Complex Structure of Galaxy Cluster Abell 1689: Evidence for a Merger from X-ray data?

0.48 MB·English

by  K. E. Andersson

Accepted forpublicationin ApJ
PreprinttypesetusingLATEXstyleemulateapjv.11/26/03
COMPLEX STRUCTURE OF GALAXY CLUSTER ABELL 1689:
EVIDENCE FOR A MERGER FROM X-RAY DATA?
K.E. Andersson1,2 and G.M. Madejski1
Accepted for publication inApJ
ABSTRACT
Abell 1689 is a galaxy cluster at z = 0.183 where previous measurements of its mass using various
techniques gave discrepant results. We present a new detailed measurement of the mass with the
data based on X-ray observations with the European Photon Imaging Camera aboard the XMM-
Newton Observatory, determined by using an unparameterized deprojection technique. Fitting the
4
total mass profile to a Navarro-Frenk-White model yields halo concentration c = 7.2+1.6 and r =
0 −2.4 200
0 1.13 0.21h−1Mpc,correspondingtoamasswhichislessthanhalfofwhatisfoundfromgravitational
±
2 lensing. Adding to the evidence of substructure from optical observations, X-ray analysis shows a
highly asymmetric temperature profile and a non-uniform redshift distribution implying large scale
n
relative motion of the gas. A lower than expected gas mass fraction f =0.072 0.008 (for a flat
a gas
±
J ΛCDM cosmology) suggests a complex spatial and/or dynamical structure. We also find no signs of
anyadditionalabsorbingcomponentpreviouslyreportedonthebasisoftheChandradata,confirming
9
2 the XMM low energy response using data from ROSAT.
Subject headings: dark matter — galaxies: clusters: individual (Abell 1689) — X-rays: galaxies:

1 clusters
v
4
0 1. INTRODUCTION initial power spectrum of fluctuations, and cosmologi-
6 cal model. However, observations often disagree with
Galaxy clusters are the largest known gravitationally
1 the numerical models. For instance, one disagreement
bound systems in the Universe. The detailed analysis of
0 regards the rotation curves of dwarf elliptical galaxies
the massdistributionofclustersisthusimportantinthe
4 which appear to be the result of a constant-density core
process of understanding the large scale structure, and
0 whereas numerical simulations predict cuspy dark mat-
the nature of dark matter. The three main methods of
/
h measuring galaxy cluster masses: virial masses from ve- ter halo profiles (Moore et al1999a). In addition, obser-
p locity dispersions of cluster galaxies, X-ray imaging and vations show fewer Milky Way satellites than predicted
- by the models (Kauffman, White, & Guiderdoni 1993;
spectroscopy of the intra-cluster medium (ICM) emis-
o
Moore et al 1999a). Clearly, to understand the nature
r sion, and the gravitational lensing of background galax-
t ies,havebeenfoundinrecentyearstobeindisagreement ofgalaxyclusters andthe dark matter they consistof,it
s is important to measure the matter distribution in clus-
a for some clusters. Generally, the X-ray estimates are in
ters via all available means. Fortunately, there are two
: good agreement with gravitational lensing for clusters
v superbX-rayobservatories,ChandraandXMM-Newton,
with a high concentrationof centralX-ray emission(the
i featuringexcellentangularresolutionandexceptionalef-
X so-called “cooling flow” clusters) but seemingly in dis-
fective area coupled with good spectral resolution, and
r agreementfor other,less centrally peakedobjects (Allen
a 1998). To obtain the estimate of the total mass (includ- those are well suited for detailed analysis of the X-ray
emitting gas of galaxy clusters
ing that due to dark matter) of a galaxy cluster from
Abell 1689 is a cluster showing a large discrepancy
itsX-rayemission–commonlyassumedtobe fromopti-
among various mass determinations, and we chose it for
cally thin, hot plasma that subtends the space between
a detailed study. It is a rich cluster, R = 4, without
galaxies – it is necessary to make the assumption of hy-
a pronounced cooling flow but with an approximately
drostatic equilibrium. This is appropriate of course only
circular surface brightness distribution suggesting a rel-
for clusters that have had time to relax into equilibrium
andhavenotexperiencedanyrecentmergerevents. Gen- atively relaxed structure. The large mass, 1015 M⊙
∼
(Tyson & Fischer 1995), and apparentsymmetric distri-
erally,it is assumed that clusters with circular isophotes
bution ofAbell 1689make it a suitable cluster for gravi-
meet this criterion.
tationallensing measurementsaswellas X-raymeasure-
Cold dark matter (CDM) hierarchal clustering is the
ments. However, the type of clusters that are believed
leading theory describing the formation of large scale
to be the most relaxed have a coolcentral component of
structure quite well. In particular, the numerical simu-
enhancedsurfacebrightness. Theabsenceofsuchacom-
lations such as Navarro, Frenk, & White (1997) (NFW)
ponentinAbell1689suggeststhattheclusterisnotfully
successfully reproduce the observed dark matter halo
relaxed. Also,thegalaxycontentoftheclusterisunusu-
profiles, which appear be independent of the halo mass,
allyspiralrichforaclusterwithhighsphericalsymmetry
1 Stanford Linear Accelerator Center, 2575 Sand Hill Road, and richness, with a galaxy type ratio E : S0 : Sp of ap-
MenloPark,CA94025, USA proximately 22 : 22 : 28 plus 25 unidentified galaxies
2 Royal Institute of Technology (KTH), AlbaNova University (Duc et al. 2002). Teague, Carter, & Gray (1990) mea-
Center(SCFAB),S-10691, Stockholm,Sweden sure a velocity dispersion of σ = 2355+238 km s−1
Electronicaddress: [email protected] 1D −183
2 Andersson & Madejski
0.201
0.188
19:30
0.180
-1:20:00 0.193(cid:9)
0.191
0.202
0.192
30
0.174 0.184
0.192
0.181
0.204
21:00
0.191
0.188(cid:9)
30
1 arcmin = 119 h-1 kpc (EdS)
22:00
34 13:11:30 26
Fig. 1.— R magnitude image of Abell 1689 from the STScI Digitized Sky Survey with all magnitude R < 18 cluster members within
the central 2′ region plotted over logarithmic X-ray contours from XMM Mos. The bump toward northeast is due to a foreground point
source. Circle sizes are proportional to galaxy R magnitudes.
