Mon.Not.R.Astron.Soc.000,??–??(2016) Printed7January2016 (MNLATEXstylefilev2.2) Extremely Isolated Galaxies I. Sample and Simulation Analysis 6 1 O. Spector⋆ and N. Brosch 0 Wise Observatory and the Raymond and Beverly Sackler School of Physics and Astronomy, 2 Tel Aviv University,TelAviv 69978, Israel n a J 5 Accepted 2015November 10.Received2015November9;inoriginalform2015July5 ] A ABSTRACT G . We have selected a sample of extremely isolated galaxies (EIGs) from the local h Universe (z < 0.024), using a simple isolation criterion: having no known neighbours p closer than 300kms−1(3h−1Mpc) in the three-dimensional redshift space (α,δ,z). - o The sample is unique both in its level of isolation and in the fact that it utilizes r HI redshifts from the Arecibo Legacy Fast ALFA survey (ALFALFA). We analysed t s the EIG sample using cosmological simulations and found that it contains extremely a isolated galaxies with normal mass haloes which have evolved gradually with little [ or no “major events” (major mergers, or major mass-loss events) in the last 3Gyr. 1 The fraction of EIGs which deviate from this definition (false positives) is 5%–10%. v For the general population of dark matter haloes it was further found that the mass 4 accretion(relativetothecurrenthalomass)isaffectedbythehaloenvironmentmainly 6 throughstronginteractionswithitsneighbours.Aslongasahalodoesnotexperience 9 major events, its Mass Accretion History (MAH) does not depend significantly on its 0 environment. “Major events” seem to be the main mechanism that creates low-mass 0 subhaloes (Mhalo <1010h−1M⊙) that host galaxies (with Mg .−14). . 1 0 Key words: galaxies:distances and redshifts – galaxies:evolution– galaxies:haloes 6 – galaxies: interactions 1 : v i X 1 INTRODUCTION AND BACKGROUND andcosmological simulations. Thecosmological simulations r were further used to estimate the properties and histories a Theresearch describedhereispartof anextensivestudyof of the dark matter (DM) haloes in which the sample EIGs starformation propertiesandevolutionofgalaxiesindiffer- reside. ent environments and of various morphological types, con- One of the unique advantages of the EIG sample we ductedinthepastfewdecades(e.g.,Brosch1983;Almoznino studyhereisthat,apartfromtheopticalredshiftdatacom- 1995; Almoznino & Brosch 1998; Brosch et al. 1998; Heller monlyusedtoestimateenvironmentdensity,italsoutilized 2001; Weingarten 2003; Brosch et al. 2008; Zitrin & Brosch HI redshifts from the Arecibo Legacy Fast ALFA survey 2008).Specifically,westudiedgalaxiesinthemostextremely (ALFALFA;Hayneset al.2011).TheALFALFAsurveyisa underdenseregionsofthelocalUniverse.Thesegalaxiesare second-generation untargeted extragalactic HI survey initi- particularly interesting since they evolved with little or no ated in 2005 (Giovanelli et al. 2005, 2007; Saintonge 2007). environmentalinterference, and aretherefore useful for val- Thissurveyutilizesthesuperiorsensitivityandangularres- idating and calibrating galaxy evolution models. Further- olution of theArecibo305m radiotelescope toconduct the more, when compared to galaxies in denser regions, they deepestevercensusofthelocalHIUniverse.ALFALFAwas illuminatetheoveralleffectsoftheenvironmentontheevo- particularly useful in verifying the isolation of the target lution of galaxies. galaxies,sincebybeinganHIsurveyiteasilymeasuresred- Wehavechosenasampleofextremelyisolated galaxies shifts of low surface brightness galaxies (LSBs) and other (EIGs)from thelocal Universe(z<0.024), basedonasim- low-luminosity late-type neighbours that are often difficult pleisolation criterion.Theneighbourhoodpropertiesofthis to detect optically butabound with HI. samplewereanalysedindetailusingbothobservationaldata Whenanalyzingtheneighbourhoodcontent,oneshould notignorethepossiblepresenceoflargeinvisiblemassesnear ⋆ E-mail:[email protected] the target galaxy. These “dark haloes” or “dark galaxies” 2 O. Spector and N. Brosch can be composed of dark as well as non-luminous baryonic Throughout this work, unless indicated otherwise, matter.Suchdarkgalaxieswithmassesupto1011M⊙might Lambda Cold Dark Matter (ΛCDM) cosmology with the exist (Tully 2005), and their number density may be com- seven-year Wilkinson Microwave Anisotropy Probe data parable to or even exceed by an order of magnitude that of (WMAP7, Bennett et al. 2011) parameters are used, in- luminous galaxies (Klypin et al. 1999). Karachentsev et al. cluding the dimensionless Hubble parameter h = 0.704. (2006) estimated that the density of dark galaxies is less We adopt here the solar g-band absolute magnitude of than∼1/20ofthepopulationofluminousgalaxies. Thises- Mg,⊙ = +5.12 (according to the Sloan Digital Sky Survey, timate came from a search for effects of interactions with SDSS,DR7web site2). neighbouringgalaxies in some 1,500 isolated galaxies. They found that no more than 0.3% of the isolated galaxies were disturbed to a noticeable level by dark galaxies. If a dark galaxy contains sufficient HI, it might be detected 2 THE SAMPLE by ALFALFA. In the latest ALFALFA catalogue (α.40; The first and possibly most crucial step in this study is se- Hayneset al.2011)199suchdarkgalaxieswerefound.Many lectingasampleofgalaxiesinextremelyunderdenseregions, of