Dust Extinction Curve Research Articles

Four Quasars above Redshift 6 Discovered by the Canada-France High-zQuasar Survey

The Canada-France High-z Quasar Survey (CFHQS) is an optical survey designed to locate quasars during the epoch of reionization. In this paper we present the discovery of the first four CFHQS quasars at redshifts greater than 6, including the most distant known quasar, CFHQS J2329-0301 at z = 6.43. We describe the observational method used to identify the quasars and present optical, infrared, and millimeter photometry and optical and near-infrared spectroscopy. We investigate the dust properties of these quasars, finding an unusual dust extinction curve for one quasar and a high far-infrared luminosity due to dust emission for another. The mean millimeter continuum flux for CFHQS quasars is substantially lower than that for SDSS quasars at the same redshift, likely due to a correlation with quasar UV luminosity. For two quasars with sufficiently high signal-to-noise ratio optical spectra, we use the spectra to investigate the ionization state of hydrogen at z > 5. For CFHQS J1509-1749 at z = 6.12 we find significant evolution (beyond a simple extrapolation of lower redshift data) in the Gunn-Peterson optical depth at z > 5.4. The line of sight to this quasar has one of the highest known optical depths at z ≈ 5.8. An analysis of the sizes of the highly ionized near-zones in the spectra of two quasars at z = 6.12 and 6.43 suggest that the intergalactic medium surrounding these quasars was substantially ionized before these quasars turned on. Together, these observations point toward an extended reionization process, but we caution that cosmic variance is still a major limitation in z > 6 quasar observations.

Dust formation and survival in supernova ejecta

The presence of dust at high redshift requires efficient condensation of grains in SN ejecta, in accordance with current theoretical models. Yet, observations of the few well studied SNe and SN remnants imply condensation efficiencies which are about two orders of magnitude smaller. Motivated by this tension, we have (i) revisited the model of Todini & Ferrara (2001) for dust formation in the ejecta of core collapse SNe and (ii) followed, for the first time, the evolution of newly condensed grains from the time of formation to their survival - through the passage of the reverse shock - in the SN remnant. We find that 0.1 - 0.6 M_sun of dust form in the ejecta of 12 - 40 M_sun stellar progenitors. Depending on the density of the surrounding ISM, between 2-20% of the initial dust mass survives the passage of the reverse shock, on time-scales of about 4-8 x 10^4 yr from the stellar explosion. Sputtering by the hot gas induces a shift of the dust size distribution towards smaller grains. The resulting dust extinction curve shows a good agreement with that derived by observations of a reddened QSO at z =6.2. Stochastic heating of small grains leads to a wide distribution of dust temperatures. This supports the idea that large amounts (~ 0.1 M_sun) of cold dust (T ~ 40K) can be present in SN remnants, without being in conflict with the observed IR emission.

Unveiling the nature of Ultraluminous Infrared Galaxies with 3-4 μm spectroscopy

We present the results of L-band spectroscopical observations of local bright Ultraluminous Infrared Galaxies (ULIRGs), performed with the Infrared Spectrometer And Array Camera (ISAAC) at the Very Large Telescope. The excellent sensitivity of the telescope and of the instrument provided spectra of unprecedented quality for this class of objects, which allowed a detailed study of the active galactic nucleus (AGN)/starburst contribution to the energy output, and of the composition of the circumnuclear absorber. We discuss the L-band spectral features of seven single sources, and the statistical properties of a complete sample of 15 sources obtained combining our observations with other published 3–4 μm spectra. Our main results are as follows. (i) When a spectral indicator suggesting the presence of an AGN (low equivalent width of the 3.3-μm emission line, steep λ–fλ spectrum, presence of an absorption feature at 3.4 μm) is found, the AGN is always confirmed by independent analysis at other wavelengths. Conversely, in all known AGNs at least one of the above indicators is present. (ii) Two new diagnostic diagrams are proposed combining the above indicators, in which starbursts and AGNs are clearly and completely separated. (iii) The above diagnostic techniques are possible with spectra of relatively low quality, which can be obtained for several tens of ULIRGs with currently available telescopes. This makes L-band spectroscopy the current best tool to disentangle AGN and starburst contributions in ULIRGs. (iv) The L-band properties of ULIRGs are heterogeneous. However, we show that all the spectral differences among ULIRGs can be reproduced starting from pure intrinsic AGN and starburst spectra and two varying parameters: the amount of dust extinction of the AGN component, and the relative AGN/starburst contribution to the bolometric luminosity. (v) Using the above decomposition model, we show that AGNs in ULIRGs have a low dust-to-gas ratio and a dust extinction curve different from Galactic. (vi) The estimate of the presence and contribution of AGNs in a complete sample shows that AGNs are hosted by approximately two-thirds of ULIRGs, but their energetic contribution is relevant (>30 per cent of the bolometric luminosity) only in ∼20 per cent of the sample.

