Astronomy & Astrophysics Far-infrared emission from intracluster dust in Abell clusters*

17 Pages·2008·0.59 MB·English

A&A 383, 367{383 (2002) Astronomy
DOI: 10.1051/0004-6361:20011613 &
(cid:13)c ESO 2002 Astrophysics
Far-infrared emission from intracluster dust in Abell clusters?
M. Stickel1, U. Klaas1, D. Lemke1, and K. Mattila2
1 Max{Planck{Institut fu¨r Astronomie, K¨onigstuhl 17, 69117 Heidelberg, Germany
2 HelsinkiUniversity Observatory,PO Box 14, 00014 Helsinki, Finland
Received 27 July 2001 / Accepted 7 November2001
Abstract. The ISOPHOT instrument aboard ISO has been used to observe extended FIR emission of six Abell
clusters. Strip scanning measurements with crossing position angles centered on the clusters were carried out at
120(cid:22)mand180(cid:22)m.Therawpro(cid:12)lesoftheI =I surfacebrightnessratioincludingzodiacallightshowa
120 (cid:22)m 180 (cid:22)m
bumptowardsAbell1656(Coma),dipstowardsAbell262andAbell2670,andarewithoutclearstructuretowards
Abell400,Abell496,andAbell4038.Aftersubtractionofthezodiacallight,thebumptowardsAbell1656isstill
present,whilethedipstowardsAbell262andAbell2670arenolongernoticable.Thisindicatesalocalized excess
ofemittingmaterialoutsidetheGalaxytowardsAbell1656withpropertiesdi(cid:11)erentfromthegalacticforeground
cirrus, while the behavior in Abell 262 and Abell 2670 can be reconciled with galactic cirrus structures localized
on the line-of-sight to these clusters. The excess of (cid:25)0.2 MJy/sr seen at 120(cid:22)m towards Abell 1656 (Coma)
is interpreted as being due to thermal emission from intracluster dust distributed in the hot X-ray emitting
intracluster medium. The integrated excess flux within the central region of 100{150 diameter is (cid:25)2.8 Jy. Since
the dust temperature is poorly constrained, only a rough estimate of the associated dust mass of MD (cid:25)107M(cid:12)
canbederived.Theassociatedvisualextinctionisnegligible(A (cid:28)0:1mag)andmuchsmallerthanclaimedfrom
V
opticalobservations.Noevidenceisfoundforintraclusterdustintheother(cid:12)veclustersobserved.Theabsenceof
any signature for intracluster dust in (cid:12)ve clusters and the rather low inferred dust mass in Abell 1656 indicates
thatintraclusterdustislikelynotresponsiblefortheexcessX-rayabsorptionseenincooling flowclusters.These
observationstherebyrepresentafurtherunsuccessfulattemptindetectingthepresumed(cid:12)nalstageofthecooling
flowmaterial,inaccordwithquiteanumberofpreviousstudiesinotherwavelengthsregions.Finally,theobserved
dimmingofthehigh-redshiftsupernovaeisunlikelybeattributabletoextinctioncausedbydustintheintracluster
or even a presumed intercluster medium.
Key words. galaxies: clusters: general { galaxies: clusters: individual: Abell 262, Abell 1656, Abell 2670 {
intergalactic medium { infrared: general
1. Introduction the cooling gas drops out of thermal equilibrium, possi-
bly leading to the formation of dense clouds and eventu-
The analysis of the X-ray and UV emission of the hot
ally to stars. X-ray spectra and image decomposition in-
(Te (cid:25) 1{10 keV) electron gas trapped in the potential dicate an extended excess absorption of soft X-rays with
well of galaxyclusters (e.g. Sarazin1986)has revealedan an HI column density of (cid:25)1021 cm−2 above the galactic
increasingly complex picture of the intracluster medium
foreground (e.g. White et al. 1991; Allen & Fabian 1997;
(ICM). High resolution X-ray images reveal the presence Sarazin1997;Allen2000).However,Arabadjis&Bregman
of small-scale structures of di(cid:11)erent temperatures (e.g.
(2000) (cid:12)nd signi(cid:12)cantly lower or even no additional ab-
Donnellyetal.1999,andreferencestherein).Highcentral sorbing column densities in a number of other clusters.
densities, lower temperatures, and cooling times signi(cid:12)- A spatially extendedEUV emissionof non-thermalorigin
cantly shorter than a Hubble time in the centers of many
(En(cid:25)lin et al. 1999, and references therein) above the ex-
clustershavebeeninterpretedasevidenceforcoolingflows pected flux from the hot X-ray gas in at least the Coma
(e.g. Fabian 1994; White et al. 1997; Allen 2000), where
and possibly other clusters seems to be fairly established
(Lieu et al. 1996a,b; Mittaz et al. 1998; Lieu et al. 1999;
Send o(cid:11)print requests to: M. Stickel, Bowyer et al. 1999), although an accompanying low en-
e-mail: [email protected] ergyX-rayexcessemissionisuncertain(Dixonetal.1996;
? Based on observations with ISO, an ESA project with in- Arabadjis & Bregman 1999). Overall, these observations
struments funded by ESA Member States (especially the PI indicate a striking departure from a nearly homogeneous
countries:France,Germany,TheNetherlandsandtheUK)and
with the participation of ISASand NASA.
Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20011613
368 M. Stickel et al.: Intracluster dust in Abell clusters
intraclustergastowardsa spatiallyinhomogeneousmulti- (Shaver 1987;Masci 1998),a signi(cid:12)cant fraction of which
phase ICM (Bonamente et al. 2001; Lieu et al. 2000). are viewed through clusters. Historically, this has been
Despite quite some observationale(cid:11)ort, neither the (cid:12)- the (cid:12)rst attempt to (cid:12)nd evidence for the existence of
nal stage of the cooling intracluster gas nor the mate- ICD (Zwicky 1957, 1962). Despite quite some e(cid:11)ort to
rial responsible for the excess absorption has been de- search for the visual extinction of background quasars
tectedatotherwavelengths(e.g.Sarazin1997;Laor1997; and galaxies seen through foreground galaxy clusters,
Koekemoeretal.1998;Miller etal.1999;Allen2000,and no common consent has been achieved whether proper-
references therein). It has been suggested that the ab- ties of background sources are actually a(cid:11)ected by the
sorbing material is present as either very cold molecular presence of dust in foreground clusters (Girardi et al.
clouds(Ferlandetal.1994)ordensecoldcloudswherethe 1992; Ferguson 1993; Maoz 1995, and references therein).
shielding allows the formation of dust grains onto which Aguirre(1999a,b)andSimonsen&Hannestad(1999)have
all volatiles have frozen (Daines et al. 1994; Fabian et al. drawn attention to the possibility that the observeddim-
1994). ming of distant type Ia supernovae might be caused by
The measured line emission of heavy elements (e.g. interveningdustratherthanresultingfromanon-zerocos-
Mushotzky et al. 1996; Fukazawa et al. 1998) clearly in- mological constant.
dicates that the ICM contains a processed component, From a comparison of Ly (cid:11) and UV continuum fluxes
likely the result of galactic winds, stripping of the inter- in 10 cooling flow clusters, Hu (1992) found an average
stellar medium (ISM) of cluster members by ram pres- excess reddening of EB−V (cid:25)0:19 above the galactic fore-
sure, or tidally removed ISM during merging with other groundabsorption.ThiswasattributedtoICDspreadout
galaxygroups.Theseprocessesalsoadddustto the ICM, over a large fraction of the cluster volume. Model calcu-
which in turn is a possible candidate for the excess X-ray lations of the extended FIR emission from ICD were pre-
absorber. The (cid:12)rst direct evidence for dust in the ICM sented by Dwek et al. (1990) and Hu (1992), who con-
is an observed oxygen K edge in an X-ray spectrum of cluded that the expected FIR flux was just within reach
the Perseus cluster attributed to neutral material, most of the available IRAS data and the then upcoming ISO
likely dust grains condensed out of the ICM (Arnaud & mission.
Mushotzky 1998). The FIR emission as a direct indicator for ICD has
In equilibrium conditions, the predicted intracluster been searched for at 60 (cid:22)m and 100 (cid:22)m on IRAS ISSA
dust (ICD) temperatures are rather uncertain with tem- plates (Wheelock et al. 1994) by Wise et al. (1993).
peratures in the range 10{20 K (Dwek et al. 1990; Removing the large scale galactic foreground cirrus emis-
Loewenstein & Fabian 1990;Braine et al. 1995),but pos- sion,evidencefordi(cid:11)useexcessFIRemissionoflowstatis-
sibly much higher (O’Dea et al. 1994; Voit & Donahue tical signi(cid:12)cance was found in the direction of severalout
1995). The thermal emission of the ICD is thus to be ex- of 56 clusters. Sub-millimeter observations of 11 cooling
pected in the FIR. If not shielded in dense clouds, the flowclusters byAnnis & Jewitt(1993)did notdetect any
highkinetictemperaturesdestroydustgrainswithatypi- emissionfromdustneartheclustercenters.Withthemore
calsizeof(cid:25)0.1(cid:22)mbysputteringovertime scalesofafew sensitive sub-millimeter bolometer array at the JCMT,
108{109years(Dweketal.1990;Tielensetal.1994;Dwek Edgeetal.(1999)detectedthedustemissionofthecentral
et al.1996).Verybig (>10(cid:22)m) dust grainsintroducedto galaxiesfrom2outof5coolingflowclusters.However,the
accountfortheobservedFIRspectraofgalaxies(Rowan{ latter two studies were insensitive to extended emission
Robinson 1992) would have a signi(cid:12)cantly longer lifetime of ICD on angular scales larger than a few arc minutes.
of theordera Hubble time. Possibly,the dust destruction Hansen et al. (2000) used ISOPHOT FIR maps to search
ismoree(cid:14)cientforsmaller(<0.1(cid:22)m)grains,leavingpref- for dust coincindent with the central galaxies of cooling
erentiallybehindthebiggergrains,whichmoreovermight flow clusters, but obtained only inconclusive results.
have a flatter extinction curve than those observedin the Comparedto FIR maps,strip-scanning measurements
Milky Way (Aguirre 1999b). Furthermore, the tempera- crossing the cluster provide a means to search a larger
tures in the central cooling flow regions of clusters might fraction of the cluster volume, and to reach the FIR fore-
generally be lower, which, in addition to the shielding by ground level far o(cid:11) the cluster centre. Furthermore, the
clouds, would contribute to an increased grain lifetime. separationofextendedintraclusterdustemissionfromthe
If dust is the absorbing material inferred from the foreground zodiacal and galactic cirrus emission can be
X-ray observations, a signi(cid:12)cant fraction of the absorbed accomplishedbyusingobservationsattwodi(cid:11)erentwave-
X-ray radiation of >(cid:24)1043ergs−1 is re-emitted in the FIR, lengthslongwardoftheIRASbands.Thiscombinedtech-
and dust would then constitute a major coolant of the nique was employed in a pilot study to search for dust in
ICM (Bregman 1992; Sarazin 1997). The FIR emission the intracluster medium of the Coma cluster (Abell 1656,
from ICD would then also be relevant for the correct in- z = 0:0232) with ISOPHOT (Lemke et al. 1996; Lemke
terpretation of measurements of the Sunyaev-Zeldovich & Klaas 1999), the photometer instrument aboard ISO
e(cid:11)ect in the sub-mm wavelength range (Khersonskii & (Kessler et al. 1996; Kessler 1999). An enhanced 120(cid:22)m
Voshchinnikov 1985; Lamarre et al. 1998). emission within the central region of (cid:25)100 ((cid:25)0.3 Mpc,
Intracluster dust may reveal itself by optical depth H = 70kms−1Mpc−1;q = 0:5) diameter of the clus-
0 0
e(cid:11)ects in cosmological investigations of distant objects terwasobserved(Stickeletal.1998).Inafollow-upstudy,
M. Stickelet al.: Intracluster dust in Abell clusters 369
Table 1. Observed Abell clusters.
