arXiv subjects

Rennan Barkana

Publications and source records attributed to Rennan Barkana.

112 records · Page 7Linked to original sources

arXiv1999

The Photo-Evaporation of Dwarf Galaxies During Reionization

During the period of reionization the Universe was filled with a cosmological background of ionizing radiation. By that time a significant fraction of the cosmic gas had already been incorporated into collapsed galactic halos with virial temperatures below about 10000 K that were unable to cool efficiently. We show that photoionization of this gas by the fresh cosmic UV background boiled the gas out of the gravitational potential wells of its host halos. We calculate the photoionization heating of gas inside spherically symmetric dark matter halos, and assume that gas which is heated above its virial temperature is expelled. In popular Cold Dark Matter models, the Press-Schechter halo abundance implies that about 50-90% of the collapsed gas was evaporated at reionization. The gas originated from halos below a threshold circular velocity of 10-15 km/s. The resulting outflows from the dwarf galaxy population at redshifts 5-10 affected the metallicity, thermal and hydrodynamic state of the surrounding intergalactic medium. Our results suggest that stellar systems with a velocity dispersion below about 10 km/s, such as globular clusters or the dwarf spheroidal galaxies of the Local Group, did not form directly through cosmological collapse at high redshifts.

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arXiv1999

A possible gravitational lens in the Hubble Deep Field South

We model an apparent gravitational lens system HDFS 2232509-603243 in the Hubble Deep Field South. The system consists of a blue V=25 mag arc separated by 0.9 arcsec from a red V=22 mag elliptical galaxy. A mass distribution which follows the observed light distribution with a constant mass-to-light ratio can fit the arc component positions if external shear is added. A good fit is also obtained with simple parameterized models, and all the models predict a forth image fainter than the detection limit. The inferred mass-to-light ratio is roughly 15 in solar units if the lens is at redshift 0.6. The lens models predict a velocity dispersion of about 280 km/s which could be confirmed with spectroscopy.

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arXiv1998

Fast calculation of a family of elliptical mass gravitational lens models

Because of their simplicity, axisymmetric mass distributions are often used to model gravitational lenses. Since galaxies are usually observed to have elliptical light distributions, mass distributions with elliptical density contours offer more general and realistic lens models. They are difficult to use, however, since previous studies have shown that the deflection angle (and magnification) in this case can only be obtained by rather expensive numerical integrations. We present a family of lens models for which the deflection can be calculated to high relative accuracy (one part in 100000) with a greatly reduced numerical effort, for small and large ellipticity alike. This makes it easier to use these distributions for modelling individual lenses as well as for applications requiring larger computing times, such as statistical lensing studies. A program implementing this method can be obtained from the author (or at http://www.sns.ias.edu/~barkana/ellip.html).

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arXiv1997

Analysis of Time Delays in the Gravitational Lens PG1115+080

We present a new method for determining time delays among the light curves of various images in a gravitational lens. The method is based on constructing a simple model for the source variation and forming a chi squared measure of the agreement of this same variation with all of the lightcurves. While inspired by Press et al. (1992a, b) our approach is different since we do not assume a Gaussian process for the source variation. We apply our method to the light curves of the quadruple gravitational lens PG1115+080 measured by Schechter et al. (1997). Unlike Schechter et al. we include correlated measurement errors in the analysis, as well as the possibility that microlensing may cause different images to vary by different factors in flux. We find a value of 25.0 (+3.3,-3.8) days (95% confidence) for the delay between components B and C (close to the 24 day value of Schechter et al). However, the ratio t_AC/t_BA of the two intermediate delays is poorly determined at 1.13 (+.18,-.17) (68% confidence), close to the value predicted by lens models (1.4) unlike the Schechter et al. value (0.7). The variation ratios of C with respect to A and of A with respect to B are both different from 1, 1.39 (+.16,-.20) and .79 (+.10,-.12) (95% confidence), respectively. This is an indication of a microlensing gradient, and this type of microlensing may allow us to conclude that the size of the quasar optical emission region is about 1000 AU.

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