Cosmological Effects of Powerful AGN Outbursts in Galaxy Clusters: Insights from an XMM-Newton Observation of MS0735+7421

0.79 MB·English

by  Myriam Gitti

COSMOLOGICAL EFFECTS OF POWERFUL AGN
OUTBURSTS IN GALAXY CLUSTERS:
INSIGHTS FROM AN XMM-NEWTON OBSERVATION OF
MS0735+7421
M. Gitti1,2, B. R. McNamara1,3,4
7
0
0 P. E. J. Nulsen 4, M. W. Wise 5
2

n
a ABSTRACT
J

2
1
We report on the results of an analysis of XMM-Newton observations of

1 MS0735+7421, the galaxy cluster which hosts the most energetic AGN outburst
v
6 currently known. The previous Chandra image shows twin giant X-ray cavities
8
( 200 kpc diameter) filled with radio emission and surrounded by a weak shock
3 ∼
1 front. XMM data are consistent with these findings. The total energy in cavities
0
and shock ( 6 1061 erg) is enough to quench the cooling flow and, since most
7
∼ ×
0 of the energy is deposited outside the cooling region ( 100 kpc), to heat the
/ ∼
h gas within 1 Mpc by 1/4 keV per particle. The cluster exhibits an upward
p ∼
- departure (factor 2) from the mean L-T relation. The boost in emissivity pro-
o ∼
r duced by the ICM compression in the bright shells due to the cavity expansion
t
s may contribute to explain the high luminosity and high central gas mass fraction
a
: that we measure. The scaled temperature and metallicity profiles are in general
v
i agreement with those observed in relaxed clusters. Also, the quantities we mea-
X
sure are consistent with the observed M-T relation. We conclude that violent
r
a
outbursts such as the one in MS0735+7421 do not cause dramatic instantaneous
departures from cluster scaling relations (other than the L-T relation). However,
if they are relatively common they may play a role in creating the global cluster
properties.
1Dept. of Physics & Astronomy, Ohio University, Clippinger Labs, Athens, OH 45701 - USA
2INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna - Italy
3
Dept. ofPhysics& Astronomy,University ofWaterloo,200UniversityAvenue West, Waterloo,Ontario
- Canada N2L 2G1
4Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 - USA
5AstronomicalInstitute,UniversityofAmsterdam,Kruislaan403,1098SJAmsterdam-TheNetherlands
– 2 –
Subject headings: galaxies: clusters: general — galaxies: clusters: individual
(MS0735.7+7421) — cooling flows — intergalactic medium — cosmology: mis-
cellaneous — X-rays: galaxies: clusters
1. Introduction
The current generation of X-ray satellites, Chandra and XMM-Newton, has shown that
the physics of the intra-cluster medium (ICM) is complex and needs to be regulated by
additional, non-gravitational processes beyond simple gravity and gas dynamics considered
in the standard Cold Dark Matter cosmological scenario (White & Rees 1978). In particular,
our understanding of cooling flow systems has radically changed. Albeit confirming the
existence of short cooling times, high densities and low temperatures in the cluster cores,
thearrivalofnewhigh-resolutionX-raydatahasshownthelackoftheemissionlinesexpected
from gas cooling below 1-2 keV and has reduced by about one order of magnitude the new
spectroscopically–derived mass deposition rates (e.g. David et al. 2001, Johnstone et al.
2002, Peterson et al. 2003 and references therein). The most plausible solution to this
so-called “cooling flow problem” is that some form of additional heating which balances
the cooling must be acting in the ICM. Among the many proposed heating mechanisms1,
one of the best candidates for supplying the energy is feedback from the Active Galactic
Nucleus (AGN) hosted by the central galaxy of the cluster (e.g., Rosner & Tucker 1989;
Tabor & Binney 1993; Churazov et al. 2001; Bru¨ggen & Kaiser 2001; Kaiser & Binney 2003;
Ruszkowski & Begelman 2002; Brighenti & Mathews 2003, Omma et al. 2004).
The possibility of AGNheating has recently become the leading idea to solve the cooling
flow problem thanks to the discovery of X-ray cavities in the ICM on scales often approxi-
mately coincident with the lobes of extended radio emission (e.g., Hydra A: McNamara et
al. 2000, David et al. 2001; Perseus: B¨ohringer et al. 1993, Churazov et al. 2000, Fabian
et al. 2000; A2052: Blanton et al. 2001, 2003; A2597: McNamara et al. 2001, Pollack et
al. 2005, Clarke et al. 2005; RBS797: Schindler et al. 2001, Gitti et al. 2006; A4059: Heinz
et al. 2002). This indicates that the radio lobes fed by AGNs have displaced the X-ray
emitting gas, creating cavities in the ICM. These outbursts can be recurrent, as it has been
1 Proposed heating mechanisms include electron thermal conduction from the outer regions of clusters
(Tucker & Rosner 1983; Voigt et al. 2002; Fabian et al. 2002; Zakamska & Narayan 2003), continuous
subcluster merging (Markevitch et al. 2001), contribution of the gravitational potential of the cluster core
(Fabian 2003), feedback from intra-cluster supernovae (Domainko et al. 2004), etc.
