Evolution Of Interstellar Medium Research Articles

Spectroscopic Study of [Mg, H, N, C, O] Species: Implications for the Astrochemical Magnesium Chemistry.

Magnesium is an abundant metal element in space, and magnesium chemistry has vital importance in the evolution of interstellar medium (ISM) and circumstellar regions, such as the asymptotic giant branch star IRC+10216 where a variety of Mg compounds bearing H, C, N, and O have been detected and proposed as the important components in the gas-phase molecular clouds and solid-state dust grains. Herein, we report the formation and infrared spectroscopic characterization of the Mg-bearing molecules HMg, [Mg, N, C], [Mg, H, N, C], [Mg, N, C, O], and [Mg, H, N, C, O] from the reactions of Mg/Mg+ and the prebiotic isocyanic acid (HNCO) in the solid neon matrix. Based on their thermal diffusion and photochemical behavior, a complex reactivity landscape involving association, decomposition, and isomerization reactions of these Mg-bearing molecules is developed, which can not only help understand the chemical processes of the magnesium (iso)cyanides in astrochemistry but also provide implications on the presence of magnesium (iso)cyanates in the ISM and the chemical model for the dust grain surface reactions. It also provides a new paradigm of the key intermediate nature of the cationic complexes in the formation of neutral interstellar species.

Dust- and inclination-corrected star formation and interstellar medium scaling relations in nearby galaxies

ABSTRACT Following from our recent work, we present a detailed analysis of star formation and interstellar medium (ISM) scaling relations, done on a representative sample of nearby galaxies. H α images are analysed in order to derive the integrated galaxy luminosity, known as a more instantaneous and accurate star formation rate (SFR) tracer, and the required photometric and structural parameters. Dust- and inclination-corrected H α luminosities, SFRs, and related quantities are determined using a self-consistent method based on previous work prescriptions, which does not require the assumption of a dust attenuation curve and use of Balmer decrements (or other hydrogen recombination lines) to estimate the dust attenuation, with the advantage of determining dust opacities and dust masses along the way. We investigate the extent to which the dust and inclination effects bias the specific parameters of these relations, the scatter and degree of correlation, and which relations are fundamental or are just a consequence of others. Most of our results are consistent with errors with other similar studies, while others come in opposition or are inconclusive. By comparing the B-band optical and H α (star-forming) discs scale lengths, we found on average the star formation distribution to be more extended than the stellar continuum emission one (the ratio being 1.10), this difference increasing with stellar mass. Similarly, more massive galaxies have a more compact stellar emission surface density than the star formation one (average ratio of 0.77). The method proposed can be applied in larger scale studies of star formation and ISM evolution, for normal low- to intermediate-redshift galaxies.

Simulations of black hole fueling in isolated and merging galaxies with an explicit, multiphase ISM

ABSTRACT We study gas inflows on to supermassive black holes using hydrodynamics simulations of isolated galaxies and idealized galaxy mergers with an explicit, multiphase interstellar medium (ISM). Our simulations use the recently developed ISM and stellar evolution model called Stars and MUltiphase Gas in GaLaxiEs (SMUGGLE). We implement a novel super-Lagrangian refinement scheme that increases the gas mass resolution in the immediate neighbourhood of the black holes (BHs) to accurately resolve gas accretion. We do not include black hole feedback in our simulations. We find that the complex and turbulent nature of the SMUGGLE ISM leads to highly variable BH accretion. BH growth in SMUGGLE converges at gas mass resolutions ≲3 × 103 M⊙. We show that the low resolution simulations combined with the super-Lagrangian refinement scheme are able to produce central gas dynamics and BH accretion rates very similar to that of the uniform high resolution simulations. We further explore BH fueling by simulating galaxy mergers. The interaction between the galaxies causes an inflow of gas towards the galactic centres and results in elevated and bursty star formation. The peak gas densities near the BHs increase by orders of magnitude resulting in enhanced accretion. Our results support the idea that galaxy mergers can trigger AGN activity, although the instantaneous accretion rate depends strongly on the local ISM. We also show that the level of merger-induced enhancement of BH fueling predicted by the SMUGGLE model is much smaller compared to the predictions by simulations using an effective equation of state model of the ISM.

Weather forecast of the Milky Way: shear and stellar feedback determine the lives of Galactic-scale filaments

ABSTRACT The interstellar medium (ISM) is an inseparable part of the Milky Way ecosystem whose evolutionary history remains a challenging question. We trace the evolution of the molecular ISM using a sample of young stellar objects (YSO) association–molecular cloud complex (YSO–MC complex). We derive their three-dimensional (3D) velocities by combining the Gaia astrometric measurements of the YSO associations and the CO observations of the associated molecular clouds. Based on the 3D velocities, we simulate the motions of the YSO–MC complexes in the Galactic potential and forecast the ISM evolution by tracing the motions of the individual complexes, and reveal the roles of shear and stellar feedback in determining ISM evolution: Galactic shear stretches Galactic-scale molecular cloud complexes, such as the G120 Complex, into Galactic-scale filaments, and it also contributes to the destruction of the filaments; while stellar feedback creates interconnected superbubbles whose expansion injects peculiar velocities into the ISM. The Galactic-scale molecular gas clumps are often precursors of the filaments and the Galactic-scale filaments are transient structures under a constant stretch by shear. This evolutionary sequence sets a foundation to interpret other gas structures.

