Chandra observations of the galaxy cluster Abell 1835

Abstract

We present the analysis of 30 ksec of Chandra observations of the galaxy cluster Abell 1835. Overall, the X-ray image shows a relaxed morphology, although we detect substructure in in the inner 30 kpc radius. Spectral analysis shows a steep drop in the X-ray gas temperature from ~12 keV in the outer regions of the cluster to ~4 keV in the core. The Chandra data provide tight constraints on the gravitational potential of the cluster which can be parameterized by a Navarro, Frenk & White (1997) model. The X-ray data allow us to measure the X-ray gas mass fraction as a function of radius, leading to a determination of the cosmic matter density of \Omega_m=0.40+-0.09 h_50^-0.5. The projected mass within a radius of ~150 kpc implied by the presence of gravitationally lensed arcs in the cluster is in good agreement with the mass models preferred by the Chandra data. We find a radiative cooling time of the X-ray gas in the centre of Abell 1835 of about 3x10^8 yr. Cooling flow model fits to the Chandra spectrum and a deprojection analysis of the Chandra image both indicate the presence of a young cooling flow (~6x10^8 yr) with an integrated mass deposition rate of 230^+80_-50 M_o yr^-1 within a radius of 30 kpc. We discuss the implications of our results in the light of recent RGS observations of Abell 1835 with XMM-Newton. Comment: 15 pages, 15 figures, accepted by MNRAS

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arXiv:astro-ph/0107311v1 17 Jul 2001
Mon. Not. R. Astron. Soc. 000, 1–15 ()Printed February 1, 2008(MN LATEX style file v2.2)
Chandra observations of the galaxy cluster Abell 1835
R. W. Schmidt,⋆S. W. Allen and A. C. Fabian
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom
Received
ABSTRACT
We present the analysis of 30ksec of Chandra observations of the galaxy cluster Abell
1835. Overall, the X-ray image shows a relaxed morphology, although we detect sub-
structure in in the inner 30kpc radius. Spectral analysis shows a steep drop in the
X-ray gas temperature from ∼12keV in the outer regions of the cluster to ∼4keV in
the core. The Chandra data provide tight constraints on the gravitational potential of
the cluster which can be parameterized by a Navarro, Frenk & White (1997) model.
The X-ray data allow us to measure the X-ray gas mass fraction as a function of radius,
leading to a determination of the cosmic matter density of Ωm= 0.40±0.09h−0.5
projected mass within a radius of ∼150kpc implied by the presence of gravitationally
lensed arcs in the cluster is in good agreement with the mass models preferred by
the Chandra data. We find a radiative cooling time of the X-ray gas in the centre of
Abell 1835 of about 3×108yr. Cooling flow model fits to the Chandra spectrum and
a deprojection analysis of the Chandra image both indicate the presence of a young
cooling flow (∼ 6×108yr) with an integrated mass deposition rate of 230+80
within a radius of 30kpc. We discuss the implications of our results in the light of
recent RGS observations of Abell 1835 with XMM-Newton.
50
. The
−50M⊙yr−1
Key words: galaxies: clusters:general – galaxies: clusters: individual:Abell 1835 –
cooling flows – intergalactic medium – gravitational lensing – X-rays: galaxies
1 INTRODUCTION
Since its launch in 1999 July, the Chandra X-ray obser-
vatory (Weisskopf et al. 2000) has observed a large num-
ber of galaxy clusters, many of which are amongst the
most massive objects of their kind. In particular, high res-
olution, spatially-resolved spectroscopy with the Advanced
CCD Imaging Spectrometer (ACIS) on Chandra is leading
to significant advances in the way we understand these sys-
tems.
As illustrated in a recent paper by Allen et al. (2001a),
Chandra observations enable us to constrain the proper-
ties of galaxy clusters with unprecedented accuracy. Chan-
dra data permit direct measurements of the X-ray gas den-
sity, temperature and total mass profiles at a resolution of
∼ 20h−1
shifts z ∼ 0.2, paving the way for detailed cosmological stud-
ies.
