Radio Observations.

In document Optical emission lines in radio sources of intermediate power (Page 49-52)

3.1:

Introduction.

A m a jo r aim of this project is to ob tain high quality, high reso lu tio n radio im ages of a statistically significant sam ple of radio galaxies. H ith erto , th e sources in o u r sam ple h ad not been subjected to any system atic stu d y using m o d ern radio synthesis techniques. E arly a tte m p ts to o b tain stru c tu ra l in fo rm atio n on some of these rad io sources were m ade by Ekers (1969) using a two elem ent interferom eter a t Parkes. T his work m ainly consisted of fittin g m odels to visibility d a ta to o b tain an g u lar sizes an d seperations of com ponents w ithin th e radio sources. For a few sources radio m aps w ith resolutions of a b o u t half an arcm in u te are available from observations using later synthesis telescopes in th e so u th ern hem isphere such as th e Mills Cross at 408 MHz or th e Molonglo Synthesis Telescope a t 843 MHz. For m ost of th e sources in our sam ple however, these resolutions are to o low to be useful. Ekers et al. (1989) used th e pre-com pletion VLA to o b ta in high resolution m aps of a large sam ple of so u th ern radio sources. A lthough this sam ple com prised m ainly low-power radio sources, a few of the objects in our sam ple are present in th is study.

Using scaled array VLA observations at 1.4 GHz and 5 GHz we have o b tain ed rad io continuum and spectral index m aps of th e sources in our sam ple w ith typical resolution of a few arcseconds. These observations have been used to determ ine th e following radio source properties:

• T h e m orphological class of th e radio source is often am biguous w ith o u t access to high resolution d a ta to determ ine th e relative positions of h o tsp o ts, an active core and radio lobes. C urrent theories of rad io sources suggest th a t m orphological class m ay strongly reflect th e underlying physics of th e radio

jets. For exam ple it is conjectured th a t class II radio sources arise from supersonic radio jets whereas class I jets are subsonic (Bicknell 1985).

• T h e flux-density of th e core com ponent of these radio sources can only be reliably o b tain ed from radio d a ta w ith resolution on th e ord er of an arcsec- ond. A ccurate core flux-densities have been o b tain ed using a fourier filtering technique discussed in section 3.4. This technique avoids confusion of th e core com ponent w ith extended em ission associated w ith th e rad io lobes. T h ere are also a num ber of quantities derived from th e radio d a ta which yield im p o rta n t physical insight into th e processes occuring in radio sources. T hese are:

• A ccurate angular sizes of th e radio sources can be d eterm in ed from high resolution radio d ata. Section 3.4 describes a m om ent analysis technique which yields radio source sizes and position angles in a sy stem atic way w hich is relatively insensitive to resolution effects.

• For well resolved radio sources, surface brightness an d size d a ta are used to determ ine m inim um pressures and m agnetic fields in th e radio em ittin g plasm a.

• Scaled a rray observations at two frequencies are used to co n stru ct sp ec tra l index m aps. These spectral index m aps when in te rp re te d w ith in some m odel fram ew ork give an estim ate of th e age of th e p lasm a a t any p o in t w ith in th e source.

• M inim um pressures and p lasm a ages are used to derive th e energy flux in th e jets supplying plasm a to th e radio lobes in some of our sources.

T his ch ap ter is prim arily concerned w ith describing th e radio observation a n d d a ta red u ctio n process. Derived q uantities such as p lasm a ages are p resen ted in c h ap te r 4. Section 3.2 contains a brief description of th e VLA an d th e o b ser­ vations w hich were carried out for this program m e, section 3.3 p resen ts a fairly d etailed description of th e d a ta reduction process. T he rad io m aps are p resen ted in section 3.4. Section 3.5 presents some su pplem entary d a ta on m illiarcsecond scale radio flux detected from a num ber of radio sources in th e sam ple. T hese d a ta come from observations w ith th e P arkes-T idbinbilla long-baseline interferom eter.

3.2:

VLA Observations.

A detailed description of th e VLA can be found in N apier, T h o m p so n a n d Ekers (1983). Here, we outline some of th e basic concepts reg ard in g th e VLA an d i t ’s o p eratio n which are necessary to an u n d erstan d in g of o u r observing strategy.

T he VLA is a radio synthesis telescope o p erated by th e N atio n al R adio A stronom y O bservatory and located near Socorro, New Mexico. At a n o rth e rn la titu d e of 34° it is capable of observing as far so u th as —45°, alth o u g h at such so u th ern declinations th e uv coverage of the in stru m en t is severely im paired. For declinations south of —12° it is desirable to observe using a h y b rid a rray configuration (w ith a longer n o rth -so u th arm ) to o b tain reasonable uv coverage an d a fairly circular beam .

T h e in stru m e n t itself consists of an array of 27 p arab o lo id a n ten n a s each of 25 m etres diam eter. T he an ten n as are arran g ed in a “wye” configuration, th e th ree arm s of which are aligned approxim ately n o rth , south-east an d south-w est. T he 351 interfero m eter baselines yield sufficient coverage in th e uv (fourier) plane th a t useful m aps of sources stronger th a n a few tens of m illijansky can be o b tain ed from observations of approxim ately fifteen m inutes d u ra tio n — th e “sn a p sh o t” m ode of observation.

T h e VLA is designed to o p erate in four sta n d a rd configurations respectively called th e A, B, C and D configurations. W ith a m axim um baseline of 1.03 km, th e D configuration is th e m ost com pact, while th e m ost extended configuration is th e A configuration w ith a m axim um baseline of 36.4 km . T h e spacing of th e an ten n as along each p a rtic u la r arm follows a logarithm ic progression a n d is arran g ed to achieve a scaling in array size (an d hence in resolution) of a facto r of th re e betw een different array configurations. Since th ree of th e available observing b an d s (L b a n d a t 1.4 GHz, C b an d at 5 GHz and U b a n d a t 15 GHz) are spaced a facto r of 3 a p a rt in frequency, different array configurations can be used to o b tain rad io m aps a t different frequencies which have sim ilar resolution (m ore precisely, they sam ple sim ilar regions in th e uv plane). For exam ple, observations a t 5 GHz in th e D configuration m atch the uv coverage of observations at 1.4 GHz using th e C configuration. T his m atch of array configuration an d observing frequency is referred to as “scaled a rra y ” m apping and is im p o rta n t for o b tain in g sp ectral index m aps of radio sources.

As m entioned above, observing so u th ern radio sources w ith th e s ta n d a rd configurations results in poor uv coverage because of th e reduced p ro je c ted length of th e n o rth -so u th com ponent of each baseline. T his results in beam s w hich are highly elongated in th e n o rth -so u th direction. M ore satisfacto ry observations can be m ade using “h y b rid ” arrays where th e south-w est an d so u th -east arm s are a rran g ed according to one configuration (e.g. th e D configuration) an d the n o rth arm is in th e next m ost extended configuration (e.g. th e C configuration).

Table 3.1: VLA Observing Schedule.

In document Optical emission lines in radio sources of intermediate power (Page 49-52)