Low-Mass X-ray Binaries and Globular Clusters Streamers and Arcs in NGC4278

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by  R. D'Abrusco

version January17,2014: fm
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
LOW-MASS X-RAY BINARIES AND GLOBULAR CLUSTERS
STREAMERS AND ARCS IN NGC4278.
R. D’Abrusco1, G. Fabbiano1 & N.J. Brassington2
versionJanuary 17, 2014: fm
ABSTRACT
Wereportsignificantinhomogeneitiesintheprojectedtwo-dimensional(2D)spatialdistributionsof
4
Low-Mass X-ray Binaries (LMXBs) and Globular Clusters (GCs) of the intermediate mass elliptical
1
galaxy NGC4278. In the inner region of NGC4278, a significant arc-like excess of LMXBs extending
0
south of the center at ∼ 50′′ in the western side of the galaxy can be associated to a similar over-
2
density of the spatial distribution of red GCs from Brassington et al. (2009). Using a recent catalog

n of GCs produced by Usher et al. (2013) and covering the whole field of the NGC4278 galaxy, we
a have discovered two other significant density structures outside the D isophote to the W and E of
J 25
the center of NGC4278, associated to an over-density and an under-density respectively. We discuss

5 the nature of these structures in the context of the similar spatial inhomogeneities discovered in
1 the LMXBs and GCs populations of NGC4649 and NGC4261, respectively. These features suggest

streamers from disrupted and accreted dwarf companions.
]
A
G 1. INTRODUCTION Here we report the case of a third galaxy, NGC4278,
. This is the third paper in a series analyzing the two- where we have been able to detect significant inhomo-
h geneityinboththe LMXBandtheGC2Ddistributions.
dimensional(2D)distributionofGCsystemsinelliptical
p NGC4278 is an elliptical galaxy at a distance of ∼ 16
- galaxies. We first reported the discovery of spiral-like
o features in the 2D distribution of the GCs of NGC4261 Mpc (Tonry et al. 2001), which was the subject of deep
r with a K-Nearest-Neighbor (KNN) technique supple- Chandra observations, resulting in a well-characterized
t catalog of LMXBs (Brassington et al. 2009, hereinafter
s mented by Monte Carlo simulations (D’Abrusco et al.
a 2013a). We then investigated the GC system of B09). This galaxyis an intermediate-size elliptical, with
[ stellar mass smaller than the giant elliptical NGC4261
NGC4649 (D’Abrusco et al. 2013b), detecting highly
and NGC4649 (see Borosonet al. 2011, for a compila-
1 significant spatial inhomogeneities. These results are
tionofproperties),whichwerethesubjectsofcompanion
v qualitatively in agreement with the discovery of com-
studies (D’Abrusco et al. 2013a,b). The spatial 2D dis-
7 plex GC systems in M87 (Strader et al. 2011) and
tribution of LMXBs in NGC4278 (see Figure 1 in B09)
4 NGC4365 (Blom et al. 2012) and reinforce the notion
suggests an arc-like feature and possible “streamers”,
7 that merging and accretion of satellites are important
motivating the present investigation.
3 in the evolution of galaxies, as predicted by the ΛCDM
. theory (e.g., Di Matteo et al. 2005). While the entire galaxy was covered with Chandra,
1 only the central region had been observed with HST
As is the case for GCs, low-mass X-ray binaries
0
at the time (Kundu & Whitmore 2001), resulting in
4 (LMXBs) can also be detected individually in ellip-
LMXB-GC identifications for the joint Chandra-HST
1 tical galaxies, with Chandra (see review, Fabbiano
coverage region. New HST observations of a field cov-
: 2006), providing another potential marker of galaxy
v ering the entire NGC4278 and the nearby NGC4283
evolution. These LMXBs may originate both from
i haverecentlybecomeavailable(Usher et al.2013)(here-
X the evolution of native stellar binary systems, or be
inafter U13), complementing the GC coverage at larger
formedbydynamicalinteractionsinGCs(Grindlay et al.
r radii. The positions of the U13 GCs suggest the pres-
a 1984; Verbunt & van den Heuvel 1995). In NGC4649,
ence of large-scale density structures. In order to get
in addition to the inhomogeneities of the GC system,
a comprehensive picture of the properties of the GCs
we have also seen a non-uniform distribution in the
populationinNGC4278,inthispaperweinvestigatethe
LMXBs (D’Abrusco et al. 2013b). Some LMXBs are as-
projectedspatialdistributionofbothcandidateU13GCs
sociatedwithGCs,andtherefore,notsurprisingly,follow
and the GC catalog from B09.
the same spatial distribution, but uncorrelated inhomo-
WediscussthedatasetsinSection2;wepresentthere-
geneities are also observed in LMXBs in the stellar field
sultsofouranalysisofthespatialdistributionofLMXBs
of NGC4649. These results have potential implications
andGCsinthecentralregionofthegalaxyNGC4278and
for our understanding of both LMXB formationand the
atlargerradiiinSection3,andexaminetheirastrophys-
generalevolutionoftheparentgalaxy. Itisthereforeim-
ical implications in Section 4. We draw our conclusions
portant to establish how frequent their occurrence may
in Section 5.
be.
