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Register a new userMolecular Hydrogen Fluorescence in the Eridanus Superbubble
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The first FUV (lambda lambda 1350-1750 A) spectral imaging observations of the Eridanus superbubble, obtained with the SPEAR/FIMS mission, have revealed distinct fluorescent emission from molecular hydrogen. In this study, the observed emission features were compared with those from a photo-dissociation region model with assumed illuminating stellar fields. The result showed rather high line ratios of I_{1580}/I_{1610}, which may imply the existence of high-temperature molecular clouds in the region. The H_2 fluorescence intensity showed a proportional correlation with H-alpha emission, indicating that the fluorescence and the recombination emission have similar physical origins.
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The nearby Orion-Eridanus superbubble, which was blown by multiple supernovae several Myr ago, has likely produced cosmic rays. Its turbulent medium, still energised by massive stars, can impact cosmic-ray transport locally. The gamma rays produced in cosmic-ray interactions with interstellar gas were used to compare the GeV to TeV cosmic-ray spectrum in the superbubble and in other regions near the Sun. We used ten years of Fermi-LAT data in the 0.25-63 GeV energy range to study the closer (Eridanus) end of the superbubble. We modelled the spatial and spectral distributions of the gamma rays produced in the different gas phases of the clouds found in this direction. We found that the gamma-ray emissivity spectrum of the gas along the outer rim and in a shell inside the superbubble is consistent with the average spectrum measured in the solar neighbourhood. This result calls for a detailed assessment of the recent supernova rate and census of massive stellar winds in the superbubble in order to estimate the epoch and rate of cosmic-ray production and to constrain the transport conditions that can lead to such homogeneity and little re-acceleration. We also found significant evidence that a diffuse cloud lying outside the superbubble, at a height of 200-250 pc below the Galactic plane, is pervaded by a 34% lower cosmic-ray flux, but with the same particle energy distribution as the local one. Super-GeV cosmic rays should freely cross such a diffuse atomic cloud without significant loss or spectral distorsion. We tentatively propose that the cosmic-ray loss relates to the orientation of the magnetic field lines threading the cirrus, which point towards the halo according to the dust polarisation data. We gathered past and present emissivity measurements near the Sun to show how the local cosmic-ray flux decreases with Galactic height and to compare this trend with model predictions.
The Orion-Eridanus superbubble, formed by the nearby Orion high mass star-forming region, contains multiple bright H$alpha$ filaments on the Eridanus side of the superbubble. We examine the implications of the H$alpha$ brightnesses and sizes of these filaments, the Eridanus filaments. We find that either the filaments must be highly elongated along the line of sight or they cannot be equilibrium structures illuminated solely by the Orion star-forming region. The Eridanus filaments may, instead, have formed when the Orion-Eridanus superbubble encountered and compressed a pre-existing, ionized gas cloud, such that the filaments are now out of equilibrium and slowly recombining.
We present high spatial resolution (2.3x1.9 or 43 pc x 36 pc at D = 3.9 Mpc) 100 GHz millimeter-wave continuum emission observations with the Nobeyama Millimeter Array toward an expanding molecular superbubble in the central region of M82. The 100 GHz continuum image, which is dominated by free-free emission, revealed that the four strongest peaks are concentrated at the inner edge of the superbubble along the galactic disk. The production rates of Lyman continuum photons calculated from 100 GHz continuum flux at these peaks are an order of magnitude higher than those from the most massive star forming regions in our Galaxy. At these regions, high velocity ionized gas (traced by H41a and [Ne II]) can be seen, and H2O and OH masers are also concentrated. The center of the superbubble, on the other hand, is weak in molecular and free-free emissions and strong in diffuse hard X-ray emission. These observations suggest that a strong starburst produced energetic explosions and resultant plasma and superbubble expansions, and induced the present starburst regions traced by our 100 GHz continuum observations at the inner edge of the molecular superbubble. These results, therefore, provide the first clear evidence of self-induced starburst in external galaxies. Starburst at the center of the superbubble, on the other hand, begins to cease because of a lack of molecular gas. This kind of intense starburst seems to have occurred several times within 10^6-10^7 years in the central region of M82.
The $l!=!+1.!!^circ3$ region in the Galactic center is characterized by multiple shell-like structures and their extremely broad velocity widths. We revisit the molecular superbubble hypothesis for this region, based on high resolution maps of CO {it J}=1--0, $^{13}$CO {it J}=1--0, H$^{13}$CN {it J}=1--0, H$^{13}$CO$^{+}$ {it J}=1--0, SiO {it J}=2--1, and CS {it J}=2--1 lines obtained from the Nobeyama radio observatory 45-m telescope, as well as CO {it J}=3--2 maps obtained from the James Clerk Maxwell telescope. We identified eleven expanding shells with total kinetic energy and typical expansion time $E_{rm kin}!sim! 10^{51.9}$ erg and $t_{rm exp}!sim! 10^{4.9}$ yr, respectively. In addition, the $l!=!+1.!!^circ3$ region exhibited high SiO {it J}=2--1/H$^{13}$CN {it J}=1--0 and SiO {it J}=2--1/H$^{13}$CO$^{+}$ {it J}=1--0 intensity ratios, indicating that the region has experienced dissociative shocks in the past. These new findings confirm the molecular superbubble hypothesis for the $l!=!+1.!!^circ3$ region. The nature of the embedded star cluster, which may have supplied 20--70 supernova explosions within 10$^5$ yr, is discussed. This work also show the importance of compact broad-velocity-width features in searching for localized energy sources hidden behind severe interstellar extinction and stellar contamination.
The advent of precise measurements of the cosmic microwave background (CMB) anisotropies has motivated correspondingly precise calculations of the cosmic recombination history. Cosmic recombination proceeds far out of equilibrium because of a bottleneck at the $n=2$ level of hydrogen: atoms can only reach the ground state via slow processes: two-photon decay or Lyman-$alpha$ resonance escape. However, even a small primordial abundance of molecules could have a large effect on the interline opacity in the recombination epoch and lead to an additional route for hydrogen recombination. Therefore, this paper computes the abundance of the H$_2$ molecule during the cosmic recombination epoch. Hydrogen molecules in the ground electronic levels X$^1Sigma^+_g$ can either form from the excited H$_2$ electronic levels B$^1Sigma^+_u$ and C$^1Pi_u$ or through the charged particles H$_2^+$, HeH$^+$ and H$^-$. We follow the transitions among all of these species, resolving the rotational and vibrational sub-levels. Since the energies of the X$^1Sigma^+_g$--B$^1Sigma^+_u$ (Lyman band) and X$^1Sigma^+_g$-C$^1Pi_u$ (Werner band) transitions are near the Lyman-$alpha$ energy, the distortion of the CMB spectrum caused by escaped H Lyman-line photons accelerates both the formation and the destruction of H$_2$ due to this channel relative to the thermal rates. This causes the populations of H$_2$ molecules in X$^1Sigma^+_g$ energy levels to deviate from their thermal equilibrium abundances. We find that the resulting H$_2$ abundance is $10^{-17}$ at $z=1200$ and $10^{-13}$ at $z=800$, which is too small to have any significant influence on the recombination history.
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