Cluster (cid:11) (cid:14) l b Redshift Richness- R
2000 2000 Abell
[(cid:14)] [(cid:14)] Class [0]
(1) (2) (3) (4) (5) (6) (7) (8)
Abell 262 01h52m46:4s +36(cid:14)0900600 136:58 −25:09 0.0161 0 110
Abell 400 02h57m38:6s +06(cid:14)0200000 170:24 −44:94 0.0238 1 75
Abell 496 04h33m37:1s −13(cid:14)1504200 209:57 −36:48 0.0327 1 56
Abell 1656 (Coma) 12h59m35:7s +27(cid:14)5703800 58:16 +88:01 0.0232 2 75
Abell 2670 23h54m13:7s −10(cid:14)2500900 81:32 −68:52 0.0761 3 26
Abell 4038 (Klemola 44) 23h47m41:9s −28(cid:14)0801900 25:08 −75:90 0.0283 2 64
Notes:
- Coordinates (Cols. 2, 3) of central sky position of ISO scans.
- Redshifts (Col. 6) and richness classes (Col. 7) taken from NED.
- Abell radii (Col. 8) according toRudnick& Owen (1977).
ISOPHOTwasusedtoobserve(cid:12)vemoregalaxyclustersin ISOPHOTdetectorinsideacharacteristicclusterAbellra-
a similar way, and the results, together with a re-analysis dius, while the nearest clusters would have required pro-
of the Coma data, are described in the following. hibitivelylongintegrationtimestocoverasigni(cid:12)cantfrac-
tion of the cluster diameter. Since a re-observation of the
Coma cluster became impossible, Abell 4038 was chosen
2. Cluster selection
as a closely resembling cluster, particularly with respect
In the (cid:12)rst intracluster dust observation with ISOPHOT tothemultipolemomentsofitsX-raymorphology(Buote
(Stickeletal.1998),theComaclusterwasselectedbecause & Tsai 1996).
it is the cluster whereearlyopticalextinction studies had TheobservedAbellclustersarelistedinTable1,which
found evidence for ICD (Zwicky 1962; Karachentsev & gives for each cluster (Col. 1) the coordinates, on which
Lipovetskii 1969), where di(cid:11)use optical emission from in- the ISOPHOT observations were centered (Cols. 2, 3),
tracluster material was already known (Welch & Sastry its galactic coordinates (Cols. 4, 5), the cluster redshift
1971; Mattila 1977), and where a detailed theoretical (Col. 6), the cluster richness class (Col. 7), and the clus-
study(Dweketal.1990)showedthatthepredicteddi(cid:11)use ter Abell radius according to Rudnick & Owen (1977)
FIR emission was within reach for ISOPHOT. (Col. 8).
Primary candidates for additional observations were
Abell 262 and Abell 2670, since Wise et al. (1993) had
3. Observations
found marginal evidence for extended FIR emission on
IRAS ISSA plates. Because the Coma cluster is known The observations were carried out with the C200 camera
to undergo merging with smaller galaxy groups (Colless of ISOPHOT (Lemke et al. 1996; Lemke & Klaas 1999),
& Dunn 1996; Vikhlinin et al. 1997; Burns et al. 1994b) a 2(cid:2)2 pixel array of stressed Ge:Ga with a pixel size of
the dust inferredfrom the di(cid:11)use FIR emissionwas inter- 890:04. For eachcluster,linear scans acrossthe cluster cen-
preted as being transferredratherrecentlyto the ICM by ter at two di(cid:11)erent position angles wereobtained, eachof
tidal stripping of cluster galaxyISM (Stickel et al. 1998). which was observed with both the C 120 and C 180 (cid:12)l-
ExtendedFIRemissionfromtheICMthusmightbefound ters(referencewavelength120(cid:22)mand180(cid:22)m, equivalent
preferentially in recent mergers or young clusters, which widths 47(cid:22)m and 72(cid:22)m, respectively) at the same sky
makes the testable prediction that old clusters should positions. The positional o(cid:11)set between subsequent sky
show weak or no extended FIR emission. It was there- measurements was 30, which resulted in almost no detec-
fore attempted to (cid:12)nd more targets alongthe sequenceof toroverlap.Theintegrationtime ateachskypositionwas
X-ray morphologies outlined by Jones & Forman (1992) (cid:25)50sforAbell1656and(cid:25)80s fortheotherclusters,dur-
and quanti(cid:12)ed as a possible dynamical and evolutionary ingwhich6{16integrationrampswith127non-destructive
sequence by Buote & Tsai (1995, 1996). Particularly, the readouts were obtained.
supposedly young clusters with irregular and the suppos- Details oftheobservationsarelistedinTable2,which
edly old virialized clusters with circular symmetric X-ray gives for each cluster (Col. 1) the date (Col. 2) and ISO
emission were considered. revolution number of the observation (Col. 3), the num-
ClustercandidatesfromthelistofBuote&Tsai(1996) ber of sky positions (Col. 4) and the fraction of the Abell
weresubjecttoconstraintsintheskyvisibilitybyISOand radius covered by the scan (Col. 5) at each position an-
in cluster redshift. The latter was necessary because at gle (Col. 6) together with the total on-target integration
toolargeredshiftsonlyveryfewnon-overlappingskyposi- time (Col. 7). Finally, Col. 8 gives the zodiacal light con-
tionswouldhavebeenpossibletoobservewiththe30 wide tributionfor 120(cid:22)mand180(cid:22)mfortheobservingdatein
370 M. Stickel et al.: Intracluster dust in Abell clusters
Table 2. Observational details.
