An X-ray cooling-core cluster surrounding a low power Compact Steep Spectrum Radio source 1321+045
aa r X i v : . [ a s t r o - ph . C O ] J un Draft version May 30, 2018
Preprint typeset using L A TEX style emulateapj v. 12/16/11
AN X-RAY COOLING-CORE CLUSTER SURROUNDING A LOW POWER COMPACT STEEP SPECTRUMRADIO SOURCE 1321+045
M. Kunert-Bajraszewska , A. Siemiginowska , A. Labiano Toru´n Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, NCU, Grudziacka 5, 87-100 Toru´n, Poland Harvard Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138 and Centro de Astrobiologia (CSIC-INTA), Carretera de Ajalvir km. 4, 28850 Torrejon de Ardoz, Madrid, Spain
Draft version May 30, 2018
ABSTRACTWe discovered an X-ray cluster in a
Chandra observation of the compact steep spectrum (CSS) radiosource 1321+045 ( z = 0 . . ± . Chandra detects the cluster emission at > σ level out to ∼ ′′ ( 240 kpc). We obtain the best fit beta model parameters of the surface brightness profile of β = 0 . ± . +1 . − . arcsec. The average temperature of the cluster is equal tokT = 4 . +0 . − . keV, with a temperature and cooling profile indicative of a cooling core. We measurethe cluster luminosity L (0 . − = 3 × erg s − and mass 1 . × M ⊙ . Keywords: active — galaxies: jets — X-rays: galaxies; clusters INTRODUCTION
Many X-ray clusters are found around radio galax-ies with large-scale radio structures which in majorityare classified as FR Is (Fanaroff & Riley 1974) with asmaller number of FR IIs (Owen & Ledlow 1997). Theseradio sources are old ( > years) and their long terminteraction with the cluster environment imprinted a richvariety of structures into the X-ray morphology, such asbubbles, shock fronts and ripples (McNamara & Nulsen2007; Fabian 2012). However, little is known about thenature of the X-ray clusters associated with young com-pact radio sources (with radio source sizes <
20 kpc),namely the Gigahertz Peaked Spectrum (GPS) and Com-pact Steep Spectrum (CSS) objects. These young (age < years) radio sources are believed to be at the begin-ning of their evolution (Readhead et al. 1996; Fanti et al.1995). If they reside in clusters the inter-cluster medium(ICM) should have not been impacted by the radio sourceand we could observe a primordial X-ray morphology ofthe cluster medium. In addition the observation of thecluster medium can provide important information aboutthe physical properties and evolution of the radio sourceitself. However, searches for luminous X-ray clusters as-sociated with GPS and CSS objects were typically un-successful (Siemiginowska et al. 2003, 2008).The only bright X-ray cluster known to host aCSS source is 3C 186 discovered by Siemiginowska et al.(2005, 2010). It is a well formed cool core X-ray cluster athigh redshift, z = 1 .
06. The central cluster galaxy hostsa radio-loud quasar with a powerful FRII-type small-scale radio morphology indicating the initial phase of itsevolution. While expanding into the cluster medium, the young radio source can potentially supply the en-ergy required to stabilize the cluster core against catas-trophic cooling. However, the high redshift location ofthis source limits the investigation of the interactions be-tween the radio source and the ICM.We discovered an X-ray emission from the galaxycluster, MaxBCG J201.08197+04.31863 (Koester et al.2007) in our
Chandra observation of a low power CSSradio source, 1321+045 at z = 0 . =71 km s − Mpc − , Ω M =0.27, Ω Λ =0.73. THE
CHANDRA
X-RAY OBSERVATIONS AND DATAANALYSIS
The
Chandra
ACIS-S observation of 1321+045 waspart of a small snapshot program targeting seven lowradio power CSS sources (in preparation). It was per-formed on 2011-12-14. The source was placed at theaim point on the back-illuminated ACIS CCD (S3) andthe observation was made in FAINT mode with 1/8CCD readout to avoid pileup. We used CIAO 4.4(Fruscione et al. 2006) and CALDB 4.5 in all the dataanalysis and
Sherpa (Freeman et al. 2001) for model-ing and fitting ( cstat with simplex method). We re-processed the data using chandra repro to apply the
Kunert-Bajraszewska et al.
