Substructure
- Xu, D.D., Mao, S., Cooper, A.P., et al. 2010, Substructure lensing: effects of galaxies, globular clusters and satellite streams, MNRAS, 408, 1721: http://adsabs.harvard.edu/abs/2010MNRAS.408.1721X
Lensing flux-ratio anomalies have been frequently observed and taken as evidence for the presence of abundant dark matter substructures in lensing galaxies, as predicted by the cold dark matter (CDM) model of cosmogony. In previous work, we examined the cusp-caustic relations of the multiple images of background quasars lensed by galaxy-scale dark matter haloes, using a suite of high-resolution N-body simulations (the Aquarius simulations). In this work, we extend our previous calculations to incorporate both the baryonic and diffuse dark components in lensing haloes. We include in each lensing simulation: (1) a satellite galaxy population derived from a semi-analytic model applied to the Aquarius haloes; (2) an empirical Milky Way globular cluster population and (3) satellite streams (diffuse dark component) identified in the simulations. Accounting for these extra components, we confirm our earlier conclusion that the abundance of intrinsic substructures (dark or bright, bound or diffuse) is not sufficient to explain the observed frequency of cusp-caustic violations in the Cosmic Lens All-Sky Survey (CLASS). We conclude that the observed effect could be the result of the small number statistics of CLASS, or intergalactic haloes along the line of sight acting as additional sources of lensing flux anomalies. Another possibility is that this discrepancy signals a failure of the CDM model.
Figure 2: Contour maps of the substructure surface mass fraction of the halo Aq-E-2, in X-projection. Left-hand panel: semi-analytic galaxies are added to the dark matter subhalo population. The mean surface mass fraction, fsub,annu, of the substructures within the 0.1 arcsec-annulus (indicated by the dashed lines) around the tangential critical curve is ∼0.16 per cent. Right-hand panel: semi-analytic galaxies and MW GCs are added to the dark subhalo population. fsub,annu∼ 0.19 per cent. GCs are more centrally distributed in the projected central region than satellite galaxies. The blue circle indicates an example of the small-scale wiggles induced by GCs.
"We conclude that the abundance of intrinsic substructures, dark or bright, bound or diffuse, cannot fully account for the observed cusp-violation frequency. Taken at face value, this lack of substructure suggests a serious problem for the CDM model. Warm dark matter models, which could reduce the satellite abundance and may help to bring the dwarf galaxy LF into agreement with observations without invoking strong feedback or photoionization effects (e.g. Sawala et al. 2010), would only make this problem worse. However, it is possible that the observed frequency of flux anomalies is strongly biased by the small number statistics of CLASS."
- Paper 2
- Paper 3
- Nierenberg, A. 2014, Detecting dark matter substructure with narrow line lensing, HST Proposal, 13732: http://adsabs.harvard.edu/abs/2014hst..prop13732N
The abundance of low mass halos is one of the key predictions of LCDM and remains at apparent odds with observations of luminous structure. Strong gravitational lensing provides a straightforward means of testing this theory as it enables the detection of dark matter subhalos at cosmological distances, without requiring the structure to contain any baryons at all. The fluxes of strongly lensed, parsec scale sources in particular, are excellent probes as they are extremely sensitive to the presence of low mass subhalos, while still being extended enough to remain unaffected by microlensing by stars which is a dominant contaminant for smaller sources. Traditionally this field has been limited to the analysis of the small number of strongly lensed, radio-loud quasars. Quasar narrow-line emission offers an alternative to radio. It is also parsec scale and microlensing free, but has the benefit of detectable in a much larger sample of systems. This proposal will combine milliarcsecond astrometry, and percent level photometry attainable with WFC3 IR grism, in order to measure spatially resolved narrow line lensing in six new systems, which cannot be studied from the ground. We have demonstrated that data of this quality can be used to detect subhalos as small as a million solar masses. This proposal will double the sample of systems which can be used to detect dark, low mass substructure using flux ratio anomalies.
