Three-dimensional mapping of the Galactic extinction curve: A new perspective on interstellar dust

New measurements of the ultraviolet surface brightness of the night sky in 71 fields in the galactic longitude range 65degree< or =l/sub i//sub i/< or =145degree are presented. The data were obtained with the Orbiting Astronomical Observatory (OAO-2) at nine wavelengths between 1500 A and 4200 A and have been corrected for the contributions due to zodiacal light and integrated starlight. The residual brightnesses were analyzed with radiative transfer models for the diffuse galactic light which incorporate a z-dependent source function. The results qualitatively confirm earlier findings for this wavelength region, yielding a wavelength dependent albedo of the interstellar grains of approximately ..cap alpha..=0.7 +- 0.1 longward of lambda3000, ..cap alpha..=0.35 +- 0.05 around the pronounced minimum near lambda2200, and ..cap alpha..=0.6 +- 0.05 at lambda1550. The true absorption nature of the bump in the interstellar extinction curve near lambda2200, as well as the increase of the albedo shortward of lambda2000 are thus reconfirmed. The phase function asymmetry factor is found to lie between g=0.6 and g=0.9 for the entire wavelength range, indicating the interstellar grains are strongly forward scattering. (AIP)

Observational results obtained to date on interstellar grains and molecules are briefly reviewed, and several promising areas for further research withSpacelab are suggested. Regarding grains, useful data can be expected on the shape of the ultraviolet extinction curve for new interstellar regions; the nature of UV extinction at short wavelengths, down to the Lyman limit; the presence or absence of structure in the UV extinction curve comparable to the visible-wavelength diffuse bands; the scattering properties of grains in new kinds of clouds and nebulae; and the polarization properties of grains in UV wavelengths. The principal advances which may be expected in observations of molecules will include the ability to probe more heavily-obscured regions, where molecular species are more abundant than in the diffuse clouds observed to date; coverage of wavelength regions (such as λλ 1400–3200) not well-studied with previous instruments such asCopernicus; and the capability of observing in optical absorption species detected in the same line of sight in radio emission, which provides unique information on cloud geometry and physical conditions.

Observational results obtained to date on interstellar grains and molecules are briefly reviewed, and several promising areas for further research with Spacelab are suggested. Regarding grains, useful data can be expected on the shape of the ultraviolet extinction curve for new interstellar regions; the nature of UV extinction at short wavelengths, down to the Lyman limit; the presence or absence of structure in the UV extinction curve comparable to the visible-wavelength diffuse bands; the scattering properties of grains in new kinds of clouds and nebulae; and the polarization properties of grains in UV wavelengths. The principal advances which may be expected in observations of molecules will include the ability to probe more heavily-obscured regions, where molecular species are more abundant than in the diffuse clouds observed to date; coverage of wavelength regions (such as λλ 1400–3200) not well-studied with previous instruments such as Copernicus; and the capability of observing in optical absorption species detected in the same line of sight in radio emission, which provides unique information on cloud geometry and physical conditions.KeywordsRadio EmissionInterstellar DustExtinction CurveScattered Light MeasurementFaint StarThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The energy stored as free radicals in the process of growth and photoprocessing of interstellar grain mantles by ultraviolet radiation in clouds is treated as the primary cause of both grain mantle destruction and gas molecule production in dark clouds. The triggering mechanism for energy release is provided by grains colliding with each other as a result of turbulence in the cores of the clouds. A quantitative derivation of a size distribution function of the general form -α3a3 e for the mantles of the core-mantle interstellar dust grains, which best determines simultaneously the mean interstellar extinction and polarization curves, is based on the dust-dust collision rate and a grain evaporation efficiency parameter, f, to be determined by experiments performed in the photochemistry laboratory at Leiden University. The same mantle destruction process which produces the size distribution function is used to provide a numerical estimate of molecule ejection into the gas phase in terms of a parameter, αM, the effective molecule concentration fraction in the evaporating mantle. The evaporation efficiency factor and the molecule concentration function required in the derivations to give a simultaneously consistent picture of the mean size distribution function and the molecule production rate, specifically for H2CO, may be inferred to be realistic representations of the laboratory results obtained to date. The molecules ejcted by the explosive process are likely to appear in initially excited states and would be subject to subsequent gas phase interactions.

