Extinction Curve Research Articles

Context. A large variety of dust attenuation and/or extinction curves has been observed in high-redshift galaxies. Some studies investigated their correlations with fundamental galaxy properties, which yielded mixed results. These variations are likely driven by underlying factors such as the intrinsic dust properties, the total dust content, and the spatial distribution of dust relative to stars. Aims. We investigate the correlations between the shape of dust attenuation curves, defined by the UV-optical slope (S) and the UV bump parameter (B), and fundamental galaxy properties. Our goal is to identify the key physical mechanisms that shape the dust attenuation curves through cosmic time in the broader context of galaxy formation and evolution. Methods. We extended the analysis of 173 dusty high-redshift (z ∼ 2 − 11.5) galaxies, whose dust attenuation curves were inferred by fitting James Webb Space Telescope (JWST) data with a modified version of the spectral energy distribution (SED) fitting code BAGPIPES. We investigate the trends between the dust attenuation parameters and different galaxy properties as inferred from the SED fitting: V-band attenuation (AV), star formation rate (SFR), stellar mass (M*), specific SFR (sSFR = SFR/M∗), mass-weighted stellar age (⟨a⟩*m), ionization parameter (log U), and metallicity (Z). For a subset of sources, we additionally explored the trends with oxygen abundance (12 + log(O/H)), which we derived using the direct Te-based method. Results. We report moderate correlations between S and AV, and B and AV. Galaxies characterized by lower (higher) AV exhibit steeper (flatter) slopes and stronger (weaker) UV bumps. These results agree with radiative transfer (RT) predictions that account for the total dust content and the relative spatial distribution of dust with respect to stars. Additionally, we find that S flattens with decreasing ⟨a⟩*m and increasing sSFR. These two trends can be explained if the strong radiation fields associated with young stars (low ⟨a⟩*m) and/or bursty galaxies (high sSFR) preferentially destroy small dust grains, which would shift the size distribution toward larger grains. Finally, the positive correlation between B and 12 + log(O/H) that emerged from our analysis might be driven by variations in the intrinsic dust properties with the gas metallicity. Conclusions. The shape of the dust attenuation curves primarily correlates with four key galaxy properties: (1) With the redshift, which traces variations in the intrinsic and/or reprocessed dust properties, (2) with AV, which reflects RT effects, (3) with the mass-weighted stellar age or sSFR, which might be driven by the radiation field strength, and (4) with the oxygen abundance, which might be linked to intrinsic dust properties. The overlap between some of these mechanisms makes it difficult to isolate their contributions, however. Further progress requires a combination of observations of a larger galaxy sample, point-like sources, spatially resolved galaxy studies, and theoretical models incorporating dust evolution in cosmological simulations.

Abstract We present, for the first time, a systematic study of quasar-associated 2175 Å dust absorbers using spectroscopic data from the Sloan Digital Sky Survey Data Release 16 (DR16). By analyzing the optical spectra and multiband magnitudes of 557,674 quasars in the redshift range of 0.7 ≤ z ≤ 2.4, we identify 843 absorbers that share the same redshifts as quasars and are believed to originate from dust in the quasar nuclei, the host galaxies, or their surrounding environments. These absorbers exhibit weak bump strengths (A bump = 0.49 ± 0.15 μm−1) and narrow widths (γ = 0.81 ± 0.14 μm−1), while their peak positions span a broad range from x 0 = 4.2 to 4.84 μm−1. Their average extinction curves resemble those of the Large Magellanic Cloud but exhibit a shallower slope. In broad-absorption-line quasars, the absorption bumps show systematic shifts in peak positions. Although further confirmation is needed, this may suggest environmental differences in dust grain properties. We find a statistically significant negative correlation between the bump strength and redshift, suggesting possible evolution in dust properties. These findings highlight the changing composition and physical conditions of dust in quasar environments, likely influenced by factors such as metallicity, radiation fields, and dust-processing mechanisms. Future studies incorporating ultraviolet and infrared data will be essential for refining the dust evolution models. Machine learning techniques and high-resolution spectroscopic follow-ups could enhance the sample completeness and provide deeper insights into the chemical properties of the dust absorbers.