these are suspected to be tidal or ram-pressure debris of referred to here as Extremely Isolated Galaxies (EIGs). By nearbygalaxies(e.g., Koopmann et al.2008).ALFALFAis, definition,these EIGs are very rare and, therefore, at mod- therefore, anextremely important toolfor testingtheisola- erate redshifts the sample is expected to be fairly small, tion of galaxies. including only a few dozen galaxies. This section describes ExtensiveopticalimagingofthesampleEIGsinbroad- the method used for selecting the sample, lists the sample bandandrest-frame Hα wasperformed usingtheWiseOb- galaxies,discussestheirobservedneighbourhoods,andcom- servatory1 (WO)onemetertelescope.This,alongwithpub- pares these with otherisolated galaxy catalogues. licobservationaldata,wereusedtomeasurethecurrentstar formation (following themethoddescribed in Spector et al. 2012) and to estimate its history. These observational re- 2.1 Isolation criterion sults will be described and discussed in detail in Spector & Brosch (in preparation). Inthelastfewdecadesgreatadvancesweremadeinredshift Thisworkattempts,amongotherthings,tohelpresolve surveys,whichnowmapthelocalUniverseinredshiftspace the question of “Nature vs. Nurture”; does the evolution withgreatprecision.Beforethesebecameavailable,isolated of galaxies depend only on their content or do their large- galaxies (IGs) had to be identified using projected coordi- scaleenvironmentshaveasignificantevolutionaryinfluence. nates alone, i.e. searching in two-dimensional space (2D). Some argue that galaxy formation is driven predominantly Radial distances had to be estimated, for example, using bythemassofthehostDMhalo,andisnearlyindependent the angular sizes of galaxies, such as done for the classical of the larger-scale halo environment (e.g., Croton & Farrar Catalogue of Isolated Galaxies (CIG; Karachentseva 1973) 2008; Tinker & Conroy 2009). This is supported by their and in Karachentsevaet al. (2010). simulationmodelsthatproducevoidgalaxiesconformingto The use of redshift data for testing the isolation of some observed statistical properties. However, since there galaxiesstarteddecadesago(Huchra& Thuan1977).Nowa- are many galaxy properties that current simulations can- days,whenthelocalUniverseismappedindetail,itispos- not predict, and since the halo mass of galaxies cannot be sible to perform accurate three-dimensional (3D) redshift directly measured, this hypothesis is hard to prove or dis- space searches. The advantages of using such strategy are prove. simplicity and straightforwardness, not having to assume Forsimilar purposes, othersamples of isolated galaxies anythingaboutthecharacteristicsofthegalaxies(size,mag- were defined and studied in “the Analysis of the interstel- nitude,etc.). lar Medium of Isolated GAlaxies” (AMIGA) international However, using redshift mapping introduces two dif- project (Verley et al. 2007; Ferna´ndez Lorenzo et al. 2013), ficulties. First is the incompleteness of most redshift in the “Two Micron Isolated Galaxy” catalogue (2MIG; databases. A galaxy that seems to be isolated might have Karachentsevaet al.2010),inthe“LocalOrphanGalaxies” neighbours for which a redshift was not yet measured. Sec- catalogue(LOG;Karachentsev et al.2011,2013),andinthe ond is the error in radial distance introduced by peculiar Void Galaxy Survey (VGS; Kreckelet al. 2012). These are velocities. Using redshift data one performs a search in 3D discussed in section 2.5. redshift space, the mathematical representation of the pro- jected coordinates: right ascension (α) and declination (δ), Insection2themethodusedforselectingthesampleof and the radial coordinate: redshift (z). It should be kept in extremely isolated galaxies (EIGs) is described, the sample mind that mapping in 3D redshift space (α, δ, z) can differ galaxies are listed, and their observed neighbourhoods are significantlyfromthetruemappinginrealspace.Forexam- discussed and compared to those of other isolated galaxy ple, although close in real space, two galaxies in a cluster surveys. Section 3 describes how cosmological simulations might have verydifferent redshifts dueto thecluster veloc- were used to analyse the EIG sample, and discusses thees- itydispersion andwillthereforeseem distantin3Dredshift timatedhalopropertiesandhistoriesoftheEIGs,aswellas space. theproperties of their neighbourhoods. In principle, independent distance measurements (not 2 www.sdss.org/dr7/algorithms/sdssUBVRITransform.html- 1 http://wise-obs.tau.ac.il/ #vega sun colors EIG - I. Sample and Simulation Analysis 3 based on redshift measurements) have the potential to im- The region downloaded from the NASA/IPAC Extragalac- proveuncertaintiescaused bypeculiarvelocities. Currently, tic Database4 (NED) for the Spring region was: 6h40m < independent distance measurements are available for more α < 17h20m, −8◦ < δ < 28◦, 1600 < cz < 7400kms−1. than 8000 galaxies in the local Universe. These were used The region downloaded from NED for the Autumn region by Tully et al. (2014) to find the limits of the “Laniakea” was: 21h00m < α < 04h00m, 12◦ < δ < 40◦, 1600 < cz < super-cluster in which we live. The accuracy of these dis- 7400kms−1. These regions allow