In this paper we present Chandra observations of the
galaxy cluster Abell 1835 at a redshift z = 0.2523. This clus-
ter is the most luminous system (LX = 2.7 × 1045ergss−1
in the 0.1-2.4 keV ROSAT band) in the ROSAT Bright-
50kpc for the most massive clusters observed at red-
⋆E-mail: rschmidt@ast.cam.ac.uk (RWS), swa@ast.cam.ac.uk
(SWA), acf@ast.cam.ac.uk (ACF)
est Cluster Sample (Ebeling et al. 1998) and has previously
been inferred to contain a strong cooling flow (with a nom-
inal mass deposition rate of ∼ 1000 M⊙yr−1; e.g., Allen
et al. 1996). The central dominant galaxy exhibits powerful
optical emission lines and a strong UV/blue continuum asso-
ciated with large amounts of ongoing star formation (Allen
1995; Crawford et al. 1999). The observed Balmer emission
line ratio also indicates significant intrinsic reddening (Allen
1995, E(B-V ) = 0.49+0.17
tection of 850µm emission from the cD galaxy by Edge et
al. (1999), which those authors attribute to emission from
warm dust heated by the young, hot stars.
−0.15). This is consistent with the de-
The great concentration of mass in the cluster is also
revealed by several features in optical images that can be
identified as gravitationally lensed background objects. In
particular, a large arc was discovered by Edge et al. (in
preparation) to the south-east of the cD galaxy. Allen et
al. (1996) used this arc to put constraints on the cluster
mass inside the radius of the arc. We will discuss several
further possibly lensed objects in this study.
The structure of this paper is as follows: Sect. 2 de-
scribes the Chandra observations and Sect. 3 the basic imag-
ing results. Sect. 4 discusses the spatially-resolved spec-
troscopy of the cluster using the Chandra data. Sect. 5
presents a detailed mass analysis using the independent X-

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Schmidt, Allen & Fabian
ray and lensing data.In Sect. 6 we examine the properties
of the central cooling flow and compare our results with
those from recent observations made with the XMM-Newton
X-ray observatory. We present our conclusions in Sect. 7.
All quantities are given throughout using a cosmology with
H0 = 50kms−1Mpc−1and q0 =1
all error bars are 1σ (68.3%) confidence intervals.
2. Unless otherwise noted
2OBSERVATIONS
Chandra observed Abell 1835 on 12 December 1999 for a
total of 19.6ksec. The target was centred in the middle of
node 1 of the back-illuminated ACIS chip S3 (ACIS-S3),
near the nominal aim point of this detector. The detector
temperature during the observation was -110
10.7ksec Chandra observation of Abell 1835 was obtained
on 29 April 2000 at a detector temperature of -120
this second exposure, the cluster was centred close to the
node boundary in the geometrical centre of the ACIS-S3
chip, in the middle of a group of five dead columns. This
has prevented us from using this second data set for the
spectral analysis presented in this paper. We have, however,
used both data sets to produce the images in Figs. 1 and 4.
◦C. A further
◦C. In
3 IMAGING
In Fig. 1 a contour plot of the 30ksec Chandra exposure
of the inner region of the galaxy cluster in the 0.3-7.0keV
energy band is shown with a side length of 3.3 arcmin-
utes (1Mpc at the distance of the galaxy cluster). About
80,000 photons are collected in this image. For comparison,
an archival 20min B-band exposure taken on 27 February
1998 with the Canada France Hawaii Telescope (CFHT) of
the central 1.5’×1.5’ is shown in Fig. 2. In the optical image,
the large arc (object A) discovered by Edge et al. (in prepa-
ration) can be seen to the south-east of the central cluster
galaxy. This arc is thought to be a gravitationally lensed
image of a galaxy far behind Abell 1835. The presence of
this arc, notwithstanding the large X-ray emission, suggests
that Abell 1835 is a very massive galaxy cluster. Allen et al.
