An Exploration of Dark Matter Science Briefing October 5, 2017 What Lurks in the Dark? An Exploration of Dark Matter Dr. Simona Murgia (UC, Irvine) Dr. Will Dawson (Lawrence Livermore National Laboratory) Carolyn Slivinski (STScI) Facilitator: Dr. Emma Marcucci (STScI)

http://nasawavelength.org/list/1929 Additional Resources Dark Matter Day: Primary Website Featured Activities: Jelly Bean Universe Find the Missing Mass – paper plate activity “Gravitational lensing” with a wine glass Basic Dark Matter Facts: Chandra Field Guide Ask an Astrophysicist Blog (archived) NASA’s Frontier Fields Additional Activities: Dark Matter Possibilities What’s the Matter?

Searching for Dark Matter with Gamma Rays Simona Murgia University of California, Irvine

Evidence for Dark Matter: A Brief Overview Evidence for dark matter is found at very different scales Galaxies Clusters of galaxies Universe

Galaxy Clusters DM gas Dark matter makes up for the missing mass The existence of dark matter was postulated by Fritz Zwicky in the 1930’s to explain the dynamics of galaxies in the Coma galaxy cluster Zwicky inferred the total mass of the cluster by measuring the velocities of its galaxies, based on Newtonian gravity. But the luminous mass (the galaxies in the cluster) was far smaller! F. Zwicky, Astrophysical Journal, vol. 86, p.217 (1937) Dark matter makes up for the missing mass gas DM Cluster stars Velocities ~ 1000 km/s R ~ Mpcs Distance ~100 Mpc (1 pc = 3.26 light yrs) Virial theorem: relates the velocity (dispersion, σ) of galaxies at some distance r from the cluster center to the enclosed mass Mtot(r) Galaxy cluster: ~1-2% stars, ~5-15% gas; the rest is dark matter

Rotation Curves of Galaxies Departures from the predictions of Newtonian gravity became apparent also at galactic scales with the measurement of rotation curves of galaxies (Rubin et al, 1970) However observed velocities stay approximately constant, i.e. stars and gas move faster then predicted! Andromeda galaxy Rotational speed Based on Newtonian dynamics, the velocity (v) of stars and gas in the galaxy should decrease with the distance (r) from the center of the galaxy. Distance from center and therefore: i.e. decreasing with r

Rotation Curves of Galaxies To reconcile theory with observations, postulate the existence of mass density not steeply falling as luminous matter density! By adding this extended matter halo, we find good agreement with observations Assume additional mass: therefore: and finally: e.g. Andromeda galaxy Dark matter makes up for the missing mass Stars+gas: 1.4 ×1011M⊙ Total mass: 1.3×1012M⊙ ~10 times more dark matter than luminous matter Stellar bulge Gas Stellar disk Dark matter Corbelli et al (2009) Andromeda galaxy

Cosmic Microwave Background Relic of a time in the early Universe when matter and radiation decoupled (protons and electron form neutral hydrogen and become transparent to photons, ~100,000s years after Big Bang) Universe was isotropic and homogeneous at large scales Very small temperature fluctuations, too small to evolve into structure observed today Require additional matter to start forming structure earlier T = 2.725 K ΔT ~ 200 μK Planck 2015 Power spectrum of matter fluctuations Observed (SDSS) baryons only Clumpiness Dodelson et al 2006 larger scales smaller scales

Dark Matter But not what it is... What data tell us about dark matter: makes up almost all of the matter in the Universe (present day Universe mostly made out of dark energy, dark matter, and small contribution from ordinary matter) interacts very weakly, and at least gravitationally, with ordinary matter is cold, i.e. non-relativistic is neutral is stable (or it is very long-lived) But not what it is...

