The Synchrotron Cosmic Web: What is it and how might we find it?

The Synchrotron
Cosmic Web !
ASKAP 2016
Tessa Vernstrom
Bryan Gaensler, Shea Brown,
MWA Collaboration
What is the Cosmic Web?!
•  Fluctuations in the primordial
matter density result in the
growth of large-scale structure
(LSS)
•  The Cold Dark Matter (CDM)
theory predicts that massive
galaxies and galaxy clusters
were built from smaller galaxies
that collided and merged
•  Result is clusters, filaments, and
voids we see today which form
a “web” like structure
(Movie: http://cosmicweb.uchicago.edu/ )
The Synchrotron Cosmic Web!
•  Intergalactic shocks from infall into and along filaments accelerate electrons and
amplify magnetic fields à producing synchrotron emission
(Keshet et al. 2004; Hoeft & Brüggen 2007; Battaglia et al. 2009; Araya-Melo et al. 2012)
•  Faint synchrotron radiation should trace large-scale structure and cosmic filaments
•  Signal should dominate other radio signals on scales ~ 10′ to 1o at frequencies ~100 MHz
MHD simulation of magnetised large-scale
structure (Brüggen et al. 2005)
Injected fields vs primordial fields (Donnert, Dolag et al.
2008)
The Synchrotron Cosmic Web!
Vazza et al., 2015, 2016 MHD simulations – 14 sq deg
Diffuse Synchrotron
Redshift bins
Sum over redshifts
Thermal gas/ X-ray
Why is it important?!
•  Could provide direct image of large-scale
structure of the universe
•  Laboratory for studying particle acceleration
in low-density shocks
•  Magnetic field strength of the intergalactic
medium
•  Direct discriminant on competing models for
the origin of cosmic magnetism
•  Discriminant on competing models of
structure formation
•  Solve the “missing baryon” problem
Planelles & Quillis (2013)
How can we detect it?!
•  Direct imaging
(Bagchi et al. 2002; Wilcots 2004; Vazza et al. 2014)
•  Statistical methods:
• 
Cross Correlation
(Brown et al. 2010, 2011)
• 
Stacking
•  Polarisation:
• 
Faraday rotation from background AGN
• 
Dispersion from fast radio bursts
• 
Also stacking and cross correlation
Direct Imaging !
•  We have detected diffuse emission in clusters
• 
• 
• 
• 
Halos
Mini-halos
Relics
But only ~50-100 detected
•  No detection of filaments yet
Complications:
•  Predicted emission faint and low surface
brightness
•  Predicted sub μJy/arcmin2 to mJy/arcmin2
•  Requires high sensitivity to large angular scales
•  Sizes on Mpc scales
•  Difficult for radio interferometer telescopes
•  Bright Galactic foregrounds
•  Bright point sources
•  Faint point source confusion
Ferretti et al., 2012
Cross Correlation!
•  Galaxy number density à traces thermal baryon distribution à should correlate with
diffuse synchrotron
Cross Correlation!
•  Galaxy number density à traces thermal baryon distribution à should correlate with diffuse
synchrotron
2MASS Galaxy Distribution coded
by redshift
(photo credit :Thomas Jarrett (IPAC/Caltech)
Simulated radio synchrotron
(credit: Klaus Dolag)
Cross Correlation!
•  Galaxy number density à traces thermal baryon distribution à should correlate with
diffuse synchrotron
•  How correlated as a function of distance or angular scale?
• 
Unknown
•  How correlated?
• 
Unknown
•  Cross correlation function: how correlated as a function of angular distance – image plane
•  Cross power spectrum: how correlated as a function of angular size – Fourier plane
•  Reasons for a positive correlation:
• 
• 
• 
• 
• 
• 
AGN (core)
Starbursts and disk emission
AGN (WAT and NAT associated with clusters)
Cluster halos
Cluster relics
Synchrotron cosmic web
•  Reasons for a negative correlation:
• 
Galactic extinction (galaxy number counts down, synchrotron up)
Increasing angular
scale
Cross Correlation with MWA!
The MWA:
• 
Frequency range: 80 – 300 MHZ
• 
2048 dual polarization dipoles
• 
Number of antenna tiles: 128
• 
Number of baselines: 8128
• 
Approximate collecting area: 2000 sq. meters
• 
Field of view: 15 - 50 deg. (200 - 2500 sq. deg.)
• 
Instantaneous bandwidth: 30.72 MHz
• 
Spectral resolution: 40 kHz
• 
Temporal resolution: 0.5 seconds
• 
Polarization: I, Q, U, V
Photo credit: Natasha Hurley-Walker
Good sensitivity to large angular scales,
low frequency, large field of view
Cross Correlation with MWA!
Field: EoR0 RA=0 Dec= -27
Full
2MASS
Point source sub
WISE
Point source & Galaxy sub
HYPER LEDA
Cross Correlation with MWA!
Take radial
average
Δθ
Δθ
Cross Correlation with MWA!
Point Sourcesà
smaller than beam
Diffuse emission à
larger than beam
Still some point source
contribution
So how much diffuse
is there ???
Cross Correlation with MWA !
Limits on Diffuse emission ?
•  But how? No model of what it should be….
•  Can get help from simulations
•  And observations
Cross Correlation!
•  Coma Cluster
With point sources
(Kronberg et al., 2007)
• 
Halo and relic
With cluster optical galaxy
positions
Without point sources
Relic
Halo
Cross Correlation with MWA !
Limits on Diffuse emission ?
•  But how? No model of what it should be….
•  Can get help from simulations
•  And observations
•  Diffuse model:
• 
• 
• 
• 
Choose sum of 3 Gaussians
Convolve with number density
Convolve with beam
Scale
•  Cross correlate model with number density
map
•  Compare to the data cross correlation and
fit
•  Point Source Model:
• 
• 
Run source finder and create model from that
Convolve with beam
Cross Correlation with MWA !
In progress / Continuing work:
•  Use model fitting to set limits on temperature and/or magnetic field
•  Effect of redshift/binning
•  Cross power
•  Different MWA field (same results?)
•  Other LSS tracers ( X-ray? )
Things to consider:
•  Beam shapes
•  Point source subtraction
•  Other effects (Galaxy, ionosphere, etc.)
•  Realistic models à need help from simulations
Ideal Observational Setup!
FIELD
FREQUENCY
•  Large area
•  Low Galactic contamination
•  Multi-wavelength coverage
•  Low (ish)
• 
• 
Too low à stronger Galaxy
Too high à weaker signal
RESOLUTION
UV COVERAGE
•  High (arcsecs)
•  Good (continuous) coverage
• 
• 
Minimize sidelobes
Deeper cleaning
• 
Point source subtraction
•  Low (arcmins)
• 
Diffuse emission
SENSITIVITY
•  Low instrumental rms
•  Good sensitivity to large and
small angular scales
Conclusions!
•  Many reasons to look for the cosmic web
• 
Missing baryons, origin of cosmic magnetism, ….
•  Many possible methods of detection
• 
Direct imaging, statistical methods, ….
•  Many new telescopes/surveys/data coming soon
• 
MWA, LOFAR, ASKAP, MeerKAT, SKA, ….
à Many reasons to think exciting new results in the near future