arXiv subjects

C. S. Kochanek

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

At least 325 records · Page 18Linked to original sources

arXiv2002

Direct Detection of CDM Substructure

We devise a method to measure the abundance of satellite halos in gravitational lens galaxies, and apply our method to a sample of 7 lens systems. After using Monte Carlo simulations to verify the method, we find that substructure comprises fraction f=0.02 (median, 0.006<f<0.07 at 90% confidence) of the mass of typical lens galaxies, in excellent agreement with predictions of CDM simulations. We estimate a characteristic critical radius for the satellites of 0.0001<b/arcsec<0.006 (90% confidence). For a satellite mass function of dn/dM M^x with x=-1.8 and M_l<M<M_h, the critical radius provides an estimate that the upper mass limit is 10^6Msun < M_h < 10^9Msun. Our measurement confirms a generic prediction of CDM models, and may obviate the need to invoke alternatives to CDM like warm dark matter or self-interacting dark matter.

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arXiv2002

Completeness in Photometric and Spectroscopic Searches for Clusters

We investigate, using simulated galaxy catalogues, the completeness of searches for massive clusters of galaxies in redshift surveys or imaging surveys with photometric redshift estimates, i.e. what fraction of clusters (M>10^14/h Msun) are found in such surveys. We demonstrate that the matched filter method provides an efficient and reliable means of identifying massive clusters even when the redshift estimates are crude. In true redshift surveys the method works extremely well. We demonstrate that it is possible to construct catalogues with high completeness, low contamination and both varying little with redshift.

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arXiv2001

Detection of CDM Substructure

The properties of multiple image gravitational lenses require a fractional surface mass density in satellites of f=0.02 (0.006 < f < 0.07 at 90% confidence) that is consistent with the expectations for CDM. The characteristic satellite mass scale, 10^6-10^9 Msun, is also consistent with the expectations for CDM. The agreement between the observed and expected density of CDM substructure shows that most low mass galactic satellites fail to form stars, and this absence of star formation explains the discrepancy between the number of observed Galactic satellites and CDM predictions rather than any modification to the CDM theory such as self-interacting dark matter or a warm dark matter component.

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arXiv2001

Dynamical Probes of The Halo Mass Function

We explore the relationship between the mass function of CDM halos and dynamical probes of the mass function such as the distribution of gravitational lens separations and the local velocity function. The compression of galactic halos by the cooling baryons, a standard component of modern models, leads to a feature in the distribution of lens separations near 3 arcsec or in the velocity function near 400 km/s. The two probes of the mass function, lens separations and the local velocity function, are mutually consistent. Producing the observed velocity function of galaxies or the separation distribution using standard adiabatic compression models requires more cold baryons, an equivalent cosmological density of Omega_b=0.02 compared to a total cosmological Omega_b=0.04, than are observed in standard accountings of the baryonic content of galaxies, Omega_b=0.006$, or our Galaxy, Omega_b<0.015. The requirement for a higher cold baryon density than is usually assigned to galaxies appears to be generic to models which use the standard adiabatic compression models for the transformation of the CDM halo by the cooling baryons. If real, this dynamical baryon discrepancy suggests either that we are neglecting half of the cold baryonic mass in standard galactic models (e.g. a MACHO, cold molecular or warm gas component), or that there is a problem with standard adiabatic compression models.

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arXiv2001

Rapid Calculation of Equatorial Rotation Curves

We derive a simple, fast one-dimensional integral for the equatorial rotation curve of a thin disk with surface density Sigma(R) modeled as a spheroid with axis ratio q. The result is simpler than standard expressions even in the limit of an infinitely thin disk (q-->0).

