Cl 1205+ 44, a fossil group at z= 0.59

21 Pages·2013·0.53 MB·English

Cl 1205+44, a fossil group at z = 0.59
M. P. Ulmer
Department Physics & Astronomy, Northwestern University, Evanston, IL 60208-2900
[email protected]
5
C. Adami, G. Covone
0
0 Laboratoire d’Astrophysique de Marseille, Traverse du Siphon, 13012 Marseille, France
2
[email protected], [email protected]

y
a
M F. Durret
Institut d’Astrophysiqe de Paris, CNRS, 98 bis Boulevard Arago, 75014 Pars, France
3
[email protected]

2
v
6 G. B. Lima Neto
8 Instituto de Astronomia, Geof´ısica e C. Atmosf./USP, R. do Mata˜o 1226, 05508-090 Sa˜o Paulo/SP, Brazil
4
[email protected]
1
0
5
0 K. Sabirli1
/ Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA
h
p [email protected]
-
o
r
t B. Holden2
s
a Department of Physics, University of California, Davis, 1 Shields Avenue, Davis, CA 95616
:
v [email protected]
i
X
r R.G. Kron
a
Fermi National Accelerator Laboratory, MS 127, Box 500, Batavia, IL 60510
[email protected]
A.K. Romer1
Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA
[email protected]
ABSTRACT
This is a report of Chandra, XMM-Newton, HST and ARC observations of an extended X-
ray source at z = 0.59. The apparent member galaxies range from spiral to elliptical and are
all relatively red (i′-K about 3). We interpret this object to be a fossil group based on the
s
difference between the brightness of the first and second brightest cluster members in the i′-
band, and because the rest-frame bolometric X-ray luminosity is about 9.2×1043 h−2 erg s−1.
70
This makes Cl 1205+44 the highest redshift fossil group yet reported. The system also contains
a central double-lobed radio galaxy which appears to be growing via the accretion of smaller
galaxies. We discuss the formation and evolution of fossil groups in light of the high redshift of
Cl 1205+44.
1
Subject headings: groups of galaxies: general — X-rays: individual (Cl 1205+44)
1. Introduction et al., 2003) independently to identify this object
as a fossil group and to obtain HST and redshift
InaROSATsurveyofextendedsources(Adami
observations. The HST and redshift data nicely
et al. 2000;Romer et al. 2001),we found a source complement the Chandra, XMM-Newton, i′- and
showing extended X-ray emission but with only a
K -band data we have obtained.
s
singlefaintoptical(R-band)counterpart. Ondeep
Our study of Cl 1205+44 also adds one more
opticalimages,the colorsofthe galaxiesinthe X-
datapointtothetopicofpreheating. Recentstud-
ray area were red enough that the program HY-
ies of simulations of preheating can be found, for
PERZderivedaphotometricredshiftgreaterthan
example, in Borgani et al. (2004). The possibility
1. This led to Chandra and XMM-Newton obser-
of preheating of the ICM in groups and clusters
vations of the object we designate as Cl 1205+44.
has taken on added significance, as it relates to
We find that the system is a fossil group at the
the Suyanev-Zel’dovich (SZ) effect, which can in-
highest redshift yet published (0.59 versus typical
fluence the interpretation of the high order power
<
values of ∼0.1). The system is particularly inter-
spectrum of the Cosmic Microwave Background
estingbecauseitallowsustoexploreevolutionary
(CMB; Lin et al. 2004).
tracksoffossilgroupsandtoconsiderscenariosfor
Inthispaper,wereporttheresultsofourmulti-
the evolution of galaxies in groups.
wavelength analysis and discuss how Cl 1205+44
Fossilgroupshavebeendefined(e.g.Jonesetal.
fitsintothelargerpictureoffossilgroupformation
2003) as being similar to groups (poor clusters)
and evolution.
in which the luminosity function of the member
WewilluseH =70kms−1Mpc−1,Ω =0.7and
galaxieshasbeenmodifiedbyaccretionofgalaxies 0 Λ
Ω =0.3hereafter. Ataredshiftof0.5915,whichis
onto the central galaxy while the rest of the sys- m
themostlikelyredshiftofCl1205+44,theangular
tem remains unevolvedfor approximately 4 Gyrs.
scale for this cosmology is 6.64 kpc/′′.
The galaxy accretion onto the D galaxy leads to
a magnitude difference between the first and sec-
2. Observations and Analysis
ond brightest galaxies (m ) of 2 or higher in the
12
rest frame R or V bands within one half a virial
2.1. Optical and IR observations
radius,whereJoneset al.(2003)use the following
to calculate the virial radius: rvir = 3.89×(T/10 We made observations in the i′ and Ks-bands
keV)0.5×(1+z)−3/2 h−1 Mpc. at the ARC telescope2. For the i′-band, we used
50
the Spicam camera for a net total exposure time
Moreover, Cl 1205+44 is interesting because
of7,200secondsatameanairmassof1.15,andfor
it harbors a double lobed radio source which is
theK -bandweusedtheGRIMIIcameraforanet
also a D (central dominant) galaxy. Such systems s
totalexposuretimeof7,200seconds. Wetooksep-
can be directly compared with the model of West
arate 10 minute exposures to acquire the i′-band
(1994). Also, the radio source simply makes the
dataand10to30secondexposurestoaccumulate
group more complex and its formation could be
related to cooling flows1 on one hand, and energy theKs data. Thei′-banddatawerereducedusing
theESO-MIDASpackageandtheK datawerere-
injection on the other (e.g. Sun et al. 2003). s
duced using DIMSUM, a NOAO/IRAF tool. The
A D galaxy that is a radio source is a tracer
zero points were computed using standard stars
for finding fossil groups which led J. S. Mulchaey
observed at the same time as the scientific data
(2003, private communication; see also Mulchaey
and at similar air masses.
