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Register a new userCosmic rays as regulators of molecular cloud properties
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Authors
Marco Padovani
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Cosmic rays are the main agents in controlling the chemical evolution and setting the ambipolar diffusion time of a molecular cloud. We summarise the processes causing the energy degradation of cosmic rays due to their interaction with molecular hydrogen, focusing on the magnetic effects that influence their propagation. Making use of magnetic field configurations generated by numerical simulations, we show that the increase of the field line density in the collapse region results in a reduction of the cosmic-ray ionisation rate. As a consequence the ionisation fraction decreases, facilitating the decoupling between the gas and the magnetic field.
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Cosmic rays (CRs) control the thermal, ionization and chemical state of the dense H_2 gas regions that otherwise remain shielded from far-UV and optical stellar radiation propagating through the dusty ISM of galaxies. It is in such CR-dominated regions (CRDRs) rather than Photon-dominated regions (PDRs) of H_2 clouds where the star formation initial conditions are set, making CRs the ultimate star-formation feedback factor in galaxies, able to operate even in their most deeply dust-enshrouded environments. CR-controlled star formation initial conditions naturally set the stage for a near-invariant stellar Initial Mass Function (IMF) in galaxies as long as their average CR energy density U_{CR} permeating their molecular ISM remains within a factor of ~10 of its Galactic value. Nevertheless, in the extreme environments of the compact starbursts found in merging galaxies, where U_{CR}sim(few)x10^{3}U_{CR,Gal}, CRs dramatically alter the initial conditions of star formation. In the resulting extreme CRDRs H_2 cloud fragmentation will produce far fewer low mass (<8 M_{sol}) stars, yielding a top-heavy stellar IMF. This will be a generic feature of CR-controlled star-formation initial conditions, lending a physical base for a bimodal IMF during galaxy formation, with a top-heavy one for compact merger-induced starbursts, and an ordinary IMF preserved for star formation in isolated gas-rich disks. In this scheme the integrated galactic IMFs (IGIMF) are expected to be strong functions of the star formation history of galaxies.
Dissociation of molecular hydrogen by secondary electrons produced by cosmic ray or X-ray ionization plays a crucial role in the chemistry of the densest part of molecular clouds. Here we study the effect of the mean kinetic energy of secondary electrons on this process. We compare predictions using a range of secondary electron energies and predictions of the cross-sections with the values in the UMIST database. We find that the predicted column densities change by nearly one dex.
We surveyed the circumnuclear disk of the Seyfert galaxy NGC1068 between the frequencies 86.2 GHz and 115.6 GHz, and identified 17 different molecules. Using a time and depth dependent chemical model we reproduced the observational results, and show that the column densities of most of the species are better reproduced if the molecular gas is heavily pervaded by a high cosmic ray ionization rate of about 1000 times that of the Milky Way. We discuss how molecules in the NGC1068 nucleus may be influenced by this external radiation, as well as by UV radiation fields.
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%
Molecular clouds, which harbor the birthplaces of stars, form out of the atomic phase of the interstellar medium (ISM). We aim to characterize the atomic and molecular phases of the ISM and set their physical properties into the context of cloud formation processes. We studied the cold neutral medium (CNM) by means of $rm HI$ self-absorption (HISA) toward the giant molecular filament GMF20.0-17.9 and compared our results with molecular gas traced by $^{13}rm CO$ emission. We fitted baselines of HISA features to $rm HI$ emission spectra using first and second order polynomial functions. The CNM identified by this method spatially correlates with the morphology of the molecular gas toward the western region. However, no spatial correlation between HISA and $^{13}rm CO$ is evident toward the eastern part of the filament. The distribution of HISA peak velocities and line widths agrees well with $^{13}rm CO$ within the whole filament. The column density probability density functions (N-PDFs) of HISA (CNM) and $rm HI$ emission (tracing both the CNM and the warm neutral medium, WNM) have a log-normal shape for all parts of the filament, indicative of turbulent motions as the main driver for these structures. The $rm H_2$ N-PDFs show a broad log-normal distribution with a power-law tail suggesting the onset of gravitational contraction. The saturation of $rm HI$ column density is observed at $sim$25$rm,M_{odot},pc^{-2}$. We conjecture that different evolutionary stages are evident within the filament. In the eastern region, we witness the onset of molecular cloud formation out of the atomic gas reservoir while the western part is more evolved, as it reveals pronounced $rm H_2$ column density peaks and signs of active star formation.
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