We report the discovery of significant mass/light offsets in the strong gravitational lensing system SDSS,J1011$+$0143. We use the high-resolution textsl{Hubble Space Telescope} (textsl{HST}) F555W- and F814W-band imaging and Sloan Digital Sky Survey
(SDSS) spectroscopy of this system, which consists of a close galaxy pair with a projected separation of $approx 4.2$ kpc at $z_{rm lens} sim 0.331$ lensing a Ly$alpha$ emitter (LAE) at $z_{rm source} = 2.701$. Comparisons between the mass peaks inferred from lens models and light peaks from textsl{HST} imaging data reveal significant spatial mass/light offsets as large as $1.72 pm 0.24 pm 0.34$ kpc in both filter bands. Such large mass/light offsets, not seen in isolated field lens galaxies and relaxed galaxy groups, may be related to the interactions between the two lens galaxies. The detected mass/light offsets can potentially serve as an important test for the self-interacting dark matter model. However, other mechanisms such as dynamical friction on spatially differently distributed dark matter and stars could produce similar offsets. Detailed hydrodynamical simulations of galaxy-galaxy interactions with self-interacting dark matter could accurately quantify the effects of different mechanisms. The background LAE is found to contain three distinct star-forming knots with characteristic sizes from 116 pc to 438 pc. It highlights the power of strong gravitational lensing in probing the otherwise too faint and unresolved structures of distance objects below subkiloparsec or even 100 pc scales through its magnification effect.
In the standard structure formation scenario based on the cold dark matter paradigm, galactic halos are predicted to contain a large population of dark matter subhalos. While the most massive members of the subhalo population can appear as luminous s
atellites and be detected in optical surveys, establishing the existence of the low mass and mostly dark subhalos has proven to be a daunting task. Galaxy-scale strong gravitational lenses have been successfully used to study mass substructures lying close to lensed images of bright background sources. However, in typical galaxy-scale lenses, the strong lensing region only covers a small projected area of the lenss dark matter halo, implying that the vast majority of subhalos cannot be directly detected in lensing observations. In this paper, we point out that this large population of dark satellites can collectively affect gravitational lensing observables, hence possibly allowing their statistical detection. Focusing on the region of the galactic halo outside the strong lensing area, we compute from first principles the statistical properties of perturbations to the gravitational time delay and position of lensed images in the presence of a mass substructure population. We find that in the standard cosmological scenario, the statistics of these lensing observables are well approximated by Gaussian distributions. The formalism developed as part of this calculation is very general and can be applied to any halo geometry and choice of subhalo mass function. Our results significantly reduce the computational cost of including a large substructure population in lens models and enable the use of Bayesian inference techniques to detect and characterize the distributed satellite population of distant lens galaxies.
We describe the methodology to include nonlinear evolution, including tidal effects, in the computation of subhalo distribution properties in both cold (CDM) and warm (WDM) dark matter universes. Using semi-analytic modeling, we include effects from
dynamical friction, tidal stripping, and tidal heating, allowing us to dynamically evolve the subhalo distribution. We calibrate our nonlinear evolution scheme to the CDM subhalo mass function in the Aquarius N-body simulation, producing a subhalo mass function within the range of simulations. We find tidal effects to be the dominant mechanism of nonlinear evolution in the subhalo population. Finally, we compute the subhalo mass function for $m_chi=1.5$ keV WDM including the effects of nonlinear evolution, and compare radial number densities and mass density profiles of subhalos in CDM and WDM models. We show that all three signatures differ between the two dark matter models, suggesting that probes of substructure may be able to differentiate between them.
