Extinction Correction Research Articles

Far-Flung Filaments of Ejecta in the Young Supernova Remnant G292.0+1.8

New optical images of the young SNR G292.0+1.8, obtained from the 0.9-m telescope at CTIO, show a more extensive network of filaments than had been known previously. Filaments emitting in [O III] are distributed throughout much of the 8 arcmin diameter shell seen in X-ray and radio images, including a few at the very outermost shell limits. We have also detected four small complexes of filaments that show [S II] emission along with [OIII]. In a single long-slit spectrum we find variations of almost an order of magnitude in the relative strengths of oxygen and sulfur lines, which must result from abundance variations. None of the filaments, with or without [S II], shows any evidence for hydrogen, so all appear to be fragments of pure SN ejecta. The [S II] filaments provide the first evidence for undiluted products of oxygen burning in the ejecta from the supernova that gave rise to G292.0+1.8. Some oxygen burning must have occurred, but the paucity of [S II]-emitting filaments suggests that either the oxygen burning was not extensive or that most of its products have yet to be excited. Most of the outer filaments exhibit radial, pencil-like morphologies that suggest an origin as Rayleigh-Taylor fingers of ejecta, perhaps formed during the explosion. Simulations of core-collapse supernovae predict such fingers, but these have never before been so clearly observed in a young SNR. The total flux from the SNR in [OIII] 5007 is 5.4 * 10**-12 ergs/cm**2/s. Using a distance of 6 kpc and an extinction correction corresponding to E(B-V) = 0.6 (lower than previous values but more consistent both with our data and radio and X-ray estimates of NH), this leads to a luminosity of 1.6 * 10**35 ergs/s in the 5007 Ang. line.

The Initial Mass Functions of Four Embedded Stellar Clusters

We present near-infrared J, H, and K images of four embedded stellar clusters in the Galaxy. We find a significant fraction of pre-main-sequence stars present in at least one of the clusters. For the clusters dominated by main-sequence stars, we determine the initial mass function (IMF) both by using the K luminosity function and a global extinction correction and by deriving individual extinction corrections for each star based on their placement in the K versus H - K color-magnitude diagram. Based on our IMFs we find a significant discrepancy between the mean IMF derived via the different methods, suggesting that taking individual extinctions into account is necessary to correctly derive the IMF for an embedded cluster.

M33 X‐7: ChASeM33 Reveals the First Eclipsing Black Hole X‐Ray Binary

The first observations conducted as part of the Chandra ACIS survey of M33 (ChASeM33) sampled the eclipsing X-ray binary M33 X-7 over a large part of the 3.45 day orbital period and have resolved eclipse ingress and egress for the first time. The occurrence of the X-ray eclipse allows us to determine an improved ephemeris of mid-eclipse and binary period as HJD (2,453,639.119 ± 0.005) ± N(3.453014 ± 0.000020) and constrain the eclipse half-angle to 26°.5 ± 1°.1. There are indications for a shortening of the orbital period. The X-ray spectrum is best described by a disk blackbody spectrum typical for black hole X-ray binaries in the Galaxy. We find a flat power density spectrum, and no significant regular pulsations were found in the frequency range of 10^(-4) to 0.15 Hz. HST WFPC2 images resolve the optical counterpart, which can be identified as an O6 III star with the help of extinction and color corrections derived from the X-ray absorption. Based on the optical light curve, the mass of the compact object in the system most likely exceeds 9 M_☉. This mass, the shape of the X-ray spectrum, and the short-term X-ray time variability identify M33 X-7 as the first eclipsing black hole high-mass X-ray binary.

