NGC 2276: a remarkable galaxy with a large number of ULXs

2.8 MB·English

by  Anna Wolter

Mon.Not.R.Astron.Soc.000,1–12(2015) Printed12January2015 (MNLaTEXstylefilev2.2)
NGC 2276: a remarkable galaxy with a large number of
ULXs
Anna Wolter,1(cid:63) Paolo Esposito,2,3 Michela Mapelli,4 Fabio Pizzolato5 and
5 Emanuele Ripamonti6
1
0 1Osservatorio Astronomico di Brera, INAF, via Brera 28, I-20121 Milano, Italy
2 2IASF–Milano, INAF, via E. Bassini 15, I-20133 Milano, Italy
3Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
n 4Osservatorio Astronomico di Padova, INAF, vicolo dell’Osservatorio 5, I-35122 Padova, Italy
a 5Dipartimento di Fisica, Universita` degli Studi di Milano, via G. Celoria 16, I-20133 Milano, Italy
J
6Dipartimento di Fisica e Astronomia ‘Galileo Galilei’, Universit`a degli Studi di Padova,

8 vicolo dell’Osservatorio 3, I-35122 Padova, Italy

]
E
Accepted2015January7.Received2015January7;inoriginalform2014September6
H
.
h
p ABSTRACT
- The starbusting, nearby (D =32.9 Mpc) spiral (Sc) galaxy NGC2276 belongs to the
o sparsegroupdominatedbytheellipticalgalaxyNGC2300.NGC2276isaremarkable
r
t galaxy,asitdisplaysadisturbedmorphologyatmanywavelengths.Thisispossiblydue
s to gravitational interaction with the central elliptical galaxy of the group. Previous
a
ROSAT and XMM–Newton observations resulted in the detection of extended hot
[
gas emission and of a single very bright (∼1041 erg s−1) ultraluminous X-ray source

1 (ULX) candidate. Here we report on a study of the X-ray sources of NGC2276 based
v
on Chandra data taken in 2004. Chandra was able to resolve 16 sources, 8 of which
4
are ULXs, and to reveal that the previous ULX candidate is actually composed of a
9
fewdistinctobjects.WeconstructtheluminosityfunctionofNGC2276,whichcanbe
9
interpretedasdominatedbyhighmassX-raybinaries,andestimatethestarformation
1
0 rate (SFR) to be ∼5–15 M(cid:12) yr−1, consistent with the values derived from optical
. andinfraredobservations.Bymeansofnumericalsimulations,weshowthatbothram
1
pressureandviscoustransfereffectsarenecessarytoproducethedistortedmorphology
0
andthehighSFRobservedinNGC2276,whiletidalinteractionhaveamarginaleffect.
5
1 Key words: galaxies: individual: NGC2276 – galaxies: star formation – X-rays:
:
v binaries – X-rays: galaxies – methods: numerical.
i
X
r
a
1 INTRODUCTION in fact revealed an extended ((cid:39)0.2 Mpc), unusually dense
((cid:39) 5.3×10−4 cm−3), hot ((cid:39)0.9 keV) and relatively metal
NGC2276 is an Sc galaxy belonging to the poor group
poor((cid:39)0.06Z(cid:12))intragroupgashalo(Mulchaeyetal.1993;
of galaxies NGC2300, which is composed of four or five
Davisetal.1996).NGC2276isundergoingstarburstactiv-
galaxies1 and dominated by the namesake elliptical galaxy.
ity. A number of Hii regions were identified in the galaxy,
Here and throughout the paper we assume a distance2 to
spread throughout its volume (Hodge & Kennicutt 1983;
NGC2276 of 32.9 Mpc and rescale published values to this
Davisetal.1997),andmanysupernovaehavebeenobserved
distance when necessary. The NGC2300 group is the first
inthelastfiftyyears(e.g.Iskudaryan&Shakhbazyan1967;
in which an X-ray emitting intragroup medium (IGM) was
Barbon, Cappellaro & Turatto 1989, Treffers et al. 1993,
observed(Mulchaeyetal.1993).ObservationswithROSAT
Dimai, Migliardi & Manzini 2005). The star formation rate
(SFR)wasestimatedbyKennicutt(1983)tobe9.5M(cid:12)yr−1
basedonthehighHαluminosity,andvaluesbetween5and
(cid:63) E-mail:[email protected] 15 M(cid:12) yr−1 can be found in the literature from different
1 Apart from NGC2300 and NGC2276, the other members are
measurements (mostly Hα and infrared observations; e.g.
NGC2268andIC455(Huchra&Geller1982).UCG03670,at16(cid:48)
Kennicutt Jr., Tamblyn & Congdon 1994, Sanders et al.
(∼0.15Mpc)fromNGC2300,islikelyanothergroupmember.
2 From the NASA/IPAC Extragalactic Database 2003; James et al. 2004).
(NED) http://ned.ipac.caltech.edu/ and adopting H0 = TheasymmetricshapeofNGC2276,witha‘bow-shock-
73kms−1 Mpc−1 like’ structure along its western edge and a ‘tail’ of gas ex-
(cid:13)c 2015RAS
2 A. Wolter et al.
tendingtowardseast,andtheunusuallyintensestarforma- The Chandra data allowed Rasmussen et al. (2006) to
tion, especially along the western edge, (the source falls in resolvedetailsoftheX-raymorphologyofNGC2276,which,
theupperrangeofthedistributionofstar-forminggalaxies; asseenatotherwavelengths,ischaracterisedbyashock-like
seee.g.Noeskeetal.2007forthescatter-plotsofgalaxiesin featurealongthewesternedgeandalowsurfacebrightness
the Aegis field) have attracted considerable attention but, tail extending to the east. Spatially resolved spectroscopy
while it is quite clear that these features are due to inter- shows that the data are consistent with the disc gas being
actionwiththeenvironment,noconsensusonthedominant pressurized at the leading western edge of NGC2276, due
physical process involved has yet been reached. to the galaxy moving supersonically through the IGM, at
Since NGC2300 and NGC2276 are relatively close to a velocity of ≈900 km s−1. Rasmussen et al. (2006) esti-
eachother(7(cid:48),about70kpc),aprimecandidateisthetidal mated that the diffuse hot gas in the disc of NGC2276 has
interaction between the two galaxies (Gruendl et al. 1993; a temperature of kT ∼ 0.3 keV and an X-ray luminosity
Davis et al. 1996, 1997; Elmegreen et al. 1991). Other pro- of L(0.3−2.0keV) =1.9×1040d2 =1.5×1040d2 erg s−1,
X 36.8 32.9
cesses involved in shaping NGC2276 are the viscous and withaninferredresidualcomponentofunresolvedX-raybi-
the ram-pressure stripping of the galactic gas in the IGM naries of about 10–15 per cent of the total flux. The IGM,
(Nulsen1982;Rasmussen,Ponman&Mulchaey2006).Ras- as observed by Chandra, has a profile consistent with the
mussen et al. (2006) questioned the importance of the tidal previous ROSAT measurements (Davis et al. 1996), a tem-
interactions and argued that turbulent viscous and ram- perature of kT ≈ 0.9 keV and Z = 0.17 Z(cid:12) (Rasmussen
pressurestripping,rampressurecompressionofthediscgas etal.2006).Thismetallicityiswellmatched,givenalsothe
and starburst outflows are more likely the main elements different method used, to the one used above.
