A&A 624, A91 (2019) https://doi.org/10.1051/0004-6361/201834653 Astronomy & cESO 2019 Astrophysics
Stellar populations in hosts of giant radio galaxies and their neighbouring galaxies A.Ku´zmicz1,2,3,B. Czerny1, andC.Wildy1 1 Center for Theoretical Physics, Polish Academy of Sciences, Al. Lotnik 32/46, 02-668Warsaw, Poland e-mail: cygnus@oa.uj.edu.pl 2 Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Krakow, Poland 3 Queen Jadwiga Astronomical Observatory in Rzepiennik Biskupi, 33-163 Rzepiennik Strzy˙zewski, Poland Received 15 November 2018 / Accepted5March 2019 ABSTRACT Context. Giant radiogalaxies (with projected linear sizeof radio structure larger than 0.7Mpc) arevery rare and unusual objects. Only ∼5% of extended radio sources reach such sizes. Understanding of the processes responsible for their large sizes is crucial to further our knowledge about the radio source’s evolution. Aims.Wecheckthehypothesisthatgiantsbecomeextremelylargeduetothespecifchistoryoftheirhostgalaxy formation,aswell asinthe contextofthe clusteror groupofgalaxies wherethey evolve. Thereforewestudythe star formation historiesin theirhost galaxies andingalaxies locatedin their neighbourhood. Methods.We studied41 giant-size radiogalaxies as well asgalaxies located withina radiusof5Mpc around giantstoverify whether theexternal conditionsof the intergalactic medium somehow infuence the internal evolutionofgalaxiesin the group/cluster.We comparedthe results witha control sampleof smaller-sizedFanaroﬀ–RileytypeII radiogalaxiesand their neighbouringgalaxies.We ft stellar continuain allgalaxy spectra using the spectral synthesis codeSTARLIGHT and provide statistical analysisof the results. Results.Wefndthathostsofgiantradiogalaxieshavealargeramountof intermediateagestellarpopulationscomparedwithsmallersizedFRIIradio sources.Thesameresultisalsovisiblewhenwecompare neighbouringgalaxieslocatedupto1.5Mpcaroundgiants and FRIIs. This may be evidence that star formation in groups with giants was triggered due to global processes occurring in the ambientintergalactic medium.These processesmayalso contributeto mechanisms responsiblefortheextremelylargesizesofgiants. Key words. galaxies: active–galaxies: structure –galaxies: nuclei 1. Introduction Amongst manytypesofextragalactic radio sources, whichcover a wide range of radio structures, morphologies and sizes, the giant radio sources (GRS) are very peculiar ones. The linear sizes of their radio structures are defned to be larger than 0.7Mpc (assumingH0 = 71kms−1Mpc−1, ΩM = 0.27, Ωvac = 0.73; Spergel et al. 2003), what is comparable with sizes of galaxy clusters. The class of GRSs is not very large. To date we know just 348 confrmed GRSs(Ku´zmicz et al. 2018)but that number is still growing, thanks to low-frequencytelescopes, high resolution radio surveys and large spectroscopic surveys. Previous studies focused on the properties of individual objects (e.g. Jamrozy et al. 2005; Subrahmanyan et al. 2006; Konaretal.2009;Orretal.2010;Machalskietal.2011),buta few studies also consider larger samples of giants. Theyconcentrateontheroleofsomefactorswhichcouldberesponsibleforthe gigantic size of radio structures. In these studies the authors con-sider the properties of the ambient intergalactic medium (IGM; Machalski et al. 2006; Subrahmanyan et al. 2008;Kuligowska et al. 2009), the advanced age of the radio structures (e.g. Mack et al. 1998;Machalski et al. 2009), recurrent radio activity (e.g. Subrahmanyan et al. 1996;Schoenmakers et al. 2000;Machalski et al. 2011),as well as the radio core and the central activegalac
tic nuclei’s(AGN) specifc properties(Ishwara-Chandra&Saikia 1999;Ku´zmicz&Jamrozy 2012). Studies carried out in recent years show that the GRSs can be used as barometers of the intergalactic medium. When the radiolobesexpand,theyfrst interactwiththeinterstellarmedium, then the intergalactic, and fnally with the intracluster medium. These interactionscanbeareasonfor asymmetriesinradio structures. Subrahmanyan et al. (2008)and Safouris et al. (2009) showed that there is a clear connection between properties of radio lobes and the distribution of neighbouringgalaxies. They showed that the asymmetries in radio morphology of some giants can be a result of inhomogeneities in the distribution of ambient intergalacticgas, which follows the large scale structure of the universe. The seriesofinvestigationsby Chenetal. (2011a,b,2012a,b) focus on the environmental properties around a few giant radio sources. In their studies theyanalyse the distribution and properties of companion galaxies around giants and fnd that they tend to lie near the radio lobes. They also show that in some cases (e.g. NGC 6251, NGC 315) the velocity dispersion of group members is not consistent with that expected from correlation curves of X-ray luminosity versus velocity dispersion (Mulchaey& Zabludoﬀ
1998). They conclude that the density ofX-ray emittinggasis unusuallylow around studiedgiantsand it can be the explanation of their extremely large sizes of radio structures. GRSs are alsoveryvaluable toolsforinvestigatingthelargescale structure of the Universe. The authors Malarecki et al. (2013, 2015), Pirya et al. (2012), Peng et al. (2015)used their large sizes to probe the distribution of the warm-hot intergalactic medium (WHIM) in flaments of the large-scale structure of Article publishedby EDP Sciences A91, page1of 20 theUniversefocusingonevidenceofradiolobe interactionswith the ambient medium. Onthe other hand,the roleoftheenvironmentforthegalaxy propertiesandevolutionisnot irrelevant.Thepastmergers infuence, forexample, thegalaxy morphology, star formation, and accretion processes. Also the cluster density is closely related to the morphological types of its galaxies. It has been shown that early-typegalaxies dominate high density environments in contrast to late-type galaxies that dominate low-density ones (Dressler 1980). There are also a few studies aimed at the con-nection between cluster environment and star formation history. Moran et al. (2005)fnd that the centralgalaxies in clusters are older than those at larger distances from cluster centre. Furthermore, Demarco et al. (2010)showed that the dense cluster envi
