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Register a new userGas and Dust Properties in the Chamaeleon Molecular Cloud Complex based on the Optically Thick HI
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Gas and dust properties in the Chamaeleon molecular cloud complex have been investigated with emission lines from atomic hydrogen (HI) and 12CO molecule, dust optical depth at 353 GHz ($tau_{353}$), and $J$-band infrared extinction ($A_{J}$). We have found a scatter correlation between the HI integrated intensity ($W_{rm HI}$) and $tau_{353}$ in the Chamaeleon region. The scattering has been examined in terms of possible large optical depth in HI emission ($tau_{rm HI}$) using a total column density ($N_{rm H}$) model based on $tau_{353}$. A nonlinear relation of $tau_{353}$ with the $sim$1.2 power of $A_{J}$ has been found in opaque regions ($A_{J}$ $gtrsim$ 0.3 mag), which may indicate dust evolution effect. If we apply this nonlinear relation to the $N_{rm H}$ model (i.e., $N_{rm H} propto tau_{353}^{1/1.2}$) allowing arbitrary $tau_{rm HI}$, the model curve reproduces well the $W_{rm HI}$-$tau_{353}$ scatter correlation, suggesting optically thick HI ($tau_{rm HI} sim$1.3) extended around the molecular clouds. Based on the correlations between the CO integrated intensity and the $N_{rm H}$ model, we have then derived the CO-to-H$_{2}$ conversion factor ($X_{rm CO}$) on $sim$1.5$^{circ}$ scales (corresponding to $sim$4 persec) and found spatial variations of $X_{rm CO}$ $sim$(0.5-3)$times$10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s across the cloud complex, possibly depending on the radiation field inside or surrounding the molecular clouds. These gas properties found in the Chamaeleon region are discussed through a comparison with other local molecular cloud complexes.
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We report a Fermi-LAT $gamma$-ray analysis for the Chamaeleon molecular-cloud complex using a total column density (NH) model based on the dust optical depth at 353 GHz ($tau_{353}$) with the Planck thermal dust emission model. Gamma rays with energy from 250 MeV to 100 GeV are fitted with the NH model as a function of $tau_{353}$, NH $propto$ $tau_{353}^{1/alpha}$ ($alpha$ $geq$ 1.0), to explicitly take into account a possible nonlinear $tau_{353}$/NH ratio. We found that a nonlinear relation, $alpha$$sim$1.4, gives the best fit to the $gamma$-ray data. This nonlinear relation may indicate dust evolution effects across the different gas phases. Using the best-fit NH model, we derived the CO-to-H2 conversion factor (XCO) and gas mass, taking into account uncertainties of the NH model. The value of XCO is found to be (0.63-0.76) $times$10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s, which is consistent with that of a recent $gamma$-ray study of the Chamaeleon region. The total gas mass is estimated to be (6.0-7.3) $times$ 10$^{4}$ Msun, of which the mass of additional gas not traced by standard HI or CO line surveys is 20-40%. The additional gas amounts to 30-60% of the gas mass estimated in the case of optically thin HI and has 5-7 times greater mass than the molecular gas traced by CO. Possible origins of the additional gas are discussed based on scenarios of optically thick HI and CO-dark H2. We also derived the $gamma$-ray emissivity spectrum, which is consistent with the local HI emissivity derived from LAT data within the systematic uncertainty of $sim$20%
We present an analysis of the HI and CO gas in conjunction with the Planck/IRAS submillimeter/far-infrared dust properties toward the most outstanding high latitude clouds MBM 53, 54, 55 and HLCG 92-35 at b = -30 deg to -45 deg. The CO emission, dust opacity at 353 GHz (tau353), and dust temperature (Td) show generally good spatial correspondence. On the other hand, the correspondence between the HI emission and the dust properties is less clear than in CO. The integrated HI intensity WHI and tau353 show a large scatter with a correlation coefficient of ~0.6 for a Td range from 16 K to 22 K. We find, however, that WHI and tau353 show better correlation for smaller ranges of Td every 0.5 K, generally with a correlation coefficient of 0.7-0.9. We set up a hypothesis that the HI gas associated with the highest Td >= 21.5 K is optically thin, whereas the HI emission is generally optically thick for Td lower than 21.5 K. We have determined a relationship for the optically thin HI gas between atomic hydrogen column density and tau353, NHI (cm-2) = (1.5 x 10^26) x tau353, under the assumption that the dust properties are uniform and we have applied this to estimate NHI from tau353 for the whole cloud. NHI was then used to solve for Ts and tauHI over the region. The result shows that the HI is dominated by optically thick gas having a low spin temperature of 20-40 K and a density of 40-160 cm-3. The HI envelope has a total mass of ~1.2 x 10^4 Msol, an order of magnitude larger than that of the CO clouds. The HI envelope properties derived by this method do not rule out a mixture of HI and H2 in the dark gas, but we present indirect evidence that most of the gas mass is in the atomic state.
