# Presentation: 015736

## new techniques for the kSZ

- Simon DeDeo (KICP)
- x

## collaborators

- Shirley Ho
- Ed Sirko
- David Spergel

- (high-z)

## physics of the kSZ

- the “first order” brother to the tSZ:
- tSZ: hot gas, random motion:
- second order term in relativistic doppler shift;
- non-thermal spectrum, parametrize with “*y*”
*traces pressure*
- kSZ: non-relativistic bulk flows; thermal spectrum
- (though second-order corrections *may* be required)
*traces momentum*

## issues for the CMB experiments

- 1. detecting it at all — a very small signal
- 2. “cleaning” off the tSZ — a much stronger signal
- 3. resolution to push past the “damping tail”
- ACT? SPT? — both can in
- principle detect it.
*(4. patchy reionization?)*

## two ways of looking at the kSZ

- 1. the “classical” version (Ostriker & Vishniac, 1987)
- 	• extract from a survey in Fourier space
- 2. the “modern” fashion (*e.g.*, Jimenez, 2005)
- 	• “circle clusters, look behind”

## the classical version (1)

- problem: in the linear regime, velocity flows are linear
- in the Limber approximation, should be no signal!
- (Kaiser, 1984)

*line of sight (up the screen) velocity*
*green, towards; orange, away*

- flows are *gradient*;
**v** and **k** are aligned:

## the classical version (2)

- insight (Ostriker & Vishniac, 1987):
- kSZ traces the *momentum*; so look to the non-linear:

- pure gradient

- “curl” component emerges

## important “global” questions

- • how does ionization fraction evolve over time?
- (feeds into cosmological parameters — viz. Doré talk)
- • how does gas trace matter?
*(effective, redshift-*
*independent smoothing*
*length is a good [1-5%]*
*guess.) — approx*
*400 kpc scale.*

## three ways to detect the kSZ

- 1. correlate the *a**lm*s
*DeDeo, Trac & Spergel (2005)*
- 2. circle the clusters
*Jimenez et al. (2005)*
- 3. reconstruct the velocity field
*DeDeo & al. (2006, in prep)*

## 1. correlate the alms

- 1. correlate the *a**lm*s
*DeDeo, Trac & Spergel (2005)*
- Slightly tricky: galaxies can be moving towards or away.
- Hence: must correlate velocity squared:
- Need to know the matter *bispectrum* to determine
- cosmological parameters.
- Do simple “tomography”: *∆z ≈ 0.1*

## 2. “circle the clusters” 

- 2. “circle the clusters”
- Look for velocity correlation function
- Jiminez et al., many others
- (note: cannot just stack as in the tSZ because velocities will average down)

## 3. reconstruct the velocity field 

- 3. reconstruct the velocity field
*DeDeo & Spergel (2005, in prep)*
- Ambitious — exciting: why not use the density field on large scales to *reconstruct *the velocity field?
*need ∆z ≈ 0.01*

## 3. reconstruct the velocity field 

- 3. reconstruct the velocity field
*DeDeo & Spergel (2006, in prep)*
- Advantages: *more information*. Get a handle on the phase of the velocity, as well as a direct (intuitive) study of both the evolution of gas, and the acceleration of flows.
- • how well can we determine the velocity field?
- 	⇒ Poisson noise
- 	⇒ avoid small-scale non-linearity
- • how well can we filter and model?

## 3. reconstruct the velocity field 

- 3. reconstruct the velocity field
*DeDeo & Spergel (2005, in prep)*
- Advantages: *more information*. Get a handle on the phase of the velocity, as well as a direct (intuitive) study of both the evolution of gas, and the acceleration of flows.
- • how well can we determine the velocity field?
- 	⇒ Poisson noise
- 	⇒ avoid small-scale non-linearity
- • how well can we filter and model?

## reconstruction (1)

- • could just use the Tully-Fisher relation to subtract off
- the Hubble flow — use this to make a template to tell you where to look in WMAP

- Shirley Ho

## reconstruction (1I)

- • use the linear density-velocity relationship — take the
-                                density, pad, and transform

## “Simple” thoughts:

- “Simple” thoughts:
- 1. the kSZ is out there, detectable, and will not strain a good galaxy survey.
- 2. the information is rich enough to constrain both physics and cosmology.
- 3. the “technology” is in place, and being improved.
- 4. another source of constraint as a check on systematics.
- 5. different dependence on H and D: seek the truly strange.