ronment stops star formation in low mass galaxies when they enter the cluster. Theyalso fnd that less massivegalaxies formed stars more recently than more massive ones. There are also numerous studies investigating the stellar pop-ulationsofradio sources(e.g. Holtetal.2007;Willsetal.2008). Theyconcentrated on the identifcation of the young stellar pop-ulations in radio galaxies to establish the timescales of radio activity relative to the merger event. The young stellar popu-lations are observed in ∼15–25% of all powerful extragalactic radio sources(Tadhunter et al. 2011). They found that in most of thosegalaxies the radio activity occurs simultaneously with the starburst and it is explained as a result of a merger event with a gas-rich galaxy. There are also a group of radio galaxies where the radio activity is triggered a long time after the starburst. Raimann et al. (2005)fnd that radiogalaxies are dominatedby intermediate age(∼1Gyr) stars, suggesting a con-nection between the radio activity andastarburst which occurred 1Gyr ago. Theyalso propose that more massive starbursts have led to more powerful radio emission. In our analysis we have attempted to answer the question of whether or not the history of giant radiogalaxy (GRG) host formation may be responsible for the growth of its radio structures. Galaxy formation is related to internal processes such as star formation, but it also depends on the global properties of the ambient medium, intergalacticgas andgalaxies which com-prise thegalaxy cluster. In this paper we present the results of a stellar population analysis for the sample of GRGs,but also extend our studies togalaxies located in the same group/cluster as giants.We useda control sampleof smaller-sized radiogalaxies to look for systematic diﬀerences between stellar populations of giants and non-giants, and to fnd such properties of GRGs which distinguish them from smaller radiogalaxies, which may be responsible for GRGs origin. The paper is organized as follows. In Sect. 2we present the sample ofgalaxies used in our analysis, in Sect. 3we describe the data reduction procedures and methods of spectral synthesis, in Sects. 4and5we discuss our results,andin Sect. 6we present the summary and the conclusions. 2. Data and sample selection 2.1. Selection of GRGs and comparison sample The sample of GRGs is extracted from the catalogue of GRSs byKu´zmicz et al. (2018). From their sample we selected 72 galaxies for which optical spectra were available in Sloan Digital Sky Survey Data Release 13 (SDSS DR13; Albareti et al. 2017). We restrict this number to 41 by including only those galaxies around which we found neighbouringgalaxies (at least onegalaxy) for which SDSS optical spectra are alsoavailable. The details of the selection process for neighbouring galaxies are presented in Sect. 2.2. In our analysis we are required to use only spectra of good quality. The selection criteria restricted the sampletonearbyGRGswith redshiftsintherangeof0.03 < z < 0.31, with mean 1.4 GHz total radio luminosity log Ptot = 24.92 and mean projected linear size D = 1.2Mpc. Asa comparisonsample,weusedthe FRII-typeradiogalaxies from Kozie -Wierzbowska&Stasi´ nska(2011), in which the authorsstudy propertiesof401FRIIradiogalaxieswithawide range of radio powers and radio structure sizes. Among them there are also 18 GRGs, and therefore we excluded them from the comparison sample. Similarly to GRGs selected for further analysis, we used only those radio sources for which we found at least one companiongalaxy with anavailable optical spectrum (see Sect. 2.2). As a result, the fnal comparison sample consists of 217 FRII radiogalaxiesina redshift rangeof0.008 < z < 0.4 and with a mean projected linear size D = 0.2Mpc. InFig. 1we presentthe characteristicsof radiogalaxies con-sideredinthispaper.Weplotthe distributionofredshifts,projected linear sizes and 1.4 GHz total radio luminosities for the sample of GRGsand FRIIs.Allofthe considered radiogalaxies are nearby objects (up to z = 0.4)witha wide rangeof radiopowers. 2.2. Selection of neighbouring galaxies We looked for neighbouringgalaxies around each radiogalaxy (GRG, and FRII from comparison sample) using SDSS DR13. We selected allgalaxies withina radiusof about ∼5Mpc from radiogalaxy host with measured spectroscopic redshifts corresponding to the redshift of radio galaxy host. We adopted the redshift diﬀerence betweena neighbouringgalaxy and the radio galaxy host equal toΔz 6 0.003 that corresponds to ∼800kms−1. The total number of neighbouring galaxies found around all of GRGs was 789 and around FRII radio galaxies was 3692. Allof selectedgalaxieswas usedin further analysis. The com-pleteness of spectroscopically selected group/cluster members depends on the completeness of SDSS. The SDSS main spec-troscopicgalaxy sampleis complete within the magnitude range 14 < r < 18. The hosts of radio galaxies are usually associated with the brightest galaxy in the group and the neighbouring galaxies are up to few magnitude fainter. For bright radio galaxy hosts (∼14 SDSS r-band magnitude) the com-pleteness of fainter spectroscopic group members is ∼90% (in the range of 14 < r < 19 mag ), but for weak radio source hosts (with ∼18 SDSS r magnitude) the completeness of spec-troscopic data below r > 18 mag is much lower. Therefore, we counted neighbouring galaxies based on the SDSS photometric data to see how spectroscopic selection can be incomplete.We counted allgalaxies which are up to fve magnitudes fainter than hosts of radio sources requiring their photometric redshift estimations to correspond to the spectroscopic redshift of the radio galaxy host with the Δzphot 6 0.02, equal to the error of SDSS photometric redshift estimations(Beck et al. 2016). InTable 1welisttheprincipal parametersof analysedGRGs, arrangingit as follows: column1 –galaxy name; columns2and 3– J2000.0galaxy coordinates; column4 – redshift; column5 – linear size; column6 – numberofgalaxiesin group withavailable spectroscopic data; column7 – number ofgalaxies within the radiusof0.5MpcfromGRG; column8 – numberofgalaxies between radius of 0.5Mpc and1Mpc around GRG; column 9 – number ofgalaxies between radius of1Mpc and 1.5Mpc around GRG; column 10 – number ofgalaxies between radius of 1.5 Mpc and 3Mpc around GRG; column 11 – number of A91, page2of 20 A.Ku´zmiczetal.: Stellar populationsinhostsofgiantradiogalaxiesandtheir neighbouringgalaxies Fig.