view Abstract Citations (917) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Dust from the Milky Way to the Magellanic Clouds Pei, Yichuan C. Abstract Interstellar dust in the Magellanic Clouds, with a weak or nearly absent 2175 A extinction feature, may be of interest in studies of galaxies in early stages of chemical evolution. To this inquiry, we extend the graphite-silicate grain model, introduced by Mathis, Rumpl, & Nordsieck and developed by Draine & Lee, from the Milky Way to the Magellanic Clouds. We find that the empirical extinction curves in the Large and Small Magellanic Clouds can be reproduced by adjusting only the relative abundances of graphite and silicate grains, while leaving all other model properties fixed to those appropriate for the Galactic extinction curve. Using the graphite-silicate models, we calculate the absorption and scattering optical depths, the mass-density ratio of interstellar dust to neutral hydrogen, and the Kramers-Kronig relation for all three galaxies. We also present a fitting function for the three extinction curves, valid not only over the observed range of wavelengths but also over the full range as predicted by the models. All the quantities we derived here are independent of the dust-to-gas ratios in the Milky Way and the Magellanic Clouds, and can be applied to other galaxies if they contain Galactic or Magellanic-type dust. Publication: The Astrophysical Journal Pub Date: August 1992 DOI: 10.1086/171637 Bibcode: 1992ApJ...395..130P Keywords: Cosmic Dust; Intergalactic Media; Interstellar Extinction; Interstellar Matter; Magellanic Clouds; Milky Way Galaxy; Chemical Evolution; Far Ultraviolet Radiation; Kramers-Kronig Formula; Astrophysics; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: INTERSTELLAR MATTER; GALAXIES: MAGELLANIC CLOUDS; ISM: DUST; EXTINCTION full text sources ADS | data products SIMBAD (3) NED (2)

Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra of stars in OB associations of M31 are used to derive the UV extinction by interstellar dust in M31 by three different methods: (1) comparing spectra of M31 star pairs, (2) comparing spectra of M31 stars to those of Galactic standard stars, and (3) comparing M31 star spectra to atmosphere models. The derived intrinsic M31 extinction curve has an overall wavelength dependence very similar to that of the average Galactic extinction curve but possibly has a weaker 2175 Å bump, however, with a significance of only 1 σ. This result is different from the LMC (30 Dor)-like curves published earlier, which contained both intrinsic M31 extinction and "foreground" extinction, and were based either on low-signal IUE spectra, or on FOS data affected by inaccuracy in the preliminary flux calibration, and were not computed with the pair method used in this work.

KNACKE1 has pointed out that quartz particles possess strong absorption bands at λ∼10µ. Although these features occur in the spectral region 3–10µ, where Johnson2 has found humps in interstellar extinction curves, by itself this cannot be taken as indicating a silica component in the interstellar grains. It has already been pointed out3 that impurities in any solid with a Debye frequency close to that of graphite would give optically active lattice bands at λ∼10µ*. Further evidence would be necessary before quartz could be considered as a serious alternative to interstellar graphite grains.

We compare the most successful and widely used map of Galactic dust extinction, provided by Schlegel, Finkbeiner, and Davis (1998, hereafter SFD), to the galaxy number counts in the Sloan Digital Sky Survey (SDSS) photometric/spectroscopic DR4 sample. We divide the SDSS survey area into 69 disjoint subregions according to the dust extinction provided by SFD and compare the surface number density of galaxies in each subregion. As expected, the galaxy surface number density decreases with increasing extinction, but only for SFD extinction values above about 0.1 to 0.2 magnitudes (depending on the band). At lower values of the SFD extinction, we find that the sky surface density of galaxies increases with increasing extinction, precisely the opposite of the effect expected from Galactic dust. We also find that the average color of the SDSS photometric galaxy sample is slightly bluer at higher SFD extinctions in this regime, again the opposite of the effect expected from Galactic dust. Even though these anomalies occur only for sight-lines with low SFD extinction values, they affect approximately 68% of the high galactic latitude sky in which galaxies and their clustering properties are normally studied. Although it would be possible to explain these effects with a mysterious component of Galactic dust, which is anti-correlated with the 100$\mu$m flux on which the SFD extinction map is based, this model is not physically plausible. Moreover, we find that the surface number density of SDSS photometric quasars does not show any similar effect, as would be expected if the explanation were an unknown Galactic dust component. Considering these results, we suggest that the far infrared (FIR) brightness of the sky in regions of true low dust extinction is significantly “contaminated” by the FIR emission from background galaxies. We show that such an explanation is both qualitatively and quantitatively consistent with the available data. Based on this interpretation we conclude that systematic errors in the SFD extinction map due to extragalactic FIR emission are quite small, on the order hundredths of a magnitude, but nevertheless statistically detectable. Unfortunately, however, these errors are also entangled in a complex way with a signal of great interest to many “precision cosmology” applications, namely the large-scale clustering of galaxies.

