Transcript No Slide Title

Temperature and Entropy Profiles of Galaxy Groups
F. Gastaldello, D. Buote, P. Humphrey, L. Zappacosta
University of California Irvine
Abstract
3 Temperature profiles
We present temperatures and entropy profiles for a
sample of 16 relaxed groups/poor clusters, observed
with Chandra and XMM, covering the temperature
range 1-3 keV and selected to have the best
available data for hydrostatic mass analysis.
The shape of the temperatures profiles (some
examples are shown in Fig.2) resemble the one
obtained for hotter, more massive clusters (e.g.
Vikhlinin et al. 2005) but with the peak of the
temperature profile occurring at smaller radii.
The temperature profiles for the systems, scaled by
the virial radius obtained by the NFW fit to the mass
profile and the gas mass weighted temperature
between 0.1-0.3 rvir (a range covered by data for
almost all the objects in the sample), show
considerable less scatter beyond 0.1 rvir.
The imprint of non-gravitational processes like
radiative cooling and AGN heating is dramatic in the
inner region (see for example the flattening of the
central temperature in the core of AWM 4).
The temperature profiles, when scaled with the virial
radius and the mass weighted temperature calculated
between 0.1-0.3 rvir show increasingly less scatter
beyond 0.1 rvir.
The entropy profiles, in particular for groups with
Mvir<1014 solar masses, show a large scatter and
they are consistent with an increasing broken power
law with steeper inner slope and flatter outer slope
which challenges theoretical predictions.
Fig.4 Scaled entropy profiles for group scale objects.
1 Introduction
X-ray observations have extensively shown that the
structure of the Intracluster medium (ICM) departs
from the self-similarity expected if only gravitational
processes are shaping its properties (e.g. Kaiser
1991, Ponman et al. 1999, Ponman et al. 2003).
Investigation of the entropy profiles is of fundamental
importance because entropy records the
thermodynamic history of the ICM, reflecting cooling
and feedback heating from supernovae and AGN (Voit
2005). High quality XMM and Chandra cluster
observations are showing that ICM entropy profiles
have the S(r)  r1.1 shape characteristic of
gravitational structure formation outside of the core,
but the overall normalization of these profiles scales
as T2/3 instead of T as in the self-similar prediction
(e.g. Pratt et al. 2006, Piffaretti 2005, Donahue et a.
2006).
It is important to extend these studies to groups/poor
clusters with temperatures less than 2 keV using
higher quality Chandra and XMM data. At this scale
the effects of feedback are more severe and should
have a more dramatic impact on temperature and
entropy profiles.
In this contribution we present measurements of the
temperature and entropy profiles of a sample of 16
groups with the best available XMM and Chandra
data, originally selected for mass determination
(Gastaldello et al. 2006, in preparation). The virial
quantities quoted in the poster are for an overdensity
of 103 appropriate for the CDM model.
NGC 1550
RGH 80
NGC 2563
AWM 4
MKW 4
A 2717
Fig.2 Temperature profiles for some representative objects of the sample. Red
symbols corresponds to Chandra data while black symbols corresponds to
XMM data. The best fit analytic profile together with the fractional residuals is
shown. When Chandra data are present in the core, XMM data are only shown
for comparison but not fitted.
Fig.3 Scaled temperature profiles. Best fit models for the temperature profiles
scaled by the virial radius obtained by mass analysis and by the gas-massweighted temperature over the radial range 0.1-0.3 rvir.
2 Data analysis
Chandra + XMM
The use of both Chandra and XMM data is essential.
The better Chandra PSF allows the use of annuli with
the smallest possible radial width, avoiding departures
from single-temperature emission which stem
primarily from a steep temperature gradient (see the
case for the XMM spectrum of the inner core of NGC
533). Point sources are better resolved, in particular
X-ray binaries in the central galaxy, showing as an
excess at energies > 3 keV in the XMM spectrum. For
this reason, when present, we use only Chandra data
in the core. At large radii the better effective area of
XMM is crucial to detect the low surface brightness
emission of groups.
Fig.1 Top panel: Left XMM spectrum
for the innermost 0-30” bin of NGC 533
folded with the best fit 1T model.
Characteristic residuals due to the
presence of different temperature
components are evident. Right Chandra
spectrum for the 5”-10” bin of NGC 533
with best fit 1T model and residuals.
Bottom panel: Temperature profile
derived from XMM (black) and from
Chandra (red) data.
4 Entropy profiles
Fig.5 Scaled entropy profiles for poor cluster scale objects.
5 Conclusions and future work
The large scatter in the entropy distribution of
groups reflect their different and complex
gasdynamical histories. The broken power law
behaviour challenges theoretical predictions and
likely require a complex balance of heating and
cooling in order to be reproduced.
Form the observational point of view for each of
these objects Chandra data are required in the core
(like for example for the interesting object AWM 4)
and XMM data at large radii, in particular with offset
observations in an attempt to follow the X-ray
emission at the largest radius possible. This is
feasible for most of the nearby objects in the sample
which show still easily detectable emission in the
central XMM pointing.
Larger, unbiased samples (not limited to relaxed
bright objects), like our ongoing Chandra project to
observe an X-ray selected flux limited sample from
the NORAS survey, will provide further insight.
The entropy profiles for groups (Mvir<1014 solar
masses) and poor clusters (Mvir in the range 1-2 1014
solar masses) are shown respectively in Fig.4 and
Fig.5, scaled by the virial radius and the natural
entropy scale for a non-radiative ICM (Svir following
the definition of Voit et al. 2006 and scaling for our
virial overdensity of 101).
Entropy profiles of groups show a large scatter and
they are consistent with an increasing broken power
law with steeper inner slope (0.8-1.3) and flatter outer
slope (0.4-0.6). A similar behaviour has been noticed
for a sample of 8 groups by Mahdavi et al. 2005.
some objects like NGC 5044, NGC 4325 and RGH 80
show a central flattening in the very inner core, a
behaviour seen in more massive clusters (e.g.
Donahue et al. 2006). The origin can be attributed to
AGN heating or merging in the case of RGH 80 (the
only object in our sample which has two dominant
elliptical galaxy in the center). NGC 2563 is showing
an entropy plateau at 0.1-0.2 rvir, most likely a fossil of
an extreme heating episode which has deeply
affected its core, explaining the low central gas
density of this object.
Entropy profiles for poor clusters show less scatter
like the ones observed in more massive objects. Local
entropy modification, like the central flattening of
AWM 4, is likely due to AGN heating.
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