1.
Distributions of redshift(top panel),projected linear size(middle panel)and 1.4GHz total luminosity(bottom panel)for GRG and FRII samples studied in this paper. galaxies between radiusof3Mpcand5Mpc aroundGRG;column12 – numberofgalaxiesin group which arefvemagnitudes fainter than host of GRG withΔzphot 6 0.02. We can see that the numberofgalaxies selected from photometricdata (column12)ismuchlargerthan numberofgalaxies selected from spectroscopic data (column 6). However,the number of photometrically selectedgalaxies should be treated with a caution because of large error of SDSS photometric redshift estimation which causes that some of selected galaxies could not belong to the same galaxy group/cluster. Our studies base onlyonthe spectroscopically selectedgalaxies despiteofthefact that in some groups the completeness can be low. However, we have not studied particular groupofgalaxies,but groupsin gen-eral, therefore the low completeness in some groups does not aﬀect the fnal results in a signifcant way. All the radio maps of GRGsandthe positionsof neighbouringgalaxies withavailable spectroscopic and photometric radshifts, are presented in AppendixA. 3. Optical analysis The spectra of giant radio galaxies, as well as galaxies from the comparison sample and all neighbouringgalaxies, were pro-cessed through the standard procedures of the Image Reduction and AnalysisFacility1 (IRAF). Each spectrumwas corrected for Galacticextinction AV taken from theNASA/IPACextragalactic database.Theextinction-corrected spectra werethen transformed totherestframeineachcaseusingthe redshiftvaluesgiveninthe SDSS.For all analysed spectra we applied the simple stellar pop-ulation (SSP) synthesis code STARLIGHT(Cid Fernandes et al. 2005)to modelthe observedspectra through fttingagalaxy spec-tral continuum.STARLIGHT code combinesN spectra froma base of individual stellar populations in search of linear combinations matching an observed spectrum. The base consists of stel-lar spectra with diﬀerent ages and metallicitiesextracted from the evolutionary synthesis modelsof Bruzual&Charlot (2003).The modelledspectrumisfttedusingaMetropolisandMarkovchain Monte Carlo techniques which explore the parameter space and searches for the minimum of χ2 between observed and modelled spectrum.For more detailssee Cid Fernandesetal. (2005).Inour modelling we used a base of 150 SSPs with 25 values of stellar ages (between1Myrand18Gyr)andsix metallicities(from 0.005 to 2.5 Z).Each SSP withagiven age and metallicity contributes tothemodelfux,anditcanbeexpressedasalight fractionpopulation vector xj, and mass fraction population vector µj. Asa result of modelling we obtain mean stellar ages, present-day stel-lar mass, mean metallicities, velocity dispersion, star formation and chemicalevolution histories.InFig. 2we presentanexample of modelled spectrum and the light fraction population vector as a function of stellar age which represents the stellar composition ofgalaxy. 4. Analysis and results 4.1. Stellar populations In our analysis we compared the parameters obtained for GRGs with those of smaller FRII radio galaxies, as well as their hosts with neighbouringgalaxies and neighbours around those two groups of radio galaxies between each other. To examine whether there are anydiﬀerences in the stellar populations, we frstly model the SSPs for eachgalaxy and then weaverage the resultant SSPs for each class ofgalaxies: for GRGs, FRII radio galaxies, neighbours of giants, and neighbours of FRIIs. Figures 3 and 4 show the mean light-weighted population vector Σxj (left column) and mass-weighted Σµ j (right column) population vector as a function of stars age t.We plotted these fgures for particular samples and for fve diﬀerent radii around radiogalaxy hosts (0.5,1, 1.5,3 and5Mpc). The Σxj and Σµj vectorsare summarizedbymetallicityandthenaveragedineach galaxysample.TopreservetheclarityofFigs.3and4,webinned the results using 12 age bins instead of 25. The results of stellar population composition in studiedgalaxies are summarized inTable 2, where the obtained SSPs are 1 http://iraf.noao.edu