for 66 cluster members, unusually high for a cluster of mass interior to 0.24 h−1 Mpc from this method is
thistemperature. PositionsandredshiftsfromDuc et al. M2D(< 0.24 h−1 Mpc) = 1.8 0.1 1014 h−1 M⊙
± ×
(2002) for all cluster members with red magnitude R < (Taylor et al. 1998). Measurement of the distortion
18andwithin2′ofthebrightestcentralgalaxyareshown of the luminosity function due to gravitational lens
in Fig. 1 together with logarithmic X-ray intensity con- magnification of background galaxies gives M (<
2D
tours from XMM Mos 1. 0.25 h−1 Mpc)=0.48 0.16 1015 h−1 M⊙ (Dye et al.
± ×
Thegravitationallensingestimate from6000bluearcs 2001). Finally, the weak gravitational shear of galax-
and arclets calibrated by giant arcs at the Einstein ra- ies in a ESO/MPG Wide Field Imager 33′ 33′ im-
×
dius of Abell 1689 gives a best fit power-law expo- age gives a mass profile with best fit NFW profile with
nent of n = 1.4 0.2 for the projected density pro- r = 1.14 h−1 Mpc and c = 4.7 or a best fit SIS
200
file from 200−h−1 k±pc to 1 h−1 Mpc (Tyson & Fischer with σ = 1028+35 km s−1 (Clowe & Schneider 2001;
1D −42
1995). (Unless otherwise stated, we assume an Einstein- King, Clowe, & Schneider 2002).
deSitter (EdS) cosmology with ΩM = 1.0,ΩΛ = 0.0 There is a good indication from the optical
and H0 = 100 h km s−1 Mpc−1.) This is steeper than data that the cluster consists of substructures.
the profile of an isothermal sphere (n = 1). The Miralda-Escud´e& Babul (1995) suggest a strong lens-
−
strong lensing analysis of two giant arcs directly gives ing model with two clumps in order to reproduce the
M2D(< 0.10 h−1 Mpc) = 1.8 0.1 1014 h−1 M⊙ positions of the brightest arcs. A larger mass clump
(Tyson & Fischer 1995). ± × (σ = 1450 km s−1) is centered on the brightest clus-
r
The mass profile derived from the deficit of lensed ter galaxy while a smaller clump (σ = 700 km s−1) is
r
red galaxies behind the cluster due to magnifica- located1′ northeastofthemainclump. Theyarriveata
tion and deflection of background galaxies suggests mass a factor 2 - 2.5 lowerfor their X-ray estimate com-
a projected mass profile of M2D(< R) 3.5 paredto theirgravitationallensingmodel. Girardi et al.
1015(R / h−1Mpc)1.3 h−1 M⊙ for R < 0.32 h≈−1 Mp×c, (1997) identify two distinct substructures centered on
close to that of an isothermal sphere (M2D R). The redshifts z = 0.175 and z = 0.184 using positional
∝
XMM Newton observation of Abell 1689 3
and redshift data of cluster galaxies from Teague et al. the count rate of approximately 2σ above the quiescent
(1990), providingfurther evidencethatthe cluster isnot period in the 0.3 10 keV band.
−
relaxed. These clumps are also aligned in the southwest
–northeastdirectionbutthelocationsdonotagreewith Vignetting correction
theonesofMiralda-Escud´e & Babul(1995). Bothstruc-
tures are found to have σ 300 400 km s−1 yielding The effective mirror collecting area of XMM-Newton
r ∼ − is notconstantacrossthe fieldof view: it decreaseswith
virial masses several times smaller than those derived
increasing off-axis angle and this decrease is energy de-
from lensing and X-ray estimates.
pendent. Thisresultsinapositiondependentdecreasein
Canthe X-rayobservationsprovideanyevidence for a
the fraction of detected events and when doing imaging
substructureinAbell1689,andwhataretheimplications
spectroscopy for extended sources, we need to account
onthemassinferredfromtheX-raydata? Inanattempt
forthis effect. By generatinganAncillary Response File
to answer this, we analyze XMM-Newton EPIC pn and
(ARF)foreachsourcespectrumregion,usingXMMSAS
EPIC Mos data to measure the mass profile of A1689
5.4.1 command arfgen we calculate an averageeffective
andto investigatethe spatialstructureofthe cluster. 2
§ area for each region considered by us (see below).
contains the details of the observations and data reduc-
tion with the XMM-Newton as well as with summary of
Background subtraction
theROSAT,AscaandChandradata; 3coversthemeth-
§
odsofspectralfittingoftheXMMdataandpresentsthe In order to correctly account for particle-induced and
analysis of cluster asymmetry; 4 derives the mass and CosmicX-rayBackground,itwouldbeoptimaltoextract
§
the slope of the mass distribution in the core; and 5 a background spectrum from the same detector region
§
presentstheinferences aboutthe structureofthe cluster collected at the same time as the source spectrum. This
inferred from the spatial analysis. The paper concludes isofcourseimpossible,andthebackgroundcanbetaken
with the summary in 6. from a source-freeregionof the detector (other than the
§
target, but near it), or can be estimated using blank
2. ABELL1689X-RAYOBSERVATIONS sky data. We adopt the former method, noting that the
2.1. XMM-Newton observation backgroundisnotentirelyconstantoverthefieldofview;
however,itcanbeassumedtobeapproximatelyconstant
Data preparation
with the exception of the fluorescent Cu line at 8 keV
Abell 1689 was observed with XMM-Newton for 40 in pn which is the strongest contaminant emission line.
ks on December 24th 2001 during revolution 374. For Using this assumption, we can effectively subtract the
imaging spectroscopy we use data from the European particlebackgroundbyextractingaspectruminasource
PhotonImagingCamera(EPIC)detectorsMos1,Mos2 free region in the same exposure. For pn data in the
and pn. Both Mos cameras were operating in the Full 7.8 8.2 keV range is excluded due to the strong spatial
−
Frame mode whereas pn was using the Extended Full dependence of the Cu internal emission.