1present address: Astronomy Center, Department of The HST data were retrieved from the archive
Physics and Astronomy, University of Sussex, Falmer, (proposal ID: 8131, PI: Mulchaey). They con-
Brighton,BN19QH sist of 6×1200 second dithered R-band WFPC2
2present address: UCO/Lick Observatories, 1156 High
(F702W filter) exposures. We reduced the data
Street,SantaCruz,CA95065
using the drizzle IRAF package (Fruchter & Hook
1Coolingflowliteratureistooextensivetocomprehensively
2002). We generated catalogs of detected objects
reference; a few references that directly relate radio lobes
to X-rays in relatively poor systems with X-ray emission
are: Carilli et al. (1994); Harris et al. (2000); McNamara 2Seehttp://www.apo.nmsu.edu fordetails.
etal.(2000)
2
using SExtractor (version 2.3 Bertin & Arnouts mostlikely in ourlist not to be a cluster member.
1996) using a detection threshold of 1.5σ and an The uncertainty in L does not affect any con-
R
analysis threshold of 2σ. clusion we draw in the discussion section of this
Weusedthefixedaperture3′′ magnitudesfrom paper. The F702W total luminosity is 6.0×1011
SExtractor for the HST magnitudes and we used LJ. Converting to the RJ band used by Jones
the SExtractor auto-magnitude feature to derive et al. (2003),we obtain LR = 5.5×1011 LJ. The
the magnitudes for the ARC data. Figure 1 K-correction to z = 0 gives a (rest frame) value
gives the magnitude histogramsin the three mag- of LR = 7.8×1011 LJ. Finally, to compare with
nitude bands available to us (in the ARC Spi- Jones et al., we use H0=50 and q0 = 0.5 which
cam field of view for i′, in the ARC GRIMII leads to LR =8.9×1011 LJ.
field of view for K and in the HST WFPC2
s
2.2. Redshift
field of view for F702W). These histograms al-
low us to estimate an upper limit to the com-
SincetheX-raydatawerenotofsufficientsignal
pleteness value of the magnitude in each bands:
to noiseratiotoconstrainthe clusterredshift(see
i′∼22.25, F702W∼26.25 and K ∼19.75. The re-
s below), we needed optical spectra of the galax-
spective solid angle coverage of the instruments
ies belonging to the system. By chance, this sys-
is: WFPC2 image: 2.2×2.2 arcmin2, i′ image:
temwasobservedbyJ.S.Mulchaey(2003,private
6×4.3 arcmin2, K image: 2×2 arcmin2.
s communication) in his ongoing fossil group sur-
In order to assess the quality of our data, vey and he kindly provided us with the value of
we plotted the magnitudes in the different bands theredshiftofthebrightestgalaxyoftheputative
against each other as shown in Figure 2. We fossil group, z = 0.5915. These data plus the po-
estimate from the results a relative 1σ uncer- sitional coincidence of the brightest group galaxy
tainty between magnitudes (limiting ourselves to (called the D galaxy hereafter) with the centroid
the brightest completeness limit) of 0.26 mag be- (see §2.3.4) of the X-ray emission firmly identi-
tween F702W and i′ and of 0.36 mag between i′ fiestheextendedX-rayemissionwiththebrightest
and Ks. This gives an upper limit to the magni- galaxy and the associated galaxies in Table 1.
tude uncertainty since some of the scatter is due
to the intrinsic color variations of the objects. 2.3. X-ray Observations
As an external test, we compared our K
s We were granted time to observe Cl 1205+44
magnitudes with the estimates for the two ob-
with the XMM-Newton (June 2003,52,200s) and
jects in the K field bright enough to be de-
s Chandra (October 2003,31800 s) satellites.
tected by 2MASS3 (including the D galaxy). We
have reasonable agreement (for the two objects, 2.3.1. Chandra data
K −K =−0.38 and +0.26) given that these
s 2MASS
objects are below the 2Mass completeness limit. The Chandra observation was made in “Very
Faint”modewithatimeresolutionof3.24secand
For use in our discussion below, we calculated
a CCD temperature of −120◦C. The data were
the total R-band luminosity. To derive this num-
reduced using CIAO version 3.0.15 following the
ber,weusedthetotalF702Wfluxenclosed4bythe
Standard Data Processing, producing new level 1
X-ray contours in Figure 3. We estimate that ap-
and 2 event files.
proximately90%ofourderivedvalueofL comes
R
fromthe galaxiesenclosedby rectanglesin Figure We have further filtered the level 2 event file,
4. Therefore, the L value is probably accurate keeping only ASCA grades 0, 2, 3, 4 and 6, and
R
to within 20%. The 20% uncertainly is also con- restricted our data reduction and analysis to the
sistentwithsubtractingtheflux of#6inFigure3 back-illuminated chip, ACIS-S3. We checked that
(seealsoTable1)which,basedonitscolors,isthe no afterglow was present and applied the Good
Time Intervals (GTI) supplied by the pipeline.