searching for neighbours tance measurements for very close galaxies and for early at distances of up to 400kms−1 (equivalent to 4h−1Mpc) type galaxies (typically located in dense regions) is ∼10%, from thecandidate galaxies. whiletheaccuracyforthemoredistantlatetypes(typically Data for these regions were downloaded from NED on located in isolated regions) is ∼20% (Courtois et al. 2012). November 13, 2012. The NED object types included in the Whenaveraged,thesegivesufficientaccuracytomapcosmic databasewere:galaxies,galaxyclusters,galaxypairs,galaxy flows.However,fortestingtheisolation of individualgalax- triples,galaxygroups,andQSO.TheSpringdatabaseregion ies in the redshift range of this research, these independent included 14273 objects, while the Autumn database region distance measurements are not accurate enough. included 3956 objects. The ALFALFA database used was the“α.40 HI source In this work we have chosen to use the simple iso- catalogue” (α.40; Hayneset al. 2011). This catalogue cov- lation criterion described in Spector & Brosch (2010). A ers 40% of the final ALFALFA survey area (∼2800deg2) galaxy is considered an EIG and is included in the andcontains15855sources.Itincludespartsoftherequired sample if it has no known neighbours closer than Spring region: 07h30m < α < 16h30m, +04◦ < δ < +16◦ 300kms−1 in 3D redshift space, and if its redshift and +24◦ < δ < +28◦, and parts of the required Au- is in the range 2000 <cz<7000kms−1. This translates tumn region: 22h < α < 03h, +14◦ < δ < +16◦ and to not having any known neighbour within a distance of +24◦ < δ < +32◦. The database covers the required red- 3h−1Mpc∼=4.26Mpc.3 shift range (1600<cz<7400kms−1). The redshift range was limited to 7000kms−1 to have Foreach EIGfoundinthesesearchestheNEDredshift reasonablecompletenessofredshiftdataaroundeachgalaxy. measurement was verified by comparing it to ALFALFA, The reason for thelower limit of 2000kms−1 is to keep the SDSS DR10, and all sources quoted by NED. The redshift sky area that has to be searched around each galaxy rel- values adopted here for the EIGs were chosen based on the atively small, since at 2000kms−1 neighbours have to be following priority list: searched for as far as 8.6◦ away. (i) Ifan optically-derivedredshift valuewith uncertainty No magnitude, HI mass or size limit was used in the < 10kms−1 exists, it was adopted. If several such values selection of candidate neighbours. The use of such limits exist, the SDSS value of the latest available data release would have somewhat reduced the level of isolation of the (usually DR10) was preferred. sample (especially for the closer EIGs), and therefore was (ii) Otherwise, if a 21cm redshift value with uncertainty not preferred. Not using such limits, however, complicates < 10kms−1 exists, it was adopted. If several such values somewhat the analysis of the sample’s isolation level (de- exist, theALFALFA valuewas preferred. scribed in section 3). (iii) Otherwise, if reasonably accurate redshift values ex- ist (∆cz < 30kms−1), the most accurate of them was adopted. 2.2 Selection process (iv) If no reasonably accurate value exists (i.e., ∆cz > 30kms−1), the galaxy was deleted from the sample. The search criterion was applied to two sky regions, one in the spring sky (Spring) and the other in the autumn sky (Autumn),asdescribedinTable1.Theseparticularregions Optical redshift measurements were preferred because were selected since they are covered by theα.40 ALFALFA they are expected to be more accurate for estimating the catalogue(Hayneset al.2011).Bothregionsincludemainly transmittance of the Hα filters in theredshifted Hα line. high Galactic latitudes. The Spring region is almost fully TheEIGs’isolationwastestedagain,usingtheadopted covered by spectroscopic data in SDSS DR10 (Ahn et al. redshift. The neighbourhoods of all EIGs were then eval- 2014). uated using data downloaded from NED for spheres of TheSpringregioncontainspartsofthefollowing large- 10h−1Mpc around each EIG. scalestructures(orderedbyincreasingredshift):Virgoclus- ter, Virgo void, Coma void, Microscopium void, and the Comawall.TheAutumnregioncontainspartsofthefollow- 2.3 The Extremely Isolated Galaxies (EIGs) ing large-scale structures (ordered by increasing redshift): Only 14 galaxies were found to be isolated, according to Delphinus void, Taurus void, Eridanus void, Perseus Pisces thesearchcriterion statedabove,intheSpringregion.This supercluster, Pegasus void, and Pisces void (Fairall 1998). correspondstoafractionof 0.4+0.3 %ofthegalaxiesinthe To allow searching for neighbours near the edges of −0.2 Spring NED dataset. In the Autumn sky region, 6 galaxies thethese search regions, larger database regions were used. (cid:0) (cid:1) were found to be isolated according to the criterion. This corresopnds to 1.5+1.8 % of the galaxies in the Autumn −0.8 NED dataset. An additional Autumn galaxy, EIG 1a-04, 3 This criterion describes the most isolated subsample of EIGs (cid:0) (cid:1) studied here. Not all sample galaxies (EIGs) pass this isolation criterion(seesection2.3). 