Combined array imaging of extragalactic radio sources

A translation

Chin. Astron. Astrophys. Vol. 20, No.1, pp. 26-31, 1996 of Acfo Astron. Sin. Vol. 36. No.3, pp. 245-251. 1995 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rinhts reserved 0275-1062/96-$24.00+0.00

SO2751062(96)00004-S

Combined array imaging of extragalactic radio sources t ZHANG ‘Shanghai

Observatory,

Fu-jun’

C. E. Akujo#l

Chinese Academy

’ University

of Nigeria,

of Sciences,

Shanghai

200090

Nigeria

Abstract Existing radio telescopes have quite different resolving powers with the result that images obtained with different instruments sometimes appear unrelated. In this paper we present the principles for combining data obtained with different arrays and show three actual examples of applying our method. Key words:

galaxies-structure-VLA

1. INTRODUCTION At present the existing radio telescope arrays have very different resolving powers. Taking as example imaging at 5GHz, for the Dutch WSRT, the resolving power is about 4 arcsec, for the US VLA-0.35 arcsec, for the British (phase II ) MERLINabout 0.05 arcsec, the EVN-0.005 arcsec, the global VLBI-0.001 arcsec. This means that the radio maps obtained with these instruments have different scales, and they sometimes show features that appear to be unrelated. In order to obtain structural information in between different scales, people have sought to carry out image treatment with combined data from different will link telescope arrays to obtain maps on intermediate scales. This type of treatment up the core-jet morphology on different scales, the small scale structure with the extended structure of the radio source. NAN Ren-dong has studied a method of combining the data from VLBI and MERLIN, for improving the imaging quality of the VLBI map in the extended componentl’l. The author smoothed the VLBI to the MERL:IN resolution and calculated the coordinate differences (AX, AY) between the peaks of the smoothed and MERLIN maps. A phase shift, calculated from the coordinate differences, is then applied to the MERLIN data so that the two set of data have a common reference point in the (u, u) plane. The data are then combined and imaging made.. In this paper we use the following method: first we take the (u, v) data observed by different types of arrays, apply various appropriate corrections, and combine them and make t Supported by National Natural Science Foundation Received 1993-12-06; revised version 1994-12-12 26

Combined

an imaging

treatment.

In order to obtain

27

Array Imaging

structural

information

an on intermediate

scale,

we must choose a suitable size for the convolution spot. If the spot is too large (such as that corresponding to the resolving power of MERLIN), then structural details will be smoothed out, if too small (such as corresponding to VLBI), then there will over-resolution. This method has been applied to the VLBI and MERLIN data of the radio source 3C 147 and the intermediate scale map provided much important information121. In this paper we shall briefly describe the basic principles by which we combine the (u, w) from different telescope arrays. We shall also give several examples of maps of actual extragalactic sources obtained with our method.

2. (u,v)

DATA

CORRECTION

AND

IMAGING

TREATMENT

before combining the data from different telescope arrays (such as VLA, WSRT, MERLIN and VLBI) to form a single system, certain corrections must be applied to the original data. We believe the following corrections are necessary:

2.1 There may be systematic

differences between the coordinate systems used by the different arrays. If such a difference exists, then we must reduce all the data to the same coordinate system. For example, between VLBI and MERLIN, the differences are AX = 369.34m, AY = 107.78m, AZ = 435.90m. Between VLA and VLBI/MERLIN, there are also differences. which must be corrected for before combining. 2.2 For historical reasons, observation by different arrays may have been made at different times. When this is the case, we must apply corrections for precession and nutation. In this respect the situation has improved. Nowadays, when application is made to EVN and VLBI for global VLBI observation, application may be made to VLA and MERLIN for joint observation. For data obtained at the same epoch, precession dn nutation corrections are, of course, unnecessary. 2.3

MERLIN

weighting

signal-to-noise nant,

and VLBI data use different

system

must

data,

be adopted.

the errors

and we must use a suitable

weights,

so, when combining

The two arrays

caused by telescope weighting

system

have different

tracking

the data,

sensitivities,

and gain calibration

a single for high are domi-

to reduce such errors.

