arXiv subjects

Tom Broadhurst

Publications and source records attributed to Tom Broadhurst.

At least 163 records · Page 9Linked to original sources

arXiv2009

Dynamical Study of A1689 from Wide-Field VLT/VIMOS Spectroscopy: Mass Profile, Concentration Parameter, and Velocity Anisotropy

We examine the dynamics structure of the rich cluster A1689, combining VLT/VIMOS spectroscopy with Subaru/Suprime-Cam imaging. The radial velocity distribution of $\sim 500$ cluster members is bounded by a pair of clearly defined velocity caustics, with a maximum amplitude of $\sim|4000|$ km/s at $\simeq$ 300 h$^{-1}$ kpc, beyond which the amplitude steadily declines, approaching zero velocity at a limiting radius of $\sim$ 2 h$^{-1}$ Mpc. We derive the 3D velocity anisotropy and galaxy number density profiles using a model-independent method to solve the Jeans equation, simultaneously incorporating the observed velocity dispersion profile, the galaxy counts from deep Subaru imaging, and our previously derived cluster mass profile from a joint lensing and X-ray analysis. The velocity anisotropy is found to be predominantly radial at large radius, becoming increasingly tangential towards the center, in accord with expectations. We also analyze the galaxy data independently of our previous analysis using two different methods: The first is based on a solution of the Jeans equation assuming an NFW form for the mass distribution, whereas in the second method the caustic amplitude is used to determine the escape velocity. The cluster virial mass derived by both of these dynamical methods is in good agreement with results from our earlier lensing and X-ray analysis. We also confirm the high NFW concentration parameter, with results from both methods combined to yield $c_{\rm vir}>13$ (1$σ$). The inferred virial radius is consistent with the limiting radius where the caustics approach zero velocity and where the counts of cluster members drop off, suggesting that infall onto A1689 is currently not significant.

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arXiv2009

z~7-10 Galaxies behind Lensing Clusters: Contrast with Field Search Results

We conduct a search for z>~7 dropout galaxies behind 11 massive lensing clusters using 21 arcmin**2 of deep HST NICMOS, ACS, and WFPC2 image data. In total, over this entire area, we find only one robust z~7 z-dropout candidate (previously reported around Abell 1689). Four less robust z-dropout and J-dropout candidates are also found. The nature of the four weaker candidates could not be precisely determined due to the limited depth of the available optical data, but detailed simulations suggest that all four could be low-redshift interlopers. We compare these numbers with what we might expect using the z~7 UV luminosity function (LF) determined from field searches. We predict 2.7 z~7 z-dropouts and 0.3 z~9 J-dropouts over our cluster search area, in reasonable agreement with our observational results, given the small numbers. The number of z>~7 candidates we find in the present search are much lower than has been reported in several previous studies of the prevalence of z>~7 galaxies behind lensing clusters. To understand these differences, we examined z>~7 candidates in other studies and conclude that only a small fraction are likely to be z>~7 galaxies. Our findings support models that show that gravitational lensing from clusters is of the most value for detecting galaxies at magnitudes brighter than L* (H<~ 27) where the LF is expected to be very steep. Use of these clusters to constrain the faint-end slope or determine the full LF is likely of less value due to the shallower effective slope measured for the LF at fainter magnitudes, as well as significant uncertainties introduced from modelling both the gravitational lensing and incompleteness.

