Galactic Extinction Research Articles

A large-scale three-dimensional model of Galactic extinction is presented based on the Galactic dust distribution model of Drimmel and Spergel (2001). The extinction A_V to any point within the Galactic disk can be quickly deduced using a set of three-dimensional cartesian grids. Extinctions from the model are compared to empirical extinction measures, including lines-of-sight in and near the Galactic plane using optical and NIR extinction measures; in particular we show how extinction can be derived from NIR color-magnitude diagrams in the Galactic plane to a distance of 8 kiloparsec.

We have used the Far Ultraviolet Spectroscopic Explorer to search for O VI λλ1031.926, 1037.617 emission in the halos of the edge-on spiral galaxies NGC 4631 and NGC 891. In NGC 4631, we detected O VI in emission toward a soft X-ray bubble above a region containing numerous Hα arcs and filaments. The line-of-sight component of the motion of the O VI gas appears to match the underlying disk rotation. The observed O VI luminosities can account for 0.2%-2% of the total energy input from supernovae (assuming a full O VI- emitting halo) and yield mass flux cooling rates between 0.48 and 2.8 M☉ yr-1 depending on the model used in the derivations. On the basis of these findings, we believe it is likely that we are seeing cooling, galactic fountain gas. No emission was detected from the halo of NGC 891, a galaxy in a direction with considerably high foreground Galactic extinction.

High-resolution R, I, and Hα images of the center of Maffei 1, the nearest normal giant elliptical galaxy (M = -20.9), have been acquired with the Hubble Space Telescope. At its distance of 3.0 Mpc, 1 pixel in the Planetary Camera field covers only 0.66 pc. The observations reveal that Maffei 1 is a type elliptical galaxy, which is typical of intermediate to massive elliptical galaxies. A source of Hα emission has been discovered in the center. It is barely resolved, with a size of 1.2 pc and an ionized gas mass of only 29 M⊙. Dust is very prominent west of the center in a complex irregular pattern. The correlation between the reddening and the deficiency in surface brightness reveals that the dust is most likely Galactic in origin and that the reddening law along the line of sight is typical of that for the diffuse interstellar medium. For the first time, globular clusters have been detected. Twenty field sources show slightly or significantly extended profiles relative to foreground stars. In I the absolute magnitude of the brightest is -9.6, which is comparable to that of M3 in the Milky Way. The spread in R-I colors, as well the mean color, of these objects is greater than that found for Galactic globular clusters, which may be due in part to a metallicity difference compared with Galactic globular clusters and in part to the variable Galactic extinction across the Wide Field Planetary Camera 2 field. Fits of integrated analytic King models to three of the brighter candidates give core radii and concentration parameters that are comparable to those of Galactic globular clusters.

Abstract We present photometric observations of six radio-loud quasars that were detected by the COMPTEL gamma-ray telescope. The data encompass seven wavebands in the optical and near infrared. After correction for Galactic extinction, we find a wide range in optical slopes. Two sources are as blue as optically-selected quasars, and are likely to be dominated by the accretion disc emission, while three others show colours consistent with a red synchrotron component. We discuss the properties of the COMPTEL sample of quasars, as well as the implications our observations have for multiwavelength modelling of gamma-ray quasars.

We determine the extinction curve in the zl = 0.83 lens galaxy of the gravitational lens SBS 0909+532 from the wavelength dependence of the flux ratio between the lensed quasar images (zs = 1.38) from 3400 to 9200 A. It is the first measurement of an extinction curve at a cosmological distance of comparable quality to those obtained within the Galaxy. The extinction curve has a strong 2175 A feature, a noteworthy fact because it has been weak or nonexistent in most estimates of extinction curves outside the Galaxy. The extinction curve is fitted well by a standard RV = 2.1 ± 0.9 Galactic extinction curve. If we assume standard Galactic extinction laws, the estimated dust redshift of z = 0.88 ± 0.02 is in good agreement with the spectroscopic redshift of the lens galaxy. The widespread assumption that SMC extinction curves are more appropriate models for cosmological dust may be incorrect.