Cluster Observing ISO # Steps Cluster Position Angle On-target Zodiacal Light at
Date Revolution Coveragea Time 120(cid:22)m/180(cid:22)m
[(cid:14)] [s] [MJy/sr]
(1) (2) (3) (4) (5) (6) (7) (8)
Abell 262 Jan. 7, 1998 784 31 0.82 45, 135 10250 2.7/1.8
Abell 400 Feb.21, 1998 829 19 0.76 40, 130 6530 5.1/2.4
Abell 496 Mar. 12, 1998 848 17 0.91 90 2950 1.9/0.9
Abell 496 Mar. 19, 1998 855 17 0.91 180 2950
Abell 1656 (Coma) Jul. 21, 1996 247 16 0.64 36, 82 3480 2.3/1.1
Abell 2670 Nov.21, 1997 736 17 1.96 19, 84 5640 3.8/1.6
Abell 4038 (Klemola 44) Dec. 24, 1997 769 19 0.89 120 3260 3.0/1.4
Abell 4038 (Klemola 44) Dec. 25, 1997 771 19 0.89 30 3260
Notes:
a Scan length in unitsof the Abell radius.
Table 3. Physical properties of observed clusters.
Cluster X-Ray N Temperature Cooling Flow GasMass L
HI XBol
Morphology [1020 cm−2] [keV] [M(cid:12)yr−1] [1012 M(cid:12)] [1044 ergs−1]
(1) (2) (3) (4) (5) (6) (7)
Abell 262 elliptical [4] 5.3 [1] 2.4 [1] 27 [3] 2.5 [3] 0.9 [1]
10.5 [4] 2.5 [2] 9 [2] 6.5 [2] 0.5 [2]
1.4 [4] 47 [7] 0.4 [4]
Abell 400 irregular [6,8] 8.8 [1] 2.5 [1] 0 [2] 11.1 [2] 0.7 [1]
2.2 [2] 0 [10] 15.0 [6] 0.6 [2]
Abell 496 single 4.4 [1] 3.9 [1] 112 [7] 2.7 [2] 5.8 [1]
symmetric [8] 12.0 [5] 4.8 [2] 138 [2] 74.1 [2] 7.9 [2]
3.3 [5] 95 [3]
Abell 1656 elliptical [11] 0.9 [1] 8.3 [1] 0 [3] 27.8 [3] 12.5 [1]
(Coma) 7.3 [2] 0 [2] 75.6 [2] 9.1 [2]
Abell 2670 double [8,9] 2.7 [1] 3.9 [1] 41 [2] 40.7 [2] 3.8 [1]
3.6 [9] 3.7 [2] 0 [9] 3.5 [2]
4.2 [9] 18 [12]
Abell 4038 elliptical [8] 1.5 [1] 3.3 [1] 87 [3] 4.3 [3] 2.2 [1]
(Klemola 44)
References:
[1] David et al. (1993); [2] White et al. (1997); [3] Peres et al. (1998); [4] David et al. (1996); [5] MacKenzie et al. (1996);
[6] Beers et al. (1992); [7] Edge et al. (1992); [8] Buote & Tsai (1996); [9] Hobbs & Willmore (1997); [10] Allen et al. (1997);
[11] Vikhlinin et al. (1997); [12] Wise et al. (1993)
Col. 2 taken from Kelsall et al. (1998) for all clusters ex- possible, several entries for each quantity together with
cept Abell 2670, which in turn was taken from the yearly the literaturereferencehavebeen listed to giveanindica-
averagedzodiacallightmapgivenbyLeinertetal.(1998). tion for the spread in the determination of these values.
Physical properties of the observed clusters are col- The six observed clusters cover the range from relaxed
lected inTable3,which givesforeachcluster (Col.1)the elliptical (Abell 262) to irregular (Abell 400) X-ray mor-
overallX-raymorphology(Col.2),thegalacticforeground phologies, from cool (Abell 262, Abell 400) to hot (Abell
column density(Col. 3),the X-raytemperatures (Col.4), 1656)X-raytemperatures,fromno(Abell400,Abell1656)
the estimated cooling flow rate (Col. 5), the derived gas to a signi(cid:12)cant (Abell 496) cooling flow rate, and about
mass (Col. 6), and the X-ray luminosity (Col. 7). Where an order of magnitude in galactic NHI column density.
