arXiv subjects

Asantha R. Cooray

Publications and source records attributed to Asantha R. Cooray.

36 records · Page 2Linked to original sources

arXiv1999

An SZ Temperature Decrement - X-ray Luminosity Relation for Galaxy Clusters

We present the observed relation between the cosmic microwave background (CMB) temperature decrement due to the Sunyaev-Zeldovich (SZ) effect and the X-ray luminosity of galaxy clusters. We discuss this relation in terms of the cluster properties, and show that the slope of the observed relation is in agreement with both the L-T relation based on numerical simulations and X-ray emission observations, and the M_gas-L relation based on observations. The slope of the Delta T_SZ-L relation is also consistent with the M_tot-L relation, where M_tot is the cluster total mass based on gravitational lensing observations. This agreement may be taken to imply a constant gas mass fraction within galaxy clusters, however, there are large uncertainties, dominated by observational errors, associated with these relations. Using the Delta T_SZ-L relation and the cluster X-ray luminosity function, we evaluate the local cluster contribution to arcminute scale cosmic microwave background anisotropies. The Compton distortion y-parameter produced by massive galaxy clusters through SZ effect is roughly two orders of magnitude lower than the current upper limit based on FIRAS observations.

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arXiv1999

An Upper Limit on Omega_matter Using Lensed Arcs

We use current observations on the number statistics of gravitationally lensed optical arcs towards galaxy clusters to derive an upper limit on the cosmological mass density of the Universe. The gravitational lensing statistics due to foreground clusters combine properties of both cluster evolution, which is sensitive to the matter density, and volume change, which is sensitive to the cosmological constant. The uncertainties associated with the predicted number of lensing events, however, currently do not allow one to distinguish between flat and open cosmological models with and without a cosmological constant. Still, after accounting for known errors, and assuming that clusters in general have dark matter core radii of the order ~ 35 h^-1 kpc, we find that the cosmological mass density, Omega_m, is less than 0.56 at the 95% confidence. Such a dark matter core radius is consistent with cluster potentials determined recently by detailed numerical inversions of strong and weak lensing imaging data. If no core radius is present, the upper limit on Omega_m increases to 0.62 (95% confidence level). The estimated upper limit on Omega_m is consistent with various cosmological probes that suggest a low matter density for the Universe.

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arXiv1999

Cluster-Cluster Strong Lensing: Expectations and Detection Methods

We calculate the all-sky number of galaxy clusters that are expected to be gravitationally lensed by foreground massive clusters. We describe the redshift and number distributions of clusters using a Press-Schechter analysis, and model the foreground lensing clusters as singular isothermal spheres. If Omega_m=0.3 and Omega_Lambda=0.7, we expect ~ 30 cluster-cluster strong lensing events that involve foreground X-ray luminous clusters with total mass greater than 7.5 x 10^14 h^-1 M_sun, or X-ray luminosity L_x (2-10 keV) 8 x 10^44 h^-2 ergs s^-1, and background clusters with total mass greater than 10^14 h^-1 M_sun. The number expected in an open universe with Omega_m = 0.3 is less than \~ 4. Because of uncertainty in sigma_8, the root-mean-square density fluctuations in spheres of radius 8 h^-1 Mpc, the exact number of such lensing events is uncertain by a factor of about 5. We examine methods to detect cluster-cluster lensing events based on optical, X-ray, and Sunyaev-Zel'dovich effect observations.

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arXiv1999

Gravitational Lensing as a Probe of Quintessence

A large number of cosmological studies now suggest that roughly two-thirds of the critical energy density of the Universe exists in a component with negative pressure. If the equation of state of such an energy component varies with time, it should in principle be possible to identify such a variation using cosmological probes over a wide range in redshift. Proper detection of any time variation, however, requires cosmological probes beyond the currently studied range in redshift of $\sim$ 0.1 to 1. We extend our analysis to gravitational lensing statistics at high redshift and suggest that a reliable sample of lensed sources, out to a redshift of $\sim$ 5, can be used to constrain the variation of the equation of state, provided that both the redshift distribution of lensed sources and the selection function involved with the lensed source discovery process are known. An exciting opportunity to catalog an adequate sample of lensed sources (quasars) to probe quintessence is now available with the ongoing Sloan Digital Sky Survey. Writing $w(z)\approx w_0 + z (dw/dz)_0$, we study the expected accuracy to which the equation of state today $w_0$ and its rate of change $(dw/dz)_0$ can simultaneously be constrained. Such a determination can rule out some missing-energy candidates, such as classes of quintessence models or a cosmological constant.