2.4 For data obtained at different times, we must allow for epoch difference density. Before combining the data, absolute calibration with standards must the original data.

in the flux be made of

2.5 All the data must have common phase reference centre. If the different arrays have a common coverage in the (u, v) plane, that is, if they have a common baseline, then a common phase reference centre can be well determined. MERLIN data are usually self-calibrated by means of VLA data. After careful corrections, the (u, u) data of the combined array are given the imaging treatment. We first use the software OLAF of Jodrell Bank to obtain a preliminary model of the radio structure, then the entire data are transferred into the software package AIPS to continue the imaging treatment. The data go back and forth between the two software a

ZHANG Fu-jun & C. E. Akujor

28

number of times until the greatest features, is reached.

possible

dynamic

range showing

many detailed

structural

For the same radio source, we should aim at using data obtained at times that are as close together as possible, so that we can suppose the structure varied very little between the two epochs. In selecting the sources, we should select as far as possible those source with a complicated, but relatively stable structure. In fact, when we wish to make a map with combined data from two or more arrays, the source must have a large extended structure. On such a scale, the change in structure between two close by epochs should be extremely small.

3.

EXAMPLES

OF IMAGING

OF EXTRAGALACTIC

SOURCES

We have already given the map of 3C 147 from combined data at 1.67 and 5GHz in a previous paper 111. The MERLIN and VLBI data provided structures on both the subarcsecond and milliarcsecond scales. The final map showed many additional details and also the connections between the features on the different scales. Application of the same method to the VLA, MERLIN and VLBI data of the radio sources 3C216 and 3C380 have also resulted in successful maps. These are shown in Figs. l-3.

Fig. 1

Images of 3C216 at 1.67GHz (a) MERLIN (c) MERLIN

+ VLBI image

image (b) VLBI image

Combined Array Imaging

29

Fig. 1 is the map of 3C216 at 5GHz. l(a) shows the MERLIN (phase 1) map. The convolution spot size is 0.25”. The peak flux is 0.68 Jy/beam. The contour lines are at (-0.125,0.125,0.25,0.50,1.0,~~~, 64.0,99.0)% of the peak. l(b) is the VLBI (EVN) map, convolution spot 25 mas, peak flux 0.45 Jy, contours (-0.5,0.5,1/O, 2.0, . . . ,64.0,99.0) % of the peak. It shows the structure on the milliarcsecond scale and covers only the central component

source.

l(a) no longer

The two component

appear

here under

Fig. l(c) shows the combined

sources

to the northwest

and southwest

shown

in

the VLBI resolution. MERLINSEVN

map.

The convolution

spot size is 50mas

and the peak flux, 0.53 Jy/beam. The contours are at (-0.125,0.125,0.25,0.5,. . -, 64.0,99.0) of the peak. This map shows the structures of both the compact central portion and the large scale extensions. The two extensions corresponds to the two components about the central component in l(a), but show many more detailed structures.

41.0 -

lb’

46-

HICHT

ASCENSKJN~B

IMO)

;

i-113.8

13.7 I

13.6 I RIGHT

Fig. 2

13.5 I

13.4 1

13.3 I

13.2 I

13.1 3

ASCENSION(BI~M)

Images of 3C216 at 5 GHz (a) VLA image (b) MERLIN image (c) VLA + MERLIN image

30

ZHANG Fu-jun & C. E. Akujor

Fig. 2 is the amp of the same source, 3C 216, at 5 GHz. 2(a) is the VLA map, convolution (-1.0,1.0,2.0,.++, 512.0)x5x10-‘Jy/beam. spot 0.4”, peak flus 0.912 Jy/b earn, contoursare spot 50 mas, peak flux 0.676 Jy/beam, con2(b) is the MERLIN (ph ase 2) map. convolution tours (-0.125,0.125,0.25,0.5;~~,64.0,99.0% of the peak. Fig. 3(c) shows the combined 0.68 Jy/beam,

contours

VLA=MERLIN

map.