(1996) estimated the projected mass inside the radius of the
arc to be between 1.4×1014M⊙ and 2×1014M⊙, if the arc
has a redshift greater than 0.7 and is situated on a critical
line of the gravitational potential of the cluster. In Fig. 2
a smaller arc can be seen to the south-west (object B) of
the central cluster galaxy. The arrow to the north-west of
the cD galaxy points to a radial feature which we discuss in
Sect. 5.2.
Except for an apparent point source ∼ 1.5 arcmin to the
east (this is discussed in Fabian et al. 2000a and Crawford
et al. 2000), the Chandra image is dominated by emission
from the hot intracluster gas of the galaxy cluster. Overall,
the cluster has a regular, relaxed morphology. The isophotes
outside the core are elliptical with an axis ratio of minor
and major axis ∼ 0.85. In Fig. 3, the average counts per sec
per arcsec2as a function of radius are shown as counted in
annuli around the galaxy cluster centre in the 0.3-7.0keV
band. The profile shows the steep surface brightness profile
typical of cooling flow clusters (e.g.,Fabian 1994). Note,
Figure 1. X-ray contours of the central 3.3×3.3arcmin (1x1Mpc)
of Abell 1835 in the 0.3-7.0keV band. The contours were drawn
at 1, 2, 4, 8, 16, 32, 64, 91 and 108 counts per 1x1arcsec pixel.
The coordinates are J2000.
B
A
Figure
(0.45×0.45Mpc) of Abell 1835. North is up and east is to the
left. The cross marks the galaxy core. The greyscale has been
wrapped once for clarity.
2. Optical B-band image of the inner 1.5’×1.5’
however, that Chandra detects a flattening of the profile
within r ∼ 30kpc.
In Fig. 4 the central 30”×30” (150kpc side length) of
Fig. 1 are shown. The image was smoothed with a Gaussian
kernel with a full-width at half-maximum of 1.6 pixels (0.8
arcsec). In the inner 20arcsec (100kpc) of the cluster Chan-
dra resolves substructure in the X-ray emission. There are
a couple of main features visible: a well-defined peak just
east of the geometrical centre (denoted by the black ring;
J2000.0 coordinates given by Chandra: RA 14:01:02.07, DEC
+2:52:43.2) of the cluster, a more inhomogeneous structure

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Chandra observations of the galaxy cluster Abell 1835
3
Figure 3. Surface brightness profile of Abell 1835 as a function
of projected separation from the cluster centre. The count rates
have been azimuthally averaged in annuli of width of 1arcsec
(4.9kpc). The data points are positioned on the abscissa at the
average of the inner and outer radius of the annuli. The data
points are shown with Poisson error bars.
to the south-west of the geometrical centre, between these
two regions an elongated strip with a reduced count rate,
and a small emission front to the south-east of the cluster
core (between the arrows in Fig. 4). The brightness gradient
towards the north-west is steeper than towards the south-
east. These features may be the sign of a merging subclump.
The number of counts per half-arcsecond pixel in the bright-
est regions in the combined 30ksec of our data is between
30 and 40. This means that the detailed structures need to
be analysed with care because the Poisson error is still of
the order of 20%. The main features, however, are robustly
detected and indicate the presence of variations of the X-ray
emission or absorption in the core of Abell 1835 on scales
of several kiloparsecs. Comparison with the optical image
(Fig. 2) shows that the emission inhomogeneities are situ-
ated inside the central cluster galaxy.
4 SPATIALLY RESOLVED SPECTRAL
ANALYSIS
4.1 Data extraction
We extract spectra from selected regions of the longer 19.6
ksec exposure Chandra data set using the CIAO software
package distributed by the Chandra X-ray Observatory Cen-
tre (CXC, web site: cxc.harvard.edu).
We only work with photons detected in the ACIS-S3
chip, which is also known as ACIS chip 7. This is a back-
illuminated chip and does not suffer from the degrada-
tion of energy resolution that affects the front-illuminated
ACIS chips on the Chandra observatory. All significant point
sources were masked out and excluded from the analysis.
Response matrix files (RMF) and ancillary response
files (ARF) were produced using a PERL script generously
provided to us by Roderick Johnstone. This script combines
individual RMFs and ARFs for each 32x32 pixel region on
the ACIS-S3 chip (generated using the mkrmf tool in the
CIAO software suite available from the CXC website) in a
photon number-weighted manner.
