1. 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)

2. 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?

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

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

5. 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

6. 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

7. 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

8. 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 = K
ΔT ~ 200 μK
Planck 2015
Power spectrum of matter fluctuations
Observed (SDSS)
baryons only
Clumpiness
Dodelson et al 2006
larger scales
smaller scales

9. 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...

10. 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

11. 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

12. 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)

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

14. 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!

15. 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

16. 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

17. 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

18. 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: ) 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:
Annihilation cross section
Dark matter particle mass

19. 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??

20. 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

21. 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
Ruled out
Annihilation cross section
Allowed
Dark matter particle mass

22. 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
Dark matter interpretation of Galactic center excess
Annihilation cross section
Dark matter particle mass

23. 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!

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

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

26. Most people are familiar with
Credit: NASA CXC

27. 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

28. 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.

29. Accelerating electrons emit photons

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

31. Far more of the mass is in the X-ray emitting intracluster plasma
And this was the picture of our universe around 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.

32. 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.

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

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

35. Gravitational lensing distorts galaxy images

40. Gravitational lensing of clusters not observed until 1990
Tony Tyson

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

42. 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)

43. 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)

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

45. 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)

46. 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.

47. 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

48. 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

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

50. Merger Scenario S N
Key
Dark Matter
Gas
+ Gas
Galaxies

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

52. 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

53. 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 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

54. Musket Ball Cluster

55. 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)

56. 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)

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

58. 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

59. 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

60. 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

61. 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

62. 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.

63. 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.

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

65. Dark Matter Activities
Carolyn Slivinski

66. ACTIVITIES
Jelly Bean Universe
Paper Plate Activity
Wine Glass demonstration

67. 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 with examples of calculations you can do to determine how many color jelly beans are in a particular jar.
Also see (Jelly Bean Universe)
Based on WMAP data; study is ongoing.
Based on

68. 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 (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.

69. “Gravitational lensing” – using an image
Abell 370
“Gravitational lensing” – using an image
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.

70. “Gravitational lensing” – using a light source
Credit: Phil Marshall
Wineglass lensing image from credit Phil Marshall.
The wineglass distortions of a candle match closely with actual gravitationally-lensed images.

71. Wineglass stem
Black Hole
mass
Magnification
Distortion
Magnification
Distortion

72. 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 or Tim Rhue at 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

73. 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
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.