A91, page3of 20 A&A 624, A91 (2019) Table 1. GRGs studied in this paper. IAU name α(2000) (h m s) δ(2000) (◦ 0 00) z D (Mpc) n n0.5 n0.5−1 n1−1.5 n1.5−3 n3−5 nm,z (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) J0003+0351 00 03 31.50 +03 51 11.3 0.095 2.03 12 2 1 1 4 3 142 J0010−1108 00 10 49.69 −11 08 12.9 0.077 0.80 20 3 2 1 6 7 99 J0042−0613 00 42 46.85 −06 13 52.6 0.124 0.85 6 0 2 1 1 1 125 J0115+2507 01 15 57.24 +25 07 20.3 0.184 1.06 3 0 2 0 0 0 147 J0120−0038 01 20 12.51 −00 38 37.8 0.235 0.71 6 0 0 0 3 2 108 J0134−0107 01 34 12.80 −01 07 28.2 0.079 1.21 54 9 2 4 12 26 168 J0135−0044 01 35 25.66 −00 44 47.3 0.156 1.06 9 0 1 0 2 5 128 J0259−0018 02 59 42.88 +00 18 40.9 0.183 0.73 5 1 0 1 0 2 159 J0751+4231 07 51 08.79 +42 31 23.6 0.203 1.19 2 0 0 0 0 1 85 J0857+0131 08 57 01.76 +01 31 30.9 0.273 1.30 2 0 0 0 0 1 95 J0858+5620 08 58 32.78 +56 20 14.7 0.240 0.87 4 0 0 0 2 1 92 J0902+1737 09 02 38.42 +17 37 51.4 0.164 1.19 3 0 0 1 1 0 90 J0914+1006 09 14 19.53 +10 06 40.5 0.308 1.71 5 1 0 0 2 1 135 J0918+3151 09 18 59.42 +31 51 40.6 0.062 0.78 31 5 3 4 9 9 93 J0926+6519 09 26 00.90 +65 19 23.0 0.140 0.78 12 2 2 0 4 3 79 J0932+1611 09 32 38.32 +16 11 57.8 0.191 0.76 2 0 0 0 0 1 86 J1004+5434 10 04 51.83 +54 34 04.4 0.047 0.81 125 9 9 12 36 58 131 J1006+3454 10 06 01.77 +34 54 10.2 0.099 4.23 12 0 5 0 4 2 69 J1021+1217 10 21 24.22 +12 17 05.3 0.129 1.97 16 1 2 3 4 5 142 J1021+0519 10 21 31.47 +05 19 01.0 0.156 2.23 9 1 1 1 2 3 111 J1032+2756 10 32 14.09 +27 56 00.2 0.085 1.04 19 3 2 3 1 9 113 J1032+5644 10 32 59.02 +56 44 53.8 0.045 0.97 84 9 9 8 18 39 74 J1111+2657 11 11 24.97 +26 57 46.6 0.034 1.12 149 10 14 12 31 81 245 J1147+3501 11 47 22.12 +35 01 08.0 0.063 0.85 31 7 5 2 7 9 88 J1247+6723 12 47 33.33 +67 23 16.5 0.107 1.35 12 0 2 1 5 3 121 J1253+4041 12 53 12.28 +40 41 23.7 0.229 1.01 2 0 0 0 1 0 90 J1308+6154 13 08 44.75 +61 54 15.3 0.162 1.48 2 0 0 0 0 1 105 J1311+4059 13 11 43.06 +40 59 00.0 0.110 0.74 15 3 3 2 1 5 102 J1327+5749 13 27 41.32 +57 49 43.4 0.120 1.61 15 3 0 3 3 5 196 J1328−0307 13 28 34.33 −03 07 45.0 0.085 1.28 33 1 4 5 7 15 142 J1345+5403 13 45 57.50 +54 03 17.0 0.163 0.80 3 0 0 1 0 1 170 J1400+3019 14 00 43.44 +30 19 18.2 0.206 2.19 7 2 0 0 1 3 140 J1409−0302 14 09 48.85 −03 02 32.5 0.138 1.37 10 2 1 0 4 2 138 J1418+3746 14 18 37.65 +37 46 24.5 0.135 1.09 14 0 3 1 4 5 153 J1428+2918 14 28 19.24 +29 18 44.2 0.087 1.42 17 0 1 1 7 7 128 J1429+0715 14 29 55.38 +07 15 12.9 0.055 0.71 47 3 5 4 10 24 116 J1507+0234 15 07 03.78 +02 34 07.2 0.124 0.83 7 0 2 1 2 1 132 J1540−0127 15 40 56.82 −01 27 10.2 0.149 0.76 7 1 1 1 1 2 112 J1555+3653 15 55 00.42 +36 53 37.4 0.247 1.34 3 0 0 1 1 0 106 J1615+3826 16 15 52.25 +38 26 31.8 0.185 0.81 5 1 0 1 1 1 173 J1635+3608 16 35 22.54 +36 08 04.7 0.165 0.90 10 1 3 2 2 1 148 divided into three age bins: young populations with stellar ages t?< 5× 108yr, intermediate populations with9 × 108yr 1010 yr. Contributions ofeachagebinaregivenin percentageof Σxj and Σµ j separately. The uncertainties of Σxj and Σµj in each sampleofgalaxies were calculatedasthe standarddeviationofthe meanvalue. In frst two rows of Fig. 3 we show the comparison of stellar populations between hosts of radiogalaxies (GRGs and FRIIs) and their neighbours. We can see that both for GRGs and smaller-sized FRIIs the hostgalaxies are dominatedby stel-lar populationswithagesabove1Gyr.In comparisonwiththeir neighbouringgalaxies theyhavea larger fractionof the oldest populations(∼10Gyr)anda smaller fractionof intermediateage stars(∼1Gyr). Thisfact canbeexplainedbythediﬀerent types ofgalaxies consideredinthe samples.The hostsof radiogalaxies are old ellipticals located predominantly in the centres of clusters/groupsofgalaxies, while their neighbours areofvarious types where the star formation processes are more common than in ellipticals. In the next step we compare the SSPs of GRGs with smallersized FRIIs. As it can be seen in Fig. 3andTable 2 the GRGs have a larger fraction of intermediate age stars(∼1Gyr) and signifcantly smaller fraction of the oldest ones(∼10 Gyr). Thisfact isevidenceof thediﬀerences in the structure of GRG hosts com-pared with hosts of smaller-sized counterparts. The same diﬀerence in SSP composition is observed for neighbours of GRGs and FRIIs. Galaxies located around GRGs have a larger fraction of middle age stars compared withgalaxies around FRIIs. It is A91, page4of 20 A.Ku´zmiczetal.: Stellar populationsinhostsofgiantradiogalaxiesandtheir neighbouringgalaxies Fig.
2.