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arXiv2001

Constraints on the long-range properties of gravity from weak gravitational lensing

Weak gravitational lensing provides a means of testing the long-range properties of gravity. Current measurements are consistent with standard Newtonian gravity and inconsistent with substantial modifications on Mpc scales. The data allows long range gravity to deviate from a 1/r potential only on scales where standard cosmology would use normal gravity but be dominated by dark matter. Thus, abnormal gravity theories must introduce two fine-tuning scales -- an inner scale to explain flat rotation curves and an outer scale to force a return to Newtonian gravity on large scales -- and these scales must coincidently match the scales produced by the dark matter theory after evolving the universe for 10 billion years starting from initial conditions which are exquisitely determined from the cosmic microwave background.

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arXiv2001

Mass Follows Light

We use comparisons between the shapes of gravitational lens galaxies and models for their mass distributions to derive statistical constraints on the alignment of the mass distribution relative to the observed lens galaxy and on the strength of tidal shear perturbations. The mass distributions are aligned with the luminous galaxies, with a 10 degree upper limit on the rms dispersion in the angle between the major axes. Statistical constraints, such as our bound on the misalignment between mass and light, are an important new approach to reducing the uncertainties in individual lens models, particularly for lenses used to estimate the Hubble constant.

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arXiv2001

Cusped Mass Models Of Gravitational Lenses

Recent observations of galaxy luminosity profiles and dark matter simulations find luminosity and mass distributions characterized by central cusps rather than finite core radii. We introduce and implement a set of cusped ellipsoidal lens models which include limits similar to the Jaffe, Hernquist, ηand NFW models and apply them to the gravitational lenses APM 08279+5255 and B 1933+503. A successful model of APM 08279+5255 with its central, odd image requires a very shallow cusp, $γ\ltorder 0.4$ where $ρ\propto r^{-γ}$ as $r\to 0$, which is similar to a core rather than the favored $1 \ltorder γ\ltorder 2$ cusps. B~1933+503, by contrast, is well modeled with a steep density cusp, $1.6 \ltorder γ\ltorder 2.0$.

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arXiv2001

Constraints on Galaxy Density Profiles from Strong Gravitational Lensing: The Case of B 1933+503

We consider a wide range of parametric mass models for B 1933+503, a ten-image radio lens, and identify shared properties of the models with the best fits. The approximate rotation curves varies by less than 8.5% from the average value between the innermost and the outermost image (1.5h^{-1} kpc to 4.1h^{-1} kpc) for models within 1 σof the best fit, and the radial dependence of the shear strength and angle also have common behavior for the best models. The time delay between images 1 and 6, the longest delay between the radio cores, is Δt = (10.6^{+2.4}_{-1.1})h^{-1} days (Ω_0=0.3, λ_0=0.7) including all the modeling uncertainties. Deeper infrared observations, to more precisely register the lens galaxy with the radio images and to measure the properties of the Einstein ring image of the radio source's host galaxy, would significantly improve the model constraints and further reduce the uncertainties in the mass distribution and time delay.

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arXiv2001

Global Probes of the Impact of Baryons on Dark Matter Halos

The halo mass function, dn/dM, predicted by hierarchical clustering models can be measured indirectly using dynamical probes like the distribution of gravitational lens image separations, dn/dDelta-theta, or halo circular velocities, dn/d v_c. These dynamical variables depend on the halo structure as well as the halo mass. Since baryonic physics, particularly cooling, significantly modifies the central density structure of dark matter halos, both observational distributions show a feature corresponding to the mass scale below which the baryons in the halo can cool (i.e. galaxies versus clusters). We use simplified but self-consistent models to show that the structural changes to the halos produced by the cooling baryons explain both distributions. Given a fixed halo mass function, matching the observed image separation distribution or local velocity function depends largely on Omega_b through its effects on the cooling time scales. These baryonic effects on the halo structure also affect the evolution of the velocity function of galaxies with redshift.

