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“Surveying the low frequency sky with LOFAR”
8-12 March 2010, Leiden, The Netherlands
INAF-Istituto di Radioastronomia, Bologna, ITALY
Cluster Radio Halos in the Cluster Radio Halos in the LOFAR eraLOFAR era
Rossella Cassano
Coll.: G. Brunetti, H.J.A. Röttgering, M. Brüggen
Radio Halos in Clusters of GalaxiesRadio Halos in Clusters of Galaxies
Optical X- ray
Coma Cluster
Galaxy cluster mass:
Barions
Dark Matter 70%
stars + dark matter hot diffuse gas
10% of stars in galaxies
15-20% of hot diffuse gas
Radio Halo
Radio Relic
Diffuse synchrotron radio sources from the ICM: Halos and Relics prove the presence of non-thermal componenets, GeV electrons (~104) and G magnetic field, mixed with the thermal ICM on Mpc scales.
Important ingredients to understand the physics
of the ICM.
The Origin of Radio HalosThe Origin of Radio Halos
=1.2 Mpce-Diffusion length=
The electron-diffusion time necessary to cover Mpc distances is >> than the electron-radiative life-time!
Tdiff
(~1010 yr) >> Tv (~108 yr)
e.g. Jaffe (1977)
Need for injection/acceleration in situ
Radio Halos are the most spectacular non thermal diffuse sources in clusters :
““Rarity” of Radio Halos & connection with cluster mergersRarity” of Radio Halos & connection with cluster mergers
RXCJ 2003-2525 Giacintucci et al. 2007
Abell 754Henry et al. 2004
Abell 2163Feretti et al. 2001
Brunetti et al. 2007 “Radio loud” GC
“Radio Quiet” GC
blue GMRT GC magenta other RH
Radio Halos are only found in non-relaxed clusters with evidences for recent /ongoing cluster mergers (e.g. Buote 2001)
The majority of Radio Quiet clusters are found in a relaxed dynamical status.
“Bullet” cluster Govoni et al. 2004
A 611
…work in progress…
Cluster-Cluster Mergers & re-acceleration scenarioCluster-Cluster Mergers & re-acceleration scenario
One possibility to explain Radio Halos is the turbulent re-acceleration scenario that assumes that electrons are accelerated by MHD turbulence generated in the cluster volume during merger events (Brunetti et al. 2001, 2004; Petrosian 2001; Ohno et al. 2002; Fujita et al. 2003; Brunetti & Blasi 2005; Cassano & Brunetti 2005; Brunetti & Lazarian 2007; Petrosian & Bykov 2008)
RXCJ 2003-2525 Giacintucci et al. 2007
Abell 754Henry et al. 2004
Abell 2163Feretti et al. 2001
“Bullet” cluster Govoni et al. 2004
Rad
io P
ower
Frequency
νs
νs
VLALOFAR RH detectable at GHz are mainly halos with larger υs ( > 1 GHz) or relatively flatter spectra (α1.1-1.5) that are associated with the most energetic and rare phenomena .
LOFAR should discover a complex population of RH, including a large number of very steep spectrum sources (α>1.5) (USSRH) that are associated with more common and less energetic phenomena .
νs
We expect a population of RH with different radio spectra, depending on the efficiency of the particle acceleration process
Rare events
More common events
νobs < νs
Low frequency
Abell 521Abell 521 : the prototype of USSRH ?
=1.9
=1.5
High frequency
Macario et al 2010
(Brunetti +al. 2008, Nature 455, 944)
=1.77A697
Dallacasa et al. 2009
Statistical Modeling of cluster RH: ingredients
χ-1 =τacc
γbχ/β νbB γb2
Semi-analityc model of cluster formation merger trees (Press & Schecther 1974; Lacey & Cole 1993)
Estimate of the turbulent energy injected in the cluster volume during merger events (Ram Pressure Stripping) and the acceleration efficiency (τacc
–1) due to MS waves.
Calculate the acceleration of fossil e due to the interaction with the turbulent waves and the ensuing Synchrotron and Inverse Compton emission spectra from the resulting electron spectra
The cosmological evolution of the magnetic field is accounted for by scaling the field with the cluster mass (cosmological MHD simulations; e.g. Dolag et al. 2002).
(cosmological “version” of turbulent-acceleration model) ( Cassano & Brunetti 2005, Cassano et al 2006 )
The fraction of GC with RH with νs>1.4 GHz is expected to increase with the cluster mass (and LX) in line with present data (from NVSS+GMRT; Cassano et al. 2008).
Cassano +al 2006
Model Expectations at Model Expectations at 00=GHz vs Observations=GHz vs Observations
fRH
The expected number of GHz RH is consistent with RH number counts from the NVSS (z=0.05-0.2; Giovannini et al. 1999) and from the GMRT RH Survey (z=0.2-0.4; Venturi et al. 2007; 2008).
Cassano +al 2010
fRH M
Fraction of galaxy clusters with radio halos at low Fraction of galaxy clusters with radio halos at low νν
νs>1.4GHz
240 MHz
150 MHz
120 MHz
74 MHz
The expected fraction of clusters with radio halos increases at low ν.
This increase is even stronger for smaller clusters (M<1015 M ⊙ ).
