arXiv subjects

C. S. Kochanek

Publications and source records attributed to C. S. Kochanek.

At least 343 records · Page 19Linked to original sources

arXiv2000

A Quantitative Study of Interacting Dark Matter in Halos

We study the evolution of Hernquist profile ``galaxies'' in the presence of self-interacting dark matter (SIDM), where the properties of the dark matter can be parameterized by one number, sigma-hat = sigma M/a^2 for a halo of mass M and break radius a. While the halos form constant density cores of size a/2 on the core radius relaxation time scale core collapse begins shortly thereafter and a steeper 1/r^2 central density cusp starts forming faster than predicted by 2-body relaxation. The formation of the steeper central cusp is accelerated if the cooling baryons adiabatically compress the dark matter. The natural consequence of SIDM is to exacerbate rather than to mitigate astrophysical problems created by dark matter density cusps.

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arXiv2000

The Host Galaxy of the Lensed Quasar Q 0957+561

Infrared images of the Q 0957+561 gravitational lens obtained with the Hubble Space Telescope show two large (\sim 5 arcsec) lensed images of the z_s=1.41 quasar host galaxy. Parts of the host galaxy are doubly-imaged like the quasar, while other parts are quadruply-imaged. The distortions of the host galaxy offer the best probe yet of the global structure of the lensing potential, which is essential for determining the Hubble constant from the measured time delay. The distortions are inconsistent with the predictions of previously published lens models, which invalidates those models and their implications for H_0. New models show that the distortions finally break the long-standing degeneracy between the shape of the lens galaxy and the tidal shear contributed by the cluster containing the lens galaxy. The shape of the lens galaxy's mass distribution must be remarkably similar to the shape of its luminosity distribution, and most models that produce reasonable values for the Hubble constant roughly match the observed ellipticity gradient and isophote twist of the lens galaxy. Also, the cluster must be non-spherical and produce a relatively small tidal shear. Although there are still degeneracies in the lens models that lead to a 25% uncertainty in the derived value of the Hubble constant, there are also strong prospects for new observations to further improve the constraints and reduce the uncertainties.

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arXiv1999

The Evolution of Gravitational Lens Galaxies

Most gravitational lens galaxies are early-type galaxies in relatively low density environments. We show that they lie on the same fundamental plane as early-type galaxies in both local and distant rich clusters. Their surface brightness evolution requires a typical star formation epoch of z=2-3, almost indistinguishable from that of rich cluster galaxies at comparable redshifts. The restricted galaxy type range of the lenses means that photometric redshifts work well even with only 1-3 filter photometry. We make preliminary measurements of the mass and luminosity functions of the lens galaxies, and find they are consistent with the standard model used for deriving cosmological limits using lens statistics. As expected for a mass-weighted sample, they are more massive and more luminous than the overall early-type galaxy population.

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arXiv1999

The Interstellar Medium of Lens Galaxies

We use observations from the CASTLES survey of gravitational lenses to study extinction in 23 lens galaxies with $0 < z_l < 1$. The median differential extinction between lensed images is $ΔE(\bv) = 0.05$ mag, and the directly measured extinctions agree with the amount needed to explain the differences between the statistics of radio and (optical) quasar lens surveys. We also measure the first extinction laws outside the local universe, including an $R_V=7.2$ curve for a molecular cloud at $z_l=0.68$ and an $R_V=1.5$ curve for the dust in a redshift $z_l=0.96$ elliptical galaxy.

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arXiv1999

The CASTLES gravitational lensing tool

We describe a series of new applications of gravitational lenses as astrophysical and cosmological tools. Such applications are becoming possible thanks to advances in the quality and quantity of observations. CASTLES (CfA-Arizona-Space-Telescope-LEns-Survey) {cfa-www.harvard.edu/castles} is an ongoing project that exploits the sensitivity and resolution of the Hubble Space Telescope (HST) at optical and infrared wavelengths to study the sample of over 50 known gravitational lenses. The observational goal of CASTLES is a uniform sample of multi-band images of all known galaxy-mass lens systems, to derive precise photometry and astrometry for the lens galaxies, all the known images, and any source or lens components that might have escaped detection. With these measurements we are investigating: (1) the properties of dust and of dark matter in lens galaxies out to z\sim1; (2) the dark matter in lens galaxies and in their environments; (3) the evolution of lens galaxies; and (4) the cosmological model, for instance by refining constraints on the Hubble constant H_0.