- Nierenberg, A.M., Treu, T., Brammer, G., et al. 2017, Probing dark matter substructure in the gravitational lens HE0435-1223 with the WFC3 grism, arXiv:1701.05188: http://adsabs.harvard.edu/abs/2017arXiv170105188N
Strong gravitational lensing provides a powerful test of Cold Dark Matter (CDM) as it enables the detection and mass measurement of low mass haloes even if they do not contain baryons. Compact lensed sources such as Active Galactic Nuclei (AGN) are particularly sensitive to perturbing subhalos, but their use as a test of CDM has been limited by the small number of systems which have significant radio emission. Radio emission is extended enough avoid significant lensing by stars in the plane of the lens galaxy, and red enough to be minimally affected by differential dust extinction. Narrow-line emission is a promising alternative as it is also extended and, unlike radio, detectable in virtually all optically selected AGN lenses. We present first results from a WFC3 grism narrow-line survey of lensed quasars, for the quadruply lensed AGN HE0435-1223. Using a forward modelling pipeline which enables us to robustly account for blending between nearby images and the main lens galaxy, we measure the [OIII] 5007 \AA flux ratios of the four lensed quasar images. We find that the lensed [OIII] fluxes and positions are well fit by a simple smooth mass model for the main lens. Our data rule out a >10^8(10^7.2)M600/M⊙ NFW perturber within ∼1."0 (0."1) arcseconds of the lensed images, where M600 is the perturber mass within its central 600 pc. The non-detection is broadly consistent with the expectations of ΛCDM for a single system. The sensitivity achieved demonstrates that powerful limits on the nature of dark matter can be obtained with the analysis of the entire sample of narrow-line lenses.
- MacLeod, C.L., Jones, R., Agol, E., and Kochanek, C.S. 2013, Detection of Substructure in the Gravitationally Lensed Quasar MG0414+0534 Using Mid-infrared and Radio VLBI Observations, ApJ, 773, 35: http://adsabs.harvard.edu/abs/2013ApJ...773...35M
We present 11.2 μm observations of the gravitationally lensed, radio-loud zs = 2.64 quasar MG0414+0534, obtained using the Michelle camera on Gemini North. We find a flux ratio anomaly of A2/A1 = 0.93 ± 0.02 for the quasar images A1 and A2. When combined with the 11.7 μm measurements from Minezaki et al., the A2/A1 flux ratio is nearly 5σ from the expected ratio for a model based on the two visible lens galaxies. The mid-IR flux ratio anomaly can be explained by a satellite (substructure), 0.''3 northeast of image A2, as can the detailed very long baseline interferometry (VLBI) structures of the jet produced by the quasar. When we combine the mid-IR flux ratios with high-resolution VLBI measurements, we find a best-fit mass between 106.2 and 107.5 M ⊙ inside the Einstein radius for a satellite substructure modeled as a singular isothermal sphere at the redshift of the main lens (zl = 0.96). We are unable to set an interesting limit on the mass to light ratio due to its proximity to the quasar image A2. While the observations used here were technically difficult, surveys of flux anomalies in gravitational lenses with the James Webb Space Telescope will be simple, fast, and should well constrain the abundance of substructure in dark matter halos.
- Oguri, M., Schrabback, T., Jullo, E., et al. 2013, The Hidden Fortress: structure and substructure of the complex strong lensing cluster SDSS J1029+2623, MNRAS 429, 482: http://adsabs.harvard.edu/abs/2013MNRAS.429..482O
We present Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3) observations of SDSS J1029+2623, a three-image quasar lens system produced by a foreground cluster at z = 0.584. Our strong lensing analysis reveals six additional multiply imaged galaxies in addition to the multiply imaged quasar. We confirm the complex nature of the mass distribution of the lensing cluster, with a bimodal dark matter distribution which deviates from the Chandra X-ray surface brightness distribution. The Einstein radius of the lensing cluster is estimated to be θE = 15.2 ± 0.5 arcsec for the quasar redshift of z = 2.197. We derive a radial mass distribution from the combination of strong lensing, HST/ACS weak lensing and Subaru/Suprime-cam weak lensing analysis results, finding a best-fitting virial mass of Mvir = 1.55+ 0.40- 0.35 × 1014 h- 1 M⊙ and a concentration parameter of cvir = 25.7+ 14.1- 7.5. The lensing mass estimate at the outer radius is smaller than the X-ray mass estimate by a factor of ˜2. We ascribe this large mass discrepancy to shock heating of the intracluster gas during a merger, which is also suggested by the complex mass and gas distributions and the high value of the concentration parameter. In the HST image, we also identify a probable galaxy, GX, in the vicinity of the faintest quasar image C. In strong lens models, the inclusion of GX explains the anomalous flux ratios between the quasar images. The morphology of the highly elongated quasar host galaxy is also well reproduced. The best-fitting model suggests large total magnifications of 30 for the quasar and 35 for the quasar host galaxy, and has an AB time delay consistent with the measured value.
