Constraining Galactic Halos with the SZ-effect

by Naureen Goheer, University of Cape Town

based on a collaboration with Kavilan Moodley (UKZN)

Galaxy morphology

galaxies

elliptic

spiral-unbarred

spiral-barred

10%

rich in gas and dust

far less evidence for young stars, gas, or dust

90%

kPc 001-5 :size

M1010 :mass sol119

kPc 001-1/10 :size

M1010 :mass sol137

spirals much better understood,focus on them

Optical part of Spiral Galaxies• two visible components

< 10% of visible mass: Bulge (stars)

~ 90 % of the visible mass:spiral arms (stars + gas&dust)

“invisible” matter component: Halo

Dark Matter Halo some indirect observational evidence for the existence of Halo

Stellar Halo, < 1% of stars

visible part

baryonic non-baryonic

DM Halo and Observations• some indirect observational evidence

for the existence of Halo through kinematic tracers, e.g:•disk galaxy rotation curves , •satellite galaxies and globular clusters, •hot gas (also around ellipticals) confirm existence of halo

• radial extents and total masses of these halos remains poorly constrained

• one possible new way of constraining amount of dark baryons is the SZ-effect

DM Halo and Theory

• might account for missing baryons (only 20% of mean baryonic density has been observed)

• DM halo required by most models of galaxy formation

• galaxy formation still not understood – have no accepted model of galaxy formation, thus no accepted halo model

• expect different halo dynamics depending on whether the galaxy at hand is starburst or quiescent; smooth halo or filaments

CMB Anisotropies

Primary Anisotropies: early effects at the last scattering surface and large scale Sachs-Wolfe effect.

Secondary Anisotropies –effects due to structure formation (nonlinear structure evolution) –gravitational effects (lensing)–scattering effects

SZ-effect: scattering of CMB photons on hot gas

The Sunyaev Zel'dovich (SZ) effect

• secondary anisotropies due to (up-)scattering of CMB photons with hot gas (keV) along the line of sight (at the centre of clusters etc.)

• thermal: due to the thermal velocities of the electrons in the gas

• kinematic: due to the bulk velocity of gaseous object

CMB=black body

scattering of CMB photons on e- in hot gas

distortion of black body spectrum

photons pick up energy and get shifted to higher frequencies

net effect

lower intensity at no effect athigher intensity at

GHz 210GHz 210

GHz 210

distinct spectral signature

allows us to distinguish signal from other sources

Thermal SZ-effect: Central decrement

Comptonization parameter=gas pressure along the line of sight

frequency dependence

dlnTT ee

thermal SZ depends on temperature and number of electrons in gas

empirically: number density of e- highest in the center and falls off radially

dlcentral decrement

• purely classical treatment • must include relativistic effects when (e.g. in clusters)

mass of object

keVTk eB 10

dlcm

TknTfT

e

eBTeCMB 2

)(

• high central decrement for clusters (higher temperature and mass)

• much smaller central decrement due to much lower mass and lower temperature in galaxies

dlnTT ee dlndTT ee integrated effect

(over entire object)

integrate over angular size

dld

BUT

• observable integrated effect (if halo is massive and hot enough)

• use integrated effect to constrain electron number density and thus the dark baryons in halo of nearby galaxies

• assuming that non-baryonic DM scales like dark baryons, this constrains the total DM content of halo

other observational constraints

• spectroscopy: can only test for specific isotopes using

• X-rays: observe Bremstrahlung etc.

SZ-effect versus X-rays

• X-ray luminosity:

• SZ-flux:

2/12)0( eex TnL

eeSZ TnS )0(

2/1

)0(

e

e

SZ

x

T

n

S

L

for extended halos with low central density, X-rays observations are less sensitive than SZ-observations!

electron temperature

central electron density

Summary and Future outlook

• 80% of the predicted baryons have not been observed• some of them might hide in the hot halos of galaxies• very difficult to directly measure the halo content

• the integrated thermal SZ-effect can be used to directly measure baryonic matter content of halo

• the new Atacama Telescope (ACT) will have high enough sensitivity to get a clear signal (better than PLANCK)

discarded slides

models of galaxy formation

• explain different halo scenarios: halos can be smooth or filled with filaments (mention models of galaxy formation)

• halo content: O VI (observed using x-rays, show example pics)

• what can we learn about models of galaxy formation

Whats nice about SZE?1) Ofcourse, the distinct spectral signature2) Measures the total thermal content of the cluster3) More or less redshift independent4) Less susceptible to messy cluster substructure, core physics (prop to density and not density squared as in XRays)

Note that at   GHz, the maximum change in intensity due to the kinematic effect coincides with the null of the thermal effect.

This, in principle, allows one to separate the two effects. The magnitude of the thermal effect for a hot, dense cluster is , and for reasonable cluster velocities the kinematic effect is an order of magnitude smaller.

210

mK 1)( thermal RJT

OR

Primary Anisotropies :early effects at the last scattering surface and large scale Sachs-Wolfe effect.

Secondary Anisotropies:secondary contributions through nonlinear structure evolution, star formation, and radiative feedback from the small scales to the large .

CMB Anisotropies

Primary Anisotropies: early effects at the last scattering surface and large scale Sachs-Wolfe effect.

Secondary Anisotropies contributions through nonlinear structure evolution, star formation, and radiative feedback from the small scales to the large .

SZ-effect: scattering of CMB photons on hot gas

The SZ-effect• Thermal Sunyaev-Zel’dovich effect: Inverse

Compton scattering of the CMB by hot electrons in the intracluster gas of a cluster of galaxies distorts the black body spectrum of the CMB. Low frequency photons will be shifted to high frequencies.

• Kinetic Sunyaev-Zel’dovich effect: The peculiar velocities of clusters produces anisotropies via a Doppler effect to shift the temperature without distorting the spectral form. Its effect is proportional to the product of velocity and optical depth.