We present initial results for counts in cell statistics of the angular distribution of galaxies in early data from the Sloan Digital Sky Survey (SDSS). We analyze a rectangular stripe 25 wide, covering approximately 160 deg2, containing over 106 galaxies in the apparent magnitude range 18 < r' < 22, with areas of bad seeing, contamination from bright stars, ghosts, and high galactic extinction masked out. This survey region, which forms part of the SDSS early data release, is the same as that for which two-point angular clustering statistics have recently been computed. The third and fourth moments of the cell counts, s3 (skewness) and s4 (kurtosis), constitute the most accurate measurements to date of these quantities (for r' < 21) over angular scales 0015-03. They display the approximate hierarchical scaling expected from nonlinear structure formation models and are in reasonable agreement with the predictions of Λ-dominated cold dark matter models with galaxy biasing that suppresses higher order correlations at small scales. The results are, in general, consistent with previous measurements in the APM, EDSGC, and Deeprange surveys. These results suggest that the SDSS imaging data are free of systematics to a high degree and will therefore enable determination of the skewness and kurtosis to 1% and less then 10%, as predicted earlier.

A detailed knowledge of the galactic extinction is mandatory to obtain the time variation of the astrophysical/kinematical parameters outside the solar neighborhood. One may hope that the GAIA photometry itself will contribute color excesses for most of the studied objects, otherwise the extinction must be deduced from the external sources. 3-D galactic models where the dust distribution is an integral part seems to be a viable solution. Some statistical information may be obtained even from a color vs. distance diagram.

The central star V4334 Sgr (Sakurai's Object) of the planetary nebula PN G010.4+04.4 underwent in 1995–1996 the rare event of a very late helium flash. It represents only one out of two such events during the era of modern astronomy (the other event was V605 Aql = Nova Aql 1919). All the other prominent objects of that type originate from events occurring several thousands of years ago (e.g. A30, A78). Thus it is of special interest for stellar evolution theory to model the detailed observations obtained during the last four years. Those models depend essentially on basic stellar parameters like effective temperature, surface gravity and stellar radius. Most of them depend strongly on the assumed distance to the object. Some models may give some constraints on this parameter, but most of them depend on the assumption as input parameter. Hence to determine a reliable distance is of considerable significance. This should be obtained through models that give us lower and upper boundaries, or through means which are independent of models. The detailed review, by using every kind of determination available up to now, leads to a Galactic foreground extinction of EB–V =0m75 ±0.05 and a distance of D = 2.0-0.6+1.0 kpc.

A detailed knowledge about the galactic extinction is mandatory to obtain the astrophysical/kinematical parameters time variation outside the solar neighbourhood. One may hope that the GAIA photometry itself will contribute to colour excess determinations for most of the sample otherwise the extinction must be deduced from external sources. 3D galactic models where the dust distribution is an integral part seems a viable solution.

