Cosmic Horseshoe Research Articles

Inner dark matter distribution of the Cosmic Horseshoe (J1148+1930) with gravitational lensing and dynamics

We present a detailed analysis of the inner mass structure of the Cosmic Horseshoe (J1148+1930) strong gravitational lens system observed with theHubbleSpace Telescope (HST) Wide Field Camera 3 (WFC3). In addition to the spectacular Einstein ring, this systems shows a radial arc. We obtained the redshift of the radial arc counterimagezs, r = 1.961 ± 0.001 from Gemini observations. To disentangle the dark and luminous matter, we considered three different profiles for the dark matter (DM) distribution: a power law profile, the Navarro, Frenk, and White (NFW) profile, and a generalized version of the NFW profile. For the luminous matter distribution, we based the model on the observed light distribution that is fitted with three components: a point mass for the central light component resembling an active galactic nucleus, and the remaining two extended light components scaled by a constant mass-to-light ratio (M/L). To constrain the model further, we included published velocity dispersion measurements of the lens galaxy and performed a self-consistent lensing and axisymmetric Jeans dynamical modeling. Our model fits well to the observations including the radial arc, independent of the DM profile. Depending on the DM profile, we get a DM fraction between 60% and 70%. With our composite mass model we find that the radial arc helps to constrain the inner DM distribution of the Cosmic Horseshoe independently of the DM profile.

Adaptive Grid Lens Modeling of the Cosmic Horseshoe Using Hubble Space Telescope Imaging

Abstract The Cosmic Horseshoe gravitational lensing system is an extraordinary example of strong gravitational lensing both due to the nearly complete Einstein ring formed in this system and due to the star-forming nature of the lensed z = 2.38 Lyman-break galaxy. In this paper, we describe the development of a new lens modeling package and the lens models produced using the Hubble Space Telescope imaging. Our new lens modeling package uses adaptive grid methods, based especially on the algorithm described by Vegetti & Koopmans. The new lens modeling package introduces the K-means method to deal with multiple background sources. We utilize two parameterized models, the singular isothermal ellipsoid and the Navarro–Frenk–White model in order to optimize the Bayesian penalty function. Color–color diagrams show two distinct colors exist in the Einstein ring, which suggests that the Cosmic Horseshoe is formed from two background galaxies or from a highly irregular galaxy. We find that the best lens model includes two components and the total mass within the Einstein ring is . The background source reconstruction shows two peaks in the source plane and the most probable lens profile is consistent with the light profile of the foreground galaxy when comparing axis ratios and position angles.

Mapping UV properties throughout the Cosmic Horseshoe: lessons from VLT-MUSE

We present the first spatially-resolved rest-frame UV study of the gravitationally lensed galaxy, the 'Cosmic Horseshoe' (J1148+1930) at z=2.38. Our gravitational lens model shows that the system is made up of four star-forming regions, each ~4-8 kpc^2 in size, from which we extract four spatially exclusive regional spectra. We study the interstellar and wind absorption lines, along with CIII] doublet emission lines, in each region to investigate any variation in emission/absorption line properties. The mapped CIII] emission shows distinct kinematical structure, with velocity offsets of ~+/-50 km/s between regions suggestive of a merging system, and a variation in equivalent width that indicates a change in ionisation parameter and/or metallicity between the regions. Absorption line velocities reveal a range of outflow strengths, with gas outflowing between -200<v(km/s)<-50 relative to the systemic velocity of that region. Interestingly, the strongest gas outflow appears to emanate from the most diffuse star-forming region. The star-formation rates remain relatively constant (~8-16 M_sol/yr), mostly due to large uncertainties in reddening estimates. As such, the outflows appear to be 'global' rather than 'locally' sourced. We measure electron densities with a range of log(Ne)=3.92-4.36 cm^-3, and point out that such high densities may be common when measured using the CIII] doublet due to its large critical density. Overall, our observations demonstrate that while it is possible to trace variations in large scale gas kinematics, detecting inhomogeneities in physical gas properties and their effects on the outflowing gas may be more difficult. This study provides important lessons for the spatially-resolved rest-frame UV studies expected with future observatories, such as JWST.

