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Register a new userDust masses and grain size distributions of a sample of Galactic pulsar wind nebulae
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We calculate dust spectral energy distributions (SEDs) for a range of grain sizes and compositions, using physical properties appropriate for five pulsar wind nebulae (PWNe) from which dust emission associated with the ejecta has been detected. By fitting the observed dust SED with our models, with the number of grains of different sizes as the free parameters, we are able to determine the grain size distribution and total dust mass in each PWN. We find that all five PWNe require large ($ge 0.1 , {rm mu m}$) grains to make up the majority of the dust mass, with strong evidence for the presence of micron-sized or larger grains. Only two PWNe contain non-negligible quantities of small ($<0.01 , {rm mu m}$) grains. The size distributions are generally well-represented by broken power laws, although our uncertainties are too large to rule out alternative shapes. We find a total dust mass of $0.02-0.28 , {rm M}_odot$ for the Crab Nebula, depending on the composition and distance from the synchrotron source, in agreement with recent estimates. For three objects in our sample, the PWN synchrotron luminosity is insufficient to power the observed dust emission, and additional collisional heating is required, either from warm, dense gas as found in the Crab Nebula, or higher temperature shocked material. For G$54.1$+$0.3$, the dust is heated by nearby OB stars rather than the PWN. Inferred dust masses vary significantly depending on the details of the assumed heating mechanism, but in all cases large mass fractions of micron-sized grains are required.
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We search for far-infrared (FIR) counterparts of known supernova remnants (SNRs) in the Galactic plane (10 deg <| l |< 60 deg) at 70-500 micron using the Herschel Infrared Galactic Plane Survey (Hi-GAL). Of 71 sources studied, we find that 29 (41 per cent) SNRs have a clear FIR detection of dust emission associated with the SNR. Dust from 8 of these is in the central region, and 4 indicate pulsar wind nebulae (PWNe) heated ejecta dust. A further 23 have dust emission in the outer shell structures which is potentially related to swept up material. Many Galactic SNe have dust signatures but we are biased towards detecting ejecta dust in young remnants and those with a heating source (shock or PWN). We estimate the dust temperature and mass contained within three PWNe, G11.2-0.3, G21.5-0.9, and G29.7-0.3 using modified blackbody fits. To more rigorously analyse the dust properties at various temperatures and dust emissivity index beta, we use point process mapping (PPMAP). We find significant quantities of cool dust (at 20-40 K) with dust masses of Md = 0.34 +/- 0.14 solar mass, Md = 0.29 +/- 0.08 solar mass, and Md = 0.51 +/- 0.13 solar mass for G11.2-0.3, G21.5-0.9, and G29.7-0.3 respectively. We derive the dust emissivity index for the PWN ejecta dust in G21.5-0.3 to be beta = 1.4 +/- 0.5 compared to dust in the surrounding medium where beta = 1.8 +/- 0.1.
Based on a one-zone evolution model of grain size distribution in a galaxy, we calculate the evolution of infrared spectral energy distribution (SED), considering silicate, carbonaceous dust, and polycyclic aromatic hydrocarbons (PAHs). The dense gas fraction ($eta_mathrm{dense}$) of the interstellar medium (ISM), the star formation time-scale ($tau_mathrm{SF}$), and the interstellar radiation field intensity normalized to the Milky Way value ($U$) are the main parameters. We find that the SED shape generally has weak mid-infrared (MIR) emission in the early phase of galaxy evolution because the dust abundance is dominated by large grains. At an intermediate stage ($tsim 1$ Gyr for $tau_mathrm{SF}=5$ Gyr), the MIR emission grows rapidly because the abundance of small grains increases drastically by the accretion of gas-phase metals. We also compare our results with observational data of nearby and high-redshift ($zsim 2$) galaxies taken by textit{Spitzer}. We broadly reproduce the flux ratios in various bands as a function of metallicity. We find that small $eta_mathrm{dense}$ (i.e. the ISM dominated by the diffuse phase) is favoured to reproduce the 8 $mu$m intensity dominated by PAHs both for the nearby and the $zsim 2$ samples. A long $tau_mathrm{SF}$ raises the 8 $mu$m emission to a level consistent with the nearby low-metallicity galaxies. The broad match between the theoretical calculations and the observations supports our understanding of the grain size distribution, but the importance of the diffuse ISM for the PAH emission implies the necessity of spatially resolved treatment for the ISM.
The dust properties in high-redshift galaxies provide clues to the origin of dust in the Universe. Although dust has been detected in galaxies at redshift $z>7$, it is difficult to constrain the dominant dust sources only from the total dust amount. Thus, we calculate the evolution of grain size distribution, expecting that different dust sources predict different grain size distributions. Using the star formation time-scale and the total baryonic mass constrained by the data in the literature, we calculate the evolution of grain size distribution. To explain the total dust masses in ALMA-detected $z>7$ galaxies, the following two solutions are possible: (i) high dust condensation efficiency in stellar ejecta, and (ii) efficient accretion (dust growth by accreting the gas-phase metals in the interstellar medium). We find that these two scenarios predict significantly different grain size distributions: in (i), the dust is dominated by large grains ($agtrsim 0.1~mu$m, where $a$ is the grain radius), while in (ii), the small-grain ($alesssim 0.01~mu$m) abundance is significantly enhanced by accretion. Accordingly, extinction curves are expected to be much steeper in (ii) than in (i). Thus, we conclude that extinction curves provide a viable way to distinguish the dominant dust sources in the early phase of galaxy evolution.
We investigate the abundance and properties (especially, grain size) of dust in galaxy halos using available observational data in the literature. There are two major sets of data. One is (i) the reddening curves at redshifts $zsim 1$ and 2 derived for Mg II absorbers, which are assumed to trace the medium in galaxy halos. The other is (ii) the cosmic extinction up to $zsim 2$ mainly traced by distant background quasars. For (i), the observed reddening curves favor a grain radius of $asim 0.03~mu$m for silicate, while graphite is not supported because of its strong 2175 AA bump. Using amorphous carbon improves the fit to the reddening curves compared with graphite if the grain radius is $alesssim 0.03~mu$m. For (ii), the cosmic extinction requires $etagtrsim 10^{-2}$ ($eta$ is the ratio of the halo dust mass to the stellar mass; the observationally suggested value is $etasim 10^{-3}$) for silicate if $asim 0.03~mu$m as suggested by the reddening curve constraint. Thus, for silicate, we do not find any grain radius that satisfies both (i) and (ii) unless the halo dust abundance is much larger than suggested by the observations. For amorphous carbon, in contrast, a wide range of grain radius ($asim 0.01$--0.3~$mu$m) is accepted by the cosmic extinction; thus, we find that a grain radius range of $asim 0.01$--0.03 $mu$m is supported by combining (i) and (ii). We also discuss the origin of dust in galaxy halos, focusing on the importance of grain size in the physical mechanism of dust supply to galaxy halos.
Pulsar Wind Nebulae (PWNe) are bubbles or relativistic plasma that form when the pulsar wind is confined by the SNR or the ISM. Recent observations have shown a richness of emission features that has driven a renewed interest in the theoretical modeling of these objects. In recent years a MHD paradigm has been developed, capable of reproducing almost all of the observed properties of PWNe, shedding new light on many old issues. Given that PWNe are perhaps the nearest systems where processes related to relativistic dynamics can be investigated with high accuracy, a reliable model of their behavior is paramount for a correct understanding of high energy astrophysics in general. I will review the present status of MHD models: what are the key ingredients, their successes, and open questions that still need further investigation.
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