Transcript Spectral properties and origin of the radio halo in Abell

Central radio galaxies in groups:
cavities, bubbles and the history of AGN heating
S.Giacintucci1,2, T. Venturi¹, J.Vrtilek 2, L. David², S.Raychaudhuri3, R.Athreya4
1. INAF, IRA, Bologna, Italy 2. Harvard-Smithsonian CfA, Cambridge, MA, USA
3. U. Birmingham, UK
4. NCRA, Pune, India
1.Introduction
Deep radio and X-ray observations of galaxy clusters and groups are providing evidence in support of the idea that the AGN activity of the central radio
galaxy plays an important role in the regulation of the cooling process: the commonness and variety of bubbles, cavities and edges observed both in the
radio and X-ray bands in groups and clusters provide direct evidence of the widespread presence of phenomena driven by the central AGN. Recent
investigations, mainly focussed on massive galaxy clusters, indicates that the energy released by the AGN during its lifetime may be enough to counteract
putative cooling flows (e.g. McNamara & Nulsen, 2007, ARAA, 45, 117). However the actual physical mechanism responsible for the heating is still unclear.
We recently started a large observational radio and X-ray project which aims to extend the study of the AGN/ICM interaction to systems smaller than
the well-studied rich clusters. The study of the AGN activity in groups is valuable for a number of reasons: although the scale of AGN outbursts is
usually less energetic and often on a smaller scale than in clusters, in the low pressure group environment the results of the radio/ICM interactions
are expected to be more apparent. More generally, the relationship between AGN and hot gas is expected to have a significant influence on the
evolution of galaxies in groups, and the understanding of the group structure and evolution is important because the group environment is the locus of the
majority of galaxies in the Universe.
Tab. 1 Galaxy group list. The 1.4 GHz flux density is from the NVSS.
2. Goals of the project and selected sample
The main goal of our project is to study the properties of the central radio
source in galaxy groups over a broad frequency range. In particular, we
want to examine the AGN effects at various phases of activity, and study
the geometry, timescales and physical mechanisms of the energy
injection. Our main tool is the study of the spectral properties of the
central radio galaxies, by means of the total and point – to - point
spectrum. This will allow us to obtain estimates of the radiative age of
the emitting electrons, which will be examined in the light of the X-ray
properties of the group environment. The progressive loss of particle
energy produces a significant steepening of the radio spectrum, so that
evidence of a former AGN activity (possibly related to presence of ghost
cavities) may be reflected only at low frequencies.
To this aim, we selected 18 galaxy groups (Tab.1) by means of a
combination of X-ray and radio criteria. All groups are well-studied in the
X-ray band, with high quality data from Chandra and/or XMM-Newton, and
show, whether in the radio or X-rays or both, structure suggestive of strong
AGN interaction with the intragroup gas.
We have been observing our sample at 610 MHz, 327 MHz and 235 MHz
with the Giant Metrewave Radio Telescope (GMRT, Pune, India). The
observational status is reported in Table 1. 16/18 groups have been
observed at 610 MHz; the completion of the 235 MHz observations will be
performed in August 2008; the completion of the 327 MHz will be carried
out as future follow up.
The full resolution of our images ranges from ~6” at 610 MHz, to ~15”
at 235 MHz. The sensitivity (1σ) in the final images ranges from ~
50μJy/b at 610 MHz to ~ 0.4 mJy/b at 235 MHz.