- Sluse, D., Kishimoto, M., Anguita, T., Wucknitz, O., and Wambsganss, J. 2013, Mid-infrared microlensing of accretion disc and dusty torus in quasars: effects on flux ratio anomalies, Astronomy and Astrophysics, 553, A53: http://adsabs.harvard.edu/abs/2013A%26A...553A..53S
Multiply-imaged quasars and active galactic nuclei (AGNs) observed in the mid-infrared (MIR) range are commonly assumed to be unaffected by the microlensing produced by the stars in their lensing galaxy. In this paper, we investigate the validity domain of this assumption. Indeed, that premise disregards microlensing of the accretion disc in the MIR range, and does not account for recent progress in our knowledge of the dusty torus, which has unveiled relatively compact dust emission. To simulate microlensing, we first built a simplified image of the quasar composed of (i) an accretion disc whose size is based on accretion disc theory, and (ii) a larger ring-like torus whose radius is guided by interferometric measurements in nearby AGNs. The mock quasars are created in the 1044.2-1046 erg/s (unlensed) luminosity range, which is typical of known lensed quasars, and are then microlensed using an inverse ray-shooting code. We simulated the wavelength dependence of microlensing for different lensed image types and for various fractions of compact objects in the lens. This allows us to derive magnification probabilities as a function of wavelength, as well as to calculate the microlensing-induced deformation of the spectral energy distribution of the lensed images. We find that microlensing variations as large as 0.1 mag are very common at 11 μm (typically rest-frame 4 μm). The main signal comes from microlensing of the accretion disc, which may be significant even when the fraction of flux from the disc is as small as 5% of the total flux. We also show that the torus of sources with Lbol ≲ 10^{45} erg/s is expected to be noticeably microlensed. Microlensing may thus be used to get insight into the rest near-infrared inner structure of AGNs. Finally, we investigate whether microlensing in the mid-infrared can alter the so-called Rcusp relation that links the fluxes of the lensed images triplet produced when the source lies close to a cusp macro-caustic. This relation is commonly used to identify massive (dark-matter) substructures in lensing galaxies. We find that significant deviations from Rcusp may be expected, which means that microlensing can explain part of the flux ratio problem.
Microlensing maps and source profiles used for our simulations are only available in electronic form at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/553/A53
- Ota, N., Oguri, M., Dai, X., et al. 2012, The Chandra View of the Largest Quasar Lens SDSS J1029+2623, ApJ, 758, 26: http://adsabs.harvard.edu/abs/2012ApJ...758...26O
We present results from Chandra observations of the cluster lens SDSS J1029+2623 at zl = 0.58, which is a gravitationally lensed quasar with the largest known image separation. We clearly detect X-ray emission both from the lensing cluster and the three lensed quasar images. The cluster has an X-ray temperature of kT = 8.1+2.0 - 1.2 keV and bolometric luminosity of L X = 9.6 × 1044 erg s-1. Its surface brightness is centered near one of the brightest cluster galaxies, and it is elongated east-west. We identify a subpeak northwest of the main peak, which is suggestive of an ongoing merger. Even so, the X-ray mass inferred from the hydrostatic equilibrium assumption appears to be consistent with the lensing mass from the Einstein radius of the system. We find significant absorption in the soft X-ray spectrum of the faintest quasar image, which can be caused by an intervening material at either the lens or source redshift. The X-ray flux ratios between the quasar images (after correcting for absorption) are in reasonable agreement with those at optical and radio wavelengths, and all the flux ratios are inconsistent with those predicted by simple mass models. This implies that microlensing effect is not significant for this system and dark matter substructure is mainly responsible for the anomalous flux ratios.