influencing the shape and SFR of NGC2276. InWolteretal.(2011),wereportedontheseveralnew
Using an XMM–Newton observation carried out in ULXs observed by Chandra in NGC2276 and on the fact
2001, Davis & Mushotzky (2004) identified a bright X- that the bright ULX candidate XMMUJ072649.2+854555
raysource(XMMUJ072649.2+854555)atthewesternedge actually consists of a number of distinct sources (see also
of the galaxy with an ultraluminous X-ray source (ULX: Suttonetal.2012).Inthiswork,wegivemoredetailsonthe
non-nuclear sources with X-ray luminosities in excess of point sources, presenting the whole Chandra data analysis,
L = 1039erg s−1 ; see e.g. Feng & Soria 2011 for a re- investigate the main mechanisms at work in shaping the
X
view) candidate. The X-ray luminosity of this object in galaxybymeansofnumericalsimulations,anddiscusstheir
the 0.5–10 keV band was L0.5−10keV = 1.1×1041d2 = influence on the X-ray source population.
X 45.7
5.7×1040d2 erg s−1 (where d stands for the distance
32.9 N
in units of N Mpc), making it one of the most luminous
known in its class (e.g. Sutton et al. 2012). They also ob- 2 ANALYSIS OF CHANDRA DATA
served that the nuclear source, undetected in the XMM–
The Chandra observation (Obs. ID 4968, see Rasmussen
Newton data,wasmuchdimmer(byafactorof∼10atleast)
et al. 2006 for further details) was performed on 2004 June
than the ROSAT/HRI nuclear source seen by Davis et al.
(1997) eight years earlier at L0.5−2keV ∼ 2×1040d2 = 23 with the Advanced CCD Imaging Spectrometer (ACIS;
1040d2 erg s−1. X 45.7 Garmire et al. 2003), and lasted about 45.8 ks. The ACIS
32.9
was operated in the standard timed exposure full-frame
ULXsaremainlyfoundingalaxieswithhighSFRs,and
mode,withthe‘veryfaint’telemetryformat,andNGC2276
indeed most might be assumed to be the extension of the
waspositionedontheback-illuminatedchipS3(CCD7).The
HighMassX-rayBinaries(HMBX)populationtohighX-ray
datawerereprocessedwiththeChandraInteractiveAnalysis
luminosities. For instance, in The Antennae (Zezas & Fab-
of Observations software package (ciao, version 4.6). 3 We
biano 2002), in the Cartwheel galaxy (Wolter & Trinchieri
removed the pixel randomization, since it deteriorates the
2004), in the NGC2207/IC2163 pair (Mineo et al. 2013)
spatial resolution, but, other than that, we followed stan-
and in NGC4088 (Mezcua et al. 2014) the X-ray luminos-
dard data reduction and analysis procedures.
ity function follows the “universal” law found by Grimm
The first step of our analysis was to run the ciao
et al. (2003) for HMXBs when scaled for the SFR. This
wavelet source-detection algorithm with sigthresh4 set to
assumption, linked to the proposed influence of low metal-
10−6, so that we expect at most one spurious source in the
licity on the evolution of massive stars (see Mapelli et al.
areaunderexaminationgiventhatweconsideronlyasingle
2010, and references therein) leads to expect not one, but
CCD. The routine found 41 sources in CCD7, of which 16
a large number of ULXs in NGC2276. Using the corre-
inthegalaxyarea,definedbyD25=177(cid:48).(cid:48)1(deVaucouleurs
lations by Mapelli et al. (2010) and assuming a metallic-
ity Z = 0.22 Z(cid:12) and a SFR of 10 M(cid:12) yr−1, we expect et al. 1991) with a signal-to-noise ratio higher than 4.
Source positions (in RA order) and statistics of the 16
about 10 ULXs. The metallicity is derived applying the
sourcesaregiveninTable1,whileinFig.1theyareshownin
Pilyugin & Thuan (2005) calibration on the de-reddened
theX-rayimageandover-plottedonanopticalimageofthe
[OIIλ 3727]/Hβ and [OIIIλ5007]/Hβ ratios measured by
galaxy. Source counts were extracted from circular regions
Kennicutt (1992) on an integrated spectrum of NGC2276.
centeredatthepositionsfoundbythesource-detectionrou-
Indeed, a high-spatial-resolution observation of NGC2276
tineandwithradius2(cid:48)(cid:48).Thesourcesareallat<2(cid:48) fromthe
takenwithChandra (Obs.ID4968,PI:Rasmussen)revealed
aimpoint, therefore the 2(cid:48)(cid:48) circle is large enough to include
thatthebrightULXcandidateisactuallycomposedofafew
differentsources(Wolteretal.2011),andshowedatotalof
8–9 sources bright enough (LX > 1039 erg s−1) during the 3 http://cxc.harvard.edu/ciao/
observationtobeclassifiedasULXs(Wolteretal.2011;Liu 4 Seehttp://cxc.harvard.edu/ciao/ahelp/wavdetect.htmlforthe
2011). parametersof wavdetect.
(cid:13)c 2015RAS,MNRAS000,1–12
ULXs in NGC2276 3
Figure 1. Left: Smoothed image (csmooth) from Chandra data in the 0.5–7 keV band. The green circle (radius=50(cid:48)(cid:48)) indicates the
region used to extract the diffuse component spectrum (associated to S7, see text for details) Right: The X-ray sources’ positions are
over-plottedontheopticalimageofNGC2276takenfromtheDigitizedSkySurvey(DSS),seehttp://archive.stsci.edu/dss/index.html.
Opencirclesandnumbersidentifydetectedpointsources;redsourcesareULXs.WhitesourceshaveL0.5−10(cid:54)1039ergs−1.
