Continuum Slope Research Articles

Hα Observations of a Large Sample of Galaxies atz∼ 2: Implications for Star Formation in High‐Redshift Galaxies

Using Hα spectra of 114 rest-frame UV-selected galaxies at z ~ 2, we compare inferred star formation rates (SFRs) with those determined from the UV continuum luminosity. After correcting for extinction using standard techniques based on the UV continuum slope, we find excellent agreement between the indicators, with = 31 M_☉ yr^(-1) and = 29 M_☉ yr^(-1). The agreement between the indicators suggests that the UV luminosity is attenuated by a typical factor of ~4.5 (ranging from no attenuation to a factor of ~100 for the most obscured object in the sample), in good agreement with estimates of obscuration from X-ray, radio, and mid-IR data. The Hα luminosity is attenuated by a factor of ~1.7 on average, and the maximum Hα attenuation is a factor of ~5. In agreement with X-ray and mid-IR studies, we find that the SFR increases with increasing stellar mass and at brighter K magnitudes to ~ 60 M_☉ yr^(-1) for galaxies with K_s 10^(11) M_☉) have had higher SFRs in the past.

The probability distribution function of the Lyman transmitted flux from a sample of Sloan Digital Sky Survey quasars

We present a measurement of the probability distribution function (PDF) of the transmitted flux in the Lyman a (Lya) forest from a sample of 3492 quasars included in the Sloan Digital Sky Survey data release 3 (SDSS DR3). Our intention is to investigate the sensitivity of the Lya flux PDF as measured from low-resolution and low signal-to-noise ratio data to a number of systematic errors such as uncertainties in the mean flux, continuum and noise estimate. . The quasar continuum is described by the superposition of a power law and emission lines. We perform a power-law continuum fitting on a spectrum-by-spectrum basis, and obtain an average continuum slope of α v = 0.59 ± 0.36 in the redshift range 2.5 < z < 3.5. We take into account that the variation in the continuum indices increases the mean flux by 3 and 7 per cent at z = 3 and 2.4, respectively, as compared to the values inferred with a single (mean) continuum slope. We compare our measurements to the PDF obtained with mock lognormal spectra, whose statistical properties have been constrained to match the observed Lya flux PDF and power spectrum of high-resolution data. Using our power-law continuum fitting and the SDSS pipeline noise estimate yields a poor agreement between the observed and mock PDFs. Allowing for a break in the continuum slope and, more importantly, for residual scatter in the continuum level substantially improves the agreement. A decrease of ∼10-15 per cent in the mean quasar continuum with a typical rms variance at the 20 per cent level can account for the data, provided that the noise excess correction is no larger than ≤10 per cent.

The Active Galactic Nuclei Contribution to the Mid‐Infrared Emission of Luminous Infrared Galaxies

We determine the contribution of AGN to the mid-IR emission of luminous infrared galaxies (LIRGs) at z > 0.6 by measuring the mid-IR dust continuum slope of 20,039 mid-IR sources. The 24 μm sources are selected from a Spitzer MIPS survey of the NOAO Deep Wide-Field Survey Bootes field and have corresponding 8 μm data from the IRAC Shallow Survey. There is a clear bimodal distribution in the 24 to 8 μm flux ratio. The X-ray-detected sources fall within the peak corresponding to a flat spectrum in νfν, implying that it is populated by AGN-dominated LIRGs, whereas the peak corresponding to a higher 24 to 8 μm flux ratio is likely due to LIRGs whose IR emission is powered by starbursts. The 24 μm emission is increasingly dominated by AGN at higher 24 μm flux densities (f24): the AGN fraction of the z > 0.6 sources increases from 9% at f24 ≈ 0.35 mJy to 74% ± 20% at f24 ≈ 3 mJy, in good agreement with model predictions. Deep 24 μm, small-area surveys, like GOODS, will be strongly dominated by starburst galaxies. AGN are responsible for ~3%-7% of the total 24 μm background.

