Galactic Extinction Research Articles

FLASH redshift survey -- I. Observations and catalogue

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.

Maximum likelihood estimates of the two- and three-dimensional power spectra of the APM Galaxy Survey

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.

A synthesis of data from fundamental plane and surface brightness fluctuation surveys

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.

Investigation of the Ultraviolet Interstellar Extinction Curve

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.

Gas, Dust, and Young Stars in the Outer Disk of M31

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 Redshift of the Optical Transient Associated with GRB 010222

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 3–5 Micron Colors of Obscured and Unobscured Active Galactic Nuclei

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.

Quasar--galaxy associations revisited

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.

Gamma‐Ray Burst Afterglows as Probes of Galactic and Intergalactic Dust

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.

The template type Ia supernova 1996X

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.

Large-scale structure of galaxies in the Ophiuchus region

The large-scale structure around the Ophiuchus cluster of galaxies in the vicinity of the Galactic Centre (l=0.°5, b=9.°5, cz=8500 km s−1) is investigated on the basis of a galaxy survey and spectroscopic observations made for a 12°×17° area. The galaxy survey was performed using six ESO/SERC Sky Survey Atlas films, and 4021 galaxies were detected in total. Recession velocities were newly obtained for 179 galaxies to make the total number of galaxies in the survey area with known velocities 219. In the distribution of bright galaxies, we identified seven new clumps of galaxies. Comparing the surface number density of bright galaxies with the Galactic extinction, which is estimated from the 100-μm flux density in the IRAS Sky Survey Atlas, we demonstrate that the seven clumps are not spurious as a result of the inhomogeneity of the Galactic extinction. Among the seven clumps, two are found to be clusters and four to be groups on the basis of the histogram of recession velocities and the number of member galaxies. The Ophiuchus cluster, two newly identified clusters, and four groups are all concentrated at 9000 km s−1. Field galaxies are also distributed centred at 8500 km s−1. Hence field galaxies occupy a common three-dimensional region with galaxies in the clusters and groups, and altogether they form a large-scale structure of supercluster size. As opposed to the overdensity in the supercluster region, the mean number density of galaxies in the velocity range 0–5000 km s−1 is only 25 per cent of the mean number density of the Universe, comparable with the density of the well-known Böotes void. Hence this nearby three-dimensional region in Ophiuchus is a void of galaxies also.

FAUST observations near the North Galactic Pole

We analyse a UV observation with FAUST in the direction of the North Galactic Pole. The region includes a cirrus cloud (G251.2+73.3) and a dark globule, and the FAUST image contains 75 UV sources. We discuss the UV source detection and their identification with optical counterparts. We use, for the first time, low-resolution spectral information as the primary means of identifying possible optical counterparts. This is complemented, and sometimes modified, by optical information available from existing data bases. The results are interpreted with the help of maps of the distribution of far-infrared emission and of the neutral hydrogen gas. We discuss the types of objects found, the degree of matching with the predictions of our UV Galaxy model, and the general behaviour of the Galactic UV extinction in this Milky Way part. We compare the UV results for this region with similar observations in the same neighbourhood, which are less affected by dust, and attempt to explain the peculiar distribution of UV magnitudes as a result of a peculiar distribution of foreground dust, which does not follow the accepted dust-to-gas relation.

Shellflow. I. The Convergence of the Velocity Field at 6000 Kilometers per Second

We present the first results from the Shellflow program, an all-sky Tully-Fisher (TF) peculiar-velocity survey of 276 Sb-Sc galaxies with redshifts between 4500 and 7000 km s-1. Shellflow was designed to minimize systematic errors between observing runs and between telescopes, thereby removing the possibility of a spurious bulk flow caused by data inhomogeneity. A fit to the data yields a bulk-flow amplitude of Vbulk = 70 km s-1 (1 σ error) with respect to the cosmic microwave background, i.e., consistent with being at rest. At the 95% confidence level, the flow amplitude is <300 km s-1. Our results are insensitive to which Galactic extinction maps we use, and to the parameterization of the TF relation. The larger bulk motion found in analyses of the Mark III peculiar-velocity catalog are thus likely to be due to nonuniformities between the subsamples making up Mark III. The absence of bulk flow is consistent with the study of Giovanelli and collaborators and flow-field predictions from the observed distribution of IRAS galaxies.

