The current status of our efforts to trace cosmic structure with 10^6
galaxies (2MASS), 10^3 galaxy clusters (NORAS II cluster survey), and precision
measurements for 10^2 galaxy clusters (HIFLUGCS) is given. The latter is
illustrated in more detail with results on the gas temperature and metal
abundance structure for 10^0 cluster (A1644) obtained with XMM-Newton.Comment: 4 pages; to be published in the Proceedings of the Conference: The
  Emergence of Cosmic Structure, College Park, MD (2002), editors: S.S. Holt
  and C. Reynolds; also available at http://www.reiprich.ne

Boehringer, Hans

Kempner, Joshua C.

Reiprich, Thomas H.

Retzlaff, Joerg

Sarazin, Craig L.

Sivakoff, Gregory R.

Skrutskie, Michael F.

English

arXiv

Sarazin, Craigh

Böhringer, Hans

Retzlaff, Jörg

Max Planck Society eDoc Server

Cosmic structure traced by precision measurements of X-ray brightest galaxy clusters in the sky

Thomas H. Reiprich

Crossref

Cosmic Structure Traced by Precision Measurements of the X-Ray Brightest Galaxy Clusters in the Sky

Reiprich, T.

Sarazin, C.

Kempner, J.

Skrutskie, M.

Sivakoff, G.

Böhringer, H.

Retzlaff, J.

