A THREE-DIMENSIONAL MAP OF MILKY WAY DUST

Context. High-resolution 3D maps of interstellar dust are critical for probing the underlying physics shaping the structure of the interstellar medium, and for foreground correction of astrophysical observations affected by dust. Aims. We aim to construct a new 3D map of the spatial distribution of interstellar dust extinction out to a distance of 1.25 kpc from the Sun. Methods. We leveraged distance and extinction estimates to 54 million nearby stars derived from the Gaia BP/RP spectra. Using the stellar distance and extinction information, we inferred the spatial distribution of dust extinction. We modeled the logarithmic dust extinction with a Gaussian process in a spherical coordinate system via iterative charted refinement and a correlation kernel inferred in previous work. In total, our posterior has over 661 million degrees of freedom. We probed the posterior distribution using the variational inference method MGVI. Results. Our 3D dust map has an angular resolution of up to 14′ (Nside = 256), and we achieve parsec-scale distance resolution, sampling the dust in 516 logarithmically spaced distance bins spanning 69 pc to 1250 pc. We generated 12 samples from the variational posterior of the 3D dust distribution and release the samples alongside the mean 3D dust map and its corresponding uncertainty. Conclusions. Our map resolves the internal structure of hundreds of molecular clouds in the solar neighborhood and will be broadly useful for studies of star formation, Galactic structure, and young stellar populations. It is available for download in a variety of coordinate systems online and can also be queried via the publicly available dustmaps Python package.

Context. Gaia parallaxes and photometric measurements open a three-dimensional (3D) era for the Milky Way, including its interstellar (IS) matter. Three-dimensional Galactic dust distributions are constructed in various ways, based on Gaia data and photometric or spectroscopic surveys. Aims. The assignment of radial motions to IS dust structures seen in 3D, or 3D kinetic tomography, would be a valuable tool allowing one to connect the structures to emission lines of the associated gas, which are now measured at increasingly higher spectral and angular resolutions, and rich in information on physical and chemical processes. To this end, one of the potential techniques is to establish a link between dust clouds and Doppler velocities of absorption lines imprinted in stellar spectra by the gas associated with the dust. This requires a relatively close correlation between the absorber column and the dust opacity. We have investigated the link between the strength of interstellar K I absorption and the opacity of the dust in front of stars in the Taurus area, and we have tested the feasibility of assigning velocities to 3D dust clouds on the basis of K I absorption data. Methods. We have obtained high spectral resolution and high signal-to-noise spectra of 58 early-type stars in the direction of the Taurus, Perseus, and California molecular clouds. We have developed a new, dual interstellar and telluric profile-fitting technique to extract the interstellar K I λλ 7665, 7699 Å absorption lines from stellar spectra and applied it to the new data and to archived spectra of 58 additional targets. In parallel, we have updated 3D dust maps reconstructed through the inversion of individual stellar light extinctions. To do so, we supplemented the catalog of extinction estimates based on Gaia and 2MASS photometry with recently published extinction catalogs based on stellar spectroscopic surveys. We used the 3D map and the set of velocity components seen in absorption to assign radial velocities to the dust clouds distributed along their paths in the most consistent way. Results. We illustrate our profile-fitting technique and present the K I velocity structure of the dense ISM along the paths to all targets. As a validation test of the dust map, we show comparisons between distances to several reconstructed clouds with recent distance assignments based on different techniques. Target star extinctions estimated by integration in the 3D map are compared with their K I 7699 Å absorptions and the degree of correlation is found comparable to the one between the same K I line and the total hydrogen column for stars distributed over the sky that are part of a published high resolution survey. We show images of the updated dust distribution in a series of vertical planes in the Galactic longitude interval 150–182.5° and our estimated assignments of radial velocities to the opaque regions. Most clearly defined K I absorptions may be assigned to a dense dust cloud between the Sun and the target star. It appeared relatively straightforward to find a velocity pattern consistent will all absorptions and ensuring coherence between adjacent lines of sight, at the exception of a few weak lines. We compare our results with recent determinations of the velocities of several clouds and find good agreement. These results demonstrate that the extinction-K I relationship is tight enough to allow one to link the radial velocity of the K I lines to the dust clouds seen in 3D and that their combination may be a valuable tool in building a 3D kinetic structure of the dense ISM. We discuss limitations and perspectives for this technique.