X
the ellipse from the detection algorithm. We expect a very
Table 1. Source detected in NGC2276. Positions from the de-
tection algorithm and net counts from the extraction regions of small contribution in the area from both the instrumental
2(cid:48)(cid:48) radius (see text). No astrometric correction has been applied and the diffuse background. The dominant source of error
andthereforethepositionaluncertaintyisthestandardChandra isstatistic.ForsourceS7,whichcorrespondstothenuclear
oneof∆(cid:54)0(cid:48).(cid:48)6(90%c.l.). region, we extracted counts in a circle of 50(cid:48)(cid:48) radius (ex-
cludingallthedetectedsourcesinthatarea)forconsistency
Number RA(J2000) Dec(J2000) NetCounts with the Rasmussen et al. (2006) analysis (see Section 4).
(0.3–5keV) The sources’ positions are plotted in Fig.1 and the extrac-
tionareaforS7isindicatedasthegreencircle.Instrumental
S1 07h26m37s.4 +85◦45(cid:48)34(cid:48).(cid:48)5 417.4±20.4
S2 07h26m43s.5 +85◦45(cid:48)00(cid:48).(cid:48)1 46.2±6.9 andcosmicbackgroundwasestimatedfromcircularregions
S3 07h26m46s.1 +85◦45(cid:48)38(cid:48).(cid:48)5 43.2±6.6 outside of the galaxy, but as close as possible to it, and ex-
S4 07h26m47s.9 +85◦45(cid:48)52(cid:48).(cid:48)3 175.8±13.3 cluding detected sources. We did not attempt to subtract
S5 07h26m48s.1 +85◦45(cid:48)54(cid:48).(cid:48)9 338.6±18.4 the diffuse gas contribution, which is, at any rate, negligi-
S6 07h26m48s.2 +85◦45(cid:48)49(cid:48).(cid:48)0 104.7±10.2 ble in the extraction area of the point sources (less than 1
S7a 07h27m13s.0 +85◦45(cid:48)16(cid:48).(cid:48)0 40.3±6.4 countpersource).Afirstindicationofthesources’spectral
S8 07h27m14s.9 +85◦46(cid:48)11(cid:48).(cid:48)5 14.2±3.9 shape can be evinced from Fig.2 in which the “true” color
S9 07h27m15s.4 +85◦45(cid:48)54(cid:48).(cid:48)9 8.2±3.0
image of the X-ray emission has been constructed. The ab-
S10 07h27m19s.7 +85◦46(cid:48)32(cid:48).(cid:48)9 19.2±4.5
sorbedorflatsourcesappearasblue/greeninthisimage.For
S11 07h27m25s.8 +85◦45(cid:48)25(cid:48).(cid:48)9 68.2±8.3
a more formal analysis, we extracted individual spectra for
S12 07h27m28s.7 +85◦45(cid:48)11(cid:48).(cid:48)1 21.3±4.7
S13 07h27m53s.3 +85◦46(cid:48)08(cid:48).(cid:48)8 53.2±7.3 the sources with more than a hundred net counts (S1, S4,
S14 07h27m58s.4 +85◦44(cid:48)37(cid:48).(cid:48)9 19.3±4.5 S5, and S6, plus the extended region around S7). For the
S15 07h28m16s.0 +85◦44(cid:48)36(cid:48).(cid:48)4 29.2±5.5 other sources (S2, S3, S8, S9, S10, S11, S12, S13, S14, S15,
S16 07h28m19s.7 +85◦44(cid:48)28(cid:48).(cid:48)5 13.3±3.7 S16), since the paucity of counts hinders individual fits, we
extracted a cumulative spectrum, which contains 336±20
a Source S7 is the center of the galaxy: for subsequent analysis
netcounts.Allthespectralanalysiswasperformedwiththe
countshavebeenextractedinacircleof50(cid:48)(cid:48)radius,excludingall xspec(v.12.8.0)fittingpackage(Arnaud1996).Resultsand
detectedsources,resultinginatotalof1413±67netcounts.
analysis of the spectra are described in the next Sections.
more than 90% of all counts 5 and is a very good match to
5 Seee.g.fig.4.7inhttp://cxc.harvard.edu/proposer/POG/html/chap4.html
(cid:13)c 2015RAS,MNRAS000,1–12
4 A. Wolter et al.
Table 2.Resultsofspectralfitforthe4ULXswithmorethan100netcountsandforthesumofthefainter
sources.Unabsorbedfluxesandluminositiesareinthe0.5–10keVenergyband.
Name Γ NH FX(0.5−10keV) L(X0.5−10keV) L(X2−10keV) χ2ν (dof)
(1021 cm−2) (10−14 ergcm−2 s−1) (1039 ergs−1) (1039 ergs−1)
S1 1.96±0.34 4.1±2.0 14.45±0.71 18.7 10.29 0.92(17)
S4 1.48+0.49 <25.0 6.78±0.51 8.73 6.39 1.47(5)
−0.25
S5 1.77±0.40 3.1±2.0 11.10±0.60 14.3 8.90 1.54(13)
S6a >3.2 6.0±4.0 2.60±0.25 3.36 2.86 1.50(1)
SUM 1.93±0.35 1.7±0.9 6.00±0.37 7.74 4.41 1.11(18)
a FluxandluminositieswerecomputedwithΓ=3.2andthefittedvalueforNH=25.3×1021 cm−2.
Table 3. Point source unabsorbed fluxes and luminosities com-
putedfromthepower-lawfittothemeanspectrumofthefainter
sources(seeSections2and3).
Name F(0.5−10keV) L(0.5−10keV) L(2−10keV)
X X X
(10−14 ergcm−2 s−1) (1039 ergs−1) (1039 ergs−1)
S2 0.96±0.14 1.24 0.70
S3 0.90±0.14 1.16 0.66
S8 0.30±0.08 0.38 0.22
S9 0.17±0.06 0.22 0.13
S10 0.40±0.09 0.52 0.29
S11 1.42±0.17 1.83 1.04
S12 0.44±0.10 0.57 0.32
S13 1.11±0.15 1.43 0.81
S14 0.40±0.09 0.52 0.29
S15 0.61±0.11 0.79 0.44
S16 0.28±0.08 0.36 0.20
etal.2004).Fortheothersources,weconsideredthecumula-
tivespectrum(seeSection2)foratotalof∼300netcounts.