An early transition to magnetic supercriticality in star formation

Magnetic fields have an important role in the evolution of interstellar medium and star formation1,2. As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas3. Here we report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the H I narrow self-absorption (HINSA)4,5 towards L15446,7—a well-studied prototypical prestellar core in an early transition between starless and protostellar phases8–10 characterized by a high central number density11 and a low central temperature12. A combined analysis of the Zeeman measurements of quasar H I absorption, H I emission, OH emission and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic flux relative to the mass, which is necessary for star formation, thus seems to have already happened during the transition from the diffuse CNM to the molecular gas traced by the HINSA. This is earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes13,14.

Laboratory Experiments on the Low-temperature Formation of Carbonaceous Grains in the ISM

Abstract The life cycle of cosmic dust grains is far from being understood and the origin and evolution of interstellar medium (ISM) grains is still under debate. In the ISM, the cosmic dust destruction rate is faster than the production rate by stellar sources. However, observations of ISM refractory matter suggest that to maintain a steady amount of cosmic grains, some supplementary production mechanism takes place. In this context, we aimed to study possible reformation mechanisms of cosmic grains taking place at low temperature directly in the ISM. The low-temperature condensation of carbonaceous materials has been investigated in experiments mimicking the ISM conditions. Gas-phase carbonaceous precursors created by laser ablation of graphite were forced to accrete on cold substrates (T ≈ 10 K) representing surviving dust grains. The growing and evolution of the condensing carbonaceous precursors have been monitored by MIR and UV spectroscopy under a number of experimental scenarios. For the first time, the possibility to form ISM carbonaceous grains in situ is demonstrated. The condensation process is governed by carbon chains that first condense into small carbon clusters and finally into more stable carbonaceous materials, of which structural characteristics are comparable to the material formed in gas-phase condensation experiments at very high temperature. We also show that the so-formed fullerene-like carbonaceous material is transformed into a more ordered material under VUV processing. The cold condensation mechanisms discussed here can give fundamental clues to fully understand the balance between the timescale for dust injection, destruction, and reformation in the ISM.

Evolution of Interstellar Medium, Star Formation, and Accretion at High Redshift

Abstract ALMA observations of the long wavelength dust continuum are used to estimate the interstellar medium (ISM) masses in a sample of 708 galaxies at z = 0.3 to 4.5 in the COSMOS field. The galaxy sample has known far-infrared luminosities and, hence, star formation rates (SFRs) and stellar masses ( ) from the optical–infrared spectrum fitting. The galaxies sample SFRs from the main sequence (MS) to 50 times above the MS. The derived ISM masses are used to determine the dependence of gas mass on redshift, , and specific SFR (sSFR) relative to the MS. The ISM masses increase approximately with the 0.63 power of the rate of increase in SFRs with redshift and the 0.32 power of the sSFR/sSFRMS. The SF efficiencies also increase as the 0.36 power of the SFR redshift evolution and the 0.7 power of the elevation above the MS; thus the increased activities at early epochs are driven by both increased ISM masses and SF efficiency. Using the derived ISM mass function, we estimate the accretion rates of gas required to maintain continuity of the MS evolution ( yr−1 at z > 2.5). Simple power-law dependencies are similarly derived for the gas accretion rates. We argue that the overall evolution of galaxies is driven by the rates of gas accretion. The cosmic evolution of total ISM mass is estimated and linked to the evolution of SF and active galactic nucleus activity at early epochs.

Turbulent energy dissipation and intermittency in ambipolar diffusion magnetohydrodynamics

The dissipation of kinetic and magnetic energy in the interstellar medium (ISM) can proceed through viscous, Ohmic or ambipolar diffusion (AD). It occurs at very small scales compared to the scales at which energy is presumed to be injected. This localized heating may impact the ISM evolution but also its chemistry, thus providing observable features. Here, we perform 3D spectral simulations of decaying magnetohydrodynamic turbulence including the effects of AD. We find that the AD heating power spectrum peaks at scales in the inertial range, due to a strong alignment of the magnetic and current vectors in the dissipative range. AD affects much greater scales than the AD scale predicted by dimensional analysis. We find that energy dissipation is highly concentrated on thin sheets. Its probability density function follows a lognormal law with a power-law tail which hints at intermittency, a property which we quantify by use of structure function exponents. Finally, we extract structures of high dissipation, defined as connected sets of points where the total dissipation is most intense and we measure the scaling exponents of their geometric and dynamical characteristics: the inclusion of AD favours small sizes in the dissipative range.