Spectral modellingusingSTARLIGHT synthesiscodeforJ1006+ 3454 GRG. Top:observed spectrumis plotedby black colour, modelled spectrumbyred and residual spectrumbythe green colour. Bottom:age distributioninthelight fraction populationvector. also clearly visible that this eﬀect is more signifcant when we take into account galaxies located closer to the radio source’s host (up to radius of 1.5 Mpc). The diﬀerence between SSPs in neighbouringgalaxies of GRGs and FRIIs is not as prominent as for radiogalaxy hosts, however we observe that the intermediate age populations in neighbours of GRGs are systematically higher than in neighbours of FRIIs, while old stellar populations are systematically lower. The above results show that giants, together with their neighbouringgalaxies, could have diﬀerent formation histories com-paringtothegroupswith smallersizedradiogalaxies.Itmaybe evidence of diﬀerent global properties of the ambient medium where those groupsevolve.Forexample, the scenarioofa close interaction or minor mergereventin the central partofagalaxy group can indicate larger star formation in the most central galaxies. We also have carried out simple statistical analyses of the typesof neighbouringgalaxiesina radiusof5Mpc from the radiogalaxy host. Based on the SDSS classifcationofgalaxies, we found that 31% of spectroscopic GRG neighbours are star forminggalaxies,6% are starburstgalaxiesand 2.4% areAGNs. In a sample of FRII neighbours we observe 27% star forming galaxies, 5.7% starburstgalaxies and 2.7%AGNs. It shows that environments around GRGs and FRIIs statistically consist of similar galaxies and the obtained larger amount of intermediate age populations in GRG neighbours is not due to the larger amountofgalaxies which formed stars more recently. In all graphs which show the distribution of ages represented by the light fraction population vector we observe a large contribution of stars with ages∼1Gyr. Some authors (e.g.Chen et al. 2010)found that the fraction of∼1Gyr stars depend on the stel-lar libraries used in spectral modelling, however qualitatively it does not aﬀect the obtained results, since we use the same ft-ting proceduresforallof ourgalaxies. Otherwise,theevidence of intermediate age stellar populations was suggested by other authors to be common in local ellipticalgalaxies (e.g. Huang& Gu 2009). 4.2. Uncertainty of STARLIGHT ftting In our studies we obtain relatively small diﬀerences between resultant SSPs for studied samples of galaxies, therefore it is important to consider the uncertainties of ftting procedure. Cid Fernandes et al. (2005)check the recovery of spectral parame
ters modelled by STARLIGHT based on mock spectra with an assumed star formation history. The synthetic spectra were per-turbed to obtain diﬀerent signal-to-noise ratios (S/N) and the error spectrum was adopted to reconstruct real observed spectrum. The authors fnd that individual components of xj are very uncertainbut the binning xj on to young, intermediate and old components gives the robust description of the star formation. Other output parameters, for example hlog t?iL, hlog t?iM also recover the input parameters well. On average our spectra have S/Nequal to 15. According to parameter uncertainties obtained by Cid Fernandes et al. (2005;listed in theirTable 1.) the mean and the dispersion between input and output values of xj in an individual objects are equal to0.62 ± 4.04%,0.01 ± 7.88%, and −0.63 ± 7.61% for young, intermediate and old stellar popu-lations respectively. Thus for an individual galaxy the dispersion is large. On the other hand, we fnally compare a sample of 41 giant radiogalaxies, so these uncertainties reduce to 0.62 ± 0.63%,0.01 ± 1.23%, and −0.63 ± 1.19%, respectively, duetothe reductionofthe dispersion.For µ j the uncertainties of parameter recoveryforasinglegalaxyareequalto0.16±1.18%, 0.93 ± 6.10% and −1.09 ± 6.54%, reducing to 0.16 ± 0.18%, 0.93 ± 0.95% and −1.09 ± 1.02%. The uncertainties are much lower than the diﬀerences we reportinTable 2. 4.3. The ages and masses Based on stellar continuum fts for individualgalaxies we determine the stellar mass, black hole mass, light and mass weighted mean stellar age hlog t?i and metallicity hlog Z?i defnedby Cid Fernandes et al. (2005)as NN XX hlog t?iL = xj log tj and hlog Z?iL = xj log Zj (1) j=1 j=1 NN XX hlog t?iM = µ j log tj and hlog Z?iM = µj log Zj. (2) j=1 j=1 In Fig. 5we plot the distributions of stellar mass, hlog t?iL, and hlog Z?iL obtained for individualgalaxies in each sample. In the frst graph, where we present the distribution of mean stel-lar mass, it can be seen that the stellar masses in hosts of radio sources are mostly higher than massesof neighbouringgalaxies, butin both samplesof neighbours we also observegalaxies with stellar masses as high as in radio sources. In the next two panels we plot the distributions of light weighted stellar ages and metallicities. Similarly to the graphs with mass distributions, the plotted parameters obtained for neighbouringgalaxies spana wide rangeofvalues and the hosts of radio sources from GRG and FRII samples are concentrated in the right end of these ranges. A91, page5of 20 Fig.
3.
Age distribution of the mean light fraction Σxj population vector(left column)and mean mass fraction Σµ j population vector(right column)for GRGs, FRIIs, neighbours of GRGs and neighbours of FRIIs.Top panels:we compare stellar populations of the GRG’shosts with their neighbouringgalaxies, in the middle panels – the hosts of smaller-sized FRII radiogalaxies with their neighbouringgalaxies, next – comparison between giants and smaller-sized FRIIs, and in the bottom panels – between neighbours of giants and neighbours of FRIIs in a radius of 0.5 Mpc from radiogalaxy host. A91, page6of 20 A.Ku´zmiczetal.: Stellar populationsinhostsofgiantradiogalaxiesandtheir neighbouringgalaxies Fig.
4.
Age distribution of the mean Σxj (left column)and meanΣµj (right column)for neighbours of GRGs and FRIIs in a diﬀerent radius around radiogalaxy host. Top panels:ina radiusof1Mpc,upper middle panels – in a radius of 1.5 Mpc, lower middle panels –ina radiusof3Mpc, and in the bottom panel –ina radiusof5Mpc. A91, page7of 20 Table 2. Summarized light and mass fraction populationvector for sam-plesof GRGs, smaller-sized FRIIs and neighbouringgalaxiesof GRGs and FRIIs, divided into three age bins: young(t?< 5× 108 yr), inter-mediate (9 × 108 yr < t?< 7.5× 109 yr), and old(t?> 1010 yr) stellar populations. Σxj Young Intermediate Old GRGs 8.1± 4.1 39.4 ± 6.2 52.5 ± 0.5 FRIIs 7.3± 3.3 26.5± 3.6 66.2 ± 0.2 R < 0.5Mpc Neighbours of GRGs 18.7 ± 6.1 50.4 ± 4.4 30.9 ± 1.1 Neighbours of FRIIs 19.6 ± 3.6 46.4 ± 2.4 34.0 ± 0.5 R < 1Mpc Neighbours of GRGs 13.9 ± 6.3 52.5 ± 4.6 33.6 ± 0.9 Neighbours of FRIIs 19.5 ± 2.5 47.0 ± 1.7 33.5 ± 0.4 R < 1.5Mpc Neighbours of GRGs 16.2 ± 6.5 50.4 ± 5.1 37.8 ± 1.0 Neighbours of FRIIs 14.9 ± 3.1 44.3 ± 2.5 40.8 ± 0.3 R < 3Mpc Neighbours of GRGs 15.4 ± 5.3 46.2 ± 4.1 38.4 ± 0.6 Neighbours of FRIIs 15.4 ± 2.5 44.3 ± 1.8 40.3 ± 0.2 R < 5Mpc Neighbours of GRGs 15.2 ± 4.4 45.3 ± 3.3 39.5 ± 0.6 Neighbours of FRIIs 16.1 ± 2.2 43.8 ± 1.5 40.1 ± 0.2 Σµj Young Intermediate Old GRGs 0.5 ± 2.6 17.1 ± 7.5 82.4 ± 0.1 FRIIs 0.1 ± 1.1 8.5 ± 3.5 91.5 ± 0.2 R < 0.5Mpc Neighbours of GRGs 0.4 ± 4.6 36.4 ± 6.1 63.2 ± 0.1 Neighbours of FRIIs 0.5 ± 1.9 35.4 ± 3.5 64.1 ± 0.1 R < 1Mpc Neighbours of GRGs 1.0 ± 3.5 34.7 ± 6.9 64.3 ± 0.3 Neighbours of FRIIs 0.6 ± 1.3 34.8 ± 2.5 64.6 ± 0.1 R < 1.5Mpc Neighbours of GRGs 0.6 ± 2.1 37.3 ± 4.2 62.1 ± 0.1 Neighbours of FRIIs 0.7 ± 1.1 35.7 ± 2.0 63.6 ± 0.1 R < 3Mpc Neighbours of GRGs 0.6 ± 1.9 36.8 ± 3.2 62.6 ± 0.1 Neighbours of FRIIs 0.7 ± 1.3 35.8 ± 2.1 63.5 ± 0.1 R < 5Mpc Neighbours of GRGs 0.6 ± 1.3 37.8 ± 2.5 61.6 ± 0.1 Neighbours of FRIIs 0.8 ± 1.2 35.6 ± 1.9 63.6 ± 0.1 To fnd anydiﬀerence in properties between the groups con-Fig.