4 http://ned.ipac.caltech.edu/ 4 O. Spector and N. Brosch Table 1.Samplesearchregions α(J2000) δ (J2000) cz(cid:2)kms−1(cid:3) Volume(cid:2)h−3Mpc3(cid:3) Spring 7h30m–16h30m 4◦–16◦ 2000–7000 5.42·104 Autumn 22h00m–03h00m 24◦–28◦ 2000–7000 9.17·103 was added to the sample although it lies outside the search 3 region. M87 ThelargerfractionofEIGsintheAutumnregioncanbe " " 2 LEDA 166859 attributedtothefactthattheSpringregionisfullycovered 3 − EIG 1s−01 by SDSS (with spectroscopic data), while the Autumn re- c p 1 gionisnot.Itmayalsobeattributedtothedifferentcompo- M sition ofboth regions, wheretheSpringregion maycontain a significantly larger fraction of cluster and wall galaxies. 3h 0 TheuseoftheALFALFAunbiasedHIdatasignificantly ! improvedthequalityof thesample. Outof32 galaxies that / −1 n passed the criterion using NED data alone, 11 galaxies did ! not pass the criterion when tested with ALFALFA data g −2 o (seven in the Spring region, and four in the Autumn re- l gion). For the seven Spring region galaxies 13 neighbours −3 were found in ALFALFA, and for the four Autumn region 1 2 3 4 5 galaxies 10 neighbours were found. r/!h−1Mpc" Thegalaxiesstudiedhereweredividedtothreesubsam- ples: Figure1. Thenumberdensity,n,inasphereofvaryingradius, 1. GalaxiesthatpassedthecriterionusingbothNEDand r,aroundthreegalaxies:EIG1s-01,LEDA166859–afieldgalaxy, ALFALFA data. M87–clustergalaxy. 2. GalaxiesthatpassedthecriterionusingNEDdata,but did not pass using ALFALFA data (had neighbours closer than 3h−1Mpc in the ALFALFA database). The galaxies of the different subsamples are listed in Tables 2 through 7. The data for each galaxy include its 3. Galaxiesforwhichthedistancetotheclosestneighbour in NED’s data is 2 – 3h−1Mpc (regardless of the distance EIGname,thefirstnamelistedforitinNED,itsALFALFA name, and its coordinates in redshift space (α, δ, cz). to theclosest neighbour in ALFALFA’s data). Notesregarding specificEIGsarelisted inappendixA. Section A2of this appendix lists theobjects that were first Subsamples1and2arecomplete,inthesensethatthey foundtobeisolated,butwereeventuallynotincludedinthe contain all catalogued galaxies that passed their criteria in sample for thevarious reasons described there. thestudiedskyregions.Subsample3isfarfrom beingcom- plete. Itcontains only those galaxies that seemed to beiso- latedinthepresentorearliersearches,butwerelaterfound 2.4 Observed neighbourhoods to have neighbours in the range 2 – 3h−1Mpc (7 in the Spring region, and 2 in the Autumn region). These include First, an example of the huge difference between the en- galaxies for which neighbours were added to NED or AL- vironments of EIGs, field galaxies and cluster galaxies is FALFAinrecentyears,aswellasgalaxieswhichhadalow- illustrated in Figure 1. This figure shows the number den- accuracy redshift valuein NEDand for which usinga more sity of galaxies, n, around EIG1s-01, around atypicalfield accurateredshiftvalueyieldedcloserneighbours.Italsocon- galaxy (LEDA 166859) and around M87, a supergiant el- tainsagalaxy,EIG3s-06,whichwasfoundbysearchingthe liptical galaxy located near the centre of the Virgo cluster. ALFALFA data alone, but had neighbours in the range 2 – For each of these three galaxies n is shown as a function 3h−1Mpc in the NED dataset. of r, the radius of a sphere around the galaxy, for which The galaxies were named according to their subsample n was calculated. The number density shown in this figure and sky region, using thefollowing format: includesgalaxieswithknownredshiftsaswellasthecentral galaxyitself,andiscalculatedforredshiftspace,i.e.without EIG BR-XX compensating for peculiar velocities. EIG1s-01hasnoneighbourscloserthan4h−1Mpcbut where: has ten neighbours at a distance of 4 – 5h−1Mpc. As can beseeninFigure1,intherangeofcalculatedsphereradius B isthegalaxy’ssubsample(1,2or3,asdescribedabove); (1<r <5h−1Mpc) the neighbourhood density of EIG 1s- R is the sky region (“s” - Spring, “a” - Autumn); 01 is about one order of magnitude lower than that of the XX is theserial numberof thegalaxy in thesubsample. typical field galaxy (LEDA 166859) and about two orders of magnitude lower than that of the cluster galaxy (M87). So,forexample,objectEIG3s-06isthesixthgalaxyin This typical example indicates that the EIGs are extreme subsample 3 of the spring sky region. field galaxies, significantly more isolated than the average. EIG - I. Sample and Simulation Analysis 5 Table2.TheEIG-1ssubsample-Springregiongalaxieswithnoneighbourscloserthan3h−1MpcinbothNED andALFALFAdata Name NEDID ALFALFAID α δ cz (J2000) (J2000) (cid:2)kms−1(cid:3) EIG1s-01 SDSSJ075041.99+144717.3 HI075041.7+144741 07:50:42.0 +14:47:17 5399± 3 EIG1s-02 2MASXJ08061617+1249401 HI080614.1+125021 08:06:16.1 +12:49:41 5694± 2 EIG1s-03 UGC04655 HI085333.4+044710 08:53:32.7 +04:46:57 6189± 1 EIG1s-04 SDSSJ092131.91+112048.2 HI092131.3+112100 09:21:31.9 +11:20:48 5670± 7 EIG1s-05 AGC208312 HI102039.6+080914 10:20:39.6 +08:09:06 5336± 5 EIG1s-06 SDSSJ102352.85+062417.0 HI102352.7+062416 10:23:52.8 +06:24:17 5587± 3 EIG1s-07 SDSSJ110414.59+050736.6 HI110418.1+050703 11:04:14.6 +05:07:37 5269± 1 EIG1s-08 SDSSJ111624.13+054352.7 - 11:16:24.1 +05:43:53 4976± 1 EIG1s-09 SDSSJ112156.76+102955.3 HI112157.6+102948 11:21:56.8 +10:29:55 4453± 2 EIG1s-10 SDSSJ124011.52+154213.8 HI124009.5+154213 12:40:11.5 +15:42:14 3916± 1 EIG1s-11 VCC1889 - 