arXiv subjects

C. S. Kochanek

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

353 records · Page 20Linked to original sources

arXiv1997

Rebuilding the Cepheid Distance Scale I: A Global Analysis of Cepheid Mean Magnitudes

We develop a statistical method for using multicolor photometry to determine distances using Cepheid variables including the effects of temperature, extinction, and metallicity and apply it to UBVRIJHK photometry of 694 Cepheids in 17 galaxies. We derive homogeneous distance, extinction and uncertainty estimates for four models, starting from the standard extragalactic method and then adding the physical effects of temperature distributions, extinction distributions, requiring positive definite extinctions, and metallicity. While we find general agreement with published distances when we make similar systematic assumptions, there is a clear problem in the standard distances because they require Cepheids with negative extinctions, particularly in low metallicity galaxies, unless the mean LMC extinction exceeds E(B-V) > 0.25. The problem can be explained by the physically expected metallicity dependence of the Cepheid distance scale, where metal-poor Cepheids are hotter and fainter than metal-rich Cepheids. For V and I we found that the mean magnitude change is -0.14 +/- 0.14 mag/dex and the mean color change is 0.13 +/- 0.04 mag/dex, with the change in color dominating the change in distance. The effect on Type Ia supernova estimates of the Hubble constant is dramatic because most were found in the metal poor galaxies with the bluest Cepheids. The Type Ia Multi-color Light Curve Shape (MLCS) method estimate for H_0 formally rises from 69 +/- 8 km/s Mpc to 80 +/- 6 km/s Mpc with the metallicity correction.

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arXiv1996

The Flat Spectrum Radio Luminosity Function, Gravitational Lensing, Galaxy Ellipticities and Cosmology

The number of lenses found in the JVAS survey of flat-spectrum radio sources for gravitational lenses is consistent with statistical models of optical surveys for lensed quasars. The 90% confidence limit on Omega_0 in flat cosmological models (Omega_0+lambda_0=1) is approximately 0.15 < Omega_0 < 2. Depending on the RLF model, we predict 2.4 to 3.6 lenses in the JVAS survey and in the first part of the fainter CLASS survey, and 0.3 to 0.6 lenses in the brighter PHFS survey for an Omega_0=1 model. The uncertainties are due to the small numbers of lenses (there are only 4 compact JVAS lenses) and the uncertainties in the radio luminosity function (RLF) caused by the lack of information on the redshift distribution of 10-300 mJy radio sources. If we force the models to produce the observed number of JVAS lenses, the mean redshift of a 50 mJy source varies from z_s=0.4 for Omega_0=0, to 1.9 for Omega_0=1, to almost 4.0 for Omega_0=2 when Omega_0+lambda_0=1. The source fluxes and redshifts of the lenses in the JVAS and CLASS surveys are consistent with the statistical models. The numbers of four-image lenses found in the JVAS survey and in surveys for lensed quasars are mutually consistent, but slightly larger than expected for models using the observed axis ratios of E and S0 galaxies. The best fits to the lens data require a projected axis ratio of b/a=0.50 with a 90% confidence range of 0.25 < b/a < 0.65.

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arXiv1996

Gravitational Lensing Limits on Early-Type Galaxies

Gravitational lenses are a unique new constraint on the structure of galaxies. We review the evidence that most lenses are early-type galaxies, the optical properties of the lens galaxies, the evidence against constant M/L models, recent work on the axis ratios of the mass distribution, and the role stellar dynamics plays in gravitational lensing.

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arXiv1996

SBS 0909+532: A New Double Gravitational Lens or Binary Quasar?

The z=1.377, B=17.0 mag quasar SBS 0909+532 A, B is a double with two images separated by 1.107 +/- 0.006 arcsec. Because the faint image has an emission line at the same wavelength as the MgII 2798 A emission line of the quasar, and lacks the broad MgIb absoption feature expected for a star with the same colors (a K star), we conclude that image B is a quasar with similar redshift to image A. The relative probabilities that the double is the smallest separation (4.8/h kpc for Ω_0=1) correlated quasar pair or a gravitational lens are \sim 1:10^6. If the object is a lens, the mean lens redshift is $ =0.5$ with 90% confidence bounds of 0.18 < z_l < 0.83 for Ω_0=1. If the lens is an elliptical galaxy, we expect it to be brighter than I < 19.5 mag. The broad band flux ratio varies with wavelength, with ΔI=0.31, ΔR=0.58, and ΔB=1.29 magnitudes, which is difficult to reconcile with the lensing hypothesis.

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arXiv1996

Shear and Ellipticity in Gravitational Lenses

Galaxies modeled as singular isothermal ellipsoids with an axis ratio distribution similar to the observed axis ratio distribution of E and S0 galaxies are statistically consistent with both the observed numbers of two-image and four-image lenses and the inferred ellipticities of individual lenses. However, no four-image lens is well fit by the model (typical $χ^2/N_{dof} \sim 20$), the axis ratio of the model can be significantly different from that of the observed lens galaxy, and the major axes of the model and the galaxy may be slightly misaligned. We found that models with a second, independent, external shear axis could fit the data well (typical $χ^2/N_{dof} \sim 1$), while adding the same number of extra parameters to the radial mass distribution does not produce such a dramatic improvement in the fit. An independent shear axis can be produced by misalignments between the luminous galaxy and its dark matter halo, or by external shear perturbations due to galaxies and clusters correlated with the primary lens or along the line of sight. We estimate that the external shear perturbations have no significant effect on the expected numbers of two-image and four-image lenses, but that they can be important perturbations in individual lens models. However, the amplitudes of the external shears required to produce the good fits are larger than our estimates for typical external shear perturbations (10-15% shear instead of 1-3% shear) suggesting that the origin of the extra angular structure must be intrinsic to the primary lens galaxy in most cases.

