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Register a new userEvolution of multiple supernova remnants
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(Abridged) Heating of the interstellar medium by multiple supernovae (SNe) explosions is at the heart of producing galaxy-scale outflows. We use hydrodynamical simulations to study the efficiency of multiple SNe in heating the interstellar medium (ISM) and filling the volume with gas of high temperatures. We argue that it is important for SNe remnants to have a large filling factor {it and} a large heating efficiency. For this, they have to be clustered in space and time, and keep exploding until the hot gas percolates through the whole region, in order to compensate for the radiative loss. In the case of a limited number of SNe, we find that although the filling factor can be large, the heating efficiency declines after reaching a large value. In the case of a continuous series of SNe, the hot gas ($T ge 3 times 10^6$ K) can percolate through the whole region after the total volume filling factor reaches a threshold of $sim 0.3$. The efficiency of heating the gas to X-ray temperatures can be $ge 0.1$ after this percolation epoch, which occurs after a period of $approx 10$ Myr for a typical starburst SNe rate density of $ u_{rm SN} approx 10^{-9}$ pc$^{-3}$ yr$^{-1}$ and gas density of $napprox 10$ cm$^{-3}$ in starburst nuclei regions. This matches the recent observations of a time delay of similar order between the onset of star formation and galactic outflows. The efficiency to heat gas up to X-ray temperatures ($ge 10^{6.5}$ K) roughly scales as $ u_{rm SN}^{0.2} n^{-0.6}$. For a typical SNe rate density and gas density in starburst nuclei, the heating efficiency is $sim 0.15$, also consistent with previous interpretations from X-ray observations. We discuss the implications of our results with regard to observational diagnostics of ionic ratios and emission measures in starburst nuclei regions.
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A systematic study of the synchrotron X-ray emission from supernova remnants (SNRs) has been conducted. We selected a total of 12 SNRs whose synchrotron X-ray spectral parameters are available in the literature with reasonable accuracy, and studied how their luminosities change as a function of radius. It is found that the synchrotron X-ray luminosity tends to drop especially when the SNRs become larger than ~5 pc, despite large scatter. This may be explained by the change of spectral shape caused by the decrease of the synchrotron roll-off energy. A simple evolutionary model of the X-ray luminosity is proposed and is found to reproduce the observed data approximately, with reasonable model parameters. According to the model, the total energy of accelerated electrons is estimated to be 10^(47-48) ergs, which is well below the supernova explosion energy. The maximum energies of accelerated electrons and protons are also discussed.
I outline the dynamical evolution of the shell remnants of supernovae (SNRs), from initial interaction of supernova ejecta with circumstellar material (CSM) through to the final dissolution of the remnant into the interstellar medium (ISM). Supernova ejecta drive a blast wave through any CSM from the progenitor system; as material is swept up, a reverse shock forms in the ejecta, reheating them. This ejecta-driven phase lasts until ten or more times the ejected mass is swept up, and the remnant approaches the Sedov or self-similar evolutionary phase. The evolution up to this time is approximately adiabatic. Eventually, as the blast wave slows, the remnant age approaches the cooling time for immediate post-shock gas, and the shock becomes radiative and highly compressive. Eventually the shock speed drops below the local ISM sound speed and the remnant dissipates. I then review the various processes by which remnants radiate. At early times, during the adiabatic phases, thermal X-rays and nonthermal radio, X-ray, and gamma-ray emission dominate, while optical emission is faint and confined to a few strong lines of hydrogen and perhaps helium. Once the shock is radiative, prominent optical and infrared emission is produced. Young remnants are profoundly affected by interaction with often anisotropic CSM, while even mature remnants can still show evidence of ejecta.
Supernova remnants (SNR) are now widely believed to be a source of cosmic rays (CRs) up to an energy of 1 PeV. The magnetic fields required to accelerate CRs to sufficiently high energies need to be much higher than can result from compression of the circumstellar medium (CSM) by a factor 4, as is the case in strong shocks. Non-thermal synchrotron maps of these regions indicate that indeed the magnetic field is much stronger, and for young SNRs has a dominant radial component while for old SNRs it is mainly toroidal. How these magnetic fields get enhanced, or why the field orientation is mainly radial for young remnants, is not yet fully understood. We use an adaptive mesh refinement MHD code, AMRVAC, to simulate the evolution of supernova remnants and to see if we can reproduce a mainly radial magnetic field in early stages of evolution. We follow the evolution of the SNR with three different configurations of the initial magnetic field in the CSM: an initially mainly toroidal field, a turbulent magnetic field, and a field parallel to the symmetry axis. Although for the latter two topologies a significant radial field component arises at the contact discontinuity due to the Rayleigh-Taylor instability, no radial component can be seen out to the forward shock. Ideal MHD appears not sufficient to explain observations. Possibly a higher compression ratio and additional turbulence due to dominant presence of CRs can help us to better reproduce the observations in future studies.
We search for far-infrared (FIR) counterparts of known supernova remnants (SNRs) in the Galactic plane (360 degrees in longitude and b = +/- 1 deg ) at 70 - 500 micron with Herschel. We detect dust signatures in 39 SNRs out of 190, made up of 13 core-collapse supernovae (CCSNe), including 4 Pulsar Wind Nebulae (PWNe), and 2 Type Ia SNe. A further 24 FIR detected SNRs have unknown types. We confirm the FIR detection of ejecta dust within G350.1-0.3, adding to the known sample of ~10 SNRs containing ejecta dust. We discover dust features at the location of a radio core at the centre of G351.2+0.1, indicating FIR emission coincident with a possible Crab-like compact object, with dust temperature and mass of Td = 45.8 K and Md = 0.18 solar mass, similar to the PWN G54.1+0.3. We show that the detection rate is higher among young SNRs. We produce dust temperature maps of 11 SNRs and mass maps of those with distance estimates, finding dust at temperatures 15 < Td < 40 K. If the dust is heated by shock interactions the shocked gas must be relatively cool and/or have a low density to explain the observed low grain temperatures.
MUSE-based emission-line maps of the spiral galaxy NGC 4030 reveal the existence of unresolved sources with forbidden line emission enhanced with respect to those seen in its own HII regions. This study reports our efforts to detect and isolate these objects and identify their nature. Candidates are first detected as unresolved sources on an image of the second principal component of the Hb, [OIII]5007, Ha, [NII]6584, [SII]6716, 6731 emission-line data cube, where they stand out clearly against both the dominant HII region population and the widespread diffuse emission. The intrinsic emission is then extracted accounting for the highly inhomogeneous emission-line background throughout the field of view. Collisional to recombination line ratios like [SII]/Ha, [NII]/Ha, and [OI]/Ha tend to increase when the background emission is corrected for. We find that many (but not all) sources detected with the principal component analysis have properties compatible with supernova remnants (SNRs). Applying a combined [SII]/Ha and [NII]/Ha classification criterion leads to a list of 59 sources with SNR-like emission lines. Many of them exhibit conspicuous spectral signatures of SNRs around 7300 Angs, and a stacking analysis shows that these features are also present, except weaker, in other cases. At nearly 30 Mpc, these are the most distant SNRs detected by optical means to date. We further report the serendipitous discovery of a luminous (M_V ~ -12.5), blue, and variable source, possibly associated with a supernova impostor.
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