Anthony P. Jones

The Evolution of Dust in the Early Universe with Applications to the Galaxy SDSS J1148+5251

Dusty hyperluminous galaxies in the early universe provide unique environments for studying the role of massive stars in the formation and destruction of dust. At redshifts above ~6, when the universe was less than ~1 Gyr old, dust could have only condensed in the explosive ejecta of Type II supernovae (SNe), since most of the progenitors of the asymptotic giant branch stars, the major alternative source of interstellar dust, did not have time to evolve off the main sequence since the onset of star formation. In this paper we present analytical models for the evolution of the gas, dust, and metals in high-redshift galaxies, with a special application to SDSS J1148+5251 (hereafter J1148+5251), a hyperluminous quasar at z = 6.4. We find that an average SN must condense at least 1 M☉ of dust to account for the observed dust mass in J1148+5251. Observationally, it is in excess of the largest dust yield of 0.02 M☉ found thus far in the ejecta of any SN. If future observations find this to be a typical SN dust yield, then additional processes, such as accretion onto preexisting grains or condensation around the active galactic nucleus, will need to be invoked to account for the large amount of dust in this and similar objects. The galaxys star formation history is still uncertain, and current observations of the gas, metal, and dust contents of J1148+5251 can be reproduced by either an intensive and short burst of star formation (ψ 103 M☉ yr-1) with a duration of 108 yr or a much lower star formation rate (ψ ≈ 100 M☉ yr-1) occurring over the lifetime of the galaxy.

The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point

Context. The evolution of amorphous hydrocarbon materials, a-C(:H), principally resulting from ultraviolet (UV) photon absorptioninduced processing, are likely at the heart of the variations in the observed properties of dust in the interstellar medium. Aims. The consequences of the size-dependent and compositional variations in a-C(:H), from aliphatic-rich a-C:H to aromatic-rich a-C, are studied within the context of the interstellar dust extinction and emission. Methods. Newly-derived optical property data for a-C(:H) materials, combined with that for an amorphous forsterite-type silicate with iron nano-particle inclusions, a-SilFe, are used to explore dust evolution in the interstellar medium. Results. We present a new dust model that consists of a power-law distribution of small a-C grains and log-normal distributions of large a-SilFe and a-C(:H) grains. The model, which is firmly anchored by laboratory-data, is shown to quite naturally explain the variations in the infrared (IR) to far-ultraviolet (FUV) extinction, the 217 nm UV bump, the IR absorption and emission bands and the IR-mm dust emission. Conclusions. The major strengths of the new model are its inherent simplicity and built-in capacity to follow dust evolution in interstellar media. We show that mantle accretion in molecular clouds and UV photo-processing in photo-dominated regions are likely the major drivers of dust evolution.

Dust evolution in the transition towards the denser ISM: impact on dust temperature, opacity, and spectral index

Context. Variations in the observed dust emission and extinction indicate a systematic evolution of grain properties in the transition from the diffuse interstellar medium (ISM) to denser molecular clouds. Aims. The differences in the dust spectral energy distribution (SED) observed from the diffuse ISM to denser regions, namely an increase in the spectral index at long wavelengths, an increase in the FIR opacity, and a decrease in temperature, are usually assumed to be the result of changes in dust properties. We investigate if evolutionary processes, such as coagulation and accretion, are able to change the dust properties of grains in a way that is consistent with observations. Methods. We use a core-mantle grain model to describe diffuse ISM-type grains, and using a discrete-dipole approximation, we calculate how the accretion of mantles and coagulation into aggregates vary the grain optical properties. We calculate the dust SED and extinction using DustEM and the radiative transfer code CRT. Results. We show that the accretion of an aliphatic carbon mantle on diffuse ISM-type dust leads to an increase in the FIR opacity by a factor of about 2 and in the FIR/submm spectral index from 1.5 to 1.8, and to a decrease in the temperature by about 2 K. We also show that the coagulation of these grains into aggregates further decreases the temperature by 3 K and increases the spectral index up to a value of ∼2. The FIR opacity is increased by a factor of 3 (7) for these aggregates (with an additional ice-mantle) compared to the diffuse ISM-dust. Conclusions. Dust evolution in the ISM resulting from coagulation and accretion, leads to significant changes in the optical properties of the grains that can explain the observed variations in the dust SED in the transition from the diffuse ISM to denser regions.