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arXiv1999

A New Wide-Separation Gravitational Lens Candidate: RXJ 0921+4529

The lens candidate RXJ 0921+4529 consists of two z_s=1.66 quasar separated by 6."93 with an H band magnitude difference of Δm=1.39. The lens appears to be a z_l=0.31 X-ray cluster, including a m_H=18.5 late-type galaxy lying between the quasar images. We detect an extended source overlapping the faint quasar but not the bright quasar. If this extended source is the host galaxy of the fainter quasar, then the system is a quasar binary rather than a gravitational lens.

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arXiv1999

Finding Gravitational Lenses With X-rays

There are $\sim 1$, 0.1 and 0.01 gravitationally lensed X-ray sources per square degree with soft X-ray fluxes exceeding $10^{-15}, 10^{-14}$ and $10^{-13} ergs/s cm^{-2}$ respectively. These sources will be detected serendipitously with the Chandra X-ray Observatory at a rate of 1--3 lenses per year of high resolution imaging. The low detection rate is due to the small area over which the HRC and ACIS cameras have the $<1\farcs5$ FWHM resolution necessary to find gravitational lenses produced by galaxies. Deep images of rich clusters at intermediate redshifts should yield one wide separation ($Δθ\gtorder 5\farcs0$) multiply-imaged background X-ray source for every $\sim 10$, 30 and 300 clusters imaged to the same flux limits.

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arXiv1999

The Quasar Pair Q 1634+267 A, B and the Binary QSO vs. Dark Lens Hypotheses

Deep HST/NICMOS H (F160W) band observations of the z=1.96 quasar pair Q 1634+267A,B reveal no signs of a lens galaxy to a 1 sigma threshold of approximately 22.5 mag. The minimum luminosity for a normal lens galaxy would be a 6L_* galaxy at z > 0.5, which is 650 times greater than our detection threshold. Our observation constrains the infrared mass-to-light ratio of any putative, early-type, lens galaxy to (M/L)_H > 690h_65 (1200h_65) for Omega_0=0.1 (1.0) and H_0=65h_65 km/s/Mpc. We would expect to detect a galaxy somewhere in the field because of the very strong Mg II absorption lines at z=1.1262 in the Q 1634+267 A spectrum, but the HST H-band, I-band (F785LP) and V-band (F555W) images require that any associated galaxy be very under-luminous less than 0.1 L^*_H (1.0 L^*_I) if it lies within less than 40 h^{-1} (100 h^{-1}) kpc from Q 1634+267 A,B. While the large image separation (3.86 arcsec) and the lack of a lens galaxy strongly favor interpreting Q 1634+267A,B as a binary quasar system, the spectral similarity remains a puzzle. We estimate that at most 0.06% of randomly selected quasar pairs would have spectra as similar to each other as the spectra of Q 1634+267 A and B. Moreover, spectral similarities observed for the 14 quasar pairs are significantly greater than would be expected for an equivalent sample of randomly selected field quasars. Depending on how strictly we define similarity, we estimate that only 0.01--3% of randomly drawn samples of 14 quasar pairs would have as many similar pairs as the observational sample.

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arXiv1999

The CASTLES project

We describe the goals of the CASTLES (CfA-Arizona-Space-Telescope-LEns-Survey) project including a sample of NICMOS images of gravitational lenses and a brief list of the preliminary findings.

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arXiv1999

Dust and Extinction Curves in Galaxies with z>0: The Interstellar Medium of Gravitational Lens Galaxies

We determine 37 differential extinctions in 23 gravitational lens galaxies over the range 0 < z_l < 1. Only 7 of the 23 systems have spectral differences consistent with no differential extinction. The median differential extinction for the optically-selected (radio-selected) subsample is E(B-V)=0.04 (0.06) mag. The extinction is patchy and shows no correlation with impact parameter. The median total extinction of the bluest images is E(B-V)=0.08 mag, although the total extinction distribution is dominated by the uncertainties in the intrinsic colors of quasars. The directly measured extinction distributions are consistent with the mean extinction estimated by comparing the statistics of quasar and radio lens surveys, thereby confirming the need for extinction corrections when using the statistics of lensed quasars to estimate the cosmological model. A disjoint subsample of two face-on, radio-selected spiral lenses shows both high differential and total extinctions, but standard dust-to-gas ratios combined with the observed molecular gas column densities overpredict the amount of extinction by factors of 2-5. For several systems we can estimate the extinction law, ranging from R_V=1.5+/-0.2 for a z_l=0.96 elliptical, to R_V=7.2+/-0.1 for a z_l=0.68 spiral. For the four radio lenses where we can construct non-parametric extinction curves we find no evidence for gray dust over the IR-UV wavelength range. The dust can be used to estimate lens redshifts with reasonable accuracy, although we sometimes find two degenerate redshift solutions.

