arXiv subjects

Tomislav Kundic

Publications and source records attributed to Tomislav Kundic.

8 recordsLinked to original sources

arXiv2000

Microlensing results from APO monitoring of the double quasar Q0957+561A,B between 1995 and 1998

If the halo of the lensing galaxy 0957+561 is made of massive compact objects (MACHOs), they must affect the lightcurves of the quasar images Q0957+561 A and B differently. We search for this microlensing effect in the double quasar by comparing monitoring data for the two images A and B - obtained with the 3.5m Apache Point Observatory from 1995 to 1998 - with intensive numerical simulations. This way we test whether the halo of the lensing galaxy can be made of MACHOs of various masses. We can exclude a halo entirely made out of MACHOs with masses between 10e(-6) Msun and 10e(-2) Msun for quasar sizes of less than 3*10e(14)cm (h = 0.6), hereby extending previous limits upwards by one order of magnitude.

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arXiv2000

Modeling Galaxy Lenses

In order to use a gravitational lens to measure the Hubble constant accurately, it is necessary to derive a reliable model of the lens surface potential. If the analysis is restricted to the locations and magnifications of point images, the derived Hubble constant depends upon the class of mass models used to fit the data. However, when there is extended emission from an Einstein ring, it may be possible to derive a potential from the observed surface brightness in a model-independent manner. This procedure is illustrated with reference to B1608+656. The multi-band images are de-reddened, de-convolved and de-contaminated so that the luminous matter and the surface brightness contours in the Einstein ring are both faithfully mapped. This intensity distribution can then be used to reconstruct the potential. Progress in implementing this program is reported. The observed incidence of multiple-imaged galaxies on the Hubble Deep Fields is an order of magnitude smaller than naively predicted on the basis of radio lens surveys, like CLASS, but consistent with the rate computed using surface photometry of candidate lens galaxies assuming standard mass to light ratios. In order to resolve this paradox, it is suggested that most galaxy lenses are located in compact groups.

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arXiv1997

The External Shear Acting on Gravitational Lens B 1422+231

In a number of multiply imaged quasar systems, a significant contribution to the lensing potential is provided by groups and clusters of galaxies associated with the primary lens. As part of an ongoing effort to gather observational data on these systems, we present spectroscopy and near-infrared and optical photometry of galaxies in the field of the quadruple lens system B 1422+231. The spectra show that the primary lens and five nearby galaxies belong to a compact group at z = 0.338. The median projected radius of this group is 35 h^{-1} kpc and its velocity dispersion is 550 km/s. A straightforward application of the virial theorem yields a group mass of 1.4 x 10^{13} h^{-1} M(sun), which provides sufficient external shear to produce the observed image configuration. This data rules out a class of models and improves the system's prospects for a measurement of the Hubble constant.

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arXiv1997

Keck Spectroscopy of the Gravitational Lens System PG 1115+080: Redshifts of the Lensing Galaxies

The quadruple system PG 1115+080 is the second gravitational lens with a reported measurement of the Hubble constant. In addition to the primary lens, three nearby galaxies are believed to contribute significantly to the lensing potential. In this paper we report accurate redshifts for all four galaxies and show that they belong to a single group at z_d = 0.311. This group has very similar properties to Hickson's compact groups of galaxies found at lower redshifts. We briefly discuss implications for the existing lens models and derive H_0 = 52 +/- 14 km/s/Mpc.

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arXiv1997

A Robust Determination of the Time Delay in 0957+561A,B and a Measurement of the Global Value of Hubble's Constant

Photometric monitoring of the gravitational lens system 0957+561A,B in the g and r bands with the Apache Point Observatory (APO) 3.5 m telescope during 1996 shows a sharp g band event in the trailing (B) image light curve at the precise time predicted from the observation of an event during 1995 in the leading (A) image with a delay of 415 days. This success confirms the "short delay," and the lack of any feature at a delay near 540 days rejects the "long delay" for this system, resolving a long-standing controversy. A series of statistical analyses of our light curve data yield a best fit delay of 417 +/- 3 days (95% confidence interval). Recent improvements in the modeling of the lens system (consisting of a galaxy and cluster) allow us to derive a value of the global (at z = 0.36) value of Hubble's constant H_0 using Refsdal's method, a simple and direct distance determination based on securely understood physics and geometry. The result is H_0 = 63 +/- 12 km/s/Mpc (for Omega = 1) where this 95% confidence interval is dominated by remaining lens model uncertainties.

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arXiv1996

Gravitational Lensing and the Extragalactic Distance Scale

The potential of gravitational lenses for providing direct, physical measurements of the Hubble constant, free from systematic errors associated with the traditional distance ladder, has long been recognized. However, it is only recently that there has been a convincing measurement of a time delay sufficiently accurate to carry out this program. By itself, an accurate time delay measurement does not produce an equivalently definite Hubble constant and the errors associated with models of the primary lens, propagation through the potential fluctuations produced by the large-scale structure and the global geometry of the universe must also be taken into account. The prospects for measuring several more time delays and the feasibility of making the corresponding estimates of the Hubble constant with total error smaller than ten percent are critically assessed.

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arXiv1996

Infrared Counterpart of the Gravitational Lens 1938+66.6

We report the detection of a very red source coincident with the gravitational lens 1938+66.6 (Patnaik et al. 1992) in K' (2.12 micron), H (1.6 micron), J (1.25 micron), and Thuan-Gunn r (0.65 micron) bands. 1938+66.6 has previously been detected as a partial radio ring indicating lensing. We find K'=17.1 +- 0.1 and r = 23.9 +- 0.2, making it a very red source with (r-K')=6.8 +- 0.25. We also observed in Thuan-Gunn g band (0.49 micron) and found g>24.5 at the 90% confidence level. We interpret our observations as a reddened gravitational lens on the basis of its optical-IR color and positional coincidence with the radio source.

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arXiv1995

Gravitational Microlensing by Random Motion of Stars: Analysis of Light Curves

We present a quantitative analysis of the effect of microlensing caused by random motion of individual stars in the galaxy which is lensing a background quasar. We calculate a large number of magnification patterns for positions of the stars slightly offset from one frame to the next, and in this way obtain light curves for fixed quasar and galaxy positions, only due to the change in the relative star positions. These light curves are analyzed to identify micro- lensing events, which are classified according to height, duration, and slope. We find that microlensing events produced by random motion of stars are shorter, steeper, and more frequent than bulk motion events. This difference is caused by the fact that in the case of random motion, caustics can move with arbitrarily high velocity, producing very short events. An accompanying video illustrates these results. For three different values of the surface mass density kappa, it shows time sequences of 1000 magnification patterns for slowly changing lens positions, together with the positions and velocity vectors of the microlensing stars. Short sequences of the video are available as MPEG movie by anonymous ftp at ftp://astro.princeton.edu/jkw/microlensing/moving_stars .

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