been targeted extensively, isotopologues such as $^{13}$CO have the advantage of being less optically thick and observations have recently become accessible across full galaxy discs. We present a comprehensive new dataset of $^{13}$CO(1-0) observations with the IRAM 30-m telescope of the full discs of 9 nearby spiral galaxies from the EMPIRE survey at a spatial resolution of $\sim$1.5kpc. $^{13}$CO(1-0) is mapped out to $0.7-1r_{25}$ and detected at high signal-to-noise through…
Methods: 12CO emission is imaged in position and position-velocity space analyzed statistically, and then compared with maps of total reddening and with models of the C+ - CO transition in H2-bearing diffuse clouds. Results: Around Zeta Oph, 12CO emission appears in two distinct intervals of reddening centered near EBV = 0.4 and 0.65 mag, of which < 0.2 mag is background material. Within either interval, the integrated 12CO intensity varies up to 6-12 K-km/s compared to 1.5 K-km/s toward Zeta Oph. Nearly 80% of the individual profiles have velocity dispersions < 0.6 km/s, which are subsonic at…
We utilize a modified astrochemistry code which includes cosmic ray attenuation in-situ to quantify the impact of different cosmic-ray models on the CO-to-H$_2$ and CI-to-H$_2$ conversion factors, $X_{\rm CO}$ and $X_{\rm CI}$, respectively. We consider the impact of cosmic rays accelerated by accretion shocks, and show that clouds with star formation efficiencies greater than 2\% have $X_{\rm CO} = (2.5 \pm 1)\times10^{20}$ cm$^{-2}$(K km s$^{-1}$)$^{-1}$, consistent with Milky Way observations. We find that changing the cosmic ray ionization rate from external sources from the canonical $ζ\a…
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 essentiall…
We derive the CO-to-H2 conversion factor, X_CO = N(H2)/I_CO, across the Perseus molecular cloud on sub-parsec scales by combining the dust-based N(H2) data with the I_CO data from the COMPLETE Survey. We estimate an average X_CO ~ 3 x 10^19 cm^-2 K^-1 km^-1 s and find a factor of ~3 variations in X_CO between the five sub-regions in Perseus. Within the individual regions, X_CO varies by a factor of ~100, suggesting that X_CO strongly depends on local conditions in the interstellar medium. We find that X_CO sharply decreases at Av < 3 mag but gradually increases at Av > 3 mag, with the transiti…
We modelled \HH\ and CO formation incorporating the fractionation and selective photodissociation affecting CO when \AV\ $\la2$mag. UV absorption measurements typically have N(\cotw)/N(\coth) $\approx 65$ that are reproduced with the standard UV radiation and little density dependence at n(H) $\approx32-1024\pccc$: Densities n(H) $\la256\pccc$ avoid overproducing CO. Sightlines observed in mm-wave absorption and a few in UV show enhanced \coth\ by factors of 2-4 and are explained by higher n(H) $\ga256\pccc$ and/or weaker radiation. The most difficult observations to understand are UV absorpti…
Far-Ultraviolet diffuse emission as a tracer of Galactic extinction at the North Galactic Pole
Tezpur University（特佐普尔大学）；Manipal Academy of Higher Education（马尼帕尔高等教育学院）；Indian Institute of Astrophysics（印度天体物理研究所）；Institute of Astronomy, 48 Pyatnitskaya St., Moscow 119017, Russia（俄罗斯莫斯科彼特尼茨卡亚街48号天文研究所）
We present the data of the Plateau de Bure Arcsecond Whirlpool Survey (PAWS), a high spatial and spectral resolution 12CO(1-0) line survey of the inner 10 x 6 kpc of the M51 system, and the first wide-field imaging of molecular gas in a star-forming spiral galaxy with resolution matched to the typical size of Giant Molecular Clouds (40 pc). We describe the observation, reduction, and combination of the Plateau de Bure Interferometer (PdBI) and IRAM-30m "short spacing" data. The final data cube attains 1.1"-resolution over the 270" x 170" field of view, with sensitivity to all spatial scales fr…
The Three-mm Ultimate Mopra Milky Way Survey. III. Data Release 6, An Atlas of Physical Conditions, Global Mass Conversion Laws, and 3D Physical Architecture of the Molecular ISM in the Fourth Quadrant
The $\rm H_2$ mass of molecular clouds has traditionally been traced by the CO(J=1-0) rotational transition line. This said, CO is relatively easily photodissociated, and can also be destroyed by cosmic rays, thus rendering some fraction of molecular gas to be "CO-dark". We investigate the amount and physical properties of CO-dark gas in two $z \sim 0$ disc galaxies, and develop predictions for the expected intensities of promising alternative tracers ([CI 609 $μ$m and [CII] 158 $μ$m emission). We do this by combining cosmological zoom simulations of disc galaxies with thermal-radiative-chemic…
Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be under-abundant and/or sub-thermally excited. We performed one of the most sensitive (down to $\mathrm{τ_{rms}^{CO} \sim 0.002}$ and $\mathrm{τ_{rms}^{HCO^+} \sim 0.0008}$) sub-millimeter molecular absorption line observations towards 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to $\mathrm{A_v< 0.32\,mag}$ and \hcop was dow…
In order to understand the origin of observed molecular cloud properties, it is critical to understand how clouds interact with their environments during their formation, growth, and collapse. It has been suggested that accretion-driven turbulence can maintain clouds in a highly turbulent state, preventing runaway collapse, and explaining the observed non-thermal velocity dispersions. We present 3D, AMR, MHD simulations of a kiloparsec-scale, stratified, supernova-driven, self-gravitating, interstellar medium, including diffuse heating and radiative cooling. These simulations model the formati…