Frame mode. The Extended Full Frame mode for pn UsinganARFgeneratedforasourceregiononaback-
is appropriate for studying diffuse sources since it has ground subtracted spectrum will not take into account
lowertimeresolutionandsoitislesssensitivetocontam- that the spectrum used as background was extracted
ination from photons being detected during readout of from a different region where the vignetting was higher,
the CCDs. These events (so called Out-Of-Time events) due to the larger off-axis distance of the background re-
show up as streaksacrossthe X-rayimage andare espe- gion. This will have the effect that the CXB compo-
ciallybothersomewhenthegoalofanobservationisspa- nent in our source spectrum will be under-subtracted
tially resolved spectroscopy of diffuse sources. All cam- and the net spectrum will contain some remnant of the
eras used the Thin filter during the observation. CXB. The particle induced background however is not
EPIC background is comprised mainly of three com- vignetted and therefore should leave no remnant.
ponents. The external particle background consists pri- To estimate the CXB component in our exposure we
marily of soft protons (E < 100 keV) being funneled takethe eventsoutsideofthe field-of-viewtobethe par-
p
through the mirrors and causing a time variable flaring ticle background. Another spectrum is then extracted
signal in the detector. The internal particle background fromalargesourcefreearea,locatedawayfromtheclus-
is mainly due to high energy particles interacting with ter. The particlebackgroundis normalizedandremoved
the detector material and causing a roughly flat spec- fromthisspectrum. Theresultingspectrumisfittedtoa
trumwithflourescentemission-linescharacteristictothe brokenpowerlawmodeltodeterminetheCXBflux. The
detector material. This component varies over the de- incorrect vignetting correction of the CXB is found to
tector surface. The third source of background is the causeanatmost2%over-estimationofthefluxwhichis
cosmic X-ray background (CXB) which is roughly con- thecaseintheoutermostannulusinouranalysis(seebe-
stant in time but varies over the sky. low). However since the vignetting is energy-dependent,
Foralldatareductionweusethe softwareandcalibra- the incorrect vignetting may cause a small ( 0.3 keV)
∼
tion data implemented in XMM Science Analysis Soft- shift in temperature for the outermostregion. The over-
ware(SAS)5.4.1. Toexcludetheeventscontaminatedby all uncertainty in the background is estimated to be at
protonflares,weproduce lightcurvesinthe 10 15keV most a few percent.
−
band where the true X-ray signal is low. We screen the We compareour backgroundsubtractionmethod with
datausingaconstraintonthetotalcountrateoflessthan themethodofusingXMMstandardblankskydatacom-
1.5 ct s−1 for Mos and 1.1 ct s−1 for pn in this band, piled by Lumb (2002); we find that both methods give
leaving an effective exposure time of 37 ks for Mos and similar fit parameters. However, given the difficulty of
29 ks for pn. This screening corresponds to a limit on normalizingthe CXB, the differenthigh-energyleftovers
4 Andersson & Madejski
from proton-flare subtraction and the different inter- is certainly not background related. It is possible that
nal particle background which occurs when using back- the pn low-energy discrepancy can be due to incorrect
ground data from a different exposure, we chose to use treatment of charge collection at lower energies (S.
in-field(ratherthanblank-sky)background. Wefindthis Snowden, priv. comm.). To resolve the discrepancy
is more robustin keeping the overallshape of the source between Mos and pn we decided to compare with Asca
spectrum uncontaminated. In all our analysis we use a GIS/SIS, ROSAT PSPC and Chandra ACIS-I data.
background region of a circular annulus with inner and
outer radii of 6′ and 8′ respectively. In fact, recent work 2.2. ROSAT, Asca and Chandra observations
by Lumb et al. (2003) suggests that the in-field back- Besides the discrepancy regarding the softest X-ray
ground method is probably more accurate. The clus- band for the Abell 1689 data between the Mos and pn
ters in the above paper, however, have smaller apparent detectors aboard XMM-Newton, the spectral fits to the
angular size which makes the background method more Chandra data presented by Xue & Wu (2002) imply a
reliable. higherabsorbingcolumn,6.7 1.5 1020cm−2 thanthe
Galactic value of 1.8 1020±cm−2×(Dickey & Lockman
×
1990). Sincesuchexcessabsorptionisnotcommonlyde-
Spectral fitting
tected in X-ray data for clusters, this requires further
Inanalyzingthevignettingcorrected,backgroundsub- investigation. To determine if there is indeed any addi-
tracted radial count-rate profile, we fit it to a conven- tionalcomponentofabsorptionbeyondthatattributable
tional beta model, S(r) (1+r/rc)−3β+0.5, where S(r) to the Galactic column – and assess the reliability of
∝
is the source surface brightness at radius r. The fit the softest energy band of the pn vs. Mos data – we
gives rc = 91.2 0.7 h−1 kpc and β = 0.72 0.01 with used the most sensitive soft (< 1 keV) X-ray data for
χ2/dof.=46us±ingdataoutto700h−1kpc. ±Clearlythis thisclusterobtainedpriortotheXMM-Newtonobserva-
modelisnotaverygoodfit;weshowithereonlyforcom- tions, collected by the ROSAT PSPC. The ROSAT ob-
pleteness and comparison with previous work, and note servation conducted during July 18-24, 1992 (available