The global gas and dust budget of the Large Magellanic Cloud: AGB stars and supernovae, and the impact on the ISM evolution

We report on an analysis of the gas and dust budget in the the interstellar medium (ISM) of the Large Magellanic Cloud (LMC). Recent observations from the Spitzer Space Telescope enable us to study the mid-infrared dust excess of asymptotic giant branch (AGB) stars in the LMC. This is the first time we can quantitatively assess the gas and dust input from AGB stars over a complete galaxy, fully based on observations. The integrated mass-loss rate over all intermediate and high mass-loss rate carbon-rich AGB candidates in the LMC is 8.5x10^-3 solar mass per year, up to 2.1x10^-2 solar mass per year. This number could be increased up to 2.7x10^-2 solar mass per year, if oxygen-rich stars are included. This is overall consistent with theoretical expectations, considering the star formation rate when these low- and intermediate-mass stars where formed, and the initial mass functions. AGB stars are one of the most important gas sources in the LMC, with supernovae (SNe), which produces about 2-4x10^-2 solar mass per year. At the moment, the star formation rate exceeds the gas feedback from AGB stars and SNe in the LMC, and the current star formation depends on gas already present in the ISM. This suggests that as the gas in the ISM is exhausted, the star formation rate will eventually decline in the LMC, unless gas is supplied externally. Our estimates suggest `a missing dust-mass problem' in the LMC, which is similarly found in high-z galaxies: the accumulated dust mass from AGB stars and possibly SNe over the dust life time (400--800 Myrs) is significant less than the dust mass in the ISM. Another dust source is required, possibly related to star-forming regions.

Damping Efficiency of the Void Mode in a Two-Phase Gas Mixture with a Phase Transition

Abstract We have discussed the wave properties in a two-phase gas mixture, which is composed of a diffuse medium [phase I (PI)] and dense clouds [phase II (PII)]. Interestingly, there are two wave modes when PII has mobility relative to PI. One is the acoustic mode, and the other is the void mode (or pattern-propagating mode). In this paper, we present a more careful discussion concerning the restoring force of the void mode. After that, we estimate the damping rate of the void mode due to the thermal process, which acts as a friction effect. We call this thermal friction. For a typical interstellar medium (ISM), where the cold cloud phase is PII and the warm phase gas is PI, friction is not so significant within the galaxy evolution timescales. However, if the coronal phase is PI, PII of the warm clouds suffers thermal friction efficiently. Thus, when the evolution of ISM is described by the classical three-phase model, we can adopt a modified two-phase model within the context of multi-phase flow; in which PI corresponds to a mixture of the hot and warm phases and PII corresponds to the cold clouds.

Morphology and Physical Structure of the Interstellar Medium

The morphology of the interstellar medium (ISM) is dependent on the wavelength at which observations are made. Morphologies at different wavelengths reveal interstellar gas components in different physical conditions, thus allow us to probe the physical structure of the ISM. The global structure of the ISM is determined by the gravitational effects of the host galaxy itself and the environment. On scales from a few parsecs to 103 parsecs, the physical structure of the ISM is largely determined by the energy feedback from massive stars. Interactions between massive stars and the ISM not only shape the ISM but also produce the multiple phases of the ISM. Multi-wavelength observations are needed to study the structure and evolution of a multi-phase ISM. The ISM of the Large Magellanic Cloud (LMC) is used to illustrate the environmental effects and the energy feedback from massive stars. The energy feedback is more complex than any recipe can describe.

Chemistry in Space

Molecules have been observed in widely disparate astronomical objects, from comets to supernova remnants. Interstellar space appear to be a real chemical laboratory, able to produce a large number of molecules, some of them relatively complex. The abundances show a very high sensitivity to local physical properties and dynamical history. This sensitivity renders molecular observations and astrochemical modelling very flexible tools for investigating the properties and evolution of the interstellar medium. A brief outline of morfology of interstellar medium is given, together with a discussion of the basic chemical processes leading to the molecular formation. Since interstellar-dust particles play a crucial role in the chemical evolution of interstellar medium, physical and chemical properties of dust are reviewed. Finally, by way of an example of modelling exercise, a toy model of the chemical evolution of interstellar gas is presented.

Stochastic evolution of refractory interstellar dust during the chemical evolution of a two-phase interstellar medium

The evolution course of refractoary interstellar dust during the chemical evolution of a two-phase interstellar medium (ISM) is studied using a simple model of the chemical evolution of ISM. It is assumed that, in this medium, the stars are born in molecular clouds, but new nucleosynthesis products and stellar return are entered into a complementary diffuse medium; the well-mixed matter of each interstellar phase is repeatedly cycled stochastically through the complementary phase and back. The dust is studied on a particle-by-particle bases as it is sputtered by shock waves in the diffuse medium, accretes an amorphous mantle of gaseous refractory atoms while its local medium joins the molecular cloud medium, and encounters the possibility of astration within molecular clouds. Results are presented relevant to the size spectrum of accreted mantles, its age spectrum and the distinction among its several lifetimes, depletion factors of refractory atoms in the diffuse gas, and isotopic anomalies.