We present deep, high-resolution optical spectra of two Galactic bulge planetary nebulae (PN), M 1-42 and M 2-36. The spectra show very prominent and rich optical recombination lines (ORLs) from C, N, O and Ne ions. Infrared spectra from were also obtained using the Short and Long Wavelength Spectrometer (SWS and LWS) on board ISO. The optical and infrared spectra, together with archival IUE spectra, are used to study their density and thermal characteristics and to determine elemental abundances. We determine the optical and UV extinction curve towards these two bulge PN using observed H i and He ii recombination line fluxes and the radio free–free continuum flux density. In the optical, the reddening curve is found to be consistent with the standard Galactic extinction law, with a total to selective extinction ratio . However, the extinction in the UV is found to be much steeper, consistent with the earlier finding of Walton, Barlow & Clegg. The rich ORL spectra from C, N, O and Ne ions detected from the two nebulae have been used to determine the abundances of these elements relative to hydrogen. In all cases, the resultant ORL abundances are found to be significantly higher than the corresponding values deduced from collisionally excited lines (CELs). In M 2-36, the discrepancies are about a factor of 5 for all four elements studied. In M 1-42, the discrepancies reach a factor of about 20, the largest ever observed in a PN. M 1-42 also has the lowest Balmer jump temperature ever determined for a PN, , 5660 K lower than its [O iii] forbidden line temperature. We compare the observed intensities of the strongest O ii ORLs from different electronic configurations, including λ4649 from , λ4072 from , λ4089 from , and λ4590 and λ4190 from the doubly excited and configurations, respectively. In all cases, in spite of the fact that the ratios of the ORL to CEL ionic abundances span a wide range from ∼, the intensity ratios of λ4649, λ4072, λ4590 and λ4190 relative to λ4089 are found to be nearly constant, apart from some small monotonic increase of these ratios as a function of electron temperature. Over a range of Balmer jump temperature from , the variations amount to about 20 per cent for the and transitions and a factor of 2 for the primed transitions, and are consistent with the predictions of the current recombination theory. Our results do not support the claim by Dinerstein, Lafon & Garnett that the relative intensities of O ii ORLs vary from nebula to nebula and that the scatter is largest in objects where the discrepancies between ORL and CEL abundances are also the largest. We find that the ORL to CEL abundance ratio is highly correlated with the difference between the temperatures yielded by the [O iii] forbidden line ratio and by the H i Balmer jump, providing the strongest evidence so far that the two phenomena, i.e. the disparity between ORL and CEL temperature and abundance determinations, are closely related. However, temperature fluctuations of the type envisaged by Peimbert are unable to explain the low ionic abundances yielded by IR fine-structure lines. The very low Balmer jump temperature of M 1-42, coupled with its very low Balmer decrement density, may also be difficult to explain with a chemically inhomogeneous composite model of the type proposed by Liu et al. for NGC 6153.

The FLAIR Shapley-Hydra (FLASH) redshift survey catalogue consists of 4613 galaxies brighter than $\bJ = 16.7$ (corrected for Galactic extinction) over a 605 sq. degree region of sky in the general direction of the Local Group motion. The survey region is an approximately $60\degr \times 10\degr$ strip spanning the sky from the Shapley Supercluster to the Hydra cluster, and contains 3141 galaxies with measured redshifts. Designed to explore the effect of the galaxy concentrations in this direction (in particular the Supergalactic plane and the Shapley Supercluster) upon the Local Group motion, the 68% completeness allows us to sample the large-scale structure better than similar sparsely-sampled surveys. The survey region does not overlap with the areas covered by ongoing wide-angle (Sloan or 2dF) complete redshift surveys. In this paper, the first in a series, we describe the observation and data reduction procedures, the analysis for the redshift errors and survey completeness, and present the survey data.

We estimate the two- and three-dimensional power spectra, P2(K) and P3(k), of the galaxy distribution by applying a maximum likelihood estimator to pixel maps of the APM Galaxy Survey. The analysis provides optimal estimates of the power spectra and of their covariance matrices if the fluctuations are assumed to be Gaussian. Our estimates of P2(K) and P3(k) are in good agreement with previous work, but we find that the errors at low wavenumbers have been underestimated in some earlier studies. If the galaxy power spectrum is assumed to have the same shape as the mass power spectrum, then the APM maximum likelihood P3(k) estimates at k≤0.19 h Mpc−1 constrain the amplitude and shape parameter of a scale-invariant CDM model to lie within the 2σ ranges 0.74≤(σ8)g≤1.28 and 0.06≤Γ≤0.46. Using the Galactic extinction estimates of Schlegel, Finkbeiner & Davis, we show that Galactic obscuration has a negligible effect on galaxy clustering over most of the area of the APM Galaxy Survey.

