Giant radio galaxies as probes of the ambient WHIM in the era of the SKA

0.29 MB·English

by  Bo Peng

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n in the era of the SKA
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h Bo Peng1,2,Ru-RongChen1,RichardStrom1,3
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E-mail: [email protected]
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[ Themissingbaryonsareusuallythoughttoresideingalaxyfilamentsaswarm-hotintergalactic

medium(WHIM).Frompreviousstudies,giantradiogalaxiesareusuallyassociatedwithgalaxy
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v groups, whichnormallytrace the WHIM. We proposeobservationswith the powerfulSKA1 to
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makeacensusofgiantradiogalaxiesinthesouthernhemisphere,whichwillprobetheambient
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4 WHIM.Theradiogalaxiesdiscoveredwillalsobeinvestigatedtosearchfordyingradiosources.
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WiththehighlyimprovedsensitivityandresolutionofSKA1,morethan6,000giantradiosources
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. willbediscoveredwithin250hours.
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AdvancingAstrophysicswiththeSquareKilometreArray,
June8-13,2014
GiardiniNaxos,Sicily,Italy
Speaker.
∗
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
GRGsasprobesofambientWHIM BoPeng
1. The WHIM
Baryonic matter -protons, neutrons, and electrons -belongs to aclass ofmassive elementary
particles which make up the atoms of all materials on Earth and in the stars. From the Big Bang
theoryandtheresultsofWMAP,baryonicmattercontributesnomorethan5percentofthecosmos,
though itremainsnotcompletely accounted for. Fromacensus oftheobservable baryonic density
in the current Universe (e.g. Shull, Smith & Danforth (2012)), all the stars, dust and gas within
galaxiesconstitutelessthan50percentofbaryonspredictedbythetheory. Byanalyzinglightfrom
distant quasars, thenumberofbaryons incosmichistory canbecounted. Itsamounthasremained
constant even going back to 10 billion years ago. Quantifying the missing baryons is essential to
obtainingacompletepictureofgalaxyformationandevolution. Themissingbaryonsaregenerally
thought toreside betweengalaxies, aswarm-hotintergalactic medium (WHIM)withtemperatures
of105 107 K.MaterialintheWHIMishighlyionizedandcanonlybeobservedatfar-ultraviolet
−
or low-energy X-ray wavelengths. This diffuse plasma isacommon prediction inthe L cold dark
matter (L CDM)cosmologies (e.g. Davé et al. (2010) ). However, because its density isrelatively
low,thedetection remainsachallenge (Davéetal.2001;Dietrichetal.2012;Werneretal.2008).
2. Giantradiogalaxies(GRGs)
Giant radio galaxies (GRGs) (linear size > 0.7 Mpc, h = 0.71) are a quite unique category,
as they are the biggest objects in the sky with large radio structures. It is generally believed that
their radio lobes are powered by beams of relativistic particles and magnetic fields from an active
nucleus. Radio structures of many known GRGs are asymmetric over their mega-parsec extents
(Lara et al. 2001; Machalski, Jamrozy & Zola 2001; Saripalli et al. 2005; Schoenmakers et al.
2000; Strom etal. 2013) (see Figure 1). Astheradio lobes expand, theyinteract withtheambient
mediumandcanactastracersofthisbarelydetectablematerial. ThelargeextentofGRGsprovides
agoodopportunity toinvestigate theambientgas.
3. Radio galaxiesand cosmology
By 1950, several compact radio sources had been associated with distant galaxies (Bolton,
Stanley &Slee1949; Ryle, Smith&Elsmore1950), andwiththeidentification ofthe verystrong
radio source Cygnus A (Baade & Minkowski 1954; Smith 1951) with a z=0.056 galaxy, the po-
tentialofradiosourcesforprobingtheUniversebecameapparent. Thisexpectationwasconfirmed
when 3C 295 was shown to be associated with a z=0.461 galaxy (Minkowski 1960). Simulta-
neously, radio source structure was found to be double lobed (Jennison & Das Gupta 1953), with
sizesrangingfromparsecstohundreds ofkilo-parsecs.
Mucheffortwentintoidentifying (andmeasuringtheredshifts of)radiosourcesfromreliable
catalogues, especially 3C. High resolution radio maps (made by the One Mile Telescope, West-
erbork and the VLA) tied down the radio source structures. The angular size measured could be
translated into linear dimensions. Inverse Compton scattering of the radio emitting high-energy
electrons becomes increasingly important at greater redshift. This effect could "snuff out" radio
sources at high z (Miley 1968) before they could attain the dimensions found among the largest
radiogalaxiesinthelocalUniverse.