M. Stickelet al.: Intracluster dust in Abell clusters 371
4. Data reduction
To push the detection of the expected weak signals at
theselongwavelengthstothelimits,andtogetcon(cid:12)dence
in detected features, severaldi(cid:11)erent methods for the sig-
nal derivation were used in parallel. Already in the case
of the Coma cluster (Stickel et al. 1998),the pairwisedif-
ferencesofconsecutiverampread-outsratherthansignals
fromthefullramp(cid:12)ttinghaveproventoprovidethemost
robust signals, particularly if only a few ramps at each
sky position are available. To get rid of pairwise readout
di(cid:11)erences a(cid:11)ected by cosmic ray hits, the robust outlier-
insensitive myriad estimator was computed and 15% of
the most deviant signals as measured by the absolute de-
viation were cut o(cid:11). This outlier removal is similar to a
median absolute deviation trimming, but instead of the
initial median, the sample myriad is used to determine Fig.1. One-dimensional model of a FIR scan measurement
the outliers. The sample myriad value in turn is a robust containingcirrusonly.Adi(cid:11)use(straight line) andalocalized
estimatorofthemode(mostcommonvalue)ofadistribu- cirrus component (Gaussian) at 120(cid:22)m (top left) and 180(cid:22)m
tion but does not require binning of the actual data set, (topright)havesimilartemperatures,hencethesummedinten-
and is easily computed by minimizing a particular cost sitiesat120(cid:22)m(bottomleft,continuous)and180(cid:22)m(bottom
function with a tuning constant set to a small value (for left, dashed) havea constant ratio (bottom right).
details see Kalluri & Arce 1998).
Separate sets of (cid:12)nal signals for each sky posi-
FCS measurements were linearly interpolated in time to
tion were derived from this tail-trimmed pairwise dis-
calibrate each sky position of each scan in each (cid:12)lter sep-
tribution by computing the mean, median, myriad
arately. Because the ISOPHOT calibration has been up-
(Kalluri & Arce 1998), and the annealing M estima-
dated since the publication of the Coma results (Stickel
tor (Li 1996). Additionally, the cumulative trimmed
et al. 1998) it was re-analyzed to get a homogeneous set
distribution was (cid:12)tted with the analytical function
of data.
y =c0(1+exp((x−c3)=c1))c2), the (cid:12)rst derivative of A comparison of the various signal derivation meth-
which resembles very closely a Gaussian. In this case, the
ods showed that the most robust results as judged by
(cid:12)nal signal value was derived from the maximum of the
thescatterinthescansignalsandI =I surface
120 (cid:22)m 180 (cid:22)m
(cid:12)rst derivative of the (cid:12)tting function.
brightnessratioswereobtainedwiththetrimmedpairwise
Signals from the much smaller distribution of ramp distribution using the myriad value as central estimator,
slopes at each sky position, obtained by (cid:12)tting (cid:12)rst order while the median and mean values of the trimmed distri-
polynomials to the deglitched readouts of each integra- bution showed a larger scatter. Therefore, only the cal-
tionramp,werederivedforcomparison.Theinitialramp- ibrated data derived from the pairwise myriad method
(cid:12)tting step was done within the ISOPHOT Interactive will be considered further. However, the gross features
Analysis PIA1 version9.0 (Gabriel et al. 1997). Since the in the I =I surface brightness ratios described
120 (cid:22)m 180 (cid:22)m
number of ramps for one sky position was rather small, below were present with decreasing con(cid:12)dence in the
a subsequent e(cid:14)cient rejection of disturbed ramp slopes I =I pro(cid:12)le of all di(cid:11)erent methods used.
120 (cid:22)m 180 (cid:22)m
was not possible. Therefore, only the robust myriad, the
annealing M estimator, and the standard PIA averaging
5. Data analysis
were applied to derive the (cid:12)nal signals.
Eventually, the signals derived by all di(cid:11)erent meth-
To search for systematic trends in the FIR color pro(cid:12)les
odswerecorrectedforsignaldependenceonrampintegra-
acrosstheclusters,theI =I ratiosforeachpo-
120 (cid:22)m 180 (cid:22)m
tion times to be consistent with calibration observations
sition angle were computed for each detector pixel sep-
(Laureijsetal.2000),dark-currentsubtracted,and(cid:12)nally
arately and subsequently averaged. If necessary, at most
flux calibrated with PIA version 9.0/Cal G version 6.0.
a single outlier at each sky position was rejected. To fur-
Fortheconversiontoanabsolutefluxlevel,measurements
therreducethescatterintheI =I ratios,peaks
120 (cid:22)m 180 (cid:22)m
of the ISOPHOT internal Fine Calibration Source (FCS)
frompointsourceswereremovedbylinearlyinterpolating
obtained at the beginning and end of each scan in each
between the two adjacent sky positions. The overall flux
(cid:12)lter were used. The signals from the two accompanying
ratio distribution was derived by averaging the individ-
ual ratios of all pixels and both PAs, applying a global
1 The ISOPHOT data presented in this paper were re-
rescaling or outlier rejection if necessary.
duced using PIA, which is a joint development by the ESA
Astrophysics Division and the ISOPHOT Consortium. The Carehas to be taken if in addition to the di(cid:11)use large
ISOPHOT Consortium is led by the Max-Planck-Institut fu¨r scaleFIRemissionfromthegalaxyalocalizedcirrusstruc-
Astronomie, Heidelberg. ture having a similar temperature is crossed. Even the
372 M. Stickel et al.: Intracluster dust in Abell clusters
Thisbehaviorcanbeexempli(cid:12)edbyaone-dimensional
model, where the di(cid:11)use cirrus component Idif might be
approximatedbyaslantedlinewithaconstantratioIdif /
120
Idif,whilethelocalizedcirrusstructuremightbemodeled
180
for simplicity by a Gaussian with a constant Idif / Idif
120 180
ratioequaltothatofthedi(cid:11)use component.Thezodiacal
light component can also be described by a slanted line
toaccountforagradientinthezodiacallightdistribution
along the scan path.
While the model pro(cid:12)le of Rcirr (Eq. (1)) is con-
stant (Fig. 1), the zodiacal light component in Rcirr+zodi
(Eq. (2)) makes the presence of a localized cirrus com-
ponent obvious, leading to a characteristic change in the
I =I ratio along the model scan, which actu-
120 (cid:22)m 180 (cid:22)m
ally reflects the pro(cid:12)le of the localized cirrus component
(Fig. 2).
