arXiv subjects

J. Lehar

Publications and source records attributed to J. Lehar.

At least 19 recordsLinked to original sources

arXiv2006

Probing the Coevolution of Supermassive Black Holes and Galaxies Using Gravitationally Lensed Quasar Hosts

In the present-day universe, supermassive black hole masses (MBH) appear to be strongly correlated with their galaxy's bulge luminosity, among other properties. In this study, we explore the analogous relationship between MBH, derived using the virial method, and the stellar R-band bulge luminosity (Lr) or stellar bulge mass (M*) at epochs of 1 < z < 4.5 using a sample of 31 gravitationally lensed AGNs and 20 non-lensed AGNs. At redshifts z > 1.7 (10--12 Gyrs ago), we find that the observed MBH--Lr relation is nearly the same (to within ~0.3 mag) as it is today. When the observed Lr are corrected for luminosity evolution, this means that the black holes grew in mass faster than their hosts, with the MBH/M* mass ratio being a factor of > 4(+2)(-1) times larger at z > 1.7 than it is today. By the redshift range 1 1.7 were fully formed bulges that passively faded to the present epoch are ruled out.

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arXiv2006

First Results from a Photometric Survey of Strong Gravitational Lens Environments

Many strong gravitational lenses lie in complex environments, such as poor groups of galaxies, that significantly bias conclusions from lens analyses. We are undertaking a photometric survey of all known galaxy-mass strong lenses to characterize their environments and include them in careful lens modeling, and to build a large, uniform sample of galaxy groups at intermediate redshifts for evolutionary studies. In this paper we present wide-field photometry of the environments of twelve lens systems with 0.24 < z_lens < 0.5. Using a red-sequence identifying technique, we find that eight of the twelve lenses lie in groups, and that ten group-like structures are projected along the line of sight towards seven of these lenses. Follow-up spectroscopy of a subset of these fields confirms these results. For lenses in groups, the group centroid position is consistent with the direction of the external tidal shear required by lens models. Lens galaxies are not all super-L_* ellipticals; the median lens luminosity is < L_*, and the distribution of lens luminosities extends 3 magnitudes below L_* (in agreement with theoretical models). Only two of the lenses in groups are the brightest group galaxy, in qualitative agreement with theoretical predictions. As in the local Universe, the highest velocity-dispersion groups contain a brightest member spatially coincident with the group centroid, whereas lower-dispersion groups tend to have an offset brightest group galaxy. This suggests that higher-dispersion groups are more dynamically relaxed than lower-dispersion groups and that at least some evolved groups exist by z ~ 0.5.

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arXiv2005

The FIRST-Optical-VLA Survey for Lensed Radio Lobes

We present results from a survey for gravitationally lensed radio lobes. Lensed lobes are a potentially richer source of information about galaxy mass distributions than lensed point sources, which have been the exclusive focus of other recent surveys. Our approach is to identify radio lobes in the FIRST catalog and then search optical catalogs for coincident foreground galaxies, which are candidate lensing galaxies. We then obtain higher-resolution images of these targets at both optical and radio wavelengths, and obtain optical spectra for the most promising candidates. We present maps of several radio lobes that are nearly coincident with galaxies. We have not found any new and unambiguous cases of gravitational lensing. One radio lobe in particular, FOV J0743+1553, has two hot spots that could be multiple images produced by a z=0.19 spiral galaxy, but the lensing interpretation is problematic.

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arXiv2003

The Redshift Distribution of Flat-Spectrum Radio Sources

The redshift distribution of flat-spectrum radio sources with 5 GHz flux densities S>5 mJy is a key component in using current radio lens surveys to probe the cosmological model. We have constructed the first flat-spectrum radio sample in the flux density range 3-20 mJy. Our new sample has 33 sources; we have determined the redshifts of 14 of these (42% complete). The low mean redshift, ~0.75, of our faintest sample needs to be confirmed by further observations to improve the sample completeness. We also increased the redshift completeness of several surveys of brighter flat-spectrum sources. While the mean redshift, ~1.1 of flat-spectrum samples fainter than 1 Jy is nearly constant, the fraction of the sources identifiable as quasars steadily drops from ~80% to ~10% as the flux density of the sources decreases.