Table 1
Best Fit Model Parameters for the X-ray cluster a R b (arcsec) Range (arcsec) Total Counts c Net Counts c kT d (keV) Norm e n e (10 − cm − ) S f (keV cm − ) t gc (10 yrs)5.3 2.0-8.6 815.0 ± . ± . +0 . − . ± . ± . ± . ± . ± . ± . +0 . − . ± . ± . ± . ± . ± . ± . +0 . − . ± . ± . ± . ± . a listed uncertainties are at 68% for one interesting parameter. b The assumed annuli are circular with the mean radius listed in the R column andranges in Range column; c A number of counts within the 0.5-7 keV energy range. d deprojected temperature; e Normalization for APEC thermalmodel defined as Norm = − π [ DA (1+ z )]2 R n e n H dV with the abundance table set to Anders & Grevesse (1989); Note that the Norm values givenby deproject are normalized to a total volume given by the outermost sphere. f Entropy; g cooling time. most recent instrument calibration. The script runs acis process events which applies the sub-pixel algo-rithm and gives the best spatial resolution images. Afterthe standard deadtime correction of 9.4% the effectiveexposure time on the source was 9.5 ksec. The X-raycentroid is located at R . A . = 13 h m s . , Decl . =+4 ◦ ′ ′′ .
45 (J2000.0).
Image Analysis
The X-ray diffuse emission covers a large part of theACIS-S CCD with the radio source located in the center(Figure 1). The smoothed X-ray image overlayed withradio contours shown in Fig. 1 seems to suggest a broadenhanced X-ray emission with two peaks in the vicin-ity of the core. Offsets between the radio core and theenhancements are consistent with the astrometric uncer-tainty of
Chandra and we did not attempt to apply anyadditional adjustments. The adjustment to the aspectsolution might be possible with a deeper observations inthe future if there are additional point sources detectedin this field.The X-ray cluster emission extends outside the fieldof view of the CCD. However, we measured the extentalong the CCD using the surface brightness profiles to-wards the north and south from the center. We used dmextract and extracted one-dimensional radial profilesassuming 18 annuli located between 3 ′′ and 100 ′′ withinPA angles of 70 ◦ -150 ◦ and 260 ◦ -335 ◦ . The cluster emis-sion is detected at 3 σ level out to about ∼ ′′ ( ∼
240 kpc)from the radio source. We fit the two profiles in
Sherpa assuming a beta1d model and obtained the core radiusof r c = 9 . +1 . − . arcsec and β -parameter of 0.58 +0 . − . .The extrapolation of the beta1 model into the circular( r = 2 ′′ ) region centered on the radio source shows theexcess X-ray emission. We associate this emission withthe radio source. An X-ray emission from the radio core
We used specextract tool to extract the X-ray spectraassuming 1.25 ′′ radius circle for the radio source and anannulus with inner and outer radii equal to 1.5 ′′ and10 ′′ respectively for the local background. This regionencloses the radio core and a part of the innermost radiostructure. The X-ray spectrum contains 36.1 ± . H = 2 . × cm − (Dickey & Lockman 1990).We first fit the background spectrum assuming theAPEC model in Sherpa (this is the cluster emissionwithin 1.5 ′′ and 10 ′′ annulus). We set the metal abun- dance to 30% of the Solar values and z = 0 .