Figure2.ThreecolorimageoftheChandradata.Smoothedwith Anabsorbedpowerlawcanbefitalsotothesedata,result-
csmoothfromciaointhe0.3–0.9keV,0.9–1.5keVand1.5–8keV inginanabsorbingcolumnN =(1.7±0.9)×1021cm−2and
H
bandsforred,blueandgreenrespectively.Manypointsourcesap-
photonindexΓ=1.9±0.4(seeFig.4).Thismeanspectrum
pearblu/greenindicatingexcessabsorptionand/orhardspectra.
wasusedtoconverttheobservedcountratesofthefaintest
sources into fluxes, see Table3.
3 POINT SOURCES AND ULXS ThenumberofsourceswithL(X0.5−10keV) (cid:62)1039erg s−1
is8.Liu(2011)classify9sourcesasULXs,howeverthetwo
Thedistributionofthedetectedsourcesfollowsroughlythe listshaveonlythe5brightestobjectsincommon(S1,S4,S5,
position of the arms of NGC2276 and the zones of high ac- S6,S11).ThefaintestonesarenotintheLiu(2011)listsince
tivity in the other bands. Contamination from background theyuseacutoffluminosityof2.×1039 erg s−1 inthe0.3–
sources may be estimated according to the logN–logS dis- 8 keV band and a different distance of D =36.8 Mpc. The
tributionofHasingeretal.(1993).Atthedetectionlimitof othersourcesintheLiu(2011)listwhicharenotlistedhere
F(0.5−2keV) =7×10−16erg cm−2 s−1(whichcorrespondsto are:NGC2276-X4,whichistheF8starSAO1148(e.g.from
X
LX ≈1038 erg s−1 at the distance of NGC2276) we expect the Simbad Astronomical Database6); NGC2276-X6 which
1.4sourcesbychanceinthetotalareacoveredbythegalaxy. isoutsidetheopticalregionofthegalaxy(D25=177(cid:48).(cid:48)1from
Therefore, at least one of the fainter sources is probably a NED) at ∼ 2(cid:48) from the nucleus of the galaxy; NGC2276-
background source. X7 which is on the border of the CCD and did not sur-
Thefewbrightsourceswithenoughcounts(∼100–400) viveourthresholds;NGC2276-X9whichisonthenextchip
to allow individual spectral fits are modeled by a simple S2 (CCD6) which we did not even analyze. A comparison
power law modified for the interstellar absorption. A satis- of XMM–Newton and Chandra images of the field of the
factory description of their spectra has a hydrogen column XMM–NewtonULXcandidateXMMUJ072649.2+854555is
densityNH ofafew1021 cm−2 (thetotalGalacticNH inthe shown in Fig.5. Instead of a single bright source, Chandra
direction of NGC2276 is ∼5.7×1020 cm−2; Kalberla et al. found 5 distinct sources (S1, S3, S4, S5, and S6; all in the
2005)andslopesbetweenΓ=1.4and2.2.Theobservednor- ULXluminosityrange,plusapossibleothersourcenearS1,
malisednetcountdistributionwiththebestfitmodelisplot- which is visible also in Fig.1 but the detection algorithm is
tedinFig.3,whilethefitresultsarelistedinTable2.These
spectra are consistent with typical results for ULXs when
only low-count-statistics spectra are available (e.g. Swartz 6 http://simbad.u-strasbg.fr/simbad/
(cid:13)c 2015RAS,MNRAS000,1–12
ULXs in NGC2276 5
Figure 4. Combined spectrum of the fainter sources (see Sec-
tion2).Thesolidlineindicatesthebest-fittingpower-lawmodel
andinthebottompanelweplottedtheratiotothemodel.
Figure5.FieldoftheULXcandidateXMMUJ072649.2+854555
as imaged by Chandra (left) and XMM–Newton (right) in the
0.3-8 keV energy band. Each image side is 55 × 46 arcsec wide.
The green contour levels are logarithmically spaced from 0.1
cts/sq.pix. Several distinct X-ray sources are resolved by Chan-
dra.
unable to pick it up) roughly following the XMM–Newton
isophotes (see also Wolter et al. 2011). The luminosity of
these sources ranges from a few 1039 to a few 1040 erg s−1
(Tables 2 and 3).
To investigate possible variability of the Chandra
sources in the region of XMMUJ072649.2+854555, we can
compare the total luminosity from Davis & Mushotzky
(2004) (L0.5−10keV = 1.1 × 1041d2 erg s−1= 5.7 ×
X 45.7
1040d2 erg s−1) to that of the region in the Chandra
32.9
dataset corresponding to the XMM–Newton point spread
function (PSF), which we chose as a circle of 30(cid:48)(cid:48) ra-
dius centered at the XMM–Newton position. This includes
sources S1, S3, S4, S5 and S6, plus the unresolved sources
and diffuse component. We extract a spectrum and fit
it with the Davis & Mushotzky (2004) best fit model
(diskbb+mekal), see their table 1) leaving the normaliza-
tionfree.WeobtainanunabsorbedluminosityL0.5−10keV =
X
5.1 × 1040d2 erg s−1, which is consistent within uncer-
Figure3. Spectroscopyofthe4brightestULXs(S1,S4,S5,S6). 32.9
tainties with the expectation. We make the same com-
Foreachsource(asindicatedineachplot),weshowthespectrum,
parison also for the other two sources detected in both
the best-fitting power-law model (solid line) and, in the bottom
the Chandra and the XMM–Newton observation, namely
panel,theratiotothemodel.
source XMMUJ072718.8+854636 (which corresponds to
(cid:13)c 2015RAS,MNRAS000,1–12
6 A. Wolter et al.
source S10), and source XMMUJ072816.7+854436 (which
corresponds to S15). If we consider the model of Davis
& Mushotzky (2004) for each of the two sources, we find
that XMMUJ072718.8+85463 is apparently fainter by one
dex (L0.5−10keV = 5.2 × 1038 erg s−1) with respect to
X
the expectation of L0.5−10keV = 4.2×1039d2 erg s−1=
X 45.7
2.2×1039d2 erg s−1). However if we consider a region of
32.9
30(cid:48)(cid:48) around S10, we detect in Chandra 158 ± 46 counts,
from the diffuse contribution of the galaxy besides the
19.2 net counts from S10: therefore it is probable that
the XMM–Newton detection was similarly contaminated
by the plasma component. A different situation is found
for XMMUJ072816.7+854436 which is quite isolated: the
Chandra luminosity for a power law spectrum is about
a factor of 1.4 higher (L0.5−10keV = 7.9 × 1038 erg s−1)
X
than expected from XMM–Newton : L0.5−10keV = 1.1 ×
X
1039d2 erg s−1= 5.7×1038d2 erg s−1, which however
45.7 32.9
we deem not compelling given the uncertainties in spectral
shapeandthedifferentresponsematricesofthetwoinstru-
ments.Inconclusion,thepossiblesmallvariationinfluxbe-
tweentheobservations,giventhedifferentspatialresolution
of the instruments, cannot be confirmed statistically.