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arXiv2002

The Evolution of a Mass-Selected Sample of Early-Type Field Galaxies

We investigate the evolution of mass-selected early-type field galaxies using a sample of 28 gravitational lenses spanning the redshift range 0 < z < 1. Based on the redshift-dependent intercept of the fundamental plane in the rest frame B band, we measure an evolution rate of d log (M/L)_B / dz = -0.56 +/- 0.04 (all errors are 1 sigma unless noted) if we directly compare to the local intercept measured from the Coma cluster. Re-fitting the local intercept helps minimize potential systematic errors, and yields an evolution rate of d log (M/L)_B / dz = -0.54 +/- 0.09. An evolution analysis of properly-corrected aperture mass-to-light ratios (defined by the lensed image separations) is closely related to the Faber-Jackson relation. In rest frame B band we find an evolution rate of d log (M/L)_B / dz = -0.41 +/- 0.21, a present-day characteristic magnitude of M_{*0} = -19.70 + 5 log h +/- 0.29 (assuming a characteristic velocity dispersion of sigma_{DM*} = 225 km/s), and a Faber-Jackson slope of gamma_{FJ} = 3.29 +/- 0.58. The measured evolution rates favor old stellar populations (mean formation redshift z_f > 1.8 at 2 sigma confidence for a Salpeter initial mass function and a flat Omega_m =0.3 cosmology) among early-type field galaxies, and argue against significant episodes of star formation at z < 1.

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arXiv2002

HST Observations of 10 Two-Image Gravitational Lenses

We report on a program to obtain HST observations of galaxy-mass gravitational lens systems at optical and infrared wavelengths. Here we discuss the properties of 10 two-image gravitational lens systems (Q0142-100=UM673, B0218+357, SBS0909+532, BRI0952-0115, LBQS1009-0252, Q1017-207=J03.13, B1030+074, HE1104-1805, Q1208+1011, and PKS1830-211). We grouped these 10 systems because they have limited lens model constraints and often show poor contrast between the images and the lens galaxy. Of the 10 lens galaxies, 7 are probably early-type galaxies, 2 are probably late-type galaxies (B0218+357 and PKS1830-211), and one was not detected (Q1208+1011). We detect the host galaxies of the z_s=4.50 lensed quasar in BRI0952-0115, the z_s=2.32 lensed quasar in HE1104-1805, and the unlensed z=1.63 quasar near LBQS1009-0252. We fit a set of four standard lens models to each lens that had sufficient constraints to compare isothermal dark matter and constant mass-to-light lens models, and to explore the effects of local tidal shears.

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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

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 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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arXiv2000

The Host Galaxy of the Lensed Quasar Q 0957+561

Infrared images of the Q 0957+561 gravitational lens obtained with the Hubble Space Telescope show two large (\sim 5 arcsec) lensed images of the z_s=1.41 quasar host galaxy. Parts of the host galaxy are doubly-imaged like the quasar, while other parts are quadruply-imaged. The distortions of the host galaxy offer the best probe yet of the global structure of the lensing potential, which is essential for determining the Hubble constant from the measured time delay. The distortions are inconsistent with the predictions of previously published lens models, which invalidates those models and their implications for H_0. New models show that the distortions finally break the long-standing degeneracy between the shape of the lens galaxy and the tidal shear contributed by the cluster containing the lens galaxy. The shape of the lens galaxy's mass distribution must be remarkably similar to the shape of its luminosity distribution, and most models that produce reasonable values for the Hubble constant roughly match the observed ellipticity gradient and isophote twist of the lens galaxy. Also, the cluster must be non-spherical and produce a relatively small tidal shear. Although there are still degeneracies in the lens models that lead to a 25% uncertainty in the derived value of the Hubble constant, there are also strong prospects for new observations to further improve the constraints and reduce the uncertainties.