Preliminary results
S1.4 GHz
(mJy)
GMRT data
Source
RA & DEC
UGC 408
NGC 315
NGC 383
NGC 507
NGC 741
00 39 18.6 +03 19 52
00 57 48.9 +30 21 09
01 07 25.0 +32 24 45
01 23 40.0 +33 15 20
01 56 21.0 +05 37 44
0.0147
235&327
0.0165
610
0.0170
610
0.0165
610
0.0185 235, 327, 610
HCG 15
02 07 37.5 +02 10 50
0.0208
610
10
NGC1407
NGC 1587
MKW 02
NGC3411
NGC4636
HGC 62
NGC5044
NGC5813
NGC5846
AWM 04
NGC6269
NGC7626
03 40 11.9
04 30 39.9
10 30 10.7
10 50 26.1
12 42 50.4
12 53 05.8
13 15 24.0
15 01 11.2
15 06 29.3
16 04 57.0
16 58 02.4
23 20 42.3
0.0059
0.0123
0.0380
0.0153
0.0031
0.0137
0.0090
0.0066
0.0057
0.0318
0.0344
0.0114
610
610
235,610
235, 610
235, 610
235, 327, 610
235, 327, 610
235, 327, 610
235, 327, 610
235, 610
-
90
120
385
38
300
5
35
15
20
650
51
130
-18 34 49
+00 39 43
-03 09 48
-12 50 42
+02 41 24
-09 12 16
-16 23 06
+01 42 07
+01 36 20
+23 55 14
+27 51 42
+08 13 02
z
(MHz)
310
640
1100
100
1000
Here we present some preliminary results of the radio/X-ray
analysis for a few radio galaxies and the results for the group
AWM04, which can be considered a prototypical study for our
aims
AWM04
NGC741/742
Right – Point-to point
Left - 610 MHz
spectral index image between
Resolution 6”x5”
4.9 GHz (VLA) and 610 MHz
Centre - 610 MHz
235-610 MHz spectral index image (colours) with
GMRT 327 MHz on DSS
235 MHz contours overlaid (S prop.to ν –α )
Spectral index trend along the jets
and fit
(GMRT) - Resolution 8”x6”
over Chandra
NGC4636 – 610 MHz over Chandra
NGC5044 – 610 MHz on Chandra
NGC507 - 610 MHz on Chandra
NGC7627 –
610 MHz over DSS2
Resolution 13 ” x 10 ”
Resolution 8” x 7” , f.c.= 1.5 mJy/b
firs contour = 2.4 mJy/b
GMRT 610 MHz
Resolution 6”x5” in all images – Average rms (1σ level) ~ 50 μJy/beam in all images
The group hosting the cD galaxy NGC741 and its companion NGC742
has a very complex and dishomogenous X-ray structure. The two
galaxies are likely in mutual tidal interaction, and a striking X-ray filament
is visible in the region between them (see central figure in the upper
frame). A spectacular large radio source dominates the radio emission. It
is unclear whether it is associated with NGC 741 or its companion. Two
compact and relatively flat spectrum components, connected by a bright
bridge of emission, are coincident with the optical nuclei of the two
galaxies.
Our 610 MHz images show that the radio emission of the central AGNs in
galaxy groups covers a wide range of morphologies (core-dominated,
twin jet, Z-shaped, typical FRI), and of linear scales (from the galactic
size of few tens of kpc, to hundreds of kpc, see the images in the lower
frame of the figure).
The high sensitivity and appropriate resolution of our GMRT images,
coupled with the high quality X-ray imaging of the intragroup gas
distribution are a promising starting point for the goals of the present
project.
AWM04 is at first glance a good example of a relaxed group, with no
significant substructure in the X-ray surface brightness. However, spectral
mapping revealed a number of features whose temperatures and
abundances diverge from the mean value of the group. Its central
cooling time of the order of 2 Gyr suggests the presence of a source of
energy injection, to prevent from catastrophic cooling (O’Sullivan et al. 2005,
MNRAS, 357, 1134). The group hosts a central wide-angle tail radio galaxy,
4C+24.36, associated with NGC6051. The overall symmetry of the radio
galaxy (shape and flux density of the jets) led us to estimate that it is
oriented at large angle to the line of sight, i.e. ~81° - 88°. Thanks to our
GMRT observations we carried out a detailed spectral analysis of the radio
galaxy, which led to an estimate of its radiative age of the order of 160 Myr.
The total energy output of the central radio galaxy fails by about two
orders of magnitude to balance the cooling. Secondary sources of
heating, such as mechanical heating of bubbles and cavities, are the most
plausible mechanism for feedback. The lack of cavities in AWM04 is still
puzzling, but we are confident that upcoming Chandra observations may
shed a light on this.
Giacintucci et al.2008, ApJ, in press (arXiv:0804.1906)