3http://www.ipac.caltech.edu/2mass/ Then, we have checked for flares using the light-
4Weexcludedthestar,whichisthebrightestobjecttoward
the top of the figure but inside the contours; it is south- 5http://asc.harvard.edu/ciao/
southwestoftheDgalaxy
3
curve in the [10–12 keV] band; no flare was de- 2.3.3. X-ray Spectral fits
tected and the total exposure time was 29,711 s.
Spectra were analyzed with XSPEC 11.3. We
We have used the CTI-corrected ACIS back-
have simultaneously fit all four spectra from the
groundevent files (“blank-sky”), produced by the
XMM-Newton MOS1, MOS2, PN and Chandra
ACIS calibration team6, available from the cal-
ACIS-S3 cameras. The spectra were rebinned so
ibration data base (CALDB). The background that we could use the standard χ2 minimization
events were filtered, keeping the same grades as
when fitting the spectra. We have applied the
the source events, and then were reprojected to mekal plasma spectral model (Kaastra & Mewe
matchtheskycoordinatesoftheCl1205+44ACIS
1993; Liedahl et al. 1995).
observation.
The photoelectric absorption (mainly due to
neutralhydrogeninourgalaxy)wascomputedus-
2.3.2. XMM-Newton data
ing the cross-sectionsgivenby Balucinska-Church
Cl 1205+44 was observed in standard Full & McCammon (1992), available in xspec. Given
Frame mode using the “thin” filter with the two the low count-rate, the hydrogen column density
EPIC MOS1 and MOS2 and the PN detectors. N could not be well constrained by the spectral
H
Thebasicdataprocessing(the “pipeline”removal fit, therefore we fixed it to the galactic value at
of bad pixels, electronic noise and correction for the cluster position. The interpolation of the HI
chargetransferlosses)wasdonewithpackageSAS map of Dickey & Lockman (1990), using the task
V5.3, thus creating calibrated event files for each nh from ftools, yields N = 1.27×1020cm−2.
H
detector. The results are shown in Figure 5. The metallic-
For the MOS1 and MOS2 cameras, following ity is poorly constrained; as shown in Table 3, at
the standard procedure, we have discarded the a1σ confidencelevelwefoundZ <0.6Z and,at
J
events with FLAG 6= 0 and PATTERN > 12; 90% confidence level, Z < 1.1Z . For complete-
J
for the PN, we have restricted the analysis to the ness in Figure 6 we show how NH and metallicity
events with PATTERN ≤ 4 and Flag =0. correlate with kT in the spectral fits.
The light-curves in the [10–12 keV] band that We restricted the spectral analysis within the
we have produced showed that there were se- energyrangewheretheclusteremissionwasabove
vere flaring events during the observation. Fil- the background and the detectors were well cali-
tering outthe periods with flaressubstantially re- brated. FortheMOS1and2cameras,thisinterval
duced the exposure times: 21,223 s, 20,861 s, and was[0.3–8.0keV],forthePN[0.5–8.0keV]andfor
16,478 s for the MOS1, MOS2 and PN, respec- ACIS-S3[0.4–7.0keV].Thespectrawereextracted
tively (from an initial 52.2 ks). insideacircleof1.4arcminradius,centeredonthe
With the cleaned event files, we have created cluster. The strong northeast X-ray point source
theredistributionmatrixfile(RMF)andancillary (probably an AGN) is outside this radius.
response file (ARF) with the SAS tasks rmfgen
2.3.4. X-ray luminosity
and arfgen for each camera and for each region
that we have analyzed. For the reasons stated above (§2.2), we assume
The backgroundwas taken into account by ex- that the cluster redshift is z = 0.5915 and fixed
tracting spectra from the blank sky templates de- this value from hereon. Table 3 summarizes the
scribed by Lumb et al. (2002), and reprojected best spectral fits, either fixing the metallicity or
to the same coordinates and roll angle of the the hydrogen column density or both. Clearly,
Cl1205+44XMM-Newton observation. Thesame themetallicityisnotwellconstrainedandweonly
filtering procedure was applied to the background haveupperlimitsforN . Fixingthemetallicityto
H
eventfiles. We giveabreakdownofthe numberof the typical value found in clusters (e.g. Fukazawa
counts used per detector in the fitting in Table 2. et al. 2000, 0.3Z ) and N to the galactic value,
J H
the mean temperature is kT = 3.0+0.3 keV. This
6http://cxc.harvard.edu/cal/Acis/WWWacis cal.html system therefore has an X-ray tem−p0e.3rature typ-
ical of a poor cluster (such as Abell 194, which
has a temperature of 2.6±0.15 keV [Nikogossyan
4
etal. 1999],andthepoorclusterRXJ0848+4456, of total flux = 56.4 ± 1.7 mJy at 20 cm (Con-
which has a temperature of 3.2±0.3 keV [Holden don et al. 2002). These facts are relevant to the
et al., 2001]) and warmer than that of a typical model proposed by West (1994). He proposed
fossil group (e.g. Jones et al. 2003). an anisotropic merger model for the origin of the
Withredshiftz=0.5915,thecorrespondingun- formation of D galaxies, and the model predicts
absorbedluminosityandfluxinthe[2.0–10.0keV] that the D-galaxy will be associated with a pow-
are (3.3±0.3)h−2×1043erg s−1 and (1.5±0.2)× erful radio source, which is the case here. Fur-
70
10−14erg s−1 cm−2. The bolometric luminosity is thermore,the model predicts that the radio lobes
(9.2±0.4)h−2 ×1043erg s−1. The cluster prop- will be aligned with the major axis of the X-ray
70
erties are summarized in Table 4. We define the emission in the cluster. Therefore, we attempted
position of the cluster to be that of the D galaxy, to determine the major axis of the X-ray emis-
as there is a peak in the contours (see Figure 3) sion, which is rather ill defined. In order to make
at this location. a determination of the major axis, we heavily
smoothed7 thedatatoproduceFigure11(seealso
For its measured bolometric luminosity, Cl
Figure 10). Then, the major axis of the X-ray
1205+44 is hotter than the best fit to the local
emissionof Cl1205+44can be defined by the line
L –T relationbasedontwo fossilgroups(Jones
X X
joining points “A” and “B” in Figure 11. In this
etal.(2003);seeourFigure7),butagreeswiththe
case the radio lobe axis is aligned within 10 de-
localrelationofNovickietal.(2002),andthevalue
grees of the X-ray axis, which is also consistent
fallswithin1σoftheL –T no-evolutionrelation
X X
withthemodelofWest. However,theopticalaxis
derived for the hzi=0.34 sample of Novickiet al.