We perform a series of comparisons between distance-independent photometric and spectroscopic properties used in the surface brightness fluctuation (SBF) and fundamental plane (FP) methods of early-type galaxy distance estimation. The data are taken from two recent surveys: the SBF Survey of Galaxy Distances and the Streaming Motions of Abell Clusters (SMAC) FP survey. We derive a relation between colour and Mg2 index using nearly 200 galaxies and discuss implications for Galactic extinction estimates and early-type galaxy stellar populations. We find that the reddenings from Schlegel et al. for galaxies with appear to be overestimated by per cent, but we do not find significant evidence for large-scale dipole errors in the extinction map. In comparison with stellar population models having solar elemental abundance ratios, the galaxies in our sample are generally too blue at a given Mg2; we ascribe this to the well-known enhancement of the α-elements in luminous early-type galaxies. We confirm a tight relation between stellar velocity dispersion σ and the SBF ‘fluctuation count’ parameter N¯, which is a luminosity-weighted measure of the total number of stars in a galaxy. The correlation between N¯ and σ is even tighter than that between Mg2 and σ. Finally, we derive FP photometric parameters for 280 galaxies from the SBF survey data set. Comparisons with external sources allow us to estimate the errors on these parameters and derive the correction necessary to bring them on to the SMAC system. The data are used in a forthcoming paper, which compares the distances derived from the FP and SBF methods.

The effect of the interstellar medium (ISM) on ultraviolet radiation is striking; there is a prominent extinction feature (a “bump”) near 2175 A (4.6 mm ). Since its discovery, the carrier of this feature has been linked to graphite dust grains. However, a study of several Galactic lines of sight using International Ultraviolet Explorer (IUE) data (E. L. Fitzpatrick & D. Massa 1988, ApJ, 328, 734) showed that the peak of this feature was tightly confined (less than 1% deviation) to 4.6 mm , while the average width of the feature was found to be about 1.0 mm 1 with approximately 10% deviation. Solid-state physics requires that, for a particle of a particular size, the width of the extinction profile cannot be changed without also changing the peak position. Therefore, graphite grains of a single shape and size cannot produce the observed feature. Other carriers have been proposed, but none can satisfy the lack of correlation between the peak and the width of the feature as noted in Fitzpatrick & Massa. As these observational constraints called into question the model of small graphite spheres as the carrier of the 2175 A feature, possible solutions were proposed. This study systematically investigates these proposed modifications to the graphite hypothesis. Most models currently in the literature focus on comparison to the Galactic average extinction curve, whereas the interesting and challenging details of this feature occur in the differences between Galactic sight lines. Our work consistently strives to meet all of the observational parameters. The major conclusions of this study are as follows: 1. New, representative optical constants for astronomical graphite can be developed using the method described by J. S. Mathis (1994, ApJ, 422, 176), so that the narrowest UV bump observed in the ISM can be reproduced. Using these properties, realistic extinction features can be generated for combinations of graphite particles of various shapes. The resulting extinction features have stationary peak positions but cannot attain the full range of widths observed in the ISM. Also, there is a correlation between peak position and width. Whereas these particles can make a significant contribution to the 2175 A feature, they cannot be the sole carriers. 2. We have demonstrated the ability of shape probability distributions to impose symmetry on the extinction profile. One or more shape distributions may exist in the ISM and would restrict the shape of the profile. Because of their asymmetric extinction profiles, grains of extreme shape (very prolate or oblate) are not favored to be the carrier of the 2175 A feature, which has implications for the mechanisms behind the polarization observed in the feature along two Galactic sight lines. 3. Using optical properties for amorphous carbon, graphite, coals, and vacuum, we determined that carbonaceous grains with core-mantle or composite structures cannot be the sole carriers of the UV feature. However, using the continuous distribution of oscillators (CDO) method (D. K. Lynch 1996, ApJ, 467, 894), optical constants for graphite in various degrees of amorphization can be constructed that can generate extinction features that satisfy all of the observational criteria which have been established for the 2175 A feature. The CDO model is similar to the model used by Mathis (1994) in that the dielectric function is represented by a Lorentzian oscillator. This model becomes more useful over a broader range of frequencies for multiple oscillators. For a distribution of different oscillators, the dielectric function may be represented as a linear superposition of the individual dielectric functions. The foundation of the CDO approximation is that the arrangement of atoms in an amorphous material is similar to the crystalline structure, with the differences, such as interatomic separations and bond angles, having a range of values represented by a statistical distribution. Therefore, a crystalline material would have plasma frequencies and damping constants represented by narrow distributions, and more amorphous materials would be represented by broader distributions. In our study, the resonance frequency was set at 2.94 mm 1 with a possible range of 0.2 mm . We then varied the damping constant in the range 0.800–1.383. By doing so, we could reproduce the UV bump along the lines of sight contained in Fitzpatrick & Massa. The CDO model is the only model in our study and in the existing astronomical literature that can satisfy all of the observational constraints. We feel that this method is an important step in the right direction for further discussion of the 2175 A feature.