12:41:46.1 +11:15:02 4725± 10 EIG1s-12 SDSSJ133156.93+133101.6 - 13:31:56.9 +13:31:02 4864± 1 EIG1s-13 SDSSJ151410.95+064449.0 - 15:14:10.9 +06:44:49 5427± 2 EIG1s-14 CGCG050-112 HI155029.2+042810 15:50:25.5 +04:28:35 6122± 17 Table 3. The EIG-1a subsample - Autumn region galaxies with no neighbours closer than 3h−1Mpc in both NEDandALFALFAdata Name NEDID ALFALFAID α δ cz (J2000) (J2000) (cid:2)kms−1(cid:3) EIG1a-01 2MASXJ00270759+2459072 HI002706.2+245912 00:27:07.6 +24:59:07 6378± 12 EIG1a-02 2MASXJ00563772+2418526 HI005632.5+241856 00:56:37.7 +24:18:53 6501± 18 EIG1a-03 AGC122211 HI023136.3+263250 02:31:36.8 +26:32:30 3691± 1 EIG1a-04 IC0238 - 02:35:22.7 +12:50:16 6008± 21 EIG1a-05 2MASXJ02535284+2630267 HI025352.1+263035 02:53:52.9 +26:30:27 6176± 3 EIG1a-06 AGESJ025917+244756 - 02:59:17.5 +24:48:43 4658± 3 EIG1a-07 AGC321304 HI220351.1+252659 22:03:51.1 +25:26:32 2692± 13 Next, specific neighbourhood data of each EIG are (8h<α<16h,9◦ <δ<11◦,3500<cz<7000kms−1)was listed. Table 8 summarizes information about the observed statistically analysed. The numberdensity of NEDgalaxies neighbourhoodoftheEIGsofsubsamplesEIG-1andEIG-2 inthisregionis0.065h3Mpc−3,whilethenumberdensityof (objectsthatpassedtheisolationcriterion).Thedistanceto ALFALFAgalaxiesinthisregionis0.039h3Mpc−3.Theav- the nearest known neighbour, d1, obtained separately from eragenumberofNEDneighbourstoadistanceof3h−1Mpc the NED and α.40 datasets, is listed. For ALFALFA, the aroundeachNEDgalaxyintheabovementionedregionwas numberof known neighboursup toa distance of 3h−1Mpc found to be 27.5±0.9 (were the uncertainty is statistical (neighbour count) is also listed (for NED it is zero by def- anddoes not includetheeffect of uncertaintiesin cz,which inition) along with the dataset coverage where “Full” indi- isexpectedtobeminor).TheaveragenumberofALFALFA catesthatthesphereofradius3h−1Mpcaroundthegalaxy neighbours to a distance of 3h−1Mpc around each NED is fully covered by the α.40 dataset and “Partial” indicates galaxyinthisregionwasfoundtobe14.3±0.5.Thisisequiv- that only a part of this sphere is covered. The table also alent to a number density of 0.243±0.008h3Mpc−3 NED lists the name of the void in which the EIG is located (or neighbours, and 0.126±0.001h3Mpc−3 ALFALFA neigh- thenamesoftwoadjacentvoids,incaseofanamelessvoid). bours, which means that the 3h−1Mpc neighbourhood of Thevoidnamesareasdefinedinthe“Atlasofnearbylarge- randomly selected NED galaxies is 3 – 4 times denser, on scale structures” of Fairall (1998). average, than theaverage density in the entire region. This Table 9 summarizes the observed neighbourhood data result is expected, given the clustered nature of galaxy dis- forsubsampleEIG-3(thosegalaxieswhichfellshortofpass- tribution in theUniverse. ingtheisolationcriterion,butwerestillstudied).Inaddition to the fields listed in Table 8, the table lists the neighbour TheALFALFAneighbourcounts(numberofALFALFA countsobtained from theNED dataset. neighbours to a distance of 3h−1Mpc listed in Table 8) of InvestigationofEIGscoordinatesinthe“atlasofnearby Spring galaxies that passed the criterion using NED data large-scale structures” (Fairall 1998) shows that most EIGs (subsamples EIG-1s and EIG-2s) and had full ALFALFA reside close to walls and filaments rather than in centres coverage were also statistically analysed. Their measured of voids. This may explain why there is no EIG with d1 > distributionfitswellaPoissondistributionwithanexpected 4.5h−1Mpc (as can be seen in Tables 8 and 9). value of 0.7+0.4 ALFALFA neighbours per EIG. Therefore, −0.3 theaveragenumberdensityofALFALFAneighbourswithin A part of the Spring sky region, in which the α.40 3h−1Mpc from these EIGs is 0.006+0.004h3Mpc−3. This −0.003 datasetcoversa3h−1Mpcradiusspherearoundeach point meansthat,onaverage,thenumberdensityaroundEIGs(1s 6 O. Spector and N. Brosch Table4.TheEIG-2ssubsample-Springregiongalaxieswithnoneighbourscloserthan3h−1MpcinNEDdata, butsomeinALFALFAdata Name NEDID ALFALFAID α δ cz (J2000) (J2000) (cid:2)kms−1(cid:3) EIG2s-01 SDSSJ075532.17+113316.7 - 07:55:32.2 +11:33:17 5842± 3 EIG2s-02 LSBCF704-V01 HI082452.4+091319 08:24:51.7 +09:13:29 6018± 2 EIG2s-04 SDSSJ124548.06+092029.0 HI124548.6+092025 12:45:48.0 +09:20:29 5740± 4 EIG2s-05 CGCG076-069 HI144932.9+134845 14:49:33.8 +13:48:25 5647± 5 EIG2s-06 CGCG050-028 HI153445.2+061813 15:34:46.1 +06:17:53 6313± 1 EIG2s-07 SDSSJ154627.10+083924.8 - 15:46:27.1 +08:39:25 3711± 1 EIG2s-08 SDSSJ161517.02+130133.0 - 16:15:17.0 +13:01:33 3650± 1 Table 5. The EIG-2a subsample - Autumn region galaxies with no neighbours closer than 3h−1Mpc in NED data, butsomeinALFALFAdata Name NEDID ALFALFAID α δ cz (J2000) (J2000) (cid:2)kms−1(cid:3) EIG2a-01 CGCG480-041 HI010617.0+253240 01:06:11.9 +25:33:06 6623± 9 EIG2a-02 FGC0362 HI025608.0+274210 02:56:08.6 +27:42:02 6473± 8 EIG2a-03 KUG2239+275 HI224205.5+274630 22:42:07.3 +27:46:11 6964± 2 EIG2a-04 AGC321226 HI225542.2+261830 22:55:44.8 +26:18:10 4372± 12 and2s) ofALFALFAgalaxies isonly 5%+3% ofthenumber 10% of thegalaxies would pass thecriterion (i.e. would not −2% density around random NED galaxies. beidentified as part of a group). TheVoidGalaxySurvey(VGS;Kreckelet al.2012)ap- pliedaredshiftspacecriterion,verydifferentfromthatused