Shock-heated Gas Research Articles

Evidence for a Fast Soft X-Ray Wind in M82 from XMM-RGS

Starburst wind models predict that metals and energy are primarily carried out of the disk by hot gas (T > 106 K), but the low energy resolution of X-ray CCD observations results in large uncertainties on the mass and energy loading. Here, we present evidence for a fast soft X-ray wind from the prototypical starburst galaxy M82 using deep archival observations from the Reflection Grating Spectrometer on XMM-Newton. After characterizing the complex line-spread function for the spatially extended outflow ( ≈4′ ), we perform emission-line fitting to measure the velocity dispersion, σ v , from O viii (0.65, 0.77 keV), Ne x (1.02 keV), and Mg xii (1.47 keV). For the T ≈ 3 × 106 K gas, O viii yields a velocity dispersion of σv=1160−90+100 km s−1, implying a wind speed that is significantly above the escape velocity (v esc ≲ 450 km s−1). Ne x ( σv=550−150+130 km s−1) and Mg xii (σ v < 370 km s−1) show less velocity broadening than O viii, hinting at a lower wind speed or smaller opening angle on the more compact spatial scales traced by the T ≈ (0.6−1) × 107 K gas. Alternatively, these higher energy emission lines may be dominated by shock-heated gas in the interstellar medium. Future synthesis of these measurements with performance verification observations of the E = 2−12 keV wind in M82 from the Resolve microcalorimeter on the X-ray Imaging and Spectroscopy Mission will inform the phase structure and energy budget of the hot starburst wind.

Spectroscopic Measurement of Electron Concentration in Shock-Heated Gas

Spectroscopic Measurement of Electron Concentration in Shock-Heated Gas

Hot gas accretion fuels star formation faster than cold accretion in high-redshift galaxies

ABSTRACT We use high-resolution ($\simeq$35pc) hydrodynamical simulations of galaxy formation to investigate the relation between gas accretion and star formation in galaxies hosted by dark matter haloes of mass $10^{12}$${{\mathrm{M}}_{\odot }}$ at $z = 2$. At high-redshift, cold-accreted gas is expected to be readily available for star formation, while gas accreted in a hot mode is expected to require a longer time to cool down before being able to form stars. Contrary to these expectations, we find that the majority of cold-accreted gas takes several hundred Myr longer to form stars than hot-accreted gas after it reaches the inner circumgalactic medium (CGM). Approximately 10 per cent of the cold-accreted gas flows rapidly through the inner CGM on to the galactic disc. The remaining 90 per cent is trapped in a turbulent accretion region that extends up to $\sim 50$ per cent of the virial radius, from which it takes several hundred Myr for the gas to be transported to the star-forming disc. In contrast, most hot shock-heated gas avoids this ‘slow track’, and accretes directly from the CGM on to the disc where stars can form. We find that shock-heating of cold gas after accretion in the inner CGM and supernova-driven outflows contribute to, but do not fully explain, the delay in star formation. These processes combined slow down the delivery of cold-accreted gas to the galactic disc and consequently limit the rate of star formation in Milky Way mass galaxies at $z \gt 2$.

Spectral Model for Calculation of Radiation Characteristics of a Shock-Heated Gas

Spectral Model for Calculation of Radiation Characteristics of a Shock-Heated Gas

GOALS-JWST: Gas Dynamics and Excitation in NGC 7469 Revealed by NIRSpec

We present new JWST NIRSpec integral field spectroscopy (IFS) data for the luminous infrared galaxy NGC 7469, a nearby (70.6 Mpc) active galaxy with a Seyfert 1.5 nucleus that drives a highly ionized gas outflow and a prominent nuclear star-forming ring. Using the superb sensitivity and high spatial resolution of the JWST instrument NIRSpec IFS, we investigate the role of the Seyfert nucleus in the excitation and dynamics of the circumnuclear gas. Our analysis focuses on the [Fe ii], H2, and hydrogen recombination lines that trace the radiation/shocked-excited molecular and ionized interstellar medium around the active galactic nucleus (AGN). We investigate gas excitation through H2/Brγ and [Fe ii]/Paβ emission line ratios and find that photoionization by the AGN dominates within the central 300 pc of the galaxy except in a small region that shows signatures of shock-heated gas; these shock-heated regions are likely associated with a compact radio jet. In addition, the velocity field and velocity dispersion maps reveal complex gas kinematics. Rotation is the dominant feature, but we also identify noncircular motions consistent with gas inflows as traced by the velocity residuals and the spiral pattern in the Paα velocity dispersion map. The inflow is 2 orders of magnitude higher than the AGN accretion rate. The compact nuclear radio jet has enough power to drive the highly ionized outflow. This scenario suggests that the inflow and outflow are in a self-regulating feeding–feedback process, with a contribution from the radio jet helping to drive the outflow.