Using the Canada-France-Hawaii Telescope, we have obtained deep high-resolution CCD images in V and I of a 28' × 28' field in the outer disk of M31 at ≈116' from the center along the major axis to the southwest and covering a range of projected galactocentric distance from about 23 to 33 kpc. The field was chosen to correspond with extended H I features recorded near the H I edge of the galaxy. The many tens of thousands of objects detected in this large field have been classified using an automatic algorithm that distinguishes unresolved from structures and provides photometry on them. For the most part the unresolved objects are stars in M31. The V-I colors of these stars are highly correlated with the column density of H I in the field. Assuming a Galactic extinction law, this yields a minimum extinction/atomic gas ratio about one-third of that in the solar neighborhood. The interstellar medium (ISM) in this outer disk of M31 therefore contains substantial amounts of dust. We have identified a population of B stars in the field whose distribution is also well correlated with the extended H I distribution. Evidently, star formation is both ongoing and wide spread in the outer disk of M31. According to the current view of the star formation process, molecular gas is therefore also expected to be present. The objects classified as resolved turn out to be a mix of background galaxies and overlapping images of foreground stars in M31. The counts and colors of the slightly objects in these ground-based CCD images therefore cannot be used for a reliable determination of the total extinction and reddening by the ISM in M31. However, the larger background galaxies are easily recognizable, and their surface density above a specific magnitude limit is anticorrelated with the H I column density, confirming that a relatively large amount of extinction is closely associated with the H I gas.

The gamma-ray burst (GRB) 010222 is the brightest GRB detected to date by the BeppoSAX satellite. Prompt identification of the associated optical transient (OT) allowed for spectroscopy with the Tillinghast 1.5 m telescope at F. L. Whipple Observatory while the source was still relatively bright (R ≃ 18.6 mag), within 5 hr of the burst. The OT shows a blue continuum with many superposed absorption features corresponding to metal lines at z = 1.477, 1.157, and possibly also 0.928. The redshift of GRB 010222 is therefore unambiguously placed at z ≥ 1.477. The high number of Mg II absorbers and especially the large equivalent widths of the Mg II, Mg I, and Fe II absorption lines in the z = 1.477 system further argue either for a very small impact parameter or that the z = 1.477 system is the GRB host galaxy itself. The spectral index of the OT is relatively steep, Fν ∝ ν-0.89±0.03, and this cannot be caused by dust with a standard Galactic extinction law in the z = 1.477 absorption system. This spectroscopic identification of the redshift of GRB 010222 shows that prompt and well-coordinated follow-up of bright GRBs can be successful even with telescopes of modest aperture.