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arXiv1999

Host Galaxies of Lensed Luminous Quasars at z~2

We present H-band observations of gravitationally lensed QSO host galaxies obtained with NICMOS on HST as part of the CfA-Arizona-Gravitational-Lens-Survey (CASTLES). The detections are greatly facilitated by the lensing magnification in these systems; we find that most hosts of radio-quiet QSOs (RQQ) at z~2 are of modest luminosity (L<L_*). They are 2-5 times fainter than the hosts of radio-loud QSOs at the same epoch. Compared to low redshifts, RQQ hosts at z~2 also support higher nuclear luminosities at given stellar host mass. This suggests that the supermassive black holes at their centers grew faster at early epochs than the stellar body of their surrounding host galaxies.

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arXiv1999

The Evolution of Gravitational Lens Galaxies

Most gravitational lens galaxies are early-type galaxies in relatively low density environments. We show that they lie on the same fundamental plane as early-type galaxies in both local and distant rich clusters. Their surface brightness evolution requires a typical star formation epoch of z=2-3, almost indistinguishable from that of rich cluster galaxies at comparable redshifts. The restricted galaxy type range of the lenses means that photometric redshifts work well even with only 1-3 filter photometry. We make preliminary measurements of the mass and luminosity functions of the lens galaxies, and find they are consistent with the standard model used for deriving cosmological limits using lens statistics. As expected for a mass-weighted sample, they are more massive and more luminous than the overall early-type galaxy population.

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arXiv1999

The Interstellar Medium of Lens Galaxies

We use observations from the CASTLES survey of gravitational lenses to study extinction in 23 lens galaxies with $0 < z_l < 1$. The median differential extinction between lensed images is $ΔE(\bv) = 0.05$ mag, and the directly measured extinctions agree with the amount needed to explain the differences between the statistics of radio and (optical) quasar lens surveys. We also measure the first extinction laws outside the local universe, including an $R_V=7.2$ curve for a molecular cloud at $z_l=0.68$ and an $R_V=1.5$ curve for the dust in a redshift $z_l=0.96$ elliptical galaxy.

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arXiv1999

The CASTLES gravitational lensing tool

We describe a series of new applications of gravitational lenses as astrophysical and cosmological tools. Such applications are becoming possible thanks to advances in the quality and quantity of observations. CASTLES (CfA-Arizona-Space-Telescope-LEns-Survey) {cfa-www.harvard.edu/castles} is an ongoing project that exploits the sensitivity and resolution of the Hubble Space Telescope (HST) at optical and infrared wavelengths to study the sample of over 50 known gravitational lenses. The observational goal of CASTLES is a uniform sample of multi-band images of all known galaxy-mass lens systems, to derive precise photometry and astrometry for the lens galaxies, all the known images, and any source or lens components that might have escaped detection. With these measurements we are investigating: (1) the properties of dust and of dark matter in lens galaxies out to z\sim1; (2) the dark matter in lens galaxies and in their environments; (3) the evolution of lens galaxies; and (4) the cosmological model, for instance by refining constraints on the Hubble constant H_0.

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arXiv1999

A New Wide-Separation Gravitational Lens Candidate: RXJ 0921+4529

The lens candidate RXJ 0921+4529 consists of two z_s=1.66 quasar separated by 6."93 with an H band magnitude difference of Δm=1.39. The lens appears to be a z_l=0.31 X-ray cluster, including a m_H=18.5 late-type galaxy lying between the quasar images. We detect an extended source overlapping the faint quasar but not the bright quasar. If this extended source is the host galaxy of the fainter quasar, then the system is a quasar binary rather than a gravitational lens.

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arXiv1999

The FIRST Efficient Gravitational Lens Survey

We have found three gravitational lenses (two are new) by observing 34 likely FIRST radio lobes with APM galaxy counterparts. We expect to find $\sim30$ such lenses in over the next few years, which will significantly improve lensing constraints on galaxy structure and cosmology.

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