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arXiv2000

B1359+154: A Six Image Lens Produced by a z=1 Compact Group of Galaxies

HST V and I-band observations show that the gravitational lens B1359+154 consists of six images of a single z_s=3.235 radio source and its star-forming host galaxy, produced by a compact group of galaxies at z_l = 1. VLBA observations at 1.7 GHz strongly support this conclusion, showing six compact cores with similar low-frequency radio spectra. B1359+154 is the first example of galaxy-scale gravitational lensing in which more than four images are observed of the same background source. The configuration is due to the unique lensing mass distribution: three primary lens galaxies lying on the vertices of a triangle separated by 0.7 arcsec (4/h kpc), inside the 1.7 arcsec diameter Einstein ring defined by the radio images. The gravitational potential has additional extrema within this triangle, creating a pair of central images that supplement the ``standard'' four-image geometry of the outer components. Simple mass models consisting of three lens galaxies constrained by HST and VLBA astrometry naturally reproduce the observed image positions but must be finely-tuned to fit the flux densities.

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arXiv2000

Evidence for Inconsistencies in Galaxy Luminosity Functions Defined by Spectral Type

Galaxy morphological and spectroscopic types should be nearly independent of apparent magnitude in a local, magnitude-limited sample. Recent luminosity function surveys based on morphological classification of galaxies are substantially more successful at passing this test than surveys based on spectroscopic classifications. Among spectroscopic classifiers, those defined by small aperture fibers (ESP, LCRS) show far stronger systematic classification biases than those defined by large apertures (APM). This effect can be easily explained as an aperture bias, whereby galaxies with globally late-type spectra are assigned earlier spectral types which depend on the redshift and luminosity of the galaxy. The effect is demonstrated by extracting successively larger aperture spectra from long-slit spectroscopy of nearby galaxies. This systematic classification bias is generic, and the 2dFGRS and SDSS surveys will show similar systematic biases in their spectral types. We suggest several methods for correcting the problem or avoiding it altogether. If not corrected, this aperture bias can mimic galaxy evolutionary effects and distort estimates of the luminosity function.

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arXiv2000

The K-Band Galaxy Luminosity Function

We measured the K-band luminosity function using a complete sample of 4192 morphologically-typed 2MASS galaxies with 7 < K < 11.25 mag spread over 2.12 str. Early-type (T < -0.5) and late-type (T > -0.5) galaxies have similarly shaped luminosity functions, alpha_e=-0.92+/-0.10 and alpha_l=-0.87+/-0.09. The early-type galaxies are brighter, M_*e=-23.53+/-0.06 mag compared to M_*l=-22.98\pm0.06 mag, but less numerous, n_*e=(0.0045+/-0.0006)h^3/Mpc^3 compared to n_*l=(0.0101+/-0.0013)h^3/Mpc^3 for H_0=100h km/s Mpc, such that the late-type galaxies slightly dominate the K-band luminosity density, j_late/j_early=1.17+/-0.12. Our morphological classifications are internally consistent, consistent with previous classifications and lead to luminosity functions unaffected by the estimated uncertainties in the classifications. These luminosity functions accurately predict the K-band number counts and redshift distributions for K < 18 mag, beyond which the results depend on galaxy evolution and merger histories.

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arXiv2000

An Efficient Search for Gravitationally-Lensed Radio Lobes

We performed an automated comparison of the FIRST radio survey with the APM optical catalog to find radio lobes with optical counterparts. Based on an initial survey covering ~3000 square degrees, we selected a sample of 33 lens candidates for VLA confirmation. VLA and optical observations of these candidates yielded two lens systems, one a new discovery (J0816+5003), and one of which was previously known (J1549+3047). Two other candidates have radio lobes with galaxies superposed, but lack evidence of multiple imaging. One of our targets (J0958+2947) is a projected close pair of quasars (8'' separation at redshifts 2.064 and 2.744). Our search method is highly efficient, with >5% of our observing targets being lensed, compared to the usual success rate of <1%. Using the whole FIRST survey, we expect to find 5--10 lenses in short order using this approach, and the sample could increase to hundreds of lensed lobes in the Northern sky, using deeper optical surveys and planned upgrades to the VLA. Such a sample would be a powerful probe of galaxy structure and evolution.

