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Observational Cosmology, Gravitational Lensing and Astrophysics Group Ian Dell’Antonio, Jeffery * Kubo , Hossein Khiabanian, Wessyl Kelly Kaitlin Goldstein, Stephan Janiszewski, Hannah Singer, Alexander Cerjan, Amandeep Gill Introduction Weak Lensing: Method A JDEM Project: Destiny The goal of observational cosmology is to determine the properties of the Universe today, and how it has evolved with time. One of the most fundamental questions to ask about the Universe is: how much matter is there, and how is it distributed? This is a deceptively hard question to answer. Most of the matter in the Universe is dark, it emits no light (of any kind!) A picture of the visible matter is not necessarily representative of what is there. To overcome this difficulty, we are using a variety of techniques that measure the mass directly, the main one being gravitational lensing. Gravitational lensing uses the fact that the presence of mass causes the space around the mass to deform. The light from distant background objects bends in response to the deformation, distorting the images of the distant galaxies. By measuring the amount the light is bent, we can measure the curvature of space and thus the mass! Measurements of the clustering of the dark matter may give us clues as to the nature of dark matter. The presence of mass causes background galaxies to appear shifted in position and stretched tangentially to the vector connecting the galaxies to the mass (see figure below). In weak lensing, the deflection of the position is not measurable. (We cannot remove the mass to see where the galaxy really is!) But the tangential ellipticity eT is. In general the distortion of a galaxy’s shape is much less than the random variation in shape and orientation from galaxy to galaxy. Our aim is to optimize the capability of DESTINY while minimizing cost. In the case of gravitational lensing, our goal is to recover the distortion in galaxy shapes that is the result of intervening mass between distant galaxies and Earth. In order to gauge the suitability of a set of telescope parameters, we simulate an image with the depth, size, and resolution of the Universe as it would be seen through the “eyes” of the telescope. Observations of Supernovae have revealed that the expansion of the Universe is accelerating. Because dark matter decelerates the expansion, this points to a new component to the Universe —the Dark Energy. Experiments like the JDEM missions and LSST will use gravitational lensing as a tool to measure dark energy and its properties. There are two types of gravitational lensing: strong lensing occurs when the curvature is great enough to cause multiple imaging—the same background object is seen in multiple directions! This yields very detailed information about the mass, but requires a significant mass density. In weak lensing, the distortion of each galaxy is too small to be reliably measured. However, the ensemble statistical effect on many background galaxies can be measured. This allows us to probe the distribution of mass even in low-density environments. Our research group is heavily involved in the study of gravitational lensing using both techniques and data from the Hubble Space Telescope as well as ground-based telescopes. However, by considering the average shape and orientation of an ensemble of galaxies in a patch of the sky, we can measure the average tangential ellipticity <eT>. If we average over enough galaxies, the random component will average to zero, allowing us to measure the lensinginduced ellipticity! To do so, small galaxy “postage stamps”, taken from Hubble Deep Fields, are scaled to reflect the size, shape, orientation, redshift, and magnitude distribution of galaxies in the real Universe. Once we simulate a full image, consisting of tens of thousands of galaxies, we add an artificial lensing signal and analyze how well we recover it. One of the critical aspects of the Destiny mission is the plan to obtain slitless spectroscopy of all the imaged fields. This will allow us to measure lowresolution (R~75) spectra of every object in the frame, and greatly improve our ability to measure distances for the lensed galaxies. Left: Artist’s rendition of the Destiny spacecraft. Above: A Schematic showing of the path of light from distant galaxies through the dark matter and how coherent distortion in the galaxy shapes are produced. As the pictures below show, however, one of the drawbacks of slitless spectroscopy is that the spectra of objects will overlap. To disentangle the spectra will require rotating the spacecraft and repeating exposures. We are generating simulations of this effect to test the optimal combination of roll angles. Above: Full simulation field 20784 by 4096 pixels, while the image above is solely 250 by 250 pixels. Weak Lensing: Mass Maps The relation between surface mass density (κ) and the shear due to the gravitational field (γ) is Our goal is to produce a map of the surface mass density. There are two classes of reconstruction methods using weak gravitational lensing data. The direct methods, approximate a local value for the shear from the observed ellipticities of the background galaxies (γT = eT). The tangential ellipticity as a function of angular distance about each point is related to the mass density at that point by There are also indirect methods of producing mass maps. Inverse methods aim to find the best fit to the data. By minimizing we are able to find the best answer for κ that satisfies the first equation. The expected ellipticity of each The Deep Lens Survey source is derived from the deflection potential (ψ) which is related to κ at xg. These methods have the benefit of not only utilizing all the shear information provided by each source galaxy, but also incorporating extra The DLS is a NOAO Survey program—it uses the 4-meter information from other observables such as magnification. telescopes at Kitt Peak in Arizona (right) and Cerro Tololo in Chile to make maps of several 4-square degree regions of the We can also produce maps with multiple sky, to a depth of about R=26. (Some 10 billion times fainter resolutions in the different parts of the observed than the naked eye can see.) field. Therefore, we achieve a uniform signal to Approximately 140 nights of 4-meter time will have been noise level and are able to better study the suballocated to this project. The background of the poster is 1.6% structure of massive clusters. of the survey area. Left: The analytical mass map of a simulated field with five clusters of galaxies with masses ranging from 1013 to 1015 Solar masses. This can be used to map out the surface density of matter and reveal the distribution of dark matter. In these methods the data must be smoothed. The picture below shows a map of the surface mass density for a four squaredegree field obtained by the Deep Lens Survey. The bright spots represent mass concentrations of about or greater than 1014 Solar masses. Above: The mass map of one of the four square-degree DLS fields. Bottom Left: The Optical data of the same field. Bottom Right: The brightest blob in the mass map is due to a know cluster of galaxies called Abell 781. Its mass is greater than 1014 Solar masses The NOAO Deep Wide-Field Survey Right: The mass map of the same field produced by an inverse method. We can also produce mass maps by only using the magnification information. The gravitational lensing changes the average number density of background galaxies by changing both the flux of the galaxies (therefore making the faint ones brighter) and the apparent solid angle. Thus, from a measurement of the local number density of galaxies, we can estimate κ. The left image shows such a mass map for the same DLS field. The Deep Lens Survey is one of several data sets currently being used to investigate cosmology and dark matter. Another, the NDWFS is a deep optical and near-infrared imaging survey that covers two 9.3 square degree fields. It was designed primarily for the study of the existence and evolution of large scale structures at redshifts z > 1 as sampled by diverse populations of objects. Our current project is using the Bootes field to detect and analyze galaxy-galaxy weak lensing as a way to constrain the dark matter in the universe. Contact For more information visit our group webpage at http://www.het.brown.edu/people/ian/astro/ or email Ian Dell’Antonio at [email protected].