Reconstructing the cosmic Horseshoe gravitational lens using the singular perturbative approach.

The reconstruction of the cosmic horseshoe gravitational lens using the perturbative method reveals the presence of significant third-order terms. The presence of these higher order terms is apparent in the numerical expansion of the perturbative fields in Fourier series. The expansion of the fields at order 2 produces a higher value of the chi-square. Expanding at order 3 provides a very significant improvement, while order 4 does not bring a significant improvement over order 3. The presence of the order 3 terms is not a consequence of limiting the perturbative expansion to the first order. The amplitude and signs of the third-order terms are recovered by including the contribution of the other group members. This analysis demonstrates that the fine details of the potential of the lens could be recovered independently of any initial assumptions by using the perturbative approach.

THE LYMAN CONTINUUM ESCAPE FRACTION OF THE COSMIC HORSESHOE: A TEST OF INDIRECT ESTIMATES* †

ABSTRACT High-redshift star-forming galaxies are likely responsible for the reionization of the universe, yet direct detection of their escaping ionizing (Lyman continuum [LyC]) photons has proven to be extremely challenging. In this study, we search for escaping LyC of the Cosmic Horseshoe, a gravitationally lensed, star-forming galaxy at z = 2.38 with a large magnification of ∼24. Transmission at wavelengths of low-ionization interstellar absorption lines in the rest-frame ultraviolet suggests a patchy, partially transparent interstellar medium. This makes it an ideal candidate for direct detection of the LyC. We obtained a 10-orbit Hubble near-UV image using the Wide Field Camera 3 (WFC3)/UVIS F275W filter that probes wavelengths just below the Lyman limit at the redshift of the Horseshoe in an attempt to detect escaping LyC radiation. After fully accounting for the uncertainties in the opacity of the intergalactic medium (IGM) and accounting for the charge transfer inefficiency in the WFC3 CCDs, we find a upper limit for the relative escape fraction of . This value is a factor of five lower than the value (0.4) predicted by the 40% transmission in the low-ion absorption lines. Though possible, it is unlikely that the nondetection is due to a high-opacity line of sight through the IGM ( % chance). We discuss several possible causes for the discrepancy between the escape fraction and the covering fraction and consider the implications for future attempts at both direct LyC detection and indirect estimates of the escape fraction.

Zooming into the Cosmic Horseshoe: new insights on the lens profile and the source shape

The gravitational lens SDSS J1148+1930, also known as the Cosmic Horseshoe, is one of the biggest and of the most detailed Einstein rings ever observed. We use the forward reconstruction method implemented in the lens fitting code Lensed to investigate with great detail the properties of the lens and of the background source. We model the lens with different mass distributions, focusing in particular on the determination of the slope of the dark matter component. The inherent degeneracy between the lens slope and the source size can be broken when we can isolate separate components of each lensed image, as in this case. For an elliptical power law model, $\kappa(r) \sim r^{-t}$, the results favour a flatter-than-isothermal slope with a maximum-likelihood value t = 0.08. Instead, when we consider the contribution of the baryonic matter separately, the maximum-likelihood value of the slope of the dark matter component is t = 0.31 or t = 0.44, depending on the assumed Initial Mass Function. We discuss the origin of this result by analysing in detail how the images and the sources change when the slope t changes. We also demonstrate that these slope values at the Einstein radius are not inconsistent with recent forecast from the theory of structure formation in the LambdaCDM model.