in this work. Kreckelet al. (2012) used SDSS DR7 data to 2.5 Comparison to other isolated galaxy samples reconstruct the density field from the spatial galaxy distri- The “Catalogue of Isolated Galaxies” (CIG; Karachentseva bution(intheredshift range900<cz<9000kms−1).Void 1973; Karachentsevaet al. 1986) was used as the basis regionswerethenidentifiedinthisdensityfieldusinga“wa- of the AMIGA international project (Verley et al. 2007; tershedfinderalgorithm”thatdoesnotassumeaparticular Ferna´ndezLorenzo et al. 2013). It defines a galaxy as iso- void size or shape. Sixty VGS sample galaxies were then lated if it has no neighbours with angular diameter in selected to be as close as possible to the centres of these the range 1a to 4a up to a projected angular distance of voids. 4 20a, where a is the angular diameter of the tested galaxy. Hirschmann et al. (2013) estimated that a sample based on Wetestedtheobservedneighbourhoodsofallgalaxiesof the AMIGA (Verley et al. 2007) 2D criterion will include thesefourcataloguesthatarewithintheEIGsearchregions a fraction of ∼18% false positives due to projection ef- (defined in Table 1). The process and datasets used were fects. For an angular diameter a = 20kpc, for example, identicaltothoseusedfortheEIGselection.Foreachofthe theKarachentseva(1973)criterioncorrespondstohavingno four catalogues, the probabilities of galaxies qualifying for neighbours with angular diameters in therange 5 to 80kpc each of the EIG subsamples (EIG-1, EIG-2 and EIG-3) are uptoadistanceof0.4Mpc.Comparedtothis,the4.26 Mpc listed in Table 10. distance criterion used in this work tests for a significantly As evident from the table, only a small fraction of the higher level of isolation. AMIGA, 2MIG and LOG catalogues may qualify as EIG-3 ThesameCIG2Dcriterionwasusedfortwoothercata- galaxies(galaxiesforwhichthedistancetotheclosestneigh- loguesofIGs.The“TwoMicronIsolatedGalaxy”catalogue bourinNED’sdatais2–3h−1Mpc).NoneoftheAMIGA, (2MIG) was created by Karachentseva et al. (2010) from 2MIG and LOG galaxies (within the regions defined in Ta- the“TwoMicronAll-SkySurvey”(2MASS;Skrutskieet al. ble 1) fitted the EIG-1 or EIG-2 criterion, and none are 2006)datausingtheselection criterionfromCIG.The“Lo- partofthesamplestudiedhere(theEIG-3subsampleisnot cal Orphan Galaxies” catalogue (LOG; Karachentsev et al. complete, i.e. does not includeall galaxies that pass its cri- 2011)wasproducedbycombininga3Dredshift-spacebased terion). However, there is one 2MIG galaxy (outside the criterion with the CIG 2D criterion. The LOG sample in- regions defined in Table 1), 2MIG 302, which is an EIG-1 cludes 520 IGs selected from a region defined by galactic galaxy(EIG1a-04).ItisnotincludedinthestatisticsofTa- latitudes |b| > 15◦ and with radial velocities smaller than ble 10 since it lies outside the search region (as mentioned 3500kms−1 relative to the centroid of the Local Group. in section 2.3). Their3Dcriterion confirmedthattheLOGsamplegalaxies Only six galaxies from the VGS catalogue are within arenotpartofgravitationally-boundgroupsthatwouldsur- the search regions of the EIG sample. Three of these pass vivetheHubbleexpansion.Itassumedthat2MASSK-band the criterion for the EIG-3 subsample, but are not part of luminosities are proportional to the total mass of galaxies. it. One galaxy, VGS 52, is an EIG-1 galaxy (EIG 1s-13). TheirK-bandluminosity-to-massrelationwastunedsothat Another galaxy, VGS 23, marginally qualifies as an EIG-1 EIG - I. Sample and Simulation Analysis 7 Table 6. The EIG-3s subsample - Spring region galaxies, for which the closest neighbour in NED data is at a distanceof2–3h−1Mpc Name NEDID ALFALFAID α δ cz (J2000) (J2000) (cid:2)kms−1(cid:3) EIG3s-01 SDSSJ104008.81+091628.5 HI104008.7+091607 10:40:08.8 +09:16:29 5420± 3 EIG3s-02 SDSSJ123814.44+100949.8 HI123813.8+100902 12:38:14.4 +10:09:50 5840± 11 EIG3s-03 CGCG043-046 HI125133.7+080242 12:51:33.5 +08:02:43 3620± 2 EIG3s-04 AGC225879 HI125829.0+121115 12:58:30.5 +12:11:22 4085± 5 EIG3s-05 CGCG047-124 HI143846.4+073700 14:38:46.8 +07:37:03 5527± 3 EIG3s-06 SDSSJ150544.49+111230.1 HI150544.8+111203 15:05:44.5 +11:12:30 3545± 2 EIG3s-07 SDSSJ151054.61+054314.7 HI151055.9+054325 15:10:54.6 +05:43:15 6436± 4 Table 7.TheEIG-3asubsample-Autumnregiongalaxies,forwhichtheclosestneighbour inNEDdataisata distanceof2–3h−1Mpc Name NEDID ALFALFAID α δ cz (J2000) (J2000) (cid:2)kms−1(cid:3) EIG3a-01 UGC12123 HI223752.8+251146 22:37:53.4 +25:11:36 4082± 2 EIG3a-02 2MASXJ01331560+2614556 HI013314.5+261508 01:33:15.6 +26:14:55 6952± 1 galaxy and was not included in the sample studied here. comparing these with random mock samples, properties of The sixth VGS galaxy does not qualify for any of the EIG theEIGs were statistically estimated. subsamples. Insection3.5propertiesofthedarkmatter(DM)haloes Distances to the closest neighbour listed in either the are analysed. These include the halo’s mass and whether NED or α.40 datasets, d1, were measured for all AMIGA, it is dominant in its immediate neighbourhood. In Section 2MIG, LOG and VGS galaxies in the EIG search regions. 