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4C 73.08
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RIGHT ASCENSION (J2000)
Figure1: WSRTmapat21cmwavelengthoftheradiogalaxy4C73.08.Thebrighterwesternlobeismore
compactthanitseasterncounterpart.AsimilareffectisseeninNGC6251(Figure.2).
NGC 6251
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RIGHT ASCENSION (J2000)
Figure2: WSRTmapmadeat92cmwavelengthoftheradiogalaxyNGC6251. LikealloftheGRGs, it
issronglypolarizedatlongwavelengths.Notehowunequalthedistancesbetweentheendsofthetwolobes
andthehostare.
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DA 240
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RIGHT ASCENSION (J2000)
Figure3: WSRTmapmadeat49cmwavelengthoftheradiogalaxyDA240.Galaxieswhicharemembers
oftheDA240grouphavetheirpositionsindicatedbysymbols.
4. Tracing the WHIMwithGRGs
As was clear from early WSRT observations of DA 240, 3C 236 and NGC 6251 (Figure 2),
giantradiogalaxiesarestronglypolarizedat608MHz(Willis,Strom&Wilson1974). Theimplied
low ambient densities were at least qualitatively consistent with the large linear sizes of the radio
components. AdefinitivestudyoffiveGRGscarriedoutbyMacketal.(1997)mappedtheirlinear
polarization at wavelengths from 92 to 2.8 cm. All of the sources were found to be polarized in
many places at the longest wavelengths, indicating an internal rotation measure of 1 rad m 2.
−
∼
The implied densities within the lobes depend upon the magnetic field strength. For equipartition
field strengths of typically 1 m G,the particle densities are of order 10 5 cm 3. In a more general
− −
consideration of densities associated with GRGs, Mack et al. (1998) conclude that the IGM with
whichtheradiocomponents interacthasadensity of1-4 10 5 cm 3.
− −
×
SeveralrecentstudieshaveexploredtheuseofGRGsforprobingtheWHIM.Inboththecases
ofSubrahmanyanetal.(2008),andSafourisetal.(2009),sensitiveradiocontinuumimagesoftwo
GRGswereused torelate theirmorphology totheambient medium ontheMpcscale. Theymade
spectroscopic observationstoinvestigategalaxydistributionwithina2 fieldsurroundingtheradio
◦
galaxy hosts. Both studies showed clear correspondence between the giant synchrotron lobes and
the surrounding galaxy distributions. Such a correspondence was also found in a series of papers
(Chen etal. 2011b, 2012a,b). Theyfound theexistence ofagalaxy group around each giant radio
galaxy(NGC6251,NGC315and4C73.08)throughopticalspectroscopicobservations. However,
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in the case of DA 240 (Chen et al. 2011a; Peng et al. 2004), the relatively symmetrical lobes
coincide withagalaxy group. Thegroup membersevenly reside along themajoraxisofthelobes
while the isolated host galaxy lies in the middle (Figure 3). The expected X-ray luminosities are
much higher than the observed values, which suggests that the matter density within the lobes is
quite low. A similar conclusion was obtained by Malarechi et al. (2013). In the investigation of a
sampleof12GRGs,theyderivedlobeenergydensitiesfromtheradioobservationsviaequipartition
arguments. The inferred pressures in the lobes of the giant radio sources, which range from 1.1
×
10 15 to2.0 10 14 Pa,arelowerthan inferred from X-rayobservations ofdense filaments. This
− −
×
demonstrates thepotential ofusingGRGstostudytheWHIMgaswithinwhichtheirlobesevolve
andinteract.