Trans-dimensional Bayesian inference for gravitational lens substructures

We introduce a Bayesian solution to the problem of inferring the density profile of strong gravitational lenses when the lens galaxy may contain multiple dark or faint substructures. The source and lens models are based on a superposition of an unknown number of non-negative basis functions (or blobs) whose form was chosen with speed as a primary criterion. The prior distribution for the blobs' properties is specified hierarchically, so the mass function of substructures is a natural output of the method. We use reversible jump Markov Chain Monte Carlo (MCMC) within Diffusive Nested Sampling (DNS) to sample the posterior distribution and evaluate the marginal likelihood of the model, including the summation over the unknown number of blobs in the source and the lens. We demonstrate the method on two simulated data sets: one with a single substructure, and one with ten. We also apply the method to the g-band image of the Cosmic Horseshoe system, and find evidence for more than zero substructures. However, these have large spatial extent and probably only point to misspecifications in the model (such as the shape of the smooth lens component or the point spread function), which are difficult to guard against in full generality.

Lensing and dynamics in two simple steps

Abstract We present a ready-to-use method to constrain the density distribution in spherically symmetric early-type galaxy lenses. Assuming a power-law density profile, then joint use of the virial theorem and the lens equation yields simple formulae for the power-law index (or logarithmic density gradient). Any dependence on orbital anisotropy can be tightly constrained or even erased completely. Our results rely just on surface brightnesses and line-of-sight kinematics, making deprojection unnecessary. We revisit three systems that have already been examined in the literature (the Cosmic Horseshoe, the Jackpot and B1608+656) and provide our estimates. Finally, we show that the method yields a good approximation for the density profile even when the true profile is a broken power law, albeit with a mild bias towards isothermality.

A study of interstellar gas and stars in the gravitationally lensed galaxy ‘the Cosmic Eye’ from rest-frame ultraviolet spectroscopy

We report the results of a study of the rest-frame UV spectrum of the Cosmic Eye, a luminous Lyman break galaxy at z=3.07331 gravitationally lensed by a factor of 25. The spectrum, recorded with the ESI spectrograph on the Keck II telescope, is rich in absorption features from the gas and massive stars in this galaxy. The interstellar absorption lines are resolved into two components of approximately equal strength and each spanning several hundred km/s in velocity. One component has a net blueshift of -70 km/s relative to the stars and H II regions and presumably arises in a galaxy-scale outflow similar to those seen in most star-forming galaxies at z = 2-3. The other is more unusual in showing a mean redshift of +350 km/s relative to the systemic redshift; possible interpretations include a merging clump, or material ejected by a previous star formation episode and now falling back onto the galaxy, or more simply a chance alignment with a foreground galaxy. In the metal absorption lines, both components only partially cover the OB stars against which they are being viewed. We tentatively associate the redshifted component with the strong damped Lyman alpha line, indicative of a column density N(H I) = (3.0 +/- 0.8) x 10(21) atoms/cm2, and propose that it provides the dust `foreground screen' responsible for the low ratio of far-infrared to UV luminosities of the Cosmic Eye. Compared to other well-studied examples of strongly lensed galaxies, we find that the young stellar population of the Cosmic Eye is essentially indistinguishable from those of the Cosmic Horseshoe and MS 1512-cB58, while the interstellar spectra of all three galaxies are markedly different, attesting to the real complexity of the interplay between starbursts and ambient interstellar matter in young galaxies (abridged).

Rest-frame ultraviolet spectrum of the gravitationally lensed galaxy “the 8 o'clock arc”: stellar and interstellar medium properties