Spatially Resolved Mid‐Infrared Spectroscopy of NGC 1068: The Nature and Distribution of the Nuclear Material

We present spatially resolved, near-diffraction-limited 10 μm spectra of the nucleus of the Seyfert 2 galaxy NGC 1068, obtained with Michelle, the mid-IR imager and spectrometer on the 8.1 m Gemini North Telescope. The spectra cover the nucleus and the central 60 × 04 of the ionization cones at a spatial resolution of approximately 04 (≈30 pc). The spectra extracted in 04 steps along the slit reveal striking variations in continuum slope, silicate feature profile and depth, and fine-structure line fluxes on subarcsecond scales, illustrating in unprecedented detail the complexity of the circumnuclear regions of NGC 1068 at mid-IR wavelengths. A comparison of photometry in various apertures reveals two distinct components: a compact (radius < 15 pc), bright source within the central 04 × 04 and extended, lower brightness emission. We identify the compact source with the AGN-obscuring torus, and the diffuse component with dust in the ionization cones. While the torus emission dominates the flux observed in the near-IR, the mid-IR flux measured with apertures larger than about 1'' is dominated instead by emission from the ionization cones; despite its higher brightness, the torus contributes <30% of the 11.6 μm flux in the central 12 region. Many previous attempts to determine the torus spectral energy distribution are thus likely to be significantly affected by contamination from the extended emission. The observed spectrum of the compact source is compared with clumpy torus models. The models require most of the clouds to be located within a few parsecs of the central engine, in agreement with recent mid-IR interferometric observations. We also present a UKIRT/CGS4 5 μm spectrum covering the R(0)-R(4) lines of the fundamental vibration-rotation band of 12CO. None of these lines was detected, and we discuss these nondetections in terms of the filling factor and composition of the nuclear clouds.

Near-infrared spectra of the leading and trailing hemispheres of Enceladus

We present individual spectra 0.8–2.5 μm of the leading and trailing hemispheres of Enceladus obtained with the CorMASS spectrograph on the 1.8 m Vatican Advanced Technology Telescope (VATT) at the Mount Graham International Observatory. While the absorption bands of water ice dominate the spectrum of both hemispheres, most of these bands are stronger on the leading hemisphere than the trailing hemisphere. In addition, longward of 1 μm, the continuum slope is greater on the leading hemisphere than the trailing hemisphere. These differences could be produced by the presence of particles on the trailing side that are smaller and/or microstructurally more complex than those on the leading side, consistent with the preferential erosion or structural degradation of regolith particle grains on the trailing side by magnetospheric sweeping. We also explore compositional differences between the two hemispheres by applying Hapke spectrophotometric mixture models to the spectra whose components include water ice and ammonia hydrate (1% NH 3⋅H 2O). We find that spectral models which include as much as 25% by weight ammonia hydrate intimately mixed with water ice and covering 80% of the illuminated area of the satellite fit the observed spectrum of both the leading and trailing hemispheres. Areal (checkerboard) mixing models of ammonia hydrate and water ice fit the leading hemisphere with 15% of the surface comprised of ammonia hydrate and the trailing hemisphere with 10% ammonia hydrate. Therefore, while these spectral data do not contain an unambiguous detection of ammonia hydrate on Enceladus, our spectral models do not preclude the presence of a modest amount of 1% NH 3⋅H 2O on both hemispheres. We examine spectral differences and similarities between both hemispheres and the tenuous E ring within which Enceladus orbits. The spectral resolution ( R = λ / Δ λ ) of these CorMASS data ( R ∼ 300 ) is comparable to but nevertheless higher than that of the Visual-Infrared Mapping Spectrometer (VIMS) ( R = 225 ) onboard the Cassini spacecraft.

The Space Density and Colors of Massive Galaxies at 2 &lt; z &lt; 3: The Predominance of Distant Red Galaxies

Using the deep multiwavelength MUSYC, GOODS, and FIRES surveys we construct a stellar mass-limited sample of galaxies at 2 1011 M☉ distributed over four independent fields with a total area of almost 400 arcmin2. The mean number density of massive galaxies in this redshift range ρ(M > 1011 M☉) = (2.2 ± 0.6) × 10-4 h Mpc-3. We present values and 25th and 75th percentiles for the distributions of observed RAB magnitudes, observed J - Ks colors, and rest-frame ultraviolet continuum slopes, M/LV ratios, and U - V colors. The galaxies show a large range in all these properties. The median galaxy is faint in the observer's optical (RAB = 25.9), red in the observed near-IR (J - Ks = 2.48), has a rest-frame UV spectrum that is relatively flat in Fλ (β = -0.4), and rest-frame optical colors resembling those of nearby spiral galaxies (U - V = 0.62). We determine which galaxies would be selected as Lyman break galaxies (LBGs) or distant red galaxies (DRGs, having J - Ks > 2.3) in this mass-limited sample. By number DRGs make up 69% of the sample, and LBGs 20%, with a small amount of overlap. By mass DRGs make up 77%, and LBGs 17%. Neither technique provides a representative sample of massive galaxies at 2 < z < 3 as they only sample the extremes of the population. As we show here, multiwavelength surveys with high-quality photometry are essential for an unbiased census of massive galaxies in the early universe. The main uncertainty in this analysis is our reliance on photometric redshifts; confirmation of the results presented here requires extensive near-infrared spectroscopy of optically faint samples.