263 for thismodel and fit the spectrum containing 1054 counts in0.5-7 keV range. The resulting best fit temperature andnormalization are kT b = 4 . +0 . − . keV and 8 . ± . × − respectively and represent the average observed valuesof the central region of the cluster. We expect onlyabout 16 ± Chandra spectrum of the radio source. These back-ground model parameters remained unchanged in fittingthe spectrum of the radio source described below.We assumed an absorbed powerlaw emission modelfor the radio source and the APEC model with thefixed parameters to the above best fit values to ac-count for the cluster emission. The resulting best fitpower law photon index is equal to Γ = 2 . +0 . − . andthe normalization to 6 . +1 . − . × − photons cm s − at 1 keV. The model unabsorbed flux of F . − =1 . ± . × − erg cm − s − corresponds to the lumi-nosity L X (0 . − ∼ × erg s − typical for a lowluminosity AGN. The photon index, however, is quitesteep and may indicate a presence of soft thermal emis-sion often observed in low luminosity AGN and explainedas a result of hot ISM of the host galaxy or a jet emission(Hardcastle et al. 2009; LaMassa et al. 2012). A higherquality spectrum is needed to understand the origin ofthis emission. Spectral analysis of the X-ray cluster
We detected 2882.2 ± . ′′ radius in the Chandra ′′ and 28 ′′ and the fourth one between 28 ′′ and35 ′′ accounts for the background (see Table 1).We assume the APEC model and fit the cluster spec-trum in each annulus. We account for the cluster 3Dvolume effects using the deproject model in Sherpa (formodel details see Fabian et al. 1981; Kriss et al. 1983;Siemiginowska et al. 2010). The best-fit model providesthe cluster temperatures, normalizations, and densitieslisted in Table 1.These numbers suggest that the cluster has a coolingcore, although this result has to be confirmed with bet-ter quality data in the future. We used the exposurecorrected image given by fluximage tool and assumedan elliptical region with 35 ′′ and 60 ′′ radii to estimatethe X-ray luminosity of the cluster to be L (0 . − =3 × erg s − . The mass of the cluster enclosed by asphere with 60 ′′ radius is about 1 . × M ⊙ assumingthe average cluster temperature of 4.4 keV, and β = 0 . Figure 1.
Chandra
ACIS-S X-ray image in 0.5-7 keV energy range overlayed with the black radio contours from the MERLIN 1.6 GHzimage. The radio contours increase by factor of 2, the first contour level corresponds to ≈ σ and amounts 0.8 mJy/beam. (Left) TheX-ray image smoothed with the Gaussian function ( σ =2 arcsec). A 30 ′′ scale bar corresponds to ∼
121 kpc. The white contours indicatethe exposed part of the CCD detector. (Right): The central region of the cluster showing the ACIS-S image smoothed with the Gaussianfunction ( σ =0.98 arcsec), the radio contours and the optical SDSS source (black cross). The 1 ′′ scale bar corresponds to ∼ which is in a broad agreement with the clusters scalingrelations in Eckmiller et al. (2011). DISCUSSION AND CONCLUSIONS
Low Power Radio Source
The radio source 1321+045 (R.A. =13 h m s .7,Decl. = +04 ◦ ′ ′′ (J2000.0)) belongs to a class ofyoung CSS radio sources (Fanti et al. 1995). It has beenobserved with MERLIN at 1.6 GHz in 2007 as a part oflarge sample of low luminosity compact (LLC) sources(Kunert-Bajraszewska et al. 2010a). The position of thecentral component visible in 1.6 GHz MERLIN image iswell correlated with the position of the optical counter-part suggesting it is a radio core (C). The two lobes (Eand W) are located on the opposite sides of the core.There is no evidence of jets or hotspots, however, thisneeds to be confirmed by observations at higher fre-quency. A total projected length of the source is equal to ∼
17 kpc and its radio luminosity, L ∼ W Hz − ( < erg s − ), places it in the FR I-FR II transitionregion.Studies of compact radio sources suggest that they ex-hibit periodic activity on timescales of 10 − years(Reynolds & Begelman 1997). On shorter timescales theradio source is not able to escape from the host galaxyand starts to recollapse within the ISM (Czerny et al.2009). Our analysis of the whole sample of LLCsources suggest that they can represent a populationof short-lived objects and undergo this phase of activ-ity many times before they become large scale FR Ior FR II (Kunert-Bajraszewska et al. 2010a,b). Whatis more, the evolution of the radio source and itsradio morphology is determined by the properties ofthe central engine: strength, accretion mode, exci-tation level of the ionized gas, and the ISM. Opti-cally many of them belong to the class of low ex-citation galaxies (LEGs), which are thought to bepowered by the accretion of hot gas (Hardcastle et al.2007; Buttiglione et al. 2010) and can be progeni-tors of large scale LEGs (Kunert-Bajraszewska et al. 2010b). The Ninth SDSS Data Release (Ahn et al.2012) gives the fluxes of the emission lines visiblein spectrum of 1321+045 and the calculated line ra-tios log [O II] λλ λ .
15 and log[O III] λ β = − .