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arXiv1999

Optical and Near Infrared Observations of the Afterglow of GRB 980329 from 15 Hours to 10 Days

We report I-band observations of the GRB 980329 field made on March 29 with the 1.34-m Tautenberg Schmidt telescope, R-, J- and K-band observations made on April 1 with the APO 3.5-m telescope, R- and I-band observations made on April 3 with the Mayall 4-m telescope at KPNO, and J- and K-band observations made between April 6 - 8 with the Keck-I 10-m telescope. We show that these and other reported measurements are consistent with a power-law fading of the optical/near infrared source that is coincident with the variable radio source VLA J0702+3850. This firmly establishes that this source is the afterglow of GRB 980329.

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arXiv1998

Gravitational Lensing Limits on the Average Redshift of Submillimeter Sources

The submillimeter universe has now been explored with the Submillimeter Common User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope, and a claim has been made to the presence of a new population of optically unidentified starforming galaxies at high redshifts (z \gtrsim 3). Such a population dramatically alters current views on the star formation history of the universe as well as galaxy formation and evolution. Recently, new radio identifications of the Hubble Deep Field submm sources have led to the suggestion that some of these sources are at low redshifts, however, submm source redshift distribution is still not well determined. Here, we present an upper limit to the average redshift by comparing the expected number of gravitationally lensed submm sources due to foreground cluster potentials to current observed statistics of such lensed sources. The upper limit depends on the cosmological parameters, and at the 68% confidence level, < 3.1, 4.8, 5.2, or 8.0 for (Omega,Lambda) values of (0.3,0.7), (0.5,0.5), (0.3,0.0) or (1.0,0.0) respectively. These upper limits are consistent with redshift distribution for 850 micron sources implied by starformation history models based on measured background radiation at far-infrared and submm wavelengths.

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arXiv1998

A Lower Limit on Omega-Lambda Using Gravitational Lensing in the Hubble Deep Field

We calculate the expected number of multiply-imaged galaxies in the Hubble Deep Field (HDF), using photometric redshift information for galaxies with $m_I < 27$ that were detected in all four HDF passbands. A comparison of these expectations with the observed number of strongly lensed galaxies places a lower limit on the current value of $Ω_m-Ω_Λ$, where $Ω_m$ is the cosmological mass density of the universe and $Ω_Λ$ is the normalized cosmological constant. Based on current estimates of the HDF luminosity function and associated uncertainties in individual parameters, our 95% confidence lower limit on $Ω_m-Ω_Λ$ is between -0.44, if there are no strongly lensed galaxies in the HDF, and -0.73, if there are two strongly lensed galaxies in the HDF. If the only lensed galaxy in the HDF is the one presently viable candidate, then, in a flat universe ($Ω_m+Ω_Λ=1$), $Ω_Λ < 0.79$ (95% C.L.). These lower limits are compatible with estimates based on high-redshift supernovae and with previous limits based on gravitational lensing.