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arXiv2009

Mass and Hot Baryons in Massive Galaxy Clusters from Subaru Weak Lensing and AMiBA SZE Observations

We present a multiwavelength analysis of a sample of four hot (T_X>8keV) X-ray galaxy clusters (A1689, A2261, A2142, and A2390) using joint AMiBA Sunyaev-Zel'dovich effect (SZE) and Subaru weak lensing observations, combined with published X-ray temperatures, to examine the distribution of mass and the intracluster medium (ICM) in massive cluster environments. Our observations show that A2261 is very similar to A1689 in terms of lensing properties. Many tangential arcs are visible around A2261, with an effective Einstein radius \sim 40 arcsec (at z \sim 1.5), which when combined with our weak lensing measurements implies a mass profile well fitted by an NFW model with a high concentration c_{vir} \sim 10, similar to A1689 and to other massive clusters. The cluster A2142 shows complex mass substructure, and displays a shallower profile (c_{vir} \sim 5), consistent with detailed X-ray observations which imply recent interaction. The AMiBA map of A2142 exhibits an SZE feature associated with mass substructure lying ahead of the sharp north-west edge of the X-ray core suggesting a pressure increase in the ICM. For A2390 we obtain highly elliptical mass and ICM distributions at all radii, consistent with other X-ray and strong lensing work. Our cluster gas fraction measurements, free from the hydrostatic equilibrium assumption, are overall in good agreement with published X-ray and SZE observations, with the sample-averaged gas fraction of = 0.133 \pm 0.027, for our sample = (1.2 \pm 0.1) \times 10^{15} M_{sun} h^{-1}. When compared to the cosmic baryon fraction f_b = Ω_b/Ω_m constrained by the WMAP 5-year data, this indicates /f_b = 0.78 \pm 0.16, i.e., (22 \pm 16)% of the baryons are missing from the hot phase of clusters.

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arXiv2008

Large Einstein Radii: A Problem for LambdaCDM

The Einstein radius of a cluster provides a relatively model-independent measure of the mass density of a cluster within a projected radius of ~ 150 kpc, large enough to be relatively unaffected by gas physics. We show that the observed Einstein radii of four well-studied massive clusters, for which reliable virial masses are measured, lie well beyond the predicted distribution of Einstein radii in the standard LambdaCDM model. Based on large samples of numerically simulated cluster-sized objects with virial masses ~ 1e15 solar, the predicted Einstein radii are only 15-25'', a factor of two below the observed Einstein radii of these four clusters. This is because the predicted mass profile is too shallow to exceed the critical surface density for lensing at a sizable projected radius. After carefully accounting for measurement errors as well as the biases inherent in the selection of clusters and the projection of mass measured by lensing, we find that the theoretical predictions are excluded at a 4-sigma significance. Since most of the free parameters of the LambdaCDM model now rest on firm empirical ground, this discrepancy may point to an additional mechanism that promotes the collapse of clusters at an earlier time thereby enhancing their central mass density.

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arXiv2008

Comparison of Cluster Lensing Profiles with Lambda CDM Predictions

We derive lens distortion and magnification profiles of four well known clusters observed with Subaru. Each cluster is very well fitted by the general form predicted for Cold Dark Matter (CDM) dominated halos, with good consistency found between the independent distortion and magnification measurements. The inferred level of mass concentration is surprisingly high, 8 = 10.4 \pm 0.9), compared to the relatively shallow profiles predicted by the Lambda CDM model, c_{vir}=5.1 \pm 1.1 (for =1.25\times 10^{15}M_{\odot}/h). This represents a 4sigma discrepancy, and includes the relatively modest effects of projection bias and profile evolution derived from N-body simulations, which oppose each other with little residual effect. In the context of CDM based cosmologies, this discrepancy implies clusters collapse earlier (z\geq 1) than predicted (z<0.5), when the Universe was correspondingly denser.

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arXiv2008

Observations of the Gas Reservoir around a Star Forming Galaxy in the Early Universe

