Diffuse Sources

Introduction

The most significant problem in searching for diffuse sources is that they are not well characterized. Most of the properties of diffuse sources, such as spectral indices and ages, are derived from detailed follow-up studies and hence are not very useful in trying to find more sources. Also, since one of the purposes of this project was to find correlations between cluster properties and the presence of diffuse sources, it was obviously not possible to use cluster properties as a guide to whether a source was a diffuse source or not. Hence our search for new diffuse sources was based on radio morphology.

We obtained images of all previously known diffuse sources (some of which are discussed in Section 1.2.2) and collated the information about their radio morphology. This information was used as a guide in our search for new diffuse sources.

Procedure

Initially we examined the MOST greyscale images, searching for any sources which had slightly extended emission. Both faint distant galaxies and cluster members appear as point sources in the MOST images. Other sources, such as radio galaxies, head-tail sources and diffuse sources are associated with extended emission. This can make the source appear asymmetric, elongated or slightly `fuzzy' in the radio greyscale image. There were 47 clusters which contained a source with extended radio emission.

For each of the 47 clusters we overlaid a DSS optical image of about $\frac{1}{3}R_A$ with MOST radio contours. Contours often reveal the nature of the radio morphology more clearly than greyscale, and the images also show which sources have possible optical hosts. We rejected sources with obvious optical counterparts and those whose radio morphology identified them as another type of radio source, for example a background lobed radio galaxy. Sources with very high intensity were not considered further since a characteristic of diffuse emission is the low surface brightness of the source. This left a group of 16 clusters with possible diffuse radio emission.

Results

For the remaining 16 clusters we made optical/radio overlays centred on the candidate diffuse sources. Some of the images are shown below and the rest are included in Appendix D. Following is a discussion of each of these sources.

    

Figure 3.1: A selection of candidate diffuse sources. Captions give names and contour values.

[A3133 2,4,6,10,30 mJy/beam] [A3376 2,3,4,5,6,8 mJy/beam] [A3389 3,4,6,9,12 mJy/beam]

A2911

This source has a bright head with some diffuse emission extending from it. There is no obvious optical counterpart, the only possible one is offset from the centre of the radio emission. However, much of the extended emission is not far above the noise and so it may not be significant. It is possible that this is a distant radio galaxy, too faint for the DSS to detect.

A3093

Although this is quite a small source, there is no optical galaxy anywhere near the peak radio emission. There is some X-ray emission associated with this cluster, but it appears to come from the central source, and does not extend throughout the cluster.

A3111

This source has two peaks in radio intensity, bridged by emission that is well above the noise. There is no obvious optical host, although there is a galaxy slightly offset from one of the peaks. The source extends outwards from the projected centre of the cluster, which is also the peak of the widespread X-ray emission. South of the source is a head-tail galaxy aligned tangential to the X-ray emission.

A3133

This is a very low surface brightness source with no optical host. Some of the emission is not far above the noise; however, it does look significant, see Figure 3.1(a). The source is located towards the cluster boundary.

A3135

This cluster has two possible diffuse sources. The one to the south is a bright source, but it is very extended and has a strange morphology. There are many optical galaxies within the area it covers and one of them is a possible optical host. There is a significant amount of X-ray emission associated with the possible host galaxy, suggesting it could be an Active Galaxy (AGN). The source to the NW is much smaller and again has no optical host.

A3164

This source has a bright head and an extended, bent tail. There is a possible optical host near the peak of the radio emission. It is likely that this is a head tail source with an extremely distorted tail, possibly due to its orbit around the cluster centre.

A3266

This cluster has a confirmed diffuse source as well as several other sources with interesting radio morphologies. This cluster is discussed in detail in Chapter 5. The diffuse source is aligned approximately tangential to the X-ray emission.

A3351

This is a small source, but it does not have any optical host and is has an asymmetric morphology. It is possible that this is a head-tail source, or a faint, distant radio galaxy.

A3372

Again, a relatively small source. This could also be a head-tail source, or a distant radio galaxy.

A3376

This is an extremely diffuse source. It has low surface brightness, covers a large area and has no optical host, see Figure 3.1(b). The source is located towards the cluster boundary, and is aligned tangential to the X-ray emission. There is at least one head-tail source in this cluster, and a large diffuse source with a higher flux density located near the peak of the X-ray emission -- see title page image. The diffuse source is also detected in the NVSS 1.4 GHz survey, although it is much fainter, see Figure D.1(l).

A3389

This is a very extended source with no optical host. See Figure 3.1(c). To the east of this source there are some large regions of low surface brightness emission which are associated with the large radio galaxy.