5.
Distributions of mean stellar mass, mean stellar age and mean centrated around GRGs and FRIIs, we determine the parameters stellar metallicity for samples of GRGs, smaller-sized FRII radio sources, and their neighbouringgalaxies. characterizing groupsasawhole(thehostofaradiogalaxywith its neighbours).We summedupthe stellar masses, hlog t?iL, and hlog Z?iLofindividualgalaxiesineachgalaxygroup separately. ples.Alsowhenwe comparethevalues characterizingthewhole In Fig. 6we plot the normalized distributions of above parame-groups with GRGs and FRIIs, we do not see the diﬀerences in ters.We obtained that both for groups with GRGs and FRIIs, all mean stellar ages, metallicity and stellar masses. The obtained of the parameters span similar ranges of values and have similar parameters for groups with giants and smaller-sized FRIIs are distribution shapes. However, we observe that the summarized nearly the same, so all considered groups look very similar. stellar massin groups with giantshave slightly highervalues However,theeﬀectsofenvironmental infuenceson internal propthaningroupswithsmallerFRIIs.Thisisevidencethatthelarger ertiesofclustermemberscanbesmallenoughtobevisible.Itcan amountof stellarmassis cumulatedingalaxiesaroundgiants. onlybewellrecognizedinstudiesofindividualgroupsforwhich InTable 3we summarizetheaveragevaluesoflightand mass-wehavegood quality spectroscopicdataforall cluster members. weighted stellar ages, metallicitiesand stellar masses obtainedfor Anysubtlediﬀerencesareusuallynot visiblewhenweaveragethe allgalaxiesin each sample.However,it canbe seen that there are large numberofvalues, because the uncertaintiesof these quan-no statistically signifcant diﬀerences between considered sam-tities become larger than presumed diﬀerences. A91, page8of 20 A.Ku´zmiczetal.: Stellar populationsinhostsofgiantradiogalaxiesandtheir neighbouringgalaxies c M∗c log e µ(t∗) s SFR(t∗)= , (3) t∗ Δlog t∗ where Mc is the total mass converted to stars throughout the ∗ c galaxy life, andµ(t∗)is the fraction of this mass in thet∗ bin. s c log e µ(t∗) s SSFR(t∗)= , (4) t∗ Δlog t∗ which measures the star formation rate with respect to the mass already converted into stars.Time-dependent star formation rates Fig.
7.
Average time-dependent star formation rate(top graph)and spe-cifc star formation rate(bottom graph)for sample of GRGs, FRIIs, neighbours of GRGs and neighbours of FRIIs. can be derived from the stellar population synthesis and they are in a good agreement with SFR estimations from Hα line (Asarietal.2007).InFig. 7we presenttheaveragedSFR(t)and SSFR(t) for each sample ofgalaxies considered in this paper. It can be seen that star formation occurred ∼1Gyr ago in GRGs started earlier andwas higher thanin FRIIgalaxies.However, there are no diﬀerences between neighbours of giants and neighbours of FRIIs. According to our results from Sect. 4.1, the neighbours of giants have larger fraction of intermediate age populations com-paredto their counterparts around FRIIgalaxies,but thiseﬀect is visibleforgalaxies locatedupto1.5Mpc fromthe host. Therefore, we plotted the same fgure as Fig. 7but for neighbouring galaxies located withina radiusof1.5Mpc.InFig.8itis clearly visible that in case of neighbours of giants the ∼1Gyr starburst also started earlier. It confrms that in groups with giants (within a radius of 1.5 Mpc) star formation processes were triggered at almost the same time indicating the specifc global conditions occurred in the intergalactic medium of a group. 4.5. Galaxy distribution around GRGs The distributionofgalaxies around GRGswaspreviously studied by several authors (e.g. Malarecki et al. 2015; Pirya et al. 2012;Chen et al. 2011a,b, 2012a,b). Theyused GRGs to probe the properties of the ambient IGM. As a result they fnd evidence of radio jet interaction with the group ofgalaxies around A91, page9of 20 A&A 624, A91 (2019) Table 3. Meanvaluesof light and mass weighted stellar ages, metallicities, and mean stellar mass for each sampleofgalaxies. Mean Mean Mean Mean Mean hlog t?iL hlog t?iM hlog Z?iL hlog Z?iM logM? [yr] [yr] [] GRGs 9.54 ± 0.49 9.96 ± 0.34 −1.82 ± 0.26 −1.65 ± 0.15 11.38 ± 0.39 FRIIs 9.71 ± 0.41 10.07 ± 0.29 −1.79 ± 0.19 −1.64 ± 0.11 11.48 ± 0.34 Neighbours of GRGs 9.11 ± 0.58 9.80 ± 0.32 −1.95 ± 0.34 −1.85 ± 0.33 10.46 ± 0.61 Neighbours of FRIIs 9.12 ± 0.59 9.80 ± 0.46 −1.95 ± 0.35 −1.86 ± 0.35 10.51 ± 0.72 Groups with GRGs 9.13 ± 0.29 9.89 ± 0.17 −1.84 ± 0.15 −1.72 ± 0.15 11.92 ± 0.24 Groups with FRIIs 9.45 ± 0.35 9.94 ± 0.23 −1.86 ± 0.18 −1.73 ± 0.16 11.89 ± 0.27 Notes. At the bottomof the table wegive the meanvalues for whole groups with GRGs and FRIIs. Fig.