We present 2 days' measurements of the hemispherical‐directional reflectance factor (HDRF) of snow made at fine spectral and angular resolution with the Automated Spectro‐Goniometer (ASG) for the range of solar zenith angles (θ0 = 40°–50°) and snow textures (surface grain size = 80–280 μm). Measurements of the stratigraphy of snow texture and density accompanied each day's suite of measurements. These measurements represent the most detailed available in terms of angular and spectral resolution. The HDRF for fine grain, faceted snow exhibited a local backscattering peak at the view zenith near the solar zenith angle, whereas those for medium grain, clustered snow did not have a local backscattering peak. The HDRF decreased at all wavelengths for an increase in grain radius from 80 μm to 280 μm. However, the decrease in HDRF in the visible wavelengths was largest at θr = 80° in the forward direction and largest for λ > 1.8 μm near θr = 30° in the backward direction. As solar zenith angle decreased from 47° to 41°, the HDRF increased near nadir for λ ≤ 1.03 μm but decreased with coherent angular structure for λ > 1.03 μm. We compared forward radiative transfer modeling results with the HDRF measurements. The forward model used single‐scattering parameters for ice spheres with radii that matched the surface‐area‐to‐volume ratio derived from stereological analysis of snow samples and a stratigraphic distribution of optical depths from measured density and modeled extinction efficiency. All HDRF models underestimated reflectance for λ > 1.30 μm and had large absolute errors in the perpendicular plane. Mean absolute RMS errors in reflectance for the fine grain, faceted snow case were 0.09 at λ = 1.3 μm and 0.14 at λ = 1.85 μm. Mean absolute RMS errors for the medium grain, clustered snow were 0.04–0.06 at λ = 1.3 μm and 0.04–0.06 at λ = 1.85 μm. The models for the more spherical medium grain snow had better overall spectral and angular fits than those for the nonspherical fine grain snow. The spherical radii inferred from the surface‐area‐to‐volume ratio from stereological analysis of snow with nonspherical particles have a greater effective path length than the actual snow particles, resulting in underestimates of hemispherical‐directional reflectance.

Light echoes occur when light from a luminous transient is scattered by dust back into our line of sight with a time delay due to the extra propagation distance. We introduce a novel approach to estimating the distance to a source by combining light echoes with recent three-dimensional dust maps. We identify light echoes from the historical supernovae Cassiopeia A and SN 1572 (Tycho) in nearly a decade of imaging from the All-Sky Automated Survey for Supernovae (ASAS-SN). Using these light echoes, we find distances of 3.6±0.1 kpc and 3.2−0.2+0.1 kpc to Cas A and Tycho, respectively, which are generally consistent with previous estimates but are more precise. These distance uncertainties are primarily dominated by the low distance resolution of the 3D dust maps, which will likely improve in the future. The candidate single degenerate explosion donor stars B and G in Tycho are clearly foreground stars. Finally, the inferred reddening towards each SN agrees well with the intervening column density estimates from X-ray analyses of the remnants.

The typical optical–UV continuum slopes observed in many type-1 active galactic nuclei (AGNs) are redder than expected from thin accretion disk (AD) models. A possible resolution to this conundrum is that many AGNs are reddened by dust along the line of sight. To explore this possibility, we stack 5000 SDSS AGNs with luminosity and redshift in bins of optical continuum slope and width of the broad Hβ emission line. We measure the equivalent width (EW) of the NaID absorption feature in each stacked spectrum. We find a linear relation between and EW(NaID), such that EW(NaID) increases as becomes redder. In the bin with the smallest Hβ width, objects with the bluest slopes, which are similar to AD predictions, are found to have , supporting the line of sight dust hypothesis. This conclusion is also supported by the dependence of the Hα/Hβ line ratio on . The implied relationship between continuum slope and dust reddening is given by , and the implied reddening of a typical type-1 AGN with is . Photoionization calculations show that the line of sight dusty gas responsible for reddening is too ionized to produce the observed sodium features. Therefore, we argue that the sodium absorption arises in regions of the host ISM that are shielded from the AGN radiation along lines of sight to the stars, and the correlation with arises since ISM columns along shielded and non-shielded sightlines are correlated. This scenario is supported by the similarity of the relation between and the Na i column implied by our results with the relation in the Milky Way found by previous studies.