We present a high signal-to-noise spectrum of a bright galaxy at z = 4.9 in 14 h of integration on VLT FORS2. This galaxy is extremely bright, i_850 = 23.10 +/- 0.01, and is strongly-lensed by the foreground massive galaxy cluster Abell 1689 (z=0.18). Stellar continuum is seen longward of the Ly-alpha emission line at ~7100 Å, while intergalactic H I produces strong absorption shortward of Ly-alpha. Two transmission spikes at ~6800 Angstroms (A) and ~7040 A are also visible, along with other structures at shorter wavelengths. Although fainter than a QSO, the absence of a strong central ultraviolet flux source in this star forming galaxy enables a measurement of the H I flux transmission in the intergalactic medium (IGM) in the vicinity of a high redshift object. We find that the effective H I optical depth of the IGM is remarkably high within a large 14 Mpc (physical) region surrounding the galaxy compared to that seen towards QSOs at similar redshifts. Evidently, this high-redshift galaxy is located in a region of space where the amount of H I is much larger than that seen at similar epochs in the diffuse IGM. We argue that observations of high-redshift galaxies like this one provide unique insights on the nascent stages of baryonic large-scale structures that evolve into the filamentary cosmic web of galaxies and clusters of galaxies observed in the present universe.

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arXiv2008

Combining Lens Distortion and Depletion to Map the Mass Distribution of A1689

[Abridged] We derive a projected 2D mass map of the well studied galaxy cluster A1689 based on an entropy-regularized maximum-likelihood combination of the lens magnification and distortion of red background galaxies registered in deep Subaru images. The known strong lensing information is also readily incorporated in this approach, represented as a central pixel with a mean surface density close to the critical value. We also utilize the distortion measurements to locally downweight the intrinsic clustering noise, which otherwise perturbs the depletion signal. The projected mass profile continuously steepens with radius and is well fitted by the NFW model, but with a surprising large concentration c_vir=13.4^{+5.3}_{-3.3}, lying far from the predicted value of c_vir ~ 5, corresponding to the measured virial mass, M_vir=(2.1\pm 0.2)\times 10^15M_{\odot}, posing a challenge to the standard assumptions defining the LCDM model. We examine the consistency of our results with estimates derived with the standard weak lensing estimators and by comparison with the inner mass profile obtained from strong lensing. All the reconstructions tested here imply a virial mass in the range, M_vir=(1.5-2.1)\times 10^15M_{\odot}, and the combined ACS and Subaru-2D mass reconstruction yields a tight constraint on the concentration parameter, c_vir=12.7 \pm 1(stat.) \pm 2.8(systematic) (c_200 ~ 10), improving upon the statistical accuracy of our earlier 1D analysis. Importantly, our best fitting profile properly reproduces the observed Einstein radius of 45 arcsec (z_s=1), in contrast to other weak lensing work, reporting lower concentration profiles, which underestimate the observed Einstein radius.

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arXiv2008

The SDSS Discovery of a Strongly Lensed Post-Starburst Galaxy at z=0.766

We present the first result of a survey for strong galaxy-galaxy lenses in Sloan Digital Sky Survey (SDSS) images. SDSS J082728.70+223256.4 was selected as a lensing candidate using selection criteria based on the color and positions of objects in the SDSS photometric catalog. Follow-up imaging and spectroscopy showed this object to be a lensing system. The lensing galaxy is an elliptical at z = 0.349 in a galaxy cluster. The lensed galaxy has the spectrum of a post-starburst galaxy at z = 0.766. The lensing galaxy has an estimated mass of $\sim 1.2 \times 10^{12} M_{\odot}$ and the corresponding mass to light ratio in the B-band is $\sim 26 M_{\odot}/L_{\odot}$ inside 1.1 effective radii of the lensing galaxy. Our study shows how catalogs drawn from multi-band surveys can be used to find strong galaxy-galaxy lenses having multiple lens images. Our strong lensing candidate selection based on photometry-only catalogs will be useful in future multi-band imaging surveys such as SNAP and LSST.

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arXiv2008

The Third Image of the Large-Separation Lensed Quasar SDSS J1029+2623

We identify a third image in the unique quasar lens SDSS J1029+2623, the second known quasar lens produced by a massive cluster of galaxies. The spectrum of the third image shows similar emission and absorption features, but has a redder continuum than the other two images which can be explained by differential extinction or microlensing. We also identify several lensed arcs. Our observations suggest a complicated structure of the lens cluster at z~0.6. We argue that the three lensed images are produced by a naked cusp on the basis of successful mass models, the distribution of cluster member galaxies, and the shapes and locations of the lensed arcs. Lensing by a naked cusp is quite rare among galaxy-scale lenses but is predicted to be common among large-separation lensed quasars. Thus the discovery can be viewed as support for an important theoretical prediction of the standard cold dark matter model.