of the D galaxy is offset by about 30 degrees with
(2002). For the purpose of later discussion, we
respectto the radio lobe axis andthe West model
have plotted the values of kT versus L for Cl
X
also predicts alignment with the galaxy distribu-
1205+44 on figures taken from Jones et al. and
tion. If there is a galaxy distribution that defines
Novicki et al. in our Figures 7 and 8.
a direction, it is the almost north-south line of
galaxies running from galaxies #7 to #8 in Fig-
2.3.5. X-ray surface brightness fits
ure 4. Overall, then, the data are not consistent
We carriedout a standardβ surface brightness with the model of West.
fittoboththeXMM-Newton dataandtheXMM-
Newton plus Chandra data combined. We used 3. Discussion
the 0.5–8 keV bands in both cases. Then with
I (b) ∝ [1 + [b/r ]2]−3β+1/2 (e.g. Sarazin 1986), 3.1. Nature of the System
x c
where I is the surface brightness as function of
b The most compelling reason for calling Cl
projected radius, b and r is the core radius. For
c 1205+44 a fossil group is that the value of m
the more robust XMM-Newton data alone case, 12
withinonehalfthe projectedvirialradius8 isvery
we found, using a maximum likelihood method:
close to that of the Jones et al.(2003)criterionof
β =0.45±0.02,andr =21′′±3′′,1σ errors. The
c being >2 forz ∼0 (for R- orV-bandrestframe),
fitappearsbettertotheeye(seeFigures9aandb)
i.e. m (i′) = 1.93. In addition, that the value
for the XMM-Newton plus Chandra case, but we 12
foundthe resultsso sensitiveto the normalization 7Note that the heavy smoothing causes the center of the
between the two data sets and the binning of the second highest contour level in the X-ray emission to be
data, that we only quote this fit as a 2σ lower differentfromthatinFigure3,buttheexactpositionofthe
bound to the core radius = 15′′; the value of β X-raypeak is not important. For, it isextremely unlikely
thattheDgalaxywouldfallsoclosetothecenterofthisX-
was again 0.45 for this minimum χ2 fit. rayemissionandnotbeassociatedwiththeX-rayemission.
Thehighestcontourlevelhastwopeaks,oneofwhichfalls
2.4. Radio Source ontheDgalaxy;seetheblackandwhiteversionofFigure
10orthecolorimageofFigure11.
Thecentralpeakwithinthesecondhighestcon- 8ForconsistencywithJonesetal. rvir =70′′ weusedtheir
tour level in Figure 3 is located on the D galaxy. cosmology,H0 =50,q0=0.5;thenthescaleis7.57kpc/′′,
TheDgalaxyisalsoadouble-lobedFIRST(White andfromtheirformulaforthevirialradius,reproducedin
−1
etal.1998)radiosourceF1205+44(seeFigure10) sthpeonidnstrtoodu7c0t′′io.n, rvirial/2 = 0.53 h50 Mpc, which corre-
5
of m is slightly smaller can be attributed to Cl sured by J. S. Mulchaey (2003, private commu-
12
1205+44being younger (by up to ∼4 Gyrs) than nication). The conclusion that these galaxies are
z = 0 fossil groups; in the Jones et al. scenario, allgroupmembers is basedonthe followingfacts:
the central galaxy grows in brightness with time theiri′ magnitudesareallsimilarexceptfortheD
as it accretes more galaxies. In older systems the galaxy;theircolorsareallsimilarexceptforgalaxy
central galaxy will have had more time to accrete #6; their apparent sizes are all similar, and their
galaxies, and hence be brighter compared to the average color (see also § 3.6) corresponds to the
remaining ones. peak in the histogram of the i′–K distribution
s
The X-ray emitting AGN we have listed in Ta- shown in Figure 12. We will assume therefore,
ble 1 is brighterthan the D galaxyin the i′-band, that except for #6, all these galaxies are cluster
so if it were a cluster member then Cl 1205+44 members. Basedontheirdisk-likemorphology,we
would certainly not be a fossil group. However, classified3outofthe6groupmembersaslatetype
wehave(withoutredshifts)twoargumentsagainst (spiral)galaxiesandtheotherthree(includingthe
this AGN being a group member: (1) its color is D galaxy) as early type (elliptical) galaxies. We
significantly bluer than the other (probable) clus- will refer to the galaxy population and colors in
ter members in Table 1; (2) the AGN becomes our discussion of the scenarios of this fossil group
the dominant galaxyand then is >∼2 (X-ray)core formation in §3.6 and §3.8 below.
radiiawayfromthecentroidoftheextendedX-ray
3.3. Possible Cooling Evolution
emission.