A re-evaluation of dust processing in supernova shock waves

Context. There is a long-standing and large discrepancy between the timescale for dust formation around evolved stars and the rapid dust destruction timescale in interstellar shocks. Aims. We use our latest estimates for dust processing to re-evaluate the dust destruction efficiency in supernova triggered shock waves, estimate the dust lifetime, and calculate the emission and extinction from shocked dust. Methods. We modelled the sputtering and fragmentation of grains in interstellar shocks for shock velocities between 50 km s −1 and 200 km s −1 . We constrained the dust destruction using our recent dust model. Finally, we coupled our code to the DustEM code in order to estimate the emission and extinction from the dust post-shock. Results. Carbonaceous grains are quickly destroyed, even in a 50 km s −1 shock, leading to a shorter lifetime than in previous studies. Silicate grains appear to be more resilient, but the new destruction lifetime that we find is similar to previous studies and short compared to the dust injection timescale. Conclusions. The calculated fraction of elements locked in grains is not compatible with the observed values and therefore implies the re-formation of dust in the dense regions of the interstellar medium. Better modelling of the silicate sputtering together with hydrodynamical simulations of interstellar shocks, appears to reduce the silicate destruction and may close the destruction-formation timescale gap.

A hidden reservoir of Fe/FeS in interstellar silicates?

Context. The depletion of iron and sulphur into dust in the interstellar medium and the exact nature of interstellar amorphous silicate grains is still an open question. Aims. We study the incorporation of iron and sulphur into amorphous silicates of olivine- and pyroxene-types and their effects on the dust spectroscopy and thermal emission. Methods. We used the Maxwell-Garnett effective-medium theory to construct the optical constants for a mixture of silicates, metallic iron, and iron sulphide. We also studied the effects of iron and iron sulphide in aggregate grains. Results. Iron sulphide inclusions within amorphous silicates that contain iron metal inclusions show no strong differences in the optical properties of the grains. A mix of amorphous olivine- and pyroxene-type silicate broadens the silicate features. An amorphous carbon mantle with a thickness of 10 nm on the silicate grains leads to an increase in absorption on the short-wavelength side of the 10 μm silicate band. Conclusions. The assumption of amorphous olivine-type and pyroxene-type silicates and a 10 nm thick amorphous carbon mantle better matches the interstellar silicate band profiles. Including iron nano-particles leads to an increase in the mid-IR extinction, while up to 5 ppm of sulphur can be incorporated as Fe/FeS nano inclusions into silicate grains without leaving a significant trace of its presence.

Destruction of Interstellar Dust in Evolving Supernova Remnant Shock Waves

Supernova generated shock waves are responsible for most of the destruction of dust grains in the interstellar medium (ISM). Calculations of the dust destruction timescale have so far been carried out using plane parallel steady shocks, however that approximation breaks down when the destruction timescale becomes longer than that for the evolution of the supernova remnant (SNR) shock. In this paper we present new calculations of grain destruction in evolving, radiative SNRs. To facilitate comparison with the previous study by Jones et al. (1996), we adopt the same dust properties as in that paper. We find that the efficiencies of grain destruction are most divergent from those for a steady shock when the thermal history of a shocked gas parcel in the SNR differs significantly from that behind a steady shock. This occurs in shocks with velocities >~ 200 km/s for which the remnant is just beginning to go radiative. Assuming SNRs evolve in a warm phase dominated ISM, we find dust destruction timescales are increased by a factor of ~2 compared to those of Jones et al. (1996), who assumed a hot gas dominated ISM. Recent estimates of supernova rates and ISM mass lead to another factor of ~3 increase in the destruction timescales, resulting in a silicate grain destruction timescale of ~2-3 Gyr. These increases, while not able resolve the problem of the discrepant timescales for silicate grain destruction and creation, are an important step towards understanding the origin, and evolution of dust in the ISM.