1.0,2.0,. . -, 512.0)x10S3

(-1.0,

Convolution

spot 60ma3,

peak flux

Jy/beam.

Fig. 3 gives the map of a familiar object, 3C380, at 5GHz. Wilkinson et a1.121 using the VLBI made many observations of this source. 3(a) is the VLA map. Convolution spot 0.35”, peak flux 2.62 Jy/beam, contours (-1.0,1.0,2.0,... , 512.0)~2~10-l3 On the VLA map, the source has an extension of over 4”, or a linear extension (He = 100,qe = 0.5). 3(b) is the MERLIN flux 2.494Jy/beam, contours (-1.0,1.0,2.0,

Jy/beam. of 16 kpc

(phase 1) map. Convolution spot 80mas, ~~~,512.0)~5~10-‘~ Jy/beam.

peak

7

0.6 -

ca0.45?,-

0

0.2-

56 -

o.o-

5i -

-0.2-

56-

-0.4-

.

0

_

-0.6-

,~

65 ,a\, 06 17.5

I

I

I

I

17.4

17.3

17.2

17.1

RIGHT ASCENSIOW(Bl950)

I

17.0

16.9

p

17.35

I -0.5 ARC

I

I

I

I

17.m

17.25

I

I

.O’

w

17.20

17.15

I

\

*

I 0.0

I 0.5

I

.

I -1.0,

SEC

17.10

RIGHT ASCENSION(BI~!I~I)

Fig. 3

Images of 3C380 at 5 GHz (a) VLA image (b) MERLIN image (c) VLA + MERLIN image

Combined

Array Imaging

31

Fig.3(c) is the combined VLA+MERLIN map. Convolution 2.57 Jy/beam, contours (-1.0,1.0,2.0, et 512.0)x2x 10m3 Jy/beam.

spot 0.15”, ;peak flux In 3(a), the two central

components

are surrounded by a low brightness area. The northwest component is resolved into two in (b) and (c). Th e central component source is also resolved in the VLBI map. In its VLBI map at 5GHz, 3C380 is a unidirectional core-jet structure, with the jet pointing

northwest. and extending some 25 mas (100 PC). Maps based on VLA, MERLIN and VLBI observations are on different scales, but after applying appropriate corrections to the data, combining them and applying imaging process ing we can obtain high-quality maps on intermediate scales. The combined maps provide more detailed maps and so are particularly useful in the study of source with extended structures.

4. PROSPECT

OF APPLICATION

TO

MILLIMETRE

WAVE

ARRAYS

Results of applying the present method to various telescope arrays at cm waves have been rather successful, but we think the method can be extended to the mm waves. Available mm wave arrays of different types again have different resolutions. At 115,GHz the best resolution of the Nobeyama Array in Japan is 4”, the Owen Valley Array has a resolution of about 2”, The NRA0 ARRAY is expected to have a resolution of I”151. If data from these instruments are sampled in some standard manner, we can then combine them and obtain maps at intermediate resolutions. It will then be useful to compare with those at 115 GHz or 230 GHz of at similar resolutions.

these combined

References [l]

NAN

[ 21

Zhang F. J. et al., MNRAS,

[ 31 [ 41

Wilkinson

[ 51

Brown

Padin

Ren-dong,

AAnS,

1989,30,

276

1991,250,650

P. N. et al., Parsec-Scale

Radio

Ft. E. et al., Radio Interferometry: Ft. L., Fbdio

Ikterferometry:

Jets, 1990,152 Theory,

Techniques

Theory, Techniques

and Applications,

and Applications,

1991,400

1991,410

maps