Zw 1718.1-0108: a highly disturbed galaxy cluster at low Galactic latitude

We report the discovery of highly distorted X-ray emission associated with the nearby cluster Zw 1718.1–0108, one of the dominant members of which is the powerful radio galaxy 3C353. This cluster has been missed by previous X-ray cluster surveys because of its low Galactic latitude (b = 19.5°), despite its brightness in the hard X-ray band (2–10 keV flux of 1.2×10–11 erg cm−2 s−1). Our optical charge-coupled device image of the central part of the cluster reveals many member galaxies which are dimmed substantially by heavy Galactic extinction. We have measured redshifts of three bright galaxies near the X-ray emission peak and they are all found to be around z = 0.028. The ASCA gas imaging spectrometer and ROSAT high-resolution imager images show three aligned X-ray clumps embedded in low surface-brightness X-ray emission extended by ∼30 arcmin. The averaged temperature measured with ASCA is kT = 4.3±0.2 keV, which appears to be hot for the bolometric luminosity when compared with the temperature—luminosity correlation of galaxy clusters. The irregular X-ray morphology and evidence for a non-uniform temperature distribution suggest that the system is undergoing a merger of substructures. Since the sizes and luminosities of the individual clumps are consistent with those of galaxy groups, Zw 1718.1–0108 is interpreted as an on-going merger of galaxy groups in a dark matter halo forming a cluster of galaxies and thus is in a transition phase of cluster formation.

Environment of the Gamma-Ray Burst GRB 971214: A Giant H ii Region Surrounded by a Galactic Supershell.

Among a number of gamma-ray bursts whose host galaxies are known, GRB 971214 stands out for its high redshift (z>/=3) and the Lyalpha emission line having a P Cygni-type profile, which is interpreted to be a direct consequence of the expanding supershell. From a profile-fitting analysis, we estimate the expansion velocity of the supershell (vexp=1500 km s-1) and the neutral column density (NHi=1020 cm -2). The redshift z=3.418 of the host galaxy proposed by Kulkarni et al. in 1998 has been revised to be z=3.425 from our profile analysis. The observed Lyalpha profile is fitted well by a Gaussian curve, which yields the Lyalpha luminosity LLyalpha=&parl0;1.8+/-0.8&parr0;x1042 ergs s-1. Assuming that the photon source is a giant H ii region, we deduce the electron number density in the H ii region ne=&parl0;40+/-10&parr0;&parl0;L/LLyalpha&parr0;0.5&parl0;R/100 pc&parr0;-1.5 cm-3, which corresponds to the illumination by about 104 O5 stars. We estimate the star formation rate to be RSF=7+/-3 M middle dot in circle yr-1 with the internal and the Galactic extinction corrected. The theory on the evolution of supernova remnants is used to propose that the supershell is at the adiabatic phase, with its radius R=18E1&solm0;253 pc, its age t=4.7x103E1&solm0;253 yr, and the density of the ambient medium n1=5.4E-1&solm0;253 cm-3, where E53=E&solm0;1053 ergs; we estimate the kinetic energy of the supershell to be Ek=7.3x1052E53 ergs. These values are consistent with the hypothesis that the supershell is the remnant of a gamma-ray burst. We note similarities between supershells found in nearby galaxies and remote primeval galaxies and propose that the gamma-ray burst may have occurred in a giant H ii region whose environment is similar to that in star-forming galaxies.

Particle Shapes and the 2175 A Feature

The Galactic interstellar extinction curve exhibits a pronounced feature at 2175 ?. The most striking characteristic of this feature is the relatively fixed nature of its central wavelength, while the width of the feature may vary. Small graphite particles are often proposed as the carrier of this UV bump. However, graphite grains of a single size and shape cannot reproduce the observed width and peak characteristics simultaneously. Here, we investigate to what extent these characteristics may be reproduced by invoking a shape distribution of prolate and oblate graphite particles. The graphite constants calculated by Draine and Lee do not yield the narrowest 2175 ? features that have been observed. We have recalculated the dielectric function of astronomical graphite such that a spherical grain of radius 50 ? produces a feature with a peak at x0 = 4.6 ?m-1 and a width ? = 0.8 ?m-1. Using grains in the Rayleigh approximation, we have calculated extinction curves for combinations of prolate, spherical, and oblate grain shapes and successfully reproduce the stationary nature of the peak. In addition, such shape variations do give a standard deviation in the width of the feature of ~1/2 the observed standard deviation. However, the absolute values of the calculated widths fall short by ~10% compared with observed widths. The observed lack of correlation between peak position and width is not reproduced by the shape variations considered here.