Discovery of optical emission from the supernova remnant G108.2−0.6 and its atomic environment

ABSTRACT We report on the first detection of optical emission from the shell-type Galactic supernova remnant (SNR) G108.2−0.6. We obtained Hα images and long-slit spectra using the 1.5-m RTT150 telescope to examine the morphological and spectral characteristics of the SNR. We detected several filaments along its north and south regions, which is consistent with its SNR nature. The spectra exhibit [S ii]/Hα ratios in the range of 0.4–1.1, indicating emission from shock-heated gas. The oxygen doublet emission lines [O i] λλ6300, 6363 detected in the south region also support the indicator of the presence of shocks. We estimate the electron density using the [S ii] 6716/6731 ratio ranging from 15 to 1800 cm−3. The spectra show a relatively low shock velocity of Vs ∼ 80 km s−1 with the pre-shock cloud density of nc ∼18–57 cm−3. The Hα/Hβ ratios show significant variation across the observed regions with extinction E(B − V) ranging from 0.22 to 1.65. We also analysed the archival H i data and estimated the kinematic distance to G108.2−0.6 of ∼0.8 kpc and its dynamical age as ∼70 ± 10 kyr.

Study of Radiation Characteristics of Shock-Heated Gases

Study of Radiation Characteristics of Shock-Heated Gases

How do supernova remnants cool? – I. Morphology, optical emission lines, and shocks

ABSTRACT Supernovae (SNe) inject ∼1051 erg in the interstellar medium, thereby shocking and heating the gas. A substantial fraction of this energy is later lost via radiative cooling. We present a post-processing module for the flash code to calculate the cooling radiation from shock-heated gas using collisional excitation data from mappings v. When applying this tool to a simulated SN remnant (SNR), we find that most energy is emitted in the EUV. However, optical emission lines ([O iii], [N ii], [S ii], H α, H β) are usually best observable. Our shock detection scheme shows that [S ii] and [N ii] emissions arise from the thin shell surrounding the SNR, while [O iii], H $\rm \alpha$, and H $\rm \beta$ originate from the volume-filling hot gas inside the SNR bubble. We find that the optical emission lines are affected by the SNR’s complex structure and its projection on to the plane of the sky because the escaping line luminosity can be reduced by 10–80 per cent due to absorption along the line of sight. Additionally, the subtraction of contaminating background radiation is required for the correct classification of an SNR on the oxygen or sulphur BPT diagrams. The electron temperature and density obtained from our synthetic observations match well with the simulation but are very sensitive to the assumed metallicity.

Anatomy of galactic star formation history: roles of different modes of gas accretion, feedback, and recycling

ABSTRACT We investigate how the diverse star formation histories observed across galaxy masses emerged using models that evolve under gas accretion from host haloes. They also include ejection of interstellar matter by supernova feedback, recycling of ejected matter and preventive feedback that partially hinders gas accretion. We consider three schemes of gas accretion: the fiducial scheme that includes the accretion of cold gas in low-mass haloes and high-redshift massive haloes as hinted by cosmological simulations; the flat scheme in which high-mass cold accretion is removed; and finally, the shock-heating scheme that assumes radiative cooling of the shock-heated halo gas. The fiducial scheme reproduces dramatic diminishment in star formation rate (SFR) after its peak as observed for the present halo mass $M_{\rm vir}\gt 10^{12.5}\, {\rm M}_\odot$ , while other two schemes show reduced or negligible quenching. This scheme reproduces the high-mass slope in the SFR versus stellar mass relation decreasing towards recent epochs, whereas other two schemes show opposite trend that contradicts observation. Success in the fiducial scheme originates in the existence of high-mass cold-mode accretion, which retards transition to the slow hot-mode accretion, thereby inducing a larger drop in SFR. Aided by gas recycling, which creates monotonically increasing SFR in low-mass haloes, this scheme can reproduce the downsizing galaxy formation. Several issues remain, suggesting non-negligible roles of missing physics. Feedback from active galactic nuclei could mitigate upturn of SFR in low-redshift massive haloes, whereas galaxy mergers could remedy early inefficient star formation.