that it is not used in the subsequent analysis. The bad from HEASARC) yielded 13.5 ks of good data. We ex-
fitaboveresultsfromthefactthattheclusteremissionis tracted the ROSAT PSPC counts from a region 3′ in
morepeakedinthecorethanthebestfitbetamodel. To radius, centered on the nominal center of X-ray emis-
obtaina generalidea ofthe properties ofthe cluster and sion. For background,we selecteda source-freeregionof
comparethis withprevious results,we extractedspectra the same PSPC image. Using these data over the nomi-
for the central 3′ (356 h−1 kpc) region, centered on the nalenergyrange0.15- 2.1keVwith the standardPSPC
X-ray centroid at 13h11m29s.4 01◦20′28′′. This radius response matrix applicable to the observation epoch, we
−
corresponds to 0.32 r or 3.9 r . Background spectra performed a spectral fit to a simple, single-temperature
200 c
were extracted from source-free regions from the same MEKAL model with soft X-ray absorption due to gas
exposure. For XMM pn we use single and double pixel with Solar abundances, using the XSPEC package as
events only whereas for XMM Mos we also use triple above. In the fit we use metal abundances of 0.27 So-
and quadruple pixel events. lar obtained in the XMM Mos fit above. While the
For spectral fitting we use the XSPEC (Arnaud limited bandpass of ROSAT PSPC precludes an accu-
1996) software package. We fit the data in ratedeterminationofthe temperature(the bestfitvalue
the 0.3 10.0 keV range using the MEKAL is kT = 4.3+1.2 keV, 90% confidence regions quoted),
− −0.8
(Mewe, Gronenschild, & van den Oord 1985; the PSPCdata providea goodmeasureofthe absorbing
Mewe, Lemen, & van den Oord 1986; Kaastra 1992; column: the best value is 1.9 0.3 1020 cm−2, cer-
Liedahl, Osterheld, & Goldstein 1995) model for the tainly consistent with the Gala±ctic va×lue. We note that
optically thin plasma and galactic absorption. With the thedifferencebetweenthetemperatureinferredfromthe
absorption fixed at the Galactic value, NH = 1.8 1020 PSPCfitandthatobtainedfromtheXMM-Newtondata
cm−2 (Dickey & Lockman 1990), and assum×ing a asaboveisaresultofthelimitedbandpassofthePSPC,
redshift z = 0.183 we arrive at a temperature of located much below the peak of the energy distribution
9.35 0.17 keV and a metal abundance of 0.27 0.03 of the cluster photons. The measurement of the absorb-
Solar±for both Mos cameras with χ2/dof. = 95±1/824. ing column, however, clearly indicates that the column
From the pn camera we get 8.25 0.15 keV and inferred from the Chandra observation by Xue & Wu
0.23 0.03 Solar with χ2/dof. = 142±4/863. The best (2002) is not correct, and might be due to instrumen-
±
fit models with residuals can be seen in Fig. 2 for tal effects. We conclude that the absorbing column is
Mos (Left) and pn (Right). The pn temperature is consistent with the Galactic value.
in disagreement with Mos data and the reason for To obtain further constraints on the absorbing col-
this effect can be seen from the residuals below 1 keV umn, we also used the Asca GIS and SIS data together
for pn (Fig. 2 (Right)). Fitting the pn data above 1 with the PSPC data for an independent constraint on
keV we get 9.33 0.20 keV and 0.24 0.03 Solar with the continuum radiation in the fitting process. We per-
χ2/dof. = 892/±722, in better agree±ment with Mos. formed standard extraction of data from all Asca de-
The temperature from Mos is in agreement with that tectors, also from a region from a region 3′ in radius
found by Asca, 9.02+−00..4300 keV (Mushotzky & Scharf for the source, and a source-free region of the same im-
1997). This observation does not suffer from pile-up, age for background. We performed a spectral fit si-
nor is the low energy spectrum sensitive to background multaneously to data from the PSPC and four Asca
subtraction. The background uncertainties below 1 keV detectors. To account for possible flux calibration dif-
for this pn spectrum are less than 1 % whereas the pn ferences, we let the normalization among all the dif-
soft excess is sometimes higher than 10 %. The excess ferent detectors run free. We used the energy range
XMM Newton observation of Abell 1689 5
Fig. 2.—BestfitMEKALmodelsfortheXMMMos(Left)andpn(Right)datafromthecentral3′ regionofAbell1689. Theabsorbing
column was fixed at the galactic value (Dickey& Lockman 1990).
Fig. 3.—The ratiooftheXMMMos(Left)andpn(Right)spectra ofAbell1689 tothebestfitmodel determinedfromtheROSATand
Asca analysis.
of 0.7 9.0 keV and 0.5 8.5 keV for the Asca GIS 9.43+0.16 keVandmetallicityof0.26 0.02Solar. While
− − −0.15 ±
andSIS camerasrespectively. Since Asca detectors (and this fitted value of absorption is formally inconsistent
in particular, the SISs) often return spectral fits with with the Galactic and ROSAT-inferred values, this is a
excess intrinsic absorption (Iwasawa,Fabian, & Nandra relatively small difference, which might be due to the
1999) we also let the absorbing column be fitted inde- slightly imperfect calibration of the Mos detectors or
pendently for the GIS, SIS and PSPC detectors. Tem- the assumption of isothermality made by us for this fit
peratureandmetalabundancesweretiedtogetherforall (since T and n are correlatedinthe fitting procedure).
H
datasets in the fit. The optical redshift z = 0.183 was We notethatusing the ROSAT valueforabsorptionwill
used. ThejointfitofROSATandAscadatagivesusthe give us a somewhat lower measure on the temperature
best fit temperature of 9.1 0.5 keV and abundances of (see below).