– 3 –
observed in some cases. For example, RBS797 shows evidence of a restarted AGN activity
with precessing jetspointing to a different direction with respect to theone of the outer radio
lobes filling the cavities (Gittiet al. 2006). The heating by AGN is thought to occur through
the dissipation of the cavity enthalpy and through shocks driven by the bubbles inflated by
the AGN. Systematic studies of a sample of X-ray cavities show that their enthalpies2 lie
between 1055 1061 erg and scales in proportion to the cooling X-ray luminosity and the
−
radio power of the host system. In more than a half of the sample the energy input from the
central radio source is currently sufficient to balance cooling (Bˆırzan et al. 2004, Rafferty et
al. 2006). The trend between X-ray luminosity and bubble mechanical luminosity, together
with the existence of short central cooling time, suggests that the AGN is fueled by a cooling
flow that is itself regulated by feedback from the AGN. The basic idea of this AGN-cooling
flow scenario is that a self-regulated equilibrium may be achieved, in which the energy input
from the AGN balances the radiative losses of the ICM over the lifetime of the cluster.
The recent discovery of giant cavities and associated large-scale shocks in three system
(MS0735+7241: McNamara et al. 2005, Hercules A: Nulsen et al. 2005a, Hydra A: Nulsen
et al. 2005b) has shown that AGN outbursts can not only affect the central regions, but
also have an impact on cluster-wide scales. In particular, the supercavities discovered in
MS0735+7421 have a diameter of about 200 kpc each and a weak cocoon shock surrounding
them has been detected. This large-scale outburst is the most powerful known so far: it
releases upward of 1061 erg into the ICM, heating the gas beyond the cooling region (Mc-
Namara et al. 2005). In this paper we want to investigate the significant consequences that
this new development has for several fundamental problems in astrophysics. One important
problem addressed by giant cavities is the so-called cluster ”pre-heating”, which is mani-
fested in the steepening of the observed luminosity vs. temperature relation for clusters
with respect to theoretical predictions that include gravity alone (e.g., Markevitch 1998).
Some extra non-gravitational energy is required to explain such a steepening (e.g., Wu et al.
2000, Voit 2005), and one possibility is that it is supplied by AGN outburst. The additional
non-gravitational heating supplied by AGN could also induce the suppression of the gas
cooling in massive galaxies required to explain the exponential turnover at the bright end
of the luminosity function of galaxies (Benson et al. 2003). This would indicate a common
solution for the two major heating problems associated with the ICM: those of cooling flow
and galaxy formation. Some cavities live longer than the time it takes to cross their own
2 The enthalpy (free energy) of the cavities is estimated from the total AGN power as H = γ PV,
γ−1
where the PV work done by the jet as it inflates the cavity is determined by measuring the cavity size and
itssurroundingpressure,andthe adiabaticindexγ isrelatedtotheinternalcompositionofthe cavities,still
unknown. In the case of a relativistic plasma γ = 4/3 and H =4PV.
– 4 –
diameters, suggesting they are in pressure balance with the surrounding medium. Since the
work required to inflate the radio lobes generally exceeds the nonthermal minimum energy
of the radio emission (e.g., De Young 2006 and references therein), in the hypothesis that
the assumptions adopted into the equipartition calculation are correct, some additional com-
ponent besides the relativistic plasma may fill the cavity and contribute to support them
internally. We want to investigate what is currently providing the necessary pressure support
to sustain the cavities. Finally, in a more general view, we want to investigate what is the
potential impact that such powerful outbursts have on the global properties of the ICM.
Understanding well the ICM physics is essential to use galaxy clusters as high-precision
cosmological tools.
We address these problems by studying the X-ray properties of the most energetic
outburst known in a galaxy cluster. MS0735+7421 (hereafter MS0735) is at a redshift of
0.216. With a Hubble constant of H = 70 km s−1 Mpc−1, and Ω = 1 Ω = 0.3, the
0 M Λ
−
luminosity distance is 1069 Mpc and the angular scale is 3.5 kpc per arcsec.