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arXiv1998

Cosmological Parameters from Statistics of Strongly Lensed Radio Sources

We calculate the expected number of strongly lensed radio sources in a sample of 6500 sources observed with the Very Large Array as part of the Cosmic Lens All Sky Survey (CLASS) during the first two sessions of its observations. A comparison between the predicted and the observed number of lensed radio sources allows a determination of the current value of Omega-Lambda. If there are six strongly lensed sources in this sample, our 95% confidence upper limit on Lambda in a flat universe is 0.79. Instead of considering a simple cosmological constant, we also consider the possibility of a quintessence scalar field responsible for the additional energy density of the universe. We present our constraints on the Omega_x-w plane. We discuss the region allowed by combined gravitational lensing statistics, high redshift Type Ia supernovae distances, and globular cluster ages. Instead of a cosmological model, we can constrain the redshift distribution of faint radio sources based on the observed gravitational lensing rate and an assumed cosmological model. If there are six strongly lensed sources, the 68% confidence upper limit on the average redshift of radio sources with flux densities less than 150 mJy at 8.4 GHz is < 1.4 + (Omega-Lambda) \pm 0.1. In order to obtain a much tighter estimate on the cosmological parameters, it is essential that the redshift distribution for radio sources at the faint flux density levels be observationally determined. We strongly recommend that statistically complete optical spectroscopic programs be carried out to obtain redshifts for a representative subsample of faint background radio sources.

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arXiv1998

Gravitational Lensing and the Hubble Deep Field

We calculate the expected number of multiply-imaged galaxies in the Hubble Deep Field (HDF), using photometric redshift information for galaxies with m_I < 27 that were detected in all four HDF passbands. A comparison of these expectations with the observed number of strongly lensed galaxies constrains the current value of Omega_m-Omega_Lambda, where Omega_m is the mean mass density of the universe and Omega_Lambda is the normalized cosmological constant. Based on current estimates of the HDF luminosity function and associated uncertainties in individual parameters, our 95% confidence lower limit on Omega_m-Omega_Lambda ranges between -0.44, if there are no strongly lensed galaxies in the HDF, and -0.73, if there are two strongly lensed galaxies in the HDF. If the only lensed galaxy in the HDF is the one presently viable candidate, then, in a flat universe (Omega_m+Omega_Lambda=1), Omega_Lambda < 0.79 (95% C.L.). These limits are compatible with estimates based on high-redshift supernovae and with previous limits based on gravitational lensing.

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arXiv1998

Cosmology with Galaxy Clusters: III. Gravitationally Lensed Arc Statistics as a Cosmological Probe

We calculate the expected number of gravitationally lensed optical, radio and sub-mm lensed sources on the whole sky due to foreground galaxy clusters for different cosmological models. We improve previous calculations of lensed arc statistics by including redshift information for background sources and accounting for the redshift evolution of the foreground lensing clusters. The background sources are described based on the redshift and optical magnitude or flux distribution for sources in the Hubble Deep Field (HDF). Using the HDF luminosity function, we also account for the magnification bias in magnitude-limited observational programs to find lensed optical arcs. Based on the results from optical arc surveys, we find that the observed number of arcs can easily be explained in a flat universe with low values for cosmological mass density of the universe. At radio wavelengths (1.4 GHz), we predict \sim 1500 lensed radio sources with flux densities greater than 10 muJy, and with amplifications due to lensing greater than 2, in a flat cosmology with Omega=0.3 and Lambda=0.7. At sub-mm wavelengths (850 mum), the number of lensed sources expected towards the same foreground lens population and cosmology is \sim 3 x 10^4. We briefly consider the possibility of using the South Pole 10-m sub-mm telescope and the Planck surveyor to identify lensed sub-mm sources. A catalog of around 100 gravitationally lensed sources at 353 GHz may be a useful by-product of Planck. (Abridged)

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arXiv1998

Gravitationally Lensed Sub-mm Sources towards Galaxy Clusters

Recent observations of galaxy clusters with the SCUBA instrument on the James Clarke Maxwell Telescope have revealed a sample of gravitationally lensed sources at sub-mm wavelengths. We extend our previous calculation on the expected number of lensed optical arcs and radio sources to study the sub-mm lensed source statistics due to foreground massive clusters. For a flat cosmology with Omega_m=0.4 and Omega_Lambda =0.6, we predict roughly 1.5 \times 10^4 lensed sub-mm sources with flux densities greater than 4 mJy at 850 microns, and with amplifications due to lensing greater than 2, towards galaxy clusters with X-ray luminosities greater than 8 x 10^44 h^-2 ergs s^-1 (0.3 to 3.5 keV band). We compare our predictions with observations from the SCUBA instrument, and briefly consider the possibility of using the South Pole 10-m sub-mm telescope and the Planck surveyor to identify lensed sub-mm sources. A catalog of around 100 gravitationally lensed sources at 353 GHz may be a useful by-product of Planck.