Nanodiamond Dust and the Far‐Ultraviolet Quasar Break

We explore the possibility that the steepening observed shortward of 1000 A in the energy distribution of quasars may result from absorption by dust, being either intrinsic to the quasar environment or intergalactic. We find that a dust extinction curve consisting of nanodiamonds, composed of terrestrial cubic diamonds or with surface impurities as found in carbonaceous chondrite meteorites, such as Allende, is successful in reproducing the sharp break observed. The intergalactic dust model is partially successful in explaining the shape of the composite energy distribution but must be discarded in the end, as the amount of crystalline dust required is unreasonable and would imply an improbable fine-tuning among the dust formation processes. The alternative intrinsic dust model requires a mixture of both cubic diamonds and Allende nanodiamonds and provides a better fit of the UV break. The gas column densities implied are of the order 1020 cm-2, assuming solar metallicity for carbon and full depletion of carbon into dust. The absorption only occurs in the ultraviolet and is totally negligible in the visible. The minimum dust mass required is of the order ~0.003r M☉, where rpc is the distance in parsecs between the dust screen and the continuum source. The intrinsic dust model reproduces the flux rise observed around 660 A in key quasar spectra quite well. We present indirect evidence of a shallow continuum break near 670 A (18.5 eV), which would be intrinsic to the quasar continuum.

Evidence for dust reddening in damped Lyα absorbers identified through Ca ii (H&K) absorption

Abstract We present a new sample of 31 Ca ii (H&K) λλ3935, 3970 absorption-line systems with 0.84 < zabs < 1.3 discovered in the spectra of Sloan Digital Sky Survey (SDSS) Data Release 3 quasars, together with an analysis of their dust content. The presence of calcium absorption together with measurements of the Mg iiλ2796, Fe iiλ2600 and Mg iλ2853 lines leads to the conclusion that the majority of our systems are damped Lyα (DLA) absorbers. The composite spectrum in the rest frame of the absorber shows clear evidence for reddening. Large and Small Magellanic Cloud extinction curves provide satisfactory fits, with a best-fitting E(B – V) of 0.06, while the Galactic dust extinction curve provides a poor fit due to the lack of a strong 2175-Å feature. A trend of increasing dust content with equivalent width of Ca ii is present. Monte Carlo techniques demonstrate that the detection of reddening is significant at >99.99 per cent confidence. The discovery of significant amounts of dust in a subsample of DLAs has direct implications for studies of the metallicity evolution of the Universe and the nature of DLAs in relation to high-redshift galaxies. The gas:dust ratio is discussed. Our results suggest that at least ∼40 per cent of the Ca ii absorption systems are excluded from the magnitude-limited SDSS quasar sample as a result of the associated extinction, a fraction similar to the upper limit deduced at higher redshifts from radio-selected surveys.

Measuring Extinction Curves of Lensing Galaxies

We critique the method of constructing extinction curves of lensing galaxies using multiply imaged QSOs. If one of the two QSO images is lightly reddened or if the dust along both sightlines has the same properties then the method works well and produces an extinction curve for the lensing galaxy. These cases are likely rare and hard to confirm. However, if the dust along each sightline has different properties then the resulting curve is no longer a measurement of extinction. Instead, it is a measurement of the difference between two extinction curves. This difference curve'' does contain information about the dust properties, but extracting a meaningful extinction curve is not possible without additional, currently unknown information. As a quantitative example, we show that the combination of two Cardelli, Clayton, & Mathis (CCM) type extinction curves having different values of R(V) will produce a CCM extinction curve with a value of R(V) which is dependent on the individual R(V) values and the ratio of V band extinctions. The resulting lens difference curve is not an average of the dust along the two sightlines. We find that lens difference curves with any value of R(V), even negative values, can be produced by a combination of two reddened sightlines with different CCM extinction curves with R(V) values consistent with Milky Way dust (2.1 < R(V) < 5.6). This may explain extreme values of R(V) inferred by this method in previous studies. But lens difference curves with more normal values of R(V) are just as likely to be composed of two dust extinction curves with R(V) values different than that of the lens difference curve. While it is not possible to determine the individual extinction curves making up a lens difference curve, there is information about a galaxy's dust contained in the lens difference curves.