We present the first detailed analysis of the rest-frame ultraviolet spectrum of the gravitationally lensed Lyman break galaxy (LBG), the “8 o'clock arc”, obtained with the intermediate-resolution X-shooter spectrograph recently commissioned on the ESO Very Large Telescope. Besides MS 1512–cB58, the Cosmic Horseshoe, and the Cosmic Eye, three other lensed LBGs at comparable redshifts, this is the fourth of such a study, usually unfeasible at high redshifts. The spectrum of the 8 o'clock arc is rich in stellar and interstellar features, and presents several similarities to the well-known MS 1512–cB58 LBG. The stellar photospheric absorption lines allowed us to constrain the systemic redshift, <i>±<i/> <i>0.0003<i/>, of the galaxy, and derive its stellar metallicity, , which is in excellent agreement with the metallicity determined from nebular emission lines. With a total stellar mass of , the 8 o'clock arc agrees with the mass-metallicity relation found for <i>z>2<i/> star-forming galaxies, although being located near the upper end of the distribution given its high mass and high metallicity. Broad HeII <i>λ<i/>1640 emission is found, indicative of the presence of Wolf-Rayet stars formed in an intense period of star formation. The 31 interstellar absorption lines detected led to the abundance measurements of 9 elements. The metallicity of the interstellar medium (ISM), (Si), is very comparable to the metallicity of stars and ionized gas, and suggests that the ISM of the 8 o'clock arc has been rapidly polluted and enriched by ejecta of OB stars. The ISM lines extend over a very large velocity range, km s<sup>-1<sup/>, from about -800 to <i>+300<i/> km s<sup>-1<sup/> relative to the systemic redshift, and have their peak optical depth blueshifted relative to the stars, implying gas outflows of km s<sup>-1<sup/>. The zero residual intensity in the strongest lines indicates a nearly complete coverage of the UV continuum by the ISM. The Ly<i>α<i/> line is dominated by a damped absorption profile on top of which is superposed a weak emission, redshifted relative to the ISM lines by about +690 km s<sup>-1<sup/> and resulting from multiply backscattered Ly<i>α<i/> photons emitted in the HII region surrounded by the cold, expanding ISM shell. A homogeneous spherical shell model with a constant outflow velocity, determined by the observations, is able to reproduce the observed Ly<i>α<i/> line profile. Furthermore, the required dust content, , is in good agreement with the attenuation measured from the Balmer decrement. These results obtained from the radiation transfer modeling of the Ly<i>α<i/> line in the 8 o'clock arc fully support the scenario proposed earlier, where the diversity of Ly<i>α<i/> line profiles in Lyman break galaxies and Ly<i>α<i/> emitters, from absorption to emission, is mostly due to variations of HI column density and dust content.

The ultraviolet spectrum of the gravitationally lensed galaxy ‘the Cosmic Horseshoe’: a close-up of a star-forming galaxy atz∼ 2

Taking advantage of strong gravitational lensing, we have recorded the rest-frame UV spectrum of the z = 2.38115 galaxy `The Cosmic Horseshoe' (J1148+1930) at higher resolution and S/N than is currently feasible for unlensed galaxies at z = 2 -3. From the analysis of stellar spectral features, we conclude that a continuous mode of star formation with a Salpeter slope gives a good representation of the UV spectrum, ruling out significant departures from a `standard' IMF. Generally, we find good agreement between the values of metallicity deduced from stellar and nebular tracers. Interstellar absorption is present over a velocity range of 1000 km/s, from -800$ to +250 km/s relative to the stars and their H II regions, and there is evidence that the outflowing interstellar gas may be patchy, covering only 60% of the UV stellar continuum. The Lya line shares many of the characteristics of the so-called Lya emitters. Its double-peaked profile can be reproduced by models of Lya photons resonantly scattered by an expanding shell of gas and dust, with 10-15% of the photons escaping the galaxy. Many of the physical properties of the Cosmic Horseshoe are similar to those of the only other galaxy at z = 2-3 studied in comparable detail up to now: MS 1512-cB58. The fact that these two galaxies have drastically different Lya lines may be due simply to orientation effects, or differences in the covering factor of outflowing gas, and cautions against classifying high-z galaxies only on the basis of spectral features, such as Lya, whose appearance can be affected by a variety of different parameters.