Using the task grppha from the FTOOLS software
package provided by the NASA High Energy Astrophysics
10 kpc
x
Figure 4. X-ray image of the central 30”×30” (150kpc×150kpc)
of Abell 1835 in the 0.3-7.0keV band. North is up and east
is to the left. The image was smoothed with a Gaussian with
a full-width at half-maximum of 0.8arcsec. The pixels are
0.5x0.5arcsec. Only pixels with at least 6 counts are plotted. The
circle marks the geometrical centre of the cluster mentioned in the
text. The cross marks the absolute position of the galaxy core (see
Fig. 2) determined by Allen et al. (1996) when plotted in the coor-
dinates given by Chandra. The uncertainties associated with the
galaxy position and the Chandra astrometry are ∼ 1arcsec. The
two arrows point at the small emission front extending between
them. The greyscale has been wrapped once for clarity.
Science Archive Research Center (HEASARC, FTOOLS
web site: heasarc.gsfc.nasa.gov/lheasoft/ftools/) we have
grouped the extracted spectra so that we have at least 20
counts in a bin corresponding to a certain range of energy
channels. We can then use Poisson error bars to account for
the uncertainty in the number of photons per bin.
We have have generated background spectra using
Maxim Markevitch’s method and program that is available
from the CXC web site. We extract the background data
from the same regions on the chip as the spectra extracted
from the Abell 1835 data set. We also scanned the light
curve of the Abell 1835 data for flares or background activ-
ity using the tool lc clean provided by Markevitch. The light
curve is stable throughout the 19.6 ksec exposure.
4.2 Colour profile analysis
In order to illustrate the spectral energy distributions from
different regions in Abell 1835 we have firstly constructed
an X-ray colour profile. Two separate images were created
in the energy bands 0.5 − 1.3 and 1.3 − 7.0 keV (0.6 − 1.6
and 1.6 − 8.8 keV in the rest frame of the source) with a
1.97 arcsec (4 raw detector pixels) pixel scale. These soft
and hard X-ray images were background subtracted and flat
fielded (using the exposure map tools available on the CXC
web site and taking full account of the spectral energy dis-

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4
Schmidt, Allen & Fabian
Figure 5. X-ray colour profile of the central 1.7arcmin (500kpc)
of Abell 1835. In this figure the ratio of the azimuthally averaged
counts in the 0.5 − 1.3 and 1.3 − 7.0 keV bands (0.6 − 1.6 and
1.6 − 8.8 keV in the rest frame of the source) is plotted as a
function of the radius of the annuli.
tributions of the detected photons). Azimuthally-averaged
surface-brightness profiles for the cluster were then con-
structed in each energy band, centred on the geometrical
centre of the X-ray emission. The colour profile formed from
the ratio of the surface brightness profiles in the soft and
hard bands is shown in Fig. 5.
From examination of Fig. 5 we see that at large radii
the observed X-ray colour ratio is approximately constant,
with a mean value of 1.05 ±0.03 (1σ error determined from
a fit to the data between radii of 200 − 400 kpc). Within
a radius of ∼30arcsec (150kpc), however, the colour ratio
rises, indicating the presence of cooler gas.
4.3Single-temperature model
We investigate now the temperature profile that causes the
colour profile shown in Fig. 5. To do this we have extracted
spectra from annuli around the geometrical centre (marked
by the black circle in Fig. 4) of the X-ray emission with
photon energies in the 0.5-7.0keV band. We use a slightly
smaller energy band for the spectral analysis than we used
for the images in order to get robust answers at soft energies.
The annuli have widths of 5, 10, 25, 50 and 100arcsec (25,
50, 125, 250 and 500kpc, respectively) and provide sufficient
counts (see Tab. 1) to study the spectral properties of the
X-ray gas.
