Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSynthetic Observations of 21cm HI Line Profiles from Inhomogeneous Turbulent Interstellar HI Gas with Magnetic Field
92
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We carried out synthetic observations of interstellar atomic hydrogen at 21cm wavelength by utilizing the magneto-hydrodynamical numerical simulations of the inhomogeneous turbulent interstellar medium (ISM) Inoue and Inutsuka (2012). The cold neutral medium (CNM) shows significantly clumpy distribution with a small volume filling factor of 3.5%, whereas the warm neutral medium (WNM) distinctly different smooth distribution with a large filling factor of 96.5%. In projection on the sky, the CNM exhibits highly filamentary distribution with a sub-pc width, whereas the WNM shows smooth extended distribution. In the HI optical depth the CNM is dominant and the contribution of the WNM is negligibly small. The CNM has an area covering factor of 30% in projection, while the WNM has a covering factor of 70%. This causes that the emission-absorption measurements toward radio continuum compact sources tend to sample the WNM with a probability of 70%, yielding smaller HI optical depth and smaller HI column density than those of the bulk HI gas. The emission-absorption measurements, which are significantly affected by the small-scale large fluctuations of the CNM properties, are not suitable to characterize the bulk HI gas. Larger-beam emission measurements which are able to fully sample the HI gas will provide a better tool for that purpose, if a reliable proxy for hydrogen column density, possibly dust optical depth and gamma rays, is available.
rate research
Read More
We present a detailed study of an estimator of the HI column density, based on a combination of HI 21cm absorption and HI 21cm emission spectroscopy. This isothermal estimate is given by $N_{rm HI,ISO} = 1.823 times 10^{18} int left[ tau_{rm tot} times {rm T_B} right] / left[ 1 - e^{-tau_{rm tot}} right] {rm dV}$, where $tau_{rm tot}$ is the total HI 21cm optical depth along the sightline and ${rm T_B}$ is the measured brightness temperature. We have used a Monte Carlo simulation to quantify the accuracy of the isothermal estimate by comparing the derived $N_{rm HI,ISO}$ with the true HI column density $N_{rm HI}$. The simulation was carried out for a wide range of sightlines, including gas in different temperature phases and random locations along the path. We find that the results are statistically insensitive to the assumed gas temperature distribution and the positions of different phases along the line of sight. The median value of the ratio of the true H{sc i} column density to the isothermal estimate, $N_{rm HI}/{N_{rm HI, ISO}}$, is within a factor of 2 of unity while the 68.2% confidence intervals are within a factor of $approx 3$ of unity, out to high HI column densities, $le 5 times 10^{23}$,cm$^{-2}$ per 1 km s$^{-1}$ channel, and high total optical depths, $le 1000$. The isothermal estimator thus provides a significantly better measure of the HI column density than other methods, within a factor of a few of the true value even at the highest columns, and should allow us to directly probe the existence of high HI column density gas in the Milky Way.
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)
We present numerical computations and analysis of atomic to molecular (HI-to-H$_2$) transitions in cool ($sim$100 K) low-metallicity dust-free (primordial) gas, in which molecule formation occurs via cosmic-ray driven negative ion chemistry, and removal is by a combination of far-UV photodissociation and cosmic-ray ionization and dissociation. For any gas temperature, the behavior depends on the ratio of the Lyman-Werner (LW) band FUV intensity to gas density, $I_{rm LW}/n$, and the ratio of the cosmic-ray ionization rate to the gas density, $zeta/n$. We present sets of HI-to-H$_2$ abundance profiles for a wide range of $zeta/n$ and $I_{rm LW}/n$, for dust-free gas. We determine the conditions for which H$_2$ absorption line self-shielding in optically thick clouds enables a transition from atomic to molecular form for ionization-driven chemistry. We also examine the effects of cosmic-ray energy losses on the atomic and molecular density profiles and transition points. For a unit Galactic interstellar FUV field intensity ($I_{rm LW}=1$) with LW flux $2.07times 10^7$ photons cm$^{-2}$ s$^{-1}$, and a uniform cosmic-ray ionization rate $zeta=10^{-16}$ s$^{-1}$, an HI-to-H$_2$ transition occurs at a total hydrogen gas column density of $4times 10^{21}$ cm$^{-2}$, within $3times 10^7$ yr, for a gas volume density of $n=10^6$ cm$^{-3}$ at 100 K. For these parameters, the dust-free limit obtains for a dust-to-gas ratio Z$^prime_d lesssim 10^{-5}$, which may be reached for overall metallicities $Z^primelesssim 0.01$ relative to Galactic solar values.
By means of 3D hydrodynamical simulations, in a separate paper we have discussed the properties of non-axisymmetric density wave trains in the outermost regions of galaxy disks, based on the picture that self-excited global spiral modes in the bright optical stellar disk are accompanied by low-amplitude short trailing wave signals outside corotation; in the gas, such wave trains can penetrate through the outer Lindblad resonance and propagate outwards, forming prominent spiral patterns. In this paper we present the synthetic 21~cm velocity maps expected from simulated models of the outer gaseous disk, focusing on the case when the disk is dominated by a two-armed spiral pattern, but considering also other more complex situations. We discuss some aspects of the spiral pattern in the gaseous periphery of galaxy disks noted in our simulations that might be interesting to compare with specific observed cases.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