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arXiv1998

Cosmology with Galaxy Clusters: II. Projection Effects on Hubble Constant and Gas Mass Fraction

It is well known that a combined analysis of the Sunyaev-Zel'dovich (SZ) effect and the X-ray emission observations can be used to determine the angular diameter distance to galaxy clusters, from which the Hubble constant is derived. The present values of the Hubble constant derived through the SZ/X-ray route have a broad distribution ranging from 30 to 70 km s^-1 Mpc^-1. We show that this broad distribution is primarily due to the projection effect of aspherical clusters, which have been modeled using spherical geometries. The projection effect is also expected to broaden the measured gas mass fraction of galaxy clusters. However, the projection effect either under- or overestimate the Hubble constant and the gas mass fraction in an opposite manner, producing an anticorrelation. Using the published data for SZ/X-ray clusters, we show that the current Hubble constant distribution is negatively correlated with the measured gas mass fraction for same clusters, suggesting that the projection effects are present in current results. If the gas mass fraction of galaxy clusters, when measured out to an outer hydrostatic radius is constant, it may be possible to account for the line of sight geometry of galaxy clusters. However, to perform such an analysis, an independent measurement of the total mass of galaxy clusters, such as from weak lensing, is needed. Using the weak lensing, optical velocity dispersion, SZ and X-ray data, we outline an alternative method to calculate the Hubble constant, which is subjected less to projection effects than the present method based on only the SZ and X-ray data. For A2163, the Hubble constant based on published SZ, X-ray and weak lensing observations is \sim 49 \pm 29 km s^-1 Mpc^-1.

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arXiv1998

Gravitationally Lensed muJy Radio Sources towards Galaxy Clusters

Galaxy clusters are expected to gravitationally lens background radio sources. However, due to the smaller surface density of radio sources, when compared to optical galaxies, such lensed events are rare. For an example, it is expected that there is no lensed radio source due to foreground galaxy clusters in the 1.4 GHz VLA FIRST survey. However, at the $μ$Jy level, the surface density of radio sources increases. Using the radio properties of the Hubble Deep Field (HDF) galaxies, we calculate the expected number of gravitationally lensed $μ$Jy rad io sources on the sky due to foreground galaxy clusters for different cosmological models. For a flat cosmology with $Ω_m=0.3$ and $Ω_Λ=0.7$, we predict $\sim$ 1500 lensed radio sources with flux densities $\sim$ 10 to 1000 muJy at 1.4 GHz. We discuss the possibility of detecting lensed muJy radio sources towards clusters with deep radio surveys. Given the recent detection of a sub-mm selected lensed muJy radio source towards A370, it is suggested that deep radio observations of clusters should already contain such lensed sources. Aided by amplification due to gravitational lensing, the search for lensed \muJy radio sources towards clusters are likely to recover star-forming galaxies at redshifts of 1 to 3.

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arXiv1998

Sunyaev-Zel'dovich Effect as a Cosmological Probe

We review recent results of Sunyaev-Zel'dovich effect (SZE) observations toward galaxy clusters. Using cm-wave receivers mounted on the OVRO and BIMA mm-wave arrays we have obtained high signal to noise images of the effect for more than 20 clusters. We present current estimates of the Hubble constant and cosmological parameters and discuss the potential of conducting statistical studies with large SZE cluster samples.

