Extragalactic Astronomy & Cosmology Dark Matter - chapter 14

1. [4097] Physics 316 Extragalactic Astronomy & CosmologyDark Matter - chapter 14 Jane Turner Joint Center for Astrophysics UMBC & NASA/GSFC 2003 Spring

2. The Question of Dark Matter What do we mean by "Dark Matter ?" material that gravitates, but has a very high mass/luminosity (M/L) ratio (aka mass/light ratio)

3. Use Gravity to look for evidence Assuming it exists, we can just use studies of the gravitational effects of dark matter on the visible (luminous) matter. (Imperfect analogy regarding the discovery of Pluto) Plus details of these effects can help determine the distribution of DM with respect to visible matter hence constrain some of the various candidates. e.g. Is DM a low space-density of verymassive objects ? a high space-density of low mass objects ? eg stars eg particles

4. Mass-to-Light Ratio Relates the mass there is to the light you see, in solar units By definition, for the Sun the ratio is 1, one solar mass to 1 solar luminosity In our part of the Galaxy M/L ~ 5 because there are many stars which are dimmer than our Sun At 30 kpc M/L ~ 30 and rising, more dim things dominating

5. 3 ways to look for evidence of DM Look at motion of luminous bodies affected by DM Measure temperature of hot gas held in galaxy/cluster by gravity Look at how clusters of galaxies bend light - gravitational lenses

6. DM in the Solar Neighbourhood ? No ! (at least not much) within our immediate solar neighbourhood (ie. within ~ 130 pc) Vast majority of gravitational mass causing vertical velocity dispersion accounted for by Stars & Gas we know about DMnot clustered around the thin diskof the Galaxy

7. What started the search for DM ? The first evidence that we were not seeing all the mass which exists came from the rotation curves of spiral galaxies Rotation curves tell you about the mass as a function of radius from M = r x v2 G Measuring spiral galaxies we expected the mass distribution to follow that of the light

8. Expectations ?

9. 1a) Spiral Galaxy Rotation Curves

10. MilkyWay: Flat Rotation Curve

11. Spiral galaxies: Flat Rotation Curves Spiral Galaxies all seem to have flat rotation curves -regardless of their mass or luminosity

12. 1b) Elliptical Galaxies Tricky, no ordered rotation to study - however, can look at widths of spectral lines, these are wide when there is a high velocity dispersion High vel dispersion occurs when the mass is high Ellipticals show wider lines than expected based on the mass we see, again, pointing to some unseen matter

13. 2) DM Evidence from Hot Halos While the most visible part of a galaxy is the globular clusters in the halo, they only make up a small part of the halo mass Hot gas shines in X-rays - need a lot of unseen mass to gravitationally bind this to the halo

14. N4631 Optical, UV, X-ray

15. X-ray Halo around M87

16. DM Evidence from Cluster Gas How about on larger scales?? There is evidence for more material than we can see between the galaxies in clusters X-ray emitting gas - can measure temperature of the gas -know how much matter needed to gravitationally bind this gas to the cluster Also, measure vel. dispersion again, but using the galaxies

17. Masses of Clusters of galaxies 1) Can find mass within a distance r of the galactic center: v is the velocity of objects moving under gravity. Can be applied to clusters of galaxies -use average speed of galaxies & the cluster radius in the orbital velocity law. 2) When we measure the temperature of hot, intracluster gas from its X-ray emission, we must first convert the gas temperature into an average speed for the gas particles. Can use which gives the approx average speeds of hydrogen nuclei in a gas of temperature T- once we find this speed, use it in the orbital velocity law.

18. DM Evidence - Coma Cluster The Coma cluster has a radius ~ 3.3 million ly (3.1x1022 m). Its intracluster medium gas has a temperature of ~ 9x107 K. 