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arXiv2000

PMN J1838-3427: A new gravitationally lensed quasar

We report the discovery of a new double-image quasar that was found during a search for gravitational lenses in the southern sky. Radio source PMN J1838-3427 is composed of two flat-spectrum components with separation 1", flux density ratio 14:1 and matching spectral indices, in VLA and VLBA images. Ground-based BRI images show the optical counterpart (total I=18.6) is also double with the same separation and position angle as the radio components. An HST/WFPC2 image reveals the lens galaxy. The optical flux ratio (27:1) is higher than the radio value probably due to differential extinction of the components by the lens galaxy. An optical spectrum of the bright component contains quasar emission lines at z=2.78 and several absorption features, including prominent Ly-alpha absorption. The lens galaxy redshift could not be measured but is estimated to be z=0.36 +/- 0.08. The image configuration is consistent with the simplest plausible models for the lens potential. The flat radio spectrum and observed variability of PMN J1838-3427 suggest the time delay between flux variations of the components is measurable, and could thus provide an independent measurement of H_0.

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arXiv2000

Multi-Frequency Analysis of the New Wide-Separation Gravitational Lens Candidate RX J0921+4529

We report the discovery of a new two-image gravitational lens candidate. The system RX J0921+4529 contains two z_s=1.66 quasars separated by 6."93 with an H band magnitude difference of $Δm=1.39$. The HST NIC2 H band images reveal an H=18.2 spiral galaxy between the quasar images, which is probably a member of a z_l=0.32 X-ray cluster centered on the field. We detect an extended source near the fainter quasar image but not in the brighter image. If this extended source is the host galaxy of the fainter quasar, then the system is a binary quasar rather than a gravitational lens. VLA observations at 3.6 cm reveal emission from the lens galaxy at the flux level of 1 mJy and a marginal detection of the brighter quasar.

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arXiv2000

The Importance of Einstein Rings

We develop a theory of Einstein rings and demonstrate it using the infrared Einstein ring images of the quasar host galaxies observed in PG1115+080, B1608+656 and B1938+666. The shape of an Einstein ring accurately and independently determines the shape of the lens potential and the shape of the lensed host galaxy. We find that the host galaxies of PG1115+080, B1608+656 and B1938+666 have axis ratios of 0.58+/-0.02, 0.69+/-0.02 and 0.62+/-0.15 including the uncertainties in the lens models. The Einstein rings break the degeneracies in the mass distributions or Hubble constants inferred from observations of gravitational lenses. In particular, the Einstein ring in PG1115+080 rules out the centrally concentrated mass distributions that lead to a high Hubble constant (H_0>60 km/s Mpc) given the measured time delays. Deep, detailed observations of Einstein rings will be revolutionary for constraining mass models and determining the Hubble constant from time delay measurements.

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arXiv2000

The Fundamental Plane of Gravitational Lens Galaxies and The Evolution of Early-Type Galaxies in Low Density Environments

Most gravitational lenses are early-type galaxies in relatively low density environments -- a ``field'' rather than a ``cluster'' population. We show that field early-type galaxies with 0 < z < 1, as represented by the lens galaxies, lie on the same fundamental plane as those in rich clusters at similar redshifts. We then use the fundamental plane to measure the combined evolutionary and K-corrections for early-type galaxies in the V, I and H bands. Only for passively evolving stellar populations formed at z > 2 (H_0=65 km/s Mpc, Omega_0=0.3, Lambda_0=0.7) can the lens galaxies be matched to the local fundamental plane. The high formation epoch and the lack of significant differences between the field and cluster populations contradict many current models of the formation history of early-type galaxies. Lens galaxy colors and the fundamental plane provide good photometric redshift estimates with an empirical accuracy of -0.03 +/- 0.11 for the 17 lenses with known redshifts. A mass model dominated by dark matter is more consistent with the data than either an isotropic or radially anisotropic constant M/L mass model, and a radially anisotropic model is better than an isotropic model.

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