There is a vast menagerie of plausible candidates for the constituents of dark matter, both within and beyond extensions of the Standard Model of particle physics. Each of these candidates may have scattering (and other) cross section properties that
are consistent with the dark matter abundance, BBN, and the most scales in the matter power spectrum; but which may have vastly different behavior at sub-galactic cutoff scales, below which dark matter density fluctuations are smoothed out. The only way to quantitatively measure the power spectrum behavior at sub-galactic scales at distances beyond the local universe, and indeed over cosmic time, is through probes available in multiply imaged strong gravitational lenses. Gravitational potential perturbations by dark matter substructure encode information in the observed relative magnifications, positions, and time delays in a strong lens. Each of these is sensitive to a different moment of the substructure mass function and to different effective mass ranges of the substructure. The time delay perturbations, in particular, are proving to be largely immune to the degeneracies and systematic uncertainties that have impacted exploitation of strong lenses for such studies. There is great potential for a coordinated theoretical and observational effort to enable a sophisticated exploitation of strong gravitational lenses as direct probes of dark matter properties. This opportunity motivates this white paper, and drives the need for: a) strong support of the theoretical work necessary to understand all astrophysical consequences for different dark matter candidates; and b) tailored observational campaigns, and even a fully dedicated mission, to obtain the requisite data.
We report the discovery of a new Einstein cross at redshift z_S = 2.701 based on Lyman-alpha emission in a cruciform configuration around an SDSS luminous red galaxy (z_L = 0.331). The system was targeted as a possible lens based on an anomalous emis
sion line in the SDSS spectrum. Imaging and spectroscopy from the W. M. Keck Observatory confirm the lensing nature of this system. This is one of the widest-separation galaxy-scale lenses known, with an Einstein radius of ~1.84 arcsec. We present simple gravitational lens models for the system and compute the intrinsic properties of the lensed galaxy. The total mass of the lensing galaxy within the 8.8 +/- 0.1 kpc enclosed by the lensed images is (5.2 +/- 0.1) x 10^11 M_sun. The lensed galaxy is a low mass galaxy (0.2 L*) with a high equivalent-width Lyman-alpha line (EW_Lya_rest = 46 +/- 5 Angstroms). Follow-up studies of this lens system can probe the mass structure of the lensing galaxy, and can provide a unique view of an intrinsically faint, high-redshift, star-forming galaxy at high signal-to-noise ratio.
Using U- through Ks-band imaging data in the GOODS-South field, we construct a large, complete sample of 275 ``extremely red objects (EROs; K_s<22.0, R-K_s>3.35; AB), all with deep HST/ACS imaging in B_435, V_606, i_775, and z_850, and well-calibrate
d photometric redshifts. Quantitative concentration and asymmetry measurements fail to separate EROs into distinct morphological classes. We therefore visually classify the morphologies of all EROs into four broad types: ``Early (elliptical-like), ``Late (disk galaxies), ``Irregular and ``Other (chain galaxies and low surface brightness galaxies), and calculate their relative fractions and comoving space densities. For a broad range of limiting magnitudes and color thresholds, the relative number of early-type EROs is approximately constant at 33-44%, and the comoving space densities of Early- and Late-type EROs are comparable. Mean rest-frame spectral energy distributions (SEDs) at wavelengths between 0.1 and 1.2 um are constructed for all EROs. The SEDs are extremely similar in their range of shapes, independent of morphological type. The implication is that any differences between the broad-band SEDs of Early-type EROs and the other types are relatively subtle, and there is no robust way of photometrically distinguishing between different morphological types with usual optical/near-infrared photometry.
The redshift distribution of near-IR selected galaxies is often used to attempt to discriminate between the classical view of galaxy formation, in which present-day luminous galaxies were assembled at early times and evolve due to the passive aging o
f their stellar populations, and that of hierarchical structure formation, in which galaxies were assembled more recently via the merging of smaller objects. We carry out such a test here, by computing the distribution of photometric redshifts of K_AB<22 galaxies in the Great Observatories Origins Deep Field Survey GOODS Southern Field, and comparing the results with predictions from a semi-analytic model based on hierarchical structure formation, and a classical `passive evolution model. We find that the redshift distributions at z <~ 1.5 of both the hierarchical and passive models are very similar to the observed one. At z>~1.5, the hierarchical model shows a deficit of galaxies, while the passive model predicts an excess. We investigate the nature of the observed galaxies in the redshift range where the models diverge, and find that the majority have highly disturbed morphologies, suggesting that they may be merger-induced starbursts. While the hierarchical model used here does not produce these objects in great enough numbers, the appearance of this population is clearly in better qualitative agreement with the hierarchical picture than with the classical passive evolution scenario. We conclude that the observations support the general framework of hierarchical formation, but suggest the need for new or modified physics in the models.