Chandra X-ray measurement of gas-phase heavy element abundances in the central parsec of the galaxy

ABSTRACT Elemental abundances are key to our understanding of star formation and evolution in the Galactic Centre. Previous work on this topic has been based on infrared (IR) observations, but X-ray observations have the potential of constraining the abundance of heavy elements, mainly through their K-shell emission lines. Using 5.7 Ms Chandra observations, we provide the first abundance measurement of Si, S, Ar, Ca, and Fe, in four prominent diffuse X-ray features located in the central parsec of the Galaxy, which are the manifestation of shock-heated hot gas. A two-temperature non-equilibrium ionization spectral model is employed to derive the abundances of these five elements. In this procedure, a degeneracy is introduced due to uncertainties in the composition of light elements, in particular, H, C, and N. Assuming that the hot gas is H-depleted but C- and N-enriched, as would be expected for a standard scenario in which the hot gas is dominated by Wolf–Rayet star winds, the spectral fit finds a generally subsolar abundance for the heavy elements. If, instead, the light elements had a solar-like abundance, the heavy elements have a fitted abundance of ∼1–2 solar. The α/Fe abundance ratio, on the other hand, is mostly supersolar and insensitive to the exact composition of the light elements. These results are robust against potential biases due to either a moderate spectral signal-to-noise ratio or the presence of non-thermal components. Implications of the measured abundances for the Galactic Centre environment are addressed.

Shock physics and shadowgraphic measurements of laser-produced cerium plasmas.

Shadowgraphic measurements are combined with theory on gas-dynamics to investigate the shock physics associated with nanosecond laser ablation of cerium metal targets. Time-resolved shadowgraphic imaging is performed to measure the propagation and attenuation of the laser-induced shockwave through air and argon atmospheres at various background pressures, where stronger shockwaves characterized by higher propagation velocities are observed for higher ablation laser irradiances and lower pressures. The Rankine-Hugoniot relations are also employed to estimate the pressure, temperature, density, and flow velocity of the shock-heated gas located immediately behind the shock front, predicting larger pressure ratios and higher temperatures for stronger laser-induced shockwaves.

Discovery of optical emission associated with the supernova remnant G107.5−1.5

ABSTRACT We present the first results from an imaging and a spectroscopic survey of the optical emission associated with supernova remnant (SNR) G107.5−1.5. We discovered optical diffuse and filamentary emission from G107.5−1.5 using the 1.5- and 1-m telescopes. The optical emissions from the North-East (NE) and North-West (NW) regions show the diffuse structure, while the South-East (SE) and East (E) regions show filamentary structure. From long-slit spectra, we found [S ii]/H α &amp;gt; 0.5 for the SE and E regions, which supports the origin of the emission being from shock-heated gas. The average [S ii]/H α ratio is found to be ∼0.4 and ∼0.3 for the NW and NE regions, respectively, indicating that the optical emission is coming from ionized gas in an H ii region. From the ratios of [S ii]λλ 6716/6731, we estimate an average electron density of ∼2400 cm−3 for the NW region, which can be attributed to the dense ionized gas. The average [S ii]λλ 6716/6731 ratios are ∼1.25 and ∼1.15 for the SE and E regions, respectively, which are indicative of low-electron density.

Self-regulation of black hole accretion via jets in early protogalaxies

ABSTRACT The early growth of black holes (BHs) in high-redshift galaxies is likely feedback regulated. While radiative feedback has been extensively studied, the role of mechanical feedback has received less scrutiny to date. Here, we use high-resolution parsec-scale hydrodynamical simulations to study jet propagation and its effect on 100 M⊙ BH accretion in the dense, low-metallicity gas expected in early protogalaxies. As the jet propagates, it shocks the surrounding gas forming a jet cocoon. The cocoon consists of a rapidly cooling cold phase at the interface with the background gas and an overpressured subsonic phase of reverse shock-heated gas filling the interior. We vary the background gas density and temperature, BH feedback efficiency, and the jet model. We found that the width of the jet cocoon roughly follows a scaling derived by assuming momentum conservation in the jet-propagation direction and energy conservation in the lateral directions. Depending on the assumed gas and jet properties, the cocoon either stays elongated to large radii or isotropizes before reaching the Bondi radius, forming a nearly spherical bubble. Lower jet velocities and higher background gas densities result in self-regulation to higher momentum fluxes and elongated cocoons. In all cases, the outward cocoon momentum flux balances the inward inflowing gas momentum flux near the Bondi radius, which ultimately regulates BH accretion. The time-averaged accretion rate always remains below the Bondi rate, and exceeds the Eddington rate only if the ambient medium is dense and cold, and/or the jet is weak (low velocity and mass loading).