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arXiv2007

Probing the Cluster Mass Distribution using Subaru Weak Lensing Data

We present results from a weak lensing analysis of the galaxy cluster A1689 (z=0.183) based on deep wide-field imaging data taken with Suprime-Cam on Subaru telescope. A maximum entropy method has been used to reconstruct directly the projected mass distribution of A1689 from combined lensing distortion and magnification measurements of red background galaxies.The resulting mass distribution is clearly concentrated around the cD galaxy, and mass and light in the cluster are similarly distributed in terms of shape and orientation. The azimuthally-averaged mass profile from the two-dimensional reconstruction is in good agreement with the earlier results from the Subaru one-dimensional analysis of the weak lensing data, supporting the assumption of quasi-circular symmetry in the projected mass distribution of the cluster.

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arXiv2007

Using Weak Lensing Dilution to Improve Measurements of the Luminous and Dark Matter in A1689

The E/SO sequence of a cluster defines a boundary redward of which a reliable weak lensing signal can be obtained from background galaxies, uncontaminated by cluster members. For bluer colors, both background and cluster members are present, reducing the distortion signal by the proportion of unlensed cluster members. In deep Subaru and HST/ACS images of A1689 the tangential distortion of galaxies with bluer colors falls rapidly toward the cluster center relative to the lensing signal of the red background. We use this dilution effect to derive the cluster light profile and luminosity function to large radius, with the advantage that no subtraction of far-field background counts is required. The light profile declines smoothly to the limit of the data, r<2Mpc/h, with a constant slope, dlog(L)/dlog(r)=-1.12+-0.06, unlike the lensing mass profile which steepens continuously with radius, so that M/L peaks at an intermediate radius, ~100kpc/h. A flatter behavior is found for the more physically meaningful ratio of dark-matter to stellar-matter, when accounting for the color-mass relation of cluster members. The cluster luminosity function has a flat slope, alpha=-1.05+-0.07, independent of radius and with no faint upturn to M_i'<-12. We establish that the very bluest objects are negligibly contaminated by the cluster V-i'<0.2, because their distortion profile rises towards the center following the red background, but offset higher by ~20%. This larger amplitude is consistent with the greater estimated depth of the faint blue galaxies, z~=2.0 compared to z~=0.85 for the red background, a purely geometric effect related to cosmological parameters. Finally, we improve upon our earlier mass profile by combining both the red and blue background populations, clearly excluding low concentration CDM profiles.

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arXiv2006

The effect of FIR emission from SDSS galaxies on the SFD Galactic extinction map

We compare the most successful and widely used map of Galactic dust extinction, provided by Schlegel, Finkbeiner & Davis (1998; hereafter SFD), to the galaxy number counts in the Sloan Digital Sky Survey (SDSS) photometric/spectroscopic DR4 sample. We divide the SDSS survey area into 69 disjoint subregions according to the dust extinction provided by SFD and compare the surface number density of galaxies in each subregion. As expected, the galaxy surface number density decreases with increasing extinction but only for SFD extinction values above about 0.1 to 0.2 magnitudes (depending on the band). At lower values of the SFD extinction, we find that the sky surface density of galaxies increases with increasing extinction, precisely the opposite of the effect expected from Galactic dust. We suggest that the far infrared (FIR) brightness of the sky in regions of true low dust extinction is significantly ``contaminated'' by the FIR emission from background galaxies. We show that such an explanation is both qualitatively and quantitatively consistent with the available data. Based on this interpretation we conclude that systematic errors in the SFD extinction map due to extragalactic FIR emission are quite small, of order hundredths of a magnitude, but nevertheless statistically detectable. (Abridged)

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arXiv2005

Can the Steep Mass Profile of A1689 Be Explained by a Triaxial Dark Halo?