2. Observation and data preparation
MS0735 was observed by XMM–Newton in April 2005 during revolution 0973. In this
paper only data from the European Photon Imaging Camera (EPIC) are analyzed and dis-
cussed, while those from the Reflection Grating Spectrometer (RGS) will be presented in
a subsequent paper. The MOS and pn detectors were both operated in Full Frame Mode
with the THIN filter, for an exposure time of 70.6 ks for MOS and 60.7 ks for pn. We use
the SASv6.5.0 processing tasks emchain and epchain to generate calibrated event files from
raw data. Throughout this analysis single pixel events for the pn data (PATTERN 0) are
selected, while for the MOS data sets the PATTERNs 0-12 are used. The removal of bright
pixels and hot columns is done in a conservative way applying the expression (FLAG==0).
To reject the soft proton flares we accumulate the light curve in the [10-12] keV band for
MOS and [12-14] keV band for pn, where the emission is dominated by the particle–induced
background, and exclude all the intervals of exposure time having a count rate higher than
a certain threshold value (the chosen threshold values are 0.4 cps for MOS and 0.7 cps for
pn). The remaining exposure times after cleaning are 50.4 ks for MOS1, 49.5 ks for MOS2
and 42.0 ks for pn. Starting from the output of the SAS detection source task, we make
a visual selection on a wide energy band MOS & pn image of point sources in the field
of view. Events from these regions are excluded directly from each event list. The source
and background events are corrected for vignetting using the weighted method described in
Arnaud et al. (2001), the weight coefficients being tabulated in the event list with the SAS
– 5 –
task evigweight. This allows us to use the on-axis response matrices and effective areas.
Unless otherwise stated, the reported errors are at 90% confidence level.
2.1. Background treatment
The background estimates are obtained using a blank-sky observation consisting of
several high-latitude pointings with sources removed (Lumb et al. 2002). The blank-sky
background events are selected using the same selection criteria (such as PATTERN, FLAG,
etc.), intensity filter (for flare rejection) and point source removal used for the observation
events. This yields final exposure times for the blank fields of 365 ks for MOS1, 350 ks for
MOS2 and 294 ks for pn. Since the cosmic ray induced background might change slightly
with time, we compute the ratio of the total count rates in the high energy band ([10-12]
keV for MOS and [12-14] keV for pn). The obtained normalization factors (1.266, 1.303,
1.283 for MOS1, MOS2 and pn, respectively) are then used to renormalize the blank field
data. The blank-sky background files are recast in order to have the same sky coordinates
as MS0735.
The usual approach to perform the background subtraction is described in full detail
in Arnaud et al. (2002). This procedure consists of two steps. In a first step, for each
product extracted from the observation event list an equivalent product is extracted from
the corresponding blank-field file and then subtracted from it. This allows us to remove
the particle background. However, if the background in the observation region is different
from the average background in blank field data, this step could leave a residual background
component. Theresidualbackgroundcomponentisestimatedbyusingblank-field-subtracted
data in a region free of cluster emission (in particular we consider an annulus lying between 8
and 10 arcmin) and then subtracted in a second step from each MOS and pn product. In our
case the residual is negative in the energy band adopted for the spatial analysis ([0.4-2] keV,
see Sect. 3.1). The residual count rate summed over the three detectors is 0.07 counts/s,
−
which represents 10% of the total background count rate in this energy band.
∼
The fluorescent emission lines3 excited by the energetic charged particles that pass
through the detector exhibit spatial variation over the detector. This effect would com-
promise the reliability of the second step of the background subtraction for spectra, as the
resulting total background spectrum (sum of the particle background spectrum, estimated in
3 Al and Si lines at 1.5 and 1.7 keV, respectively, in the MOS data, and Ni, Cu and Zn lines around 8
keV in the pn data.
– 6 –
the blank-field-subtracted cluster region, and the residual background component, estimated
in the blank-field-subtracted outer annular region) shows fluorescence lines which are shifted
by a few energy channels with respect to the MS0735 spectrum. Therefore the residual
background component is neglected for the purpose of the spectral analysis.
3. Morphological analysis
The adaptively smoothed, exposure corrected MOS1 count rate image in the [0.3-10]
keV energy band is presented in Fig. 1. The smoothed image is obtained from the raw
image corrected for the exposure map (that accounts for spatial quantum efficiency, mirror
vignetting and field of view) by running the task asmooth set to a desired signal-to-noise
ratio of 20. Regions exposed with less than 10% of the total exposure are not considered.
The inner part of the cluster shows the high surface brightness characteristic of a cool-
ing flow. We notice a sharp central surface brightness peak at a position 07h41m44s.06 +
74◦14′38′′.62 (J2000). At small radii, two strong depressions having a diameter of about 150
kpc (see Sect. 3.1) are visible on opposite side of the cluster center in the NE-SW direction.