XMM-Newton spectroscopy of high-redshift quasars

XMM‐Newton observations of 29 high-redshift (z > 2) quasars, including seven radio-quiet, 16 radio-loud and six broad absorption line (BAL) objects, are presented; due to the high redshifts, the rest-frame energy bands extend up to ∼30‐70 keV. Over 2‐10 keV, the quasars can be well fitted in each case by a simple power law, with no strong evidence for iron emission lines. The lack of iron lines is in agreement both with dilution by the radio jet emission (for the radio-loud quasars) and the X-ray Baldwin effect. No Compton reflection humps at higher energies (i.e. above 10 keV in the rest frame) are detected either. Over the broad-band (0.3‐ 10 keV), approximately half (nine out of 16) of the radio-loud quasars are intrinsically absorbed, with the values of NH generally being 1 × 10 22 to 2 × 10 22 cm −2 in the rest frames of the objects. None of the seven radio-quiet objects shows excess absorption, while four of the six BAL quasars are absorbed. The radio-loud quasars have flatter continuum slopes than their radio-quiet counterparts (� RL ∼ 1.55; � RQ ∼ 1.98 over 2‐10 keV), while, after modelling the absorption, the underlying photon index for the six BAL quasars is formally consistent with the non-BAL radio-quiet objects. Ke yw ords: galaxies: active ‐ quasars: general ‐ X-rays: galaxies.

Extreme X-Ray Behavior of the Low-Luminosity Active Nucleus in NGC 4395

We present the results of a 17 ks Chandra observation of the nearby dwarf spiral galaxy NGC 4395, focusing on the X-ray properties of the moderate-mass black hole that resides in its nucleus. Chandra affords the first high-quality, broadband X-ray detection of this object that is free of contamination from nearby sources in the field. We find that the nuclear X-ray emission is unresolved in the Chandra image and confirm the rapid, large-amplitude X-ray variability reported in previous studies. The spectrum of the nuclear source shows evidence for absorption by an ionized medium. There is also evidence for spectral variability over the course of the Chandra observation, although contrary to prior reports, it appears to be uncorrelated with fluctuations in the hard X-ray count rate. It is possible that the short-term spectral variability results from column density changes in the ionized absorber. By far the most unusual high-energy property of NGC 4395 is the shape of its spectrum above 1 keV. The Chandra data indicate a power-law photon index of Gamma \approx 0.6, which is much flatter than the Gamma \approx 1.8 X-ray spectra typical of active galactic nuclei and the slope of the nuclear X-ray spectrum measured from an earlier ASCA observation of NGC 4395. This extreme flatness and dramatic long-term variability of the X-ray spectrum are unprecedented among active galactic nuclei. A variety of possibilities for the origin of the flat continuum slope are considered; none provides a fully satisfactory explanation.

The XMM-Newton spectrum of the high-zoptically-obscured QSO RX J1343.4+0001: a classic radio quiet QSO

We present a 30 ks XMM-Newton observation of the z = 2.35 type II radio quiet quasar RX J1343.4+0001. These data provide the first good quality X-ray spectrum for this object. We measured a continuum slope r = 1.85 ± 0.10 with only an upper limit on the column density of the absorbing material of N z H ≤ 10 22 cm -2 as well as an Fe Ka emission line at the 3σ confidence level. We do not find therefore a highly absorbed object nor a truly flat spectrum as suggested on the basis of previous less sensitive ROSAT and ASCA measurements. The N z H upper limit is fully consistent with the optical extinction 3 < A v < 10 inferred from IR observations. The Fe Ka line is consistent with fluorescence from neutral iron and, noteworthy, is one of the most distant observed so far. The X-ray spectral properties of RX J1343.4+0001 agree well with the steep continuum slope (Γ 1.9) being independent of increasing redshift and luminosity as inferred by X-ray studies of large samples of RQ QSOs.