Tobeafossilgroup,thelowerlimitJonesetal. PerhapsthefactthatCl1205+44ishotterthan
(2003) place to LXbol is 1 × 1042 h−502 ergs s−1, thetwoz=0fossilgroupswithmeasuredtemper-
which Cl 1205+44 easily meets. Although Jones atures discussed by Jones et al. (2003) is due to
et al. do not set an upper limit to the X-ray cooling between z ∼ 0.6 and z ∼ 0. We now con-
temperature or luminosity, the LXbol (corrected sider this possibility. We used the cooling time
to H0 = 75 for their Figure 3, and our Figure 7) equation from Sarazin (1986) for our calculations
pointfallsintheregionwheretheJonesetal. fos- (see §3.5 fordetails). To derive a lowerbound, we
sil group sample lies. Cl 1205+44 is also distin- used our β =0.45, r =15′′ model fit and we de-
c
guished from “normal clusters” in that the LXbol rivedanaveragecoolingtime of6.5 Gyrswithin1
value is about a factor of two lower at kT ∼ 3 core radius and ∼ 11 Gyrs within 2r . The time
c
keV than the cluster sample compiled by Lumb betweenz=0.59andz=0.2(the highestredshift
et al. (2004) for clusters at z ∼ 0.4−0.6. In con- inthe sampleonwhichJonesetal. basetheirdis-
trast, the LXbol– LR point lies approximatelyhalf cussionof fossilgroupformationand evolution)is
way between the Jones et al. lines for “normal” only 3 Gyrs. Even the longer time of 4 Gys (the
X-ray bright groups and fossil groups. On the averageageoffossilgroupsintheJonesetal. sce-
other hand, the X-ray luminosity and kT values nario) is less than the 6.5 Gyrs we derived for the
for Cl 1205+44 are similar to the values Holden average cooling time within the core. We derive
et al. (2001) assigned to an extended X-ray emit- a 2σ lower bound of ∼ 4 Gyrs at the very core
tingregiontheysimplyclassifiedasa“cluster.” In which could lead to cooling in the center of the
theend,however,them12valueofnearly2(within cluster. It is unlikely, therefore, that the ICM of
one half the projected rvir) and LXbol >1042 ergs high redshift fossil groups is hotter than that of
s−1 meet the primary Jones et al. criteria, which low redshift ones due to cooling between z ∼ 0.6
leads us to conclude Cl 1205+44 is fossil group, and z ∼ 0.0. We defer a discussion of the energy
and we will assume it is a fossil group in what input to the ICM until §3.8.1.
follows.
The issue of cooling, gas infall (or the suppres-
sion of gas infall), resulting heating due to gas in-
3.2. Fossil Group Members
fall, etc. is a complicated one and has been dis-
We must make an assumption about galaxy cussed extensively in the literature, see for exam-
membership, since redshifts for the specific galax- ple, some recent works (Clarke et al. 2004; Kaas-
ies are not available to us, and except for the D tra et al. 2004; Peterson et al. 2004, and refer-
galaxynoneofthosemarkedinFigure4wasmea- ences therein). The purpose of the above discus-
6
sion was not to argue for or against cooling per over 1 Gyr.
se, but rather to determine if it is possible for Furthermore, there is the lack of a correlation
the gas to have cooled enough between z ∼ 0.6 betweenthe radioandthe X-rayemission. There-
and 0.1 to explain the temperature difference be- fore we have no evidence that the radio source
tween Cl 1205+44 and nearby fossil groups. We is responsible for extra energy input to the intra
haveshownthatifweassumethesimplestcircum- clustermedium(ICM)ofCl1205+44. Regardless,
stances, i.e. a collisionless gas without a tangled Sunetal.(2003)suggestthatcDgalaxiescanpro-
magnetic field, the gas could just barely cool over videheatingtotheICMviagalacticwindsover10
this z ∼ 0.6-0.1 time interval. Then, since heat Gyr.
input is likely in any event, we conclude that the
nearby fossil groups do not have lower tempera- 3.5. Gas Mass and Total Mass
tures than Cl 1205+44because ofcooling. It may
Inordertocomparewithpreviouswork,itisin-
just be that hot fossil groups such as Cl 1205+44
terestingtocalculatethegasmassinvariousways.
arerareperunit volumebut arethe ones thatare
Belowwegivethe valuesbasedonthe assumption
easiest to detect at high z.
of a core radius of 21′′. The values are approxi-
3.4. The Radio andX-ray Source Relation mately 40% lower if the other best lower limit of
15′′ is used. Thereforethese results are simply for
The issue is whether or not there is evidence qualitative comparison with previous work.
for the radiosource interacting with the ICM and
We used the relationships between X-ray sur-
producing X-rays via the inverse Compton effect.
face brightness and mass as described in Sarazin
Theradiolobes,aswediscussedin§2.4,seemtobe
(1986). We assumed the electron density is 1.1
aligned with the emission that defines the major
times the proton density and a mean molecular
axis in the X-ray emission, but there are no radio
weight of 0.6 for the gas. Within 100 kpc for the
lobes oneither the FIRST (White etal.1998)im- XMM model, we find a gas mass ∼8×1011 M .