The authors present new HST STIS NUV-MAMA and STIS CCD observations of the BL Lac object AO 0235+164 and the intervening damped Ly {alpha} (DLA) line at z{sub {alpha}} = 0.524. The line profile gives N(HI) = 5 {+-} 1 x 10{sup 21} cm{sup -2} and, combined with the H I 21 cm absorption data leads to a spin temperature of Ts = 220 K {+-} 60 K. Those spectra also show a strong, broad feature at the expected position of the 2175 {angstrom} graphitic dust feature at z{sup {alpha}} = 0.524. Assuming a Galactic type dust extinction curve at z{sub {alpha}} = 0.524 gives a dust-to-gas ratio of 0.19 Galactic, but the fit, assuming the underlying, un-reddened spectrum is a single power-law, is poor in the far-UV. A dust-to-gas ratio of 0.19 Galactic is similar to the LMC, but the AO 0235+164 spectrum does not fit the LMC extinction curve, or the SMC extinction curve (which has practically no 2175 {angstrom} feature). A possible interpretation includes dust similar to Galactic, but with less of the small particles that produce the far-UV extinction. The metallicity of the z{sub {alpha}} = 0.524 absorber, estimated from the observed N(HI) and excess X-raymore » absorption (beyond Galactic) derived from contemporaneous and archival ASCA and ROSAT.« less

The authors present new HST STIS NUV-MAMA and STIS CCD observations of the BL Lac object AO 0235+164 and the intervening damped Ly {alpha} (DLA) line at z{sub {alpha}} = 0.524. The line profile gives N(HI) = 5 {+-} 1 x 10{sup 21} cm{sup -2} and, combined with the H I 21 cm absorption data leads to a spin temperature of Ts = 220 K {+-} 60 K. Those spectra also show a strong, broad feature at the expected position of the 2175 {angstrom} graphitic dust feature at z{sup {alpha}} = 0.524. Assuming a Galactic type dust extinction curve at z{sub {alpha}} = 0.524 gives a dust-to-gas ratio of 0.19 Galactic, but the fit, assuming the underlying, un-reddened spectrum is a single power-law, is poor in the far-UV. A dust-to-gas ratio of 0.19 Galactic is similar to the LMC, but the AO 0235+164 spectrum does not fit the LMC extinction curve, or the SMC extinction curve (which has practically no 2175 {angstrom} feature). A possible interpretation includes dust similar to Galactic, but with less of the small particles that produce the far-UV extinction. The metallicity of the z{sub {alpha}} = 0.524 absorber, estimated from the observed N(HI) and excess X-ray absorption (beyond Galactic) derived from contemporaneous and archival ASCA and ROSAT.

THE diffuse interstellar bands, which have been known for about 36 years1, are still one of the mysteries of astronomy. One hope has been that with definite identification of these bands will come not only further information about interstellar matter and its condition but, more specifically, more information about the nature of interstellar grains. Even without identification, however, there have been properties of the lines which could conceivably carry information about their origin. When van de Hulst2,3 considered the question of the origin of the diffuse lines from the point of view of the optics of small particles containing absorptive impurities, he noted that the shape of the extinction curve in the neighbourhood of the absorption lines could be one of various possibilities, depending on the size of the particles. Although this result was based on an approximate low index of refraction theory it turned out to be quite correct—at least qualitatively—as confirmed by detailed Mie theory computations4–7. As a consequence of these extensive calculations it was conclusively shown that if the 4430 band (the strongest of the diffuse bands) is symmetric there can be no simple accounting for these bands as originating from absorbing atoms or molecules dispersed through the interior of the grain, because the effective grain sizes needed to account for the shape of the extinction curve are such as to produce highly asymmetric additional extinction about the average. But new observational data on extinction have been reinterpreted8,9 and it now seems that the 4430 band is probably highly asymmetric and indeed may actually consist of a lower extinction at the shorter wavelengths followed by an additional extinction at the longer wavelengths, the whole being similar to a dispersion curve as predicted for the small dielectric or dirty ice particles4 which give rise to the interstellar extinction. We have followed up this idea and have extended the calculations to polarization10 as well as extinction in the 4430 region produced by several of the proposed interstellar grain models including silicates and graphite. In so doing we have found that the polarization may be the more useful and definitive way of getting information about the grains11.