We note that source S6 is possibly coincident with a
Figure6. TheX-rayluminosityfunctionofNGC2276computed
triple radio source of total L = 9.51×1036 erg s−1,
5GHz inthe2–10keVbandbyusingthesourcesdetectedinthegalaxy
which Mezcua et al. (2013) suggest could be the first evi-
areabyChandra.Thenucleushasbeenremoved,aswellasoneof
dence of an extended jet in a non-nuclear source. The pres- thefaintsources,toaccountfortheexpectedcontaminationfrom
ence of jets in ULXs has been invoked in a few cases (see background sources (see Section3). Thesolidlines represent the
also Cseh et al. 2014) but the evidence is not firm yet. The universalXLFforHMXBbyGrimmetal.(2003),normalizedto
source has a very blue (hard) color in Fig.2 which might 5(long-dashedline)and15M(cid:12) yr−1 (short-dashedline).
indicate an excess absorption. This however is not enough
to imply that the source is a background source since high
levelsofabsorptionareknowntobeintrinsictomanyULXs. is lower (0.05 counts pixel−1) and therefore we corrected
onlysourceswithlessthan10counts.Justtogiveanindica-
tion,theslopeoftheresultingXLFisα =−0.71±0.03for
N
3.1 The X-ray luminosity function
NGC2276andα =−0.75±0.05fortheCartwheel.These
C
In order to construct the X-ray luminosity function (XLF) numbers become lower (αN = −0.66 and αC = −0.60) if
ofNGC2276,wehavetoassesstwopossiblesourcesofcon- we restrict the fit to the logLX (cid:54)39.8, consistent with the
tamination: spurious, unwanted sources which add to the ‘universal’ luminosity function slope of α = −0.60 found
XLF, and missing sources due to detection uncertainties. by e.g. Grimm et al. 2003; Swartz et al. 2011; Mineo et al.
We also do not include the central source (S7). To account 2012aforhighmassX-raybinaries(HMXBs)andproposed
for the possible presence of spurious contamination from toscalewiththeSFR.TheXLFfromGrimmetal.(2003)is
background sources among the Chandra point sources, we alsoplottedinFig.6fortwodifferentvaluesoftheSFRof5
randomlyexcludedoneofthethreefaintestsources,follow- and 15 M(cid:12) yr−1. Also, by comparing the total X-ray lumi-
ing the results of Section3 for a chance coincidence of 1.4 nosity or the number of ULXs with the formulas of Grimm
source at the faintest fluxes. The contamination becomes et al. (2003), similar results are obtained: SFR = 5 and
less than 0.2 sources in the entire galaxy, as defined by 5.5 M(cid:12) yr−1, respectively. These values are in the same
the D25 diameter, at fluxes F (cid:62) 10−14 erg cm−2 s−1. To rangeofSFRderivedinotherwavebands(seeSect.1).Also
X
correct for incompleteness, we base our assessment on fig. eq. (20) from Mineo et al. (2012a) gives a predicted num-
12 from Kim & Fabbiano (2003) in which the detection ber of sources with LX > 1038 erg s−1= 3.2×SFR (cid:62) 16,
probabilitiesasafunctionofbackgroundcountsandsource consistentwiththenumberofsourcesfound,takingintoac-
counts are plotted. The background includes both diffuse count that at least one is spurious and the incompleteness
emission from the galaxy and field background and is esti- of detection at the limit.
matedinthecaseofNGC2276tobeatalevelof0.15counts
pixel−1 (by averaging total counts in the galaxy region, up
to D25, after excluding detected sources). We use the sec-
4 THE DIFFUSE EMISSION COMPONENT
ond panel, relative to a 2(cid:48) off-axis, since the whole galaxy
iswithinthislimit.Weapplytheinterpolatedcorrectionto The nuclear source of NGC2276 was previously detected
all sources with less than 22 counts. This is a conservative withROSAT asvariable(Davisetal.1997)atL(0.5−2keV) ∼
X
estimate.InFig.6,wecomparetheresultingXLFwiththat 2×1040d2 = 1040d2 erg s−1. Notably, the source was
45.7 32.9
oftheCartwheelgalaxy,oneoftherichestgalaxiesinULXs not detected in the 2004 XMM–Newton observation, which
(Wolter & Trinchieri 2004). The Cartwheel XLF was cor- set an upper limit of L(0.5−10keV) <1.2×1039d2 erg s−1
X 32.9
rectedwiththesameprocedure:inthiscasethebackground on its emission. However, a correct comparison of the flux
(cid:13)c 2015RAS,MNRAS000,1–12
ULXs in NGC2276 7
Figure 8.Spectrumofthecentralregionofthegalaxy(seeSec-
tion4).Thesolidlineindicatesthebest-fittingpower-lawmodel
andinthebottompanelweplottedtheratiotothemodel.
2.1 × 1040 erg s−1, about 12 per cent of the total lumi-
nosity and consistent with the expectation for the unre-
solved point source component derived from the K band
Figure 7.Profileinthedirection280–370◦,inredvs.theoppo- luminosity of NGC2276 LK = 10.7, which corresponds to
siteone:110–200◦,inblack.Thegreendashedlineindicatesthe L2−10keV ∼ 1040 erg s−1, by using the relation in Kim &
X
extent of the PSF computed via chart, normalized to the first Fabbiano (2004). Since there should also be a fraction of
datapoint. unresolved HMXBs, this leaves little room for a powerful
AGN.