J
age or the NVSS (Condon et al. 2002)image that Within one core radius the gas mass is 1.9×1012
coincide with the X-ray emission features called
M . If we assume no temperature gradient, we
J
“A” and “B” in §2.4. Therefore, this alignment
derive a dynamical mass (e.g., Sarazin, 1986) of
is probably accidental. Regardless of whether we 1 × 1013 M . If we use the Sun et al. (2003)
J
assumethealignmentisaccidentalornot,thecon-
value of M/L of 5 and our value of L , we find
R
tribution of inverse Compton flux from the radio the total galaxy mass of Cl 1205+44is 7.5×1012
lobes to the 1-10keVX-rayemissionappearsneg-
M whichisover10timeshigherthantheirvalue
J
ligible as there is no (detectable) X-ray enhance- of about 5×1011 M for the group (not classi-
J
ment associated with the radio lobes.
fied as a fossil group, however) surrounding NGC
BasedontheratiooftheDgalaxytoNGC1550 1550. Within 100kpc, their value of the gas mass
(atthe center ofanX-raybrightgroup;Sun etal. (M )ofabout3×1011M isabout2timeslower
gas J
(2003)20cmfluxes(56.4±1.7mJy/16.6±1.6mJy, than our value for Cl 1205+44. We also calcu-
fromtheVLANVSS;Condonetal.,2002)andon lated M from Allen et al. (2001) (see also Sun
2500
the measured redshifts, we find that the radio lu- etal.2003)whoderivedaformulaforM using
2500
minosity of the D galaxy in Cl 1205+44 is about a set of X-ray luminous relaxed clusters. Then,
1.4×104 timesthatofNGC1550. Thetotalradio M = 2.7 × 1013 M × (T/1.37)1.5/E(z) =
2500 J
flux(∼2.5×10−14ergscm−2 s−1 assumingaflux 3×1013 M . E(z) = H/H = (Ω (1+z)4 +
J 0 r,0
spectral index of −0.7 and that the flux extends Ω (1 + z)3 + Ω + (1 − Ω )(1 + z)2)1/2; for
m,0 Λ,0 0
from0.1GHz to10GHz)iscomparableto the to-
Ω = 0 as assumed here, this can be simplified
r,0
tal X-ray flux (and luminosity) of the gas. But if to (1+z)(1+zΩ +Ω (1+z)2−Ω )(1/2).
m,0 Λ,0 Λ,0
we assume the radio source is only “radio active”
(e.g. Lara et al. 2004, and references therein) for 3.6. Galaxy Colors
108 years,plusthatitprobablycaninjectnomore
To compare with other galaxies at this red-
than 10% of its maximum amount of energy out-
shift we have used the Sloan Digital Sky Survey
put into the ICM, this reduces the total energy
inputto1%ofthe totalenergyoutputoftheICM
7
(SDSS)9 and the 2MASS catalog. For galaxies ters that formed earlier in these simulations tend
bright enough to have spectra measured with the to have fewer galaxies and less continuous infall
SDSS,wefindthatthecolorsofthosegalaxiesatz (to produce the BO effect) than the systems that
between0.5and0.6aretypicalofthoseinTable1 formed later. These simulations predict, then,
assumed to be members of CL 1205+44. that the galaxy to total cluster mass ratio should
The six galaxies for which we have colors that be lowerforthe earliestformedclusterscompared
weassociatewithCl1205+44areallrelativelyred, to later ones. With a sample of one fossil group
even the late-type galaxies. In contrast, Butcher withredgalaxies,however,itisprematuretomake
& Oemler (1984) found a high fraction of blue a comparison between the data and the simula-
galaxiesatthisredshiftcomparedtolowredshifts. tions at this level.
The Butcher-Oemler effect can be explained by
3.7. The D Galaxy
assuming that at 0.6 compared to 0, there is a
higherfractionofgalaxiesthathavejustfalleninto The D galaxy of our system is similar to the
the cluster and have not had their gas stripped z = 0.25–0.5 simulated galaxies in West (1994).
yet (e.g. Kauffmann 1995). Hence, the newer (at Our value of the i′ magnitude of 19.27 converts
leastspiral)clustermemberstendtobe bluer,the into a rest frame absolute R magnitude of −24.1
J
higher the redshift. For Cl 1205+44, the fossil (luminosity distance = 3.4×103 Mpc, where we
group could have formed at z of about 2, could
used 0.7 for the K correction for an elliptical at
have aged about 4 Gyrs, and could have had no z = 0.59). This value of −24.1 is about 0.5 mag-
galaxy infall since birth. The only galaxy popula-
nitudes brighter than the brightest cluster galaxy
tion evolution that has taken place has been the magnitudes compiledby Collins et al.(2003). Re-
merging of galaxiesinto the centralD galaxyplus cent galaxy mergers may be responsible for this
ram pressure stripping of the gas from the galax-
relatively high brightness.
ies. Then,therearenorecentinfallspiralsandthe
Note that ongoing minor mergers still appear
spirals have the same colors as the elliptical (un-
to be visible on the HST image (see Figure 13).
dertheassumptionthatallbutgalaxy#6andthe
These ongoing mergers at a relatively early stage
AGNinTable1areclustermembers;for,asnoted
of this fossil group history do not favor the fossil
in §3.1 it would be peculiar to have the brightest
group formation scenario of Mulchaey & Zablud-
clustergalaxysofarremovedfromtheclustercen-
off(1999),whoproposedaformationofthese sys-
ter). Inthis hypothesis,the spiralshavehadtheir
tems with an unusual initial luminosity function.