Photoexcitation and Relaxation Properties of a Spin-Crossover Solid in the Case of a Stable High-Spin State

The molecular solid [Fe(II)L(2)](ClO(4))(2).CH(3)CN where L is 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine provides a stable high-spin (HS) state at low temperature. Photoexcitation and subsequent relaxation have been studied using light-induced excited state spin trapping [LIESST(H --> L)] in the 700-850 nm range, determination of T(LIESST), relaxation curves at different temperatures, and temperature dependence of the light-induced spin equilibrium under constant irradiation. The measured photoinduced population of the metastable low-spin (LS) state (<30%) was drastically limited by the concomitant L --> H photoprocess. The absence of static light-induced thermal hysteresis and the stretched exponential shape of the relaxation curves respectively revealed the absence of sizable interactions and a large spreading of the activation energies attributed to the ligand flexibility. The whole data set has been simulated using a linear rate equation, with a simplified correction for the bulk extinction of light in the powder sample.

Investigating the Break in the Cepheid Period‐Luminosity Relation and Its Implications

Investigating the Break in the Cepheid Period‐Luminosity Relation and Its Implications

The Reddening of Red Supergiants: When Smoke Gets in Your Eyes

Deriving the physical properties of red supergiants (RSGs) depends on accurate corrections for reddening by dust. We use our recent modeling of the optical spectra of RSGs to address this topic. First, we find that previous broadband studies have underestimated the correction for extinction in the visible, and hence the luminosities (if derived from V); the shift in the effective wavelengths of the standard B and V bandpasses necessitates using an effective value of the ratio R'V=4.2 to correct broadband photometry of RSGs if RV=3.1 for early-type stars viewed through the same dust, where we have assumed the standard reddening law of Cardelli and coauthors. Use of the Fitzpatrick reddening law would lead to R'V=3.8, as well as slightly lower values of extinction derived from spectrophotometry, but results in slightly poorer fits. Second, we find that a significant fraction of RSGs in Galactic OB associations and clusters show up to several magnitudes of excess visual extinction compared to OB stars in the same regions; we argue that this is likely due to circumstellar dust around the RSGs. We also show that the RSG dust production rate (as indicated by the 12 ?m excess) is well correlated with bolometric luminosity, contrary to what has been found by earlier studies. The stars with the highest amount of extra visual extinction also show significant near-UV (NUV) excesses compared to the stellar models reddened by the standard reddening law. This NUV excess is likely due to scattering of the star's light by the dust and/or a larger average grain size than that typical of grains found in the diffuse interstellar medium. Similar excesses have been attributed to circumstellar dust around R Coronae Borealis stars. Finally, we estimate that the RSGs contribute dust grains at the rate of 3×10-8Msolar yr-1 kpc-2 in the solar neighborhood, comparable to what we estimate for late-type WCs, 1×10-7Msolar yr-1 kpc-2. In the solar neighborhood this represents only a few percent of the dust production (which is dominated by low-mass AGBs), but we note that in low-metallicity starbursts, dust production by RSGs would likely dominate over other sources.

Follow-up near-infrared spectroscopy of ultraluminous infrared galaxies observed by ISO

We present low resolution near-infrared spectroscopy of an unbiased sample of 24 ultraluminous infrared galaxies (ULIRGs), selected from samples previously observed spectroscopically in the mid-infrared with the Infrared Space Observatory (ISO). Qualitatively, the near-infrared spectra resemble those of starbursts. Only in one ULIRG, IRAS 04114-5117E, do we find spectroscopic evidence for AGN activity. The spectroscopic classification in the near-infrared is in very good agreement with the mid-infrared one. For a subset of our sample for which extinction corrections can be derived from Pa-alpha and Br-gamma, we find rather high Pa-alpha luminosities, in accordance with the powering source of these galaxies being star formation.[Fe] emission is strong in ULIRGs and may be linked to starburst and superwind activity. Additionally, our sample includes two unusual objects. The first, IRAS F00183-7111, exhibits extreme [Fe] emission and the second, IRAS F23578-5307, is according to our knowledge one of the most luminous infrared galaxies in H2 rotation-vibration emission.

