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Register a new userStudy of the Cosmic Rays and Interstellar Medium in Local HI Clouds using Fermi-LAT Gamma-Ray Observations
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An accurate estimate of the interstellar gas density distribution is crucial to understanding the interstellar medium (ISM) and Galactic cosmic rays (CRs). To comprehend the ISM and CRs in a local environment, a study of the diffuse $gamma$-ray emission in a mid-latitude region of the third quadrant was performed. The $gamma$-ray data in the 0.1--25.6~GeV energy range of the Fermi Large Area Telescope (LAT) and other interstellar gas tracers such as the HI4PI survey data and the Planck dust thermal emission model were used, and the northern and southern regions were analyzed separately. The variation of the dust emission Dem with the total neutral gas column density NH was studied in high dust-temperature areas, and the NH/Dem ratio was calibrated using $gamma$-ray data under the assumption of a uniform CR intensity in the studied regions. The measured integrated $gamma$-ray emissivities above 100~MeV are $(1.58pm0.04)times10^{-26}~mathrm{photons~s^{-1}~sr^{-1}~Hmbox{-}atom^{-1}}$ and $(1.59pm0.02)times10^{-26}~mathrm{photons~s^{-1}~sr^{-1}~Hmbox{-}atom^{-1}}$ in the northern and southern regions, respectively, supporting the existence of a uniform CR intensity in the vicinity of the solar system. While most of the gas can be interpreted to be HI with a spin temperature of $T_mathrm{S} = 125~mathrm{K}$ or higher, an area dominated by optically thick HI with $T_mathrm{S} sim 40~mathrm{K}$ was identified.
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We report an analysis of the interstellar gamma-ray emission from the Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the Fermi Large Area Telescope. They are among the nearest molecular cloud complexes, within ~300 pc from the solar system. The gamma-ray emission produced by interactions of cosmic-rays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained gamma-ray emissivities above 250 MeV are (5.9 +/- 0.1(stat) (+0.9/-1.0)(sys)), (10.2 +/- 0.4(stat) (+1.2/-1.7)(sys)), and (9.1 +/- 0.3(stat) (+1.5/-0.6)(sys)) x10^(-27) photons s^(-1) sr^(-1) H-atom^(-1) for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively. Whereas the energy dependences of the emissivities agree well with that predicted from direct CR observations at the Earth, the measured emissivities from 250 MeV to 10 GeV indicate a variation of the CR density by ~20% in the neighborhood of the solar system, even if we consider systematic uncertainties. The molecular mass calibrating ratio, Xco = N(H2)/Wco, is found to be (0.96 +/- 0.06(stat) (+0.15/-0.12)(sys)), (0.99 +/- 0.08(stat) (+0.18/-0.10)(sys)), and (0.63 +/- 0.02(stat) (+0.09/-0.07)(sys)) x10^20 H2-molecule cm^(-2) (K km s^(-1))^(-1) for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, suggesting a variation of Xco in the vicinity of the solar system. From the obtained values of Xco, the masses of molecular gas traced by Wco in the Chamaeleon, R CrA, and Cepheus and Polaris flare regions are estimated to be ~5x10^3, ~10^3, and ~3.3x10^4 Msolar, respectively. A comparable amount of gas not traced well by standard HI and CO surveys is found in the regions investigated.
We report an analysis of the interstellar gamma-ray emission from nearby molecular clouds Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the {it Fermi} Large Area Telescope (LAT). They are among the nearest molecular cloud complexes, within $sim$ 300 pc from the solar system. The gamma-ray emission produced by interactions of cosmic-rays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained gamma-ray emissivities from 250 MeV to 10 GeV for the three regions are about (6--10) $times$ 10$^{-27}$ photons s$^{-1}$ sr$^{-1}$ H-atom$^{-1}$, indicating a variation of the CR density by $sim$ 20% even if we consider the systematic uncertainties. The molecular mass calibration ratio, $X_{rm CO} = N{rm (H_2)}/W_{rm CO}$, is found to be about (0.6--1.0) $times$ 10$^{20}$ H$_2$-molecule cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ among the three regions, suggesting a variation of $X_{rm CO}$ in the vicinity of the solar system. From the obtained values of $X_{rm CO}$, we calculated masses of molecular gas traced by Wco in these molecular clouds. In addition, similar amounts of dark gas at the interface between the atomic and molecular gas are inferred.
A study of the interstellar medium (ISM) and cosmic rays (CRs) using Fermi Large Area Telescope (LAT) data, in a region encompassing the nearby molecular clouds MBM 53, 54, and 55 and a far-infrared loop-like structure in Pegasus, is reported. By comparing Planck dust thermal emission model with Fermi-LAT gamma-ray data, it was found that neither the dust radiance (R) nor the dust opacity at 353 GHz (tau353) were proportional to the total gas column density N(Htot) primarily because N(Htot)/R and N(Htot)/tau353 depend on the dust temperature (Td). The N(Htot) distribution was evaluated using gamma-ray data by assuming the regions of high Td} to be dominated by optically thin atomic hydrogen (HI) and by employing an empirical linear relation of N(Htot)/R to Td. It was determined that the mass of the gas not traced by the 21-cm or 2.6-mm surveys is ~25% of the mass of HI in the optically thin case and is larger than the mass of the molecular gas traced by carbon monoxide by a factor of up to 5. The measured gamma-ray emissivity spectrum is consistent with a model based on CR spectra measured at the Earth and the nuclear enhancement factor of <=1.5. It is, however, lower than local HI emissivities reported by previous Fermi-LAT studies employing different analysis methods and assumptions on ISM properties by 15%-20% in energies below a few GeV, even if we take account of the statistical and systematic uncertainties. The origin of the discrepancy is also discussed.
Latest precise cosmic-ray (CR) measurements and present gamma-ray observations have started challenging our understanding of CR transport and interaction in the Galaxy. Moreover, because the density of CRs is similar to the density of the magnetic field, gas, and starlight in the interstellar medium (ISM), CRs are expected to affect the ISM dynamics, including the physical and chemical processes that determine transport and star formation. In this context, observations of gamma-ray emission at MeV energies produced by the low-energy CRs are very important and urgent. A telescope covering the energy range between ~0.1 MeV and a few GeV with a sensitivity more than an order of magnitude better than previous instruments would allow for the first time to study in detail the low-energy CRs, providing information on their sources, their spectra throughout the Galaxy, their abundances, transport properties, and their role on the evolution of the Galaxy and star formation. Here we discuss the scientific prospects for studies of CRs, ISM (gas, interstellar photons, and magnetic fields) and associated gamma-ray emissions with such an instrument.
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%
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