1. The Dark Side of the Universe Sukanya Chakrabarti (FAU)

2. what’s the universe made of? The visible part of the universe is a tiny fraction!

3. how do we know dark matter exists? can we figure out where it is?

4. Nomenclature pc: typical distance between stars [3x10 18 cm]. Orion: D ~ 400 pc ~ 1200 Lyrs. R d: Scale length of spiral galaxies: ~ 3 kpc Galaxies are made up of: gas, stars and dark matter. Sound speeds of cold gaseous component in galaxies ~ 7 km/s; effective sound speeds of stellar component: ~ 30 km/s

5. The History of Dark Matter: Discovery of Neptune Urbain Le Verrier: Aug 13 1846: predicted azimuth of Neptune

6. History: Dark Matter Flat Rotation Curves: Rubin et al.1978, Bosma 1978. Σ ☆ ∝ exp (-r/R d ). R d ~ 3 kpc. Unseen dark mass producing flat rotation curves. 2005 Millennium Simulation. Cold dark matter paradigm: accretion

7. Galaxy composition Stars -- 10 11 stars Gas -- 10 10 M sun Dark Matter Halo -- makes up the rest. Total mass of our galaxy: 10 12 M sun

8. Gravitational lensing

9. how do we know dark matter exists?... you can infer its existence from its gravitational effect (tides on the earth’s ocean from the moon) can we figure out where it is?

10. Overview Galaxies with optical companions : Proof of Principle Milky Way Inferring distribution of dark matter in galaxies Cold gas as tracer of perturbing dark-matter dominated dwarf galaxies

11. The Cold Dark Matter Paradigm & the distribution of galaxies Simulations correctly recover the observed distribution of galaxies on large scales (Davis et al. 1985; Springel et al. 2006) Springel et al. 2006

12. Dark Matter: Structure on Sub-Galactic Scales

13. Diemand et al. 2008 - theory predicts: should be ~1000 sub-halos with M> 10 7 M sun, ~ 1 sub-halo of mass 10 10 M sun Where are the rest? Can you find them by looking for their signatures on gas disks? Dark Sub-Halos: Expectations from Simulations

14. Large Magellanic Cloud

15. Sagittarius Dwarf

16. Infrared (2MASS) Map of Milky Way

17. Tidal Imprints (footprints) of Dark Subhalos on Outskirts of Galaxies Coldest Component Responds the Most! (by ratio of inverse sound speed squared). Gas has short-term memory. Maximize rate of detection of dark subhalos by looking for their tidal footprints on atomic hydrogen gas disks. Atomic hydrogen (HI) Maps! Footprints of Dark Sub-Halos

18. Disturbances in HI disks in Local Spirals: Proof of Principle

19. M51 a m (r)=∫Σ(r, ϕ )e -im ϕ d ϕ Local Fourier Amplitudes of HI data: Metric of Comparison to simulations HI Map optical image

20. M51 Simulation Comparison Chakrabarti, Bigiel, Chang & Blitz, 2011 3-D stereoscopic rendering shown at AAS 2011

21. Tidal Interaction of Satellite with Galaxy forced response inferring satellite mass & location from ripples in galactic disk -- throwing pebbles in a pond

22. Variance Vs Variance Best-fits -- close to origin on variance vs variance plot (S 1 -S 1-4 ), shown at best-fit time. “Variants” include varying initial conditions (ICs), interstellar medium (ISM), star formation prescription, orbital inclination, etc. Our estimate of M s (1:3) close to observational numbers.

23. Galaxies with known optical companions contd. ~1:100 satellite, R peri = 7kpc (close agreement with Koribalski & Sanchez 09) (global fourier amplitudes) Method works for 1:3 - 1:100 mass ratio satellites

24. HI Map of Milky Way HI maps: Levine, Blitz & Heiles 2006. What caused these structures well outside the solar circle? a m (r)=∫Σ(r, ϕ )e -im ϕ d ϕ

25. Simulations Parameter space survey of simulations to explain observed disturbances in HI map of Milky Way. Chakrabarti & Blitz 2009. MsMs R peri inclination 1:10- 1:1000 0.1-50kpc f gas (0.1- 0.3), EQS (isothermal- multi- phase)

26. Initial Conditions, Orbits -- what really matters? Not very sensitive to initial conditions (for parameters comparable to spirals). CB09 -- M s and R peri are what really matter

27. Hunting for the Dark Dwarf Galaxy Why haven’t we seen it yet? Known Milky Way companions have been discovered so far in the optical bands. Huge obscuration in the plane! Prediction for azimuth of satellite (Chakrabarti & Blitz 2011)

28. Infrared (2MASS) Map of Milky Way but 2MASS is not deep enough. GLIMPSE survey of galactic plane will search for putative dwarf galaxy

29. In Closing...

30. Gravitational Lensing Lensing and Tidal Analysis are complementary methods to probe CDM sub-structure and dark matter and do not rely on the stellar light distribution Vegetti et al. 2010 -- characterizing dwarfs through gravitational imaging

31. Statistics from many galaxies! and our own (GLIMPSE) Gives us a chance to understand statistical viability of Tidal Analysis 1800s -- > 2012

32. Summary & Future Analysis of perturbations in cold gas on outskirts of galaxies constrains mass,R,and azimuth of dark (or luminous) perturbers. New method to characterize satellites (to see dark galaxies). Method tested for satellites with mass ratio: ~1:100 - 1:3. Extended to infer dark matter density profile of spirals. Next decade - complex interplay of baryons and dark matter. Computational study of galaxy evolution & cosmology on galactic and sub-galactic scales. From the skeletal structure of the universe to understanding the guts of galaxy evolution.