Abstract The simple stellar population models produced by stellar population and spectral synthesis (SPS) codes are used as spectral templates in a variety of astrophysical contexts. In this paper, we test the predictions of four commonly used stellar population synthesis codes (YGGDRASIL, BPASS, FSPS, and a modified form of GALAXEV, which we call GALAXEVneb) by using them as spectral templates for photometric spectral energy distribution (SED) fitting with a sample of 18 young stellar clusters. All clusters have existing Hubble Space Telescope (HST) Cosmic Origins Spectrograph far-UV spectroscopy that provides constraints on their ages as well as broadband photometry from HST Advanced Camera for Surveys and Wide Field Camera 3. We use model spectra that account for both nebular and stellar emission, and additionally test four extinction curves at different values of R V . We find that for individual clusters, choice of extinction curve and SPS model can introduce significant scatter into the results of SED fitting. Model choice can introduce scatter of 34.8 Myr in age, a factor of 9.5 in mass, and 0.40 mag in extinction. Extinction curve choice can introduce scatter of up to a factor of 32.3 Myr in age, a factor of 10.4 in mass, and 0.41 mag in extinction. We caution that because of this scatter, one-to-one comparisons between the properties of individual objects derived using different SED fitting setups may not be meaningful. However, our results also suggest that SPS model and extinction curve choice do not introduce major systematic differences into SED fitting results when the entire cluster population is considered. The distribution of cluster properties for a large enough sample is relatively robust to user choice of SPS code and extinction curve.

Abstract Using the Hubble Space Telescope/Space Telescope Imaging Spectrograph, ultraviolet (UV) extinction curves have been measured in M31 along 13 new sight lines, increasing the M31 sample to 17. This sample covers a wide area of M31, having galactocentric distances of 5–16 kpc, enabling the analysis of UV extinction curve variations over a large region of an external galaxy similar to the Milky Way with global galactic characteristics such as metallicity for the first time. No correlation is found between the extinction parameters and galactocentric distance, which might be expected if there is a radial metallicity gradient in M31. Most of the new UV extinction curves presented here are significantly different from the average extinction curves of the Milky Way, Large Magellanic Cloud (LMC), and Small Magellanic Cloud (SMC), but the average M31 extinction curve is similar to the average extinction curve in the 30 Dor region of the LMC. The wide range of extinction curves seen in each individual Local Group galaxy suggests that global galactic properties such as metallicity may be less important than the local environmental conditions, such as density, UV radiation field, and shocks along each sight line. The combined behavior of the Milky Way, LMC, SMC, and now M31 UV extinction curves supports the idea that there is a family of curves in the Local Group with overlapping dust grain properties between different galaxies.

Abstract Dust is a fundamental component of the interstellar medium (ISM) within galaxies, as dust grains are highly efficient absorbers of UV and optical photons. Accurately quantifying this obscuration is crucial for interpreting galaxy spectral energy distributions (SEDs). The extinction curves in the Milky Way (MW) and Large Magellanic Cloud (LMC) exhibit a strong feature known as the 2175Å UV bump, most often attributed to small carbonaceous dust grains. This feature was recently detected in faint galaxies out to z = 7.55, suggesting rapid formation channels. Here we report the detection of a strong UV bump in a luminous Lyman-break galaxy at zprism = 7.11235, GNWY-7379420231, through observations taken as part of the NIRSpec Wide GTO survey. We fit a dust attenuation curve that is consistent with the MW extinction curve within 1σ, in a galaxy just ∼700 Myr after the Big Bang. From the integrated spectrum, we infer a young mass-weighted age (t⋆ ∼ 22 − 59 Myr) for this galaxy, however spatially resolved SED fitting unveils the presence of an older stellar population (t⋆ ∼ 252 Myr). Furthermore, morphological analysis provides evidence for a potential merger. The underlying older stellar population suggests the merging system could be pre-enriched, with the dust illuminated by a merger-induced starburst. Moreover, turbulence driven by stellar feedback in this bursty region may be driving PAH formation through top-down shattering. The presence of a UV bump in GNWY-7379420231 solidifies growing evidence for the rapid evolution of dust properties within the first billion years of cosmic time.