A3505

This is a relatively small source, with very low flux density. It has no optical host near the peak radio emission, but it does have a galaxy near one end. This could be a very distorted head-tail source.

A3553

This is a relatively small source located near the projected centre of the cluster. There is a large source, possibly a head-tail, to its south.

A3566

This is a long, narrow source which bends around at right angles. It has very low surface brightness, no obvious optical counterpart and is located towards cluster boundary. This could be a background FRI radio galaxy.

A3716

This source has two clear peaks in its radio emission which are surrounded by much lower flux diffuse emission. It is located towards the cluster boundary. This could be two head-tail sources.

A3851

A small, faint source located near the edge of the cluster. It appears to have two slight peaks in the radio emission, and could be a distant radio galaxy.

A3880

A large source which extends from the centre of the cluster, right out to $\frac{1}{3}R_A$. The peak in radio emission is centred on the dumbbell cD galaxy. It is possible that there are two sources here as the bridge between them has very low flux density.

   
Confidence Rating

The next chapter deals with correlations between cluster properties and the presence of diffuse sources. To increase the validity of the results we have given each of the 17 sources (in 16 clusters -- there are 2 candidates in A3135) one of the following ratings:

1: The source is likely to be a diffuse source. It has most of the characteristic properties of a diffuse source; large extent, low surface brightness, no optical counterpart.

2: It is uncertain whether the source is a diffuse source. There is no optical galaxy aligned with the peak radio emission but it does not have all the properties of a diffuse source.

3: The source is unlikely to be a diffuse source.

The 14 sources with a rating of 1 or 2 will be used as `positive detections' for the statistical analysis in Chapter 4.

Summary

 

Table: Properties of the candidate diffuse sources. RA is given to the nearest 5'' and Dec to the nearest 1'. Dist is the projected distance of the diffuse source from the cluster centre ($\pm$ 10 kpc). Size is the linear extent of the diffuse source ($\pm$ 10 kpc), measured along its longest axis. Peak is the peak flux density ($\pm$ 0.5 mJy), and Int is the integrated flux density ($\pm 5\%$). P₈₄₃ is the power in units of $10^{23} \mbox{ WHz}^{-1}$. The Ratings are described in Section 3.4.

ACO	Source Location		Dist	Size	S843 mJy		P843	Rating
	RA	Dec	kpc	kpc	Peak	Int
A2911	01:27:10	-37:36:00	640	210	4	118	1	2
A3093	03:10:40	-47:31:00	400	200	<1	11	<1	2
A3111	03:17:45	-45:45:00	90	550	5	246	37	1
A3133	03:31:50	-45:53:00	760	370	1	40	3	1
A3135	03:34:58	-38:51:00	570	290	2	55	5	3
A3135	03:34:10	-39:00:00	450	700	32	2937	258	2
A3164	03:46:00	-56:59:00	1100	430	5	213	17	3
A3266	04:31:00	-61:22:00	450	410	3	176	14	1
A3351	05:31:45	-38:25:00	280	180	3	79	12	2
A3372	05:55:45	-34:46:00	150	260	1	20	3	2
A3376	06:00:10	-40:02:00	350	560	1	140	6	1
A3389	06:23:40	-64:53:00	430	210	1	119	2	1
A3505	12:08:45	-34:24:00	250	380	<1	11	2	2
A3553	13:19:20	-37:07:00	220	230	3	59	3	2
A3566	13:38:55	-35:25:00	450	220	1	34	2	2
A3716	20:52:00	-52:46:00	680	360	20	754	34	1
A3880	22:27:40	-30:30:00	340	870	44	1756	130	3

We have found 17 candidate diffuse sources from a sample of Abell southern clusters and have classified 14 of them as `positive detections' for the purpose of statistical analysis. We based our search on the radio morphologies of previously studied diffuse sources. The peak and integrated flux densities, linear extent and distance from the cluster centre were measured for each source. The calculation of integrated flux densities will be discussed further in Section 5.3.2. We calculated the power, P₈₄₃, of each source from its integrated flux density, S₈₄₃, using the inverse square law

$$P_{843} = 4 \pi d^2 S_{843}$$ (3.1)

where d is the distance to the source, given by the Hubble relation

$$cz = H_0 d \qquad (z<<1)$$ (3.2)

Table 3.1 shows the quantitative characteristics of the candidate diffuse sources. These are comparable with the characteristics of previously studied diffuse sources. On average our sources are smaller than previously studied ones, however this is probably due to selection effects in earlier surveys -- in low angular resolution surveys the larger sources are easier to detect than the smaller ones.