We use SDSS photometry of 73 million stars to simultaneously obtain best-fit main-sequence stellar energy distribution (SED) and amount of dust extinction along the line of sight towards each star. Using a subsample of 23 million stars with 2MASS photometry, whose addition enables more robust results, we show that SDSS photometry alone is sufficient to break degeneracies between intrinsic stellar color and dust amount when the shape of extinction curve is fixed. When using both SDSS and 2MASS photometry, the ratio of the total to selective absorption, $R_V$, can be determined with an uncertainty of about 0.1 for most stars in high-extinction regions. These fits enable detailed studies of the dust properties and its spatial distribution, and of the stellar spatial distribution at low Galactic latitudes. Our results are in good agreement with the extinction normalization given by the Schlegel et al. (1998, SFD) dust maps at high northern Galactic latitudes, but indicate that the SFD extinction map appears to be consistently overestimated by about 20% in the southern sky, in agreement with Schlafly et al. (2010). The constraints on the shape of the dust extinction curve across the SDSS and 2MASS bandpasses support the models by Fitzpatrick (1999) and Cardelli et al. (1989). For the latter, we find an $R_V=3.0\pm0.1$(random) $\pm0.1$(systematic) over most of the high-latitude sky. At low Galactic latitudes (|b|<5), we demonstrate that the SFD map cannot be reliably used to correct for extinction as most stars are embedded in dust, rather than behind it. We introduce a method for efficient selection of candidate red giant stars in the disk, dubbed "dusty parallax relation", which utilizes a correlation between distance and the extinction along the line of sight. We make these best-fit parameters, as well as all the input SDSS and 2MASS data, publicly available in a user-friendly format.

Context. Gaia data and stellar surveys open the way to the construction of detailed 3D maps of the Galactic interstellar (IS) dust based on the synthesis of star distances and extinctions. Dust maps are tools of broad use, also for Gaia-related Milky Way studies. Aims. Reliable extinction measurements require very accurate photometric calibrations. We show the first step of an iterative process linking 3D dust maps and photometric calibrations, and improving them simultaneously. Methods. Our previous 3D map of nearby IS dust was used to select low-reddening SDSS/APOGEE-DR14 red giants, and this database served for an empirical effective temperature- and metallicity-dependent photometric calibration in the Gaia G and 2MASS Ks bands. This calibration has been combined with Gaia G-band empirical extinction coefficients recently published, G, J, and Ks photometry and APOGEE atmospheric parameters to derive the extinction of a large fraction of the survey targets. Distances were estimated independently using isochrones and the magnitude-independent extinction KJ−Ks. This new dataset has been merged with the one used for the earlier version of dust map. A new Bayesian inversion of distance-extinction pairs has been performed to produce an updated 3D map. Results. We present several properties of the new map. A comparison with 2D dust emission reveals that all large dust shells seen in emission at middle and high latitudes are closer than 300 pc. The updated distribution constrains the well-debated, X-ray bright North Polar Spur to originate beyond 800 pc. We use the Orion region to illustrate additional details and distant clouds. On the large scale the map reveals a complex structure of the Local Arm. Chains of clouds of 2–3 kpc in length appear in planes tilted by ≃15° with respect to the Galactic plane. A series of cavities oriented along a l ≃ 60–240° axis crosses the Arm. Conclusions. The results illustrate the ongoing synergy between 3D mapping of IS dust and stellar calibrations in the context of Gaia. Dust maps provide prior foregrounds for future calibrations appropriate to different target characteristics or ranges of extinction, allowing us in turn to increase extinction data and produce more detailed and extended maps.

To fully exploit the potential of quasars as probes of cosmic chemical evolution and the internal gas dynamics of galaxies it is important to understand the selection effects behind the quasar samples and in particular if the selection criteria exclude foreground galaxies with certain properties (most importantly a high dust content). Here we present spectroscopic follow-up from the 10.4-m GTC telescope of a dust-reddened quasar, eHAQ0111+0641, from the extended High A_V Quasar (HAQ) survey. We find that the z=3.21 quasar has a foreground Damped Lyman-alpha Absorber (DLA) at z=2.027 along the line of sight. The DLA has very strong metal lines due to a moderately high metallicity (with an inferred lower limit of 25% of the solar metallicity), but a very large gas column density along the line-of-sight in its host galaxy. This discovery is further evidence that there is a dust bias affecting the census of metals, caused by the combined effect of dust obscuration and reddening, in existing samples of z>2 DLAs. The case of eHAQ0111+0641 illustrates that dust bias is not only caused by dust obscuration, but also dust reddening.