TheuBVIPhotometric System. II. Standard Stars

Paper I of this series described the design of a CCD-based photometric system that is optimized for ground-based measurements of the size of the Balmer discontinuity in stellar spectra. This "uBVI" system combines the Thuan-Gunn u filter with the standard Johnson-Kron-Cousins BVI filters and can be used to discover luminous yellow supergiants in extragalactic systems and post–asymptotic giant branch stars in globular clusters and galactic halos. In the present paper we use uBVI observations obtained on 54 nights with 0.9 m telescopes at Kitt Peak and Cerro Tololo to construct a catalog of standardized u magnitudes for standard stars taken from the 1992 catalog of Landolt. We describe the selection of our 14 Landolt fields and give details of the photometric reductions, including red leak and extinction corrections, transformation of all the observations onto a common magnitude system, and the establishment of the photometric zero point. We present a catalog of u magnitudes of 103 stars suitable for use as standards. We show that data obtained with other telescopes can be transformed to our standard system with better than 1% accuracy.

The Spectral Irradiance of the Moon

Images of the Moon at 32 wavelengths from 350 to 2450 nm have been obtained from a dedicated observatory during the bright half of each month over a period of several years. The ultimate goal is to develop a spectral radiance model of the Moon with an angular resolution and radiometric accuracy appropriate for calibration of Earth-orbiting spacecraft. An empirical model of irradiance has been developed that treats phase and libration explicitly, with absolute scale founded on the spectra of the star Vega and returned Apollo samples. A selected set of 190 standard stars are observed regularly to provide nightly extinction correction and long-term calibration of the observations. The extinction model is wavelength-coupled and based on the absorption coefficients of a number of gases and aerosols. The empirical irradiance model has the same form at each wavelength, with 18 coefficients, eight of which are constant across wavelength, for a total of 328 coefficients. Over 1000 lunar observations are fitted at each wavelength; the average residual is less than 1%. The irradiance model is actively being used in lunar calibration of several spacecraft instruments and can track sensor response changes at the 0.1% level.

The Hα Galaxy Survey

We study the two main corrections generally applied to narrow-band Hα fluxes from galaxies in order to convert them to star formation rates, namely for [Nii] contamination and for extinction internal to the galaxy. From an imaging study using carefully chosen narrow-band filters, we find the [Nii] and Hα emission to be differently distributed. Nuclear measurements are likely to overestimate the contribution of [Nii] to total narrow-band fluxes. We find that in most star formation regions in galaxy disks the [Nii] fraction is small or negligible, whereas some galaxies display a diffuse central component which can be dominated by [Nii] emission. We compare these results with related studies in the literature, and consider astrophysical explanations for variations in the [Nii]/Hα ratio, including metallicity variations and different excitation mechanisms. We proceed to estimate the extinction towards star formation regions in spiral galaxies, firstly using Brγ/Hα line ratios. We find that extinction values are larger in galaxy nuclei than in disks, that disk extinction values are similar to those derived from optical emission-line studies in the literature, and that there is no evidence for heavily dust-embedded regions emerging in the near-IR, which would be invisible at Hα. The numbers of galaxies and individual regions detected in Brγ are small, however, and we thus exploit optical emission line data from the literature to derive global Hα extinction values as a function of galaxy type and inclination. In this part of our study we find only a moderate dependence on inclination, consistent with broad-band photometric studies, and a large scatter from galaxy to galaxy. Typical extinctions are smaller for late-type dwarfs than for spiral types. Finally, we show that the application of the type-dependent extinction corrections derived here significantly improves the agreement between star formation rates calculated using Hα fluxes and those from far-infrared fluxes as measured by IRAS. This again supports the idea that heavily dust-embedded star formation, which would be underestimated using the Hα technique, is not a dominant contributor to the total star formation rate of most galaxies in the local Universe.