gas removed via ram pressure stripping, and they
An alternative proposal is the evolutionary sce-
havebeenclustermemberssinceits formationap-
nario of fossil groups proposed by Borne et al.
proximately 4 Gyrs ago. The key issue is what
(2000) and Jones et al. (2003). This links com-
suppressesgalaxyinfallforfossilgroupscompared
pact groups of galaxies and giant elliptical galax-
to typical groups and clusters.
iesviaanultra-luminousinfraredgalaxy(ULIRG)
Fossil groups have probably formed in initially
phase. The D galaxy of our system (which is also
above average over dense regions (so that the a radio source) has an i′-K color very similar to
s
group collapsed early), which, however, were not thatofULIRGs(fromNED10)inthe 2MASSsur-
sufficiently rich in matter and galaxies to sustain
vey. This D galaxy could be an ULIRG that is
growth of the group beyond some point in time. justturningon,exceptthattheK -bandfluxfalls
s
The propertyofhavingrelativelya lowtotalmat-
well below other ULIRGs (e.g. Yun et al. 2004).
ter value and a negligible blue galaxy fraction is Therefore if the D galaxy were formed in the pro-
directly related to arriving at an over density suf- cess suggested by West (1994), then the ULIRG
ficient for collapse earlier than more massive sys-
phase has probably already passed.
tems. For example, simulations by Gao et al.
Continuing with the idea of D galaxy evolu-
(2004) have shown that the galaxy infall rate and
tion in galaxy groups, we refer to Collins et al.
number of galaxies in a cluster is related to the
(2003). They show that in poor groups the vari-
age of the universe when the cluster formed; clus-
ancebetweencentralgalaxypropertiesfromgroup
9http://www.sdss.org/dr2/ 10http://nedwww.ipac.caltech.edu/
8
to group is quite large (factors of 10 or more) in (2002). Babul et al. made predictions of initial
terms of total optical output. Thus, we expect energy input which they comparedwith the data.
there would be a variation in the optical prop- The two fossilgroupsfoundby Jones etal.(2003)
erties of the D galaxies in fossil groups such as with strong enough signal to measure a tempera-
Cl 1205+44, unless there is a direct correlation ture were consistent with an entropy floor of 100
between the kT value and the D galaxy stellar keV cm2. In contrast, Cl 1205+44 lies a factor
content. Only a high redshift (>∼ 0.6) survey of of two or more above the 100 keV cm2 line and
fossilgroupswillbe ableto shed lightonthe vari- instead is nearly on the line occupied by normal
anceofinitialconditionsandasurveyingeneralis richclusters and normalgroups (initial energy in-
needed to shed light on the relationship between put 427 keVcm2; see Figure 7). This implies that
kT and the D galaxy. eitherCl1205+44isreallynotafossilgroup,con-
trary to our classification, or that it is unlike the
3.8. Scenarios nearby fossil groups,and that we were able to de-
tectCl1205+44preciselybecauseitismuchmore
To summarize a formation scenario for Cl
luminous than low z fossil groups. This could be
1205+44: There was an initial potential well of
due tothe resultofarelativelyhigh(comparedto
dark matter. Energy injection occurred via some
theaveragefossilgroup)entropyfloorinputinthe
process or processes such as perhaps supernovae,
ICM, or indicate that Cl 1205+44 simply is more
radio galaxies, or ULIRGs. This process heated
massive that other fossil groups.
the gas as galaxiesfell into the potential well at a
Opposed to the model of Babul et al. (2002),
redshift of about 2. The galaxies passed through
wefindamuchlowervalueforthe pseudoentropy
the ICM several times and the gas and dust were
swept out of the spirals. This ram pressure strip- = Tne−2/3 keV cm2 of 85 keV cm2. We derived
pingsuppressedcontinuousstarformationandleft this value at r = 0.1r200 = 0.1rvirial from our β
thespiralstobecomeasredastheellipticals. The fit(rc =21′′),whereweestimateanaveragekT=
systemformedinarelativevoidofgalaxiessothat 3 keVand an electron density at the core to be ∼
there was no continuous infall of galaxies to keep 8.1×10−3cm−3,i.e. 6.5×10−3 cm−3 at0.1rvirial.
the systemfedwith blue spiralfield galaxies. The This implies some refinement to the Babul et al.
centralgalaxygrewtobecomearelativelylargeD model such as continuous rather than impulsive
galaxy by merging with other galaxies to form a heating (pre-heating)atformation. Note that our
m value >∼ 2. The entire evolutionary sequence entropyfora3keV(thetemperatureoftheICMof
12
took about 4 Gyrs. Cl1205+44)temperatureis lowerthan wasfound
for3keVvirializedobjects(including clustersand
TheabovefitswiththeJonesetal.(2000,2003)
groups) at z <∼0.2, by Ponman et al. (2003).
scenario, which assumes that the time it takes
for the merging of galaxies to form a bright cen- The fact that the entropywe find is lowerthan
tral galaxy is approximately 4 Gyrs and no con- thatfoundformorenearbyclustersandgroupsat
tinuous field galaxy infall has taken place. The the sametemperature is consistentwiththe “neg-
fact that the D galaxy in Cl 1205+44 appears to ative” entropy evolution suggested by Maughan
have nearly completed the merging process and is et al. (2004). Maughan et al. proposed a form
approximately 4 Gyrs younger than z ≃ 0 fossil E(z)−4/3 based on self-similar scaling and on the
groups, implies that (at least some) fossil groups evolution of the critical density of the Universe
at z = 0 are much older than 4 Gyrs. with z. The scaled-by-temperature (i.e. divided
by kT) entropies for similar temperature objects
3.8.1. Consequences found by Ponman et al. (2003) at the same ra-
dius (0.1r ) have values of about 75 cm2, which
vir
Energy to heat the ICM above and beyond ifscaledfromz=0toz=0.5915byE(z)−4/3 cor-
gravitational infall results in an entropy (defined
respondto 49cm2,whichis withinafactoroftwo
here as the pseudo entropy kTne−2/3; where ne is of the value for Cl 1205+44 = 28 cm2 at 0.1r .