We have carried out a CO (J = 2 right arrow 1) survey of the Scutum Arm region of the Milky Way molecular ring. Our goals are to compare CO (J = 2 right arrow 1) maps of individual Galactic clouds with the large-scale CO (J = 2 right arrow 1) emission from the Galactic plane, and to predict the CO (J = 2 right arrow 1) appearance of a Galactic cloud ensemble in an external galaxy. The angular resolution and spatial coverage of our survey are compatible with the existing CO (J = 1 right arrow 0) survey of this region by Sanders et al., which we use for comparison. We identify 34 molecular clouds in our map region; their relationships between size and line width and between virial mass and luminosity are consistent with the relationships seen in CO (J = 1 right arrow 0) emission. However, we note that previous studies have shown considerable variation in these relationships; we attribute much of this variation to differences in their cloud definition algorithm and statistical method. We find that the median ratio of integrated emission in the two lowest CO transitions for the clouds in our sample is I(2 right arrow 1)/ I(1 right arrow 0) = 0.69, implying that the typical emitting region in the line of sight contains cold gas that is not actively star-forming. Our conclusion that the molecular ring emission is not dominated by star-forming regions is consistent with other large-scale studies of the Milky Way. Our distributions of cloud size and temperature also imply that relatively massive molecular clouds that lac k star formation, such as Maddalena's cloud, are not rare in the inner Galaxy.

We present a new three-dimensional map of dust reddening, based on Gaia parallaxes and stellar photometry from Pan-STARRS 1 and 2MASS. This map covers the sky north of a decl. of −30°, out to a distance of a few kiloparsecs. This new map contains three major improvements over our previous work. First, the inclusion of Gaia parallaxes dramatically improves distance estimates to nearby stars. Second, we incorporate a spatial prior that correlates the dust density across nearby sightlines. This produces a smoother map, with more isotropic clouds and smaller distance uncertainties, particularly to clouds within the nearest kiloparsec. Third, we infer the dust density with a distance resolution that is four times finer than in our previous work, to accommodate the improvements in signal-to-noise enabled by the other improvements. As part of this work, we infer the distances, reddenings, and types of 799 million stars. (Our 3D dust map can be accessed at doi:10.7910/DVN/2EJ9TX or through the Python package dustmaps, while our catalog of stellar parameters can be accessed at doi:10.7910/DVN/AV9GXO. More information about the map, as well as an interactive viewer, can be found at argonaut.skymaps.info.) We obtain typical reddening uncertainties that are ∼30% smaller than those reported in the Gaia DR2 catalog, reflecting the greater number of photometric passbands that enter into our analysis.

Extragalactic astronomy relies on the accurate estimation of source photometry corrected for Milky Way dust extinction. This has motivated the creation of a number of “Galactic” dust maps. We investigate whether these maps are contaminated by extragalactic signals using the clustering-redshift technique, i.e., by measuring a set of angular cross-correlations with spectroscopic objects as a function of redshift. Our tomographic analysis reveals imprints of extragalactic large-scale structure patterns in nine out of 10 Galactic dust maps, including all infrared-based maps as well as “stellar” reddening maps. When such maps are used for extinction corrections, this extragalactic contamination introduces redshift- and scale-dependent biases in photometric estimates at the millimagnitude level. It can affect both object-based analyses, such as the estimation of the Hubble diagram with supernovae, as well as spatial statistics. The bias can be appreciable when measuring angular correlation functions with low amplitudes, such as lensing-induced correlations or angular correlations for sources distributed over a broad redshift range. As expected, we do not detect any extragalactic contamination for the dust map inferred from 21 cm H i observations. Such a map provides an alternative to widely used infrared-based maps but relies on the assumption of a constant dust-to-gas ratio. We note that, using the Wide-field Infrared Survey Explorer 12 μm map sensitive to polycyclic aromatic hydrocarbons (PAHs), an indirect dust tracer, we detect the diffuse extragalactic PAH background up to z ∼ 2. Finally, we provide a procedure to minimize the level of biased magnitude corrections in maps with extragalactic imprints.