On Determining Extinction from Reddening

The influence of shifts in effective wavelengths on ratios of total to selective extinction is examined, primarily to determine how to evaluate the Galactic extinction of extragalactic bodies in a way that minimizes systematic errors. In the process, a new procedure is developed for evaluating the Galactic or extragalactic extinction of any source in any filter from any index of reddening. The amount of dust along a sightline is quantified by the optical depth at 1 μm, which has the advantage of being roughly equal numerically to E(B-V). The optical depth can be derived iteratively from a color excess using an appropriate spectral energy distribution (SED) for the reddening probe, and a monochromatic law of reddening which delivers a value of AV/E(B-V) characteristic of the obscuring medium when applied to the spectrum of a reference source for which this ratio is known. Knowledge of the optical depth then facilitates the determination of the extinction of any source in any filter without concern as to the shape of the spectrum of the probe. The ratio of total to selective extinction for stars and galaxies is synthesized for a variety of filter combinations in order to examine variations with type, tilt, optical depth, and redshift. For this purpose, representative integrated SEDs spanning the space ultraviolet to the near-infrared are constructed for galaxy types E, Sab, Sbc, Scd, and Im, all at well-defined inclinations. In addition, an algorithm to adjust the shapes of the SEDs for tilt is developed. Along the main sequence, the classical ratio of total to selective extinction, AV/E(B-V), increases by 23% from O5 to M6. At late types, there are differences as high as 17% between evolved and unevolved stars. Along the Hubble sequence, AV/E(B-V) decreases by 5% from E to Im. The value for elliptical galaxies falls near the locus for the main sequence, not the giant branch. Correlated against B-I, AV/E(B-V) for star-forming galaxies is systematically lower than for stars of the same color by up to 5%. It increases much more rapidly with tilt than with the optical depth of Galactic dust, although neither dependence is strong. For both stars and galaxies, AV/E(B-V) varies dramatically with the redshift. Changes of 16% for a Type Ia supernovae and 22% for a Cepheid are seen out to z = 0.4. For elliptical galaxies, a variation of 30% can be expected out to z = 1, the precise form of which being dependent upon the ultraviolet excess. Even infrared ratios of total to selective extinction, such as AH/E(B-V), change significantly with color and redshift because of differential shifts in the effective wavelengths of B and V. As a gauge of reddening, E(V-I) is greatly preferable to E(B-V), because it is much less sensitive to color and redshift, yet more sensitive to the optical depth of dust. A demonstration is given on how to quantify upper limits to Galactic extinction which should be placed on studies of high-redshift supernovae, to reduce the redshift dependence of extinction corrections to a range that is insignificant compared with residuals supporting accelerated universal expansion. When the new technique for evaluating extinction corrections is applied to Cepheids in M31, distances for fields at different radii become less dispersed, confirming that the period-luminosity relation is not very sensitive to metallicity. However, the discrepancy between the Cepheid and maser distances to NGC 4258 cannot be attributed to the method of handling the extinction.

The radial extinction profiles of late-type galaxies

We have used UV (FOCA) and FIR (IRAS) images of six nearby late type galaxies to study the radial variation of the UV extinction (deduced from the FIR/UV ratio). We compare the UV extinction gradient with other extinction indicators (Balmer decrement) and search for a relation between the extinction, the metallicity and the gas surface density among our galaxies. We detect in our small sample a clear relation between extinction and metallicity. These observed relations are used to calibrate an empirical recipe useful for extinction correction in the UV, visible and near-infrared images of late type galaxies.

Subaru Deep Survey. V. A Census of Lyman Break Galaxies atz ≃ 4 and 5 in the Subaru Deep Fields: Photometric Properties