3.6massaccretionhistories(MAHs)areanalysed.Next,the Basedonthese,theprobabilitydistributionfunction(PDF) neighbourhoodsoftheEIGsareanalysedintermsofgalaxy ofd1 wascalculatedforeachcatalogue(Figure2).Forcom- number density (section 3.7), halo number density (section parison, Figure 3 shows the PDF of d1 for each of the EIG 3.8)andDMmassdensity(section3.9).Finally,thetidalac- subsamples and for the EIG-1 and EIG-2 subsamples to- celerationexertedonEIGsbytheirneighbouringDMhaloes gether (all galaxies that passed theisolation criterion using is analysed (section 3.10). theNED dataset). It is evident from these figures that the d1 of AMIGA, Asdiscussedinsection2.1,theneighbourhoodmeasure- 2MIGandLOGgalaxiesistypicallysignificantlylowerthan mentislimitedbytwofactors:incompletenessoftheredshift the d1 of EIG galaxies. The average d1 of the tested galax- data(i.e.,redshiftdataisnotavailableforasignificantfrac- ies was 0.83h−1Mpc for AMIGA, 0.74h−1Mpc for 2MIG tion of the galaxies), and peculiar velocities that introduce and 1.19h−1Mpc for LOG, compared to 3.54h−1Mpc for an error in the distance measurement. Due to these, the the EIG-1 subsample, 1.58h−1Mpc for the EIG-2 subsam- actual neighbourhood of an individual EIG may differ sig- ple and 2.39h−1Mpc for the EIG-3 subsample. The av- nificantlyfromwhatitseemstobefromthedatainTables8 erage d1 of the EIG-1 and EIG-2 subsamples together is and9,orfromthenumberdensityfunctions,suchasshown 2.86h−1Mpc. The PDF for the VGS catalogue reaches in Figure 1. higher d1 values compared to the other three catalogues. However, as a sample, rather than individually, the The average d1 measured for thesix tested VGSgalaxies is probabilities of neighbourhood properties can be derived 2.39h−1Mpc. using cosmological simulations. These simulations describe mockuniverseswithdetailedinformationonDMhaloesand galaxies that reside in them. By applying the same search WeconcludethattheEIGsamplestudiedhereisindeed process used previously to select the EIG sample on these extremein its measurable isolation. The use of HI redshifts mock universes, mock EIG samples were created for which from ALFALFAprovedtobeakeyfactorinidentifyingthe the simulated properties were calculated. The distribution most extremely isolated subsample (EIG-1). of these properties in the mock EIG samples serves as an estimateoftheprobabilitydistributionfunctions(PDFs)of theseproperties in the real EIG sample. ThederivationprocessofthePDFsincludedthefollow- 3 PROPERTIES ESTIMATED USING ing steps: COSMOLOGICAL SIMULATIONS This section describes the analysis of cosmological simula- • Definingpointsofviewandskyregionsinthemockuni- tionsweperformed toestimatepropertiesoftheEIG-1and verses, simulating the observer and the sky region in which EIG-2subsamples.Two cosmological simulations wereused themock EIGs are searched for. forthisanalysis(describedinsection3.1)usingwhichmock • Estimating “completeness” functionsoftheNEDdata, EIG samples were created (sections 3.2 through 3.4). By which definefor each given observable magnitude, the frac- 8 O. Spector and N. Brosch Table 8.ObservedneighbourhoodofEIGs(subsamples1&2) NED ALFALFA a EIG d1 d1 Neighbour Coverage Void name h−1Mpc h−1Mpc count a name 1s-01 (cid:2) 4.19 (cid:3) (cid:2) 4.32 (cid:3) 0 Partial Canis Major 1s-02 3.22 3.95 0 Full Canis Major 1s-03 4.35 5.99 0 Partial Canis Major b 1s-04 3.02 3.01 0 Full Ursa Major – Canis Major b 1s-05 3.01 3.05 0 Full UrsaMajor – Hydra b 1s-06 3.11 3.11 0 Partial UrsaMajor – Hydra 1s-07 3.27 5.07 0 Partial UrsaMajor – Hydrab b 1s-08 4.02 4.18 0 Partial Leo – Hydra b 1s-09 3.07 3.30 0 Full Leo – Hydra 1s-10 3.02 3.22 0 Partial Coma 1s-11 3.91 4.04 0 Full Coma 1s-12 3.28 3.96 0 Partial Coma 1s-13 5.46 3.73 0 Partial Microscopium 1s-14 3.45 3.40 0 Partial Microscopium 1a-01 3.20 3.69 0 Partial Pisces 1a-02 3.28 3.33 0 Partial Pisces 1a-03 3.95 4.98 0 Partial Taurus 1a-04 3.35 3.21 0 Partial Pisces 1a-05 3.10 3.10 0 Partial Pisces 1a-06 4.49 4.49 0 Partial Taurus 1a-07 4.42 4.43 0 Partial Delphinus 2s-01 3.24 1.07 2 Full Canis Major 2s-02 3.61 0.66 1 Full Canis Major 2s-04 3.00 1.90 2 Full Coma 2s-05 3.32 0.94 3 Partial Microscopium 2s-06 3.31 1.57 3 Partial Microscopium b 2s-07 3.43 2.37 1 Full Virgo – Microscopium 2s-08 3.21 1.71 1 Partial Microscopium 2a-01 3.11 1.19 1 Partial Pisces 2a-02 3.30 1.63 4 Partial Pisces 2a-03 3.47 1.60 4 Full Pegasus 2a-04 3.14 2.74 1 Partial Pegasus a“Neighbour count”and“Coverage” refertoasphereofradius3h−1MpcaroundtheEIG. bAvoidbetweenthetwovoids,whosenamesarelisted. Table 9.ObservedneighbourhoodofEIGs(subsample3) NED ALFALFA a EIG d1 Neighbour d1 Neighbour coverage Void name h−1Mpc count a h−1Mpc count a name b 3s-01 (cid:2) 2.86 (cid:3) 1 (cid:2) 3.14 (cid:3) 0 Full Ursa Major – Hydra 3s-02 2.29 1 1.96 3 Full Coma 3s-03 2.87 1 0.97 4 Partial Coma 3s-04 2.98 1 2.98 1 Partial Coma 3s-05 2.83 1 5.12 0 Full Microscopium 3s-06 2.44 4 3.71 0 Partial Virgo 3s-07 2.73 1 3.82 0 Partial Microscopium 3a-01 2.40 1 2.39 1 Partial Pegasus 3a-02 2.37 1 2.37 1 Partial Pisces a“Neighbourcount” and“Coverage”refertoasphereofradius3h−1MpcaroundtheEIG. bAvoidbetween thetwovoids,whosenamesarelisted. EIG - I. Sample and Simulation Analysis 9 Table 10.OtherIGcatalogues -probabilityofqualificationasEIGs a Fraction [number] qualifying as Catalogue EIG-1 EIG-2 EIG-3 None AMIGA (0+8)% [0] (0+8)% [0] (7+12)% [3] (93+5 )% [40] −0 −0 −5 −12 2MIG (0+7)% [0] (0+7)% [0] (4+10)% [2] (96+3 )% [47] −0 −0 −3 −10 LOG (0+26)% [0] (0+26)% [0] (9+29)% [1] (91+7 )% [10] −0 −0 −7 −29 VGS (33+37)% [2] (0+39)% [0] (50+31)% [3] (17+40)% [1] −24 −0 −31 −14 aInsquarebracketsarethenumbersoftestedIGsofeachcatalogue thatqualify foreachEIGsubsample(or,under“None”,thatdonotqualifyforany subsample). 