Evidence for the presence of dust in intervening QSO absorbers from the Sloan Digital Sky Survey

We find evidence for dust in the intervening QSO absorbers from the spectra of QSOs in the Sloan Digital Sky Survey Data Release 1. No evidence is found for the 2175 A feature which is present in the Milky Way dust extinction curve.

Dust and Diffuse Interstellar Bands in theza= 0.524 Absorption System toward AO 0235+164

The authors present new HST STIS NUV-MAMA and STIS CCD observations of the BL Lac object AO 0235+164 and the intervening damped Ly {alpha} (DLA) line at z{sub {alpha}} = 0.524. The line profile gives N(HI) = 5 {+-} 1 x 10{sup 21} cm{sup -2} and, combined with the H I 21 cm absorption data leads to a spin temperature of Ts = 220 K {+-} 60 K. Those spectra also show a strong, broad feature at the expected position of the 2175 {angstrom} graphitic dust feature at z{sup {alpha}} = 0.524. Assuming a Galactic type dust extinction curve at z{sub {alpha}} = 0.524 gives a dust-to-gas ratio of 0.19 Galactic, but the fit, assuming the underlying, un-reddened spectrum is a single power-law, is poor in the far-UV. A dust-to-gas ratio of 0.19 Galactic is similar to the LMC, but the AO 0235+164 spectrum does not fit the LMC extinction curve, or the SMC extinction curve (which has practically no 2175 {angstrom} feature). A possible interpretation includes dust similar to Galactic, but with less of the small particles that produce the far-UV extinction. The metallicity of the z{sub {alpha}} = 0.524 absorber, estimated from the observed N(HI) and excess X-raymore » absorption (beyond Galactic) derived from contemporaneous and archival ASCA and ROSAT.« less

A supernova origin for dust in a high-redshift quasar.

Interstellar dust plays a crucial role in the evolution of the Universe by assisting the formation of molecules, by triggering the formation of the first low-mass stars, and by absorbing stellar ultraviolet-optical light and subsequently re-emitting it at infrared/millimetre wavelengths. Dust is thought to be produced predominantly in the envelopes of evolved (age >1 Gyr), low-mass stars. This picture has, however, recently been brought into question by the discovery of large masses of dust in the host galaxies of quasars at redshift z > 6, when the age of the Universe was less than 1 Gyr. Theoretical studies, corroborated by observations of nearby supernova remnants, have suggested that supernovae provide a fast and efficient dust formation environment in the early Universe. Here we report infrared observations of a quasar at redshift 6.2, which are used to obtain directly its dust extinction curve. We then show that such a curve is in excellent agreement with supernova dust models. This result demonstrates a supernova origin for dust in this high-redshift quasar, from which we infer that most of the dust at high redshifts probably has the same origin.

A Quantitative Comparison of the Small Magellanic Cloud, Large Magellanic Cloud, and Milky Way Ultraviolet to Near‐Infrared Extinction Curves

We present an exhaustive quantitative comparison of all the known extinction curves in the Small and Large Magellanic Clouds (SMC and LMC) with our understanding of the general behavior of Milky Way extinction curves. The RV-dependent CCM relationship of Cardelli, Clayton, and Mathis and the sample of extinction curves used to derive this relationship are used to describe the general behavior of Milky Way extinction curves. The ultraviolet portion of the SMC and LMC extinction curves are derived from archival IUE data, except for one new SMC extinction curve, which was measured using Hubble Space Telescope Space Telescope Imaging Spectrograph observations. The optical extinction curves are derived from new (for the SMC) and literature UBVRI photometry (for the LMC). The near-infrared extinction curves are calculated mainly from 2MASS photometry supplemented with DENIS and new JHK photometry. For each extinction curve, we give RV = A(V)/E(B - V) and N(H I) values that probe the same dust column as the extinction curve. We compare the properties of the SMC and LMC extinction curves with the CCM relationship three different ways: each curve by itself, the behavior of extinction at different wavelengths with RV, and the behavior of the extinction curve Fitzpatrick and Massa fit parameters with RV. As has been found previously, we find that a small number of LMC extinction curves are consistent with the CCM relationship, but the majority of the LMC and all the SMC curves do not follow the CCM relationship. For the first time, we find that the CCM relationship seems to form a bound on the properties of all the LMC and SMC extinction curves. This result strengthens the picture dust extinction curves exhibit of a continuum of properties between those found in the Milky Way and the SMC bar. Tentative evidence based on the behavior of the extinction curves with dust-to-gas ratio suggests that the continuum of dust extinction curves is possibly caused by the environmental stresses of nearby star formation activity.