8.
Average time-dependent star formation rate(top graph)and spe-cifc star formation rate(bottom graph)for sample of GRGs, FRIIs, neighboursofGRGswithinaradiusof1.5Mpcand neighboursofFRIIs within a radius of 1.5 Mpc. some GRGs. Theyalso fnd that in most giants, the shorter jet is brighter, suggesting asymmetries in the IGM which may not be apparentinthe distributionof neighbouringgalaxies(Piryaetal. 2012). Also the asymmetries and deformations of radio lobes indicate the infuence of environment on to the radio source. Malareckietal. (2015)fndthat thereisa tendencyforGRG’s lobes to grow in directions that avoid dense regions that have large numberofgalaxies(i.e. perpendicularto flaments)onboth small and large scales. Hence theystate that GRGs can grow to large sizes due to their specifc location in large scale structure of Universe. In Appendix A we plot the maps of all GRGs from our sample and marked all spectroscopically confrmed neighbours as well as photometric candidates for galaxy group mem-bers. Eight of the GRGs from our sample was also studied by Pirya et al. (2012; J0926+6519, J1006+3454, J1147+3501, J1247+6723, J1311+4059, J1328−0307, J1400+3019, J1428+ 2918, J1635+3608). Based on the distribution of spectroscopically identifed galaxies that we study in this paper, we can see that many GRGs have radio lobes directed towards the less dense regions in the cluster (as it was shown by Malarecki et al. 2015), but there are some evident examples of GRGs where the radio jets are directed to the denser regions (e.g. J0134−0107, J0918+3151, J1004+5434, J1021+1217, J1032+5644, J1311+4058 and J1429+0715). In these radio sources we observe neighbouring galaxies located along the radio lobes with few or nogalaxies in the orthogonal direction. AnotherexampleofsuchagiantisDA240 studiedby Chenetal. (2011b)who showthat neighbouringgalaxies lie along the major axis of the radio source. It shows that GRGs are located in relativelyvariousenvironmentsand future studies are neededtoverify if their orientation in large scale structure of Universe can be a signifcantfactor responsible for their sizes.Wealso note thata large fraction (45%) of GRGs from our sample have at least one relatively close neighbouringgalaxy 0.2Mpcaway from GRG’s host, while in a comparison sample of smaller sized FRIIs we observe close neighboursin36%of radiogalaxies,but this ten-dencyalso have to be tested in future studies. 5. Discussion There are many factors which are closely related to star for-mation in galaxies. We can distinguish two groups of them: thefactors relatedtophysical propertiesof individualgalaxies (e.g. mass, luminosity, morphological type,gas richness, etc.), and the environmentalfactors(galaxy interactions and mergers, tidal forces, cold streams, gas stripping, strangulation, density of IGM, etc.) Some of them can trigger a starburst, and some of them can suppress star formation. The results obtained for groups with GRGs and FRIIs indicate the importance of environmentalfactors becausewe observehigher fractionsof ∼1Gyr aged star formation not only in hosts of GRGsbut also in their neighbouringgalaxies. 5.1. Merger events Galaxy interactions and mergers are thought to be a major pro-cess drivinggalaxy formation and theymay be responsible for triggering the star formationin interactinggalaxies.However,it is well known that majorgalaxy mergers are rare in the nearby A91, page 10 of 20 A.Ku´zmiczetal.: Stellar populationsinhostsofgiantradiogalaxiesandtheir neighbouringgalaxies Universe (e.g. Patton et al. 2000).We also know that the major-ityofgalaxy stellar masswas reachedatthe cosmictime corresponding to z ∼ 2–3 (e.g. Stott et al. 2013). This star formation epoch is visible as the peak of light and mass fraction population vectorsinFigs. 3and4near10Gyrs. Asignifcant fraction of intermediate age stars(∼1Gyr) are visible in all samples considered in this paper. This was also foundby Raimannetal. (2005)fora sampleof24 radiogalax
ies. They suggest that there is a connection between starburst episodes occurring1Gyragoandtheradioactivityatthepresent time. Theyalso state that the starbust was a consequence of, for example, interaction witha passingexternalgalaxy, or merger. This scenariois also proposedby Huang&Gu (2009)although theydo not fnd anyobvious evidence of morphological disturbanceina sampleoflow redshift ellipticalgalaxies. Theyalso state that the merger events leading to star formation are relatively minor and that the morphological disturbances could not already be visible. It is also confrmed by the observations that massive galaxies(>1010 M)passed one or two major merger events within z < 1.2(Conselice et al. 2009). According to these results it is possible that star formation which happened ∼1Gyr ago is a result of mergers at z ∼ 1. Merger events could be a good explanation of ∼1Gyr star-bursts in GRGs and smaller sized radiogalaxies. However, it is more signifcant for centralgalaxies of the group. Thegalaxies located at larger distances from the centre are not disturbed by the central merger, so the ∼1Gyr star formation visible in these galaxies is likely to have other origin. 5.2. Environment Global star formationmaybea resultofenvironmentalfactors. The environmental infuence on star formation has been studied by many authors. For example the studies of Hoyle et al. (2005a,b)show that galaxies located inside voids have higher star formation ratesthangalaxiesin denserregionsandthatthey are still forming stars at the same rate as in the past. However theGRSsaremostlylocatedoutof cosmicvoids(Ku´zmiczetal. 2018). Also Ceccarelli et al. (2008)fnd that bluergalaxies witha wide range of luminosity and local density, which are located at the void peripheries, show increased star formation. They explain this eﬀect as a consequence of lower accretion and the merger historyofgalaxiesarrivingatvoidwallsfromthe emptier inner void regions. 