The local interstellar medium (ISM) is suffused with dark gas, identified by excess infrared and gamma ray emission, yet undetected by standard ISM tracers such as neutral hydrogen (HI) or carbon monoxide emission. Based on observed dust properties from Planck, recent studies have argued that HI mixed with dust is strongly saturated and that dark gas is dominated by optically-thick HI. We test this hypothesis by reproducing this model using data from Planck and new 21 cm emission maps from GALFA-HI -- the first large-area 21cm emission survey with comparable angular resolution to Planck. We compare the results with those from a large sample of HI column densities based on direct observations of HI optical depth, and find that the inferred column density corrections are significantly lower than those inferred by the Planck-based model. Further, we rule out the hypothesis that the pencil-beam HI absorption sight lines preferentially miss opaque blobs with small covering fraction, as these structures require densities and pressures which are incompatible with ISM conditions. Our results support the picture that excess dust emission in the local ISM is not dominated by optically-thick HI, but is rather a combination of intrinsic changes in dust grain emissivities and H2 missed by CO observations.
We present synthetic Hi and CO observations of a simulation of decaying turbulence in the thermally bistable neutral medium. We first present the simulation, with clouds initially consisting of clustered clumps. Self-gravity causes these clump clusters to form more homogeneous dense clouds. We apply a simple radiative transfer algorithm, and defining every cell with <Av> > 1 as molecular. We then produce maps of Hi, CO-free molecular gas, and CO, and investigate the following aspects: i) The spatial distribution of the warm, cold, and molecular gas, finding the well-known layered structure, with molecular gas surrounded by cold Hi, surrounded by warm Hi. ii) The velocity of the various components, with atomic gas generally flowing towards the molecular gas, and that this motion is reflected in the frequently observed bimodal shape of the Hi profiles. This conclusion is tentative, because we do not include feedback. iii) The production of Hi self-absorption (HISA) profiles, and the correlation of HISA with molecular gas. We test the suggestion of using the second derivative of the brightness temperature Hi profile to trace HISA and molecular gas, finding limitations. On a scale of ~parsecs, some agreement is obtained between this technique and actual HISA, as well as a correlation between HISA and N(mol). It quickly deteriorates towards sub-parsec scales. iv) The N-PDFs of the actual Hi gas and those recovered from the Hi line profiles, with the latter having a cutoff at column densities where the gas becomes optically thick, thus missing the contribution from the HISA-producing gas. We find that the power-law tail typical of gravitational contraction is only observed in the molecular gas, and that, before the power-law tail develops in the total gas density PDF, no CO is yet present, reinforcing the notion that gravitational contraction is needed to produce this component. (abridged)
Comparison analyses between the gas emission data (HI 21cm line and CO 2.6 mm line) and the Planck/IRAS dust emission data (optical depth at 353 GHz tau353 and dust temperature Td) allow us to estimate the amount and distribution of the hydrogen gas more accurately, and our previous studies revealed the existence of a large amount of optically-thick HI gas in the solar neighborhood. Referring to this, we discuss the neutral hydrogen gas around the Perseus cloud in the present paper. By using the J-band extinction data, we found that tau353 increases as a function of the 1.3-th power of column number density of the total hydrogen (NH), and this implies dust evolution in high density regions. This calibrated tau353-NH relationship shows that the amount of the HI gas can be underestimated to be ~60% if the optically-thin HI method is used. Based on this relationship, we calculated optical depth of the 21 cm line (tauHI), and found that <tauHI> ~ 0.92 around the molecular cloud. The effect of tauHI is still significant even if we take into account the dust evolution. We also estimated a spatial distribution of the CO-to-H2 conversion factor (XCO), and we found its average value is <XCO> ~ 1.0x10^20 cm-2 K-1 km-1 s. Although these results are inconsistent with some previous studies, these discrepancies can be well explained by the difference of the data and analyses methods.
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