REST-FRAME OPTICAL SPECTRA OF THREE STRONGLY LENSED GALAXIES ATz∼ 2

We present Keck II NIRSPEC rest-frame optical spectra for three recently discovered lensed galaxies: the Cosmic Horseshoe (z = 2.38), the Clone (z = 2.00), and SDSS J090122.37+181432.3 (z = 2.26). The boost in signal-to-noise ratio (S/N) from gravitational lensing provides an unusually detailed view of the physical conditions in these objects. A full complement of high S/N rest-frame optical emission lines is measured, spanning from rest frame 3600 to 6800 A, including robust detections of fainter lines such as Hγ, [S II]λ6717,6732, and in one instance [Ne III]λ3869. SDSS J090122.37+181432.3 shows evidence for active galactic nucleus activity, and therefore we focus our analysis on star-forming regions in the Cosmic Horseshoe and the Clone. For these two objects, we estimate a wide range of physical properties. Current lensing models for the Cosmic Horseshoe and the Clone allow us to correct the measured Hα luminosity and calculated star formation rate. Metallicities have been estimated with a variety of indicators, which span a range of values of 12+ log(O/H) = 8.3-8.8, between ~0.4 and ~1.5 of the solar oxygen abundance. Dynamical masses were computed from the Hα velocity dispersions and measured half-light radii of the reconstructed sources. A comparison of the Balmer lines enabled measurement of dust reddening coefficients. Variations in the line ratios between the different lensed images are also observed, indicating that the spectra are probing different regions of the lensed galaxies. In all respects, the lensed objects appear fairly typical of ultraviolet-selected star-forming galaxies at z ~ 2. The Clone occupies a position on the emission-line diagnostic diagram of [O III]/Hβ versus [N II]/Hα that is offset from the locations of z ~ 0 galaxies. Our new NIRSPEC measurements may provide quantitative insights into why high-redshift objects display such properties. From the [S II] line ratio, high electron densities (~1000 cm–3) are inferred compared to local galaxies, and [O III]/[O II] line ratios indicate higher ionization parameters compared to the local population. Building on previous similar results at z ~ 2, these measurements provide further evidence (at high S/N) that star-forming regions are significantly different in high-redshift galaxies, compared to their local counterparts.

Two new large-separation gravitational lenses from SDSS

We present discovery images, together with follow-up imaging and spectroscopy, of two large separation gravitational lenses found by our survey for wide arcs (the CASSOWARY). The survey exploits multicolor photometry of Sloan Digital Sky Survey to find multiple blue components around red galaxies. CASSOWARY~2 (or the Cheshire Cat) is composed of two massive early-type galaxies at z = 0.426 and 0.432 respectively lensing two background sources, first a star-forming galaxy at z = 0.97 and second a high redshift galaxy (z> 1.4). There are at least three images of former source and probably four or more of latter, arranged in two giant arcs. The mass enclosed within larger arc of radius 11 arcsecs is about 33 x 10^{12} solar masses. CASSOWARY~3 comprises an arc of three bright images of a z = 0.725 source, lensed by a foreground elliptical at z = 0.274. The radius of arc is about 4 arcsecs and enclosed mass is 2.5 x 10^{12} solar masses. Together with earlier discoveries like Cosmic Horseshoe and 8 O'Clock Arc, these new systems, with separations intermediate between arcsecond separation lenses of typical strong galaxy lensing and larger separation cluster lenses, probe very high end of galaxy mass function.

Models of the Cosmic Horseshoe gravitational lens J10044112

We model the extremely massive and luminous lens galaxy in the Cosmic Horseshoe Einstein ring system J1004+4112, recently discovered in the Sloan Digital Sky Survey. We use the semilinear method of Warren & Dye, which pixelizes the source surface brightness distribution, to invert the Einstein ring for sets of parametrized lens models. Here, the method is refined by exploiting Bayesian inference to optimise adaptive pixelization of the source plane and to choose between three differently parametrized models: a singular isothermal ellipsoid, a power-law model and a Navarro, Frenk & White (NFW) profile. The most probable lens model is the power law with a volume mass density p α r -1.96+0.02 and an axis ratio of ∼0.8. The mass within the Einstein ring (i.e. within a cylinder with projected distance of ∼30 kpc from the centre of the lens galaxy) is (5.02 ± 0.09) x 10 12 M ⊙ , and the mass-to-light ratio is ∼30. Even though the lens lies in a group of galaxies, the preferred value of the external shear is almost zero. This makes the Cosmic Horseshoe unique amongst large separation lenses, as almost all the deflection comes from a single, very massive galaxy with little boost from the environment.