High-resolution, 3D radiative transfer modeling - I. The grand-design spiral galaxy M 51

Context. Dust reprocesses about half of the stellar radiation in galaxies. The thermal re-emission by dust of absorbed energy is considered to be driven merely by young stars so is often applied to tracing the star formation rate in galaxies. Recent studies have argued that the old stellar population might be responsible for a non-negligible fraction of the radiative dust heating. Aims. In this work, we aim to analyze the contribution of young (≲100 Myr) and old (~10 Gyr) stellar populations to radiative dust heating processes in the nearby grand-design spiral galaxy M 51 using radiative transfer modeling. High-resolution 3D radiative transfer (RT) models are required to describe the complex morphologies of asymmetric spiral arms and clumpy star-forming regions and to model the propagation of light through a dusty medium. Methods. In this paper, we present a new technique developed to model the radiative transfer effects in nearby face-on galaxies. We construct a high-resolution 3D radiative transfer model with the Monte-Carlo code SKIRT to account for the absorption, scattering, and non-local thermal equilibrium (NLTE) emission of dust in M 51. The 3D distribution of stars is derived from the 2D morphology observed in the IRAC 3.6 μm, GALEX FUV, Hα, and MIPS 24 μm wavebands, assuming an exponential vertical distribution with an appropriate scale height. The dust geometry is constrained through the far-ultraviolet (FUV) attenuation, which is derived from the observed total-infrared-to-far-ultraviolet luminosity ratio. The stellar luminosity, star formation rate, and dust mass have been scaled to reproduce the observed stellar spectral energy distribution (SED), FUV attenuation, and infrared SED. Results. The dust emission derived from RT calculations is consistent with far-infrared and submillimeter observations of M 51, implying that the absorbed stellar energy is balanced by the thermal re-emission of dust. The young stars provide 63% of the energy for heating the dust responsible for the total infrared emission (8−1000 μm), while 37% of the dust emission is governed through heating by the evolved stellar population. In individual wavebands, the contribution from young stars to the dust heating dominates at all infrared wavebands but gradually decreases towards longer infrared and submillimeter wavebands for which the old stellar population becomes a non-negligible source of heating. Upon extrapolation of the results for M 51, we present prescriptions for estimating the contribution of young stars to the global dust heating based on a tight correlation between the dust heating fraction and specific star formation rate.

Interstellar and circumstellar grain formation and survival

Dust formation is primarily associated with stars in their dying throes, e.g. when low–mass stars reach the red–giant or asymptotic–giant branch (AGB) phase of their evolution, or when massive stars explode as supernovae (SNe). While the contribution of AGB stars to the galactic dust budget is significant, both in terms of variety and quantity, that due to SNe is not yet clear. AGB stardust formation includes grains of amorphous and crystalline silicates, hydrogenated carbons, silicon carbide and graphite. However, not all of these materials have yet been detected in circumstellar regions or in the interstellar medium (ISM). The derived lifetimes for these materials in the ISM appear to be short compared with the time–scale for the formation of new dust. Thus a grain lifetime and propagation problem is posed. Apparently, it is also necessary to reform and grow grains in the ISM, through accretion and coagulation processes, in order to explain interstellar dust observations. This paper discusses dust formation in circumstellar and interstellar environments, dust sources and their contributions to the galactic dust budget, and dust survival and propagation in the ISM.

Dust models post-Planck: constraining the far-infrared opacity of dust in the diffuse interstellar medium

We compare the performance of several dust models in reproducing the dust spectral energy distribution (SED) per unit extinction in the diffuse interstellar medium (ISM). We use our results to constrain the variability of the optical properties of big grains in the diffuse ISM, as published by the Planck collaboration. We use two different techniques to compare the predictions of dust models to data from the Planck HFI, IRAS and SDSS surveys. First, we fit the far-infrared emission spectrum to recover the dust extinction and the intensity of the interstellar radiation field (ISRF). Second, we infer the ISRF intensity from the total power emitted by dust per unit extinction, and then predict the emission spectrum. In both cases, we test the ability of the models to reproduce dust emission and extinction at the same time. We identify two issues. Not all models can reproduce the average dust emission per unit extinction: there are differences of up to a factor

A framework for resolving the origin, nature and evolution of the diffuse interstellar band carriers?

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