Dark Matter Candidates None of the known elementary particles has the right properties to be the dark matter Need new particles and new theories beyond the Standard Model of particle physics! Image credit: G. Bertone

Detect energy it deposits Dark Matter Searches INDIRECT SEARCHES COLLIDER SEARCHES DIRECT SEARCHES Dark Matter Standard Model Standard Model Standard Model Dark Matter Dark Matter Find its annihilation byproducts Detect energy it deposits Produce it in the lab Fermi-LAT CDMS XENON100 Large Hadron Collider PAMELA IceCube

Indirect Dark Matter Searches Very rich search strategy, multi-messenger and multi-wavelength Gamma rays are particularly good probes to learn about the particle nature of dark matter via its annihilations DARK MATTER DISTRIBUTION ANNIHILATION PROCESS Simulated Milky Way-like dark matter halo: very dense at its center, large number of substructures + Via Lactea II (Diemand et al. 2008)

Gamma rays from Dark Matter Annihilation Galactic center Dark matter substructures Pieri et al, arXiv:0908.0195

Indirect Detection Results - Gamma Ray If a signal is detected, we can learn about the mass of the dark matter particle, how often it annihilates, how it is distributed in space, and constrain underlying theories Dark matter particle mass Annihilation cross section (how often annihilations occur) Detection!

Indirect Detection Results - Gamma Ray If a signal is not detected, we can rule out many possibilities Dark matter particle mass Annihilation cross section (how often annihilations occur) Ruled out Allowed

Fermi Mission The Large Area Telescope The Fermi Large Area Telescope (LAT) observes the gamma-ray sky in the 20 MeV to >300 GeV energy range with unprecedented sensitivity Orbit: 565 km, 25.6o inclination, circular. The LAT observes the entire sky every ~3 hrs (2 orbits) Fermi LAT is a pair conversion telescope: gamma ray converts to electron-positron pairs which are recorded by the instrument Fermi LAT

The Fermi LAT Gamma-Ray Sky Fermi LAT data 4 years, E > 1 GeV A potential dark matter signal must be disentangled from other more conventional (and brighter!) processes that produce gamma rays

A Dark Matter Signal from the Galactic Center? An excess in the Fermi LAT GC data consistent with dark matter annihilation was first claimed in 2009 (Goodenough and Hooper, arXiv:0910.2998.) More recent analyses are consistent with these results Properties of the dark matter particle and underlying particle physics model can be inferred However, other more mundane gamma-ray sources such as pulsars could explain the excess Image credit: NASA/T. Linden, U. Chicago C. Karwin et al, arXiv:1612.05687 Annihilation cross section Dark matter particle mass

galactic interstellar emission Caveats The determination of the Galactic center excess heavily relies on modeling of the gamma- ray emission from other processes (the excess is a small fraction of the total emission observed toward the Galactic center!) Modeling of the gamma-ray sky is complex, and improvements are crucial to confirm the properties of the excess and to conclusively determine whether it originates from dark matter or something else! = + + data sources galactic interstellar emission isotropic + dark matter??

Dark Matter Substructures Optically observed dwarf spheroidal galaxies: largest dark matter substructures predicted by simulations Excellent targets for gamma-ray dark matter searches Very rich in dark matter Expected to be free from other gamma ray sources, and therefore a potential signal is more easily interpreted compared to the Galactic center

Dwarf Spheroidal Galaxies Search for a signal in 25 dwarf spheroidal galaxies. No significant emission is found The limits probe a dark matter explanation of the Galactic center excess Fermi LAT Collaboration, arXiv 1503.02641 Ruled out Annihilation cross section Allowed Dark matter particle mass

Dark matter interpretation of Galactic center excess Dwarf Spheroidal Galaxies Search for a signal in 25 dwarf spheroidal galaxies. No significant emission is found The limits probe a dark matter explanation of the Galactic center excess Fermi LAT Collaboration, arXiv 1503.02641 Dark matter interpretation of Galactic center excess Annihilation cross section Dark matter particle mass

Summary/Outlook Thank you! Evidence for dark matter is overwhelming Many experiments have been relentlessly searching for dark matter particle candidates Gamma rays have been able to test and rule out many possibilities An intriguing excess originating from the Galactic center has been found; however, more work and improved understanding of the gamma-ray sky are necessary to determine its nature, dark matter or otherwise Thank you!