41 (see Buttiglione et al. 2010, fordefinitions) are consistent with 1321+045 being a LEG.We used ITERA (Groves & Allen 2010) to generateemission-line diagnostic (or BPT, after Baldwin et al.(1981)) diagrams and to compare the emission-lineratios of 1321+045 with predictions for starburst(Leitherer et al. 1999; Dopita et al. 2006; Levesque et al.2010), dusty and dust-free AGN (Groves et al. 2004a),and shock (Allen et al. 2008) models. We found thatthe emission line ratios of 1312+045 are consistent withAGN-dominated photoionization with small contribu-tions from star-formation models. None of the shockmodels (with and without precursor) reproduce the fluxratios, suggesting that jet-induced shocks are not presentin the ISM of 1321+045 or their contribution to the ion-ization of the optically emitting gas is negligible.
Interactions between the Radio Source and the ICM
The presence of distortions and cavities in the X-ray image of a cluster can signal interactions betweenthe central galaxy (the radio source) and the ICM(Hlavacek-Larrondo et al. 2012). Some clusters havemultiple pairs of cavities filled with low frequency radioemission, probably caused by the previous radio activ-ity phase. The high resolution radio observations showthat the reborn radio source can reside inside the clustercore and multiple cavities provide the evidence for theprevious outbursts (O’Sullivan et al. 2012; Clarke et al.2009; Tremblay et al. 2012). Figure 1 shows X-ray emis-sion from the 1321+045 cluster in 0.5-7 keV energy rangeoverlayed with radio contours from the MERLIN 1.6 GHzobservations. There is no presence of any ripples anddiscontinuities in the current X-ray image of a 1321+045cluster but rather uniform emission without indicationsof an interaction with the radio jets, or any signature ofthe previous activity.Inspection of the VLSS image of 1321+045 at 74 MHz
Kunert-Bajraszewska et al. (Cohen et al. 2007) shows that the low frequency emis-sion extends out to ∼
120 kpc. This low frequency radioemission is uniform and there is no trace of more ex-tended components which could indicate the older phaseof radio activity.The synchrotron spectrum of 1321+045 consist of fourpoints and is steep from 74 MHz to 5 GHz with the index α = 0 .
95 (defined as S ∝ ν − α ). We used a simple modelof synchrotron emission (Kunert-Bajraszewska et al.2009) to reproduce the observed spectrum in order to findthe value of the magnetic field. We took the size of theradio lobe, assumed an equipartition between the parti-cle energy and the magnetic field energy and a value ofthe Doppler factor δ = 1. We obtained the best fit valueof B = 1 . × − G which gives the magnetic pressurein each radio lobe to be ∼ × − dyn cm − . Basedon the cluster central density and temperature we esti-mate a central thermal pressure of ∼ × − dyn cm − .Taking into account the uncertainties in determining thedeprojected temperature and density (only three annuli)and the value of the thermal pressure we conclude thatthe cluster environment could limit the growth of theweak radio source.Given the value of the magnetic field and the spectrumbreak frequency, ν br we can estimate the synchrotron ageof the source 1321+045. The break frequency indicatesthe critical point in which the radio spectrum changesits spectral index and the value of ν br depends on theelapsed time since the source formation (Murgia et al.1999). In the case of older objects the break frequency ismoved toward lower frequencies. The synchrotron spec-trum of 1321+045 does not show the self-absorption peakor the break frequency in the range 74 MHz − ∼ and ∼ years,respectively. The ν br lower than 74 MHz indicates thatthe population of electrons has cooled down to low en-ergies and the source after a typical for CSS sourceslifetime (10 − years) started to fade away. As wealready suggested the evolutionary paths of young ra-dio AGNs are probably determined by the propertiesof their central engines, namely the HEG/LEG path(Kunert-Bajraszewska et al. 2010b). However, in someobjects the surrounding environment could be also animportant factor influencing the evolution. SUMMARY
Chandra observations are required to confirm this result.The optical analysis rule out the presence of jet-induced shocks in the ISM of 1321+045. We speculate that thislow power small scale radio galaxy did not have enoughenergy to get out of the host galaxy and it is now in acoasting phase. ACKNOWLEDGMENTS
This research has made use of data obtained by theChandra X-ray Observatory, and
Chandra
X-ray Center(CXC) in the application packages CIAO, ChIPS, andSherpa. This research is funded in part by NASA con-tract NAS8-03060. Partial support for this work wasprovided by the
Chandra grants GO1-12124X.REFERENCESgrants GO1-12124X.REFERENCES