2. DATA
1Harvard-SmithsonianAstrophysicalObservatory,60Garden The B09 catalog lists 236 X-ray sources with 0.3-8.0
Street, Cambridge,MA02138, USA
2Center for Astrophysics Research, University of Hertford- keVluminositiesrangingfrom3.5×1036to∼2×1040 ergs
shire,CollegeLaneCampus,Hatfield,Hertordshire,AL109AB, s−1, andalsoincludes a listofGCs fromaHST WFPC2
UK observation (Kundu & Whitmore 2001), partially cover-
2
Contaminants were excluded using a sample of spectro-
X−ray sources outside HST footprint
29.31 LLFMMieXXldBB LssM iinnX BRBsleude GGCCss 40 arcsecs sticoonpiacnadllybycovnifisurmaleidnsGpCecstitoondoeftearlmlGinCetchaendoipdtaimteaslwsehloesce-
half-light radii exceeded 10 pc (U13). The final cata-
29.30 lcoogntoafmUin1a3tiGonC,ccoanntdaiidnast7e1s,6wsohuicrhcesis.Uvi1r3tufaolulyndfrteheatfrtohme
GC candidates follow a bimodal color distribution, and
eg]29.29 tthherersehdoldangd−bzlu=e1c.l0a7s8se,slecaadninbgetosep37a0rarteeddGusCinsgaandco3l4o6r
d
ation [29.28 bGlCuesG(FCigsu(rseee3T, alebflte)1s)h.oTwhsethspaatttiahledriesdtrisbuubtpioonpuolfaUtio1n3
n
cli is more centrally concentrated than the blue subpopu-
e
D29.27 lation, as commonly observed in elliptical galaxies (see
e.g.Strader et al.2006).U13 alsocheckedthe azimuthal
29.26 N dpilsettreibcuotvieornagoefG(rC<s2in.5t′)h,efirnegdiionngstohfaNtGthCe4w27h8olweitphopcuomla--
tion of GCs and the two color classes separately are all
29.25 ctioonns.isFteignutrwei3th(raiguhnt)ifocormmlpyarreasndthoempaozsiimtiounthsaolfdthisetrGibCus-
W from B09 with those of the U13 common area. The two
185.06 185.05 185.04 185.03 185.02 185.01 185.00 distributions are similar on the regionof overlap,except
Right Ascension [deg]
for the core of the galaxy where the U13 catalog seems
Figure 1. LMXBsinNGC4278. GC-LMXBsinblueandredGCs to be affected by a larger incompleteness than the cat-
areindicatedwithblueandorangesymbols,whilefieldLMXBsare
alog used by B09. The overall agreement between the
plottedasblacktriangles. TheoutersolidlinerepresentstheChan-
drafootprint,andtheenclosedareashowstheHSTfootprintused two GC catalogs shows that hidden systematics are not
toextracttheGCsample. TheellipserepresentstheD25 isophote likelytoaffectourresults. AsfortheLMXBdata,anyin-
ofNGC4278fromdeVaucouleurs etal.(1991). Themajoraxisof completenesseffectwouldhaveonlyaradialdependence,
NGC4278isalsoshown(deVaucouleursetal.1991).
because ofthe radialworseningofthe Chandra PSFand
of the source crowding in the center. These effects will
Table 1 not affect any 2D inhomogeneity.
SamplesofLMXBsandGCsusedinthispaper.
3. ANALYSIS&RESULTS
Ntot NGCs Nfield
LMXBsa 236[171] 80{34/46} 42(114) We haveanalyzedthe 2Ddistributions of LMXBs and
Ntot Nred Nblue GCs, following D’Abrusco et al. (2013a), where a de-
GCsb 266 121 145 tailed explanation of the method used in this paper is
GCsc 716 360 356
given. In brief, for each sample of LMXBs and GCs, we
aIn square brackets, the number of LMXBs located inside the
have first generated density maps using the K-Nearest
D25 isophote of NGC4278. In curlybrackets, the number of GC-
LMXBsassociatedtoblueandredGCsrespectively. Inparenthe- Neighbor (KNN) method of Dressler (1980). This den-
sis,thenumberofLMXBslocatedoutsidetheHSTfootprint. sity is based on the local distribution of GCs, i.e. on the
bCatalog of GCs from B09, covering the central region of distance of the K-th closest GCs from the points where
NGC4278.
cCatalogofGCsfromU13,coveringthewholefieldofthegalaxy the density is estimated. We have evaluated the density
NGC4278. in each knot of a regular grid covering the region of the
sky where GCs and LMXBs are found. For a given K
ingthegalaxy(Table1). Figure1showsthe2Ddistribu- value, the point-density is estimated as:
tion of these X-ray sources, which suggests some spatial
features, in particular a region with a relative lack of K
LMXBs in the south between 25′′ and 40′′, followed by DK= (1)
A (d )
D K
an arc-like region richer in LMXBs. Thirteen out of a
total of 23 LMXBs located in the region of the arc are where K is the index of the nearest neighbor used to
associated with red GCs, with 5 LMXBs associated to calculate the density and A =π·d2 is the area of the
D K
blue GCs and 5 field LMXBs. The 2D distributions of circlewithradiusequaltothedistanceoftheK-thnear-
red (left) and blue (right) GCs separately are shown in est neighbor d . The uncertainty on the KNN density
K
Figure 2. The red GCs are centrally concentrated and scales with the square root of K, so that the relative
sparse in the N-W corner of the observed region. They fractional error is inversely proportional to the square
also seem to be approximately aligned along the major rootofK. Inotherwords,the fractionalaccuracyof the
axis of the galaxy, with the exclusion of an over-density methodincreaseswithincreasingK attheexpenseofthe
in the S-W corner of the footprint. The blue GCs, on spatial resolution. In this paper, we have reconstructed
the other hand, are more evenly distributed and do not the density maps of the samples considered using values
show any clear over- or under-density. of K ranging from 2 to 10. While the “optimal” value
U13usedfivearchivalHSTACSpointingstostudythe of K depends on the typical size (expressed as number
propertiesoftheGCsysteminNGC4278outtolargera- of members) of the spatial structures that we want to
dialdistances. ThesampleofGCcandidatewasselected highlight, this number cannot be determined a priori.
based on the g−z color, the z magnitude and the half- For this reason, we consider an interval of “reasonable”
lightradiiofthesourcesextractedfromtheHSTimages. K values that are roughly proportional to the average
3
29.31 20 arcsecs 29.31 20 arcsecs
●● ● ● ● ●
Declination [deg]29.2829.2929.30 N ●●●●●●●●● ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● ●●●●●●●●●●● ●● Declination [deg]29.2829.2929.30 N ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●
●●●● ● ●● ● ● ● ●●
29.27 ●●●●●●●●●●●●●● ●● ●● 29.27 ●●● ●●● ●●● ● ● ●●
W W
185.05 185.04 185.03 185.02 185.01 185.00 185.05 185.04 185.03 185.02 185.01 185.00
Right Ascension [deg] Right Ascension [deg]
Figure 2. Positions of red GCs (left) and blue GCs (right) in NGC4278. The ellipse represents the D25 isophote of NGC4278
from deVaucouleursetal. (1991). The outlines identify the footprint of the HST observations used by B09 to extract the catalog of
GCs. ThemajoraxisofNGC4278thegalaxyisalsoshown(deVaucouleursetal.1991).