The Extinction toward the GRB 970228 Field

We determine the local Galactic extinction toward the field of gamma-ray burst GRB 970228 using a variety of methods. We develop a maximum-likelihood method for measuring the extinction that compares galaxy counts in different bands in the field of interest to those in a field of known extinction, and we apply this method to the GRB 970228 field. We also measure the extinction by comparing the observed broadband colors of stars in the GRB 970228 field to the colors of library spectra of the same spectral type. Finally, we estimate the extinction using the neutral hydrogen column density and the amount of infrared dust emission toward this field. Combining the results of these methods, we find a best-fit Galactic extinction in the optical of AV = 1.09, which implies that the intrinsic afterglow of GRB 970228 is significantly brighter and bluer than it appears.

GRB 970228 and GRB 980329 and the nature of their host galaxies

We find that the local galactic extinction towards the field of gamma-ray burst GRB 970228 is , which implies a substantial dimming and change in the spectral slope of the intrinsic GRB 970228 afterglow. We measure a color for the extended source coincident with the afterglow. Taking into account our measurement of the extinction toward this field, this color implies that the extended source is most likely a galaxy undergoing star formation, in agreement with our earlier conclusion (Castander & Lamb 1998). In a separate analysis, we find that the inferred intrinsic spectrum of the GRB 980329 afterglow is consistent with the predictions of the simplest relativistic fireball model. We also find that the intrinsic spectrum of the afterglow is extincted both by dust (source frame 1 mag), and that the shape of the extinction curve is typical of young star-forming regions like the Orion Nebula but is not typical of older star-forming or starburst regions. The ≈ 2 mag drop between the R and the I bands can be explained by the far-ultraviolet non-linear component of the extinction curve if , and by the 2175 Å bump if ; other redshifts are not consistent with the observational data, given our general model.

Paschen Lines and the Reddening of the Radio Galaxy 3C 109

This paper presents spectrophotometric observations from 0.8 to 2.5 μm of the broad emission line radio galaxy 3C 109. Three Paschen lines plus Hα are measured and combined with a published value for Hβ to estimate the reddening for the continuum source and the region producing the broad emission lines. A key factor in this determination is the foreground reddening due to the Galaxy. The recent COBE/IRAS maps of Schlegel, Finkbeiner, & Davis indicate a Galactic extinction of E(B-V) = 0.57, more than twice the value inferred from the H I measurements. If the former value is adopted, the in situ reddening is E(B-V) = 0.77. This value implies, per the analysis of Goodrich & Cohen, that 3C 109 is an object of extremes, displaying a dereddened UV-optical continuum that is extraordinarily blue (fν ~ ν1.5) and harboring dust grains that are more efficient polarizers than any found in the Milky Way. Both extremes hint at some additional factor or mechanism, and a few possibilities are discussed. Regardless, the significant amount of internal reddening indicated by infrared spectroscopy is consistent with results from previous optical polarimetry and spectropolarimetry, optical and X-ray spectroscopy, and infrared photometry, in demonstrating that 3C 109 is an obscured quasar—a luminous object whose nuclear radiations from the soft X-ray to the near-infrared are absorbed, scattered, polarized, and reradiated further into the infrared by a circumnuclear envelope of dust.

Dust Properties of NGC 4753

We report on BVR surface photometry of a lenticular galaxy, NGC 4753, with prominent dust lanes. We have used the multicolor broadband photometry to study dust extinction as a function of wavelength and derived the extinction curve. We find the extinction curve of NGC 4753 to be similar to the Galactic extinction curve in the visible region, which implies that the sizes of dust grains responsible for optical extinction are similar to those in our Galaxy. We derive the dust mass from optical extinction as well as from the far-infrared fluxes observed with IRAS. The ratio of the two dust masses, Md,IRAS/Md,optical, is 2.28 for NGC 4753, which is significantly lower than the value of 8.4 ± 1.3 found previously for a large sample of elliptical galaxies. The total mass of the observed dust within NGC 4753 is about a factor of 10 higher than the mass of dust expected from loss of mass from red giant stars and destruction by sputtering and grain-grain collisions in low-velocity shocks and sputtering in supernova-driven blast waves. We find evidence for the coexistence of dust and Hα-emitting gas within NGC 4753. The current star formation rate of NGC 4753, averaged over the past 2 × 106 yr, is estimated to be less than 0.21 M⊙ yr-1. A substantial amount of dust within NGC 4753 exists in the form of cirrus.