thenumberofelectronspercm−3andkTisinkeV) vir
These results then appear to be more consistent
floor. The initial energy injection and its conse-
withcontinuousratherthanimpulsiveinitialheat-
quences were discussed in detail by Babul et al.
ing to the ICM.
9
If there were some initial energy injection at Our main conclusion is that Cl 1205+44 is a
z = 2 (whatever its cause), then this injection fossil group. A formation scenario is that the
couldaffectthehighorderportionoftheCMBvia group formed in a peak density region of rela-
the SZ effect fromclusters and groupsof galaxies. tively low total mass at z ∼ 2. The formation
The possibility of this energy input and its effect regions did not have enough matter to sustain
on SZ measurements has been discussed by Lin continuous infall of galaxies and gas, and such a
et al. (2004) in the context of deriving cosmolog- scenario is consistent with the simulation carried
ical parameters based on the power spectrum of out by Gao et al. (2004). Beyond the formation
the CMB. Their impulsive heating model requires of a central dominant galaxy via accretion, fossil
energy injection at z = 2. They fit their models groupsappearold,i.e.,fossil-likepreciselybecause
tothe L -T normalclusterandgroupdatathat they have had little or no evolution in terms of
X X
are shown here in Figure 7, and from this we see galaxy infall since their inception. And, some fos-
that Cl 1205+44 is consistent with the Lin et al. silgroupscouldbeconsiderablyolderthan4Gyrs.
model(seealsoBorganietal.2004). Thefactthat No evolution of the galaxy population (at least in
preheatingoccurredinthe Linetal. modelnearz the case of Cl 1205+44)is implied because of the
∼2 is consistentwith our suggestionthat atleast dearthofbluespirals,thekT-L valuepointon
Xbol
somefossilgroups,suchasCl1205+44,formedat theno-evolutiontracksforrichclustersofgalaxies
z ∼2. However, if continuous heating models can (Novicki et al. 2002),and our cooling calculations
also be shown to fit the data, then the amount of of the IGM are consistent with (but cannot ex-
initial impulsive heating suggested by Lin et al. clude) no significant cooling of CL 1205+44 in 3
willnotbeaslargeasthey suggested,andthe im- Gyrs.
pact on the high order CMB observations will be Some preheating of the gas in groups and clus-
less. ters via supernovae or an active galaxy phase is
likely to have occurred even in fossil groups (or
4. Summary and Conclusions normalgroups)asevidencedbytherelativelyhigh
temperature of CL 1205+44, but it also appears
Wehavefound,viaaROSATsurveyofarchival
likely that there is continuous rather than impul-
data, a group of galaxies with a relatively red
sive heating at group formation, which results in
population. HYPERZ calculations based on the
“negative” evolution of the pseudo entropy (e.g.
i′- and K -band data suggested the system might
s Maughan et al. 2004). However, as the kT ver-
>
have z ∼ 1. Subsequent redshift work found that
sus L of Cl 1205+44, fits within 1σ the no-
the redshiftis = 0.5915,andChandra plus XMM- Xbol
evolution with z of Novicki et al. (2002), the neg-
Newton observationsconfirmedtheexistenceofX-
ative evolution for entropy and the no-evolution
ray emitting gas in the group. We classify this
models for kT versus L (and galaxy popula-
group as a fossil group based on its m in i′ Xbol
12 tion) may need to be fine tuned to be made con-
and on the fact that its X-ray luminosity exceeds
sistent with each other.
1×1042h−2ergss−1. Thismakesitbyfarthehigh-
50 Also,theamountofpreheatinghasimplications
estredshiftfossilgroupyetreported. Thetemper-
for the interpretation of high order measurements
aturefortheICMisabnormallyhighcomparedto
of the CMB. Our data are consistent with “neg-
two z ∼ 0 fossil groups with a similar L , and
Xbol
ative” evolution of the pseudo entropy which im-
the L –L point lies approximately half way
Xbol R
plies,ononehand,alesserimpactoftheSZ effect
between the best fit lines for normal and fossil
from high z clusters than suggested by Lin et al.
groups Jones et al. (2003); Cl 1205+44has a kT–
(2004). On the other hand, that the kT versus
L value comparabletoanotherz ∼0.6system
Xbol
L value for our data and many other clusters
that was classified as a cluster (RX J0848+4456; Xbol
is consistent with no evolution, suggests that per-
Holdenetal.,2001). But,althoughtherearesome
haps a significantfaction of ICM heating did take
similarities with normal groups and clusters, we
placenearz =2, andhencedoes havea significant
have classified it as a fossil group based on the
primary criteria of m >∼ 2 in R- or V-band rest effect on the high order terms of the CMB.
12
frame and an X-ray luminosity ≥1×1042h−2 The discovery and X-ray and optical measure-
50
ments of still more z ≥ 0.6 clusters and fossil
10