5. Prospects withSKA1
A significant fraction of known giant radio galaxies were identified from the surveys of the
WSRTandVLA:WENSS,NVSSandFIRST.TheSKA1_SURwillbeequippedwith96dishesin
Western Australia, including 36dishes ofASKAP(latitude isabout 27 S).AtLband, itwillhave
◦
ahigh surveyspeed figureofmerit(FoM)ofabout 156timesgreater thantheVLA,and1.5times
better sensitivity aswellasa1.6to49timeshigher angular resolution (Table1). Ifitcarries outa
survey similar tothe NVSS,withintegration times of10 minutes and abandwidth of500 MHzat
L band, the noise level should achieve 9.1 m Jy/beam. Since the GRGs usually have intermediate
power between 1025 and 1026 W/Hz at 1.4 GHz, if the SNR is presumed (based on experience)
to be 150 (Machalski, Jamrozy & Zola 2001; Schoenmakers et al. 2000), the luminosity distance
will be 7825 Mpc (z 1.2). Since the minimum elevation angle is about 15 degree (similar to
∼
the ASKAP), SKA1_SUR can observe the sky up to +48 (more than 80 percent of the whole
◦
sky). If the Galactic latitude is set to be b > 12.5 to avoid the confusion of the Milky Way,
◦
| |
the sky coverage will then be reduced to about 27,000 deg2. Adopting a number density of 10 7
−
Mpc 3 (Saripalli et al. 2005), the SKA1_SURis expected to detect about 12,000 GRGsin a total
−
observing timeofabout 250 hours (formula listed in the Appendix). Evenat anearly phase (50%
capability), SKA1_SURwillstillbeabletodetectabout6,000GRGs,whichcanbeidentifiedwith
counterparts obtained from optical surveys, such as SDSS,2dF, and the planned LSST.The GRG
samplewillthenincrease bymorethananorderofmagnitude.
Atthesametime,theSKA1_MIDwillbeequippedwith254dishesinSouthAfrica,including
64 dishes from MeerKAT. Compared with the VLA, this will provide 6.2 times better sensitivity
and74timeshighersurveyspeed. Moreover,theangularresolution willbe6.4to200timesbetter.
After GRG candidates have been obtained from the SKA1_SUR, SKA1_MID can be used for
a deeper investigation, especially for low surface brightness lobes, which is important for their
use as cosmological probes. If the SKA2_MID could be equipped with PAFs, the survey speed
withSKA2_MIDwouldbegreatlyincreased, andthiswillmaketheSKA2_MIDquiteapowerful
instrument fordiscovering moreofthesefaint,extendedradiosourcesathigherredshift.
Furthermore,sincethefrequencyn ofradiosynchrotronemissionisproportionaltothesquare
oftheelectrons’energyE andtheambientmagneticfieldB(n E2B),whichisexpectedtobelow
∼
and weak in the diffuse radio lobes of GRGs, detection of new radio structures in GRGs is likely
fromobservations usingtheSKA1lowfrequencyarray. Theremayevenbediscoveryofnewones
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GRGsasprobesofambientWHIM BoPeng
Table 1: Summaryofsystemsensitivity,surveyspeedandangularresolutionofSKA1,VLA,WSRTatL
band,andLOFARatlowfrequency(extractedfromDewdneyetal.(2013)).
JVLA WSRT SKA1_MID SKA1_SUR LOFAR SKA1_LOW
A /T m2/K 265 124 1630 391 61 1000
sys sys
deg2m4K 2 1.76E4 3.84E3 1.3E6 2.75E6 5.21E4 2.7E7
SurveySpeedFoM −
Resolution arcsec 1.4-44 16 0.22 0.9 5 11
FoV deg2 0.25 0.25 0.49 18 14 27
Bandwidth MHz 1000 160 250 500 4 250
(like the discovery of GRGUGC09555 with LOFAR).Thelow frequency array of SKA1willbe
equipped with 250,000 antennas in Western Australia, with 16 times higher sensitivity and 520
timesquicker surveyspeedcomparedwithLOFAR.
Recently,Luo&Sadler(2010)modelledtheevolutionofFRIandIIradiosources,findingthatthe
latterevolvewithdecreasingmonochromaticradiopower,whilethepoweroftheformerincreaseswithage.
Thefinalstageforbothseesrapidlydecliningemissionastheradiosourcefadesaway,thelobesreceiving
littleornoenergyfromthehostnucleusorjets. Asthelobesdecline,theirradiospectrawillsteepen,soa
dyingradiogalaxyshouldhavelobeswithasteepradiospectrumandnojetsornuclearcomponent.Finding
candidatesforthisGRGphasewillbewell-suitedtotheSKA1capabilityatlowfrequencies.