Lawrence Livermore National Lab Will Dawson Lawrence Livermore National Lab LLNL-PRES-739383 This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC

Galaxy Cluster Mass ~ 1015 Solar Masses Abell 1689 NASA, ESA, E. Jullo (JPL/LAM), P. Natarajan (Yale) and J-P. Kneib (LAM)

Most people are familiar with Credit: NASA CXC

Astronomer’s Periodic Table Why is there primarily Hydrogen and He? But even this periodic table is too complex if for me so I made my own version. Credit: NASA CXC

A new component to clusters Just as mercury moves faster because it experiences a stronger force of gravity… The gas is moving so fast in clusters that it ionizes.

Accelerating electrons emit photons

Chandra X-ray Map of the Cluster Plasma Abell 1689 X-ray: NASA/CXC/MIT/E.-H Peng et al; Optical: NASA/STScI

Far more of the mass is in the X-ray emitting intracluster plasma And this was the picture of our universe around 1930. You had hydrogen and He, about 10% of which was contained in stars and the remaining 90% was in the form of gas. However in 1933 this picture of the universe was dramatically changed.

Cosmologist’s Periodic Table Dark Matter Why is there primarily Hydrogen and He? But even this periodic table is too complex if for me so I made my own version.

Gravitational lensing best tool for studying dark matter Zwicky (1937)

Mass warps space-time and alters the path of light Works for all types of matter

Gravitational lensing distorts galaxy images

Gravitational lensing of clusters not observed until 1990 Tony Tyson

Weighing clusters with weak gravitational lensing Abell 1689 Tyson et al. (1990)

The first gravitational lensing mass map Now you can measure both how much mass there is and where it is. Found a discrepancy between the total mass and stellar mass. Abell 1689 Tyson et al. (1990)

Thanks to Hubble a lot has improved in past 20 years Abell 1689 NASA, ESA, E. Jullo (JPL/LAM), P. Natarajan (Yale) and J-P. Kneib (LAM)

Much higher resolution mass maps Abell 1689 NASA, ESA, E. Jullo (JPL/LAM), P. Natarajan (Yale) and J-P. Kneib (LAM)

For some clusters the X-ray plasma and dark matter distributed similarly Abell 1689 Abell 1689 X-ray: NASA/CXC/MIT/E.-H Peng et al; Optical: NASA/STScI NASA, ESA, E. Jullo (JPL/LAM), P. Natarajan (Yale) and J-P. Kneib (LAM)

Merging galaxy clusters are an exception Bullet Cluster X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

Merger Scenario S N Key Dark Matter Gas + Gas Galaxies Now let me briefly summarize how we believe this cluster reached its observed state. So what I will show on the right is a simplified movie where dark matter is represented by… First of all we believe that the North cluster began in the South and the South cluster began in the North. Key Dark Matter Gas + Gas Galaxies

Merger Scenario S N Gravitational Attraction Key Dark Matter Gas + Gas The two clusters were gravitationally attracted to one another. Key Dark Matter Gas + Gas Galaxies

Merger Scenario S N Key Dark Matter Gas + Gas Galaxies And began speeding towards one another. Key Dark Matter Gas + Gas Galaxies

Merger Scenario S N Key Dark Matter Gas + Gas Galaxies

Merger Scenario N+S Key Dark Matter Gas + Gas Galaxies Eventually they collided. Key Dark Matter Gas + Gas Galaxies

Merger Scenario N S Momentum Momentum Key Dark Matter Gas + Gas Now this is the part of the movie where the plot really begins to thicken. The galaxies of each cluster pass right through one another. This isn’t due to any new physics it is just that there is so much space between the galaxies that any one galaxy’s chance of hitting another is very small. So their momentum carry them right on through. The gas however is diffuse and more evenly occupies space, thus a collision between two gas particles is much more likely. And much of the gas actually gets stopped in the middle. Key Dark Matter Gas + Gas Galaxies

Merger Scenario N C S Key Dark Matter Gas + Gas Galaxies The interesting thing is that when we map the mass of the cluster it is located with the galaxies not the gas. And as I mentioned earlier the mass in galaxies is 1/7 that of the mass of the gas. Thus there must be some other mass component. Dark matter. And because it is located with the galaxies and not with the gas we can infer that it must be collisionless like the galaxies, unlike the gas. So by comparing and contrasting the dark matter’s behavior with that of the galaxies and gas we are able to constrain it properties. And find that it has an effective cross-section less than that of a neutrino. The current tightest constraint on dark matter by Randall et al. 2008 is an order of magnitude greater and was achieved by using complex hydrodynamic computer simulations of the Bullet Cluster. Key Dark Matter Gas + Gas Galaxies