Thermal infrared photometry in the L and M' bands and L-M' colors of type 1 and type 2 active galactic nuclei (AGNs) are presented. After combining our observations with photometric data at similar wavelengths taken from the literature, we find that the excess of L-M' colors of type 2 AGNs (37 sources, 50 data points) relative to type 1 AGNs (27 sources, 36 data points), due to dust extinction, is statistically detectable but very small. We next investigate the L-M' colors of type 2 AGNs by separating less dust obscured type 2 AGNs and highly dust obscured type 2 AGNs. In both cases, the L-M' colors are similar to the intrinsic L-M' color of unobscured AGNs, and the L-M' color excess of the latter highly dust obscured type 2 AGNs, due to dust extinction, is much smaller than that expected from the Galactic dust extinction curve. Contamination from starbursts and the time lag of flux variation are unlikely to explain this small L-M' color excess, which is best explained if the dust extinction curve in the close vicinity of AGNs is fairly flat at 3–5 μm, as a result of a size increase of the absorbing dust grains through coagulation.

Gravitational lensing predicts an enhancement of the density of bright, distant QSOs around foreground galaxies. We measure this QSO–galaxy correlation wqg for two complete samples of radio-loud quasars, the southern 1Jy and Half-Jansky samples. The existence of a positive correlation between z � 1 quasars and z � 0.15 galaxies is confirmed at a p = 99.0% significance level (> 99.9% if previous measurements on the northern hemisphere are included). A comparison with the results obtained for incomplete quasar catalogs (e.g. the Veron-Cetty and Veron compilation) suggests the existence of an ‘identification bias’, which spuriously increases the estimated amplitude of wqg for incomplete samples. This effect may explain many of the surprisingly strong quasar–galaxy associations found in the literature. Nevertheless, the value of wqg that we measure in our complete catalogs is still considerably higher than the predictions from weak lensing. We consider two effects which could help to explain this discrepancy: galactic dust extinction and strong lensing.

The amount and properties of high-redshift galactic and intergalactic (IG) dust are largely unknown, but could be investigated using multi-wavelength photometry of high-z objects that have a known intrinsic spectrum. Observations of gamma-ray burst (GRB) afterglows appear to support the theoretical model of an adiabatic blast wave expanding into an external medium. In this model, the synchrotron peak flux is independent of frequency, providing a flat spectrum when observed over time, and therefore allowing straightforward measurement of the relative attenuation of afterglow flux in widely separated bands. Applying this method to dust extinction, we show that for a sample of afterglows which have been corrected by galactic extinction, comparison between the number counts of peak fluxes in X-ray versus optical can provide constraints on an intergalactic component of dust. A similar technique can probe the redshift-dependence of extinction in GRB-forming regions without requiring an assumed relation between extinction and reddening by the dust. Probing systematic changes in extinction with redshift - particularly in IG and/or non-reddening dust - is crucial to a proper interpretation of the Type Ia Supernova Hubble diagram and similar observations, and useful in understanding GRB progenitor environments.

UBVRIJ photometry and optical spectra of the Type Ia SN 1996X obtained at ESO during a one-year-long observational campaign are presented, and supplemented by late time HST photometry. Spectroscopically, SN 1996X appears to be a `normal' SN Ia. The apparent magnitude at maximum was B=13.24 +/- 0.02, and the colour B-V=0.00 +/- 0.03. The luminosity decline rate, Delta m_B(15) = 1.31 +/- 0.08, is close to average for a SN Ia. The best estimate of the galactic extinction is A_B=0.30 +/- 0.05, and there is evidence that reddening within the parent galaxy is negligible. Detailed comparison of the light and colour curves of various `normal' SNe Ia shows that the assumption that multicolour light curves can be described simply as a one-parameter family is not perfect. Together with problems in the calibration of the templates, this may explain the discrepancies in the distance modulus derived adopting different calibrations of the absolute magnitude vs. light curve shape relations. Indeed we found that M_B ranges from -19.08 to -19.48 and M-m range from 32.02 to 32.48 depending on the method used (Hamuy et al. 1996a,Phillips et al. 1999, Riess et al. 1998a). Computations of model light curve and synthetic spectra for both early- and late-times, confirm that 1996X is a normal Type Ia SN and that a satisfactory fit can be obtained using a W7 progenitor structure only if we adopt the short distance. A larger distance would imply a too large Ni mass for this fainter than average SNIa.