Search for MACHOSOutlineWhat are MACHOS?Dark MatterBaryonic dark matterCandidatesDetectionDetectionProblems with detectionEffortsMACHOEROS 2OthersFindingsFuture workReferences1Search for MACHOSMark ZimmermanApril 17, 20072Outline• What are MACHOS?• Dark matter• The need for baryonic dark matter• Candidates• Detection method• Problems/concerns with detection• Collaborations• Findings• Future work3What are MACHOS?• Massive Compact Halo Object• Any celestial mass that is both dense (not a diffuse gas cloud) and dark (not visible in any electromagnetic band)• In some sense, defined by our inability to see it?4Dark Matter• First observed by Fritz Zwicky in 1933• Galactic rotation curves do not fall off as quickly as expectedGalaxy NGC3198 from Begeman 1989http://astro.berkeley.edu/~mwhite/darkmatter/rotcurve.html5Baryonic dark matter• Using Big Bang nucleosynthesis, the amount of baryonic matter in the universe can be estimated• This is done by measuring the He/H ratio today, since most of the He was produced in the early big bang• This happened when neutrons were first able to decay into protons – the density of baryonic matter determines how many neutrons found protons to turn into He before they decayed6Candidates• Brown dwarfs• Dim white dwarfs• Neutron stars• Black holes• Planets (Jupiter like objects, aborted stars)• Very dim stars7Detection• First proposed by Paczynski• Uses GR to get around the electromagnetic invisibility• Gravitational lensing8Detection• “micro”lensingOSLSOLEDDDcGMR24=9A microlensing event observed by the OGLE collaboration10Problems with detection• Proportional increase in brightness isn’t very helpful• So only real measured quantity is lensing duration• Assuming you know where the source is, this quantity still varies with 3 variables– Lens distance– Lens transverse velocity– Lens mass11Efforts• Most efforts have focused on the LMC (Large Magellanic Cloud) or SMC (Small Magellanic Cloud)• They are distant enough to probe our own galactic halo, but close enough to resolve millions of stars• Some effort has also been put into looking toward the Galactic bulge• Probability of observing lensing/Optical depth• Even if the whole halo was MACHOs, the optical depth would still be small enough to require that millions of stars be monitored12MACHO• An American collaboration observing in Australia• Monitored 8.6 million stars in the LMC• In order to do this, they had little time coverage on each star – blind to short duration (low mass) events• 528 events observed, 450 of which are clean and follow the model• With 7 years worth of data, they measured6101.2−×≈τ13EROS 2• A French collaboration observing in Chile• Monitored ~7 million stars for 6.7 years in the LMC• Carefully accounting for backgrounds due to variable stars and source confusion, they observed only one microlensing event in this time• This corresponds to• This is at variance with the MACHO findings71036.0−×<τ14Others• EROS 1, which monitored only ~150,000 stars but with high time sampling, to rule out any large contribution from low mass lenses• OGLE 1 and 2: Polish experiments observing in Chile with similar methods and a wealth of publications1.3m Warsaw Telescope - Las Campanas Observatory, Chilehttp://bulge.princeton.edu/~ogle/15Findings• Despite disagreements in the data, all collaborations seem to agree that MACHOs do not compose all of the dark matter in the galactic halo• Current estimates are below 30% for the mass fraction of the halo composed of MACHOs• Most common mass for MACHOs is ~0.5 solar masses• Data also suggests that the galactic dark matter halo may not be perfectly spherical16N-body simulation of a galactic dark matter halo http://relativity.livingreviews.org/open?pubNo=lrr-2002-4&page=node7.html17Future work• The collaborations will continue to collect more data• Now focusing in on the SMC and galactic bulge as well• Trying to correct for problems like variable stars, achromatic events, reconcile experimental differences18References• [1] Spiro, M., Aubourg, E., and Palanque-Delabrouille, N., Nuclear Physics B (Proc. Suppl.) 80 95-108 (2000)• [2] Mancini, L., Jetzer, P., Scarpetta, G, AIP Conf. Proc. 2003, Vol. 695, Issue 1, p339-347• [3] Paczynski B., Astroph. J. 301, 503 (1986)• [4] Thomas, C.L. et al., Astrophys. JS. (2005) http://wwwmacho.mcmaster.ca/Bulge/catalogPaper.pdf• [5] Tisserand, P. et al., Astron. & Astrophys. (2006) http://xxx.lanl.gov/PS_cache/astro-ph/pdf/0607/0607207.pdf• [6] Holopainen, J,. et al., Mon. Not. R. Astron. Soc. 368, 1209-1222 (2006)• [7] Gates, E., Gyuk, G., and Turner, M., Phys. Rev. D, 53,