Withthehighlyimprovedsystemsensitivity,surveyspeedandangularresolutionofSKA1,thereshould
be morethan oneorderof magnitudeincreasein the numberofGRGs fromabout140atpresent(a small
fractionofwhichhaveredshiftsgreaterthan0.5(Komverg&Pashchenko2009)).Withtheirabilitytotrace
the ambient IGM, more GRGs will help illuminate the distribution and physical state of the WHIM and
identifywhetheritcanexplainthemissingbaryonproblem. Andwithmoredetailsonhigh-zGRGs,being
able to compare them with low redshift sources will help to clarify how giant radio galaxies evolve, and
especiallyhowtheirenvironmentsaffecttheirradiostructures,andwhethertherearesignificantdifferences
ingroupsize,composition,morphology,etc.,withcosmicepoch.
Atlowredshifts,SKA1withitsexcellentsensitivityandangularresolutionwillbeabletoinvestigate
GRGgroupsinunprecedenteddetail.Inseveralgroups(likeDA240,Chenetal.(2011a);Pengetal.(2004);
and 4C 73.08, Chen et al. (2012b); Strom et al. (2013)) we find that a large fraction of the companion
galaxiesareweakradiosources. TheSKA1sensitivitywillenableustoexploretheweakradioemissions
ofcompaniongalaxiesfurther. Inaddition,anumberofthegroupgalaxiesappeartobespiralgalaxies. 21
cmHIobservationsofthemarelikelytoturnupanumberofdetections.OnecanusetheHIdatatoestimate
theamountofmasspresent,andstudythegroupkinematics.
6. Conclusions
OurgoalwithSKA1istocarryoutalarge-scalesurveytosearchforgiantradiogalaxies. Thesurvey
should be unbiased and complete down to a well-defined brightness or flux density, over a specified sky
area. TheGRGsfoundwillbeusedto traceoutthe WHIM,whilewealso hopetodetectcandidatedying
RGs,objectsoflowsurfacebrightnesswithlittleevidenceforcompactfeatures(jets,nuclearcomponents),
but with a steep radio spectrum. The high-z GRGs will be used to investigate how giant radio sources
evolve. Someoftherelativelynearbygroups(likethatsurroundingDA 240)appeartohaveseveralspiral
companions. Weplantoobservetheminthe21cmHIlinetostudythemotionandmassesofGRGgroup
members.
Among the questions we hope to answer, we note the following: Can one use the GRGs to detect
the WHIM? And if so, can we pinpoint the missing baryons? Can one use GRGs and their surrounding
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GRGsasprobesofambientWHIM BoPeng
companionstotracethedistributionandphysicalstateoftheWHIM?Finally,arethereenoughhigh-zgiant
radiogalaxiestoinvestigatehowradiogalaxiesevolve?
Acknowledgements
This work was partially supported by the Chinese Ministry of Science and Technology under the
StateKeyDevelopmentProgramforBasicResearch,GrantNos. 2013CB837900and2012CB821800,the
ProjectsofInternationalCooperationandExchange,NSFC,GrantNo. 11261140641,thekeyresearchpro-
gramoftheChineseAcademyofSciences(CAS),GrantNo.KJZD-EW-T01,andtheOpenProjectProgram
oftheKeyLaboratoryofRadioAstronomy.RGSwishestothanktheChineseAcademyofSciencesforthe
awardofavisitingprofessorship,whenmostofthisresearchwasdone.
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Noiselevelwithtwopolarizations:(Jy/beam)
Appendix s = Ssys (A.7)
√2Bt
A. Formulaused to estimatethe
Systemequivalentfluxdensity:
number ofGRGsforSKA_SUR
2kbTsys
Ssys= (A.8)
A
Numberofgiantradiogalaxies: eff
N=r ×Vco (A.1) FBoorltzSmKaAn1c_oSnUstRan,ts:eknbsi=tiv1i.t3y8s×pe1c0i−fi2c3atJio/Kn: Aeff =
Tsys
Numberdensity:r =10−7(Mpc−3) 391m2K−1;bandwidth: B=500MHz
Comovingvolume:(Mpc3) Integratedtime:t =10minuteswithNVSSobserv-
1 ingmodel
Vco= 3AskyD3co (A.2)
Skycoverage:(deg2) B. Estimateofthetotal observing time
Asky=D RA D sin(Dec) D l D sin(b) (A.3)
× − × t A
sky
Comovingdistance:(Mpc) t= (B.1)
FoV
DL ForSKA1_SUR,fieldofview: FoV =18 (deg2);
Dco= (A.4)
1+z integrated time: t =10 minutes with NVSS observing
Redshiftz:(www.icosmos.co.uk) model;skycoverage:A (equationA.3)
sky
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