MPG.PuRe

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Cosmic Structure Traced by Precision
Measurements of the X-Ray Brightest Galaxy
Clusters in the Sky
Thomas H. Reiprich∗†, Craig L. Sarazin∗, Joshua C. Kempner∗∗, Michael F.
Skrutskie∗, Gregory R. Sivakoff∗, Hans Böhringer‡ and Jörg Retzlaff‡
∗Department of Astronomy, University of Virginia, PO Box 3818, Charlottesville, VA 22903-0818
†thomas@reiprich.net, http://www.reiprich.net
∗∗Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-67 Cambridge, MA 02138
‡Max-Planck-Institut für extraterrestrische Physik, PO Box 1312, 85741 Garching, Germany
Abstract. The current status of our efforts to trace cosmic structure with 106 galaxies (2MASS),
103 galaxy clusters (NORAS II cluster survey), and precision measurements for 102 galaxy clusters
(HIFLUGCS) is given. The latter is illustrated in more detail with result on the gas temperature
and metal abundance structure for 100 cluster (A1644) obtained with XMM-Newton.
Galaxy clusters have been very important tools to study cosmic tructure and deter-
mine cosmological parameters. Even moderately sized samples yield competitive statis-
tical constraints, e.g.,HIFLUGCS (Fig. 1), [1]. However, accuracy is currently limited
by systematic uncertainties. Luckily major improvements are now feasible, e.g., by tak-
ing advantage of new multiwavelength data and especially Chandra and XMM-Newton
X-ray observations. We’ve started a project to tackle systema ic uncertainties in flux,
temperature, gas and gravitational mass estimates by detailed analyses of the 63 X-ray
brightest galaxy clusters in the sky (HIFLUGCS). Many of these clusters have already
been observed by Chandra and XMM-Newton. Data are accumulating in the archives.
The first out of further nine approved Chandra observations proposed by us is being
carried out today (2002-12-16).
The NORAS II cluster survey contains about 800 X-ray selected galaxy clusters
(Böhringer, Retzlaff et al., in prep.). In order to test for systematic effects caused by
different selection techniques — which must be well understood to obtain precise cos-
mological constraints — we started correlating the NORAS IIclusters with the 2MASS
extended source catalog (the latter shown in Fig. 1, [2]) andcolor selected point source
catalog. Furthermore galaxy overdensities at cluster position will be determined in or-
der to estimate richness–mass relations and mass-to-lightratios.
In the following we illustrate possible improvements from the X-ray side on an espe-
cially tough example. XMM-Newton observations of A1644 show a very complicated
surface brightness distribution on all scales (Reiprich etal., in prep.). A main clump
and a smaller sub clump are easily identified. The emission surrounding both core re-
gions is highly non-spherical. And the core region of the main clump itself contains a
displaced core-within-a-core. How much do X-ray flux, gas, and gravitational mass es-
timates based on precision measurements differ from simpleestimates where, e.g., the
RXJ2344
HCG94
A3921
A3888
A3827A3822
IIZw108
A3695
A2319
ZwCl1742
OPHIUCHU
TRIANGUL
A3627
A2151w
N5846N5813
A1914
A1800A1775
A3560
A1689
A35320A3528sn
M49
A1413
S636
A0644
PKS0745
UGC03957
A3395n A0548eA0548w
S540
A0539
3C129
S405
PERSEUS
AWM7
N499
A2877
A2734
A4059A4038
A2657
A2634
A2597
A2589
S1101
A3667
A2255
A2256 A2244
A2204
A2199
A2163
A2 47
A2142 A2063
A2065
MKW SA2052A20 9
MKW8
A3581
A1795
A3571A3562
A3558
A1736
N5044
COMA
A1651
A1650
A1644
A3526
N4636ZwCl1215MKW4
A1367
A1060 HYDRA_AA0754
A0576
sA 391 A3376
A0496
A3266
EXO0422
N1550
A0478
A3158
IIIZw54
2A0335
FORNAX
A3112
A0401A0399
A0400
A0262
507
A0133
A0119
A0085
CYGNUS_A
VIRGO
5*10^5  2MASS  Galaxies10^2 HIFLUGCS Galaxy Clusters
FIGURE 1. HIFLUGCS and other local X-ray galaxy clusters shown in Galactic coordinates on top of
the distribution of half a million 2MASS galaxies. Qualitatvely both populations seem to nicely trace an
underlying matter distribution with clusters being located at the peaks.
whole cluster is treated as a spherical cow as might be done ifonly ROSAT All-Sky Sur-
vey data were available? How much do mass estimates differ whn only a broad beam
overall gas temperature estimate is available?
The X-ray flux (fX) ratio between the main and sub clump is about 3:1. This means
instead of one cluster in a flux-limited sample withfX ≈4×10
−11 erg/s/cm2 one actually
has two clusters with aboutfX ≈ 3 and 1× 10
−11 erg/s/cm2 each. This is quite an
important difference, especially for construction of luminosity and mass functions!
The intraclucter gas temperature is a fundamental observabl for a mass determination
based on the hydrostatic assumption. One first step of refineme t compared to broad
beam temperature estimates is the construction of radial temperature profiles for the
main and sub clump. To our surprise we found that each of the two temperature profiles
appears very similar to temperature profiles of relaxed, apparently undisturbed clusters:
a drop in the center to about 1/2 to 1/3 of the ambient gas temperatur , an isothermal
structure in the outer parts, and weak indications for a slight temperature drop in the
very outermost regions accessible. This appears to be good news: this cluster may
not be as complicated as it seemed, the temperature structure may not be affected by
the interaction of the two sub clumps (which are located at about the same redshift).
Note, however, that simple broad beam temperature estimates would still be biased
low compared to the ambient gas temperature due to cool emission in the dense (high