±
0.25 0.06 Solar, in agreement with the values quoted Finally, we reduced the Chandra data for Abell 1689
±
by Mushotzky & Scharf (1997), and the absorption (for using the mostrecentreleaseofdata reduction software;
the PSPC data) 1.75 0.08 1020 cm−2, in agreement specifically,duetothedegradationoftheChandraACIS-
± ×
withtheGalacticHI21cmdata. Weshallusethisvalue I low-energy response correction for the charge transfer
for the absorption in the subsequent analysis. inefficiency (CTI) is necessary, and we applied this to
This best fit ROSAT/Asca model was compared with theChandradata. AsofChandradataanalysissoftware
thedatafromthesameregionintheXMM-Newtoncam- package CIAO ver. 2.3 this correction can be applied
eras giving an unfitted reduced χ2 of 1.39 for pn and in the standard event processing. It is also necessary to
1.06forbothMoscamerascombined. Theratioofthese account for the ACIS excess low-energy absorption due
spectra to the ROSAT/Asca model can be seen in Fig. to hydrocarbon contamination. We used the acisabs
3. From this result we conclude that Mos low energy code for the correction to the auxiliary response func-
response is more consistent with previous data and sub- tion. The event grades used in the ACIS analysis were
sequently,wechoosetoignoreallpndatabelow1.1keV. GRADE=0,2,3,4 and 6.
Re-fitting the XMM data from the above region using TheclusterwasobservedwiththeChandraACIS-Ide-
0.3 10.0 keV for Mos data and 1.1 10.0 keV for tector array at two separate occasions for 10 ks each on
− −
pn leaving the absorbing column as a free parameter 2000-04-15 and 2001-01-07. Spectra were extracted for
yields n = 1.08 0.16 1020 cm−2, a temperature of the central 3′ region centered on the X-ray centroid at
H
± ×
6 Andersson & Madejski
Fig. 4.—The ratio of the Chandra ACIS-I spectra of Abell 1689 to the best fit model determined from the ROSAT and Asca analysis,
before (Left)and after (Right)the acisabs correction. Data used for the spectrum include eventswith ACIS grades 0, 2, 3, 4, and 6.
13h11m29s.4 01◦20′28′′ followingtheCIAO2.3Science gion. Pointsources with intensities greaterthan 3σ over
−
Threads for extended sources. Fig. 4 shows the ratio the average are excluded. The outer radii of the annuli
of the combined Chandra data to the best fit model de- are as follows: 15′′, 25′′, 35′′, 47.5′′, 60′′, 75′′, 95′′, 125′′,
termined from ROSAT and Asca above. The left spec- 165′′, 230′′ and 350′′.
trumisbeforeandtherightaftertheacisabscorrection. Average cluster properties were determined using all
Fitting the corrected spectrum to an absorbed MEKAL annular spectra simultaneously out to 693 h−1 kpc
model in the energy range 0.3 - 7.0 keV yields absorp- (0.61 r , 7.6 r ) using the same spectral fitting pro-
200 c
tionof1.7 0.6 1020cm−2,atemperatureof12.4 1.1 cedures as in Section 2.1.0. We use the optical redshift
± × ±
keV and abundances of 0.34 0.10 Solar using the op- of z = 0.183 (Teague et al. 1990) and the line of sight
tical redshift z=0.183. The di±screpant absorption found absorptionof N =1.75 1020 cm−2 as measured from
H
×
by Xue & Wu (2002) (using data 0.7 - 9.0 keV) is ap- ROSAT data, and also consistent with the Galactic col-
parently corrected for by acisabs and the value is in umn density (Section 2.2). The mean temperature of
agreement galactic absorption. However there is still a the cluster is found to be kT = 9.00+0.13 keV and the
−0.12
largediscrepancyintemperature,whichalsocanbeseen mean abundance 0.25 0.02 Solar. We note that leav-
from the high energy ends of Fig. 4. Part of this effect ing the redshift as a fr±ee parameter gives a best fit red-
can possibly be attributed to the high energy particle shift z = 0.173 0.003 (90% confidence range) which
backgroundbut more likely to uncorrectedinstrumental is considerably le±ss than measured via optical observa-
effects. Repeating all above steps for single pixel events tions. Considering Mos and pn data separately gives
only(GRADE=0)inordertoachievehigherspectralac- z =0.171 0.002 and z =0.178 0.003 respectively.
curacygivesusabestfittemperatureof7.2 0.4keV.We Inorder±totakeintoaccountth±ethree-dimensionalna-
±
cannotaccountforthedifferencesbetweenthetwoChan- ture of the cluster we consider the spectrum from each
dra data sets (using GRADE=0 vs. GRADE=0,2,3,4 of the 11 annuli to be a superposition of spectra from a
and 6 events) nor between the results of the Chandra number of concentric spherical shells intersected by the
and XMM spectral fits. We note here that the photon same annulus. The sphericalshells havethe same spher-
statistics resulting from the XMM observation is supe- icalradiiastheprojectedradiioftheannuli. Weassume
rior to that in the Chandra data, and since our analysis that each shell has a constant temperature, gas density
does not require the superior angular resolution of the andabundance. Thevolumeforeachannulus/shellinter-
Chandramirror,welimittheanalysisbelowtotheXMM sectionis calculatedto determine howlargea fractionof
data. emissionfromeachshellshouldbeattributedtoeachan-
nulus. Assigninga spectralmodelto eachsphericalshell
3. SPECTRALANALYSIS
we can simultaneously fit the properties of all spherical
3.1. Temperature and metallicity distribution for a shells.
spherically symmetric model In practice we use the method of
Arabadjis, Bautz, & Garmire (2002) where we have
To obtain a radial profile of cluster gas temperature,
a matrix of 11 11 MEKAL models with absorption in
abundance, and density, we first make the assumption
×
XSPEC. In the process of fitting the data in XSPEC,
that the cluster is spherical and that above properties
eachdatagroup(consistingofthreedatafiles: pn,Mos1
are only functions of radius. For this, we divide the im-
age of the cluster into 11 concentric annuli out to 5′50′′ and Mos2 spectra for each annulus) is fitted using
(693 h−1 kpc) centered on the X-ray centroid. For each the same set of models. For the central annulus (the
one with zero inner radius so it’s actually a circle),
annulus, we extract spectra from all EPIC cameras,and
which will intersect all 11 spherical shells, we will need
we set the inner and outer radii of each region by re-
to apply 11 MEKAL models to represent these. This
quiring that each annulus contains at least 9000 counts
per each Mos camera and 13000 for pn. This allows us meansthatwe havetoapply11MEKALmodelsto each
datagroup (annulus) with absorption where each model
to derive a reliable estimate of temperature in each re-
XMM Newton observation of Abell 1689 7
Fig. 5.— The radial metallicity distribution of Abell 1689 as Fig. 6.— The radial temperature distribution of Abell 1689 as
derived from the spectral deprojection fitting. derived from the spectral deprojection fitting.