A Massive Star Is Born: How Feedback from Stellar Winds, Radiation Pressure, and Collimated Outflows Limits Accretion onto Massive Stars

Massive protostars attain high luminosities as they are actively accreting and the radiation pressure exerted on the gas in the star’s atmosphere may launch isotropic high-velocity winds. These winds will collide with the surrounding gas producing shock-heated (T ∼ 107 K) tenuous gas that adiabatically expands and pushes on the dense gas that may otherwise be accreted. We present a suite of 3D radiation-magnetohydrodynamic simulations of the collapse of massive prestellar cores and include radiative feedback from the stellar and dust-reprocessed radiation fields, collimated outflows, and, for the first time, isotropic stellar winds to model how these processes affect the formation of massive stars. We find that winds are initially launched when the massive protostar is still accreting and its wind properties evolve as the protostar contracts to the main sequence. Wind feedback drives asymmetric adiabatic wind bubbles that have a bipolar morphology because the dense circumstellar material pinches the expansion of the hot shock-heated gas. We term this the “wind tunnel effect.” If the core is magnetized, wind feedback is less efficient at driving adiabatic wind bubbles initially because magnetic tension delays their growth. We find that wind feedback eventually quenches accretion onto ∼30 M ⊙ protostars that form from the collapse of the isolated cores simulated here. Hence, our results suggest that ≳30 M ⊙ stars likely require larger-scale dynamical inflows from their host cloud to overcome wind feedback. Additionally, we discuss the implications of observing adiabatic wind bubbles with Chandra while the massive protostars are still highly embedded.

Fermi LAT Gamma-ray Detection of the Recurrent Nova RS Ophiuchi during its 2021 Outburst

We report the Fermi LAT γ-ray detection of the 2021 outburst of the symbiotic recurrent nova RS Ophiuchi. In this system, unlike classical novae from cataclysmic binaries, the ejecta from the white dwarf form shocks when interacting with the dense circumstellar wind environment of the red giant companion. We find the LAT spectra from 50 MeV to ∼20–23 GeV, the highest-energy photons detected in some subintervals, are consistent with π 0-decay emission from shocks in the ejecta as proposed by Tatischeff & Hernanz for its previous 2006 outburst. The LAT light curve displayed a fast rise to its peak >0.1 GeV flux of ≃6 × 10−6 ph cm−2 s−1 beginning on day 0.745 after its optically constrained eruption epoch of 2021 August 8.50. The peak lasted for ∼1 day and exhibited a power-law decline up to the final LAT detection on day 45. We analyze the data on shorter timescales at early times and found evidence of an approximate doubling of emission over ∼200 minutes at day 2.2, possibly indicating a localized shock-acceleration event. Comparing the data collected by the American Association of Variable Star Observers, we measured a constant ratio of ∼ 2.8 × 10−3 between the γ-ray and optical luminosities except for a ∼5×smaller ratio within the first day of the eruption likely indicating attenuation of γ rays by ejecta material and lower high-energy proton fluxes at the earliest stages of the shock development. The hard X-ray emission due to bremsstrahlung from shock-heated gas traced by the Swift-XRT 2–10 keV light curve peaked at day ∼6, later than at GeV and optical energies. Using X-ray derived temperatures to constrain the velocity profile, we find the hadronic model reproduces the observed >0.1 GeV light curve.

Cold-mode and hot-mode accretion in galaxy formation: an entropy approach

ABSTRACT We have analysed two cosmological zoom simulations with $M_{\rm vir}\sim 10^{12}{\rm \, M}_\odot$ from the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) series, both with and without feedback. We show that an entropy criterion based on the equation of state of the intergalactic medium can successfully separate cold- and hot-mode accretion. The shock-heated gas has non-negligible turbulent support and cools inefficiently. In the simulations without feedback, only a small fraction (≲20 per cent) of the stellar mass comes from baryons that have been in the hot circumgalactic medium, although quantitative conclusions should be taken with caution due to our small-number statistics. With feedback, the fraction is larger because of the reaccretion of gas heated by supernovae, which has lower entropies and shorter cooling times than the gas heated by accretion shocks. We have compared the results of NIHAO to predictions of the GalICS 2.1 semi-analytic model of galaxy formation. The shock-stability criterion implemented in GalICS 2.1 successfully reproduces the transition from cold- to hot-mode accretion.