The skies of the Southern hemisphere contain many of the objects important in the studies of galactic structure and stellar evolution. Apart from the Magellanic Clouds, the two nearest galaxies, there is a most interesting area of the Milky Way - that near the direction of the galactic centre. The greater part of it can be observed at observatories situated in the Northern hemisphere, but the region between l11 = 290° and lII = 350° is inaccessible. There is a marked disparity between our knowledge of the Northern and Southern Milky Ways, and this is reflected particularly in our understanding of spiral structure, the optical interpretation of which depends almost entirely on Northern hemisphere observations.The main object of this thesis is to add to the knowledge of the structure of the Southern Milky Way by: (a) A study of the size and distribution of H II regions. (b) The studies of two star concentrations - a visual grouping of early-type stars, and a young cluster. In (a), a photographic survey for the detection of H II regions in the Southern Milky Way is carried out. Their distribution and apparent sizes as a function of galactic longitude are used to construct a qualitative representation of spiral structure which is compared with the current models. The aims of the investigations carried out in (b) are to study the characteristics of visual groupings of early-type stars and to aid the determination of distances of spiral arms in the Southern Milky Way.

The diffuse interstellar bands (DIBs) probably arise from complex organic molecules whose strength in local galaxies correlates with neutral hydrogen column density, N(H i), and dust reddening, E(B−V). Because Ca ii absorbers in quasar (QSO) spectra are posited to have high N(H i) and significant E(B−V), they represent promising sites for the detection of DIBs at cosmological distances. Here we present the results from the first search for DIBs in nine Ca ii-selected absorbers at 0.07 &amp;gt; zabs &amp;gt; 0.55. We detect the 5780-Å DIB in one line of sight at zabs= 0.1556; this is only the second QSO absorber in which a DIB has been detected. Unlike the majority of local DIB sight-lines, both QSO absorbers with detected DIBs show weak 6284-Å absorption compared with the 5780-Å band. This may be indicative of different physical conditions in intermediate redshift QSO absorbers compared with local galaxies. Assuming that local relations between the 5780-Å DIB strength and N(H i) and E(B−V) apply in QSO absorbers, DIB detections and limits can be used to derive N(H i) and E(B−V). For the one absorber in this study with a detected DIB, we derive E(B−V) = 0.23 mag and log N(H i) ≥ 20.9, consistent with previous conclusions that Ca ii systems have high H i column densities and significant reddening. For the remaining eight Ca ii-selected absorbers with 5780-Å DIB non-detections, we derive E(B−V) upper limits of 0.1–0.3 mag.

We present optical polarization data of 110 stars in the direction of NGC 6633. From multi-wavelength BVRI measurements of 64 of these stars we obtained a wavelength of maximum polarization that yielded an estimate of total-to-selective extinction ratio R V = 3.18 ± 0.04 over the observed region. The distribution of the polarization position angles overall reveals a fairly uniform Galactic magnetic field. The fractional V-band polarization of the cluster’s members is between 0.8% and 1.4%, clearly above the 0.5% average value measured for the nearby field stars in the sample. We analyze the V-band polarization and maximum polarization as a function of color excess and extinction based on dust maps, Gaia, and uvbyβ photometry to discuss the polarization efficiency. The interpretation depends on the set of color excess or extinction data used, but for the majority of stars the polarization is confined below the empirical p max ∼ 9%E(B − V) upper limit for the maximum polarization. On average, the normalized Stokes parameters show a slight offset of the foreground and background stars in comparison to the cluster’s members, but a significant overlap for many stars along the line of sight is present. We discuss a comparison of our optical polarization measurements to the Planck data in the context of the interpretation that the dust causing the infrared emission is also responsible for the optical polarization. We suggest that the polarization we measure for stars at the cluster’s distance around 400 pc is due to foreground dust between 100 and 350 pc and that multiple clouds along the line of sight could be the reason for some of the variations we observe.

We present BVRI polarimetric measurements of nine bright stars in a 15′-square region centered on Stock 19. Seven of them satisfy the Serkowski equation for star light polarization due to interstellar dust. This allowed us to estimate the degree of maximum polarization (pmax) and the wavelength of maximum polarization (λmax). Along this line of sight, pmax ranges from 0.332% to 0.948%, and the average λmax of 0.542 ± 0.018 μm yields a total-to-selective extinction ratio RV = 3.04 ± 0.11. Four of the observed stars were previously listed as high-probability members of Stock 19. However, based on their Gaia distances, they are spread out along the line of sight and show a correlation between polarization and distance that is typical for the diffuse interstellar medium. The extinction, as obtained from dust maps, and the Planck 353 GHz polarization are low for this line of sight, additionally suggesting that these stars might not represent an actual cluster. The polarization parameters of three of the observed stars indicate the possible presence of an intrinsic polarization component, likely due to circumstellar material.