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arXiv1998

Results from the CASTLES Survey of Gravitational Lenses

We show that most gravitational lenses lie on the passively evolving fundamental plane for early-type galaxies. For burst star formation models (1 Gyr of star formation, then quiescence) in low Omega_0 cosmologies, the stellar populations of the lens galaxies must have formed at z_f > 2. Typical lens galaxies contain modest amounts of patchy extinction, with a median differential extinction for the optical (radio) selected lenses of E(B-V) = 0.04 (0.07) mag. The dust can be used to determine both extinction laws and lens redshifts. For example, the z_l=0.96 elliptical lens in MG0414+0534 has an R_V=1.7 +/- 0.1 mean extinction law. Arc and ring images of the quasar and AGN source host galaxies are commonly seen in NICMOS H band observations. The hosts are typically blue, L < L_* galaxies.

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arXiv1998

The Infrared Einstein Ring in the Gravitational Lens MG1131+0456 and the Death of the Dusty Lens Hypothesis

We have obtained and modeled new NICMOS images of the lens system MG1131+0456, which show that its lens galaxy is an H=18.6 mag, transparent, early-type galaxy at a redshift of about z_l = 0.85; it has a major axis effective radius R_e=0.68+/-0.05 arcsec, projected axis ratio b/a=0.77+/-0.02, and major axis PA=60+/-2 degrees. The lens is the brightest member of a group of seven galaxies with similar R-I and I-H colors, and the two closest group members produce sufficient tidal perturbations to explain the ring morphology. The host galaxy of the MG1131+0456 source is a z_s > 2 ERO (``extremely red object'') which is lensed into optical and infrared rings of dramatically different morphologies. These differences imply a strongly wavelength-dependent source morphology that could be explained by embedding the host in a larger, dusty disk. At 1.6 micron (H), the ring is spectacularly luminous, with a total observed flux of H=17.4 mag and a de-magnified flux of 19.3 mag, corresponding to a 1-2L_* galaxy at the probable source redshift of z_s > 2. Thus, it is primarily the stellar emission of the radio source host galaxy that produces the overall colors of two of the reddest radio lenses, MG1131+0456 and B~1938+666, aided by the suppression of optical AGN emission by dust in the source galaxy. The dusty lens hypothesis -- that many massive early-type galaxies with 0.2 < z_l < 1.0 have large, uniform dust opacities -- is ruled out.

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arXiv1997

Why Quasar Pairs Are Binary Quasars And Not Gravitational Lenses

We use simple comparisons of the optical and radio properties of the wide separation (3'' to 10'') quasar pairs to demonstrate that they are binary quasars rather than gravitational lenses. The most likely model is that all the pairs are binary quasars, with a one-sided 2--sigma (1--sigma) upper limit of 22% (8%) on the lens fraction. Simple models for the expected enhancement of quasar activity during galaxy mergers that are consistent with the enhancement observed at low redshift can explain the incidence, separations, redshifts, velocity differences, and radio properties of the binary quasar population. Only a modest fraction (< 5%) of all quasar activity need be associated with galaxy mergers to explain the binary quasars.

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arXiv1997

The Optical Properties of Gravitational Lens Galaxies as a Probe of Galaxy Structure and Evolution

We combine photometry and lens modeling to study the properties of 17 gravitational lens galaxies between z=0.1 and 1. Most of the lens galaxies are passively evolving early-type galaxies, with a few spirals. The colors, scale lengths, and ellipticities of lens galaxies are similar to those of the general population of early-type galaxies, although there may be a deficit of apparently round lens galaxies produced by the inclination dependence of lensing cross sections. The projected mass distributions are aligned with the projected light distributions to <~ 10 deg, except in the presence of a strong external tidal perturbation, suggesting that dark matter halos have orbits that are significantly modified by interactions with the baryonic component and are not far out of alignment with the stars. Lens galaxies obey image separation/lens luminosity correlations analogous to the Faber-Jackson and Tully-Fisher relations, which are consistent with standard dark matter lens models. The lens galaxy mass-to-light ratios decrease with redshift as d(log M/L_B)/dz = -0.3\pm0.1 (-0.5\pm0.1) for Omega_0=1 (0.1), thus providing direct evidence of passive evolution for a sample of early-type galaxies in low-density environments. The evolution-corrected mass-to-light ratios are generally larger than predicted by constant M/L dynamical models, although there is significant scatter; with improved photometry, lens galaxy mass-to-light ratios would better distinguish between constant M/L and dark matter models. These conclusions are limited primarily by the quality of lens galaxy photometry.