Spectral-line Survey of the Region of Massive Star Formation W51e1/e2 in the 4 mm Wavelength Range

We present the results of a spectral-line survey of the W51e1/e2 star-forming region at 68–88 GHz. 79 molecules and their isotopologues were detected, from simple diatomic or triatomic molecules, such as SO, SiO, and CCH, to complex organic compounds, such as CH3OCH3 or CH3COCH3. A number of lines that are absent from the Lovas list of molecular lines observed in space were detected, and most of these were identified. A significant number of the detected molecules are typical for hot cores. These include the neutral molecules HCOOCH3, CH3CH2OH, and CH3COCH3, which are currently believed to exist in the gas phase only in hot cores and shock-heated gas. In addition, vibrationally excited C4H and HC3N lines with upper-level energies of several hundred Kelvins were found. Such lines can arise only in hot gas with temperatures on the order of 100 K or higher. Apart from neutral molecules, various molecular ions were also detected. Some of these (HC18O+, H13CO+, and HCS+) usually exist in molecular clouds with high visual extinctions. Potential formation pathways of complex organic molecules and hydrocarbons, along with nitriles, are considered. These formation routes are first discussed in the context of laboratory experiments elucidating the synthesis of organic molecules in interstellar ices in cold molecular clouds, followed by sublimation into the gas phase in the hot core stage. Thereafter, we discuss the predominant formation of hydrocarbons and their nitriles in the gas phase through bimolecular neutral–neutral reactions.

Deep optical study of the mixed-morphology supernova remnant G 132.7+1.3 (HB3)

ABSTRACT We present optical CCD images of the large supernova remnant (SNR) G 132.7+1.3 (HB3) covering its full extent for the first time, in the emission lines of Hα +[N ii], [S ii], and [O iii], where new and known filamentary and diffuse structures are detected. These observations are supplemented by new low-resolution long-slit spectra and higher resolution images in the same emission lines. Both the flux-calibrated images and spectra confirm that the optical emission originates from shock-heated gas since the [S ii]/Hα &amp;gt; 0.4. Our findings are also consistent with the recently developed emission-line ratio diagnostics for distinguishing SNRs from H ii regions. A multiwavelength comparison among our optical data and relevant observations in radio, X-rays, gamma-rays and CO bands, provided additional evidence on the interaction of HB3 with the surrounding clouds and clarified the borders of the SNR and the adjacent cloud. We discuss the supernova (SN) properties and evolution that led to the current observables of HB3 and we show that the remnant has most likely passed at the pressure driven snowplow phase. The estimated SN energy was found to be (3.7 ± 1.5) × 1051 erg and the current SNR age (5.1 ± 2.1) × 104 yr. We present an alternative scenario according to which the SNR evolved in the wind bubble cavity excavated by the progenitor star and currently is interacting with its density walls. We show that the overall mixed morphology properties of HB3 can be explained if the SN resulted by a Wolf−Rayet progenitor star with mass $\sim \! 34 \rm ~M_{\odot }$.

Supernova induced processing of interstellar dust: impact of interstellar medium gas density and gas turbulence

ABSTRACT Quantifying the efficiency of dust destruction in the interstellar medium (ISM) due to supernovae (SNe) is crucial for the understanding of galactic dust evolution. We present 3D hydrodynamic simulations of an SN blast wave propagating through the ISM. The interaction between the forward shock of the remnant and the surrounding ISM leads to destruction of ISM dust by the shock-heated gas. We consider the dust processing due to ion sputtering, accretion of atoms/molecules, and grain–grain collisions. Using 2D slices from the simulation time series, we apply post-processing calculations using the paperboats code. We find that efficiency of dust destruction depends strongly on the rate of grain shattering due to grain–grain collisions. The effective dust destruction is similar to previous theoretical estimates when grain–grain collisions are omitted, but with grain shattering included, the net destruction efficiency is roughly one order of magnitude higher. This result indicates that the dust-destruction rate in the ISM may have been severely underestimated in previous work, which only exacerbates the dust-budget crises seen in galaxies at high redshifts.

Diverse Star Formation History in Presence of Cold-Mode Gas Accretion: From Solar Neighborhood to Distant Galaxies

AbstractExistence of cold-mode gas accretion along with the hot-mode accretion of the shock-heated gas can explain the bimodality in the elemental abundance of the Milky Way disk stars as well as the mass-dependence of galaxy morphology represented by mass ratios of thin disks, thick disks, and bulges.