Musket Ball Cluster

Musket Ball Cluster Galaxy Density Contours zphot = 0.53±0.1 Hubble Space Telescope Image: STScI Subaru 8m Telescope Image: Subaru Telescope, NAOJ KPNO 4m Mayall Telescope Image: NOAO/AURA/NSF Keck 10m Telescope Image: Laurie Hatch Galaxy Density Contours zphot = 0.53±0.1 Discovered in the DLS via its weak lensing shear signal. This is a color composite postage stamp using DLS BVR. The white contours show the galaxy density for photo-z selected galaxies, cluster redshift +/- 1 sigma. They begin at 200 / Mpc^2 and increase in increments of 50. Dawson et al. (2012a)

Weak Gravitational Lensing Mass Map Hubble Space Telescope Image: STScI Subaru 8m Telescope Image: Subaru Telescope, NAOJ Mass Map with Galaxy Density Contours (white) Be sure to really stress similarities of ground and space based results. Subaru: -50 galaxies/arcmin^2, used full p(z) tomography (briefly state what tomography is) HST: -140 galaxies/arcmin^2, used Dan Coe’s HUDF photo-z’s as a function of magnitude bin HST Dawson et al. (2012a)

X-ray Gas Map Chandra Space Telescope Credit: NASA/CXC/Berry Point source subtracted and adaptively smoothed. Signal to Noise of each peak.

Dissociative Merger N C S Key Dark Matter Gas + Gas Galaxies Put it all together and you’ve got a dissociative merger. Key Dark Matter Gas + Gas Galaxies

4 ways to constrain sDM with dissociative mergers Gas and dark matter offset ≠0 ≠0 Markevitch et al. originally introduced three means of constraining the dark matter cross-section using dissociative mergers. The first involves relating the scattering depth of the gas with that of the dark matter. Since the dark matter is offset from the gas we know that it’s scattering depth must less than that of the gas. Key Dark Matter Gas + Gas Galaxies

Significant DM-Gas Offset enables sDM constraint Mass Map with Galaxy Density Contours (white) and X-ray contours (red) Weak lensing peaks to X-ray peak offset: 1.4′±0.3 Following work of Markevitch et al. 2004 Well this doesn’t really help. 𝜎 𝐷𝑀 𝑚 𝐷𝑀 ≲7 𝑐𝑚 2 𝑔

4 ways to constrain sDM with dissociative mergers Gas and dark matter offset Slowing of the subclusters M/L ratio of subclusters Galaxies and dark matter offset ≠0 ≠0 A fourth way to constrain the dark matter cross-section was introduced by Randall et al. (2008). Using SIDM simulations they were able to estimate the expected offset dark matter from the galaxies as a function of the dark matter cross-section. Since the centroid in the bullet were consistent to within measurement errors they were able to place an upper limit on the dark matter cross-section. This is my favorite method since it requires the fewest assumptions. This is the method that I believe we should focus on. Key Dark Matter Gas + Gas Galaxies

The Musket Ball mass & galaxy maps generally agree, but… Surface mass density S/N map Galaxy density (white contours) Centroid errors; 68%, 95% Confidence (black contours) 4.5 2.5 0.5 -1.5 -3.5 6.5 Surface Mass Density S/N So here is the weak lensing mass S/N map based on HST ACS observations. And over plotted in white are the galaxy number density contours. The black contours give the 68 and 95% confidence intervals on the respective centroids. Now we only have weak lensing measurements, where as the bullet cluster had strong lensing + weak lensing, so the Musket Ball’s weak lensing centroid is not as well constrained. Plus the Musket Ball is not quite as massive so that also worsens the constraint. In fact the North centroid errors are so large that it is essentially useless. However the more massive Southern subcluster has better constraints so lets take a closer look at it.

The Musket Ball shows an offset between galaxies and WL 4.5 2.5 0.5 -1.5 -3.5 6.5 Surface Mass Density S/N 19” Weak Lensing Centroid Galaxy Centroid There is a problem! These two centroids aren’t consistent. They are offset by 19 arcsec.