Dust Extinction Curves and Lyα Forest Flux Deficits for Use in Modeling Gamma‐Ray Burst Afterglows and All Other Extragalactic Point Sources

Since gamma-ray burst afterglows were first detected in 1997, the relativistic fireball model has emerged as the leading theoretical explanation of the afterglows. In this paper, we present a very general, Bayesian inference formalism with which this, or any other, afterglow model can be tested, and with which the parameter values of acceptable models can be constrained, given the available photometry. However, before model comparison or parameter estimation can be attempted, one must also consider the physical processes that affect the afterglow as it propagates along the line of sight from the burst source to the observer. Namely, how does extinction by dust, both in the host galaxy and in our galaxy, and absorption by the Lyα forest and by H I in the host galaxy, change the intrinsic spectrum of the afterglow? Consequently, we also present in this paper a very general, eight-parameter dust extinction curve model and a two-parameter model of the Lyα forest flux deficit versus redshift distribution. Using fitted extinction curves from Milky Way and Magellanic Cloud lines of sight, and measurements of Lyα forest flux deficits from quasar absorption line systems, we construct a Bayesian prior probability distribution that weights this additional, but necessary, parameter space such that the volume of the solution space is reduced significantly, a priori. Finally, we discuss the broad applicability of these results to the modeling of light from all other extragalactic point sources, such as Type Ia supernovae.

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.

Dust extinction curves and Lyα forest flux deficits for use in modeling gamma-ray burst afterglows and all other extragalactic point sources

Dust extinction curves and Lyα forest flux deficits for use in modeling gamma-ray burst afterglows and all other extragalactic point sources

Age and Dust Degeneracy for Starburst Galaxies Solved?

A spectral evolution model of galaxies that includes both stellar and dust effects is newly built. xApplying the model to 22 nearby starburst galaxies, we have shown that far infrared luminosity of galaxies helps to break the age-dustiness degeneracy. We have derived a unique solution of age and the dustiness for each starburst galaxy. The resulting starburst ages and optical depths are in the range $10 \le t (Myr) \le 500$ and $0.5 \le \tau_{V} \le 5.0$, respectively. The result is robust and is almost independent of model assumptions such as dust distributions, extinction curves, and burst strengths. With the rapidly growing sensitivity of submillimeter detectors, it should become possible in the near future to determine the age and $\tau_{V}$ of star-forming galaxies at redshifts $z \simeq 3$ and beyond. Accurate estimates of $\tau_{V}$ for Lyman-break galaxies and high-z galaxies might require a substantial revision of the previously claimed picture of star formation history over the Hubble time.

Are high-redshift quasars hidden by dusty galaxies?

A Monte Carlo technique has been developed to compute the joint probability distribution for the reddening and extinction of quasars by galaxies along the line of sight with a realistic dust extinction curve including the 220 nm feature. Galaxies are treated as disks with random inclinations, having exponential or modified Gaussian radial profiles. This technique is used to find the distribution of the reddened quasars on multicolor diagrams such as the (U-J, J-F) plots used by Koo and Kron to find faint quasars. The multicolor search technique is not a simple UV excess approach, and the resulting color selection does not add significantly to the flux selection discussed by Wright, Ostriker and Heisler, and Heisler and Ostriker. The quasar reddening trend observed by Wright and Malkan is about 0.35 +/- 0.50 times the mean reddening of the selected sample predicted by the Heisler and Ostriker model. Models which match both the observed reddening trend and the Lyα absorber column density distribution can still have significant effects on quasar counts: count reduction factors at z = 3 of 2.7-4 instead of the factor of 22 reduction given by the Heisler and Ostriker model. These models have dust densities as high as 0.23 mag of visual extinction per Hubble radius.

Colors of reflection nebulae. II - The excitation of extended red emission

New BVI color-difference measurements are reported for the reflection nebulae NGC 7023, NGC 2068, and CED 201. The data are obtained through BVI imaging with a CCD detector and are analyzed together with previously published measurements for the reflection nebula NGC 2023. All four nebulae are substantially redder in the (V, I) range than expected on the basis of dust scattering alone, a result of the presence of extended emission in the I band. The relative strength of the I excess reaches a maximum in nebular regions where the color in the (B, V) range is bluest. These facts are interpreted in terms of a model which explains the extended I emission as a luminescence process excited by UV radiation from the illuminating star(s). Model fits identify the wavelength region between 1800 and 2500 A of the illuminating star's spectrum as the source of the energy of excitation. It appears possible that the 2200 A band in the dust-extinction curve is a gate for the excitation energy, and it is likely that the I excess emission and the extended near-IR emission recently discovered in reflection nebulae are similar in nature and origin.