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arXiv1995

LensMEM: A Gravitational Lens Inversion Algorithm Using the Maximum Entropy Method

We present a new algorithm for inverting poorly resolved gravitational lens systems using the maximum entropy method (MEM). We test the method with simulations and then apply it to an 8 GHz VLA map of the radio ring lens MG1654+134. We model the lens as a singular isothermal sphere embedded in an external shear field and find the critical radius of the lens is $b=0\parcs9820$, the dimensionless shear is $γ=0.0771$, and the position angle of the shear is $θ=100\pdeg8$. These results are consistent with the results obtained by Kochanek (1995) using a complementary inversion algorithm based on Clean.

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arXiv1995

A Technical Memorandum On Core Radii In Lens Statistics

Quantitative estimates of lensing probabilities must be self-consistent. In particular, for asymptotically isothermal models: (1) using the $(3/2)^{1/2}$ correction for the velocity dispersion overestimates the expected number of lenses by 150\% and their average separations by 50\%, thereby introducing large cosmological errors; (2) when a core radius is added to the SIS model, the velocity dispersion must be increased; and (3) cross sections and magnification bias cannot be separated when computing the lensing probability. When we self-consistently calculate the effects of finite core radii in flat cosmological models, we find that the cosmological limits are independent of the core radius.

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arXiv1995

Gravitational Lenses and the Structure of Galaxies

Nearly singular isothermal mass distributions with small core radii are consistent with stellar dynamics, lens statistics, and lens models as a model for E/S0 galaxies. Models like the de Vaucouleurs model with a constant mass-to-light ratio are not. While the isothermal distributions are probably an oversimplification, E/S0 galaxies (at least in projection) must have significant amounts of dark matter on scales of an effective radius.

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arXiv1995

Is There A Cosmological Constant?

We present limits on the cosmological constant from the statistics of gravitational lenses using newly completed quasar surveys, new lens data, and a range of lens models. The formal limit is $λ_0 < 0.66$ at 95\% confidence in flat cosmologies, including the statistical uncertainties in the number of lenses, galaxies, quasars, and the parameters relating galaxy luminosities to dynamical variables. The limit holds for either softened isothermal or de Vaucouleurs models of the lens galaxies, suggesting that the radial mass distribution in the lens galaxy is not a significant systematic uncertainty. The cosmological limits are unaffected by adding a small core radius to the isothermal lens models or by the luminosity of the lens galaxy. Inconsistent models of the effects of a core radius make significant errors in the cosmological constraints. Extinction in E/S0 galaxies can significantly reduce the limits on the cosmological constant, but changing the expected number of lenses by a factor of two requires a minimum of 100 times more dust than is observed locally in E/S0 galaxies. Rapid evolution of the lenses is the most promising means of evading these limits. However, a viable model must not only reduce the expected number of lenses but do so without significantly altering the average image separations, magnitudes, redshifts, and the relative properties of optically and radio selected lenses.

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arXiv1995

Deprojection of Axially Symmetric Objects

The deprojection of axisymmetric density distributions is generally indeterminate to within the addition of certain axisymmetric distributions (konus densities) that are invisible in projection. The known class of konus densities is expanded considerably here through the introduction of {\em semikonus functions}. These functions are closed with respect to multiplication in ordinary space, and the real part of an arbitrary polynomial of semikonus functions is a konus function. This property facilitates the construction of semikonus (and konus) functions with tailored properties, such as asymptotic forms. We also develop a simple technique for constructing several classes of konus distributions with arbitrary density profiles in the equatorial plane.

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arXiv1995

The Mass Distribution of the Lens Galaxy in MG1131+0456

We present the results of modeling 5, 8 and 15 GHz maps of the gravitational lens MG1131+0456 using the LensClean algorithm. Two models for the mass distribution in the lens were fit: a de Vaucouleurs model and a model with the two-dimensional potential phi = b^{2-alpha}(r^2+s^2)^{alpha/2}/2alpha, both in an external shear field. The best fit de Vaucouleurs model has an effective radius of R_e = 0.83+-0.13 arcseconds, larger than that measured at optical wavelengths. The best fit ``alpha model'' has alpha=0.6+-0.2 and a core radius of s = 0.19 +- 0.07 arcseconds. An isothermal model (alpha=1) is inconsistent with the data. The best alpha model fits the 8 GHz map significantly better than the best de Vaucouleurs model. Although it is difficult to distinguish visually the original image and the reconstructions, none of the models is statistically consistent with the data. We believe the primary problem is that an external shear is an inadequate model for the angular structure of the lens galaxy. The finite core radius in the models is required by the structure of the extended ring, so we confirm that the central component in the 8 GHz map is a central, lensed image of the source rather than the lens galaxy. The model lens positions, the position angle of the shear, and the mass interior to the ring are determined very precisely by the lens models. The time delay between the compact components is predicted with an uncertainty we estimate to be at most +- 9%; the formal uncertainty is +- 4%.

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