been studied in detail by e.g. Pratt & Arnaud (2002)
represents the properties of one spherical shell. The whofindthatcorrectingforthe PSFredistributiongives
normalization of each shell model is set to be the ratio a profile that is consistent with an uncorrected profile.
of the volume that shell occupies in the cylinder that is Abell 1689 has a temperature profile without large tem-
the annulus/shell intersection to the volume it occupies perature variations and no large central flux concentra-
in the central annulus. Abundance and temperature tion. Weconcludethattheeffectoffluxredistributionin
are tied together for the models representing the same our case will be small and we do not attempt to correct
spherical shell. Of course not all annuli intersect every for this. The PSF is also weakly energy dependent, and
shell, and for those shells not intersected by the annulus to quantify its possible effect on the observed tempera-
to which they are attributed, the model normalization ture profile, we calculate the energy dependent flux loss
will be zero. The matrix of MEKAL models is thus from the central annulus and the effect on the central
triangular and can be fitted directly to the spectra we temperature. The difference in flux loss between various
have extracted from annular regions in the data. This energy bands (ranging from 1.5 to 7.5 keV) for the on-
approach allows for all data to be fitted simultaneously, axis PSF is 3%. We find that for a cluster with an
∼
and we do not have the problem with error propagation assumed temperature of 9 keV, this could give an error
which occurs when starting to fit the outermost annulus of the central temperature by at the most 0.5 keV. We
and propagating inward subtracting contributions from note that this is a maximum difference since in practice,
each previous shell. thefluxgainedfromouterannulicouldsomewhatreduce
The metallicity profile (Fig. 5) from the deprojection this effect by working in the opposite manner.
showssignsofincreasingabundancetowardthecenterof The luminosity of the cluster in the 0.5 10.0 keV
−
the cluster. Intemperature(Fig. 6)wefind anapparent band, calculated from the best fit model above (with
decrease for large radii (kT <8 keV), an effect that has z =0.183)givesLX (EdS) =1.02 1045 h−2 ergs s−1 for
×
been seen in analysis of other clusters with XMM (see an EdS (Ω = 1.0, Ω = 0.0) Universe or L =
M Λ X (CDM)
e.g. Pratt & Arnaud (2002)). Gas dynamic simulations 1.21 1045 h−2 erg s−1 for a CDM (Ω = 0.3, Ω =
M Λ
of the formation of galaxy clusters also show a decline 0.7) ×Universe. This corresponds to a bolometric lu-
of temperature at large radii (Evrard & Metzler 1996). minosity of L = 1.63 1045 h−2 erg s−1 or
bol (EdS)
We do not find a significant cooling in the cluster core L = 1.94 1045 h−2 e×rg s−1. All above val-
bol (CDM)
with the highest temperature (kT 9.5 keV) near the ×
∼ ues should include 10% as the absolute calibration er-
core radius: in fact, we will show in section 3.2 that the
ror of XMM. From Chandra analysis, Xue & Wu (2002)
temperature profile is not symmetric around the cluster find L =1.66 0.64 1045 h−2 ergs s−1 whereas
center. Wenotethatforcompleteness,wealsoperformed bol(EdS) ± ×
Mushotzky & Scharf (1997) find L = 1.77
the above analysis with the best-fit value of the redshift bol(EdS) ×
inferred from the X-ray data alone, and while the ex- 1045 h−2 ergs s−1 from Asca data, both in agreement
act values of temperature and elemental abundances are with our results. This does not provide any new in-
slightly different, about 0.2 keV lower for temperature, formation regarding the location of Abell 1689 in the
the general trends in the radial runs of the parameters Luminosity-Temperature(Mushotzky & Scharf1997)re-
are the same. lation and it is still in a close agreement with the trend
The limited point spread function (PSF) of the XMM suggested by other clusters.
mirrors is a potential problem especially for the annuli
3.2. Asymmetry analysis
locatedclosetothecentersincethosearenotmuchlarger
thanthe PSFFWHM of 6′′. Someofthe flux incident With the good quality XMM data, it is possible to
∼
on the central (circular) region will be distributed over verify the result of Xue & Wu (2002) that there is no
the outer annuli and vice versa. This flux redistribu- discrepancy between the optical and X-ray centers of
tion will have the effect of smoothing out the measured the cluster. We determine the center of X-ray emis-
temperature profile since all annuli will have some flux sionforAbell1689usingXIMAGE commandcentroid.
that originally belong in other annuli. This effect has We also include a measurement of the lensing center
8 Andersson & Madejski
17:00
18:00
19:00
-1:20:00
21:00
22:00
23:00
1 arcmin = 119 h-1 kpc (EdS)
24:00
13:11:40 30 20
Fig. 7.—Spatial distributionof spectral fit temperatures (6-10 keV)with superimposed logarithmic X-ray contours forA1689. 6keVis
marked as black, 10 keV is white, and intermediate temperatures are various shades of gray.
oftheICMradialtemperaturedistributionaswellasthe
Table 1. Comparison of the position of the cluster center of Abelloffset of optical and X-ray redshifts prompted us to an-
1689frommembergalaxies,gravitationallensing andX-raydata
alyze the spatial structure of the cluster. In Fig. 7 we
show the spatial temperature distribution. Spectra were
Method Center Ref.
extractedinrectangularregionsandfittedusingthesame
R.A.(J2000.) Dec. (J2000.)
method as in Section 2.1.0. The temperature in the fig-
X-ray(ROSAT) 13h11m29s.1 −01◦20′40′′ 1 ure scales linearly from6 keV(black) to 10 keV (white).