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arXiv1997

Limits on Cosmological Models From Radio-Selected Gravitational Lenses

We are conducting a redshift survey of 177 flat-spectrum radio sources in 3 samples covering the 5GHz flux ranges 50-100, 100-200 and 200-250 mJy. So far, we have measured 124 redshifts with completenesses of 80%, 68% and 58% for the bright, intermediate, and faint flux ranges. Using the newly determined redshift distribution we can derive cosmological limits from the statistics of the 6 gravitational lenses in the JVAS sample of 2500 flat-spectrum radio sources brighter than 200 mJy at 5GHz. For flat cosmological models with a cosmological constant, the limit using only radio data is Omega_0 > 0.27 at 2-sigma (0.47 < Omega_0 < 1.38 at 1-sigma). The limits are statistically consistent with those for lensed quasars, and the combined radio + optical sample requires Omega_0 > 0.38 at 2-sigma (0.64 < Omega_0 < 1.66 at 1-sigma) for our most conservative redshift completeness model and assuming that there are no quasar lenses produced by spiral galaxies. Our best fit model improves by approximately 1-sigma if extinction in the early-type galaxies makes the lensed quasars fainter by Delta m = 0.58 +- 0.45 mag, but we still find a limit of Omega_0 > 0.26 at 2-sigma in flat cosmologies. The increasing fraction of radio galaxies as compared to quasars at fainter radio fluxes (rising from ~10% at 1 Jy to ~50% at 0.1 Jy) explains why lensed optical emission is common for radio lenses and partly explains the red color of radio-selected lenses.

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arXiv1997

The geometry of the quadruply imaged quasar PG 1115+080; implications for Ho

Time delay measurements have recently been reported for the lensed quasar PG 1115+080. These measurements can be used to derive Ho, but only if we can constrain the lensing potential. We have applied a recently developed deconvolution technique to analyse sub-arcsecond I band images of PG 1115+080, obtained at the Nordic Optical Telescope (NOT) and the Canada France Hawaii Telescope (CFHT). The high performance of the deconvolution code allows us to derive precise positions and magnitudes for the four lensed images of the quasar, as well as for the lensing galaxy. The new measurement of the galaxy position improves its precision by a factor of 3 and thus strengthens the constraints on the lensing potential. With the new data, a range of models incorporating some of the plausible systematic uncertainties yields Ho = 53 (+10/-7) km/s/mpc.

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arXiv1997

Gravitational Lensing By Spiral Galaxies

We study gravitational lensing by spiral galaxies, using realistic models consisting of halo, disk, and bulge components combined to produce a flat rotation curve. Proper dynamical normalization of the models is critical because a disk requires less mass than a spherical halo to produce the same rotation curve---a face-on Mestel disk has a lensing cross section only 41% as large as a singular isothermal sphere with the same rotation curve. The cross section is sensitive to inclination and dominated by edge-on galaxies, which produce lenses with an unobserved 2-image geometry and a smaller number of standard 5-image lenses. Unless the disk is unreasonably massive, disk+halo models averaged over inclination predict \lesssim 10% more lenses than pure halo models. Finally, models with an exponential disk and a central bulge are sensitive to the properties of the bulge. In particular, an exponential disk model normalized to our Galaxy cannot produce multiple images without a bulge, and including a bulge reduces the net flattening of edge-on galaxies. The dependence of the lensing properties on the masses and shapes of the halo, disk, and bulge means that a sample of spiral galaxy lenses would provide useful constraints on galactic structure.

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arXiv1997

Determining the Hubble Constant from the Gravitational Lens PG 1115+080

For the quadruple gravitational lens PG 1115+080, we combine recent measurements of the time delays with new lens models to determine the Hubble constant H_0. We explore the effects of systematic uncertainties in the lens models on the estimates of H_0, and we discuss how the uncertainties can be reduced by future observations. We find that the lens cannot be fit by an isolated lens galaxy, but that it can be well fit by including a perturbation from the nearby group of galaxies. To understand the full range of systematic uncertainties it is crucial to use an ellipsoidal galaxy and to let the group position vary. In this case, the existing constraints cannot break degeneracies in the models with respect to the profiles of the galaxy and group and to the position of the group. Combining the known time delays with a range of lens models incorporating most of the plausible systematic effects yields H_0 = 51_{-13}^{+14} km s^{-1} Mpc^{-1}. The constraints on the lens models, and hence on H_0, can be improved by reducing the standard errors in the lens galaxy position from 50 mas to \sim10 mas, reducing the uncertainties in the time delays to \sim0.5 days, and constraining the lens mass distribution using HST photometry and the fundamental plane. In particular, the time delay ratio r_{ABC} = Δτ_{AC} / Δτ_{BA} may provide the best constraint on the mass profile of the galaxy.

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