Table 2
FractionsofsimulatedresidualmapsofthepopulationofGCsandLMXBswithnumberofextremepixels(i.e.,pixelswithdensityvalues
exceeding the90-thpercentileoftheobservedpixelresidualdistribution)largerthanthenumberofobservedextremepixelsinthe
residualmap. Valuesinparenthesisrefertothefractionofsimulateddensitymapswithatleastonegroupofcontiguous extremepixelsas
largeasthegroupsofcontiguous extremepixelsintheobservedresidualmaps. Thesefractionsweredeterminedbycountingthenumber
ofsimulateddensitymapswithatleastonegroupofcontiguous extremepixelsequalorlargerthanthegroupofcontiguous extreme
pixelsobservedover-densityregions.
Residuals
K=7 K=8 K=9
AllGCs(red+blue)a 3.3%(0.2%) 0%(0%) 0%(0%)
RedGCs 0.2%(0.3%) 0%(0%) 0%(0%)
BlueGCs 1.2%(0%) 0%(0%) 0%(0%)
AllLMXBs 12.3%(5.6%) 2.5%(0%) 1.2%(0%)
LMXBsinGCs 9.8%(2.1%) 1.8%(0.3%) 0.2%(0%)
FieldLMXBs 19.5%(4.2%) 1.4%(0.1%) 0%(0%)
AllGCs(red+blue)b 1.9%(0.1%) 0%(0%) 0%(0%)
RedGCs 7.8%(4.3%) 2.5%(1.2%) 0.4%(0%)
BlueGCs 9.3%(5.4%) 3.9%(2.0%) 0.1%(0.1%)
aGCdataset fromB09
bGCdataset fromU13
density of sources of the samples for which the density and K=9 respectively.
maps are reconstructed. Moreover, large values of K
would result in over- and under-density structures that
are smoothed over large areas and that have lost part of Table 3
the spatial information they carry. Significance ofthemainover-densitystructures oftheLMXBs
We produced residual maps, by subtracting from the (“A”,“B”,“C”)(Figure4,rightpanel),GCsfrom(B09)(“D”,
“E”,“F”)(Figure6,midandrightpanels)andU13GCs(“G”,
observed density maps a smooth distribution based on
“H”,“I”,“L”,“M”)(Figure7,lowerpanels)residualmaps. In
the observed radial number density profiles. Using the parenthesis,thesignificanceofthegroupsofpixelinthe(B09)
smoothdistributionasseedforMonteCarlosimulations, GCsresidualmapscorrespondingtotheover-densitiesdefinedin
wehavethenestimatedthe statisticalsignificanceofany the(B09)LMXBssignificancemap.
inhomogeneities apparent in the residual map. Given LMXBs AllGCs RedGCs BlueGCs
the relatively sparsedata we haveused 5000simulations A ∼8σ (∼0σ) (∼6σ) (∼0σ)
in each case. Table 2 shows the percentage of simulated B ∼5σ (∼1.5σ) (∼0σ) (∼2σ)
C >10σ (∼0σ) (∼0σ) (∼0σ)
distributionsofLMXBsandGCswith“extreme”number
D - - ∼7.5σ -
of pixels, i.e. with density values exceeding those of the E - - ∼2.5σ -
90% of pixels in the observeddensity map. The fraction F - - - ∼4σ
ofspatiallyclusteredsimulateddistributions ofGCsand G - ∼10σ >10σ ∼10σ
H - >6σ ∼10σ >5σ
LMXBsfromB09becomesnegligibleforK=8andK=9
I - ∼5σ >10σ ∼4σ
respectively, while the same fractions for all GCs and L - <1.5σ ∼2.7σ ∼0σ
red/blueU13GCsbecomeconsistentwithzeroforK=7 M - ∼4σ <1.5σ ∼3.2σ
4
● Red GCs 2 arcmin 29.31 Brassington et al. 2009 20 arcsecs
Declination [deg]29.2429.2629.2829.3029.3229.34 N●●Blu●●●●e●●● ●G●●●●●●C●●●●●●s●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● Declination [deg]29.2729.2829.2929.30 Usher et al. 2013
29.22 185.10 W 185.05 185.00 184.95 185.05 185.04 185.03 185.02 185.01 185.00
Right Ascension [deg] Right Ascension [deg]
Figure 3. Left: U13GCsinNGC4278. BlueandredGCsareindicatedwithblueandorangesymbols.Theoutersolidlinerepresentsthe
footprint of the HST ACS observations used to extract the U13 catalog, and the enclosed area shows the HST footprint used to extract
the GCs sample from B09. The ellipses represent the D25 isophotes of NGC4278 and NGC4283 from deVaucouleurs etal. (1991). The
majoraxisofNGC4278isalsoshown(deVaucouleurs etal.1991). Right: distributionoftheGCsextractedfromU13(solidsymbols)and
fromB09(opensymbols)inthecentralregionofNGC4278. TheblacksolidlinerepresentthefootprintoftheHSTACSobservationsused
inB09. TheD25 ellipticalisophoteandthemajoraxisofNGC4278arealsoshown.
−−
● −
29.31 LKM=9XB●s ● ● 40 arcsecs 29.31 LKM=9XBs −− 40 −arcsecs
● ● ●
Declination [deg]22229999....22237890 ●●● ●●● ●●●● ● ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● ● ●●●●●●● ●●●●●●●● Declination [deg]22229999....22237890 − −− −B−−−−−−−−||−−−−|||−−−−−−|||−−−−−−|||||−−−−||C−−−−A−−−−−−|−−−||−−−−−−−|||−−−−−−−||||−−−−−−−−||||||
N ● ● ● N
29.26 ● ● 29.26
● ● ●
● ●
●
29.25 ● 29.25
●
W W
185.06 185.05 185.04 185.03 185.02 185.01 185.00 185.06 185.05 185.04 185.03 185.02 185.01 185.00
Right Ascension [deg] Right Ascension [deg]
Figure 4. Left: K=9densitymapsofthesampleofLMXBsinNGC4278. Arbitraryisodensitycontoursshowthehigher-densityregions
ineachmap. Right: PositionsoftheK=9residualswithsignificancelargerthan1σ,2σ and3σ obtained fromtheresidualmapsderived
fromthedistributionofthewholecatalogofLMXBsinNGC4278. Allthenegativeresiduals(bluepixels)havesignificancebetween1and
2σ. Positive(orange) pixels>2σ areindicatedwithahorizontalline;>3σ withacross.