Dust attenuation in galaxies has often been used as a proxy for the extinction of point sources, such as supernovae, even though this approach ignores fundamental differences between the two cases. We present an analysis of the impact of geometric effects and scattering within dusty media on recovered galaxy dust properties. We employed SKIRT, a radiative transfer code, to simulate observations of point sources embedded in dust clouds, as well as spiral and elliptical galaxies. We examined various galaxy morphologies, inclinations, and instrument apertures. We find that in galaxies the scattering of light into the line of sight and the presence of sources at different depths within the galaxy make attenuation fundamentally different from extinction. For a medium with an intrinsic extinction curve slope of R_V=3.068, we recover effective attenuation curve slopes, R_ V_ eff ranging from $0.5$ to $7$, showing that the two are not analogous, even for local resolved observations. We find that R_ V_ eff greatly depends on dust density, galaxy morphology, and inclination, the latter being the most significant. A single simulated galaxy, viewed from different angles, can qualitatively reproduce the well-known relation between attenuation strength, A_ V_ eff and R_ V_ eff observed for star-forming galaxies. An increase in dust density leads to higher R_ V_ eff across all inclinations, which, assuming a correlation between stellar mass and dust density, explains the increase in R_ V_ eff with mass observed in star-forming galaxies. We cannot explain the observed differences in R_ V_ eff between star-forming (higher R_ V_ eff ) and quiescent (lower R_ V_ eff ) high-mass galaxies, suggesting intrinsically lower R_V values for ellipticals. We conclude that highly attenuated regions of simulated face-on galaxies yield R_ V_ eff within a $10%$ error of the intrinsic extinction R_V of the medium, allowing different dust types to be distinguished. For edge-on spirals, the median R_ V_ eff for low A_ V_ eff regions appears to better approximate the extinction R_V.

Abstract The dust extinction curve is typically parameterized by a single variable, R(V), in optical and near-infrared wavelengths. R(V) controls the slope of the extinction-versus-wavelength curve, and is thought to reflect the grain-size distribution and composition of dust. Low-resolution, flux-calibrated BP/RP spectra from Gaia have allowed the determination of the extinction curve along sightlines to 130 million stars in the Milky Way and Magellanic Clouds. We show that these extinction curves contain more than a single degree of freedom—that is, that they are not simply described by R(V). We identify a number of components that are orthogonal to R(V) variation, and we show that these components vary across the sky in coherent patterns that resemble interstellar medium (ISM) structure. These components encode variation in the 770 nm extinction feature, intermediate-scale and very broad structure, and a newly identified feature at 850 nm, and they likely trace both dust composition and local conditions in the ISM. Correlations of the 770 and 850 nm features with R(V) suggest that their carriers become more abundant as the carrier of the 2175 Å feature is destroyed. Our 24 million extinction-curve decompositions and feature equivalent-width measurements are publicly available at doi:10.5281/zenodo.14005028.

We present new Galactic reddening maps of the high Galactic latitude sky using DESI imaging and spectroscopy. We directly measure the reddening of 2.6 million stars by comparing the observed stellar colors in g−r and r−z from DESI imaging with the synthetic colors derived from DESI spectra from the first two years of the survey. The reddening in the two colors is on average consistent with the Fitzpatrick (1999) extinction curve with RV=3.1. We find that our reddening maps differ significantly from the commonly used Schlegel et al. (1998) (SFD) reddening map (by up to 80 mmag in E(B−V)), and we attribute most of this difference to systematic errors in the SFD map. To validate the reddening map, we select a galaxy sample with extinction correction based on our reddening map, and this yields significantly better uniformity than the SFD extinction correction. Finally, we discuss the potential systematic errors in the DESI reddening measurements, including the photometric calibration errors that are the limiting factor on our accuracy. The E(g−r) and E(r−z) maps presented in this work, and for convenience their corresponding E(B−V) maps with SFD calibration, are publicly available.