We use the software package XSPEC (Arnaud 1996) to
model the spectra with the MEKAL plasma emission model
(Kaastra & Mewe 1993; Liedahl et al. 1995) multiplied by
the PHABS (Balucinska-Church & McCammon 1992) pho-
toelectric absorption model
MODEL1 = PHABS(NH) × MEKAL(T;Z;K).(1)
Figure 6. Temperature kT as a function of radius r from the
cluster centre out to 3.3arcmin (1Mpc). For data points at radii
? 30arcsec (150kpc), we have plotted the temperatures derived
using models with the absorption fixed at the Galactic value.
The free model parameters are indicated in brackets: the
absorbing equivalent hydrogen column density NH, the gas
temperature T, the metallicity Z (in units of solar abun-
dance), and a normalization factor K.
An account of the relevant parameters and the model
fits is given in Table 1. All our fits are statistically accept-
able, with reduced χ2values of around or slightly above
unity. Since the absorbing column density and the metallic-
ity are not well constrained in the outer annuli, we have also
carried out a second analysis in which we grouped the data
into three larger regions. This procedure eliminates free pa-
rameters and thus yields more robust estimates. The results
are shown in Table 2.
It can be seen from Table 2 that the spectra from annuli
with inner radii larger than 30arcsec (150kpc) are consis-
tent with Galactic absorption (Dickey & Lockman 1990).
We have thus repeated the isothermal model fits according
to eq. (1) for these annuli with fixed Galactic absorption.
The corresponding best-fit models are also given in Table 1.
For the outermost annuli, this leads to a smaller predicted
temperature with smaller uncertainties.
In Fig. 6 the results from the model fitting for the am-
bient gas temperature T are plotted. For annuli with inner
radii ?30arcsec (150kpc), we have fixed the absorbing col-
umn density to the Galactic value. The best-fit temperatures
reveal a strong drop from kT ∼ 12keV in the outer regions
of the cluster down to 4keV in the centre of the cluster. We
note that the photons in the annuli have been emitted in a
hollow cylinder with a cross-section given by the annulus.
The measured temperatures are fits to the spectrum from
this whole region and thus have to be regarded as average
(emission weighted) temperatures.
We have plotted the metallicity and absorption re-
sults of Table 2 in Fig. 7. The metallicity appears approxi-

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Chandra observations of the galaxy cluster Abell 1835
5
Table 1. Isothermal model fits. The model described in eq. (1) with the free parameters equivalent hydrogen column density NH,
temperature T and metallicity Z was fitted to spectra extracted from annuli around the geometrical centre of Abell 1835 with inner
radii r1 and outer radii r2. For annuli with r1? 250kpc (50arcsec) we have modelled the X-ray spectra from the annuli both with free
equivalent hydrogen column density NH(top block of models), and with fixed Galactic absorption (Dickey & Lockman 1990, bottom
block of models). Since neither the metallicity nor the absorbing column density can be smaller than zero, the lower limit has always been
fixed accordingly for these parameters. In columns 6 and 7 we list the total number of counts and the expected number of background
counts in the 0.5-7.0keV band. In the last three columns the χ2values, the number of degrees of freedom (DOF) and the reduced
¯ χ2= χ2/DOF are given.