5.3. Cold streams The other possibleexplanationof global star formationin groups ofgalaxiesisthe interactionofgroupgalaxieswithcold(104K– 105K)intergalacticgas which penetrates thegalaxies. Such cold fows are flamentary and clumpy(Kereš et al. 2005),particularly in the low density environment. The star formation caused by cold streams occurs only in low-massgalaxy halos(<1012 M). Formoremassivehalosthecold streamispreheatedinastanding shock to nearly virial temperature of 106K and star formation does not follow(Dekel&Birnboim 2006). Therefore, when we consider cold streams as an explanation of higher star formation occurring ∼1Gyrago,itcanonlybethe caseforlowmassgalaxy halos. The evidence of cold streams passing through the galaxy group should be visible in the ages of stellar populations. The typical velocities of cold streams are ∼104kms−1(Zinger et al. 2018)and to pass the distance equal to the assumed diameter of the group(∼10Mpc)it needs about1Gyr.The intermediate-age stellar populations have ages in the range of9 × 108yr < t?< 7.5× 109yr, so we should be able to see evidence of higher star formationin the whole group along the cold stream.For our sample of GRGs we do not see any evidence that the starburst occurred along anypreferred direction which could correspond to the cold stream direction. In some cases the ∼1Gyr starburst is initially visible in centralgalaxies of the group, and in some cases it initially occurs at the edges. However, we do not know the sizes and geometries of these supposed cold streams and it is possible that theycould pass through thegalaxy group in more complicated ways. 6. Conclusions In this paper we studied the stellar populations of 41 GRGs and galaxies which belong to the groups around them.We compare our results witha sampleof 217 smaller-sized FRII radiogalaxies and their neighbours in order to fnd systematic diﬀerences in properties of the GRG’s hosts and their environment, which can be responsible for the origin of large scale radio lobes. The main conclusions of this work can be summarized as follows: Theaverage stellar populationsin samplesofgalaxies –the GRG hosts, FRII hosts, neighbours of GRGs and neighbours of FRIIs – are dominatedby old stellar populations(t?> 1010 yr) but they also comprise signifcant fraction of intermediate age populations with ages9 × 108yr < t?< 7.5× 109yr. The GRG’shosts have larger intermediate age stellar populations compared to smaller-sized FRIIs, in which the larger fraction of the oldest populations with ages above t?> 1010 yr can be observed. The sameeﬀect canbe seenfor neighbouringgalaxies locatedupto1.5Mpc from radiogalaxy host –the neighbours of giants have larger fractions of intermediate age populations compared to their counterparts around FRIIgalaxies. We do not fnd diﬀerencesinthe meanvaluesof stellar mass, hlog t?iL, and hlog Z?iL obtained for each sample of galaxies. Also, the diﬀerences in these parameters derived for individual groups ofgalaxies are statistically insignifcant, indicating that groups with GRGs and groups with smaller-sized FRIIs are sim-ilar. Based on the distribution of neighbouring galaxies around GRGs, we found that radio jets are usually oriented towards the regions with smaller numbers of surroundinggalaxies, however there are also a fraction of giants with jets oriented towards the dense regions. Therefore, future detailed studies are needed to confrm the scenario of specifc orientation of GRGs in large scale structure of Universe, postulated as a possible explanation of large sizes of giants. The larger fraction of intermediate age stellar populations in GRGs and their neighbouringgalaxies canbeexplained as, for example, a result of past merger events or cold streams penetrating the group ofgalaxies, which can trigger star formation. The smaller radio sources also have a large fraction of inter-mediate age stellar populations but this number is lower than in GRGs. This means that in groups with GRGs, the processes responsible for star formation could be globally more eﬃcient and they not only occurred in the central ellipticalgalaxy,but also in surrounding members of the group. These processes have larger signifcance on spatial scales of 1.5 Mpc around the radio source. Therefore, either the global properties of the intergalactic mediumorpasteventsthat happenedinthegalaxy groupscanbe responsible for the giant sizes of radio structures. Both mergers and cold streams may also supply the centralAGN of the radio source.This indicatesthatinsuchgalaxiesthe centralblackhole A91, page 11 of 20 is fedbynewmaterial and the radio activity mode may persist for a longer time, or it occurs more frequently than in smaller radio sources. This scenario may support the idea that the longer activity phaseof centralAGNin GRGs maybe responsible for giant radio source sizes. The obtained results show that future studies of larger samples of GRGs with accompanying multi-object spectroscopy can be very helpful in investigations of GRGs ori-gin and evolution in cluster environments. Acknowledgements. This project was supported by the Polish National Center of Science under decision UMO-2016/20/S/ST9/00142. 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A.1.
Distributionofgalaxies around giant radiogalaxies studiedin this paper. Allof the 1.4GHz radio maps were taken from NVSS survey. The red plus symbols denote the neighbouring galaxies within the radius of 5Mpc from GRGs host, for which spectroscopic redshifts were available and have Δz 6 0.003. The host of GRGs are marked as red pluses inside the red boxes.With the black crosses we indicate thegalaxies withavailable spectroscopic redshiftsbutwithlowsignaltonoiseratio –thisarenotusedtoour studies.Theblueboxes denotegalaxiesthat arefve magnitudesfainterthanthehostgalaxyofGRGwith Δzphot 6 0.02.In eachmapweplotthe circlesof0.5Mpc,1Mpcand5Mpc radius around GRGs host. A91, page 13 of 20 A.Ku´zmiczetal.: Stellar populationsinhostsofgiantradiogalaxiesandtheir neighbouringgalaxies A91, page 15 of 20 A.Ku´zmiczetal.: Stellar populationsinhostsofgiantradiogalaxiesandtheir neighbouringgalaxies A91, page 17 of 20 A91, page 18 of 20 A.Ku´zmiczetal.: Stellar populationsinhostsofgiantradiogalaxiesandtheir neighbouringgalaxies A91, page 19 of 20 Fig.
A.1.
continued. A91, page 20 of 20