Studying Galaxy Clusters
with Astro-H
Silvano Molendi
IASF-Milano/INAF
ICM in Galaxy Clusters
Largest gravitationally bound structures in the
local universe.
Subject of intense research
a) as cosmological tools
b) interesting structures
a) and b) are of course connected as some
understanding of the structures is required to
use clusters as cosmological tools.
Setting the context
Basic properties of the ICM
•
The ICM is tenuous, typical densities 10-4
to a few 10-2 cm-3
•
hot, temperatures from 107 to 108 K (1-10
keV)
•
highly ionized: H, He completely ionized
heavier elements partially ionized
Chemically enriched, heavy elements such as
O, Si and Fe are present in almost solar
proportions
•
X-ray emission
Thermal bremsstrhalung + line emission
Setting the context
Basic properties of the ICM
X-ray emission form clusters
is extended
ASTRO-H
ASTRO-H will carry the first micro-calorimeter
We have had high resolution spectroscopy in X-ray
for more than decade now, so is ASTRO-H really
going to be a major step forward?
The large improvement in collecting area at Fe K line
will afford considerable insight for many
astrophysical sources.
However, for extended sources, for which high res.
spectroscopy has been possible only in the most
limited sense, SXS will produce a true quantum leap
The importance of motion
Show movies
Disclaimer
• Many things can and will be done, in this
presentation I will cover some of the most
important.
• Not an exhaustive presentation.
Motions in the ICM
Indirect evidence of motions is now abundant
Shocks in the ICM
Bullet cluster
Best example of shock
Core of sub-structure
already gone through
cluster
From RH conditions
and density jump
Mach number ~ 3
Vshock ~ 4700 km/s
Markevitch & Vikhlinin (2007)
Motions in the ICM
Shocks now seen in a few systems point to
mildly supersonic gas motions
These are rare systems, one of the reasons is the
shocks occur exactly in the plane of the sky
There are likely similar systems seen at
different angles (e.g. A576 Dupke+07) these will
be accesible with SXS
Motions in cores
Indirect evidence of gas motions in the cores
of many “relaxed” (Cool Core) systems
Core of Perseus cluster
Highly structured
Enhanced emission
Cavities
Fabian+03
Radio Bubbles
Cavities coincide
with radio lobes
associated to the
AGN at the
center
Fabian+03
Ripples and weak shocks
Fabian+03
Radio cavities expand and rise buoyantly, in doing so they
set the ICM in motion, generating ripples and weak shocks
Cold Fronts
Ghizzardi, Rossetti &SM 2010
• CF are contact
discontinuities (SB
jumps) discovered by
Chandra (Markevitch
& Vikhlinin 2007)
• Found in merging
systems where they
mark dense subcluster
cores that have
survived a merger
Cold Fronts
Ghizzardi, Rossetti &SM 2010
• Found in non merging
systems were they result
from the displacement of
lowest entropy gas from
the bottom of the
potential well.
• Small pressure jump
across the CF point to
subsonic motions
• Search for cold fronts in
a representative sample
of nearby systems shows
that ALL relaxed clusters
host at least 1 CF
Gas Motions in clusters
• Indirect evidence for ubiquitous subsonic gas motion
in cluster cool cores
• Subsonic motions are quite likely one of the defining
traits of these systems, (the reason this statement
is seldom made is that we have, thus far, lacked
direct observations of gas motions)
• Open questions are:
what is the kinetic energy vs. thermal energy?
is the flow laminar or turbulent?
what is the viscosity of the ICM?
• Indirect evidence for mildly supersonic motion in a
few systems, typically those were shock is
propagating in the plane of the sky.
SXS on ASTRO-H
• A 6eV resol @ Fe Kα line corresponds to a velocity
resolution of 300 km/s
• The sound speed in the ICM is
cs = (5/3·p/ρ)1/2 = 1460 km/s·(kT/8 keV) 1/2
ASTRO-H well suited to measure subsonic motions in ICM
• Most important line is the Fe Kα
• For most T He-like Fe Kα is the most prominent
Fe He-like Kα
Laminar flow
Different regions with Δv=200km/s
Turbulent flow
Region with vTurb=200km/s
Laminar vs Turbulent flow
• Some line broadening may be present even if
turbulence is completely absent (limited angular
resolution & line of sight effects)
• The combination of high spectral resolution and
moderate angular resolution should allow us to
distinguish between flows dominated by laminar and
turbulent motions.
• Intermediate cases will be more difficult but not
impossible to characterize.
ICM Thermo-dynamic structure
• XMM-Newton Grating (Peterson+01 ) and CCD data
(SM & Pizzolato 01) falsified the cooling-flow model
that predicted strong multi-phaseness of ICM in
cluster cores (temperatures from a few keV down to
0.1 keV).
• Evidence, in some instances, of suppression of
conduction, presence of multi-temperature structure:
1. suppression of conduction across cold
fronts e.g. A3667, Ettori & Fabian 2001
2. presence of blobs/filaments of cooler gas
in the radio arms of M87 (SM 2002, etc.)
ICM Thermo-dynamic structure
The limited spectral resolution of CCDs and spatial
resolution of gratings does not allow us to address the
issue of moderate multi-phaseness in the ICM at large
SXS will change this
ICM Spectra at CCD resolution
―1
―2
―4
―8
keV
keV
keV
keV
ICM Spectra at SXS resolution
―1
―2
―4
―8
keV
keV
keV
keV
Fe L-shell emission
T up to 3keV
Ar Kα line emission
He-like
H-like
Ratio of H to He-like
2keV <T < 8keV
Fe Kα line emission
He-like
H-like
Ratio of H to He-like
2keV <T < 10keV
ICM Thermo-dynamic structure
• The relative intensity of Fe L-shell lines and the
ratio of H-like Kalpha to He-like Kalpha lines for
O,Mg,Si,S,Ar,Ca & Fe are all dependent upon
temperature and can be used to provide independent
estimates of T and assess presence of even modest
multiphase-ness.
• As for the study of line broadening some apparent
multi-T will be present even if the gas is single phase
(limited angular resolution & line of sight effects),
however these effects can be accounted for
Summary
The SXS will produce a quantum leap in our
understanding of the ICM
It will:
 directly detect motions within the ICM for the
first time
 characterized the thermodynamic structure of
the ICM
 improve our understanding of metals in the ICM
NuSTAR
• The relative intensity of Fe L-shell lines and the
ratio of H-like Kalpha to He-like Kalpha
lines(FWHM)
for
10 arcsec
O,Mg,Si,S,Ar,Ca & Fe are all dependent
13 x upon
13 arcmin
temperature and can be used to provide1 keV
independent
@ 60 keV
estimates of T and assess presence of 0.5
even
keV @modest
6 keV
multiphase-ness.
• As for the study of line broadening some apparent
multi-T be present even if turbulence is the gas is
single phase (limited angular resolution & line of sight
effects), however these effects can be accounted
for
< 48 h
NuSTAR
• Launched on June 13th in
LEO
• First Light on June 28th
• Thus far everything has gone well
• Mast deployment successful
• Instruments functioning within expectations