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

Abstract Accurate interpretation of observations relies on the interstellar dust extinction law, which also serves as a powerful diagnostic for probing dust properties. In this study, we investigate the multiwavelength extinction law of the quiescent, starless molecular cloud Coalsack and explore its potential variation across different interstellar environments: the surrounding region, the nearby high Galactic latitude region, the inner dense region, and the inner diffuse region. Using a sample of 368,524 dwarf stars selected from Gaia DR3 as tracers, we establish the effective temperature T eff–intrinsic color relations to derive the intrinsic color indices and optical−mid-infrared (MIR) color excess (CE) for 20 bands. Linear fits to the CE–CE diagrams provide CE ratios, which are subsequently converted into relative extinction. The resulting extinction curves for different environments exhibit steep slopes in the near-infrared (NIR) and flat profiles in the MIR. In the optical–NIR range, the Coalsack extinction law is consistent with the R V = 3.1 of S. Wang & X. Chen and B. S. Hensley & B. T. Draine, while in the MIR, it follows the R V = 5.5 of J. C. Weingartner & B. T. Draine, similar to the results of active star-forming clouds. At an angular resolution of 1 . ′ 3 , our extinction map reveals fine cloud structures. No correlation is found between R V and E B,V for E B,V > 0.3 mag, implying a uniform optical extinction law in the Coalsack cloud. The derived average R V value is 3.24 ± 0.32.

Abstract Research efforts incorporating machine learning (ML) are currently focused on developing replacements for the toxic and bio‐accumulative per‐ and polyfluorinated alkyl substances in fire suppressing foams. In the following work, ensembles of 10 artificial neural networks (ANN) were trained on a fire suppression database, described by Sudol et al., correlating area under the curve values obtained from 19‐cm gasoline and heptane pool fire extinction curves to the molecular descriptors of surfactants within various firefighting foams. These ANN model ensembles were then used to evaluate proposed surfactant structures to predict the firefighting effectiveness prior to laboratory synthesis. The two most promising surfactants were a tetrasiloxane diglucoside and a chlorotrisiloxane‐polyethyleneoxide (PEO). These surfactants were synthesized, and their fire extinction performances were assessed via 19‐cm gasoline and heptane pool fire experiments to validate the ANN predictions. The synthesis of the demonstrably high‐performing tetrasiloxane diglucoside surfactant is considered a successful ML application in the context of fluorine‐free firefighting surfactant research and development. Meanwhile, the synthesis of the low‐performing chlorinated PEO surfactant, which failed to meet predicted performance expectations, demonstrates the need for both comprehensive training data sets and the proper consideration of modeling redundancies to safeguard against unreliable ML‐derived performance predictions.

Interstellar dust grains cause extinction (absorption and scattering) of light from background astronomical sources. The spectral shape of the extinction curve depends on the dust composition. We used low-resolution optical spectra to measure the extinction curve of 130 million stars. By inverting these data, we mapped the extinction curve parameter R(V) within the Milky Way in three dimensions and within the Magellanic Clouds in two dimensions. These maps provide improved extinction corrections for astronomical observations. We find that R(V) varies with extinction, consistent with dust grains growing by accretion in low-extinction regions and by coagulation in higher-extinction regions. Star-forming regions have high R(V) values, indicating either preferential destruction of small dust grains or additional supply of large dust grains in those regions.

Optimizing gold nanorods for photonics and optoelectronics applications remains a challenging task due to the complexity of balancing efficiency, cost, and scalability in the manufacturing process. Genetic algorithms (GAs) offer a promising approach for addressing these challenges by providing high-quality solutions within feasible computational times. GAs are particularly effective for optimizing nanophotonic materials, as they can navigate extensive solution spaces and handle multiple objectives concurrently. This capability is crucial due to the interdependence of optical and geometric properties in such materials. In this work, we review recent advances in computational methods for material optimization and apply a multi-objective genetic algorithm (MOGA) to identify the optimal geometric parameters for enhancing the optical properties of gold nanorods. Our approach leverages recent literature to explore solution spaces that improve extinction curves, with results validated through statistical analysis and finite element method simulations. This research contributes to advancing the efficacy of photonic nanomaterials, which are essential for applications such as rapid light-switching devices.