We investigate the photometric properties of Lyman break galaxies (LBGs) at z = 3.5-5.2 based on large samples of 2600 LBGs detected in deep (i' ≲ 27) and wide-field (1200 arcmin2) images taken in the Subaru Deep Field (SDF) and the Subaru/XMM-Newton Deep Field (SXDF) using broadband B, V, R, i', and z' filters. The selection criteria for the LBG samples are examined with 85 spectroscopically identified objects, and the completeness and contamination of the samples are estimated from Monte Carlo simulations based on a photometric-redshift catalog of the Hubble Deep Field-North. We find that these LBG samples are nearly rest-frame UV magnitude-limited samples, missing systematically only 10% of red high-z galaxies (in number), which are a dusty population with E(B - V) ≳ 0.4. We calculate luminosity functions (LFs) of the LBGs with the estimated completeness and contamination and find (1) that the number density of bright galaxies (M1700 < -22 ; corresponding to SFR ≳ 100 h M☉ yr-1 with extinction correction) decreases significantly from z = 4 to 5 and (2) that the faint-end slope of the LFs of LBGs may become steeper toward higher redshifts. We estimate the dust extinction of z ≃ 4 LBGs with M < M* (≃-21) from UV-continuum slopes and obtain E(B - V) = 0.15 ± 0.03 as the mean value. The dust extinction remains constant with apparent luminosity but increases with intrinsic (i.e., extinction-corrected) luminosity. We find no evolution in dust extinction between LBGs at z = 3 and 4. We investigate the evolution of UV-luminosity density by integrating the LFs of LBGs and find that the UV-luminosity density at 1700 Å, ρUV, does not significantly change from z = 3 to 5, i.e., ρUV(z = 4)/ρUV(z = 3) = 1.0 ± 0.2 and ρUV(z = 5)/ρUV(z = 3) = 0.8 ± 0.4; thus, the cosmic star formation rate (SFR) density (with correction for dust extinction) remains constant within the error bars, or possibly has a slight decline, from z = 3 to 5. We estimate the stellar mass density from the cosmic SFR thus obtained and find that this stellar mass density is consistent with those derived directly from the stellar mass function at z = 0-1 but exceeds those at z ~ 3 by a factor of 3. We find that the ratio of the UV-luminosity density of Lyα emitters (LAEs) to that of LBGs is ≃60% at z ≃ 5, and thus about half of star formation probably occurs in LAEs at z ≃ 5. We obtain a constraint on the escape fraction of UV ionizing photons (i.e., UV continuum in 900 Å) produced by LBGs, fesc ≳ 0.13. This implies that the escape fraction of LBGs may be larger than that of star-forming galaxies at z = 0.

The field surrounding NGC 7603: Cosmological or non-cosmological redshifts?

We present new observations of the field surrounding the Seyfert galaxy NGC 7603, where four galaxies with different redshifts - NGC 7603 (z = 0.029), NGC 7603B (z = 0.057) and two fainter emission line galaxies (z = 0.245 and z = 0.394) - are apparently connected by a narrow filament, leading to a possible case of anomalous redshift. The observations comprise broad and narrow band imaging and intermediate resolution spectroscopy of some of the objects in the field. The new data confirm the redshift of the two emission-line objects found within the filament connecting NGC 7603 and NGC 7603B, and settles their type with better accuracy. Although both objects are point-like in ground based images, using HST archive images we show that the objects have structure with a FWHM = 0.3-0.4 arcsec. The photometry in the R-band obtained during three different campaigns spread over two years does not show any signs of variability in these objects above 0.3−0. 4m ag. All the above information and the relative strength and width of the main spectral lines allow us to classify these as HII galaxies with very vigorous star formation, while the rest of the filament and NGC 7603B lack star formation. We delineate the halo of NGC 7603 out to 26.2 mag/arcsec 2 in the Sloan r band filter and find evidence for strong internal distortions. New narrow emission line galaxies at z = 0.246, 0.117 and 0.401 are also found at respectively 0.8, 1.5 and 1.7 arcmin to the West of the filament within the fainter contour of this halo. We have studied the spatial distribution of objects in the field within 1.5 arcmin of NGC 7603. We conclude that the density of QSOs is roughly within the expected value of the limiting magnitude of our observations. However, the configuration of the four galaxies apparently connected by the filament appears highly unusual. The probability of three background galaxies of any type with apparent B-magnitudes up to 16.6, 21.1 and 22.1 (the observed magnitudes, extinction correction included) being randomly projected on the filament of the fourth galaxy (NGC 7603) is ≈3 × 10 −9 . Furthermore, the possible detection of very vigorous star formation observed in the HII galaxies of the filament would have a low probability if they were background normal-giant galaxies; instead, the intensity of the lines is typical of dwarf HII galaxies. Hence, a set of coincidences with a very low probability would be necessary to explain this as a fortuitous projection of background sources. Several explanations in terms of cosmological or non-cosmological redshifts are discussed.