X-ray(Chandra) 13h11m29s.45 −01◦20′28′′.06 2 The errors on the temperature of the inner 16 regions
Lensing 13h11m29s.6 −01◦20′29′′ 1
are 0.5keV,whiletheerrorsontheouter8regionsare
Optical 13h11m29s.44 −01◦20′29′′.4 3 ∼
X-ray(XMM) 13h11m29s.4 −01◦20′28′′ 4 ∼1.0 keV. We clearly see an asymmetry in the temper-
ature around the cluster center with an overall increase
toward the northeast.
References. — (1) Allen (1998) ; (2) Xue&Wu (2002); (3)
To check the consistency of these results and to in-
Ducetal.(2002);(4)Thisstudy.
crease our accuracy, we re-group the data in larger spa-
tial regions and perform a fit using the same above pro-
cedure. We first fit the data keeping the redshift frozen
fromDuc et al.(2002) andthe X-raymeasurementfrom at the optical value. Temperatures derived from this fit
ROSAT by Allen (1998) (Table 1). We find that all val- areshownin Fig. 8 (Left). Thereafter, we leavethe red-
uesagreewithin 3′′ exceptthe ROSATestimate,the off- shift as a free parameter and re-fit the data. The fitted
setofwhichweattributetouncertaintiesinROSATHRI redshifts for these regions are shown in Fig. 8 (Right).
astrometry. This apparently perfect agreement among All errors in Fig. 8 are 90% confidence limits. In the
X-ray, lensing, and optical centers leads us to conclude temperaturemap,weseeacleardiscrepancybetweenthe
that the ICM density peak and the central dominant northernandsouthernpartoftheclusterwithahintofa
galaxy is probably located at the bottom of the dark temperaturegradientinthesouthwest–northeastdirec-
matterpotentialwell. Still,theapparentnon-uniformity
XMM Newton observation of Abell 1689 9
9.30+-0.34 keV 0.172+-0.003
8.99+-0.30 keV 0.185+-0.006
8.29+-0.37 keV 0.167+-0.008
8.08+-0.31 keV 0.170+-0.009
1 arcmin = 119 h-1 kpc (EdS) 1 arcmin = 119 h-1 kpc (EdS)
Fig. 8.— The spatial distribution of temperature (left) and redshift (Right) in the central region of Abell 1689 with 90% confidence
intervals.
tion. The redshift map reveals a high redshift structure sentially the same peak position of 8.051 0.001 keV.
±
to the east at z =0.185 0.006 separated from the rest This corresponds to a possible artificial redshift offset of
±
of the cluster at z 0.17. Analyzing the Mos and pn maximum ∆z =0.0005. To explain the difference in the
∼
dataseparatelyforthishighredshiftregiongivesabroad offsetmeasuredbyusasaninstrumentaleffect,wewould
minimuminχ2 atz =0.187 0.008forMoswhereaspn havetohaveanoffset(sayattheFeKlineat6.7keV)of
±
shows several minima in the z =0.165 0.200 range. 62eV,andnotonthe orderof1eV,asinferredfromthe
−
This region may indicate a subcluster falling inward Cu K instrumental line. Hence we conclude that there
awayfromtheobserver. Thisisfurthersupportedbythe is no gain shift that could alter our redshift measure-
optical data, indicating that there are also high-redshift ments between pn CCDs 4 and 7. According to XMM
giant elliptical galaxies in this region (Fig. 1): it is in- calibration documentation (Kirsch 2003) the magnitude
teresting to speculate if this is actually the remains of a of calibration errors for pn & Mos should be no larger
cluster core? Especially intriguing is the apparent coin- than 10 eV.
cidence between the smaller subcluster as suggested by
4. MASSPROFILE
strong lensing (Miralda-Escud´e & Babul 1995) and our
high redshift gas region approximately 1′ northeast of 4.1. Mass calculation
the main cluster. Another possibility is that the red- Ifweassumethattheclusterissphericalwithasmooth
shift variation is due to large bulk motions of the intra- static gravitational potential and that the intra-cluster
cluster gas. It has been shown in gas dynamic simula- medium is a pressure-supported plasma, we can employ
tions that clusters with apparently relaxed X-ray pro- thehydrostaticequilibriumequation. ThecircularX-ray
filescanhavecomplexgas-velocityfieldsandbefarfrom isophotes (Fig. 1) generally indicate that a cluster is in
relaxed (Evrard& Metzler 1996). This kind of motion dynamicalequilibrium. The hydrostaticequationcanbe
could give rise to non-thermal emission from shocks etc. written as (Sarazin 1988) :
In our analysis we cannot distinguish between the two
kT (r) r d ln T (r) d ln ρ (r)
possibilities of bulk motion and subclustering. M(r)= g g + g (1)
Thismeasurementofnon-uniformredshiftdistribution − Gmpµ (cid:18) d ln r d ln r (cid:19)
ofthe X-rayemitting gasis important,andtoverifyifit where M(r) is the enclosed total gravitating mass en-
couldbeduetoinstrumentaleffects,weinvestigatedthis closed within a sphere of a radius r, T (r) and ρ (r)
g g
in more detail. The data in the regions above are from are temperature and density of the ICM at r, µ is the
different CCD chips in the pn data whereas for Mos all mean particle weight and m is the proton mass. Using
p
data are from the same chip. Since the pn camera pro- thetemperatureandnormalizationsfromthespectralde-
videsabouthalfofthedata,wewanttoverifythatthere projectionfittingwecalculatethetotalgravitatingmass.
are no gain shifts between the CCDs, which, if present, Errors are treated by error propagation.
could easily cause such an effect. Most of the cluster Themassdistributionisfittedtoasingularisothermal
emission is on the CCD chips 4 and 7. To test for any sphere (SIS)
possible gain offset, we extracted the data from each of 2σ2r
the chips individually to verify the position of the inter- M(r)= r (2)
G
nalfluorescentCuKαline,withthedominantcomponent
where σ is the 1-dimensional velocity dispersion, which
at 8.0478 keV. The spectra in the range 7.8 8.2 keV r
− is used here for a comparison with previous mass esti-
are fitted to a Gaussian profile: both datasets yield es-
mates.