r1 (kpc)
r2 (kpc)kT (keV)
Z (solar)
NH(1020cm−2)countsexp. background
χ2
DOF¯ χ2
0 25
50
4.0+0.3
−0.3
4.7+0.2
−0.2
7.5+0.5
−0.4
8.5+0.7
−0.7
9.1+0.8
−0.7
8.0+0.8
−0.6
12.5+3.7
−1.9
16.9+5.1
−3.2
26.0+2.8
−11.2
0.24+0.07
−0.07
0.31+0.05
−0.06
0.28+0.06
−0.07
0.39+0.10
−0.10
0.35+0.08
−0.09
0.30+0.09
−0.10
0.49+0.27
−0.24
0.20+0.45
−0.20
0.00+0.52
−0.00
3.93+0.82
−0.82
3.97+0.66
−0.64
3.27+0.58
−0.55
2.29+0.66
−0.65
3.00+0.60
−0.58
4.75+0.75
−0.74
1.86+0.96
−1.14
0.07+2.11
−0.07
0.00+0.29
−0.00
4791
8300
10776
7680
10527
7838
5466
7003
8322
2.4
8.5
27.0
51.2
138.7
262.6
378.0
1123.4
3075.7
148.1
218.1
233.8
186.3
203.6
177.0
135.6
159.2
198.0
122
161
193
172
195
164
124
169
199
1.21
1.35
1.21
1.08
1.04
1.08
1.09
0.94
1.00
25
50
100
150
250
375
500
750
100
150
250
375
500
750
1250
150
250
375
500
750
250
375
500
750
1250
9.7+0.6
−0.6
10.0+0.8
−0.8
11.9+1.9
−1.4
13.1+2.1
−1.6
12.5+3.5
−2.4
0.35+0.09
−0.09
0.32+0.12
−0.12
0.48+0.26
−0.23
0.22+0.21
−0.12
0.23+0.36
−0.23
2.30
2.30
2.30
2.30
2.30
10527
7838
5466
7003
8322
138.7
262.6
378.0
1123.4
3075.7
205.0
188.1
135.8
162.3
207.6
196
165
125
170
200
1.05
1.14
1.09
0.95
1.04
Table 2. Model parameters for isothermal models using combined sets of data. The spectra from annuli around the cluster centre as
used in Table 1 have been fitted simultaneously using the same absorbing column density and metallicity for all annuli in the range
r1→ r2 and by allowing for different temperatures in each of the annuli of Table 1. The last three columns, as in Table 1, describe the
number of degrees of freedom and the quality of the fits.
r1 (kpc)
r2 (kpc)
Z (solar)
NH
χ2
DOF¯ χ2
(1020cm−2) (1020cm−2)
0 500.29+0.05
−0.04
0.31+0.06
−0.05
0.31+0.07
−0.06
3.94+0.51
−0.51
2.88+0.44
−0.43
2.55+0.38
−0.36
367.0
422.0
898.9
285
367
857
1.29
1.15
1.05
50
150
150
1250
mately constant with radius. We find tentative evidence that
the isothermal model requires excess absorption above the
Galactic value in the inner 100kpc.
As an aside, we mention that if we apply model (1)
to the spectrum extracted from a circle with a radius of
30arcsec (150kpc) and leave the Ni abundance as an ad-
ditional free parameter, we find a Ni abundance of 2.3 ±
0.8times solar in this region. This relatively high Ni abun-
dance is relevant for models of Type Ia supernovae and
their enrichment of the intergalactic medium (e.g., Dupke
& White 2000).
4.4 Spectral deprojection analysis
The results discussed in the previous section are based on
the analysis of projected spectra. In order to determine the
effects of projection, we have also carried out a deprojection
analysis of the Chandra spectra using the method described
by Allen et al. (2001a). This method decomposes the ob-
served annular spectra (Table 1) into the contributions from
the X-ray gas emission from nine spherical shells.
The data for all nine annular spectra were fitted simul-
taneously in order to correctly determine the parameter val-
ues and confidence limits. The spectral model used therefore
has 2n+1 free parameters (where n = 9 is the number of an-
nuli), corresponding to the temperature and emission mea-
sure in each spherical shell and the overall emission-weighted
metallicity (the metallicity is linked to take the same value
at all radii, yielding a metallicity Z = 0.32+0.03
sorbing column density was fixed at the Galactic value. The
temperature profile determined with the spectral deprojec-
tion code is shown in Fig. 8.
−0.04). The ab-
5X-RAY MASS ANALYSIS
5.1The mass model
The observed X-ray surface brightness profile (Fig. 3) and
deprojected X-ray gas temperature profile (Fig. 8) may to-
gether be used to determine the X-ray gas mass and total
mass profiles in the cluster. For this analysis, we have used
an enhanced version of the deprojection code described by
White, Jones and Forman (1997) and have followed a sim-
ilar method to that described by Allen, Ettori & Fabian
(2001a). Spherical symmetry and hydrostatic equilibrium
are assumed.
A variety of simple parameterizations for the cluster
mass distribution were examined, to establish which could