Context. Gamma-ray burst (GRB) afterglows are powerful probes for studying the different properties of their host galaxies (e.g., the interstellar dust) at all redshifts. By fitting their spectral energy distribution (SED) over a large range of wavelengths, we can gain direct insights into the properties of the interstellar dust by studying the extinction curves. Unlike the dust extinction templates, such as those of the average Milky Way (MW) or the Small and Large Magellanic Cloud (SMC and LMC), the extinction curves of galaxies outside the Local Group exhibit deviation from these laws. Altogether, X-ray and gamma-ray satellites as well as ground-based telescopes, such as Neil Gehrels Swift Observatory (Swift) and Gamma-Ray Optical and Near-Infrared Detector (GROND), provide measurements of the afterglows from the X-ray to the NIR, which can be used to extract information on dust extinction curves along their lines of sight (LoS). The study presented in this paper undertakes such a photometric study, comprising a preparatory work for the SVOM mission and its ground-based follow-up telescope COLIBRI. Aims. We propose a simple approach to parameterize the dust extinction curve of GRB host galaxies. The model used in this analysis is based on a power law form with the addition of a Loretzian-like Drude profile with two parameters: the extinction slope, γ, and the 2175 Å bump amplitude, Eb. Methods. Using the g′r′i′z′JHKs GROND filter bands, we tested our dust extinction model and explored the parameter space in extinction and redshift by fitting SEDs of simplified simulations of GRB afterglow spectra based on different extinction curve templates. From a final sample of 10 real Swift/GROND extinguished GRBs, we determined the quantities of the dust extinction in their host and measured their extinction curves. Results. We find that our derived extinction curves are in agreement with the spectroscopic measurements reported for four GRBs in the literature. We compared four other GRBs to the results of photometric studies where fixed laws were used to fit their data. We additionally derived two new GRB extinction curves. The measured average extinction curve is given by a slope of γ = 1.051 ± 0.129 and Eb = 0.070 ± 0.036, which is equivalent to a quasi-featureless in-between SMC-LMC template. This is consistent with previous studies aimed at deriving the dust host galaxy extinction where we expect that small dust grains dominate in GRB environment, yielding a steeper curve than the mean MW extinction curve.

Carbonaceous compounds are a significant component of interstellar dust and the composition and structure of such materials is therefore of key importance. We present 1.5 μm‐15 μm spectra of a plasma polymerized carbonaceous material produced in RF discharge under low pressure, using C2H2 as a precursor component. The plasma polymerization process described here provides a convenient way to make carbonaceous interstellar dust analogues under controlled conditions and to compare their characteristics to astronomical observations. Here, we focus on a comparison to the IR spectra of interstellar dust in the light of the criteria for “good” carbonaceous interstellar dust analogue. The UV bump at extinction curve 217.5 nm is another key feature for understanding the abundance of carbon in interstellar media. We present here some preliminary results of UV extinction on plasma polymerized astro‐analogue.

The incompatibilities between the standard theory of interstellar extinction and observations

Interstellar dust extinction curves provide valuable information about dust properties, including the composition and size of the dust grains, and are essential to correct observations for the effects of interstellar dust. In this work, we measure a representative sample of near-infrared (NIR; 0.8–5.5 μm) spectroscopic extinction curves for the first time, enabling us to investigate the extinction at wavelengths where it is usually only measured in broad photometric bands. We use IRTF/SpeX spectra of a sample of reddened and comparison stars to measure 15 extinction curves with the pair method. Our sample spans A(V) values from 0.78 to 5.65 and R(V) values from 2.43 to 5.33. We confirm that the NIR extinction curves are well fit by a power law, with indices and amplitudes differing from sight line to sight line. Our average diffuse NIR extinction curve can be represented by a single power law with index α = 1.7, but because of the sight line-to-sight line variations, the shape of any average curve will depend on the parental sample. We find that most of the variation in our sample can be linked to the ratio of total-to-selective extinction R(V), a rough measurement of the average dust grain size. Two sight lines in our sample clearly show the ice extinction feature at 3 μm, which can be fitted by a modified Drude profile. We find tentative ice detections with slightly over 3σ significance in two other sight lines. In our average diffuse extinction curve, we measure a 3σ upper limit of A(ice)/A(V) = 0.0021 for this ice feature.