The Hα Luminosity Function of Morphologically Classified Galaxies in the Sloan Digital Sky Survey

We present a study of the H$\alpha$ line emission from a sample of 1482 optically-selected, morphologically-classified bright galaxies (median redshift of 0.05) derived from the Sloan Digital Sky Survey. The luminosity function is calculated for each morphological class and for the total sample. The luminosity function fitted with the Schechter form gives a slope $\alpha=-1.43\pm 0.10$ for the total sample and the H$\alpha$ luminosity density is $10^{39.31\pm 0.04{+0.10\atop-0.07}}h$ erg s$^{-1}$Mpc$^{-3}$, where the first error is statistical and the second is systematic. This value is consistent with that derived by Gallego et al. (1995), but this agreement is caused by a fortuitous cancellation of their neglect of stellar absorption that affects the estimate of extinction corrections and a significant sample incompleteness of emission line galaxies. The fraction of H$\alpha$ emitters monotonically increases from early (a few % for ellipticals) to late types (100% for irregular galaxies), whereas strong emitters exist in all classes of morphological types. We find that 83% of the luminosity density comes from spiral galaxies, 5% from irregular galaxies, and 9% from early type galaxies; a small number of morphologically disturbed galaxies contribute by 3%.

Phenomenon of extinction in textured materials analysis as a source of both physical error and information on microstructure characteristics

The correction of pole density in the analysis of textured materials by means of an original X-ray diffraction technique is proposed. In the framework of the conventional model of mosaic crystals and the dynamic theory of X-ray diffraction, a method for the separation and determination of the primary and secondary extinction parameters is developed. From these parameters, microstructural features in textured materials, such as the average domain size, the domain misorientation angle and the dislocation density at the domain boundaries are evaluated. The essential influence of the extinction phenomenon on the directly calculated pole densities and the incorrect interpretation of the texture modifications is demonstrated for the case of aluminum samples after cold rolling and different stages of thermal annealing. Conventional determination of the pole densities for low-index reflections shows a decrease in their magnitudes after the first and second stages of recrystallization; on the contrary, second-order reflections indicate an increase in the pole densities. Extinction corrections remove this inconsistency.

Electron deformation density in titanium diboride chemical bonding in TiB 2

Titanium diboride, TiB 2, crystallizes in the AlB 2-type structure, hexagonal P6/ mmm. The conventional, free atom crystal structure refinement led to R=2.23%, and including extinction corrections to R=1.58%. Multipole refinements with multipoles up to order four (hexadecapole) reduced the R value to 1.21%. Difference density maps revealed charge deficiencies on the boron sites and broadbands of charge accumulations between the boron atoms indicating a graphitic B-delocalization of the boron sp 2 hybrid orbitals.

Erratum: “Follow‐UpHubble Space Telescope/Space Telescope Imaging Spectroscopy of Three Candidate Tidal Disruption Events” (ApJ, 592, 42 [2003

The fluxes of the emission lines in the HST/STIS spectra of NGC 5905 given in Table 3 were incorrect by multiplicative factors of 2.5 from the G430L spectrum, and 0.5 from the G750M spectrum. This error does not affect the diagnostic line ratios from neighboring lines or the principal conclusions of the paper. A corrected version of Table 3 is given here. The only quantitative change in interpretation comes from the revised Balmer decrement in the nucleus of NGC 5905, H /H 1⁄4 2:50 0:34, which indicates that there is no reddening in the line of sight, and is now consistent with the lack of X-ray absorption in the ROSAT flare spectrum. Thus, we no longer make extinction corrections, and the density estimates for the lineemitting gas calculated in x 4.3 are lowered. The corrected H luminosity of 5:1 10 ergs s 1 implies fcnH 330 cm , while the corrected [O iii] luminosity of 3:5 10 ergs s 1 implies ne fc 240 cm . These constraints, in addition to the unchanged [S ii] line ratio, which requires ne 500 cm , are consistent with photoionized clouds of nH 10 and a large covering fraction fc. The soft X-ray luminosity predicted in x 4.3 for the Seyfert nucleus in NGC 5905, using the correlation of soft X-ray luminosity with H , is 9 10 ergs s , now an order of magnitude below the lowest value observed by the ROSAT PSPC (LX 2 10 ergs s ). It is still argued that the latter detection might also be partly associated with its nuclear starburst. The corrected luminosity of H is 10 times less than the peak H luminosity powered by the flare as calculated in x 4.3, while He ii k4686 is still not detected and can be assigned an upper limit of 1:5 10 ergs s . These corrected values are still evidence that the emission lines are not powered by the flare; He ii k4686 would be expected to be even stronger than H for gas ionized by the flare. In x 4.4 we referred to the possibility of ‘‘a gross error in the H /H measurement in the STIS spectrum due primarily to stellar absorption in those lines.’’ Instead, there was a numerical error, not a physical one.