Abstract The James Webb Space Telescope (JWST) observations have revolutionized extragalactic research, particularly with the discovery of little red dots (LRDs), which have been discovered as a population of dust-reddened broad-line active galactic nuclei (AGNs). Their unique V-shaped spectral feature, characterized by a red optical continuum and a UV excess in the rest frame, challenges us to discern the relative contributions of the galaxy and AGN. We study a spectral energy distribution (SED) model for LRDs from rest-frame UV to infrared bands. We hypothesize that the incident radiation from an AGN, characterized by a typical SED, is embedded in an extended dusty medium with an extinction law similar to those seen in dense regions such as Orion Nebula or certain AGN environments. The UV−optical spectrum is described by dust-attenuated AGN emission, featuring a red optical continuum at λ > 4000 Å and a flat UV spectral shape established through a gray extinction curve at λ < 3000 Å, due to the absence of small-size grains. There is no need for additional stellar emission or AGN scattered light. In the infrared, the SED is shaped by an extended dust and gas distribution (γ < 1; ρ ∝ r −γ ) with characteristic gas densities of ≃10–103 cm−3, which allows relatively cool dust temperatures to dominate the radiation. As a result, these dust structures shift the emission energy peak from near-infrared to mid-infrared bands in the rest frame; for sources at z ~ 4–7, the corresponding wavelengths shift from the JWST/MIRI to Herschel range. This model, unlike the typical AGN hot torus models, can produce an infrared SED flattening that is consistent with LRD observations through JWST MIRI. Such a density structure can arise from the coexistence of inflows and outflows during the early assembly of galactic nuclei. This might be the reason why LRDs emerge preferentially in the high-redshift Universe younger than 1 billion years.

Context. First detected in 1965, the mysterious ultraviolet (UV) extinction bump at 2175 Å is the most prominent spectroscopic feature superimposed on the interstellar extinction curve. Its carrier has remained unidentified over the six decades since its first detection, although many candidate materials have been proposed. Aims. Widely seen in the interstellar medium of the Milky Way as well as several nearby galaxies, this bump was recently also detected by the James Webb Space Telescope (JWST) at the cosmic dawn in JADES-GS-z6-0, a distant galaxy at redshift z ≈ 6.71, corresponding to a cosmic age of just 800 million years after the big bang. Differing from that of the known Galactic and extragalactic interstellar sightlines, which always peak at ~2175 Å, the bump seen at z ≈ 6.71 peaks at an appreciably longer wavelength of ~2263 Å and is the narrowest among all known Galactic and extragalactic extinction bumps. Methods. Here we show that the combined electronic absorption spectra quantum chemically computed for a number of polycyclic aromatic hydrocarbon (PAH) molecules closely reproduce the bump detected by JWST in JADES-GS-z6-0. Results. This suggests that PAH molecules had already been pervasive in the Universe at an epoch when asymptotic giant branch stars had not yet evolved to make dust.

Abstract The first all-sky, high-resolution, 3D map of the optical extinction curve of the Milky Way revealed an unexpected steepening of the extinction curve in the moderate-density, “translucent” interstellar medium (ISM). We argue that this trend is driven by growth of the total mass of polycyclic aromatic hydrocarbons (PAHs) through gas-phase accretion. We find a strong anticorrelation between the slope of the optical extinction curve—parameterized by R(V)—and maps of the PAH mass fraction (relative to the total dust mass)—parameterized by q PAH—derived from infrared emission. The range of observed q PAH indicates PAH growth by a factor of ∼2 between A V ≃ 1 and 3. This implies a factor-of-2 stronger 2175 Å feature, which is sufficient to lower R(V) by the observed amount. This level of PAH growth is possible given rapid accretion timescales and the depletion of carbon in the translucent ISM. Spectral observations by JWST would provide a definitive test of this proposed explanation of R(V) variation.

Abstract For aspherical interstellar dust grains aligned with their short axes preferentially parallel to the local magnetic field, the amount of extinction per grain is larger when the magnetic field is along the line of sight and smaller when in the plane of the sky. To the extent that optical extinction arises from both aligned and unaligned grain populations with different extinction properties, changes in the magnetic field orientation induce changes in its wavelength dependence, parameterized by R V ≡ A V /E(B − V). We demonstrate that the measured total and polarized extinction curves of the diffuse Galactic interstellar medium imply R V varies from 3.21 when the magnetic field is along the line of sight (ψ = 0) to R V = 3.05 when in the plane of the sky (ψ = 90°). This effect could therefore account for much of the large-scale R V variation observed across the sky (σ(R V ) ≃ 0.2), particularly at high Galactic latitudes.