Properties of Lyα emitters around the radio galaxy MRC 0316–257

Observations of the radio galaxy MRC 0316–257 at $z = 3.13$ and the surrounding field are presented. Using narrow- and broad-band imaging obtained with the VLT, 77 candidate Ly α emitters with a rest-frame equivalent width of >15 A were selected in a ∼ 7´$\,\times\,7$´ field around the radio galaxy. Spectroscopy of 40 candidate emitters resulted in the discovery of 33 emission line galaxies of which 31 are Ly α emitters with redshifts similar to that of the radio galaxy, while the remaining two galaxies turned out to be [$\ion{O}{ii}$] emitters. The Ly α profiles have widths ( FWHM ) in the range of 120–800 km s -1 , with a median of 260 km s -1 . Where the signal-to-noise was large enough, the Ly α profiles were found to be asymmetric, with apparent absorption troughs blueward of the profile peaks, indicative of absorption along the line of sight of an $\ion{H}{i}$ mass of at least $2 \times 10^2 {-} 5 \times 10^4$ M $_{\odot}$. Besides that of the radio galaxy and one of the emitters that is a QSO, the continuum of the emitters is faint, with luminosities ranging from 1.3 L * to L * . The colors of the confirmed emitters are, on average, very blue. The median UV continuum slope is $\beta = -1.76$, bluer than the average slope of LBGs with Ly α emission ($\beta \sim -1.09$). A large fraction of the confirmed emitters ( ∼ 2/3) have colors consistent with that of dust-free starburst galaxies. Observations with the Advanced Camera for Surveys on the Hubble Space Telescope show that the emitters that were detected in the ACS image have a range of different morphologies. Four Ly α emitters ( ∼ 25%) were unresolved with upper limits on their half light radii of $r_{\rm h} ∼ 19%) show multiple clumps of emission, as does the radio galaxy, and the rest ( ∼ 56%) are single, resolved objects with $r_{\rm h} α emitters are on average smaller than LBGs. The average star formation rate of the Ly α emitters is 2.6 M $_{\odot}\;{\rm yr}^{-1}$ as measured by the Ly α emission line or M $_{\odot}\;{\rm yr}^{-1}$ as measured by the UV continuum. The properties of the Ly α galaxies (faint, blue and small) are consistent with young star forming galaxies which are still nearly dust free. The volume density of Ly α emitting galaxies in the field around MRC 0316–257 is a factor of $3.3^{+0.5}_{-0.4}$ larger compared with the density of field Ly α emitters at that redshift. The velocity distribution of the spectroscopically confirmed emitters has a dispersion of 640 km s -1 , corresponding to a FWHM of 1510 km s -1 , which is substantially smaller than the width of the narrow-band filter (${\it FWHM} \sim 3500$ km s -1 ). The peak of the velocity distribution is located within 200 km s -1 of the redshift of the radio galaxy. We conclude that the confirmed Ly α emitters are members of a protocluster of galaxies at $z \sim 3.13$. The size of the protocluster is unconstrained and is larger than $3.3 \times 3.3$ Mpc 2 . The mass of this structure is estimated to be >$3{-}6 \times 10^{14}$ M $_{\odot}$ and could be the progenitor of a cluster of galaxies similar to e.g. the Virgo cluster.

Spectral Classification of Quasars in the Sloan Digital Sky Survey: Eigenspectra, Redshift, and Luminosity Effects