Dust enshrouded star-forming activity in Arp 299

We present mid-infrared spectro-imaging ($5{-}16~\mu$m) observations of the infrared luminous interacting system Arp 299 (= Mrk 171 = IC 694+NGC 3690) obtained with the ISOCAM instrument aboard ISO. Our observations show that nearly 40% of the total emission at 7 and $15~\mu$m is diffuse, originating from the interacting disks of the galaxies. Moreover, they indicate the presence of large amounts of hot dust in the main infrared sources of the system and large extinctions toward the nuclei. While the observed spectra have an overall similar shape, mainly composed of Unidentified Infrared Bands (UIB) in the short wavelength domain, a strong continuum at ~$13~\mu$m and a deep silicate absorption band at $10~\mu$m, their differences reveal the varying physical conditions of each component. For each source, the spectral energy distribution (SED) can be reproduced by a linear combination of a UIB “canonical” spectral template and a hot dust continuum due to a $230{-}300$ K black body, after independently applying an extinction correction to both of them. We find that the UIB extinction does not vary much throughout the system ($A_{\rm V} \la5$ mag) suggesting that most UIBs originate from less enshrouded regions. IC 694 appears to dominate the infrared emission of the system and our observations support the interpretation of a deeply embedded nuclear starburst located behind an absorption of about 40 magnitudes. The central region of NGC 3690 displays a hard radiation field characterized by a [Neiii]/[Neii] ratio ≥ 1.8. It also hosts a strong continuum from 5 to $16~\mu$m which can be explained as thermal emission from a deeply embedded ($A_{\rm V}\sim60$ mag) compact source, consistent with the mid-infrared signature of an active galactic nucleus (AGN), and in agreement with recent X-ray findings.

Bright Lyman Break Galaxy Candidates in the Sloan Digital Sky Survey First Data Release

We report the discovery of six compact starburst galaxy candidates with redshifts 2.3 < z < 2.8 and r-band magnitudes 19.8-20.5 in the Quasar Catalog of the Sloan Digital Sky Survey (SDSS) First Data Release. The SDSS spectra resemble the composite spectrum of Lyman break galaxies (LBGs) at z ? 3 (albeit with some differences and broader spectral lines), but the objects are 4-5 mag brighter than an L* LBG. These objects could be extremely luminous LBGs, normal LBGs amplified by gravitational lensing, or a rare class of broad absorption line (BAL) quasars. In the first case, star formation rates inferred from the UV continuum luminosities, with no correction for dust extinction, are ~300-1000 M? yr-1, similar to those of ultraluminous infrared galaxies, but in these UV-bright objects the star formation is evidently not obscured by high dust column densities. The SDSS images show no evidence of multiple imaging or foreground lensing structures. The spectra have fairly broad absorption features and prominent high-ionization absorption but do not show high-ionization emission lines and do not resemble known BAL quasars. A rough estimate of the luminosity function of the objects is above a Schechter function extrapolation from LBGs at fainter magnitudes. Improved optical spectra and observations at X-ray, IR, and submillimeter wavelengths could help determine the nature of these remarkable objects.