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arXiv1998

Cosmology with Galaxy Clusters: I. Mass Measurements and Cosmological Parameters

Under the assumption that the gas mass fraction of galaxy clusters estimated out to an outer hydrostatic radius is constant, it is possible to constrain the cosmological parameters by using the angular diameter distance relation with redshift. We applied this to a sample of galaxy clusters from redshifts of 0.1 to 0.94, for which published gas and total masses are available from X-ray data. After scaling the gas fraction values to the r_500 radius (Evrard, Metzler & Navarro 1996), we find an apparent decrease in gas fractions at high redshifts, which can be scaled back to the mean gas fraction value at z ~ 0.1 to 0.2 of (0.060 \pm 0.002) h^{-3/2}, when Ω_m = 0.55^{+0.35}_{-0.23} (1-σstatistical error, Ω_m + Ω_Λ =1). However, various sources of systematic errors can contribute to the change in gas mass fraction from one cluster to another, and we discuss such potential problems in this method.

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arXiv1998

Radio Emission towards Lensed Arcs in Galaxy Clusters

We present initial results from a search to detect radio emission at 1.4 GHz from gravitationally lensed arcs towards galaxy clusters. Using the FIRST survey observations of galaxy cluster fields, we have clearly detected 1.4 GHz emission at the location of bright arc towards the galaxy cluster Abell 370. The possibility that this emission is associated with a projected galaxy superimposed on the arc cannot be completely ruled out. However, the detection of CO(2-1) line towards this arc by Casoli et al. (1996) and the increase in 1.4 GHz diffuse emission near the arc region strongly suggest a lensed origin for detected radio emission.

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arXiv1998

Radio Sources in Galaxy Clusters at 28.5 GHz

We present serendipitous detections of radio sources at 28.5 GHz (1 cm), which resulted from our program to image thermal Sunyaev-Zeldovich (SZ) effect in 56 galaxy clusters. We find 64 radio sources with fluxes down to 0.4 mJy, and within 250 arcseconds from the pointing centers. The spectral indices (S ~ ν^-α) of 54 sources with published low frequency flux densities range from -0.6 to 2 with a mean of 0.77, and a median of 0.84. Extending low frequency surveys of radio sources towards galaxy clusters CL 0016+16, Abell 665, and Abell 2218 to 28.5 GHz, and selecting sources with 1.4 GHz flux density greater than 7 mJy to form an unbiased sample, we find a mean spectral index of 0.71 and a median of 0.71. We find 4 to 7 times more sources predicted from a low frequency survey in areas without galaxy clusters. This excess cannot be accounted for by gravitational lensing of a background radio population by cluster potentials, indicating most of the detected sources are associated with galaxy clusters. For the cluster Abell 2218, the presence of unsubtracted radio sources with 28.5 GHz flux densities less than 0.5 mJy, can only contribute to temperature fluctuations at a level of 10 to 25 \muK. The corresponding error due to radio point source contamination in the Hubble constant derived through a combined analysis of 28.5 GHz SZ images and X-ray emission observations ranges from 1% to 6%.

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arXiv1997

Stellar Occultation Observations of Saturn's North-Polar Temperature Structure

We have observed a stellar occultation of GSC5249-01240 by Saturn's north polar region on November 20, 1995 from NASA's Infrared Telescope Facility (IRTF). This is the first recorded occultation by the polar region of a giant planet. The occulted region extends 88 km in vertical height and 660 km in horizontal length, over a region from 82.5 to 85 degrees in planetocentric latitude and from 20 to 30 degrees in planetocentric longitude. Based on isothermal model fits to the light curve, we find an equivalent temperature of 130 \pm 10 K at a pressure level of 1.6 \pm 0.1 \mubar, which corresponds to a half-light latitude of 83.2 \pm 0.2 degrees and longitude of 24.1 \pm 0.5 degrees. Using numerical inversion procedures, we have retrieved the temperature profile of the occulted region, which suggests an increase in temperature (with radius) of 14.5 K between 6 and 10 \mubar. We also find temperature fluctuations of 1 to 5 K along the path probed by the occultation; if the observed temperature gradients of these fluctuations apply to the vertical direction only, then this region is super-adiabatic. More likely these thermal gradients are due to a combination of diffractive scintillations and horizontal temperature variations. Given that isothermal model fits and numerical inversions cannot separate individual contributions to observed temperature fluctuations, such as from vertical variations, horizontal variations, and scintillations, this occultation requires a further study.

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