RGS observations of cool gas in cluster
cores
Jeremy SandersInstitute of Astronomy
University of Cambridge
A.C. Fabian, J. Peterson, S.W. Allen, R.G. Morris,J. Graham, R.M. Johnstone
Sanders et al, 2008, MNRAS, 385, 1186Sanders et al, in prep
Cooling in cluster cores Many cluster cores have steeply peaked X-ray
surface brightness profiles, i.e. short mean radiative cooling times.
Suppose there is a luminosity L emitted from within a cooling region rcool
To offset radiation lost through cooling, if the cluster is in steady state and there is no heating, there should be a mass deposition rate of
Measured values from SB profiles gave values of 10-1000s solar masses per year – a “cooling flow”
luminosity of region is: - radiation of thermal energy - PdV work done on gas entering rcool kT
mμLM
52
Lack of cool X-ray emitting gas
see also Peterson et al 01, 03, Kaastra et al 01, 03, Tamura et al 01,...
Spectra imply less than 10% of cooling rates expected from luminosity profiles
Only significant gas down to 1/2 to 1/3of outer temperature
AGN heating?
Fe XVII lines (indicatingtemperature ~ 0.7 keV)missing
de Plaa et al
2A 0335+096
The Centaurus clusterInner core of the
Centaurus cluster
200 ks Chandra observation
RGB image
Sanders & Fabian (2002, 2006)
Fabian, Sanders et al (2005)
10 kpc
Centaurus temperature map
Temperature (keV)
Crawford et al 2005
Chandra 200ks observation200 ks Chandra observation
Projected temperature in Centaurus varies from
3.7 keV in outskirts
to 0.7 keV in the cool plume
Centaurus cross dispersion image
wavelength
1D image
With the RGS spectrometer we get a spectrum of the cluster as a function of one spatial dimension.
150 ks total XMM exposure
Inner 160 arcsec width (30 kpc)Central inner 60 arcsec (13 kpc)60-160 arcsec (13-30 kpc)
Ionisation equilibrium
Values taken from Mazzotta et al 1998
By measuring the distribution of ionisation states of metals we canfind the gas temperature, or distribution of gas temperature
Inner 60 arcsec width (16 kpc)
strong
missing
Line ratios — constraints on kT
Implies average temperature of gas 3.5 to 5.2 × 106 K
Ratio of flux of emission lines and compared to CHIANTI model
Spectral fitting limits on gas kT
Multi temperature model(components fixed in temperature but
varying in emission measure)
Cooling flow model(mass deposition in absence of heating)
Factor of 10 in temperature > factor of 10 in mass deposition
Spectral fitting limits on gas kT
Multi temperature model(components fixed in temperature but
varying in emission measure)
Cooling flow model(mass deposition in absence of heating)
Factor of 10 in temperature > factor of 10 in mass deposition
tcool ~ 107 yr!
New RGS spectra
156ks
138ks
HCG 62
O VIII
Fe XVII
156 ksHCG 62 is a compact group of galaxies
Prominent radio bubbles with ~1057 erg energy content
Temperature varies in spatially resolved studies from 1.5 to 0.7 keV (Morita et al 2006)
High abundance arc (Gu et al 2007)
HCG 62: temperature distribution8 component VAPEC model
Shows clear break in temperature distribution at 0.6 keV
Emission measure drops by more than an order of magnitude
Lower temperature consistent with CCD results from Chandra/XMM (Morita et al 2006)
HCG 62: mass deposition rate8 component VMCFLOW cooling flow model in steps of temperature
In the absence of cooling, there is a very sharp drop in mass deposition rate at 0.6 keV, by at least an order of magnitude
tcool ~ 3×108 yr
Conclusions Varied behaviour of cool cores observed by RGS
Wide range in temperature in Centaurus (factor 10)
Narrow range in HCG 62 (factor 2)
Both show strong variation in emission measure with temperature
Implies there is always close feedback It is important to study a wide range of clusters
with deep observations with XMM We have a deep Abell 1835 observation soon
Star formation in this object ~ mass deposition rate (100 Solar masses/year)
Sound waves in Centaurus
Fourier high-pass-filtered image
Sanders & Fabian (submitted)
Centaurus SB profiles and residuals
λ ~ 9 kpc
Power calculation Use deprojection factor to convert from
ripple amplitude to intrinsic pressure ampl. Assuming spherical waves, this implies
9×1042 erg s-1 sound wave power at r=30 kpc, where tcool=3.3 Gyr
9 kpc wavelength implies 107 yr period (similar to Perseus)
Luminosity emitted within 30 kpc is 1.3×1043 erg s-1
Using period and energy of radio bubbles, implies power of 2×1043 erg s-1
HCG 62: extra absorption8 component VMCFLOW cooling flow model in steps of temperature
Extra column density of 5×1020 cm-2 added into model for three lowest temperature components
HCG 62: ignoring OVII8 component VMCFLOW cooling flow model in steps of temperature
Only fitting spectrum from 7 to 20 A
Very coolest gas
Very coolest X-ray gas at 0.35 keV has a mean radiative cooling time of only ~107 yr
Line profiles
Emission measure maps
Best fittingsmoothingsizes fromRGS linewidths
Centaurus temperature and metals25 kpc
Centaurus in different elements
Sanders & Fabian (2006)
Significant amount from Type II supernovae (~30%)
Therefore large amount of star formation in the Centaurus cluster
Either massive initial burst
Continuous ~5 M/yr Likely to have been
undisturbed for ~ 8 Gyr
Close heating/cooling balance
Sanders & Fabian (2006)Supernova fractions