3.1. LMXBs and GCs in the central region quadrant, indicated as “A” in Figure 4, is ∼8σ signifi-
cant. We calculate the significance of a each single fea-
Here we discuss the results of the residual maps ob-
ture by counting the fraction of simulated distributions
tained using the B09LMXBs andGC samples. Figure 4
ofLMXBsorGCswithgroupsofspatiallyclusteredpix-
shows the results for the LMXB distribution. The den-
els with similar average significance. We found that the
sity map (left) shows that LMXBs are centrally concen-
radial “streamer” extending to the NW of the center of
trated, with one significant over-density located along
the galaxy, indicated as “B” in the Figure 4, has 5σ sig-
the southern major axis and extending west-ward. The
nificance. The other two apparent “streamers”point to-
existence of this over-density is confirmed by the pixel
wards E and N, but their statistical significance is low.
map of residuals with significance larger than 1σ (right,
Figure 4). In particular, the “arc” at R∼1′ in the SW Repeating the analysis for GC and field LMXBs (within
5
Figure 5. Positions of the>1σ residuals for field (left) and GCs-LMXBs (right) samples of LMXBs for K=9. Positive (orange) pixels
>2σ are indicated with a horizontal line;>3σ with a cross. In all plots the D25 elliptical isophote of NGC4278 and the footprints of
the Chandra (and HST, for the field-GC classes) observations used to extract the GCs and LMXBs catalogs respectively are shown for
reference.
the HST footprint) separately (Figure 5), we only find of the > 1σ significance pixels - mid panel in Figure 7)
an excess corresponding to the “arc” with the red GCs. are the large over-density associated to the region occu-
Distinct 2Dfeatures canalsobe seeninthe GC distri- pied by the nearby galaxy NGC4283 in the N-E corner
bution,athigheroverallstatisticalsignificance. Figure6 of the observed field (“G”), the significant over-density
showsthedensitymaps(upperpanels)andresidualmaps alongtheD ontheWsideofNGC4278(“H”),andthe
25
(lowerpanels)forall,redandblueGCsrespectively. The largeunder-densitylocatedontheEsideoftheNGC4278
residualmapofthe entire(red+blue)GCssampledoes galaxy, south of the NGC4283 galaxy (“I”). All of these
not displays significant inhomogeneities, while the resid- structures are clearly visible in the residual maps gener-
ual maps for red and blue GCs (obtained for for K=8) atedeither using the entire GC sample or the two colors
show clear localized over-densities. As already discussed sub-samples,withsignificancelargerthan8σ inallthree
in Section 2, the red GCs follow the directionof the ma- cases. A peculiar but less significant structure (< 1.5σ
jor axis of NGC4278, while blue GCs tend to cluster at forallGCs, ∼2.7σ forredGCs),locatedwithin the D
25
larger radial distances. Significant excesses of red GCs isophote of the NGC4278 galaxy and west of the cen-
can be seen in the mid-lower panel in Figure 6 in the ter of the galaxy (“L”), is visibile in the residual maps
area of the LMXB arc (>7.5σ, “D”), and, with a lower produced using all GCs and red GCs. This elongated
significance, just north of the region of the NW LMXB structure is similar to the “streamer”(“B”) identified in
streamer (> 2.5σ, “E”). The significance map for blue the residual map of the sample of LMXBs from the cat-
GCs (lower-right panel in Figure 6) shows one moder- alog presented by B09 (see Figure 4). While this struc-
atelysignificantover-density(∼4σ,“F”)overlappingthe ture is not significant per se, the fact that it is close to
position of LMXBs “streamer” indicated as “B” in Fig- a high-significance under-density makes it unlikely that
ure 4. A summary of the significances of the main over- it is due to a random density contrast. Another feature
density structures seen in the LMXBS and GCs residual that seems to point towards the center of NGC4278 is
maps (right plot in Figure 4 and lower mid and right clearly visible in the residual map generated from the
plots in Figure 6 respectively) is presented in Table 3. entire GCs sample (“M” in lower-left plot of Figure 7).
This over-density is significant at a ∼ 4σ level in the
residual map of all GCs and at a ∼ 3.2σ level in the
3.2. The GC distribution at larger radii
residual map generated by blue GCs shown in Figure 7
Using U13, we produced density and residual maps of (lower right).
the entire sample of GCs and of the two color classes Figure 8 shows the position of the pixels associatedto
obtained with the threshold g−z =1.078 for values of ≥ 1σ residuals for the red GCs (left) and the blue GCs
K ranging from 2 to 10 (see Section 3 for more details) (right) for K=9 in the region of the field covering the
and with N = 5000 simulations. The probability of a region of NGC4278 covered by the samples of GCs and
simulated map showing the same number of “extreme” LMXBsfromB09. Thesolidblackandgreenlinesrepre-
pixelsoftheobserveddensitymapbecomesnegligiblefor sentthecontoursoftheover-densityintheresidualmaps
K=7 and K=9 for all and red/blue GCs, respectively. of the LMXBs and GCs of the B09 paper. Even though
The residual maps for all, red and blue U13 GCs are the > 1σ pixels in the significance maps produced with
shown in Figure 7 (upper panels). The most significant U13 GCs are not as spatially clustered as in the maps
features ofthe residualmaps (clearlyshownin the maps
6
29.31 GCs 20 arcsecs 29.31 Red GCs 20 arcsecs 29.31 Blue GCs 20 arcsecs
Declination [deg]29.2829.2929.30 N K=●●●8●●●●●●●● ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● Declination [deg]29.2829.2929.30 N K=●●8●● ●●●●● ●● ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● ●●●● ●●●●●●●●●● ● ●● Declination [deg]29.2829.2929.30 N K=●8●●●●●● ●●●●● ●● ●● ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● ●●●●●●●●●●●●●●●●●
29.27 W ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● ● ●●● ●● 29.27 W ●●●●●●●●●●●● ●● ●● ●●●●● ●● ● ●● 29.27 W ●● ● ●●●● ● ●●●●●● ● ●●
185.05 185.04 185.03 185.02 185.01 185.00 185.05 185.04 185.03 185.02 185.01 185.00 185.05 185.04 185.03 185.02 185.01 185.00
Right Ascension [deg] Right Ascension [deg] Right Ascension [deg]
29.31 GCs 20 arcsecs 29.31 Red GCs 20 arcsecs 29.31 Blue GCs 20 arcsecs
K=8 K=8 K=8
29.30 29.30 29.30
F
− E − −| −|
Declination [deg]29.2829.29 −| − − Declination [deg]29.2829.29 −| −| D Declination [deg]29.2829.29 − −| −− − −| −
N N N − −|
− − − −|
29.27 29.27 − −| −−| −−| −−| − 29.27 −
− −| −| −| −
W W W
185.05 185.04 185.03 185.02 185.01 185.00 185.05 185.04 185.03 185.02 185.01 185.00 185.05 185.04 185.03 185.02 185.01 185.00
Right Ascension [deg] Right Ascension [deg] Right Ascension [deg]
Figure 6. Upperpanels: fromlefttoright,K=8densitymapsofall,redandblueGCsoftheNGC4278. Arbitraryisodensitycontours
show the higher-density regions. Lower panels: fromleft to right, K=8 residualmaps of all, redand blue GCs of the NGC4278 sample.