19. DM Evidence - Coma Cluster To find the mass from the temperature of the X ray–emitting gas, we first use the given formula to find the average speeds of the hydrogen nuclei in the gas: Plugging this velocity into the orbital velocity law, we find: recalling that the Sun's mass is 2.0 x 1030 kg -> 2.1x 1014 M

20. 3) Gravitational lensing Adding the mass of visible stars and gas falls way short of these estimates! M/L ~ 100-400 So, how is this invisible stuff distributed? Use gravitational lensing to map it Angle of deflection  mass

21. Summary

22. DM Candidates REMINDER: Protons & neutrons belong to a category of particles called baryons, so ordinary matter is sometimes called baryonic matter (Technically, a baryon is a particle made from three quarks) By extension, extraordinary matter is called nonbaryonic matter Usually divided between Baryonic DM candidates Non-Baryonic DM candidates Serious candidates have been proposed with masses ranging from axions, 10-71 Msun to 106 Msun Black Holes

23. Baryonic DM Problems If baryonic DM is the key then we are talking brown dwarfs, planets (anything else would emit in opt. IR , X-rays or something!) then we have problems as to why we can’t detect it (!) Or, can we imagine new objects….suggestion: MACHOs -Massive Compact Halo Objects which will gravitationally lense background stars

24. Baryonic DM Problems Detect MACHOs by monitoring stars - look for var in star types which are not intrinsically variable

25. A MACHO Brown dwarf, 0.05 Solar masses Dozens of such events found, but estimates suggest MACHOS cannot provide enough mass, they are typically a few hundreths of a solar mass -only 20% of the halo mass DM must be predominantly something else

26. DM Candidates Non-Baryonic DM candidates CDM “cold” Dark Matter -weakly-interacting particles moving slowly enough to collect into galaxies HDM “hot” Dark Matter, faster moving particles such as neutrinos

27. Non-Baryonic DM Problems If non-baryonic DM is the key , it’s a major source of gravitational forces in the Universe, responsible for the structures we see today. Particles in HDM schemes move fast - can make large structures OK (walls etc) - cannot make small structure (galaxies etc) Particles in CDM schemes move slowly - can make small structures (galaxies) - cannot make large structures (walls etc) Hence the new ideas for a combination of both Mixed Dark Matter (MDM) models

28. Non-Baryonic DM Candidates Detected ! … but not heavy enough.. Would not “clump” enough detected neutrinos suggest theres just not enough of them Only popular HDM candidate… Neutrinos (those with Mc2 < 100eV) Most popular CDM candidates Collectively known as WIMPs (Weakly Interacting Massive Particles) new , yet undetected massive particles predicted by Grand Unified Theories…eg. gauge bosons Many, particle detectors & accelerators looking for WIMPs … none found yet

29. Gravity OK ? Worth noting that several people have suggested that we may have our theory of Gravity wrong ! …shouldn’t be absolutely discounted yet !

30. Future Progress ? ? WIMPs any day/year now … ? Certainly accelerators are getting towards the right mass range to make some of these particles thus place constraints on models (even if the particles are not all the DM)

31. Structure Formation If most matter is dark it must have affected how structure in the universe formed So, from existing structures, infer something about DM If DM dominated in early universe, models predict galaxies & proto-galaxies formed, separated as universe expanded, then in some places gravity held groups together So, smaller objects formed before conglomerates….a “bottom up” scenario

32. Structure Formation <-- 200 Mpc --> Bottom-up models suggest we should see structures merging -we do see this by examination of velocities across large areas <-- Merging clusters seen by Chandra

33. Structure Formation Clusters combine to make very large structures

34. Structure Formation Clusters combine to make very large structures

35. Largest Scales Universe appears more uniform on largest scales

36. Anyway, back to DM… So, how much of the stuff is there & why do we care? If theres a lot of DM everything will collapse in on itself If theres little DM the expansion will continue unchecked Critical density is that for which gravity will be strong enough to just halt the expansion of the universe 10-29 g cm-3 Need to know how to determine age and fate of the universe Estimates of age depend on how much we think gravity has held back expansion Fate depends on whether gravity ultimately “wins”

37. Anyway, back to DM… So, how much of the stuff is there & why do we care? Need to know to determine age and fate of the universe Estimates of age depend on how much we think gravity has held back expansion Fate depends on whether gravity ultimately “wins” If theres a lot of DM everything will collapse in on itself If theres little DM the expansion will continue unchecked  < 1 is a coasting universe  >1 is a recollapsing universe  =1 is a critical universe, just enough matter to halt expansion

38. Anyway, back to DM… So, how much of the stuff is there & why do we care? Recent evidence suggest the acceleration may be speeding up !!! Galaxies receding from each other increasingly faster Universe will become cold and dark most quickly Ideas under the names dark energy, quintessence and the cosmological constant, 