Fig.2. One-dimensional model of a FIR scan measurement On the other hand, if no localizedcirrus structurebut
with cirrus and zodiacal light. In addition to the di(cid:11)use insteadanICDcomponentdescribedbyIICD =RICD(cid:2)
120 (cid:22)m
(continuous straight line) and a localized cirrus component
I ispresentwithatemperaturedi(cid:11)erentfromthe
180 (cid:22)mIDC
(Gaussian), a zodiacal light component (dashed line) with a galactic cirrus, i.e. RICD 6=Rcirr, then the ratio
much higher temperature is present at 120(cid:22)m (top left) and
180(cid:22)m (top right). Although thesummed intensities (bottom Idif +IICD IICD
left) look similar tothecasewithoutzodiacal light,theirratio 120 120 =Rcirr+(RICD−Rcirr) (cid:2) 180 (3)
Idif +IICD Idif +IICD
(bottom right) does reflect the pro(cid:12)le of the localized cirrus 180 180 180 180
structure as a dip. is already varying across the cluster due to the non-
constant ICD component, reflecting the ICD brightness
small contribution from the zodiacal light, which is weak
pro(cid:12)le. Adding the zodiacal light only changes the level
at FIR wavelengths, will break the degeneracy of the dif-
of variation in the observed overallI =I ratio,
fuseandlocalizedcirruscomponentintheI =I 120 (cid:22)m 180 (cid:22)m
120 (cid:22)m 180 (cid:22)m but not the presence of a dip or bump originating in the
ratio and mimic an additional extended FIR emitting
ICD component.
component.
This behavior provides the possibility to distinguish
Speci(cid:12)cally, if the 120(cid:22)m and 180(cid:22)m surface bright-
in the zodiacal light subtracted I =I ratio an
nesses of the di(cid:11)use cirrus component are related by 120 (cid:22)m 180 (cid:22)m
additionalICDcomponentwithasurfacebrightnessvary-
Idif =Rcirr (cid:2) Idif, and the brightnesses of a localized
120 180 ing on scales of a clusters diameter from a localized cir-
cirrus structure with a similar temperature are given
rus component. If a dip or bump is still present in the
by Iloc =Rcirr (cid:2) Iloc, the I =I ratio Rcirr of
120 180 120 (cid:22)m 180 (cid:22)m I =I ratioafterthesubtractionofthezodiacal
galactic cirrus alone is given by 120 (cid:22)m 180 (cid:22)m
lightcomponent,itcanbeattributedtoanadditionalFIR
Idif +Iloc component outside the Galaxy and most likely identi(cid:12)ed
120 120 =Rcirr: (1)
Idif +Iloc withthethermalFIRemissionofICD.Ontheotherhand,
180 180
adiporbumpwhichdisappearsintheI =I ra-
Forsimilartemperaturesofbothcomponents,thisratiois 120 (cid:22)m 180 (cid:22)m
tio with the subtraction of the zodiacal light component
nearlyconstant,andalmostindependentofthebrightness
is most likely due to a localized cirrus structure. Only
pro(cid:12)lesofbothcirruscomponents.Foranadditionalzodi-
if the ICD properties (temperature, grain composition,
acallightcomponentthebrightnessesatbothwavelengths
are related by Izodi =Rzodi (cid:2) Izodi, where Rzodi 6= Rcirr etc.) closely resemble that of galactic cirrus, a character-
120 180 istic change of the I =I ratio from ICD will
becauseofthedi(cid:11)erenttemperaturesofgalacticcirrusand 120 (cid:22)m 180 (cid:22)m
also disappear with the subtraction of the zodiacal light
zodiacal light.
component, and there is then no possibility to separate
The total observedI =I ratio is then given
120 (cid:22)m 180 (cid:22)m
the two components with the measurements at two FIR
by
wavelengths.
Idif +Idif +Izodi
120 120 120 =Rcirr
Idif +Idif +Izodi
180 180 180 6. Results
Izodi
+ (Rzodi−Rcirr) (cid:2) 180 (cid:1) (2)
Idif +Idif +Izodi 6.1. Abell 262
180 180 180
Althoughthebrightnessesofthedi(cid:11)usecirruscomponent Abell262is aspiralrichclusterwithanE/Dtypecentral
andthezodiacallightvaryonlyslightlyacrossthecluster, galaxy(NGC708)showingdustlanesanddi(cid:11)usegaswith
theoverallratioisnotconstantduetothevaryingbright- lineemission.TheclusterX-raytemperatureisratherlow,
ness of the localized cirrus structure Idif , and directly and there is independent evidence for a central cooling
180 (cid:22)m
reflects the brightness pro(cid:12)le of this component. flow (Table 3). Giovanelli & Haynes (1985) found that a
M. Stickelet al.: Intracluster dust in Abell clusters 373
Fig.5. The surface brightness ratios I =I after
120 (cid:22)m 180 (cid:22)m
subtraction of the zodiacal light, averaged over all four de-
tector pixels, along PA 45(cid:14) (asterisks) and PA 135(cid:14) (squares)
as a function of distance from the cluster center of Abell 262.
Southlies to theleft at negative distances, north to theright.
Fig.3. TheobservedbrightnessdistributionsofthefourC200
detector pixels at 120 (cid:22)m for Abell 262 along PA 45(cid:14) (top)
and PA 135(cid:14) (bottom). South lies to the left at negative dis-
tances, north to the right. The brightness level is correct only
for the lowest data stream. For clarity, the other three pixel
data streams are o(cid:11)set arbitrarily. The 180(cid:22)m brightnessdis-
tributions (not shown) are quitesimilar.