The cavities are surrounded by a bright X-ray emission of elliptical shape, in the form ex-
pected by a radiococoon. The elliptical discontinuity in theX-raysurface brightness appears
more evident in the Chandra data and has been interpreted as a weak shock (McNamara et
al. 2005). Beyond the cavity region the cluster maintains a slightly elliptical morphology
up to large radii, showing hints of structures that could be the results of past radio activity
that created older cavities.
3.1. Surface brightness profile
We consider different sectors to the north and south (N sector: between 10◦ west and
60◦ east of north; S sector: between 15◦ east and 60◦ west of south) and to the east and
west (E sector: between 30◦ north and 75◦ south of east; W sector: between 30◦ south and
80◦ north of west) in order to include and exclude the cavity regions, respectively. We also
consider a full 360◦ sector in which we mask the cavities (hereafter undisturbed cluster).
For each sector, we compute a background-subtracted, vignetting-corrected, radial surface
brightness profile in the [0.4-2] keV energy band for each camera separately. For the pn
– 7 –
Fig. 1.— MOS1 image of MS0735 in the [0.3-10] keV energy band. The image is corrected
for vignetting and exposure and is adaptively smoothed (signal-to-noise ratio = 20).
– 8 –
data, we generate a list of out-of-time events4 (hereafter OoT) to be treated as an additional
background component. The effect of OoT in the current observing mode (Full Frame) is
6.3%. The OoT event list is processed in a similar way as done for the pn observation event
file. The profiles for the three detectors are then added into a single profile, binned such
that at least a signal-to-noise ratio of 3 is reached.
The cluster emission is detected up to 1.3 Mpc ( 6 arcmin). In Fig. 2 we show the
∼
X-ray surface brightness profile for the sector containing the northern cavity compared from
that of the undisturbed cluster. We note that the data of the undisturbed cluster appear
regular, while those for the N sector show a clear deficit of emission between radii 50 and
∼
200 kpc relative to the other directions. The S sector profile shows a behavior similar to
∼
the N one, although the depression is less pronounced, whereas the E and W sectors appear
regular.
Thesurfacebrightnessprofileoftheundisturbedcluster isfittedintheCIAOtoolSherpa
with various parametric models, which are convolved with the XMM point spread function
(PSF). The overall PSF is obtained by adding the PSF of each camera (Ghizzardi 2001),
estimated at an energy of 1.5 keV and weighted by the respective cluster count rate in the
[0.4-2] keV energy band. A single β-model (Cavaliere & Fusco Femiano 1976) is not a good
description of the entire profile: a fit to the outer regions shows a strong excess in the center
as compared to the model (see Fig. 3). The centrally peaked emission is a strong indication
of a cooling flow in this cluster. We find that for 100 kpc <r <1150 kpc the data can be
described (χ2 1.65 for 87 d.o.f.) by a β-model with a cor∼e ra∼dius r = 195 4 kpc and a
red ∼ c ±
slope parameter β = 0.77 0.01 (3 σ confidence level). The single β-model functional form
±
is a convenient representation of the gas density profile in the outer regions, which is used
as a tracer for the potential. This best fit model is thus used in the following to estimate
the cluster gas and total mass profiles (see Sect. 6).
We also consider a double isothermal β-model and find that it can account for the entire
profile, when the very inner andouter regions areexcluded: for10 kpc<r <1150kpc thebest
∼ ∼
fit parameters are r = 200 4 kpc, β = 0.77 0.01, r = 156 8 kpc, β = 4.67 0.56;
c1 1 c2 2
± ± ± ±
χ2 1.87 for 96 d.o.f. By assuming a common β value we find: r = 215 4 kpc,
red ∼ c1 ±
r = 30 2 kpc, β = 0.79 0.01; χ2 2.08 for 97 d.o.f.
c2 ± ± red ∼
4
Out-of-time events are caused by photons which arrivewhile the CCD is being readout, and are visible
in an uncorrected image as a bright streak smeared out in RAWY.
– 9 –
Fig. 2.— Background subtracted, azimuthally-averaged radial surface brightness profile of
the N sector data in the [0.4-2] keV range. The dotted red line shows the profile in full
360◦ sector with masked cavities (undisturbed cluster). A deficit of emission in the N sector
between radii of 50-200 kpc is visible.
∼
– 10 –
Fig. 3.— Background subtracted, azimuthally-averaged radial surface brightness profile in
the [0.4-2] keV range. The best fit β-model fitted over the 100 - 1150 kpc region is over-
plotted as a solid red line. In the central region, the observed surface brightness profile shows
a strong excess as compared to the extrapolation of the model.