Constraining Galactic Halos
with the SZ-effect
by Naureen Goheer,
University of Cape Town
based on a collaboration with
Kavilan Moodley (UKZN)
Galaxy morphology
spirals much
better understood,
focus on them
rich in gas and dust
spiral-unbarred
90%
mass : 10  10 M sol
9
galaxies
11
size : 5 - 1 00 kPc
spiral-barred
10%
far less evidence for young stars, gas, or
dust
elliptic
mass : 10  10 M sol
7
13
size : 1/10 - 1 00 kPc
Optical part of Spiral Galaxies
• two visible components
~ 90 % of the visible mass:
< 10% of visible mass:
spiral arms (stars + gas&dust)
Bulge (stars)
“invisible” matter component: Halo
Stellar Halo,
< 1% of stars
visible part
Dark Matter Halo
some indirect observational evidence for the
existence of Halo
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
Secondary
Anisotropies
–effects due to
structure formation
(nonlinear structure
evolution)
–gravitational effects
(lensing)
–scattering effects
Primary
Anisotropies:
early effects at the
last scattering
surface and large
scale Sachs-Wolfe
effect.
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
allows us to
distinguish signal
from other sources
distinct spectral signature
lower intensity at   210 GHz
no effect at   210 GHz
higher intensity at   210 GHz
scattering of CMB
photons on e- in hot gas
photons pick up energy
and get shifted to
higher frequencies
distortion of black
body spectrum
net effect
Thermal SZ-effect: Central decrement
empirically: number density of e- highest in the
center and falls off radially
k B Te
 T  f ( )TCMB
T  Tne e Tn e dl 2 dl
mec
dl
central
decrement
frequency dependence
Comptonization parameter
=gas pressure along the line of sight
thermal SZ depends on temperature and number of electrons in gas
mass of object
• purely classical treatment
• must include relativistic effects when k B Te  10 keV
(e.g. in clusters)
 T  T e  n e dl
integrated effect
(over entire object)
 T  T e  d   n e dl
integrate over angular size
• high central decrement for clusters (higher
temperature and mass)
d
• • much
smallerintegrated
central decrement
observable
effect (if due
halotois massive
much lower mass and lower temperature in
and hot enough)
galaxies
• use integrated effect to
constrain electron
BUT
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
dl
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:
Lx
S SZ
L x  n e ( 0 ) Te
2
1/ 2
S SZ  n e ( 0 )Te

ne (0)
Te
1/ 2
electron
temperature
central electron
density
for extended halos with low central
density, X-rays observations are less
sensitive than SZ-observations!
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 signature
2) Measures the total thermal content of the cluster
3) More or less redshift independent
4) Less susceptible to messy cluster substructure, core
physics (prop to density and not density squared as in XRays)
Note that at   210 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
(  T RJ ) thermal  1 mK
reasonable cluster velocities the
kinematic effect is an order of
magnitude smaller.
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
Secondary
Anisotropies
contributions
through nonlinear
structure
evolution, star
formation, and
radiative feedback
from the small
scales to the large
.
Primary
Anisotropies:
early effects at the
last scattering
surface and large
scale SachsWolfe effect.
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.