1 1 AMIGA 2MIG 1" 0.8 1" 0.8 − − c c p p M 0.6 M 0.6 h h ! ! 0.4 0.4 / / F F D D P 0.2 P 0.2 0 0 0 1 2 3 4 5 0 1 2 3 4 5 d /!h−1Mpc" d /!h−1Mpc" 1 1 1 1 LOG VGS 1" 0.8 1" 0.8 − − c c p p M 0.6 M 0.6 h h ! ! 0.4 0.4 / / F F D D P 0.2 P 0.2 0 0 0 1 2 3 4 5 0 1 2 3 4 5 d /!h−1Mpc" d /!h−1Mpc" 1 1 Figure 2. PDFofthedistancetotheclosestneighbour,d1,fortheAMIGA,2MIG,LOGandVGSisolatedgalaxycatalogues. tion of galaxies in the search region for which a redshift servable” datasets. These are divided to “Springmock EIG measurement was available in theNEDdataset. The“com- samples”, which simulate subsamples EIG-1s and EIG-2s pleteness”functionsoftheSpringandAutumnregionswere (together), and “Autumnmock EIG samples”, which simu- measured separately, since they are significantly different late subsamples EIG-1a and EIG-2a (together). (the Spring region is fully covered by SDSS, while the Au- tumnregion is not).The “completeness” functionswere es- • Creating “mock random samples” by randomly se- timated separately for each of thesimulations. lecting a thousand galaxies from each “mock observable” • Creating“mockobservable”datasets,eachincludingall dataset.These“mockrandomsamples”areusedasreference coordinates and simulation IDs of galaxies, randomly se- to the “mock EIG samples”, when evaluating their proper- lected using the “completeness” function. These “mock ob- ties’ PDFs. servable” datasets imitate the data that would have been availablefromNED,hadthe“mockuniverses”beenthereal • “Measuring” simulated properties of “mock EIG sam- Universe.Foreachsimulation,pointofview,andskyregion, ples” and “mock random samples” galaxies, and creating several such “mock observable” datasets were created. histograms that estimate the PDFs of these properties in • Creating “mock EIG samples” by applying the sample thereal UniverseEIGsample, and in real Universerandom selectionprocess(describedinsection2.2)onthe“mockob- galaxies. 10 O. Spector and N. Brosch 1 1 EIG−1 EIG−2 1" 0.8 1" 0.8 − − c c p p M 0.6 M 0.6 h h ! ! 0.4 0.4 / / F F D D P 0.2 P 0.2 0 0 0 1 2 3 4 5 0 1 2 3 4 5 d /!h−1Mpc" d /!h−1Mpc" 1 1 1 1 EIG−1 & EIG−2 EIG−3 1" 0.8 1" 0.8 − − c c p p M 0.6 M 0.6 h h ! ! 0.4 0.4 / / F F D D P 0.2 P 0.2 0 0 0 1 2 3 4 5 0 1 2 3 4 5 d /!h−1Mpc" d /!h−1Mpc" 1 1 Figure 3. PDFof the distance to the closest neighbour, d1, foreach subsample: “EIG-1”, “EIG-2” and“EIG-3”, and forthe “EIG-1 &EIG-2”subsamplestogether. Note that, at the time this analysis was performed no 3.1.1 Millennium II-SW7 (Mill2) cosmological simulation claimed to estimate theHI content ofgalaxies withreasonable accuracy5.Therefore, the“com- The Millennium II-SW7 simulation (Mill2; Guo et al. 2013) made publically available by the Virgo Consor- pleteness” functions were defined for the luminous content tium (Lemson & et al. 2006) is an updated version of the only, and not for HI content (21cm fluxes). The estimated Millennium-II simulation (Boylan-Kolchin et al. 2009) in PDFs discussed below, therefore, relate more closely to the whichthestructuregrowthinaΛcolddarkmatter(ΛCDM) EIG-1 and EIG-2 subsamples together (all galaxies that universewas scaled to parameters consistent with WMAP7 passedtheisolationcriterionusingtheNEDdataset),rather Bennett et al. (2011). The properties of galaxies were sim- than to each subsample separately. ulated using the semi-analytical model (SAM) described in As already shown in section 2.3, the use of ALFALFA Guo et al. (2013). significantly improves the quality of the sample. Therefore, Mill2 simulates a cube with edge length of the isolation properties of the EIG-1 subsamples are ex- 104.311h−1Mpc, and uses 21603 particles of mass pectedtoexhibitsignificantlymoreisolated-likePDFscom- pared to the PDFs estimated here (for EIG-1 and EIG-2 Mp = 8.5024 · 106h−1M⊙ each. It uses the following together). cosmological parameters: h = 0.704, ΩΛ = 0.728 (density parameterfordarkenergy),Ωm =0.272(densityparameter for matter), Ω = 0.045 (density parameter for baryonic b matter),ns =0.961(normalization ofthepowerspectrum), and σ8 = 0.807 (amplitude of mass density fluctuation in 3.1 Simulations 8h−1Mpcsphere at z=0). Two typesof halo classifications are defined in Mill2: The following twocosmological simulations were used inde- pendentlyfortheEIGshistoryandneighbourhoodanalysis. • Friendsof Friends(FOF)groups - definedwith b=0.2 (Boylan-Kolchin et al. 2009)6. 5 The only simulation that estimates HI content to date is Il- 6 Inthe FOFmethod (Davisetal.1985)all particlepairssepa- lustris(Vogelsbergeretal.2014).Itsdatabecamepubliconlyon ratedbylessthanafraction,b,ofthemeaninterparticlesepara- April2015(Nelsonetal.2015),thus Illustriswasnotconsidered tion(linkinglength)arefound.Eachdistinctsubsetofconnected fortheanalysispresentedhere. particlesisthendefinedasanFOFgroup.