We study 16,707 quasar spectra from the Sloan Digital Sky Survey (SDSS) (an early version of the First Data Release; DR1) using the Karhunen-Loeve transform (or principal components analysis). The redshifts of these quasars range from 0.08 to 5.41, the i-band absolute magnitudes from -30 to -22, and the resulting rest-frame wavelengths from 900 to 8000 A. The quasar eigenspectra of the full catalog reveal the following: first order—the mean spectrum; second order—a host-galaxy component; third order—the UV-optical continuum slope; fourth order—the correlations of Balmer emission lines. These four eigenspectra account for 82% of the total sample variance. Broad absorption features are found not to be confined in one particular order but to span a number of higher orders. We find that the spectral classification of quasars is redshift and luminosity dependent; as such there does not exist a compact set (i.e., less than ≈10 modes) of eigenspectra (covering 900–8000 A) that can describe most variations (i.e., greater than ≈95%) of the entire catalog. We therefore construct several sets of eigenspectra in different redshift and luminosity bins. From these eigenspectra we find that quasar spectra can be classified (by the first two eigenspectra) into a sequence that is defined by a simple progression in the steepness of the slope of the continuum. We also find a dependence on redshift and luminosity in the eigencoefficients. The dominant redshift effect is a result of the evolution of the blended Fe II emission (optical) and the Balmer continuum (the small bump, λrest ≈ 2000–4000 A). A luminosity dependence is also present in the eigencoefficients and is related to the Baldwin effect—the decrease of the equivalent width of an emission line with luminosity, which is detected in Lyα, Si IV+O IV], C IV, He II, C III] and Mg II, while the effect in N V seems to be redshift dependent. If we restrict ourselves to the rest-wavelength regions 1150–2000 A and 4000–5500 A, the eigenspectra constructed from the wavelength-selected SDSS spectra are found to agree with the principal components by Francis et al. and the well-known Eigenvector-1 of Boroson & Green, respectively. ASCII formatted tables of the eigenspectra are available.

Linking the Power Sources of Emission‐Line Galaxy Nuclei from the Highest to the Lowest Redshifts

This dissertation searches for common grounds for the diversity of properties exhibited by the emission-line nuclei of galaxies, from large look-back times to the local universe. I present results of (1) a program of high signal-to-noise spectroscopy for 44 quasars using the MMT and Keck obz 4 servatories; (2) a detailed analysis of the ultraviolet and optical spectral behavior of 22 narrow-line Seyfert 1 (NLS1) galaxies based on archival Hubble Space Telescope (HST) spectra; (3) an in-depth investigation of the proposed link between NLS1s and quasars by means of comparison of composite z 4 spectra and a principal component analysis; (4) a simulation of Seyferts/quasars designed to explore the role of dust in modifying their observed spectral energy distribution; and (5) a sensitive search for accretion signatures in a large sample of nearby emission-line galaxy nuclei, employing a quantitative comparison of the nebular line flux ratios in small (HST) and large (ground-based) apertures. The lowand high-redshift quasars are found to be very similar in their emission characteristics, although differences exist. In particular, the data bolster indications of supersolar metallicities in the luminous, sources, which supports z 4 scenarios that assume substantial star formation concurrent or preceding the quasar phenomena. An in-depth analysis of quasar selection effects is performed, based on comparisons of distributions of equivalent widths for quasar ensembles characterized by different detection techniques. The measurements are consistent with a common parent population, suggesting that the trends identified here are representative of high-z QSOs. The quasars are found to be more metal enriched and z 4 more spectroscopically heterogeneous than the NLS1s, indicating that a close connection between these objects remains doubtful. NLS1s reveal redder UV-blue continua than those measured in other quasars and Seyferts. The sources with UV line absorption are in general less powerful and show redder spectra, suggesting that luminosity-dependent dust absorption may be important in modifying their continua. A recedingtorus–like geometry seems to explain these trends and other observed correlations between quasar luminosity and continuum slope. This relatively simple framework also predicts a number of absorbed AGNs that is consistent with recent modeling and observations of the X-ray background light and extragalactic point source population. Finally, in most of the nearby emission-line nuclei, the expected increased AGN-like behavior at smaller scales is not seen, although the nuclear emission is resolved. This suggests that these sources are not necessarily powered by accretion onto a compact object and that the composite model proposed for the LINER/H ii transition nuclei (which assumes a central accreting-type nucleus surrounded by star-forming regions) is not generally supported. Only about 10% of the narrow-line objects reveal broad Ha features, thus changing their spectral classification from type 2 to type 1.

What controls the C iv line profile in active galactic nuclei?