We observe that our revised temperature and luminos-
wouldneedadetailedmodelingofthesurroundingemission,
ity have essentially no impact on the results of Rasmussen
given the different resolution of the instruments that have
etal.(2006)(actually,thehigherluminosityandlowertem-
observedNGC2276throughtheyears.SourceS7,coincident
perature enforce their conclusion that diffuse X-ray emis-
withtheopticalnucleusofthegalaxy,isextendedasweshow
sion from the western edge does not arise from a shock-
inFig.7,wherewecompareitwiththeChandraPSFderived
compressed gas component).
withchartwithstandardproceduresatthepositionofthe
If we compute the total emission from the D25 of the
nucleus. We plot two different profiles in the NE and SW galaxy, then the diffuse component is L0.5−2keV ∼ 2.5×
directions (110 to 200◦ and 280 to 370◦, respectively) to 1041 erg s−1: we can compare it with theXfindings of Mineo
showthedifferentextentatlargeradiiinthetwodirections, et al. (2012b) that Ldiffuse/SFR = 1.5 × 1040 erg s−1 /
as already noted by Rasmussen et al. (2006). (M(cid:12) yr−1), albeit withXa large scatter of ∼ a dex and very
SincethediffuseemissionofNGC2276andinitsprox-
modeldependent.AgainthisgivesaSFRintherange5–25
imity was studied in detail by Rasmussen et al. (2006), we M(cid:12) yr−1.
justverifytheconsistenceofourresultsforthediffuseemis-
We recall also that the source is immersed in an IGM
sion of the disc (after removing the point sources) for their
component: we report the parameters found by Davis et al.
region A (see their figure 2). We used an extraction region
(1996) using ROSAT data, rescaled to our distance, which
as similar as possible to that of Rasmussen et al. (2006): a
we will exploit for the subsequent simulations. They found
circle of 50(cid:48)(cid:48) radius, visible in Fig.1. an extent of about 25(cid:48) (0.24 Mpc) for the X-ray emitting
We make a more general hypothesis that there might
IGM, centered close to, but not exactly on, the central
be intrinsic absorption in the galaxy, and we bin the spec-
elliptical galaxy NGC2300. The surface brightness of the
trum so as to have a minimum of 100 counts per bin,
medium is described by a King function with core radius
but we checked that the values are not very dependent r ∼ 4.3(cid:48) and index β = 0.41, and has a luminosity of
0
on binning. In this way, in addition to the Galactic ab- Lbol =1.12×1042d2 =5.8×1041d2 erg s−1. The mass
sNorpt=ion(,4.w2+e1m.7)ea×su1r0ed21acnma−b2sofrobrintghecobleusmtnfiteq(uχi2va=len1t.0to1 ofXthehotintragroup45.g7asisMgas =(13.22.59±0.13)×1012 M(cid:12).
H −1.3 ν Rasmussenetal.(2006)findsimilarvalues,buttheROSAT
for 13 degrees of freedom (dof)). The resulting spectrum
larger field of view is better suited to measure this large
is plotted in Fig.8. This results in a lower temperature of
extent component.
kT = 0.18+0.05 keV and an only poorly constrained pho-
−0.02
ton index Γ = 4±2, but we note that the power-law com-
ponent is necessary to properly fit the spectrum and we
5 WHERE DOES THE MORPHOLOGY OF
freeze it to Γ=1.7, appropriate to represent both a low lu-
NGC2276 COME FROM?
minosity active galactic nucleus (AGN) component and/or
the unresolved point source component. The corresponding Whether the perturbed morphology of NGC2276 is the re-
total luminosity is L(0.5−2keV) = 1.8×1041 erg s−1. The sult of tidal interactions with NGC2300, or of ram pres-
X
luminosity of the power-law component is L(0.5−2keV) = sure stripping is still an open question. The simplest way
X
(cid:13)c 2015RAS,MNRAS000,1–12
8 A. Wolter et al.
to estimate the importance of tidal forces, without running 6 NUMERICAL SIMULATIONS
a simulation, is to compare the centripetal acceleration of
To study the morphology of NGC2276 in more detail
NGC2276(a )totheradialacceleration(a )exertedbythe
c r than with our order-of-magnitude calculations, we ran N-
othergalaxiesinthegroup,whichisdominatedbyNGC2300
body/smoothed-particle hydrodynamics (SPH) numerical
and by the hot gas component:
simulations of the interaction between NGC2276 and its
GM(r) environment. We used the gadget-2 code (Springel et al.
a (r)= , (1)
c r2 2001; Springel 2005).
(cid:20) (cid:21) Theinitialconditionsaretakentothesimplestpossible
1 1
ar(D)=GMgroup(D) D2 − (D+r)2 , (2) level.Inallthesimulations,weputNGC2276onaparabolic
orbitaroundNGC2300(assumedtositatthebottomofthe
where r and M(r) are the three-dimensional distance from gravitational potential well of the group). The two galaxies
the center of NGC2276 and the mass of NGC2276 inside start from an initial separation of 170 kpc, 85 Myr before
r, respectively, while M (D) is the mass of the group theperiapsis.Atperiapsis,thegalaxiesreachtheirminimum
group
enclosed within a distance D from the center of NGC2300. distance of 160 kpc, and after another 85 Myr they reach
their present separation of 170 kpc. We ran the simulations
To derive the actual distance between NGC2276 and
for 1 Gyr, but the most relevant effects occur during (and
NGC2300 we used the velocity of NGC2276 with respect
to the IGM, derived assuming that the IGM is coupled to within ∼< 200 Myr after) the periapsis passage.
We describe the elliptical galaxy NGC2300 as a rigid
NGC2300. Rasmussen et al. (2006) derive a Mach number
potential,toreducethecomputingtime.Wefurtherassume
of M = 1.70±0.23, corresponding to a three-dimensional
velocity of v = v ×M = 865±120 km s−1. The distance that the total contribution is due to the dominant compo-
s
nent of dark matter, described by a Navarro, Frenk and
D=x/cosαisthenderivedbymeasuringx=83.2kpc(the
White (NFW) profile (Navarro et al. 1997):
distance between the centers of the two galaxies projected
on the plane of the sky) and cosα = ∆vr/v, where ∆vr = ρ(x)= ρ0 , (4)
421 km s−1 is the relative recession velocity between the x(1+x)2
twogalaxies(Rasmussenetal.2006).Thethree-dimensional
wherex= r,withr =R /candR =(M G/H2)1/3
distance between the two galaxies is then 170 kpc, under rs s 200 200 tot 0
(H = 73 km s−1 Mpc−1 being the Hubble constant, M
these assumptions. 0 tot
being the total mass and c being the concentration param-
For r = 13 kpc (which is approximately the isopho-
eter), and
tal radius of NGC2276) and M(13kpc) = 2×1011 M(cid:12),
we obtain ac = 1.6 × 10−8 cm s−2. For D = 170 kpc, ρ = 200ρcritc3 , (5)
ar(D)∼5×10−10 cm s−2. Thus, we expect tidal forces to 0 3 (ln(1+c)−c/(1+c))
be small today, when compared with the self-gravity of the
whereρ isthecurrentcriticaldensityoftheUniverse.We
galaxy.Letusconsidernowtheimportanceofram-pressure crit
stripping.Thedistancefromthecenterofagalaxyatwhich assumeMtot =1.3×1013 M(cid:12) andconcentrationparameter
c=10.
ram-pressurestrippingbecomesimportantcanbeevaluated
The second ingredient is the IGM. We perform three
as (Yoshida et al. 2008)
differentsimulationsfordifferentchoicesoftheIGMprofiles.