Pixels are color-coded according to the number of σ the pixel deviates from the average. Darker colors indicate larger residuals: blue,
negative; orange, positive. The small +,− and = signs within each pixel indicate positive, negative or null residuals respectively. The
footprintoftheHSTobservations usedtoextract thecatalog ofGCs andtheD25 ellipsesofNGC4278andtheiso-densitycontours from
theresidualmapsoftheLMXBsdistributionarealsoshown.
generated from the B09 GCs, they are mostly located luminous sources. These results may be questioned by
within the outlines of the structures from the residuals following deeper studies because of source variability
mapsofLMXBsandGCsfromB09(comparewithpixels (A. Zezas, work in preparation). In the case of both
occupied by over-densities in Figure 4 and Figure 6 for NGC4278andNGC4649thelistsofLMXBswerederived
LMXBs and GCs respectively). In more detail, a group from the analysis of several coadded deep observations
of positive residual pixels in the red GCs map are lo- taken over varied time spans, which would average over
cated in the external region of the arc feature (S-W of variability effects. Moreover, in NGC4278 a later snap-
the center of NGC4278). Another less significant struc- shot, not included in B09, was obtained to study long-
ture is approximatelyplacedalongthe N-E streamer. In term variability of luminous LMXBs (Brassington et al.
the residual maps generated with the blue GCs, a fairly 2012). The full data set confirms the spatial features
significant over-density (∼ 3σ) is located on the N tip discussed in this paper (see Figure 1 of Pellegrini et al.
of the streamerfeature, while few positive pixels overlap (2012)).
the innermost section of the S-W arc. Thearcandtheover-densityatlargeradiiareseenonly
in the 2D distribution of LMXBs associated with GCs.
4. DISCUSSION The 2D spatial distributions of red and blue GCs also
Our analysis demonstrates the presence of significant show significant features. In particular, within the cen-
inhomogeneity in the projected spatial distribution of tral region of the galaxy, an over-density in the red GC
LMXBsinNGC4278,includinganarctothesouthofthe distribution is coincident with the LMXB arc, and fea-
nucleus, a radial streamer, and an over-density at large turescanalsobe seenlooselycorrelatedwith the LMXB
radii. Thesefeaturesarereminiscentofthosereportedin streamer. At larger radii, apart from a significant over-
the elliptical galaxies NGC4261 (Zezas et al. 2003) and densityofGCsassociatedwiththepositionofthegalaxy
NGC4649(D’Abrusco et al.2013b). Thestudyofthe2D NGC4283, two main features dominate the spatial dis-
distribution of LMXBs in NGC4261 (Zezas et al. 2003) tribution of GCs: a significant under-density region lo-
was based on a relatively short single Chandra observa- cated E of NGC4278 and close to the D25 isophote of
tion,leadingtothedetectionofasmallnumberofhighly NGC4283, most evident in the red GCs map, and a sig-
7
GCs 2 arcmin Blue GCs 2 arcmin Red GCs 2 arcmin
Declination [deg]29.2429.2629.2829.3029.3229.34 N++K++−+++=−−−++++++++7−+++−++−−−+−++++−−+++++−++−−−−−−+++++++−−−−−−−−−−−+−+−−−++++−=+=++++−−−−−−−−+++−+−+−−−−−−−−+−=−+++++−−+−−−−−−−−+−−++−+−−−−++++=++=+++++=−−−−−−++++−+++++−−−−−++−−+++−−−++++−−−−−++−−+++−−−−++++−−−+++−+−=+=+++−−−−−−+−−+++−−−−−−−−++−−+=++++++++−−−−−−−−−−−−−++−−++++++−−−−=−+++−+−−−−−−+−+−−++−−−−−−−−+−−+=+++++++−−−−−−−−−−−−+−−+=−−++++++−−−−=+=+−−+−+−−−−++−−−−−−+−−+−+−==++++++−−−−++++++−+−+++++−+++++−++++++−−−−−−−+−−−+++++−−−−−−++−+−−−++−++=++−−−−−−−−−+−−−−++−−−−−−−−−−−−−−+−+−==−−−−−−−−−−−−−−++−−−−=−−−−−−−−−−++++−−−−−−−−−−−−−+−+++−+−−=−−−−−=−−−−−+−−−−+−−−−−−−−−−−−−−−+−−++−−−−++==++−−−+−−−−−+−−+−−+−−−+++++=−−+++++++−−−−+−+++++−−+−+++++−−−+−+++−−++=++−−−−−−−−+++−+++++−−++−===+=+−++−+=−−−−−−++−−+−−++−−−−−−++−−++−====++−−−−−−−+−++−−+−++−−−−−−++++−−=+=+−−−−−−−−−++++−−+−−−−−++==++−++++=−−−−−−−−++++−+−−+−−−++=+++−−−−++−−−−−+−+−−−−−++++−−−−−−−−+++===+−−−−−−−−−++++++−−+−−−−+−−+++++++−−−−−−++−−+++++++++−−−=−+++−++=−−−−+++−+−−−+−−−+−−−−−+++===++++−−−−−−−−−−−−−−−+++=+++−−−+=++−−−−+−−−−−−−−+−−−−−−−−===+−−−−−−−−−−−−−−−−=+−−−−−−−− Declination [deg]29.2429.2629.2829.3029.3229.34 N++K++++++=−++++++++++9−++++−+=−+++++++−−−−=+−++++−+−+++++++++−−−−−−−−+−+=+++++−+−−+++++−−−−−−−−−−+−−++++++++−−−+−−−++−++++−−−−−+−−++−−−++−=−+−++++++−−−++++++=−−−−−−−+++−−++++++++−−−−++++−++−+++−−−−−−−−−+++++−−−−+=+++++++++−−++++−−−−−−−−−−−−−−−=−=−++++++++−−−−+++−−−−−−−−−−−−−−+=+−−−−++++++−++−−+=−−−−−−−−−−−−−−++−−=−−−−++−++−−+++−−−−−−−−−−+−−−−−==−−−++−+−+−−−+−+−−−−−+−−−−=−−−++−−−−+===−−++++++−−−−++++++++====+−−−−−+++−−−−−++−−−−+−−−−−−+−+−−−+−−−−−−+++−=++++−++−−−−−−−−−−−−++++−+++−−−−−−−−++−−−−−−−−−−−−−−−+−+−−−−−−−++−−−−−−−++−−−−=−−−−−−−−−−−++=−+−−−−−−++−+−−++−−−+−−−−−−−−−−−−−+−−−++=−−−−−+++−−−+++++−−−−−−−−−−−−−−+++−=+++−−++−+−−+−−+−−−−−−−−−−−−−+++−+=−−=−−++−+−++++++−−−−−−+++=−+==+−++++++++++++−+++++−−−−−−++++−−+++−+++−=+−++++−=−++++−−−−−−+−−−−++++++−−+++++++−−+++−+−−−−−++−+=++−−−+++−−++−++++++−−++−−−−+−+++−=+++−−−−−=++++++++++++−−−−++−−−−++++++++++−−−−−+++++++−−−++++−+−−−−−+++++++++−+−−−++++−−−−+−−+−−−++=++++++−−−−−−+++++++++++=+−−−−−−++++++++−−+−−−+++−−−−−−−−−−+++==−−+−−−+++−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− Declination [deg]29.2429.2629.2829.3029.3229.34 N−−K−++−−−=−++−−−+−+++9−+−+++++−−+=++−−−−−++−++++−++++−−−+++−−+++++−−−−+++−−=++−−+++++−−−−−+++++−−−−−−−−−−−−++++++++−−−−++−−−−−+−+++++++−−−−−−++++++++−−−−−−−−−++++++−−−−−−−−−−+−−−−−+−−+−+++++++−−−+−−−−−+−−+−+++++++−−−−−−+++++−−−−−−−−+++=+++=+−−−−−−+++−−−−−−−−+++−−−+−+++−−−−−−=−−−+++++++−+−−−−+−−−=++++−++−−−−−+−−−++−+−−−+++++−−−−−++−+−−==−++++++−+−−−−−−−+−++−−−−−−−=+−+++=++−++−−−−−−++++−+−++++−−++−++−−+=−+++++++−−−−−++++++++−−+++−−−−−+=+++++++++−+++=++−−−−−−−−−−++++−−−−−==+−−−++−−−−−−−+−++−−−−−−−−−−−−−−−−+−−−−−−−−−−−−−+++−−−−−−−−+−−−−−−−−−+−====−−−+−−+++++−−−−−−−−+++−−−−=++−+−−−+−−−−−−−−++−−−−−−−−−+−++=−++=−−+−−−−−+−−−−−−−+++++−−−+−+++++++=+−−−++++−+−−++−−−+−++−−++−−++−−−−−+−++−−++−−−−+−++++=+−−−+−+−−−++=++++=+++−−−++−+−++−−−−−−−+−−+−−++−=−−−−−−+−−−−−−−+−−+−++++−−−−−−−+−−−+++++−−−+++−++++−−−−−−−−−+++−−−−+++=+−−−−−−−−−−−−−−+++−−−−−−−−+++−−−−−−−−−−−−+++++++−−−−+−−−−−−−++++++==+++−−−−=++−−−−−++−−−−−−++−−−−+++−−−−−−−−++++++++−−−−−−−−+++++++++−−−−−−+++−−−−−−−−−−+=++++++=−−−−−−−−−−−++++++−−−−−−−−+++==++−−−−−−−+−−−−−−−−−=++−−===−−=−−−−−−−−
29.22 185.10 W 185.05 185.00 184.95 29.22 185.10 W 185.05 185.00 184.95 29.22 185.10 W 185.05 185.00 184.95
Right Ascension [deg] Right Ascension [deg] Right Ascension [deg]
GCs 2 arcmin Red GCs Blue GCs 2 arcmin
Declination [deg]29.2829.3029.3229.34 K−|=−−−−−||||7−−−−−−||||−−−−−|||−−−|G−−−−−−−||||−−|M−−−−−||−|−−−−||− − −−−|−|−| −| −−−−−−|||−−−−−|||H Declination [deg]29.2829.3029.3229.34 K=−9−−−−||||−−−−−−−−−||||||−−−−−−−−−−−−−|||||G−−−−−−−−−−|||||−−−−−−|||−−−−−−|||−−−−−||−−−−−||||−−−−−−|||−−−−−−−−−||−−|L−| −| −−−−−||−−−|−−−H| Declination [deg]29.2829.3029.3229.34 K−=−−−−||9−−−−−−−||||−−−−−−−||||−−−−−||G−− −−M−− − −−−||−−|− −−−−−|||− −−|−|−−−−−|−−−−−||H
29.2429.26 N −−|−−| −− − −− −− 29.2429.26 N I −−−− − −−−−−|| 29.2429.26 N −−|−| − −−|− −−|
29.22 185.10 W 185.05 185.00 184.95 29.22 185.10 W 185.05 185.00 20 arcse1c8s4.95 29.22 185.10 W 185.05 185.00 184.95
Right Ascension [deg] Right Ascension [deg] Right Ascension [deg]
Figure 7. Upper panels: fromleftto right, residualmaps of all,red andblue GCs of NGC4278 fromthe U13 catalog, forK=7, K=9
andK=9respectively. Pixelsarecolor-codedaccordingtothenumberofσ eachpixeldeviates fromtheaverageofthesimulateddensity.