The steep mass profile of A1689 derived from recent detailed lensing observations is not readily reconciled with the low concentration halos predicted by the standard CDM model. However, halo triaxiality may act to bias the profile constraints derived assuming a spherically symmetric mass distribution, since lensing relates only to the projected mass distribution. A degree of halo triaxiality is inherent to the CDM structure formation, arising from the collision-less nature of the dark matter. Here we compare the CDM-based model predictions of triaxial halo with the precise lensing measurements of A1689 based on the ACS/HST and Subaru data, over a wide range of 10kpc<r<2Mpc. The model lensing profiles cover the intrinsic spread of halo mass and shape, and are projected over all inclinations when comparing with the data. We show that the model parameters are only weakly constrained and strongly degenerate mainly because of the lack of information along the line of sight. In particular, the limits on the concentration parameter become less restrictive with increasing triaxiality. Yet, by comparing the obtained constraints with expected probability distributions for the axis ratio and concentration parameters computed from numerical simulations, we find that ~6% of cluster-size halos in the CDM model can match the A1689 lensing observations at the 2-sigma level, corresponding to cases where the major-axis of the halo is aligned with the line of sight. Thus halo triaxiality could reduce the apparent discrepancy between theory and observation. This hypothesis needs to be further explored by a statistical lensing study for other clusters as well as by complementary three-dimensional information derived using X-ray, kinematics, and SZ effect observations.

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arXiv2005

The ALHAMBRA Survey: For a systematic Study of Cosmic Evolution

ALHAMBRA is a project to gather data to sample a fraction of the Universe with enough precision to follow the evolution of its content and properties with z, a kind of Cosmic Tomography. It is defined as a 4 square degrees photometric survey with 20 contiguous, equal width, medium band filters covering from 3500 to 9700 A, plus the JHKs NIR bands. It was optimized to get (for a fixed observing time) the maximum number of objects with accurate classification and redshift and to be sensitive to relatively faint emission features. The observations will be carried out with the 3.5m telescope in Calar Alto (Spain). We intend to reach the limit AB=25 in all the optical filters from the bluest to 8300 A, and from 24.7 to 23.4 for the remainder. The expected limit in the NIR is fixed at Ks=20, H=21, J=22. The homogeneous and contiguous spectral coverage will result in several hundred thousand objects with accurate SED identification and z-values. The accuracy of the survey will allow to make detailed studies, including galactic, without the need for any further follow-up. Indeed, it will provide exciting targets for large telescopes, the GTC in particular.

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arXiv2004

The Surprisingly Steep Mass Profile of Abell 1689, from a Lensing Analysis of Subaru Images

Subaru observations of A1689 (z=0.183) are used to derive an accurate, model-independent mass profile for the entire cluster, r<2 Mpc/h, by combining magnification bias and distortion measurements. The projected mass profile steepens quickly with increasing radius, falling away to zero at r~1.0 Mpc/h, well short of the anticipated virial radius. Our profile accurately matches onto the inner profile, r<200 kpc/h, derived from deep HST/ACS images. The combined ACS and Subaru information is well fitted by an NFW profile with virial mass, (1.93 \pm 0.20)10^15 M_sun, and surprisingly high concentration, c_vir=13.7^{+1.4}_{-1.1}, significantly larger than theoretically expected (c_vir~4), corresponding to a relatively steep overall profile. A slightly better fit is achieved with a steep power-law model that has its 2D logarithmic slope -3 and core radius theta_c~1.7' (r_c~210 kpc/h), whereas an isothermal profile is strongly rejected. These results are based on a reliable sample of background galaxies selected to be redder than the cluster E/S0 sequence. By including the faint blue galaxy population a much smaller distortion signal is found, demonstrating that blue cluster members significantly dilute the true signal for r~400 kpc/h. This contamination is likely to affect most weak lensing results to date.