(cid:18) (cid:19) Thefirstisa‘dry’simulation,withnoIGM.Thisisacontrol
h GM M
r = ln ∗ ISM , (3) simulationaimingtoisolatethepurelytidaleffectsbetween
rp 2 2πv2ρIGMh4 the galaxies.
ThesecondandthirdsimulationsassumethattheIGM
where h is the scale-length of the disc, M∗ and MISM is present, but with different distributions. In both cases
are the total stellar mass and inter-stellar medium mass of thedistributionsarecenteredonNGC2300.Inbothsimula-
thegalaxy,v istherelativevelocitybetweenthegalaxyand tions,thegasisassumedtobeadiabatic,withinitialtemper-
the IGM, and ρIGM is the local density of IGM. If we as- ature2×106K,thetotalmassofgasisMg =1.25×1012M(cid:12)
sumeh=3kpc,M∗ =2.8×1010 M(cid:12),MISM =8×109 M(cid:12), and the mass of a single gas particle is 106 M(cid:12) (for a total
v = 850 km s−1 and ρIGM = 5×10−27 g cm−3, which are number of 1.25×106 particles).
consistent with the observations of NGC2276 (Davis et al. In the second simulation, the IGM has a NFW profile7
1996; Rasmussen et al. 2006) and its environment (we use with concentration c=10.
the same values for the simulations described in the next Finally,inthethirdsimulationthegasisdistributedas
Section), we obtain rrp = 6.5 kpc, which is larger than h a King model (King 1962):
butstillinsidetheisophotalradiusofNGC2276(13.2kpc).
Thus, we expect ram-pressure to be quite effective in per- (cid:20) (cid:16)r (cid:17)2(cid:21)−3β+0.5
ρ(r)=ρ 1+ , (6)
turbing the morphology of NGC2276. 0 rc
Eveniffromequations(1)and(2)wehaveshownthat
tidalforcesarenotasefficientasrampressure,itisreason-
7 The NFW profile usually refers to dark matter, and not to
abletoexpectthatthereisaninterplaybetweentidalforces
baryonicmatter.TherationaleofadoptingaNFWprofileforthe
and ram pressure, leading to an enhancement of NGC2276
baryonicIGMisthefollowing.Forlargeradii,theNFWhasthe
strippingwithrespecttothecaseinwhichrampressurewas asymptotic form (cid:37) ∝ r−3 to be compared with the (cid:37) ∝ r−2 of
the only process at work (see e.g. the argument in Yoshida an isothermal sphere. The NFW would model an IGM distribu-
etal.2008).Inaddition,viscousstripping(e.g.Nulsen1982) tionconcentratedclosetoNGC2300,whiletheisothermalsphere
might also play a role, removing matter from the ISM. modelsamorewidespreadIGM.
(cid:13)c 2015RAS,MNRAS000,1–12
ULXs in NGC2276 9
where ρ = 3.1 × 10−3 cm−3 (from Davis et al. 1996), No gas is stripped from the galactic disc, and the tails
0
β =0.41 (from Rasmussen et al. 2006) and r =4.0(cid:48) (from observed by Rasmussen et al. (2006) are not reproduced.
c
Rasmussen et al. 2006).
The third ingredient is a model of NGC2276. We
parametrize it, according to Widrow & Dubinski (2005), as 6.2 NFW simulation
thesumofthreedifferentcomponents:bulge,discanddark
The evolution of the disc galaxy changes significantly, if
matterhalo.Thetotalnumberofparticlesinthedarkmat-
the IGM is accounted for. As we expect from our order-
terhalo,bulgeanddiscoftheNGC2276modelare2×106,
of-magnitude estimate, ram-pressure stripping and viscous
105 and6×105,respectively.Thesofteninglengthis100pc.
stripping are efficient, and lead to the formation of tails al-
The dark matter halo is modeled as a NFW potential
ready during the periapsis passage (see Fig.9). We also see
with Mtot = 6×1011 M(cid:12) and c = 10. The bulge is rep- that the front side of the galaxy disc is perturbed, recalling
resented by a S´ersic profile, according to the Prugniel &
the asymmetric ‘bow-shock-like’ shape of NGC2276.
Simien (1997) deprojection ρb(r) ∝ (r/re)−p exp(r/re)1/n, The total mass in the tails is ∼6×108 M(cid:12) after the
where we assume re = 0.83 pc, p = −1 and n = 1.83. The first periapsis passage (it will rise to ∼1×109 M(cid:12) at the
normalization of the profile is set by the total mass of the
end of the simulation). A part of this mass forms clumps
bulge Mb =6×109 M(cid:12). that are visible in Fig.9. The cooling time in the galaxy
The gas and the stellar component are distributed as discis∼104−5 yr,whileitis10–20Myrintheram-pressure
an exponential Hernquist disc (Hernquist 1993):
stripped tails. The dynamical time t is 200 Myr and 10
dyn
M Myr in the galaxy disc and in the tails, respectively. Thus,
ρ (R,z)= d exp(s−R/h) sech2(z/z ), (7)
d 4πh2z 0 star formation occurs in the galaxy disc and might occur
0
also in the tails.
where R and z are the cylindrical coordinates, M =3.6×
d Finally, a small but significant fraction of the gas com-
1010 M(cid:12) isthetotalmassofthedisc,h=3kpcisthescale- ponent of the NGC2276 model (8.6×108 M(cid:12), i.e. 6.6 per
length of the disc and z =0.3 pc is the scale-height of the
0 centoftheentiregascomponent)iscompletelystrippedand
disc.Thedisciscomposedofbothgasandstellarparticles,
lost from the galaxy during the entire simulation.
withthetotalmassofgasandstarsbeingM =0.8×1010
d,g
and Md,∗ =2.2×1010 M(cid:12), respectively.