emissivity) centers. For instance, the temperature estimate in a large annulus around the
main clump — taken as the ambient temperature — is a factor of 1.1–1.15 higher than
a broad beam temperature estimate including both clumps. SinceM ∝ T1.5−2.0 [e.g., 3]
this factor translates into a factor of 1.15–1.32 in a mass estimate!
The next step we took is the subdivision of annuli into regular segments. Figure 2
shows selected regions as well as surface brightness contours overlaid onto a hardness
12h57m00s12h58m00s 57m50s 40s 30s 20s 56m50s 40s
-17d20m00s
-17d30m00s
16m00s
18m00s
14m00s
22m00s
24m00s
26m00s
28m00s
32m00s
R18
R29R28R27
R20
R26R32
R24R31
R22R30
FIGURE 2. Hardness ratio map of the galaxy cluster A1644. Background,exposure, and vignetting
corrected, adaptively smoothed, combined MOS1-MOS2-pn cou t rate image ratio for the energy bands
(0.3–2.0keV)/(2.0–10.0keV). Soft emission appears bright and hard emission dark. Also shown are
surface brightness contours for the (0.3–2.0keV) image andregions selected for spectral analysis.
ratio (HR) map. Preliminary temperature (metal abundance)estimates for the segments
based on standard spectral model fits are shown in the left (right) hand side of Fig. 3.
Now it becomes obvious that the apparently regular temperature profiles are misleading
and only due to averaging over (too) large regions. The temperatures of the segments to
the East of the main clump (R22, R24, and R26) are all significatly lower than almost
all regions to the West (R27–R32). This may indicate that thegas in the regions between
the two clumps has been heated up by adiabatic compression oreven shocks.
Having found irregularities in the temperature structure afurther step is to select
regions based on the HR map to directly search for cool/metalrich trails (bright) or
hot spots (dark). The significance of brightness fluctuations s again evaluated by direct
spectral fits. Note that not all artifacts, e.g., inexact exposure correction close to CCD
chip boundaries, have been removed in the HR map in Fig. 2. Thespectral analysis,
however, is not affected by this. The region to the South of the sub clump (R18) appears
fairly bright and therefore soft, whereas a small region to the Southeast (R20) appears
hard. The spectral fit results (Fig. 3) reveal that indeed thetemperature estimate for R18
is significantly lower than the estimate for the rest of this annulus (R19) and especially
than that for R20. Furthermore the metal abundance estimatefor R18 appears enhanced.
These findings together with the surface brightness structure might imply that to the
South we see gas that has been removed from the cooler center of th sub clump possibly
by the combined effect of some energy source related to the central galaxy of the sub
FIGURE 3. Gas temperature (left) and metal abundance (right) estimates of selected regions based on
simultaneous spectral fits to background and vignetting corre ted spectra from the MOS1, MOS2, and pn
detectors aboard XMM-Newton. Note that R21=R27+R30, R23=R28+R31, R25=R29+32, and R19=full
annulus–R18; see Fig. 2. Statistical error bars show the 90%confidence level for one interesting parameter.
clump (a radio source) and the movement through intracluster gas of the main clump.
Should more detailed modeling confirm the significance of theabundance excess then
other possibilities like cooling of intracluster gas onto the moving sub clump (as seen in
A1795 on much smaller scales, [4]) would be more difficult to reconcile with the data.
In summary the temperature structure of A1644 shows that this system is quite com-
plicated as indicated by the surface brightness structure.Int restingly no evidence for
substructure has been found in optical and near-infrared observations [5]. The next step
is to attempt an improved mass estimate and compare it to moresimple estimates as
generally applied to clusters at higher redshift and in larger samples. Note that this de-
tailed study required only 12 ks of good data. Chandra and XMM-Newton observations
of the completeHIFLUGCS are quite cheap but offer a great opportunity for clusters in
the era of precision cosmology. Comparison of cluster X-rayd ta to the wealth of new
multiwavelength data, e.g., 2MASS, will also help to reducesystematic uncertainties.
We acknowledge the benefit from the dedicated work of the NORAS II and 2MASS
teams. This work was supported by NASA XMM-Newton Grant NAG5-10075. THR
acknowledges support by the Celerity Foundation through a Post-Doctoral Fellowship.
The XMM-Newton project is an ESA Science Mission with instruments and contribu-
tions directly funded by ESA Member States and the USA (NASA). This publication
makes use of data products from 2MASS, which is a joint project of the University of
Massachusetts and IPAC/Caltech, funded by NASA and NSF.
REFERENCES
1. Reiprich, T. H., and Böhringer, H.,ApJ, 567, 716–740 (2002).
2. Jarrett, T. H., Chester, T., Cutri, R., Schneider, S., Skruts ie, M., and Huchra, J. P.,AJ, 119, 2498–2531
(2000).
3. Finoguenov, A., Reiprich, T. H., and Böhringer, H.,A&A, 368, 749–759 (2001).
4. Fabian, A. C., Sanders, J. S., Ettori, S., Taylor, G. B., Allen, S. W., Crawford, C. S., Iwasawa, K., and
Johnstone, R. M.,MNRAS, 321, L33–L36 (2001).
5. Tustin, A. W., Geller, M. J., Kenyon, S. J., and Diaferio, A. J, 122, 1289–1297 (2001).


Cosmic Structure Traced by Precision Measurements of the X-Ray Brightest
  Galaxy Clusters in the Sky

Cosmic Structure Traced by Precision Measurements of the X-Ray Brightest Galaxy Clusters in the Sky

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