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Register a new userThermal and turbulent properties of the Warm Neutral Medium in the solar neighborhood
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The transition from the diffuse warm neutral medium (WNM) to the dense cold neutral medium (CNM) is what set the initial conditions to the formation of molecular clouds. The properties of the turbulent cascade in the WNM, essential to describe this radiative condensation process, have remained elusive in part due to the difficulty to map out the structure and kinematics of each H I thermal phases. Here we present an analysis of a 21 cm hyper-spectral data cube from the GHIGLS HI survey where the contribution of the WNM is extracted using ROHSA, a Gaussian decomposition tool that includes spatial regularization. The distance and volume of the WNM emission is estimated using 3D dust extinction map information. The thermal and turbulent contributions to the Doppler line width of the WNM were disentangled using two techniques, one based on the statistical properties of the column density and centroid velocity fields, and another on the relative motions of CNM structures as a probe of turbulent motions. We found that the volume of WNM sampled here, located at the outer edge of the Local Bubble, shows thermal properties in accordance with expected values for heating and cooling processes typical of the Solar neighbourhood. The WNM has the properties of sub/trans-sonic turbulence, with a turbulent Mach number at the largest scale probed here (l = 130 pc) of Ms = 0.87 +- 0.15, a density contrast of 0.6 +- 0.2, and velocity and density power spectra compatible with k-11/3. The low Mach number of the WNM provides dynamical conditions that allows the condensation mode of thermal instability (TI) to grow freely and form CNM structures, as predicted by theory.
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We present 21-cm absorption measurements towards 12 radio continuum sources with previously identified thermally-unstable warm neutral medium (WNM). These observations were obtained with the Expanded Very Large Array (EVLA) and were complemented with the HI emission spectra obtained with the Arecibo Observatory. Out of 12 sources, HI absorption was detected along 5 lines of sight (seven new absorption features in total), resulting in a detection rate of ~42%. While our observations are sensitive to the WNM with a spin temperature T_s<3000 K, we detected only two wide absorption lines with T_s=400-900 K. These temperatures lie above the range allowed for the cold neutral medium (CNM) by the thermal equilbrium models and signify the thermally unstable WNM. Several absorption features have an optical depth of only a few x10^{-3}. While this is close or lower than what is theoretically expected for the CNM, we show that these weak lines are important for constraining the fraction of the thermally unstable WNM. Our observations demonstrate that, for the first time, high bandpass stability can be achieved with the VLA, allowing detection of absorption lines with a peak optical depth of ~10^{-3}.
We report a deep Giant Metrewave Radio Telescope (GMRT) search for Galactic H{sc i} 21cm absorption towards the quasar B0438$-$436, yielding the detection of wide, weak H{sc i} 21cm absorption, with a velocity-integrated H{sc i} 21cm optical depth of $0.0188 pm 0.0036$~km~s$^{-1}$. Comparing this with the H{sc i} column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of $3760 pm 365$~K, one of the highest measured in the Galaxy. This is consistent with most of the H{sc i} along the sightline arising in the stable warm neutral medium (WNM). The low peak H{sc i} 21cm optical depth towards B0438$-$436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the H{sc i} 21cm absorption and emission spectra. This yields a gas kinetic temperature of $rm T_k leq (4910 pm 1900)$~K, and a spin temperature of $rm T_s = (1000 pm 345)$~K for the gas that gives rise to the H{sc i} 21cm absorption. Our data are consistent with the H{sc i} 21cm absorption arising from either the stable WNM, with $rm T_s ll T_k$, $rm T_k approx 5000$~K, and little penetration of the background Lyman-$alpha$ radiation field into the neutral hydrogen, or from the unstable neutral medium, with $rm T_s approx T_k approx 1000;K$.
Dynamic and thermal processes regulate the structure of the multi-phase interstellar medium (ISM), and ultimately establish how galaxies evolve through star formation. Thus, to constrain ISM models and better understand the interplay of these processes, it is of great interest to measure the thermal pressure ($P_{rm th}$) of the diffuse, neutral gas. By combining [C II] 158 $mu$m, HI, and CO data from 31 galaxies selected from the Herschel KINGFISH sample, we have measured thermal pressures in 534 predominantly atomic regions with typical sizes of $sim$1 kiloparsec. We find a distribution of thermal pressures in the $P_{rm th}/ksim10^3-10^5$ K cm$^{-3}$ range. For a sub-sample of regions with conditions similar to those of the diffuse, neutral gas in the Galactic plane, we find thermal pressures that follow a log-normal distribution with a median value of $P_{rm th}/kapprox3600$ K cm$^{-3}$. These results are consistent with thermal pressure measurements using other observational methods. We find that $P_{rm th}$ increases with radiation field strength and star formation activity, as expected from the close link between the heating of the gas and the star formation rate. Our thermal pressure measurements fall in the regime where a two-phase ISM with cold and warm neutral medium could exist in pressure equilibrium. Finally, we find that the midplane thermal pressure of the diffuse gas is about $sim30$% of the vertical weight of the overlying ISM, consistent with results from hydrodynamical simulations of self-regulated star formation in galactic disks.
About 20% of stars in the solar vicinity are in the Hercules stream, a bundle of stars that move together with a velocity distinct from the Sun. Its origin is still uncertain. Here, we explore the possibility that Hercules is made of trojans, stars captured at L4, one the Lagrangian points of the stellar bar. Using GALAKOS--a high-resolution N-body simulation of the Galactic disk--we follow the motions of stars in the co-rotating frame of the bar and confirm previous studies on Hercules being formed by stars in co-rotation resonance with the bar. Unlike previous work, we demonstrate that the retrograde nature of trojan orbits causes the asymmetry in the radial velocity distribution, typical of Hercules in the solar vicinity. We show that trojans remain at capture for only a finite amount of time, before escaping L4 without being captured again. We anticipate that in the kinematic plane the Hercules stream will de-populate along the bar major axis and be visible at azimuthal angles behind the solar vicinity with a peak towards L4. This test can exclude the OLR origin of the Hercules stream and be validated by Gaia DR3 and DR4.
In the cold neutral medium, high out-of-equilibrium temperatures are created by intermittent dissipation processes, including shocks, viscous heating, and ambipolar diffusion. The high-temperature excursions are thought to explain the enhanced abundance of CH$^{+}$ observed along diffuse molecular sight-lines. Intermittent high temperatures should also have an impact on H$_2$ line luminosities. We carry out simulations of MHD turbulence in molecular clouds including heating and cooling, and post-process them to study H$_2$ line emission and hot-gas chemistry, particularly the formation of CH$^+$. We explore multiple magnetic field strengths and equations of state. We use a new H$_2$ cooling function for $n_{rm H} leq 10^5,{rm cm}^{-3}$, $Tleq 5000,{rm K}$, and variable H$_2$ fraction. We make two important simplifying assumptions: (i) the ${rm H}_2/{rm H}$ fraction is fixed everywhere, and (ii) we exclude from our analysis regions where the ion-neutral drift velocity is calculated to be greater than 5 km/s. Our models produce H$_2$ emission lines in accord with many observations, although extra excitation mechanisms are required in some clouds. For realistic r.m.s. magnetic field strengths ($approx 10$ $mu$G) and velocity dispersions, we reproduce observed CH$^+$ abundances. These findings contrast with those of Valdivia et al. (2017). Comparison of predicted dust polarization with observations by {it Planck} suggests that the mean field $gtrsim 5 mu$G, so that the turbulence is sub-Alfvenic. We recommend future work treating ions and neutrals as separate fluids to more accurately capture the effects of ambipolar diffusion on CH$^+$ abundance.
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