Darker colors indicate larger residuals: blue, negative; orange, positive. The small +,− and = signs within each pixel indicate positive,
negative or null residuals respectively. The footprint of the HST observations used to extract the catalog of GCs and the D25 ellipses of
NGC4278andNGC4283arealsoshown. Lowerpanels: sameasabove,withonlypixelsassociatedtopositiveandnegativeresidualslarger
than1σ shown.
nificant over-density on the W side of NGC4278, which 1985), NGC4649 hosts a low-luminosity nuclear ra-
is clearly visible in all residual maps. dio source (Dressel & Wilson 1985) and NGC4278 has
As discussed in our previous work (Zezas et al. 2003; a LINER nucleus, which is also detected in the ra-
Bonfini et al.2012;D’Abrusco et al.2013a,b),thesespa- dio (Younes et al. 2010; Terashima & Wilson 2003). Of
tial features are strongly suggestive of the effects of the three galaxies, NGC4649 is the only one associ-
galaxy growth by accretion of satellites and merging atedwith aprominentluminous extendedhothalo(e.g.,
with neighboring galaxies. These phenomena have been Humphrey et al. 2013).
observed in our Local Group, and are connected with These three galaxies all present some sign of
streams of GCs (e.g., Belokurov et al. 2006; Ibata et al. merging or accretion of external matter, other
1994;McConnachie et al.2009). GCsarelikelytobethe than our results: NGC4261 has clear boxy
individually detectable fossil remnants of these events isophotes (Nieto & Bender 1989), usually sugges-
in more distant galaxies (see e.g., the simulation of tive of merging (Kormendy & Bender 1996). Recent
Pen˜arrubia et al. 2009). Similarly, X-ray binaries are kinematical measurements (Arnold et al. 2013) suggest
alsoindividuallydetectabletracersoftheirparentstellar that NGC4649 might be a major dry merger remnant,
population (Fabbiano 2006). showing disk-like outer rotation, which suggests a mas-
The three elliptical galaxies we have investigated so sive lenticular galaxy (S0) progenitor. NGC4649 has a
far, present a range of properties. NGC4261 and nearneighbor,thespiralgalaxyNGC4647,withwhichit
NGC4649 are giant ellipticals with logLK=11.4 LK,⊙, is likely to be tidally interacting (de Grijs & Robertson
while NGC4278 is of intermediate stellar mass, with 2006). Both NGC4261 and NGC4649 are early-type
logLK =10.8 LK,⊙ (de Vaucouleurs et al. 1991); all of SuperNova (SN) Ia host galaxies. SN 1939B, located
themhaveoldstellarpopulations,withagesrangingfrom close to the outer side of the large-scale arc struc-
12 to 16.3 Gy (see compilation in Borosonet al. 2011). ture of GCs discussed by (D’Abrusco et al. 2013a)
While NGC4649 is in Virgo, NGC4261 and NGC4278 in the S-W quadrant of NGC4261, was discovered
are in small groups (see Garcia 1993). NGC4261 hosts by Zwicky & Minkowsky (1939) and was later classified
a luminous radio AGN (3C270, Birkinshaw & Davies as type Ia SN. Type Ia SN 2004W (Moore et al. 2004)
8
29.31 Red GCs LGMCXs B(Bs r(aBsrsainssgitnognt oent aelt. a2l0. 0290)09) 29.31 Blue GCs LGMCXs B(Bs r(aBsrsainssgitnognt oent aelt. a2l0. 0290)09)
K=9 K=9
29.30 29.30
− − − −
| |
− − − − −
| |
g] g]
ation [de29.29 −| −| −| −−| ation [de29.29 −| − −−
n n
cli cli
De −De
29.28 29.28 −| − −|
N N −| −|
−
29.27 29.27 −
W W
20 arcsecs 20 arcsecs
− −
185.05 185.04 185.03 185.02 185.01 185.00 185.05 185.04 185.03 185.02 185.01 185.00
Right Ascension [deg] Right Ascension [deg]
Figure 8. Residual maps for red GCs (left) and blue GCs (right) of the U13 catalog for K = 9, zoomed into the region observed
by Kundu&Whitmore (2001) and used to extract the catalog of GCs used in B09. The D25 isophote of NGC4278 and the major axis
from are shown. The thick green and blue lines show the over-densities in the residual maps derived from the distributions of catalog of
LMXBsandGCsdiscussedinB09.
was located 51.6′′ W and 78.7′′ S of the nucleus of sult ofconduction heating of the incoming materialby a
NGC4649, where a low significance over-density of morediffuse hotter X-rayemitting ISM (Pellegrini2011,
GCs has been determined (see right plot in Figure 3 and references therein). However,the inhomogeneity we
of D’Abrusco et al. (2013b)). The presence of type Ia havediscoveredtellsadifferent,perhapscomplementary,
SNs in these two galaxies hints at intermediate age story. Given the old stellar age and the regular appear-
stellar population that could be linked to past merger ance of the stellar spheroid, any LMXBs originating for
and/or interaction events. NGC4278 is relatively more theevolutionofnativebinarysystemsinthestellarfield,
isolated, although also has a near neighbor, NGC4283. ought to be distributed smoothly, closely following the
Moreover, it has a complex multi-phase ISM (see stellar surface brightness. If the GC populations them-
Pellegrini et al. 2012). selveswerealsoformedearlyinthehistoryofNGC4278,
NGC4278hasalargemassiveHIring,extendingfor37 thesamewouldholdforthedistributionofGCsandtheir
kpc (Raimond et al. 1981), which may be slowly accret- associated LMXBs. While this is true overall in the ra-
ing(Morganti et al.2006). Thisdiskextendsdownto∼1 dial distribution of LMXB number density (see Figure
kpc, where a ionized rotating disk is found (Sarzi et al. 4 of B09), our results clearly show that there are local
2006). Morganti et al. (2006) show that the HI ring in perturbations to this picture. Therefore some of the ac-
NGC4278 has a complex structure, with two faint tail- cretion events in NGC4278 may have involved the dis-
like structures at large radii, pointing towards S-W and ruption of dwarf companions or perhaps result in some
N-E of the center of the galaxy respectively (see Fig- localized star formation, that while not visible against
ure 1 in Morganti et al. (2006)). While both HI tails the preponderantold stellar populationmay be revealed
are more spatially extended than the over-densities in by the LMXB and GC inhomogeneities.
the GCs and LMXBs distributions discussed in this pa-
per, we notice that the GCs over-density H (lower pan- 5. SUMMARYANDCONCLUSIONS
els of Figure 7) overlaps the base on the S-W HI tail-
We have discovered statistically significant spatial in-
like structure. Morganti et al. (2006) discuss the origin
homogeneities in the 2D distributions of LMXBs and
of the neutral gas in early-type galaxies, arguing that
GCs in the intermediate mass elliptical NGC4278. In
while merger or accretion events are thought to be the
particular,inthe innerregionofthegalaxywedetectan
origin of some of the structures observed in the spatial
arc-like feature in the LMXB distribution, which is also
distribution of HI, the lack of perturbation in the stellar
reflected in a local over-densityof the 2D distribution of
population of some galaxies suggests that cold accretion
redGCs. Atlargerradii,thedistributionofGCsisdom-
from the Inter-Galactic Matter (IGM) could also be re-
inatedbyaverysignificantover-densitylikelyassociated
sponsible for the presence of neutral gas. The HI disk
to the nearby galaxy NGC4283. Two other significant
in NGC4278 is co-located with a disk of X-ray emission
densitystructuresarevisibleoutside ofthe D isophote
and with complex and irregular dusty filaments, which 25
to the W andE of the center of NGC4278,associatedto
may be streaming towards the nucleus (Pellegrini 2011;
an over-density and an under-density respectively.