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arXiv2004

Probing Halos of Galaxies at Very Large Radii using Background QSOs

(ABRIDGED) Gaseous halos of nine nearby galaxies (with redshifts cz < 6000 km/s) were probed at large galactocentric radii using background quasars observed with HST GHRS and STIS. The projected quasar-galaxy separations range from 55 to 387 h^{-1}_{75} kpc. Lyαabsorption lines were successfully detected in the spectra of five quasars, at impact parameters of up to 170 h^{-1}_{75} kpc from the center of the nearby galaxy, and in each case at wavelengths consistent with the galaxy's redshift. Our observations include the lowest redshift Lyαlines detected to date. HI velocity fields were obtained at the VLA for three of the galaxies in our sample (and in one case was available from the literature), to derive their rotation curves. When comparing the inner rotation curves of the galaxies with the velocity at large radius provided by the Lyαline it is apparent that it is very difficult to explain the observed Lyαvelocity as due to gas in an extended rotating disk. In most cases one would need to invoke large warps in the outer gas disks and also thick gas disks in order to reconcile the observed velocities with the predicted ones. In light of these results we conclude that in a typical galaxy there is no longer detectable atomic gas corotating in an extended disk at radii > 35 α^{-1}, where α^{-1} is the stellar disk exponential scale-length. The cosmic web is the most likely origin for the detected Lyαlines.

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arXiv2004

Strong Lensing Analysis of A1689 from Deep Advanced Camera Images

We analyse deep multi-colour Advanced Camera images of the largest known gravitational lens, A1689. Radial and tangential arcs delineate the critical curves in unprecedented detail and many small counter-images are found near the center of mass. We construct a flexible light deflection field to predict the appearance and positions of counter-images. The model is refined as new counter-images are identified and incorporated to improve the model, yielding a total of 106 images of 30 multiply lensed background galaxies, spanning a wide redshift range, 1.0$<$z$<$5.5. The resulting mass map is more circular in projection than the clumpy distribution of cluster galaxies and the light is more concentrated than the mass within $r<50kpc/h$. The projected mass profile flattens steadily towards the center with a shallow mean slope of $d\logΣ/d\log r \simeq -0.55\pm0.1$, over the observed range, r$<250kpc/h$, matching well an NFW profile, but with a relatively high concentration, $C_{vir}=8.2^{+2.1}_{-1.8}$. A softened isothermal profile ($r_{core}=20\pm2$\arcs) is not conclusively excluded, illustrating that lensing constrains only projected quantities. Regarding cosmology, we clearly detect the purely geometric increase of bend-angles with redshift. The dependence on the cosmological parameters is weak due to the proximity of A1689, $z=0.18$, constraining the locus, $Ω_M+Ω_Λ \leq 1.2$. This consistency with standard cosmology provides independent support for our model, because the redshift information is not required to derive an accurate mass map. Similarly, the relative fluxes of the multiple images are reproduced well by our best fitting lens model.

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arXiv2004

Monitoring lensed starlight emitted close to the Galactic Center

We describe the feasibility of detecting the gravitational deflection of light emitted by stars moving under the influence of the massive object at the Galactic center. Light emitted by a star orbiting behind the central mass has a smaller impact parameter than the star itself, and suffers the effect of gravitational lensing, providing a closer probe of the central mass distribution and hence a stricter test of the black hole hypothesis. A mass of $4.3\times 10^{6} M_{\odot}$ causes a $0.1-2\rm mas$ deviation in the apparent position of orbiting stars projected within $10^{\circ}$ of the line of sight to the galactic center. In addtion, we may uniquely constrain the distance to the center of the galaxy because lensing deflections constrain the ratio $\rg/R_{0}$ of the Schwarzschild radius to the distance to the black hole, $R_{o}$, whereas the ratio $\rg/R_{o}^{3}$ is obtained by fitting the orbit.

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