The high-ionization lines in active galactic nuclei (AGN), such as C iv, tend to be blueshifted with respect to the lower-ionization lines, such as Hβ, and often show a strong blue excess asymmetry not seen in the low-ionization lines. There is accumulating evidence that the Hβ profile is dominated by gravity, and thus provides a useful estimate of the black hole mass in AGN. The shift and asymmetry commonly seen in C iv suggest that non-gravitational effects, such as obscuration and radiation pressure, may affect the line profile. We explore the relation between the Hβ and C iv profiles using the ultraviolet (UV) spectra available for 81 of the 87 z≤ 0.5 PG quasars in the Boroson & Green sample. We find the following. (1) Narrow C iv lines (full width at half-maximum, FWHM < 2000 km s−1) are rare (∼2 per cent occurrence rate) compared with narrow Hβ lines (∼20 per cent). (2) In most objects where the Hβ FWHM < 4000 km s−1 the C iv line is broader than Hβ, but the reverse is true when the Hβ FWHM > 4000 km s−1. This argues against the view that C iv generally originates closer to the centre, compared with Hβ. (3) C iv appears to provide a significantly less accurate, and possibly biased estimate of the black hole mass in AGN, compared with Hβ. (4) All objects where C iv is strongly blueshifted and asymmetric have a high L/LEdd, but the reverse is not true. This suggests that a high L/LEdd is a necessary but not sufficient condition for generating a blueshifted asymmetric C iv emission. (5) We also find indications for dust reddening and scattering in ‘normal’ AGN. In particular, PG quasars with a redder optical–UV continuum slope show weaker C iv emission, stronger C iv absorption and a higher optical continuum polarization.

Free‐Free Spectral Energy Distributions of Hierarchically Clumped HiiRegions

In an effort to understand unusual power-law spectral slopes observed in some hypercompact HII regions, we consider the radio continuum energy distribution from an ensemble of spherical clumps. An analytic expression for the free-free emission from a single spherical clump is derived. The radio continuum slope (with F_\nu \nu^\alpha) is governed by the population of clump optical depths N(tau), such that (a) at frequencies where all clumps are thick, a continuum slope of +2 is found, (b) at frequencies where all clumps are optically thin, a flattened slope of -0.11 is found, and (c) at intermediate frequencies, a power-law segment of significant bandwidth with slopes between these two limiting values can result. For the ensemble distribution, we adopt a power-law distribution N(tau) tau^{-\gamma}, and find that significant power-law segments in the SED with slopes from +2 to -0.11 result only for a relatively restricted range of $\gamma$ values of 1 to 2. Further, a greater range of clump optical depths for this distribution leads to a wider bandwidth over which the intermediate power-law segment exists. The model is applied to the source W49N-B2 with an observed slope of \alphab +0.9, but that may be turning over to become optically thin around 2 mm. An adequate fit is found in which most clumps are optically thin and there is little shadowing of rearward clumps by foreground clumps (i.e., the geometrical covering factor C<<1). The primary insight gained from our study is that in the Rayleigh-Jeans limit for the Planck function that applies for the radio band, it is the distribution in optical depth of the clump population that is solely responsible for setting the continuum shape, with variations in the size and temperature of clumps serving to modulate the level of free-free emission.

XMM – Newton observations of the Seyfert 1 galaxy ESO 141-G55

We report on an observation of the Seyfert 1 galaxy ESO 141–G55 performed in October 2001 with the EPIC MOS cameras and Reflection Grating Spectrometers (RGS) on board the XMM – Newton observatory. We find the hard (3–10 keV) continuum slope, including reflection, to be somewhat flatter (Γ=1.72±0.06) than that of a typical broad-line Seyfert 1 galaxy. The spectrum shows a weak emission line at 6.45 ± 0.04 keV with a measured equivalent width of ≈ 40 eV. A broad spectral feature is observed around 7 keV that can be fitted by an absorption edge at 7.6 ± 0.1 keV. The extrapolation of the primary power law continuum to energies lower than 2 keV indicates the presence of a soft excess component contributing 45± 3% of the overall flux in the 0.3–2.0 keV energy range. This soft-excess cannot be explained solely by enhanced reflection from a highly ionized disk.

Rapid Oscillations in Cataclysmic Variables. XVI. DW Cancri

ABSTRACTWe report photometry and spectroscopy of the nova‐like variable DW Cancri. The spectra show the usual broad H and He emission lines, with an excitation and continuum slope characteristic of a moderately high accretion rate. A radial‐velocity search yields strong detections at two periods, 86.1015(3) minutes and 38.58377(6) minutes. We interpret these as respectively the orbital period Porb of the binary, and the spin period Pspin of a magnetic white dwarf. The light curve also shows the spin period, plus an additional strong signal at 69.9133(10) minutes, which coincides with the difference frequency 1/Pspin - 1/Porb. These periods are stable over the 1 year baseline of measurement. This triply‐periodic structure mimics the behavior of several well‐credentialed members of the “DQ Herculis” (intermediate polar) class of cataclysmic variables. DQ Her membership is also suggested by the mysteriously strong sideband signal (at νspin - νorb), attesting to a strong pulsed flux at X‐ray/EUV/UV wavelengths. DW Cnc is a new member of this class, and would be an excellent target for extended observation at these wavelengths.