Fig.6. The overall zodiacal-light subtracted surface bright-
ness ratio I =I for Abell 262, averaged over both
120 (cid:22)m 180 (cid:22)m
position angles and all detector pixels.
signi(cid:12)cant fraction of the galaxies inside one Abell radius
are de(cid:12)cient in HI, while Bravo-Alfaro et al. (1997) have
shownthatanumberofthesegalaxieshaveanasymmetric
HIdistribution.Bothresults aremostlikelythe resultsof
the interaction of the cluster galaxies with the ICM.
The 100 (cid:22)m IRAS HIRES image shows an elongated
cloud-likepatchofFIRemission,roughlyorientedataPA
(cid:25)50(cid:14),overlaidby(cid:12)lamentarystructures.OneISOPHOT
scanwasexecutedparalleltothisgeneralelongation,while
the other one was perpendicular. The extended 100 (cid:22)m
Fig.4. TherawsurfacebrightnessratiosI =I ,av- emission region lies at the edge of a much larger cirrus
120 (cid:22)m 180 (cid:22)m
eraged over all four detector pixels, along PA 45(cid:14) (asterisks) structure, and David et al. (1996) argued that the excess
andPA135(cid:14)(squares)asafunctionofdistancefromthecluster X-ray absorber required by the ROSAT PSPC observa-
centerofAbell262.Southliestotheleftatnegativedistances, tions is actually galactic cirrus rather than an absorber
northtotheright.BothPAsshowamarkedsouth-northasym- physically associated with the cluster. The inferred col-
metry and a broad depression closely aligned with the cluster umn density of galactic HI is about twice the value usu-
center.
allyusedfortheanalysisofX-raydata(Starketal.1992),
374 M. Stickel et al.: Intracluster dust in Abell clusters
indicating signi(cid:12)cant fluctuations in the galactic HI col-
umn density on angular scales <2(cid:14).
However, from emission line ratios Hu (1992) found
evidenceforanexcessreddeningofEB−V (cid:25)0:20mag,in-
dicating a cluster internalabsorbercontaining dust. Wise
etal.(1993)attemptedtosubtractthegalacticforeground
cirrus from IRAS ISSA plates and found evidence for
residual di(cid:11)use extended emission at 60 (cid:22)m and 100 (cid:22)m,
which was attributed to intracluster dust, too.
Both ISOPHOT scans show for both position angles
the broad bump of extended FIR emission, and con(cid:12)ned
to one sky position, the presence of an unresolvedcentral
point source (Fig. 3). It can not be decided whether this
FIR point sourceis due to the central cD galaxy,because
a spiralgalaxyis projected onto its outskirts (Fanti et al.
1982).ThisconfusionwasalreadynotedbyBregmanetal.
(1990). Several other compact sources appear only in the
data streams of individual pixels, indicating non-central
crossings.
The raw I =I surface brightness ratios in-
120 (cid:22)m 180 (cid:22)m
cluding zodiacal light for both position angles, separately
averaged over the four detector pixels, agree remarkably
well (Fig. 4), even in the marked north { south asymme-
try,andshowaverybroadminimumsomewhato(cid:11)-center
fromthenominalclustercentreposition.Remarkably,the
broad depression completely disappears in the zodiacal-
light subtracted surface brightness ratios along both PAs
(Fig. 5),leavingonlyanalmostlinearlyincreasingoverall
I =I ratio (Fig. 6).
120 (cid:22)m 180 (cid:22)m Fig.7. TheobservedbrightnessdistributionsofthefourC200
detector pixels at 120(cid:22)m for Abell 400 along PA 40(cid:14) (top)
6.2. Abell 400 and PA 130(cid:14) (bottom). South lies to the left at negative dis-
tances, north to theright. The brightness level is correct only
AdynamicalanalysisofAbell400wascarriedoutbyBeers for the lowest data stream. For clarity, the other three pixel
et al. (1992), who concluded that this spiral rich cluster datastreams are o(cid:11)set arbitrarily. The180(cid:22)m brightness dis-
tributions(not shown) are quitesimilar.
actually consists of two bound sub-clusters currently un-
dergoingmerging.TheirregularX-raymorphology(Beers
et al. 1992; Buote & Tsai 1996) and the absence of a
cooling flow (White et al. 1997; Allen & Fabian 1997)
also indicates a young dynamical cluster age. The overall
X-raytemperatureof(cid:25)2.5keVisratherlow(Davidetal.
1993; White et al. 1997).
Although Abell 400 lies at a much higher galactic lat-
itude than e.g. Abell 262, both ISOPHOT scans (Fig. 7)
show nevertheless a much higher absolute level of the
120(cid:22)m surface brightness. The 100(cid:22)m IRAS HIRES im-
age shows quite a structured region with large patches
of cirrus foreground towards Abell 400. Two very bright
point sources were detected with one detector pixel only
along PA 40(cid:14) and PA 130(cid:14), which have been identi(cid:12)ed
with PGC 11141 (IRAS F02542+0600) and UGC 2444
(IRAS 02558+0606), respectively. Both scans also indi-
cateinthedatastreamsofseveralpixelsthepresenceofa Fig.8. The raw surface brightness ratios I120 (cid:22)m=I180 (cid:22)m av-
eraged over all four detector pixels, along PA 40(cid:14) (asterisks)
much weakercompact sourcecon(cid:12)ned to the skyposition
andPA130(cid:14) (squares)asafunctionofdistancefromthecenter
centered on the cluster.
ofAbell400.Thepro(cid:12)lesalongthetwoPAsareratherdissim-
The I120 (cid:22)m=I180 (cid:22)m raw surface brightness ratios ilar with structures on angular scales of 100{200. South lies to
(Fig. 8) along the two PAs are quite structured on an-
theleft at negative distances, north to theright.
gular scales of 100{200 without a good overall agreement
M. Stickelet al.: Intracluster dust in Abell clusters 375
Fig.9. The surface brightness ratios I =I after
120 (cid:22)m 180 (cid:22)m
subtraction of the zodiacal light, averaged over all four de-
tector pixels, along PA 40(cid:14) (asterisks) and PA 130(cid:14) (squares)
as a function of distance from the center of Abell 400. South
lies to the left at negative distances, north to the right. The
overallshapesofbothPAsshowaflatterregiontowardssouth,
and a steepening towards north.