Lauer et al.2005;Tang et al.2011). Thesefeatureshave
These features are suggestive of streamers from
also been interpreted as due to slow accretion of the
disrupted and accreted dwarf companions, of which
HI gas, unaccompanied by star formation, given the old
LMXBs and GCs may be the fossil remnants.
stellar population age; the X-ray disk may be the re-
NGC4278 is the third galaxy where we have de-
9
tected this type of feature. Similarly for two pre- DiMatteo,T.,Springel,V.,&Hernquist,L.2005,Nature,433,
vious cases, NGC4261 (D’Abrusco et al. 2013a) and 604
NGC4649 (D’Abrusco et al. 2013b), the availability of Dressel,L.L.,&Wilson,A.S.1985,ApJ,291,668
Dressler,A.1980,ApJ,236,351
observations has provided us with the ability to charac-
Fabbiano,G.2006, ARA&A,44,323
terizethewholeGCpopulationofthegalaxyandthereby Fabbiano,G.,Brassington,N.J.,Lentati,L.,etal.2010, ApJ,
identify the large-scale disturbances with the system. 725,1824
Our exploratorywork begs the question of how frequent Garcia,A.M.1993,A&AS,100,47
these features may be. If they are common, they may Grindlay,J.E.,Hertz,P.,Steiner,J.E.,Murray,S.S.,&
Lightman,A.P.1984, ApJ,282,L13
revealed widespread continuing galaxy evolution by ac-
Humphrey,P.J.,Buote, D.A.,Brighenti,F.,Gebhardt, K.,&
cretion, in agreementwith the predictions of the ΛCDM Mathews,W.G.2013,MNRAS,430,1516
theory (Di Matteo et al. 2005). We have demonstrated Ibata,R.A.,Gilmore,G.,&Irwin,M.J.1994,Nature,370,194
that the study of the 2D distributions of GCs systems Khochfar,S.,Emsellem,E.,Serra,P.,etal.2011, MNRAS,417,
(and LMXBs, when available) is a valuable tool in these 845
Kormendy,J.,&Bender,R.1996, ApJ,464,L119
investigations. The nextstep willbe to obtaingoodsys-
Kundu,A.,&Whitmore,B.C.2001, AJ,121,2950
tematic opticalcoverageofa wellchosensample ofellip- Lauer,T.R.,Faber,S.M.,Gebhardt, K.,etal.2005,AJ,129,
tical galaxies. 2138
McConnachie,A.W.,Irwin,M.J.,Ibata, R.A.,etal.2009,
Nature,461,66
WethankJ.StraderandA.Zezasforusefuldiscussions Mineo,S.,Fabbiano,G.,D’Abrusco,R.,etal.2013,
and comments that have helped to improve the paper. arXiv:1306.6490
Moore,M.,Li,W.,Filippenko,A.V.,Chornock,R.,&Foley,
This work was partially supported by the Chandra X-
R.J.2004,IAUCirc.,8286,2
rayCenter(CXC),whichisoperatedbytheSmithsonian
Morganti,R.,deZeeuw,P.T.,Oosterloo,T.A.,etal.2006,
AstrophysicalObservatory(SAO)under NASA contract MNRAS,371,157
NAS8-03060. Nieto,J.-L.,&Bender,R.1989,A&A,215,266
Pellegrini,S.2011,ApJ,738,57
Pellegrini,S.,Wang,J.,Fabbiano, G.,etal.2012,ApJ,758,94
REFERENCES
Pen˜arrubia,J.,Walker,M.G.,&Gilmore,G.2009,MNRAS,399,
1275
Arnold,J.A.,Romanowsky,A.J.,Brodie,J.P.,etal.2013, Raimond,E.,Faber,S.M.,Gallagher,J.S.,III,&Knapp,G.R.
arXiv:1310.2607 1981,ApJ,246,708
Belokurov,V.,Zucker,D.B.,Evans,N.W.,etal.2006,ApJ,647, Sarzi,M.,Falco´n-Barroso,J.,Davies,R.L.,etal.2006,MNRAS,
L111 366,1151
Birkinshaw,M.,&Davies,R.L.1985,ApJ,291,32 Strader,J.,Brodie,J.P.,Spitler,L.,&Beasley,M.A.2006, AJ,
Blom,C.,Forbes,D.A.,Brodie,J.P.,etal.2012,MNRAS,426, 132,2333
1959 Strader,J.,Romanowsky, A.J.,Brodie,J.P.,etal.2011,ApJS,
Bonfini,P.,Zezas,A.,Birkinshaw,M.,etal.2012,MNRAS,421, 197,33
2872 Tang,Y.,Gu,Q.,Zhang,S.,&Tang,B.2011,MNRAS,414,1827
Boroson,B.,Kim,D.-W.,&Fabbiano,G.2011,ApJ,729,12 Terashima,Y.,&Wilson,A.S.2003,ApJ,583,145
Brassington,N.J.,Fabbiano, G.,Kim,D.-W.,etal.2009,ApJS, Tonry,J.L.,Dressler,A.,Blakeslee,J.P.,etal.2001,ApJ,546,
181,605 681
Brassington,N.J.,Fabbiano, G.,Zezas,A.,etal.2012,ApJ,755, Verbunt,F.,&vandenHeuvel,E.P.J.1995,X-rayBinaries,457
162 Younes,G.,Porquet,D.,Sabra,B.,etal.2010,A&A,517,A33
D’Abrusco,R.,Fabbiano,G.,Strader,J.,etal.2013,ApJ,773,87 Usher,C.,Forbes,D.A.,Spitler,L.R.,etal.2013,MNRAS,2309
D’Abrusco,R.,Fabbiano, G.,etal.,sub.toApJ Zezas,A.,Hernquist,L.,Fabbiano, G.,&Miller,J.2003,ApJ,
deVaucouleurs,G.,deVaucouleurs,A.,Corwin,H.G.,Jr.,etal. 599,L73
1991, S&T,82,621 Zwicky,F.&Minkowki,R.1939,IAUCirc.,774,1
deGrijs,R.,&Robertson,A.R.I.2006,A&A,460,493