HE 0047-1756: A new gravitationally lensed double QSO

The quasar HE 0047-1756, at z=1.67, is found to be split into two images 1.44" apart by an intervening galaxy acting as a gravitational lens. The flux ratio for the two components is roughly 3.5:1, depending slightly upon wavelength. The lensing galaxy is seen on images obtained at 800 nm and 2.1 \mu; there is also a nearby faint object which may be responsible for some shear. The spectra of the two quasar images are nearly identical, but the emission line ratio between the two components scale differently from the continuum. Moreover, the fainter component has a bluer continuum slope than the brighter one. We argue that these small differences are probably due to microlensing. There are hints of an Einstein ring emanating from the brighter image toward the fainter one.

Seasonal change in Titan's haze 1992–2002 from Hubble Space Telescope observations

Images from the Hubble Space Telescope (HST) document the seasonal migration of haze in Titan's atmosphere. Image profiles show darkening of the north relative to the south at 439 nm (blue) but no change at 619 nm. The limb profile at 889 nm has inverted, becoming north‐bright, a variation consistent with haze transport towards the winter hemisphere by winds. The complex altitude‐time variation of the north‐south haze differences are indicated in resolved spectra acquired with the Space Telescope Imaging Spectrograph (STIS): the continuum slope of a north/south ratio spectrum changes sign, becoming red, between 2000 and 2002, although the 889 nm band had already reversed by 2000, suggesting the haze distribution changes most rapidly at high altitudes.

Rest-frame optical continua ofL∼L*,z&gt; 3 quasars: probing the faint end of the high-zquasar luminosity function

Near-IR photometry for 20 radio-loud z>3 quasars, 16 of which are radio- selected, are presented. These data sample the rest-frame optical/UV continuum, which is commonly interpreted as emission from an accretion disk. In a previous study, we compared the rest-frame optical/UV continuum shapes of 15 optically bright (V 3 quasars with those of 27 low redshift (z~0.1) ones that were matched to the high redshift sample in evolved luminosity (i.e. having luminosities ranging from 1-7 times the characteristic luminosity, L*, where L*~(1+z)^{~3}) to look for signs of evolution in the central engines. We found the continuum shapes at z~0.1 and z>3 similar, consistent with no significant change in the ratio mdot/M, where mdot is the accretion rate with respect to the Eddington rate and M is the black hole mass. This study expands our earlier high redshift sample to lower luminosity, away from extreme objects and towards a luminosity overlap with lower redshift samples. The distribution of rest-frame optical/UV continuum shapes for this fainter sample is broader, extending further to the red than that of the brighter z>3 one. Three quasars from this fainter sample, two radio-selected and one optically-selected, have optical continuum slopes alpha<-1 (F_{nu}~nu^{alpha}). The optically-selected one, LBQS0056+0125, appears to be reddened by dust along the line of sight or in the host galaxy, whereas the radio-selected ones, PKS2215+02 and TXS2358+189, could derive their red continua from the contribution of a relatively strong synchrotron component to the rest-frame optical. These objects may represent a bridge to a population of very red high redshift quasars to which ongoing or future near-IR, optical and deep X-ray surveys will be sensitive.

Integral field spectrophotometry of gravitationally lensed QSOs with PMAS

AbstractWe present spatially resolved spectrophotometric observations of multiply imaged QSOs, using the Potsdam Multi‐Aperture Spectrophotometer (PMAS), with the intention to search for spectral differences between components indicative of either microlensing or dust extinction. For the quadruple QSO HE 0435–1223 we find that the continuum shapes are indistinguishable, therefore differential extinction is negligible. The equivalent widths of the broad emission lines are however significantly different, and we argue that this is most likely due to microlensing. Contrariwise, the two components of the well‐known object UM 673 have virtually identical emission line properties, but the continuum slopes differ significantly and indicate different dust extinction along both lines of sight. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)