Fig.11. The observed brightness distributions of the four
C200 detector pixels at 120(cid:22)m for Abell 496 along PA 90(cid:14)
(top) and PA 180(cid:14) (bottom). The brightness level is correct
only for the lowest data stream. For clarity, the other three
pixel data streams are o(cid:11)set arbitrarily. The 180(cid:22)m bright-
ness distributions (not shown) are quitesimilar.
Fig.10. The overall zodiacal-light subtracted surface bright-
ness ratio I =I for Abell 400, averaged over both
120 (cid:22)m 180 (cid:22)m
position angles and all detector pixels.
ofthetwopro(cid:12)les.However,theratioatthescancrossing
isinalmostperfectagreement,therebyprovidinganinde-
pendentcheckofthe fluxcalibration.After subtractionof
the zodiacal light, the I =I surface brightness
120 (cid:22)m 180 (cid:22)m
ratios (Fig. 9) of the two PAs are in much better agree-
ment, showing a flatter part towardssouth and a gradual
steepening towards north. This overall behavior is quite
easily seen in the overall I =I surface bright-
120 (cid:22)m 180 (cid:22)m
ness ratio (Fig. 10).
Fig.12. The raw surface brightness ratios I =I ,
120 (cid:22)m 180 (cid:22)m
averagedoverallfourdetectorpixels,alongPA90(cid:14) (asterisks,
6.3. Abell 496
from east towards west) and PA 180(cid:14) (squares, from south
The relaxed single symmetric X-ray morphology (Buote towards north) as a function of distance from the center of
& Tsai 1996), the signi(cid:12)cant cooling flow (Table 3), and Abell 496. The pro(cid:12)le is nearly constant along PA 90(cid:14), while
PA 180(cid:14) shows an almost linear increase.
the double-nucleus central cD galaxy with very small
velocity dispersion (Tonry 1985) indicates a dynami-
cally old, relaxed cluster. There is evidence for an X-ray
376 M. Stickel et al.: Intracluster dust in Abell clusters
Fig.13. The surface brightness ratios I =I after
120 (cid:22)m 180 (cid:22)m
subtractionofthezodiacallight,averagedoverallfourdetector
pixels, along PA 90(cid:14) (asterisks, from east towards west) and
PA 180(cid:14) (squares, from south towards north) as a function of
distance from thecenter of Abell 496.
Fig.15. The observed brightness distributions of the four
C200 detector pixels at 120(cid:22)m for Abell 1656 along PA 82(cid:14)
(top) and PA 36(cid:14) (bottom). The brightness level is correct
only for the lowest data stream. For clarity, the other three
pixel data streams are o(cid:11)set arbitrarily. The 180(cid:22)m bright-
ness distributions (not shown) are quitesimilar.
Fig.14. The overall zodiacal-light subtracted surface bright-
nessratioI =I forAbell496,averagedoverallfour
120 (cid:22)m 180 (cid:22)m
detector pixels.
The color pro(cid:12)les after subtraction of the zodiacal light
(Fig. 13) do not changevery much, still showing a steady
absorptioncolumndensityabovethegalacticvalueacross increase from south to north and a rather flat but wig-
the cluster (White et al. 1994; MacKenzie et al. 1996; gling behavior along PA 90(cid:14). Nevertheless, both pro(cid:12)les
Allen et al. 2001), consistent with the excess reddening were combined to check for the presence of structure on
ofEB−V (cid:25)0:20magderivedfromemissionlineratios(Hu scales of 100{200. The resulting pro(cid:12)le (Fig. 14), however,
1992). shows only a gentle rise.
TheISOPHOTscansalongbothPAs(Fig.11)arede-
void of strong point sources. In the data streams of two
6.4. Abell 1656 (Coma)
pixels along PA 180(cid:14), there is an indication for a weak
compactsourcecon(cid:12)nedto a singlesky position.Abroad Thereisnowoverwhelmingevidenceforrecentorongoing
hump o(cid:11)set to the east from the cluster center is visible merging processes in the Coma cluster from optical and
along PA 90(cid:14), and a similar but weaker bump appears to X-ray data (Colless & Dunn 1996; Vikhlinin et al. 1997;
be superposed on the generally decreasing surface bright- Burns et al. 1994b;Neumann et al. 2001).The early evi-
ness pro(cid:12)les along PA 180(cid:14). denceforintraclusterlight(Welch&Sastry1971;Mattila
The two rawI =I surface brightness ratios 1977) has recently been con(cid:12)rmed with the detection of
120 (cid:22)m 180 (cid:22)m
including the zodiacal light (Fig. 12) along PA 180(cid:14) and additional low surface brightness structures interpreted
PA90(cid:14)arestrikinglydi(cid:11)erent.Theformerisalmostmono- as tidal debris from recent galaxy interactions (Gregg &
tonically increasing from south towards north across the West 1998; Trentham & Mobasher 1998). With an X-ray
cluster,whilethelattervariesmuchlessacrossthecluster. temperature of (cid:25)8 keV, Abell 1656 is the hottest of the