10 Andersson & Madejski
The predicted density profile from CDM hierarchal
clustering according to Navarro et al. (1997) for dark
matter halos is
ρ(r) δ
c
= (3)
ρ (z) (r/r )(1+r/r )2
crit s s
where ρ (z) is the critical energy density at halo red-
crit
shift z and δ is characteristic density defined by
c
200 c3
δ = (4)
c
3 [ln(1+c) c/(1+c)]
−
wherec=r /r istheconcentrationofthehalodefined
200 s
as the ratio of the virial radius r to r , which in turn
200 s
isacharacteristicradiusintheNFWmodel. Thecritical
Fig. 9.—Spherical mass profile of Abell1689 (pluses) with best
density at redshift z for a flat (Ω0 =1) Universe is fit NFW model (solid line) and singular isothermal sphere model
(dot-dashed). The singularisothermal sphere isclearly notavery
ρ (z)= 3H02[Ω (1+z)3+Ω ] (5) good fit to the data (see also Table 2). For comparison, the mass
crit M Λ of the intra-clustergas is included (asterisks).
8πG
where H is the Hubble constant, Ω and Ω are the
0 M Λ
current contributions of matter and vacuum energy re-
spectively to the energy density of the Universe. inthe Allen et al. (2003) sampleandsignificantlylower
More recent numerical studies suggest a steeper core thanthemean. Wefindthatinourestimatefgas hasnot
slope and a sharper turn-over from small to large radii convergedtoaconstantandthismighthelpexplainpart
(Moore et al 1999). Both models can be generalized as of the discrepancy. However,it may be the case that for
many clusters f does not converge until well beyond
ρ gas
ρ(r)= 0 (6) r2500.
(r/rs)γ[1+(r/rs)α](β−γ)/α Comparingthemassandtemperatureatr2500 ofAbell
1689 to the M-T relation derived for a set of relaxed
(Zhao1996),whereγandβcharacterizethedensityslope
clusters (Allen & Fabian 2001) shows a low mass for the
at small and large radii respectively whereas α deter-
temperature of Abell 1689. The M-T relation predicts
minesthesharpnessintheturn-over. Moststudiesagree
on β = 3 but the value of γ is still being debated. The H(z)/H0 M2500 = 4.5 1014M⊙( 10%) for a 9 keV
× × ±
clusterwhereH(z)istheHubbleconstantattheredshift
NFW and Moore profiles fit into the parameter space
ofthecluster. ForAbell1689wefindH(z)/H M =
(α,β,γ) as (1,3,1) and (1.5,3,1.5) respectively. 0 2500
We choose to fit our data to the NFW model (Eq. 3) 2.4 1014M⊙( 15%),significantlylower. The×aboveval-
× ±
ues were derived assuming a flat ΛCDM cosmologywith
which, when integrated over r, yields
Ω = 0.3, Ω = 0.7 and h = 0.7. The unusually low
M Λ
M(r)=M [ln(1+r/r )+(1+r/r )−1 1] (7) massmaybe due tothe factthatthe massofAbell 1689
0 s s
× −
seems to increase steadily beyond r . However, we
where M = 4πρ δ r3. We find that the data give the 2500
0 c c s note that a lower mass (than would be predicted by the
best fit for the NFW model with c=7.2+1.6 and r =
−2.4 200 M-Trelation)is notanuncommonfeaturefor unrelaxed
1.13 0.21h−1 MpcwhereastheSISfitgivesσr =918 clusters (cf. Smith et al. (2003)).
27 km± s−1. The total mass data and models are show±n For completeness, we note that the calculated total
in Fig. 9 together with the mass of the X-ray emitting massincludestheICMandgalaxymasscontributionsas
gas Mgas. Model parameters are summarized in Table well as the dark matter. The proper way of fitting the
2. For a cosmology with ΩM = 0.3 and ΩΛ = 0.7, the NFW model would be to subtract these contributions
best fit NFW model changes to c = 7.7+−12..76 and r200 = prior to performing the fit. The NFW model for the
1.31 0.25 h−1 Mpc. actual dark matter halo is not used in this paper; we
±
Another cosmologically important quantity for clus- only calculate the total mass profile.
ters is the fraction of the total mass that is in the X-
ray emitting gas, f = M /M . Allen et al. 4.2. Core slope
gas gas TOT
(2003) analyze data from the Chandra Observatory for
The mass data were fitted to a simple power law
10 dynamically relaxed clusters between z = 0.09 and (M(< r) rγ) in the ranges 20 90 h−1 kpc and
z = 0.46 and measure an average redshift independent 200 500 ∝h−1 kpc. We find the b−est fit of the slope
f =0.108 0.014 at the r radius (where the total −
gas 2500 of the matter profile to to be 1.73 0.34and 0.72 0.32
±
massdensityis2500timesthecriticaldensityatthered- ± ±
for small and large radii, respectively. This corresponds
shiftofthecluster). Thecosmologywherethisisvalidis tototalmassdensityslopes(ρ rα)ofα= 1.27 0.34
a flat ΛCDM cosmology with ΩM = 0.291+−00..004306 assum- and α = 2.28 0.32, in go∝od agreement−with±what
ingΩ h2 =0.0205 0.0018andh=0.72 0.08. Theer- is expected−from±numerical simulations of CDM hierar-
b
± ±
roronf aboveisthermsdispersionoftheAllen et al. chal clustering. We note here that Bautz & Arabadjis
gas
(2003) sample which is comparable to the individual er- (2002) have measured the density profile of Abell 1689
rorsonf . With ourbest fit NFW modelin the above for r < 100 h−1 kpc using Chandra data and found
gas
cosmology we find f = 0.072 0.008 for Abell 1689 α 1.3.
gas
± ∼−
at the r radius. This is lower than all the clusters We do not observe a flattening of the core density
2500