ThegascomponentoftheNGC2276modelisassumed
6.3 Isothermal sphere simulation
to be adiabatic, with initial temperature of T = 2×104 K
(Rasmussen et al. 2006). We do not introduce a cooling Thethirdsimulationisverysimilartothesecondone.Even
function, but verify ‘a posteriori’ if the cooling time t in this simulation, ram pressure produces a deformation in
cool
isshorterthanthedynamicalfreefalltimet toestimate the front side of the galaxy disc and leads to the formation
dyn
the rate of star formation. In particular, we estimate the of tails and clumps. The total mass in the tails is 6×108
cooling time as M(cid:12) aftertheperiapsispassage,andraisesto7.5×108 M(cid:12)
at the end of the simulation, with no significant difference
3 nkT
tcool = 2 n n Λ(Z,T,n), (8) with respect to the second simulation. In the tails, tcool ∼
e p t ∼ 10 Myr, suggesting that star formation is possible.
dyn
wherenisthetotalgasdensity,neisthedensityofelectrons, In the disc galaxy, tcool ∼ 104−5 yr << tdyn ∼ 300 Myr,
n is the density of protons, k is the Boltzmann constant, indicating that star formation should occur.
p
T isthetemperature,andΛ(Z,T,n)isthecoolingfunction, Thetotalmassofgasthatislostfromthediscgalaxyby
as a function of n, T and of the metallicity Z. We used the theendofthesimulationis1.5×108 M(cid:12),i.e.∼12percent
cooling function of Sutherland & Dopita (1993). oftheentireinitialgasmassintheNGC2276model.Thisis
We assume that gas is able to form stars if nearly a factor of two larger than in the second simulation.
ThedifferencecanbeexplainedwiththefactthattheIGM
tcool <tdyn <tlife, (9) densitygoesasρIGM ∝r−3 and∝r−2 inthesecondandin
the third simulation, respectively. Since the density of the
wheret isthelifetimeofagasclumporstructure,derived
life IGMatlargedistancesfromtheellipticalgalaxyinthethird
directly from the simulations.
simulation is higher than that in the second simulation, we
Herebelowweshortlysummarizetheresultsofoursim-
expect that ram pressure stripping will last longer than in
ulations.
the second simulation (i.e. it is important even when the
disc galaxy is far from periapsis).
6.1 Dry simulation
6.4 Summary of simulation results and caveats
Our order-of-magnitude estimate shows that tidal forces
should not be very important for NGC2276. On the other In conclusion, we found that ram-pressure stripping and
hand, even in the simulation without IGM we observe the viscous stripping are the dominant processes and can lead
formation of (not strongly pronounced) tidal arms in both to both the formation of the tails and the deformation of
thestellarandthegaseouscomponents.Inaddition,thegas the front side of the disc galaxy. Tidal forces contribute
disc thickens. The regions where condition 9 is satisfied are marginally, by producing tidal arms and thickening the
veryrareinthedrysimulation,andweexpectthatstarfor- gaseousdisc.Wefoundafactorof2–5lowerSFRthaninthe
mation is not enhanced by the interaction, even during the observations, but we are aware that our very approximate
periapsis passage. treatment of gas underestimates both cooling and SFR.
(cid:13)c 2015RAS,MNRAS000,1–12
10 A. Wolter et al.
Moreover,theversionoftheSPHtechniqueadoptedin
gadget-2isknowntopoorlyresolvedynamicalinstabilities
and mixing (e.g. Agertz et al. 2007). The main reason is
thattheSPHformulationadoptedingadget-2inaccurately
handlesthecaseoflow-densityregionsincontactwithhigh-
densityones.Asaconsequence,gasturbulenceisartificially
suppressed,andgasstrippingduetorampressuremightbe
underestimated with respect to other formalisms (e.g. grid
codes). In particular, we expect that the gas mass in the
stripped tails is underestimated in our simulations, and the
morphology of the resulting tails is quite smoother than it
should be. This implies that our results can be regarded
as lower limits for the ram-pressure effects. Bearing these
caveats in mind, we conclude that our simulations are able
to qualitatively reproduce the peculiar shape of NGC 2276,
and to confirm the importance of ram pressure and viscous
stripping.
7 SUMMARY & CONCLUSIONS
We have analyzed a Chandra observation of NGC2276, a
spiral galaxy in the group of the elliptical NGC2300. We
havepaidparticularattentiontothepointsourcesdetected
in the area of the galaxy. We find that a large number of
ULXsarepresent,whichmakesNGC2276oneoftherichest
environments for this still enigmatic bright sources.
We have seen in Wolter et al. (2011) that six of the X-
raybrightsources,fourofwhichareULXs,arepositionally
coincident (1(cid:48)(cid:48)–2(cid:48)(cid:48)) with Hii regions (from Hodge & Kenni-
cutt1983;Davisetal.1997).Howeverthespatialresolution
at the distance of NGC2276 is not enough to warrant a
physicalassociation.Atleastfivesupernovaehavebeenob-
served in the last fifty years (see Section1 for references),
twoclosetothecenterandtwointhewesternregionwhere
ULXs are found.
All observations concur on linking the high activity
with the presence of a large number of ULXs, although
thereisnoone-to-onecorrespondencebetweenpeaksindif-
ferent bands, as is already seen in other galaxies, like the
Cartwheel(Wolter&Trinchieri2004).Radioemissionisen-
hanced throughout the galaxy body: one of the sULX is
possibly associated to either a radio bubble or an extended
jetstructure(Mezcuaetal.2013),howeverthecomplexra-
diofieldmakestheinterpretationdifficult.Thesourcehasa
very hard spectrum: we cannot exclude the possibility that
it is a chance superposition with a background AGN.
We detect 15 sources associated with the galaxy
NGC2276 in the Chandra observation, out of which 8 are
above the ULX luminosity threshold. We derive an XLF
andconfirmthepreviousestimatesoftheSFRofNGC2276
in the range SFR = 5–15 M(cid:12) yr−1. The value is close to
those of the Antennae and the Cartwheel, which are fitted
with SFR = 7.1 M(cid:12) yr−1 (Zezas & Fabbiano 2002) and
∼ 20 M(cid:12) yr−1 (Wolter & Trinchieri 2004), respectively.
These numbers reconcile expectation from Mapelli et al.
Figure 9. Color-coded density map stars (top panel) and gas (2010) with observations.
(centralandbottompanel)att=300MyrintheNFWsimulation. We know that moderate variability is rather common
The boxes measure 300 kpc per edge. Top panel: the density of in ULXs (see e.g. Crivellari, Wolter & Trinchieri 2009 and
starshasbeenprojectedalongthez-axis;central(bottom)panel: Zezas et al. 2006) and in recent years a few transient ULXs
thedensityofgashasbeenprojectedalongthez-axis(y-axis).A have also been identified (e.g. Sivakoff et al. 2008; Middle-
tailstructureiswellvisibleinthecentralandbottompanel. tonetal.2012,2013;Soriaetal.2012;Espositoetal.2013).
(cid:13)c 2015RAS,MNRAS000,1–12