We are improving the dark matter constraint by studying more systems Galactic light Total mass X-rays Radio waves Golovich+ 2017 Benson+ 2017

Dark Matter Activities Carolyn Slivinski

ACTIVITIES Jelly Bean Universe Paper Plate Activity Wine Glass demonstration

Energy Distribution of the Universe The Chandra activity has a clickable pie chart, with information available about each of the pie segments. A handout is found at http://chandra.si.edu/resources/handouts/lithos/piehandout.pdf with examples of calculations you can do to determine how many color jelly beans are in a particular jar. Also see http://nasawavelength.org/list/1929 (Jelly Bean Universe) Based on WMAP data; study is ongoing. Based on http://chandra.harvard.edu/resources/flash/univ_pie.html

Paper Plate Activity – Find the Hidden Mass Use a screwdriver to poke a hole in the center of 2 paper plates, then separate the plates. Arrange 6 quarters symmetrically across the center line of one paper plate. Add a 7th quarter in a random location, then tape or glue the second paper plate on top. Use the screwdriver to spin the plate. One side should tilt down. Try to find a location for an 8th quarter on the top plate which will balance the spinning plate (tape it down so it’s firmly attached!). Then measure and mark a location that is located opposite from that 8th quarter. The 7th quarter should be underneath that mark! Check your results by holding the plates up to a strong light. This resource can be found at http://nasawavelength.org/list/1929 (Find the Missing Mass – Paper Plate Activity) Just like this activity, all of astronomy involves indirect measurements and investigations. It would have been very easy for us to tear apart the two paper plates in order to discover where the "hidden matter" was. Every day, scientists wish they could do that very thing to the Universe! But they can’t. So when we study a subject such as dark matter, it is important to understand the tools at hand to probe its nature -- since we cannot just take the easy way out! If you shine a light through the paper plate to locate the "hidden matter", it is as if you were applying the concept of gravitational lensing to locate dark matter. Lastly, you can think of balancing torques in paper plates as analogous to the rotational curve observations of galaxies that is being done now to reveal even more about dark matter. This activity is based on materials created by Sonoma State University.

“Gravitational lensing” – using an image Abell 370 “Gravitational lensing” – using an image http://hubblesite.org/image/4024/news_release/2017-20 Gravitational Lensing – Abell 370 Using the base of a wineglass, you can replicate the effect that gravitational lensing has on our views of distant objects in space. Slide the wineglass over an image, or a series of gridlines, to see how the images will warp and stretch.

“Gravitational lensing” – using a light source Credit: Phil Marshall Wineglass lensing image from http://kipac-web.stanford.edu/research/gravitational_lensing; credit Phil Marshall. The wineglass distortions of a candle match closely with actual gravitationally-lensed images.

Wineglass stem Black Hole mass Magnification Distortion Magnification Distortion

ASTC partnership A Professional Development opportunity – How to Use NASA Resources; future funding resources available Seven webinars to be held in 2018, with these goals: Increase knowledge of NASA Astrophysics-related concepts Improve familiarity of NASA Astrophysics resources and ways to use them Utilize real NASA data Interact with NASA Subject Matter Experts To participate in this webinar series, contact Wendy Hancock at whancock@astc.org or Tim Rhue at trhue@stsci.edu by December 31, 2017 As a follow-on to this webinar series, there will be an opportunity to apply for $2,500 mini-fund resources to be competitively awarded to selected institutions, in order to implement or facilitate programming, produce exhibits, etc., using Universe of Learning resources. 58

To ensure we meet the needs of the education community (you To ensure we meet the needs of the education community (you!), NASA’s UoL is committed to performing regular evaluations, to determine the effectiveness of Professional Learning opportunities like the Science Briefings. If you prefer not to participate in the evaluation process, you can opt out by contacting Kay Ferrari <kay.a.ferrari@jpl.nasa.gov>. This product is based upon work supported by NASA under award number NNX16AC65A to the Space Telescope Science Institute, working in partnership with Caltech/IPAC, Jet Propulsion Laboratory, Smithsonian Astrophysical Observatory, and Sonoma State University. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Aeronautics and Space Administration.