We present a catalog of individual stellar and dust extinction properties along close to 500,000 sight lines in the southwest bar of the Small Magellanic Cloud (SMC). The catalog is based on multiband Hubble Space Telescope photometric data spanning near-ultraviolet to near-infrared wavelengths from the Small Magellanic Cloud Investigation of Dust and Gas Evolution survey (SMIDGE) covering a 100 × 200 pc area. We use the probabilistic technique of the Bayesian Extinction And Stellar Tool (BEAST) to model the spectral energy distributions of individual stars in SMIDGE and include the effects of observational uncertainties in the data. We compare BEAST-derived dust extinction properties with tracers of the interstellar medium, such as the emission from the 12CO (2–1) transition (I(CO)), the dust mass surface density (Σdust) from far-IR emission, the H i column density (N(H i)) from the 21 cm transition, and the mass fraction of polycyclic aromatic hydrocarbons (PAHs; q PAH, derived from IR emission). We find that the dust extinction (A(V)) in the SMIDGE field is strongly correlated with Σdust and I(CO), and less so with N(H i) and q PAH, and suggest potential explanations. Our extinction measurements are also sensitive to the presence of the 2175 Å bump in the extinction curve toward UV bright stars. While most do not show evidence for the bump, we identify ∼200 lines of sight that are 2175 Å bump candidates. Furthermore, we find distinct structures in the dust extinction–distance distributions that provide insights into the 3D geometry of the SMC.

Abstract The 2175Å bump is a prominent absorption feature at ultraviolet (UV) wavelengths in dust extinction and attenuation curves. Understanding the relative strength of this feature is important for making accurate dust corrections at both low- and high-redshift. This feature is postulated to arise from polycyclic aromatic hydrocarbon (PAH) dust grains; however, the carrier has not been definitively established. We present results on the correlation between the 2175Å feature and PAH abundances in a spatially-resolved manner for 15 local galaxies in the PHANGS-JWST survey that have NUV and mid-IR imaging data from Swift/UVOT and JWST/MIRI, respectively. We find a moderate positive correlation between the 2175Å feature strength and PAH abundance (Spearman’s coefficient, $0.3 \lesssim \rho \lesssim 0.5$ ), albeit with large intrinsic scatter. However, most of this trend can be attributed to a stronger negative correlation of both quantities with SFR surface density and specific-SFR (proxies of ionising radiation; $\rho\sim-0.6$ ). The latter trends are consistent with previous findings that both the 2175Å carrier and PAHs are small grains that are easily destroyed by UV photons, although the proxy for PAH abundance (based on photometry) could also be influenced by dust heating. When controlling for SFR surface density, we find weaker correlations between the 2175Å feature and PAH abundances ( $\rho \lesssim 0.3$ ), disfavouring a direct link. However, analyses based on spectroscopic (instead of photometric) measurements of the 2175Å feature and PAH features are required to verify our findings. No significant trends with gas-phase metallicity or galactocentric radii are found for the 2175Å feature and PAHs; however, the metallicity range of our sample is limited ( $8.40 \lt 12+\log[\mathrm{O/H}] \lt 8.65$ ). We provide prescriptions for the strength of the 2175Å feature and PAHs in local massive (metal-rich) galaxies with SFR surface density and specific-SFR; however, the former should be used with caution due to the fact that bump strengths measured from Swift/UVOT are expected to be underestimated.

Theoretical infrared (IR) and electronic absorption spectra of the C20 fullerene and its nitrogen-substituted heterofullerenes in gas and water solvent are studied and discussed in terms of astronomical observations. The replacement of a carbon atom by nitrogen results in two stable heterofullerenes, which is confirmed by their HOMO to LUMO energy gap. The ionization potential and electron affinity of these molecules are reported. Theoretical calculations performed at the B3LYP/6-311++G(d,p) level of density functional theory (DFT). The effect of water solvent is studied using the integral equation formalism polarized continuum model (IEFPCM) at the same level of theory. Effects of substitution on the electronic and absorption spectra of these molecules are studied. The results of the C20 fullerene and its heterofullerenes show spectra with peaks at 6.2, 6.67, 7.0, 7.7, 8.5, 11.3, and 12.8 μm, which have corresponding features in observed spectra of the planetary nebulae Tc1 and NGC 7027, and the reflection nebulae NGC 2023 and NGC 7023. The electronic absorption spectra of these molecules are also calculated by time-dependent DFT (TD-DFT) and discussed in relation to the ultraviolet bump feature at 217 nm in the interstellar extinction curve. We estimate the transition wavelength, oscillator strength, and symmetry using the AOMix program.