Radio halo luminosity functions at 120 MHzRadio halo luminosity functions at 120 MHz
tot at 120 MHzs < 600 MHz
s < 600 MHz
s > 600 MHz
s > 600 MHz
n H(P
)xP
[G
pc h
70-1]-3
P120 [Watt/Hz] P120 [Watt/Hz]
The low-power end of the RHLF is dominated by RH with s < 600 MHz, i.e., halos with a synchrotron radio
spectrum >1.9 between 240 and 600 MHz (f-).
For a given cluster mass (and B), radio halos with smaller values of s have lower monochromatic radio luminosity at a given frequency 0< s
Cassano et al. 2010
LOFAR surveys & detection of RH LOFAR surveys & detection of RH
MS3 commisioning survey 120 MHz, rms 0.5 mJy, beam 30"30"
Tier 1: The Large Area Survey 120 MHz, rms 0.1 mJy, beam 5"5"
About half of RH flux is emitted within 0.5 RH
Given the typical brightness profiles of RH this approach would lead to the detection, in the case 1 , at several , of the central part of the halos => we would identify
(at least) candidates RH.
Govoni et al. 2004 Brunetti et al. 2007
We consider a beam of 25"25" to increase the sensitivity to extended emission:
Injection of “fake” RH in the u-v data set
RH
1 for the NVSS survey
2 for the GMRT RH surveys
(Brunetti et al. 2007, Cassano et al. 2008, Venturi et al. 2008)“empty” field
fH=0.28 mJy fH=0.32 mJy fH =0.45 mJy
NVSS field 0=1.4 GHz, rms=0.45 mJy/b beam= 45"45"
1) 2) 3)
RH Number Counts in LOFAR surveys: IRH Number Counts in LOFAR surveys: I
The expected number of RH with 120 <s < 600 MHz (USSRH) increases with
increasing the survey sensitivity
120 <s < 600 MHz
s > 120 MHz
• fmin(z) of Mpc scale RH assuming different values of “ rms” that mimic possible LOFAR observations;
• LOFAR sky coverage: Northern hemisphere (>0) and high Galactic latitudes (|b|>20)
rms=0.25, 0.5, 1, 1.5 mJy/b
rms=0.25 mJy/b => rms=0.2 mJy/b and =1-1.3300 RH at z <0.6 50% with s <600 MHz
rms=0.5 mJy/b => rms=0.2-0.25 mJy/b and =2.5-2=> rms=0.5 mJy/b and =1
rms= 1 mJy/b => rms=0.5 mJy/b and =2
200 RH at z <0.6 33% with s <600 MHz
70 RH at z <0.6 30% with s <600 MHz MS3 ?
Tier 1?
Ebeling, Edge & Henry 2001
fx>3·10-12 erg s-1cm-2 fx>10-12 erg s-1cm-2fx>310-12 erg s-1cm-2
RH Number Counts in LOFAR surveys: IIRH Number Counts in LOFAR surveys: II•Searching for RH in X-ray selected cluster samples with LOFAR surveys;
•Catalogs of X-ray clusters in the northern hemisphere: z<0.3 eBCS (Ebeling et al. 1998, 2000) NORAS (Böhringer et al. 2000) : 33% of the sky: |b|> 20 and 0°
0.3<z<0.6 MACS (Ebeling et al. 2001) 55% of the sky: |b|> 20 and -40° 80°
s > 120 MHz
120 <s < 240 MHz
240 <s < 600 MHz
600 <s < 1400 MHz
rms=1 mJy/b rms=0.25 mJy/b
RH Number Counts in LOFAR surveys: IIRH Number Counts in LOFAR surveys: II
rms=0.25 mJy/b => rms=0.2 mJy/b and 1-1.3 130 RH at z<0.6 (out of 400 clusters in eBCS and MACS sample) 40% with s< 600
MHz
rms= 1 mJy/b => rms=0.5 mJy/b and =2 70 RH at z<0.6, 20 RH with s< 600
MHz
s > 1.4 GHz
Combining radio and X-ray selection criteria we derive:
rms=1 mJy/b
LLRR-L-LXX luminosity correlation at low frequency luminosity correlation at low frequency
rms=0.25 mJy/b
USS halos observed at 120 MHz should be less luminous than those with flatter spectra in clusters with the same M (or Lx).
The bulk of USS halos visible at low frequency are expected to be associated with galaxy clusters of intermediate X-ray luminosity, LX3-51044.
By making use of Monte-Carlo procedures we show that the presence of these RH in LOFAR surveys at 120 MHz would cause a steepening and a broadening of the LL120120-L-LXX correlation with respect to that observed at 1.4 GHz.
0=120 MHz • observed halos
120-240 MHz
240-600 MHz
600-1400 MHz
>1400 MHz
Cassano 2010
ConclusionsConclusions
Radio Halos are possibly due to turbulence acceleration occuring in clusters during merger events
Radio Halos are expected to be a complex population of radio sources whose spectral properties should be intrinsically related to the dynamical status of the hosting clusters
LOFAR is expected to discover >300 RH in the Tier 1 Large Area Survey and up to 50-100 in the MSSS survey (20-25 RH are presently known)
Future is bright ! ….
LOFAR follow-up of eBCS+MACS clusters 130 RH in the Tier1 LAS
The radio—X-ray luminosity correlations should steepen at lower freq.